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GUIDELINES FORTHEDESIGN OF FLEXIBLE PAVEMENTS (Fourth Revision)
Published by:
INDIAN ROADS CONGRESS Kama Koti Marg, Sector-6, R.K. Puram, New Delhi-110 022 June 2017 Price : ` (Plus Packing & Postage)
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First Published
: September, 1970
Reprinted
: December, 1976
First Revision
: December, 1984
Reprinted
: October, 1990 (Incorporates Amendment No. 1, September 1988
Reprinted
: April, 1995
Second Revision
: July, 2001
Reprinted
: March, 2002
Reprinted
: July, 2004
Reprinted
: April, 2005
Reprinted
: June, 2006
Reprinted
: June, 2007
Reprinted
: December, 2007
Reprinted
: September, 2008
Reprinted
: October, 2009
Reprinted
: July, 2011
Third Revision Fourth Revision
: July, 2012 2017
(All Rights Reserved. No part of this publication shall be reproduced, translated or transmitted in any form or by any means without the permission of the Indian Roads Congress)
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CONTENTS Page No. Personnel of Highways Specifications and Standards Committee 1.
Introduction
2.
Scope of Guidelines
3.
General
4.
Traffic
4.1
General
4.2
Traffic growth rate
4.3
Design life
4.4
Vehicle damage factor
4.5
Distribution of Commercial traffic over the carriageway
4.6
Computation of design traffic
5.
Sub-grade
5.1
Requirements of CBR for Sub-grade
5.2
Effective CBR
5.3
Determination of resilient modulus
6.
Principles of Pavement Design
6.1
Pavement Model
6.2
Fatigue in bottom layer of bituminous pavement
6.3
Rutting in Pavement
6.4
Top-down cracking in bituminous layer
6.5
Cementitious Sub-base and base
7.
Pavement Composition
7.1
General
7.2
Sub-base layer
7.3
Base layer
7.4
Bituminous layer
8.
Perpetual Pavement 5
(i)
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9.
Pavement Design Procedure
9.1
Using IITPAVE
9.2
Using Design Catalogue
9.3
Material Properties
10.
Pavement Design Catalogues
10.1
Granular base and granular Sub-base
10.2
Cemented base and cemented sub-base with crack relief interlayer of aggregate
10.3
Cemented base and cemented sub-base with SAMI at the interface of cemented base and the bituminous layer
10.4
Foamed bitumen/bitumen emulsion treated RAP
10.5
Cemented base and granular sub-base with crack relief layer of aggregate interlayer
10.6
Other pavement composition
11.
Internal Drainage in Pavement
12.
Design in Frost Affected Areas
13.
Summary of Design Procedure and use of IITPAVE software
Annex-I:
Consideration in Design of Bituminous Pavement
Annex-II:
Worked out Examples Illustrating the Design Method
Annex-III:
Equivalence of thickness of bituminous mixes of different moduli
Annex-IV: Annex-V: Annex-VI:
Preparation of Laboratory Test Specimens for CBR Test and Selection of Sub-grade CBR Drainage layer Recommendation for Bituminous Wearing Courses for Flexible Pavement
Annex-VII:
Selection of Grade of Binders and Mixes for Bituminous Courses
Annex-VIII:
Resilient Modulus of Granular Materials
Annex-IX:
Reclaimed Asphalt Pavement and Mix Design
Annex-X:
Pavement Layers with Chemical Stabilized Materials
Annex-XI :
Properties of Cementitious base and Sub-base
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PERSONNELOF THE HIGHWAYS SPECIFICATIONS AND STANDARDS COMMITTEE ABBREVIATIONS All symbols are explained where they occur first. Some of the symbols are, AAAT
-
Average Annual Air Temperature
AAPT
-
Average Annual Pavement Temperature
AMAT
-
Average Monthly Air Temperature
AMPT
-
Average Monthly Pavement Temperature
AASHTO
-
American Association of State Highway and Transportation Officials
ASTM
-
American Society of Testing and Materials
AUSTROADS
-
Association of Australian and New Zealand Road Transport and Traffic Authorities.
BC
-
Bituminous Concrete
BIS
-
Bureau of Indian Standards
BM
-
Bituminous Macadam
Cs
-
Spacing of Transverse Cracks
CBR
-
California Bearing Ratio
CFD
-
Cumulative Fatigue Damage
CTB/CT
-
DBM
-
Cement Treated Base-includes all type of Cement Chemical stabilized bases Dense Bituminous Macadam
E
-
Elastic Modulus of Cementitious Layer
GB
-
Granular Base
GDP
-
Gross Domestic Product
GSB
-
Granular Sub-base
Ic
-
Crack Infiltration Rate per Unit Length
IRC
-
Indian Roads Congress
Kp
-
Infiltration Rate Per Unit Area of Un-Cracked Pavement Surface
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MR
-
Resilient Modulus
MRUP
-
Modulus of Rupture
MEPDG
-
Mechanistic Empirical Pavement Design Guide
msa
-
Million Standard Axles
MORTH
-
Ministry of Road Transport & Highways
MSS
-
Mixed Seal Surfacing
Nc
-
No. of Longitudinal Cracks
Nf
-
Cumulative No. of Repetitions for Fatigue Failure
NR
-
Cumulative No .of Repetitions for Rutting Failure
PC
-
Premix Carpet
qi
-
Water Infiltration Rate Per Unit Area
SAMI
-
Stress Absorbing Membrane Interlayer
RAP
-
Reclaimed Asphalt Pavement
RF
-
Reliability Factor
SDBC
-
Semi-Dense Bituminous Concrete
SD
-
Surface Dressing
SDP
-
State Domestic Product
UCS
-
Unconfined Compressive Strength
Va
-
Volume of Air Voids
Vb
-
Volume of Bitumen
VDF
-
Vehicle Damage Factor
VG
-
Viscosity Grade
Wp
-
Width of Pavement Subjected to Infiltration
Wc
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Length of Transverse Cracks
WBM
-
Water Bound Macadam
WMM
-
Wet Mix Macadam
εt
-
Horizontal Tensile Strain
εv
-
Vertical Subgrade Strain
µ
-
Poisson’s Ratio
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µε
-
Micro Strain
GUIDELINES FOR THE DESIGN OF FLEXIBLE PAVEMENTS 1 INTRODUCTION 1.1The first guidelines on design of flexible pavement published in 1970 were based on (i) California Bearing Ratio of subgrade and (ii) traffic in terms of number of commercial vehicles more than 3 tonnes laden weight. They were revised in 1984 in which design traffic was considered in terms of cumulative number of equivalent standard axle load of 80 kN and design charts were provided for traffic up to 30 msa using an empirical approach. 1.2The second revision was carried out in 2001 when pavements were required to be designed for traffic up to150msa because of faster growth of commercial vehicles. The revised guidelines used a semi-mechanistic approach based on the results of the MORTH’s research scheme R-56 implemented at IIT Kharagpur. The software, FPAVE was developed for the analysis and design of flexible pavements. Multilayer elastic theory was adopted for stress analysis of the layered elastic system. A large number of data collected from different parts of India under various research schemes of MORTH implemented by academic Institutes and CRRI were used for the development of fatigue and rutting criteria from field performance data and laboratory tests. 1.3 The third revision was carried out in 2012 to include new development around the world from sustainability considerations. The volume of tandem, tridem and multi-axle vehicles had increased manifold and heavier axle loads had increased considerably due to better network of roads. Attention was focused on fatigue resistant bituminous mixes with high viscosity binders for heavy traffic with a view to construct high performance long life bituminous pavements. 1.4 The fourth revision is done to include the feedback from the professionals on the performance bituminous pavements for the heavily trafficked highways. The experience on the use of new form of construction and materials such as stone matrix asphalt, Gap Graded Rubberised bitumen mix, modified bitumen, foamed bitumen, bitumen emulsion, warm mix asphalt, cementitious bases and sub-bases were utilized in improving the guidelines. The current version fine tunes the third revision to address frequent early failure of bituminous concrete and Dense Bituminous Macadam which is of great concern to all involved with highway projects. Guidelines include check for minimum thickness of cement treated bases and granular subbases for structural safety against the construction traffic. High modulus bituminous bases has also been included in the guidelines since this is being commonly being used in Europe, and now being practiced in South Africa, Australia and some states in USA. Conventional construction material like aggregates is becoming progressively scarce on
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account of environmental concerns as well as legal restrictions on quarrying while the construction activity has expanded phenomenally. This has shifted focus from large scale use of conventional aggregates to use of local, reclaimed asphalt pavement materials, recycled concrete and engineered marginal aggregates in construction. Accordingly, refinement has been carried out in the current revision of IRC 37 on use of new and alternate materials in the current design practices. A designer can use his/her sound engineering judgment consistent with local environment using a semi-mechanistic approach for design of pavements. Sinha,A.V. Bose, Dr. Sunil Nirmal, S.K.
- Convenor - Co-Convenor - Member-Secretary Members
Basu, Chandan Bhanwala, Col. R.S. Bongirwar, P.L. Das, Dr.Animesh Dushaka, Vanlal Gajria, Maj. Gen. K.T. Jain, Dr. M.C. Jain, R.K. Jain, Rajesh Kumar Jain, Dr. S.S. Kandhal, Prof. Prithvi Singh
Katare, P.K. Krishna, Prabhat Lal, Chaman Nigam, Dr. S.K. Pachauri, D.K. Pandey, R.K. Sarma, Dr. Sivaram B. Sharma, S.C. Tyagi, B.R. Wasson,Ashok Yadav,Dr.V.K.(Rep.ofDGBR) Corresponding Members
Bhattacharya, C.C. Dongre, Dr. Raj
Justo, Dr. C.E.G. Sharma, S.K.
Ex-Officio Members President, IRC Director General (RD) & SS, MORTH Secretary General, IRC
(Yadav, Dr. V.K., VSM) (Indoria, R.P.)
1.6 The revised draft was prepared by Prof. B.B.Pandey of IIT Kharagpur after a feedback during an open house discussion of IRC: 37-2012 at NHAI headquarters on 9th April 2016 and comments of various consssultants and the members of the H-2 committee on 6th May 2017during the H-2
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committee meeting. four years after the publications of the guidelines. Experts from different organization presented their views on various clauses and some concerns about a few clauses were shared. The revised document considering the comments from consultants, field performance, experience of the users and that of IIT Kharagpur is presented before the Flexible Pavement Committee (H-2) 1.7 The finalized document was submitted to the Highways Specifications and Standards Committee (HSS) of IRC for their consideration. The document was approved by the Highways Specifications and Standards Committe (HSS) in its meeting held on 23.9.2011. The Executive Committee in its meeting held on 7.10.2011approved the document for Placing before Council. The document was approved by the IRC Council in its meeting held on 3.11.2011 at Lucknow. The DG (RD) & SS authorized the Convenor of the Flexible Pavement Committee (H-2) to incorporate the comments offered by the Council members. The comments have been incorporated and the document has been finalized for printing as one of the revised Publications of IRC.
2 SCOPE OF THE GUIDELINES 2.1 The Guide lines shall apply to the design of new flexible pavements for Expressways, National Highways, State Highways, Major District Roads and other categories of roads predominantly carrying motorized vehicles. Users are expected to use their skill, experience and engineering judgment considering the local environment and past pavement performance in the respective regions while selecting a pavement composition 2.2There can be many compositions for a strong and durable flexible pavement with bituminous surfacing of which a few given in the following are discussed in the guidelines. : (i) Granular base and sub-base (ii) Cementitious bases and cementitious sub-bases with a crack relief layer of aggregate interlayer below the bituminous surfacing (iii)
Cementitious bases and cementitious sub-bases with SAMI in-between bituminous surfacing and the cementitious base layer for retarding the reflection cracks in to the bituminous layer
(iv) (v) (vi)
Reclaimed Asphalt Pavement(RAP) base treated with foamed bitumen/bitumen emulsion Cementitious subbase, WMM base and bituminous surfacing High Modulus Bituminous mixes with stiffer binders. Cost and availability of materials will determine choice of different layers
Use of deep strength long life bituminous pavement is also included in the guidelines in the light of wide international experience. Thickness design with different compositions are illustrated with examples 2.3 These guidelines shall not be straight way applied to overlay design for which Falling Weight Deflectometer (IRC: 115-2014) or IRC:81-1997 should be used (32,33). 2.4 The guidelines may require revision from time to time in the light of future development and
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experience in the field. Towards this end, it is suggested that all the organizations intending to use the guidelines should keep a detailed record of the year of construction, subbgrade CBR, soil characteristics including resilient modulus, pavement composition and specifications, traffic, pavement performance, overlay history, climatic conditions etc. and provide feedback to the Indian Roads Congress for further revision.
3 GENERAL 3.1 The Mechanistic-Empirical approach adopted in IRC:37-2001 and IRC:37-2012 was retained in the current guidelines also. For a design life up to 20 msa, a reliability of 80% is recommended for the determination of pavement thickness. It implies that the pavement reaches the end of the design life when 20% of the surface area may have develop about 20% fatigue cracking or have a rutting of 20mm in the design period whichever occurs earlier. For design traffic exceeding 20 million standard axles, the cracking and rutting have been restricted to 10 percent of the area. The cracking and rutting models are based on the data of the research schemes of the Ministry of Road Transport & Highways, Government of India, under which pavement performance data were collected from all over India by academic institutions and Central Road Research Institute (56,59,60) to evolve the fatigue and rutting criteria for pavement design using a semi-analytical approach. In the absence of any further research in the field to modify or refine these models, the same models are considered applicable to these guidelines as well. These revised guidelines retain the scope of pavement design of the last version which included alternate materials like cementitious and reclaimed asphalt materials for sustainability, and subjecting them to a rigorous stress analysis using the software IITPAVE in view of good performance of alternate materials such as cement treated bases and recycled Asphalt pavement stabilized with foamed bitumen. The material properties of these alternate materials, such as Resilient/Elastic Modulus, were extensively tested in laboratories in the country, especially IIT, Kharagpur. Conservative values of material properties are suggested in these Guidelines because of variation in test results on materials from different sources. The basic approach from National Standards of Australia, South Africa and MEPDG of the USA as well as those adopted in some of the satisfactorily performing pavements constructed in the country using cementitious and RAP bases was also considered. The material properties that affect the design greatly however should be tested in laboratory as per the standard test procedures. 3.2 The experience on a number of high volume highways constructed during the last fifteen years using the guidelines of IRC: 37-2001and IRC: 37-2012 shows that the common mode of distress has been flushing, and rutting in the bituminous layer (51, 53 and 63). Surface cracking of the bituminous layer (i.e. top down cracking) within a year or two of traffic loading followed by pothole formation during the monsoon were also reported from different parts of India (39, 53 63,74,74). Published literatures on fatigue and rutting of different types of bituminous mixes have helped in better understanding of these problems (22, 26, 28, 40, 44, 45, 49 and 53). The present guidelines strongly recommend that these problems need serious consideration. Bituminous mix design needs to be considered an integral part of pavement design exercise with a view to provide (i) fatigue and moisture resistant mixes in the bottom bituminous layer (ii) rut and moisture resistant bituminous
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mix for the intermediate layer and (iii) rut, moisture, crack and age resistant wearing course. 3.3 Bituminous pavements with cementitious bases and cementitious subbases recommended in the 2012 version of the guidelines have given good performance in different parts of India. The low strength cementitious materials are cost effective when marginal aggregates are used.. They may develop only fine shrinkage and traffic induced cracks after the construction and the long term effective layer moduli would be lower than those determined in the laboratory by unconfined compression test commonly used for cement concrete. Elastic Moduli of such layers are to be judiciously selected to ensure a good long term performance as a structural layer. Their fatigue fracture behavior of the cementitious base is analyzed on the same principles that are applied to concrete pavements. Only low strength cementitious bases are recommended since high strength rigid bases may develop wide shrinkage cracks which may reflect to the bituminous surface. Due to lower strength requirement of the cemented sub-bases and bases, the required compressive strength can be easily achieved even by stabilizing local and marginal materials. While their strength may be low, it is essential to ensure a reasonable level of durability by ‘wetting’ and ‘drying’ test for the cementitious bases. The thickness of cement bound base must be sufficient to carry the construction traffic. 3.4 The Guidelines recommend that the following aspects should be given consideration while designing better performing pavements: (i)
Incorporation of design period of more than fifteen years.
(ii)
Computation of effective CBR of subgrade for pavement design.
(iii)
Use of rut resistant surface and binder layer.
(iv)
Use of fatigue and moisture resistant bottom bituminous layer.
(v)
Selection of surface layer to prevent top down cracking.
(vi)
Use of bitumen emulsion/foamed bitumen treated Reclaimed Asphalt Pavements in base course.
(vii)
Consideration of stabilized sub-base and base with locally available soil and aggregates.
(viii)
Design of drainage layer where necessary.
(ix)
Computation of equivalent single axle load considering (a) single axle with single wheels (b) single axle with dual wheels (c) tandem axle and (d) tridem axles.
(x)
Design of long life pavements with deep strength bituminous layer. Each of the items listed above has been discussed in these guide lines at appropriate places.
3.5 Load associated failure is considered as the mode of failure in these guidelines as environmental effects on bituminous layers are built-in in the calibration of rutting and fatigue equations from pavement performance.
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4 TRAFFIC 4.1 General
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4.1.1 The pavement design method considers design traffic in terms of the cumulative number of standard axles (80kN) to be carried by the pavement during the design life. Axle load spectrum data are required where cementitious bases are used for evaluating the fatigue damage of such bases for heavy traffic. Following information is needed for estimating design traffic: (i) Initial traffic after construction in terms of number of Commercial Vehicles per Day (CVPD). (ii)
Traffic growth rate during the design life in percentage.
(iii)
Design life in number of years.
(iv)
Spectrum of axle loads.
(v)
Vehicle Damage Factor (VDF).
(vi)
Distribution of commercial traffic over the carriageway.
4.1.2 Only the number of commercial vehicles having gross vehicle weight of 30 kN or more and their axle loads are considered for the purpose of design of pavement. 4.1.3Assessmentof the present day average traffic should be based on seven-day-24-hour count made in accordance with IRC: 9-1972 (72). 4.2 Traffic Growth Rate 4.2.1 The present day traffic has to be projected for the end of design life at growth rates (‘r’) estimated by studying and analyzing the following data: (i)
The past trends of traffic growth; and
(ii)
Demand elasticity of traffic with respect to macro-economic parameters (like GDP or SDP) and expected demand due to specific developments and land use changes likely to take place during design life.
4.2.2 If the data for the annual growth rate of commercial vehicles is not available or if it is less than 5 per cent, a minimum growth rate of 5 per cent should be used (IRC:SP:84-2014). 4.3 Design Life 4.3.1 The design life is defined in terms of the cumulative number of standard axles in msa that can be carried before a major strengthening, rehabilitation or capacity augmentation of the pavement is necessary. 4.3.2 It is recommended that pavements for National Highways and State Highways should be designed for a minimum design life of 15 years. Expressways and Urban Roads may be designed for a life of 20 years or higher using innovative design adopting high fatigue and rut resistant bituminous mixes. In the light of experience in India and abroad, very high volume roads with design traffic greater than 300 msa and long life pavements can also be designed using the principles stated in the guidelines. For other categories of roads, a design life of 10 to 15 years may be adopted.
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4.3.3 Stage construction for major highways may be adopted in exceptional conditions only when the traffic projection is very uncertain. In such cases, thickness of granular layer should be provided for the full design period. The bituminous layer of a pavement designed for a short life of ten years will get cracked after the design period and any bituminous overlay will need raising of levels of shoulder and the median kerb. The overlay over a cracked and damaged bituminous layer will have a shorter life due to reflection cracks under heavy traffic condition. Major strengthening/rehabilitation of pavements after a short design period in the stage construction may lead to high life cycle cost due to higher cost of rehabilitation at an early stage. In case of cemented bases and sub-bases, stage construction may lead to early failure because of high flexural stresses in the cemented layer. 4.4 Vehicle Damage Factor 4.4.1 The guidelines use Vehicle Damage Factor (VDF) in estimation of cumulative repetitions of standard axles load for thickness design of pavements. In case of cemented bases, cumulative fatigue damage principle, applicable to rigid pavement, is used for determining fatigue life of cementitious bases for heavy axle load repetitions using the data for the spectrum of axle loads. 4.4.2 The Vehicle Damage Factor (VDF) is a multiplier to convert the number of commercial vehicles of different axle loads and axle configuration into the number of repetitions of standard axle load of magnitude 80 kN. It is defined as equivalent number of standard axles per commercial vehicle. The VDF varies with the vehicle axle configuration and axle loading. 4.4.3 The equations for computing equivalent single axle load factors for single, tandem and tridem axles given below should be used for converting different axle load repetitions into equivalent standard axle load repetitions. Since the VDF values in the AASHO Road Test for flexible and rigid pavement are not much different, for heavy duty pavements, the computed VDF values are assumed to be same for bituminous pavements with cemented and granular bases. 𝐴𝑥𝑙𝑒 𝑙𝑜𝑎𝑑 𝑖𝑛 𝐾𝑁 4 ) ---------------4.1 65
Single axle with single wheel on either side ˭ (
𝐴𝑥𝑙𝑒 𝑙𝑜𝑎𝑑 𝑖𝑛 𝐾𝑁 4 ) -------------80
4.2
𝐴𝑥𝑙𝑒 𝑙𝑜𝑎𝑑 𝑖𝑛 𝐾𝑁 4 ) -------------148
4.3
Single axle with dual wheel on either side ˭ (
Tandem axle with dual wheel on either side ˭ (
𝐴𝑥𝑙𝑒 𝑙𝑜𝑎𝑑 𝑖𝑛 𝐾𝑁 4 ) ------------224
Tridem axle with dual wheel on either side ˭ (
4.4
.
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4.4.4 Multi-axle vehicles may consist of combination of the able axle classes 4.1 to 4.4 as given above. VDF should be arrived at carefully by carrying out specific axle load surveys on the existing roads for a minimum of 24 hours in each direction. Minimum sample size for survey is given in Table 4.1.Light Commercial Vehicles and Pickups contribute little to VDF in comparison with two axles and multi-axle vehicles. Axle load survey should not have any bias for loaded or unloaded vehicles. On some sections, there may be significant difference in axle loading in two directions of traffic. In such situations, the VDF should be evaluated direction wise. Each direction can have different pavement thickness for divided highways depending upon the future axle load pattern. Table 4.1 Sample Size for Axle Load Survey Total number of Commercial Vehicles per day
Minimum percentage of Commercial Traffic to be surveyed
<3000
20 per cent
3000 to 6000
15 per cent
>6000
10 per cent
4.4.5 Axle load spectrum The spectrum of axle load in terms of axle weights of single, tandem, tridem and multi-axle should be determined and compiled under various classes with class intervals of 10kN,such as 10 kN, 20 kN and 30 kN for single, tandem and tridem axles respectively. 4.4.6 Where sufficient information on axle loads is not available and the small size of the project does not warrant an axle load survey, the default values of vehicle damage factor as given in Table 4.2 may be used. Table 4.2 Indicative VDF Values Initial traffic volume in terms of commercial vehicles per day
Terrain Rolling/Plain
Hilly
0-150
1.5
0.5
150-1500
3.5
1.5
More than 1500
4.5
2.5
4.5 Distribution of Commercial Traffic over the Carriageway 4.5.1 Distribution of commercial traffic in each direction and in each lane is required for determining the total equivalent standard axle load applications to be considered in the design. In the absence of adequate and conclusive data, the following distribution may be assumed until more reliable data on placement of commercial vehicles on the carriageway lanes are available:
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(i)
Single-lane roads
Traffic tends to be more channelized on single-lane roads than two-lane roads and to allow for this concentration of wheel load repetitions, the design should be based on total number of commercial vehicles in both directions. (ii)
Two-lane single carriageway roads
The design should be based on 50 per cent of the total number of commercial vehicles in both directions. If vehicle damage factor in one direction is higher, the traffic in the direction of higher VDF is recommended for design. (iii)
Four-lane single carriageway roads
The design should be based on 40 per cent of the total number of commercial vehicles in both directions. (iv)
Dual carriageway roads
The design of dual two-lane carriageway roads should be based on 75 per cent of the number of commercial vehicles in each direction. For dual three-lane carriageway and dual four-lane carriageway, the distribution factor will be 60 percent and 45 percent respectively. 4.5.2 Where there is no significant difference between traffic in each of the two directions, the design traffic for each direction may be assumed as half of the sum of traffic in both directions. Where significant difference between the two streams exists, pavement thickness in each direction can be different and designed accordingly. For two way two lane roads, pavement thickness shall be the same for both the lanes even if VDF values are different in different directions, and designed for higher VDF. For divided carriageways, each direction may have different thickness of pavements if the axle load patterns are significantly different but the possibility of future traffic growth with enhanced economy should also be considered. 4.6 Computation of Design Traffic 4.6.1 The design traffic in terms of the cumulative number of standard axles to be carried during the design life of the road should be computed using the following equation:
𝑁=
365×[(1+𝑟)𝑛 −1] 𝑟
×𝐴×𝐷×𝐹
-------
4.5
Where, N= Cumulative number of standard axles to be catered for in the design in terms of msa. A=Initial traffic in the year of completion of construction in terms of the number of Commercial Vehicles Per Day (CVPD). D=Lane distribution factor (as explained in para 4.5.1). F = Vehicle Damage Factor (VDF).
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n = Design life in years. r=Annual growth rate of commercial vehicles in decimal (e.g., for 5 percent annual growth rate, r =0.05 The traffic in the year of completion is estimated using the following formula: A= P(1 + r)x
4.6
Where, P= Number of commercial vehicles per day as per last count. x=Number of years between the last count and the year of completion of construction.
5 SUBGRADE 5.1 Requirements of CBR for Subgrade The top 500 mm of the subgrade immediately below the the pavement can be made up of in-situ material, select soil, or stabilized soil forming the foundation of a pavement. It should be well compacted to limit the rutting due to additional densification during the service life. It shall be compacted to a minimum of 97 per cent of laboratory dry density achieved with heavy compaction as per IS: 2720 (Part 8) for Expressways, National Highways, State Highways, Major District Roads and other heavily trafficked roads. IRC: 36 “Recommended Practice for the Construction of Earth Embankments for Road Works” should be followed for guidance during planning and execution of work. The select soil forming the top 500mm of the subgrade should have a minimum CBR of 8 percent for roads having traffic of 450 commercial vehicles per day or higher. The guidelines for preparation of samples, testing and acceptance criteria are given in sub-paras given below. The in-situ CBR of the subgrade soil can also be determined from the Dynamic Cone Penetrometer (60°cone) from the following relation (ASTM-D6951-09) (11). Log10CBR = 2.465 – 1.12 log10N
5.1
Where N = mm/blow 5.1.1 Selection of dry density and moisture content for test specimen 5.1.1.1 The laboratory test conditions should represent the field conditions as closely as possible. Compaction in the field is done at a minimum of 97 per cent of the laboratory density at moisture content corresponding to the optimum moisture content. In field, the subgrade undergoes moisture variation depending upon local environmental factors, such as, the water table, precipitation, soil permeability, drainage conditions and the extent to which the pavement is impermeable to moisture, which affect the strength of the subgrade in terms of CBR. In high rainfall areas, lateral infiltration through unpaved shoulder, median, porous and cracked wearing surface may have significant effect on the subgrade moisture condition.
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As a general practice, the worst field moisture is simulated by soaking the specimens in water for four days so that the subgrade CBR is not underestimated.. 5.1.1.2 Number of tests, design value and tolerance limit. Where different types of soils are used in subgrade, a minimum of six to eight average CBR values (average of three tests) for each soil type along the alignment will be required for determination of th design CBR. The CBR value corresponding to 90 percentile should be adopted as the design CBR. 90 percent of the CBR values are equal or greater than the design value for high volume roads such as Expressways, National Highways and State Highways. For other categories of roads, design can be th based on 80 percentile of laboratory CBR values. Method of computation of 90 percentile CBR is given in Annexure IV. Pavement thickness on new roads may be modified at intervals as dictated by the changes in soil profile and identification of the homogeneous sections though it will be found inexpedient to do so frequently from practical considerations. The maximum permissible variation within the CBR values of the three specimens should be as indicated in Table 5.1. Table 5.1 Permissible Variation in CBR Value CBR ( per cent)
Maximum variation in CBR value
5
±1
5-10
±2
11-30
±3
31 and above
±5
Where variation is more than the above, the average CBR should be the average of test results from at least six samples and not three. 5.2 Effective CBR 5.2.1 Where there is significant difference between the CBRs of the top 500mm of the select subgrade and embankment soils, the design should be based on effective CBR which can be determined as per the fundamental procedure given in the following(ref 4).
(i) Using IITPAVE software, determine the maximum deflection due to a single wheel load of 20000 N and a tyre pressure of 0.56 MPa for a two layer elastic system for a given thickness of the select subgrade soil and the embankment soil considering a poison’s ratio from 0.35. (ii) Find the resilient modulus of a single layer elastic system which gives the same deflection as the two layer elastic system as per the following equation 2(1−𝜇 2 )𝑝𝑎 Mr = …….. 5.2 𝛿 Where MR= the effective resilient modulus of the subgrade for pavement design, p = pressure over
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circular area of radius= a , δ= surface deflection at the centre of the loaded area of a single wheel over 500mm or any other thickness of the select subgrade and the embankment soil. Poisson’s ratio has little effect on the effective CBR . Effective CBR= (Mr/17.6)^(1/0.64) for CBR>5 It is the effective resilient modulus (Mr) value rather than the CBR that is used in design. An example for computation of effective resilient modulus is given in Annexure=II. (iii) In case the borrow material is placed over rocky foundation, the effective CBR may be larger than the CBR of the borrow material. Use of the CBR of the borrow material may be adopted for pavement design with proper safeguards against development of pore water pressure between the foundation and the borrow material. 5.3 Determination of Resilient Modulus of the subgrade The behaviour of the subgrade is essentially elastic under the fast moving traffic loading with negligible permanent deformation in a single pass. Resilient modulus is the measure of its elastic behaviour determined from recoverable deformation in the laboratory tests. It is an important parameter for design and the performance of a pavement. This can be determined in the laboratory by conducting repeated tri-axial tests as per procedure specified in AASHTO T307-99 (2003)(1).Since such a facility is not widely available and is very expensive, the default resilient modulus can be estimated from generally acceptable correlations given as: The relation between resilient modulus and the effective CBR is given as: MR(MPa) = 10 * CBR
for CBR ≤5
= 17.6 * (CBR)0.64 for CBR > 5
… 5.3
MR= Resilient modulus of subgrade soil. The CBR of the subgrade should be determined as per IS: 2720 (Part 16) (38) at the most critical moisture conditions likely to occur at the site. The test should be performed on remoulded samples of soils in the laboratory. The pavement thickness should be based on 4-day soaked CBR value of the soil, remoulded at placement density and moisture content ascertained from the compaction curve. In areas with rainfall less than 1000 mm, four day soaking is too severe a condition for well protected subgrade with thick bituminous layer and the strength of the subgrade soil may be underestimated. If data is available for moisture variation in the existing in-service pavements of a region in different seasons, moulding moisture content for the CBR test can be based on field data. Wherever possible the test specimens should be prepared by static compaction. Alternatively dynamic compaction may also be used. Both procedures are described in brief in Annex-IV.
6 PRINCIPLES OF PAVEMENT DESIGN (Users of these guidelines are advised to read this section in conjunction with Annexs I to XI for a better appreciation of the context and the requirements of pavement design) 6.1 Pavement Model: A flexible pavement displays a near elastic behaviour under a fast moving
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traffic and it is, therefore, modeled as an elastic multilayer structure. Stresses and strains at critical locations (Fig.6.1) are computed using a linear layered elastic model. The Stress analysis software IITPAVE can be used for the computation of stresses and strains in flexible pavements. Tensile strain, Єt, at the bottom of the bituminous layer and the vertical subgrade strain, Єv, on the top of the subgrade are conventionally considered as critical parameters for pavement design to limit cracking and rutting in the bituminous and non-bituminous layers respectively. The computation indicates that tensile strain due to heavy wheel loads near the surface close to the edge of a wheel can be sufficiently large to initiate longitudinal surface cracking followed by transverse cracking much before the flexural cracking of the bottom layer if the mix tensile strength is not adequate. This is due to (i) deficiency of the binder in the mix and (ii) brittleness of the mix because of faster ageing caused by oxidation at higher temperatures prevalent in plains of India. It is therefore necessary that the top layer should be both rut and age resistant so that resurfacing intervals can be longer.
Fig. 6.1: Different Layers of a Flexible Pavement
6.2 Fatigue in Bottom Layer of Bituminous Pavement and Fatigue Life 6.2.1 With load repetitions, the tensile strain induced at the bottom of the bituminous layer causes micro cracks, which go on widening and expanding till the load repetitions are large enough for the cracks to propagate to the surface over an area that is unacceptable from the point of view of long term serviceability of the pavement. The phenomenon is called fatigue fracture of the bituminous layer and the number of load repetitions in terms of standard axles that cause fatigue failure denotes the fatigue life of the pavement. In these guidelines, cracking in 20 per cent area has been considered for traffic up to 20 msa and 10 per cent for traffic beyond that.
6.2.2 Fatigue model was calibrated in the R-56(56) studies using the pavement performance data collected during the R-6 (59) and R-19 (60) studies sponsored by MORTH. Two fatigue equations having 80 and 90 percent reliabilities were fitted and the equations were further modified to reflect effect of air void and volume of bitumen for the conventional bituminous mixes designed by Marshall method. The equations are given as:
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Nf= 1.6064 * C * 10–04x [1/εt] 3.89* [1/MR]0.854 For 80% reliability
6.1
Nf= 0.5161 * C * 10–04x [1/εt] 3.89* [1/MR]0.854 For 90% reliability
6.2
Where, Nf= fatigue life in number of cumulative standard axles, εt= Maximum Tensile strain at the bottom of the bituminous layer, and MR= resilient modulus of the bituminous layer depending on selection of binder and temperature The strain, is computed for a standard axle load of 80 kN, each wheel of a dual wheel carries a load of 20kN at 0.56 MPa tyre pressure
Mechanistic design (3,62) recommend a factor ‘C’ to be introduced in fatigue models to take into account the effect of air voids (Va) and volume of bitumen (Vb), which is given by the following relationships C = 10M, and
𝑉𝑏
𝑀 = 4.84 (𝑉𝑎+𝑉𝑏 − 0.69)
(6.3)
6.2.3 Well-known fatigue models also (3, 8 and 62) include the above approach to take into account the effect of volume of bitumen and air voids in the bituminous mix. Equations 6.1and 6.2 show that changes in volume air voids (Va) and volume of bitumen (Vb) have an impact on the fatigue life. Vb can vary from 10% for DBM I mix to 14% for mixes with 14mm and 19mm maximum nominal size aggregates used in European countries and California (76). For example, if bitumen content i s i n c r e a s e d above the optimum bitumen content to have an air void of 3% and if the volume of bitumen is 13 per cent, the fatigue life would be increased by more than three times (Fig I-2 of Annexure-I ). Higher fatigue life due to increased bitumen content for some mixes at air void around 7% was verified from laboratory tests at IIT Kharagpur (40) as shown in Fig.I-1. The recommendation in these guidelines is to target low air voids and higher bitumen content for the lower bituminous layer to obtain a fatigue and moisture resistant mix. Since DBM mixes consisting of larger size of aggregates are used in base and binder courses in India and UK as opposed to much smaller aggregate size used in European and US practice (76,77,78), the volume of binder cannot be increased beyond 11.5% and 12.5% for DBM I and DBM II mixes respectively for 3% air void. The mix with modified bitumen also has improved fatigue lives than the mix with neat bitumen (Fig.I-1 in Annexure I) and increasing the binder content will improve the fatigue life and moisture resistance of the mixes considerably. Modulus values of DBM-1 and DBM-2 mixes are higher due to good interlocking of larger size aggregates. Extreme care should be taken to eliminate segregation of during the construction due to the presence of large size aggregates. 6.2.4 Eq.6.1 is recommended for use for traffic up to 20 msa and Eq. 6.2, for traffic greater than 20 msa. Choice of a binder will depend upon the pavement temperature as well the traffic as discussed in the Annexure-VII. A soft bitumen such as VG10 may be suitable for snow bound area in high altitudes to minimize transverse cracking due to shrinkage. Local experience, however, shall determine the binder selection. Volume of stiffer bitumen in DBM layer can be increased by adjustment of aggregates passing through 4.76 mm and 2.36mm sieve during mix design process to
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increase the voids in the mineral aggregates (VMA) and to accommodate more binder for durability. The guidelines recommend that the designer should consider these aspects with a view to obtain a higher life of bituminous mixes. 6.2.5 If DBM is laid in two layers, it is recommended that the lower DBM mix be designed for 2.5% air void and compacted to an air void of 4% or less (MS2, 2014). In such cases, an effective air void of 3% with the actual volume of effective binder may be considered for the pavement design. If DBM is laid in a single layer, the mix shall be designed for 3% air void and it shall be compacted to an air void of 4.5% or less and the pavement shall be designed for an effective air void(Va) of 3.5% and effective volume of bitumen(Vb) in the bottom rich DBM mix. Va and Vb are determined considering the lost bitumen due to its absorption into the pores of the aggregates. In such cases, the restriction of 75% of voids in mineral aggregates filled with bitumen (Vfb) shall not apply. For the bitumen rich DBM1, the volume of bitumen may approach 11.5% while for the DBM 2, the value may be 12.5%. Bottom rich DBM layer is necessary to eliminate cracking of DBM when it is exposed to traffic during the construction when the laying of bituminous layer continues on the other side. VG30 shall not be used in bottom rich DBM in flexible pavements carrying traffic higher than 10msa in regions where the maximum pavement temperature exceeding 580C. Some design examples of pavements for different air voids and effective volumes of bitumen are given in the Annexure II. Mix design issue is presented in Annexure-VII. A discussion on the effect of modulus of mixes, air voids and volume of bitumen on fatigue behaviour of bituminous mixes is also presented in Annexure-I. The current worldwide trend is to design a bituminous layer with greater amount of harder bitumen to have a long life by limiting rutting, fatigue cracking and moisture damage even in a colder climate. 6.3 Rutting in Pavement 6.3.1 Rutting is the permanent deformation in pavement occurring longitudinally along the wheel path. It may partly be caused by densification and plastic deformation in the subgrade and other nonbituminous layers which would reflect to the overlying bituminous layers to take a deformed shape like a trough. The bituminous mixes also may undergo equal amount of rutting due to secondary compaction and plastic deformation at higher temperatures under heavy traffic. Excessive rutting greatly increases the roughness of the pavement and therefore, it has to be limited to a certain value to maintain a good riding quality. In these guidelines the limiting rutting of 20mm is targeted in 20 percent of the length for design traffic up to 20 msa and 10 percent of the length for the design traffic beyond that. 6.3.2 Rutting model Like the fatigue model, a rutting model also was developed in the R-56(56) project using the pavement performance data collected during the R-6(59) and R-19(60) studies. The models at 80 per cent and 90 per cent reliability are given as: N = 4.1656 x 10-08[1/εv]4.5337
… 6.4
N = 1.41x 10-8x[1/εv]4.5337
… 6.5
Where,
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N = Number of cumulative standard axles, and εv= Vertical strain in the subgrade As can be seen, the model considers the vertical strain in subgrade as the only index for rutting, which is a measure of bearing capacity of the subgrade. The rutting in granular layer also is lower when the vertical subgrade elastic strains are limited to those given by Equations 6.4 and 6.5. A granular layer founded on a strong subgrade has a high resilient modulus and resists rutting. Rutting in the bituminous layers also occurs due to the shear deformation and secondary compaction and its magnitude can be as high as the rutting in non-bituminous layer including the subgrade. This needs to be addressed by providing rut resistant bituminous mixes using a higher viscosity bitumen or modified bitumen. 6.4 Tyre pressure and stress computation The standard axle load equivalency factors developed at AASHO road test for developing pavement design model are based on the tyre pressure of 0.56 MPa and the standard load of 80kN. Same equivalency factors are adopted in the current IRC: 37 and therefore the tyre pressure of 0.56 MPa is adopted in the guidelines. In the R-56 Research Scheme also, the tyre pressure of 0.56 MPa and a standard axle load of 80 kN were considered for the calibration of fatigue and rutting equation. For the cement treated bases, spectrum of axle loads determined from axle load survey data is used for pavement design and a realistic tyre pressure of 0.80 MPa is applied for the analysis of stresses in such pavements. It is to be noted that the wearing course is affected greatly by the high tyre pressure and its effect in deeper layers is much less due to faster decrease of the surface pressure with depth. Hence a strong wearing course material such as Stone Matrix Asphalt or a material of equal strength must be used to avoid damage to upper layer of pavements due to high tyre pressure. 6.5. Cumulative Fatigue Damage and Rutting in Bituminous Mixes A flexible pavement is subjected to spectrum of axle loads of trucks and multi-axle vehicles applied through single, tandem, tridem axles, and their magnitudes are often much higher than the standard axle loads. Fatigue damage is large due to heavier axles compared to those with lower loads. Axle load equivalency factor for converting axles of carrying loads of different magnitudes into repetitions of equivalent standard single axle load of magnitude 80 kN as per the AASHTO guidelines is not strictly valid for the heavy traffic in India since (i) the maximum single load was limited to 13 ton during the AASHO road test and (ii) the site had freezing conditions in winter. Other countries have developed different vehicle damage factors. It is necessary to use principle of cumulative damage to determine the total rutting, area of fatigue crackling and loss of riding quality in pavements due to each class of axle load for the design of flexible pavements. The fatigue damage principle may be stated as 𝑛𝑖
Cumulative fatigue damage(CFD) = ∑𝑘1 𝑁𝑖 Where ni = number of expected axle load repetitions of load level i Ni= Fatigue life at load level i
6.6
26
k = number of temperature levels in different seasons. When CFD=1,the pavement is deemed to have failed in fatigue due large scale cracking. Fatigue damage is to be computed hourly for the whole year. Similar models are to be developed for rutting and riding quality. The above approach has to be calibrated from field performance considering rutting ,cracking and riding quality. Development of fatigue cracking takes place when the temperature is lower in winter which is short in India. The bituminous, granular and the subgrade layers undergo rutting due to the repeated axle loads. The rutting is greater in the bituminous layer during the summer while it is large in the subgrade and the granular layers in wet season. For estimating the rutting, each of the layers are sub-layered, stresses due to spectrum of axle loads are computed in different sublayers in different seasons and permanent deformations are determined in each sublayer from their stress – strain relations . The rutting in each layer is computed by adding the permanent deformations in each sub-layer. Sum of all the permanent deformations in subgrade, granular subbase, granular base and bituminous layers in each season in the design period is the total rutting. Material properties are to be evaluated season wise for each layer before the development of the rutting models. Each state has a different climate (3) and calibration factors will be accordingly different. Researchers and consultants may consider the above approach to evolve a performance based pavement design model for different regions. 6.6 Top Down Cracking in Bituminous Layer While fatigue cracking is conventionally considered as a ‘bottom-up cracking’ phenomenon, ‘topdown cracking’ followed by raveling and pothole formation during the rains have frequently been observed within two to three years of construction on high volume roads because of (i) faster ageing of the wearing course at higher temperatures in plains of India (ii) deficiency of the binder and (ii) surface cracking due to high tensile stresses close to tyre edge by heavy axle loads. These guidelines recommend moisture, age and fatigue resistant gap graded rubberized bitumen mix – GGRB ( IRC:SP:107) or Stone Matrix Asphalt –SMA (IRC:SP:79) with PMB40 for high volume pavements with design traffic exceeding 20 msa to eliminate frequent maintenance, a practice widely adopted at the international level. Intervals of overlays also are expected to be much larger as experienced in other countries. Trials on both GGRB mixes and SMA have given good performance on National Highways in India. For traffic exceeding 10 msa and less than 20 msa, BC with modified binder is recommended. Traffic projection for the selection of the binder for the DBM and the wearing course shall be done on the basis of 15/20 years of design traffic and the regional temperature as discussed in Annexure VII. Users should exercise their knowledge and experience in selecting the wearing course notwithstanding the above recommendations. 6.7 Cementitious Sub-base and Base 6.7.1 Cementitious materials normally crack due to shrinkage and temperature changes even without pavement being loaded. Such materials having low cement content develop fine cracks and have to be preferred to high cement content mixes producing wider cracks. While making a judgment on the strength values for design, the reduction in strength due to the cracked condition caused by shrinkage and the construction traffic needs to be fully recognized. The elastic modulus (E) recommended for design is lower than their laboratory values obtained from unconfined compression test. The extent of reductions proposed has generally been in agreement with practices followed in the national standards
27
of other countries like Australia, South Africa, MEPDG of the USA etc. A number of pavements with cementitious bases have been constructed in different parts of the country using the recommendations of IRC:37-2012 and the performance is reported to be good, The structural evaluation of bituminous pavements with cementitious base and subbase in Madhya Pradesh and West Bengal was done with FWD and the pavement was found to be strong. New pavements constructed with these materials need to be closely monitored by Falling Weight Deflectometer (FWD) for the extended period of fifteen years for the evaluation of material properties for future guidance. These guidelines permit construction with cementitious materials in the interest of saving the environment and using the local and marginal materials after stabilization depending on the location. Validated results of cemented pavement layers from India and other countries and use of a sound analytical tool can result in good performing pavements at an affordable cost consistent with environmental requirement. 6.7.2 Fatigue cracking in cementitious bases The analysis of fatigue cracking of cement treated layers is recommended at two levels because of very heavy axle loads on highways in India. Preliminary thickness of the cemented layer is first evaluated from fatigue consideration in terms of cumulative standard axles. At the second level, the cumulative fatigue damage due to individual axles is calculated based on a model which uses ‘stress ratio’ (the ratio of actual stresses developed due to a class of wheel load and the flexural strength of the material) as the parameter. The computation of stresses due to spectrum of axle loads is done by the IITPAVE program. An excel sheet can be used to calculate the fatigue damage of each class of wheel loads and summed up for the entire axle load spectrum to obtain the cumulative fatigue damage(CFD). The CFD of wheel loads should be less than one during the design life of a pavement. If it is greater than one, the section has to be changed and iteration done again. The first model in terms of cumulative standard axle load repetitions adopted from the Australian experience may not always work in India because of very heavy axle loads, while the second one (Eq 6.8) provides a very scientific approach for fatigue analysis of cementitious base in which fatigue damage is computed for axle loads of different magnitudes. First level (Eq 6.7) gives the initial cross section in a few trials only. The second level analysis alone may be used for thickness design without considering the first level but it may take a few more trials. Thickness may increase by 40 to 50 mm in most cases after second level analysis is done. Thickness of CTB should also be safe for the construction traffic as illustrated in Annex-II. The two fatigue equations are given below: 6.7.2.1 Fatigue life in terms of standard axles for first level analysis
𝑁=
12 113000 ( 0.804 +191) 𝑅𝐹 ⌊ 𝐸 𝜀 ⌋ --𝑡
Where, RF= Reliability factor for cementitious materials for failure against fatigue. =1 for Expressways, National Highways and other heavy volume roads.
6.7
28
=2 for others carrying less than 10 msa. N = Fatigue life of the cementitious material. E = Elastic modulus of cementitious material. €t= tensile strain in the cementitious layer, microstrain. 6.7.2.2 Fatigue Equation for second level analysis The spectrum of axle loads has to be compiled under various axle load classes for fatigue analysis of cementitious bases. One pass of tandem and tridem axles respectively may be taken as equivalent to two and three passes of single axles respectively because axles located at distances of 1.30 m or higher from each other do not have any significant overlapping of stresses. For example, if a tridem axle carries a load of 45 tons, this is equivalent to three passes of single axles of weight 15 tons each. The fatigue life is given by the following equation 𝐿𝑜𝑔𝑁𝑓𝑖=
0.972−(𝜎𝑡/𝑀𝑅𝑢𝑝 ) 0.0825
6.8
Nfi= Fatigue life at axle load of class i. Logarithm has a base 10. σt= tensile stress under cementitious base layer. MRup= 28 day flexural strength of the cementitious base, 𝜎𝑡/𝑀𝑅𝑢𝑝 = 𝑠𝑡𝑟𝑠𝑠 𝑟𝑎𝑡𝑖𝑜 The fatigue criterion is considered satisfied if Σ(ni/Nfi) is less than 1,where ni is the expected number of axles of load level ‘i’ in the design period. In cemented bases proposed in the guidelines, neither vertical subgrade strain nor tensile strain in the bituminous layer is critical in many cases. 6.7.3: Checking the thickness of cementitious base for the construction traffic: Three axle dumpers carrying the WMM material for the crack relief layer over CTB may weigh as much as 320 kN. While low strength cementitious subbase may crack and form interlocking blocks due to construction traffic, the cementitious base (CTB) forming an important load bearing layer shall not be allowed to crack. For example, after the construction of the CTB, WMM is brought to the site on dumpers and loaded into the paver for laying. It should be checked that the flexural stress caused by the dumper is less than the seven day flexural strength of the CTB. Minimum thickness of CTB may also be governed by construction traffic.
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7 PAVEMENT COMPOSITION 7.1 General A flexible pavement covered in these guidelines consists of different layers as shown in Fig. 7.11.
Bituminous layer Aggregate interlayer/SAMI for cementitious base Untreated aggregate, aggregates/ RAP treated with cement /Foamed bitumen/ bitumen emulsion base base Granular/ subbase Figure 7.11cementitious Different Layers of Bituminous pavements
Figure 7.11 Layered Bituminous Pavements
The sub-base and the base layers can be granular (unbound) or chemically stabilized with additives such as cement, lime, flyash and other cementitious stabilizers. In case of pavements with cementitious base, a crack relief layer provided between the bituminous layer and the cementitious base delays the reflection crack in the bituminous layer. This may consist of dense graded crushed aggregates of thickness 100 mm to 150mm of WMM conforming to IRC/MORTH specifications or the Stress Absorbing Membrane Interlayer (SAMI) of elastomeric modified binder at the rate of about 2 litre/m2 covered with light application of 10 mm aggregates to prevent picking up of the binder by construction traffic (16). WMM should be compacted to 100% of the modified dry density to act as a strong crack relief layer (67) The well compacted base layer of Wet Mix Macadam (64) serves as a strong support for the compaction of Dense Bituminous Macadam and bituminous Concrete. The base layer may also consist of reclaimed asphalt pavement/granular materials stabilized with bitumen emulsion or foamed bitumen. Reclaimed asphalt pavement materials when treated with foamed bitumen or bitumen emulsion with or without fresh aggregates are required to have the required minimum indirect tensile strength to be considered as a base layer. Industrial waste such as steel slag aggregates can also be used as a granular or cement stabilized layer if its expansive properties have been arrested by proper curing. The sub-base layer serves three functions (i) provides a strong support for the compaction of WMM/WBM layer (ii) protects the subgrade from overstressing and (iii) serves as drainage and filter layer. The thickness of sub-base, whether bound or unbound, should meet these functional requirements.
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7.2 Sub-base layer 7.2.1 Unbound sub-base layer 7.2.1.1 Sub-base materials may consist of natural sand, moorum, gravel, laterite, kankar, brick metal, and crushed stone, crushed slag and reclaimed crushed concrete/reclaimed asphalt pavement or combinations thereof meeting the prescribed grading and physical requirements. When the sub-base material consists of combination of materials, mixing should be done mechanically either using a suitable mixer or adopting mix-in-place method. The sub-base should have sufficient strength and thickness to serve the construction traffic as illustrated in Annexure II. Since the top 500mm of the subgrade is strong with a CBR of 8, a minimum thickness of 150mm is recommended if there is no need of any drainage layer. It is most economical to lower the value of vertical subgrade strain in the pavement design process by increasing the thickness of GSB layer rather than increasing the thickness of WMM or bituminous layers 7.2.1.2 Granular sub-base (GSB) should conform to MORTH Specifications for Road and Bridge Works (64). These specifications and the specified grain size distribution of the sub-base material should be strictly enforced in order to meet strength, filter and drainage requirements of the GSB. When the sub-base is used as a drainage layer, Los Angeles abrasion value should be less than 40 to prevent excessive crushing during the rolling to prevent loss of the required permeability. The minimum thickness of compacted GSB shall be 150 mm. 7.2.1.3 The sub-base may sometimes be required to be laid two layers, the lower layer forms the separation/ filter layer ( subase grading I,II,V and VI of Morth) to prevent intrusion of subgrade soil into the pavement while the upper GSB forms the drainage layer (GSB grading III and IV of MORTH) to drain away any water that may enter through surface cracks particularly in high rainfall area with bituminous layer thinner than 100mm. Filter and drainage layers can be designed as per IRC: SP: 42-2014 (35), IRC: SP: 50-2013(36) and other information given in Annexure V. Commercially available synthetic geo-composite having adequate horizontal permeability may also be used both as a separation and drainage layer in locations where aggregates are very costly but its strengthening effect should not be considered in pavement design. Grid lock geocell filled with aggregates can also be used as a drainage layer. Minimum thicknesses of drainage and filter layers shall be 100mm each. The subbase layer is being provided below the median also in continuation with the that below the pavement, a nonwoven geotextile may be provided over the GSB in the median part so that the fines do not enter into the GSB and choke it. If the median has a clay soil, there is little chance of water migrating into lower layer due to its low permeability and the geotextile may be dispensed with. 7.2.1.4 Resilient Modulus The design parameter for granular sub-base is resilient modulus (MR), which is given by the following equation: MRgsb= 0.2h0.45* MRsubgrade
…7.1
31
Where h = thickness of sub-base layer in mm MR value of the sub-base is dependent upon the MR value of the subgrade since weaker subgrade doesn’t permit higher modulus of the upper layer because of deformation under loads. 7.2.2 Bound sub-base layer 7.2.2.1 The material for bound subbase may consist of soil, aggregate or soil aggregate mixture modified with chemical stabilizers such as cement, lime, lime-flyash, commercially available stabilizers1 etc. The drainage layer of the subbase may consist of coarse/open graded aggregates bound with about 2% to2.5% cement/ bitumen emulsion for stability during the construction. If soil stabilised with cementitious material is used as a sub-base when granular material is not easily available, commercially available geo-composites possessing the necessary horizontal permeability can be used to serve both as a drainage and filter/separation layer. Drainage and separation layers are essential when water is likely to enter into pavements from the shoulder, median, bottom or through the cracks in surface layer. There is little chance of moisture entering from the pavement surface with paved shoulder when bituminous layer has a thickness of 100mm or higher. 7.2.2.2 Strength Parameter The relevant design parameter for bound sub-bases is the elastic modulus E, which can be estimated from the unconfined compressive strength of the material. The cementitious granular sub-base must have minimum 7-day UCS of 1.5 to 3 MPa. The elastic modulus of the bound sub-base, E , (12) is given by the following equations: Ecgsb= 1000 * UCS
…7.2
Where UCS = 28 day strength of the cementitious granular material. It should be ensured that minimum UCS is attained in the field and laboratory strength values should be 1.5 times higher (MORTH) than the required field strength due to variability in the construction ( 64). Equation 7.2 gives a value in the range of 2000 to 4000 MPa. Since the sub-base acts as a platform for the construction traffic, low strength cemented sub-base is expected to crack during the construction and a design value of 600 MPa is recommended for the stress analysis. Poisson’s ratio may be taken as 0.25. The cemented subbase shall be cured for a minimum of three days before the construction of the upper layer. If the stabilized soil sub-bases have 7-day UCS values in the range 0.75 to 1.5 MPa, the recommended E value for design is 400 MPa with Poisson’s ratio of 0.25. 7.3 Base Layer 7.3.1 Unbound base layer The base layer may consist of wet mix macadam, water bound macadam, crusher run macadam, reclaimed concrete etc. Relevant specifications of IRC/MORTH are to be adopted for the construction. The minimum thickness shall not be less than 150mm. South African Standard TRH4(68) recommends use of Water Bound Macadam base for heavy traffic in wet where labour is
32
available since it performs better due to good interlocking of larger size aggregates When both sub-base and the base layers are made up of unbound granular layers, the composite resilient modulus of the granular sub-base and the base is given as: … 7.3
MR_granular=0.2* h0.45MRsubgrade Where h = thickness of granular sub-base and base, mm Poisson’s ratio of granular bases and sub-bases may be taken as 0.35. 7.3.2 Cementitious bases
7.3.2.1 Flexural strength of a cementitious base is critical to the satisfactory performance of a bituminous pavement. Unconfined compressive test is usually carried out in mix design for quick evaluation of strength parameters. Cemented base layers may consist of aggregates or soil-aggregate mixture stabilized with chemical stabilizers such as cement, lime, lime-flyash or other commercially available stabilizers which are required to give a minimum strength of 4.5 to 7 MPa in 7/28 days. While the conventional cement stabilized material should attain the above strength in seven days (IRC:SP-89-2010(30)), granular materials and soil-aggregate mixture stabilized with lime, pozzolanic stabilizers, lime-flyash etc. should meet the above strength requirement in 28 days since strength gain in such materials is a slow process. MORTH (64) recommends the laboratory strength to be 1.5 times the desired strength in the pavement to account for the variability. Even Reclaimed Asphalt materials are being stabilized in-situ with cement worldwide for the rehabilitation of severely damaged pavements. Though the initial modulus of the cementitious bases may be in the range 10000 to 15000 MPa, the long term modulus of the cemented layer may be much lower due to cracks caused by shrinkage, expansion-contraction and construction traffic (67,68). Australian guidelines (16) recommend use of Equation 7.2 for the cemented layer. Curing of cemented bases shall be done for a minimum period of seven days before the commencement of the construction of the upper layer for achieving the required strength as described in IRC: SP-89, and curing should start immediately by spraying bitumen emulsion, wet jute mat or periodical mist spray of water without flooding or other methods. If cementitious base must meet the durability criterion as laid down in 7.3.2.3. Pavement design is to be done accordingly. Note: Where commercially available stabilisers are used, the stabilized materials should meet the additional requirement of leachability and concentration of heavy metals apart from the usual requirements of strength and durability.
7.3.2.2 Strength parameter Flexural strength is required for carrying out the fatigue damage analysis of the cement treated base. MEPDG recommends that the modulus of rupture for chemically stabilized bases can be taken as 20 percent of the 28 day unconfined compressive strength on cylinder. Laboratory tests IIT Kharagpur do confirm that the recommendation is conservative (17, 18). The above is recommended in these guidelines. The following default values of modulus of rupture corresponding the 28 day UCS of 7 MPa, 5.25 MPa and 3.5 MPa are recommended for cementitious bases (MEPDG). Cementitious stabilized aggregates Lime—flyash-soil
1.40 MPa
- 1.05 MPa
33
Soil cement
- 0.70 MPa
Poisson’s ratio of the cemented layers may be taken as 0.25. Relation between UCS, indirect tensile strength(ITS) and Flexural Strength(FS) should be established for all stabilized layers for quality control since ITS is easy to determine on the cores taken from the field. FS is about 1.5 times the ITS. 7.3.2.3 While minimum size of sample of beam for cement stabilized aggregate for flexure test can be 100m x 100mmx500mm, the beam size for flexural tests for stabilized soil with hydraulic binders( cement, lime, lime-flyash and other commercially available cementitious binders) can be from 50mm x50mm x250mm to 75mm x75mm x 375mm, and third point loading shall be applied at a rate of 1.25mm per minute, same as that of the CBR loading frame. 7.3.2.4 Durability criteria Even if the minimum UCS criterion is met, the minimum cementitious material in the bound base layer should be such that in a wetting and drying test (BIS: 4332 (Part IV) - 1968) (19), the loss of weight of the stabilized material does not exceed 14 percent after 12 cycles of wetting and drying. In cold and snow bound regions like Arunachal Pradesh, Jammu & Kashmir, Ladakh, Himachal Pradesh etc., durability should also be determined by freezing and thawing test and the loss of weight should be less than 14 percent after 12 cycles (BIS: 4332 (Part IV) – 1968) (19). The cementitious base meeting the strength requirements of IRC: SP: 89-2010 is found to meet the criteria of durability requirement (16). 7.3.2.5 Strength gain with time The cementitious materials keep on gaining strength over a long time period and 28 day strength gives a conservative design. It should be recognized that the cemented materials may possibly degrade also with time depending upon the loads, temperature and moisture conditions, freeze-thaw cycles and quantity of chemical stabilizers. 7.3.2.6 Crack relief layer A SAMI layer using elastomeric modified bitumen provided over the cementitious layer delays the cracks propagating into the bituminous layer (12). A crack relief layer of wet mix macadam of thickness 100mm to150 mm sandwiched between the bituminous layer and treated layer is much more effective in arresting the propagation of cracks from the cementitious base to the bituminous layer (16, 24, 67 and 66). The sandwiched aggregate layer between two strong layers become stiffer under heavier loads because of confinement. The WMM layer should be compacted to a minimum of 100% of dry density (67,68).If shoving and deformation are expected in the WMM layer due to unavoidable construction traffic, the granular layer may be treated with 2 to 3 per cent of SS2 bitumen emulsion in the WMM plant during blending of aggregates to avoid regrading. 7.3.2.7 Modulus of crack relief layer The resilient modulus of a well graded granular layer such as Wet Mix Macadam provided between the cementitious and bituminous layers is dependent upon the confinement pressure under wheel load (3, 16, 27, 67 and 68). The modulus may vary from 250 to 1000 MPa depending upon the gradation
34
and a typical value of 450 MPa (16, 67 and 68) is suggested for the sandwiched aggregate layer for the analysis of pavements. Strong support from the cementitious base results in higher modulus than what is given by Eq. 7.3 when a granular layer rests on a weak layer. Poisson’s ratio of the aggregate relief layer may be taken as 0.35. A brief description on strength and resilient modulus of cementitious materials is given in Annex VIII. 7.3.2.7 Quality Control during construction of cement treated base and subbase: At the construction site itself, samples of cement stabilized bases and subbases should be compacted in cubes by vibratory compactor with square plate used in the laboratory for mix design and the cubes should be sealed in plastic bags to prevent escape of moisture and cured in the laboratory. To check the density of the compacted layer, standard sand jar apparatus may be used for the compacted layer up to 200mm thickness. In a number of projects, the compacted thickness of the stabilized layer is as large as 300mm when a single pass mechanized stabilizer with heavy vibratory sheep foot roller for compaction is used. In such cases, the density of the compacted layer has to be determined at two depths. For 300mm compacted thickness, density for the top 150mm thickness is determined first by sand jar apparatus and then the next 150mm is taken up . A minimum of 98% of the laboratory density must be achieved in each layer. 7.3.3 Bitumen emulsion/foamed bitumen treated reclaimed asphalt pavement base. The base may be made up of milled material from reclaimed asphalt pavements treated with foamed bitumen or bitumen emulsion. The long term resilient modulus of the material with bitumen emulsion(SS2)/ foamed bitumen may be taken as 800 MPa The laboratory values may range from 600 to 1200 MPa (52, 61). For foamed asphalt, VG30 is recommended for the foamed asphalt mix for a better strength2 due to high temperature in plains of India. The fine mist of bitumen sprayed over RAP does not blend with bitumen of RAP due to lower temperature and presence of moisture. _
The above mentioned resilient modulus value can be ensured if the Indirect Tensile Strength (ASTM: D6931) (14) of the 102 mm diameter Marshall specimen of the Emulsion/Foamed Bitumen treated material has a minimum value of 100 kPa in wet condition and 225 kPa in dry condition at deformation rate of 50 mm/minute (59) at 25°C. The recommended Poisson’s ratio is 0.35. Mix design and test methods are given in Annexure IX. It is recommended that the modulus test be carried out to confirm that the pavement is not under designed. Bitumen emulsion mix can be laid in-situ by a train of equipment in which milling, addition of bitumen emulsion, water, lime or cement, mixing operations and compaction are carried out continuously. In case of small works, wet mix plant can be used for mix preparation with bitumen emulsion which can be laid by a paver. RAP mixes with foamed asphalt also are being laid in India by in-place or in-plant methods on a large number of pavements for rehabilitation /widening avoiding use and transportation costly and precious aggregates. 2
California Department of Transportation uses PG64-16 bitumen (closer to VG30 bitumen) for foamed bitumen. In India also, VG30 is being used now in cold recycling with foamed bitumen .
7.4 Bituminous layers 7.4.1 The Resilient Modulus (MR) of bituminous mixes is another input for the analysis of stresses in flexible pavements. Its magnitude depends upon the grade of binder, the temperature, Frequency /
35
loading time, air void, volume of bitumen, texture, gradation of aggregates, maximum size of aggregates, state of packing of aggregates, orientation of the test sample while testing and viscosity of the binder. VG40 is most suitable for the hot climate in plains of India. Modified binders also are suitable both for hot and cold climate. The modulus of mixes with modified binders varies a great deal depending upon the modifier, duration of blending and the extent of air blowing of base bitumen. It may vary from a low of about 1600 MPa to a high of 3000 MPa at 35°C, lower modulus is not a reflection on quality of the modified binder. Though an elastomer modified binder gives a lower modulus of the bituminous hot mix, its fatigue live, which controls the thickness of the bituminous layer, is much higher. Lower modulus is due to large recoverable deformation in elastomer binders. It is to be noted that the modulus values change continuously with a change in temperature and it can be as high as 5000MPa at lower temperatures at midnight to less 1000 MPa at 650 C at noon when the bitumen becomes too soft. Indicative values of resilient modulus values of the bituminous materials at 350Cwith different binders are given in Table 7.1. These are based on l a b o r a t o r y tests at IIT Kharagpur in Indirect Tensile mode as per ASTM 4123 now revised as ASTM: D7369-09(12). The European standard equivalent to ASTM 4123 is EN 12697-26 and it is available commercially. A loading pulse of 0.1 second followed by a rest period of 0.9 second is adopted for Indirect Tensile Test. These findings are in agreement with those given in some other literature as well for similar binders e.g., Monismith et al. (20), AUSTROADS and others. VG30 bitumen may have a viscosity from 2400 poise to 3600 poise at 600C while for VG40, the viscosity range is 3200 to 4800 poise. Hence modulus values of both VG30 and VG40 may be close to each other in some cases when viscosity overlaps. The guidelines, therefore recommend that the modulus values of mixes with normal and modified binders should be determined from a repeated Indirect Tensile Strength test. Figure 7.2 shows a schematic diagram of indirect tensile test as per EN 12697-26 ( similar to ASTM D4123).Total recoverable horizontal deformation is measured and the modulus computed considering a Poisson’ ratio of 0.35, a value commonly recommended in the method. The test is simple to carry out as compared to that that prescribed in ASTM: D7369-09(12) which gives inconsistent result for 150mm DBM 1 mixes because of larger size aggregates. Dynamic Load
LVDT
1
Marshall sample
Figure 7.2 Indirect tensile test
2
36
Resilient modulus of a Marshall specimen is given as
𝑀𝑟 =
𝑃(𝜇+0.27) 𝑡𝐻𝑟
7.4
Where p= cyclic load (10% of the Indirect tensile strength), µ= Poison’s ratio (range 0.35 to 0.40), Hr= Recoverable horizontal deformation across the diameter (sum of recoverable deformation of LVDTs 1 and 2 in Figs 7.2) and t=thickness If the test parameters are Load=1500 N, diameter of specimen(D)=102mm, height(t)= 60.75mm, Recoverable horizontal deformation (Hr) = 4 micron (0.004mm), assuming µ=0.35 Mr= 3827 MPa. Higher Poisson’s ratio gives higher modulus. A regression equation (49) given below for 102mm diameter specimens can be used for the determination of MR value of the dense graded bituminous mixes if test facility is not available. For 102mm diameter DBM 2 mix, Aggregates retained on 26.5 mm should be replaced with those passing 26.5mm and retained on 19mm for mix design if 150mm diameter mould and compaction hammer are not available. MR(MPa)=2.6975 x ITS + 724.46 for mixes with neat bitumen -7. 5 R² = 0.84 MR(MPa)=1.1991 x ITS + 1170 for mixes with modified bitumen …. 7. 6 2 R = 0.89 Where, ITS =Indirect Tensile Strength in kPa, MR= Resilient Modulus in MPa. 102mm diameter specimens give marginally higher ITS value than 150mm sample for the same aggregate size. Higher aggregate size gives higher modulus values. ITS is to be determined in the Marshall loading frame at a speed of 50mm/minute as explained in Appendix IX. Over 30 number of 150mm DBM samples from the construction sites containing VG40 bitumen from different parts of India were tested at 350C at IIT Kharagpur and the results along with the regression equation are shown in Figure 7.3. It can be seen that most Mr values are greater than 3000 MPa.
Mr of DBM,MPa
37
5000 4500 4000 3500 3000 2500 2000 1500 1000 500 0
y = 11.088x - 3015.8 R² = 0.6842
450
500
550 600 ITS of DBM in kPa
650
700
Figure 7.3 Mr of DBM with VG40 vs Indirect Tensile strength for 150mm marshall specimen at 350C The regression equation is valid for ITS values from 460 kPa to 650 kPa. Large scatter is due to VG40 from different sources used by contractors in different parts of the country When a number of 150mm diameter DBM samples were cast in IIT Kharagpur laboratory with V40 bitumen from a single source (Haldia refinery), 68% of the Mr values were between 3090 and 3700 MPa and 95% of the values were between 2710 MPa and 4030 MPa. 10 to 15% variation in the modulus of bituminous mixes has a minor effect on the pavement design. A change in modulus of ± 500 MPa may result in thickness variation of about 5 mm per 100mm thickness of the DBM layer having a modulus of 3000 MPa. Examples are solved in Annexure II to illustrate the effect of modulus on thickness of bituminous layers. An additional thickness of about 10 mm of the DBM layer is to be provided to take care of undulations in WMM layer considering that 10mm variation in surface irregularity of the WMM/WBM layer is permitted in MORTH. A small portion of DBM acts as a profile correcting course. Maximum modulus of the DBM layer may be taken as 3000 MPa with VG40 bitumen even if higher modulus is obtained in laboratory tests, and for VG30 bitumen, the recommended upper limit is 2000 MPa. Moduli at 350C of DBM mixes can be determined in the laboratory or from the regression equation shown in Figure 7.3. For temperatures less than 350C, Equations 7.5 and 7.6 can be used for pavement design. The values in Table 7.1 agree with majority of laboratory results. 7.4.2 Diameter of Test samples for ITS and Resilient Modulus Test: The DBM mix is the main structural layer of a flexible pavement and forms bulk of the thickness of a bituminous layer for high volume roads with granular bases and sub-bases. Unlike wearing course, it ages slowly since it is situated at 40mm to 50mm below the surface. Its modulus plays an important role in lowering the tensile strain in its bottom and vertical strain on the top of the subgrade. Since the maximum nominal size of aggregate in DBM can be 40mm and the largest dimension may be still higher, 150mm diameter test specimen shall be adopted for the Mr test. Higher aggregate size gives higher modulus of the bituminous mixes because of increased interlocking.
38
7.4.3 Modified Bitumen for wearing course and Resilient Modulus: The mixes with modified bitumen are known to (i) age slowly (ii) give improved fatigue lives and higher rut resistance notwithstanding lower resilient modulus values obtained in the laboratory for some polymers and CRMB. Even if the modulus of the wearing course such as SMA/Gap Graded Rubberized bitumen mix with modified bitumen (IRC:SP:79, IRC:SP:107) is lower, the modulus assigned to the DBM layer may be adopted for the wearing course also for the stress computation because (i) the wearing course is under direct compression below the wheel loads and the dynamic modulus values at a speed of 50 to 60 kmph (corresponding to the frequency of 10 hz) under the compression will be higher and (ii) the wearing course will age faster and the modulus will increase. As per the international practice (MS2), only modified binder such as PMB40 or rubberized bitumen (Equivalent to PG76 or higher as per US practice) are recommended in Stone Matrix Asphalt. In case of gap graded mixes with rubberized bitumen, binder containing at least 20% crumb rubber ( IRC:SP:107) shall be used. 7.4.4 DBM Mixes with Modified Bitumen: The mixes with modified bitumen can have a fatigue life that can be three to ten times of mixes with neat bitumen as illustrated in Fig. I-1 in Annexure I. While the wearing courses with PMB/CRMB can be milled and recycled easily since they become brittle due to ageing due to exposure to sun, the DBM I and DBM II mixes with modified bitumen can be used in lower layers if their recyclability can be established from the experience in India or abroad. Mixes with elastomeric binders may cause problems in milling and recycling due to stickiness, and pavement rehabilitation can pose a serious problem for lack of technology. Sustainability issues have to be kept in mind. 7.4.5 High viscosity binder is recommended for the DBM mixes to limit rutting and loss of riding quality. Their fatigue lives depend upon the test method whether the tests are done under constant strain or constant stress modes. Stiffer mixes have higher fatigue lives under control stress mode and lower fatigue lives under control strain mode. For thin bituminous layer, constant strain mode is applicable while for thicker mixes, constant stress mode is more appropriate. Fatigue life of mixes can be further improved by reducing the air voids and increasing the volume of binder. Low air voids and higher bitumen content may, however, induce bleeding and rutting if softer binders are used. These guidelines, therefore, recommend that these factors should be taken into account while designing bituminous mixes. 7.4.5 The Poisson’s ratio of bituminous layer depend upon the pavement temperature and a value from 0.35 to 0.40 recommended for temperature up to 35°C and value of 0.50 for higher temperatures. Stress computation is not very sensitive to minor changes in Poisson’s ratio. Fatigue equation at any pavement temperature from 20°C to 40°C can be evaluated by substituting the appropriate value of the resilient modulus of the bituminous mix, air void and volume of bitumen. Examples of typical designs are given for a temperature of 35°C. It is to be noted that the bituminous mixes harden with time and modulus may increase due to ageing than what is given in the Table 7.1. Hence field performance is to be periodically recorded for future guidance. Annex. VII gives various
39
considerations for the selection of binders and mixes in the light of international experience. Table 7.1 Typical values of Resilient Modulus and of Bituminous Mixes, MPa* Mix type
Temperature °C 20
25
30
35
40
BC and DBM for VG10 bitumen
2300
2000
1450
1000
800
BC and DBM for VG30 bitumen
3500
3000
2500
2000
1250
BC and DBM for VG40 bitumen
6000
5000
4000
3000
2000
BC and DBM for Modified Bitumen (IRC: SP: 53-2010)
5700
3800
2400
1600 to 2800
1300
BM with VG 10 bitumen
500 MPa at 35°C
BM with VG 30 bitumen
700 MPa at 35°C
RAP treated with 3-4 percent bitumen emulsion/ 800 MPa at 35°C foamed bitumen (2-2.5) percent residual bitumen and 1.0 percent cementitious material. *Thickness of bituminous layer is affected by about 5mm per 100mm of the layer for a variation of about 15% in modulus of 3000 MPa
7.4.5 Quality issues in DBM and BC: Though mix design is done in the laboratory much ahead of the start of the bituminous construction, it is to be ensured that the material that is being laid has the necessary parameters as per the mix design because of inherent variability in a large scale construction. In case of two or three lane paving, at least two sets of loose samples from locations close to paver from each lane shall be taken during the paving and a part of the sample shall be used to prepare three specimens in Marshall moulds in the field itself with 75 blows compaction on each face to form the reference density for comparing the density of cores of the compacted mass at the same location. Another part of the loose samples may be used for the determination of the maximum theoretical density and the grading so that remedial measures are taken early. This should be done at every 250m.
8 LONG LIFE PAVEMENTS 8.1 Long Life Pavement: A pavement having a life of fifty years or longer is termed as Long Life th Pavement(LLP), also known as perpetual pavement. As per Asphalt Institute, MS-4, 7 edition 2007, if the tensile strain caused by the traffic in the bituminous layer is less than 70 micro strain, the endurance limit of the material, the bituminous layer never cracks. Similarly if vertical subgrade strain is less than 200 micro strain, there will be practically very little rutting in subgrade. For the climatic conditions on plains of India where Average Annual Pavement Temperature may be close to
40
350C, the corresponding limiting strains may be taken as 81and 200 micro strains respectively due to lower resilient modulus of the bituminous layer. Design of such a pavement is illustrated in the guideline. Different layers are so designed and constructed that only the surface layer would need replacement from time to time with a new layer. There is now enough evidence worldwide that deep strength bituminous pavements suffer damage only at the top and nowhere else. The pavement composition can be so selected that LLP can be constructed at a marginally higher initial cost but at a lower life cycle cost. A design example is given in Annexure II 8.2 Pavement Design for 300 msa: Pavements are commonly designed in India for a design life of 15 to 20 years when the total traffic in most cases is about 150 msa or less. Design method presented in the guidelines can be used for design of pavement for full concession periods when the traffic level may reach even 300 msa or higher. Annexure II illustrates a design for such a pavement.
9 PAVEMENT DESIGN PROCEDURE 9.1 Using IITPAVE Any combination of traffic and pavement composition can be analyzed using IITPAVE. The designer can apply his/her judgment and experience in the choice of pavement materials and layer thickness. The standard wheel load, tyre pressure, number of layers, the layer thickness of individual layers and the layer properties are the user specified inputs in the Program, which gives strains at critical locations as outputs. The adequacy of design is checked by the Programme by comparing these strains with the allowable strains as predicted by the fatigue and rutting models. A satisfactory pavement design is achieved through iterative process by varying layer thicknesses or, if necessary, by changing the pavement layer materials. Stress analysis may sometimes give very low thickness from fatigue and rutting models and users are required to select the minimum thickness from practical considerations such as maximum size of aggregates, construction traffic and availability of materials. (See section 13 for Procedures for using IITPAVE and Annex II for worked out examples). 9.2 General Design Process A number of examples are presented to illustrate the design process for different types of pavement compositions for a given subgrade CBR and a given traffic. The users are required to carry out the design independently to optimize the thickness for the specific CBR and the design traffic for the selected combinations considering the basic equations for fatigue and rutting as shown in the examples in Cl. 10.0. Where practicable, synthetic Geo-composite
layer may be used for
separation/drainage above the subgrade to replace costly granular subbase. Cement treated marginal
41
aggregates can also be used to optimize the cost. Reclaimed asphalt pavement materials from the damaged pavements or recycled concrete can be treated with cement/ foamed bitumen/ bitumen emulsion for the construction of bases and subbases. Thus there is a wide choice of materials for optimization of pavement design. A strong subgrade and a higher modulus subbase will provide a durable pavement. Following points may be kept in view while designing a pavement. i.
For thicker bituminous layer(>100mm), use bituminous mix with higher modulus while for thinner bituminous surfacing(≤50mm) over granular base and subbase, the mix with lower modulus is preferred to allow greater flexibility due to large deflection caused by wheel load in thinner pavements
ii.
For snow bound area, use softer binder to limit thermal transverse cracking
iii.
To ensure long term riding quality, stabilization of expansive soil, good compaction of embankment, and other pavement layers is necessary.
iv.
Greater reduction in tensile strain at the bottom of the bituminous top (BT) can attained by increasing the thickness of the BT. Reduction in the vertical subgrade strain due to increase in the thickness of BT is marginal
v.
Greater decrease in vertical subgrade strain can be brought about by increasing the thickness of granular subbase. The effect on tensile strain in the BT due to increase in the thickness of GSB is very small
vi.
Though Wet Mix Macadam base is a necessity for major highways due to large volume of work, Water Bound Macadam(WBM) can be used in lower category of roads since research ( 50) has indicated that WBM is highly resistant to influence of water, and the South African standard, TRH4,(68) recommends such a base for heavy traffic in wet regions. For thick bituminous surface layer, both WBM and WMM have similar behaviour.
vii.
Compaction of WMM requires a good support from GSB and hence thicknesses of GSB as indicated in the catalogue is necessary.
9.3 Material Properties Regardless of the design procedure, it is essential that the material properties are adopted after conducting relevant tests on the materials. Where all test facilities are not available, at least those tests must be carried out, which can validate the assumed design properties. In Annex XI, the type of tests for the stabilized materials required as well as the range of values for material properties are given based on typical test results and experience in India and other countries. The values as
42
suggested may be adopted for pavement design as default values but not without validation by subjecting the materials to such tests which can be easily carried out in any laboratory (e.g. Unconfined Compressive Strength) to validate the assumed design values.
10. INDICATIVE DESIGN CATALOGUES FOR DIFFERENT TRAFFIC AND SUBGRADE CBR The process of design of pavements and indicative thickness of pavements with different compositions are illustrated in Plates 1 to 15. The thickness of DBM layer can vary from a minimum of 50mm to 175mm or higher as required by the design traffic and it is laid in a single layer or in two layers depending upon the thickness. If the DBM thickness is up to 75mm, DBM 2 is laid in a single layer. If the thickness is greater than 75mm and less than 100mm, DBM 1 is adopted. For thickness greater than 100mm, combination of DBM1 and DBM 2 or two layers of DBM2 can be considered for the pavement composition. For the single layer DBM, the mix design is done for 3.0% air void and compacted to air void of 4.5% or lower and the pavement design is done for 3.5% air void considering actual volume of bitumen. For the two layer DBM, the bottom layer is designed for 2.5% air void, compacted to air void of 4% or lower and the pavement is designed for 3% air void and actual volume of bitumen. The upper DBM is designed for 4% air void and compacted to an air void of less than 7. In some solved examples, thickness of the bituminous layer has been computed considering an air void of 3% and a volume of bitumen (Vb) of 11.5% for the bottom DBM1 layer in a two layer DBM. In a real case, actual value of Vb and an air void of 3.0% should be considered for design when the mix has been designed for 2.5% air void. For the single layer DBM, the pavement should be designed for an AV of 3.5% and the actual volume of bitumen though the mix is designed for 3% air void. For traffic exceeding 20 msa, a wearing course of SMA or Gap Graded mixes Rubberised Bitumen (GGRB) should be preferred because of their durability. SMA with PMB and GGRB have much higher life than BC with PMB or CRMB. For traffic up to 20 msa but greater than 10msa, BC with PMB/CRMB shall be preferred and for traffic up to 10 msa, a wearing course BC/SDBC/ MSS/ Premix carpet with seal coats with neat bitumen can be considered. Two coat surface dressing has given good performance of pavements in South Africa with traffic less than 5 msa. Thicknesses given in Plates 1 to 15 form an initial pavement composition for further iteration for pavement design. The suggested pavement compositions are not exhaustive. There can be many equally good or even better combinations and designers should examine alternatives for every project
43
to suit the local environment. The final thicknesses may be greater or lower than what is given in the plates. During the preparation of Detailed Project Report, even preliminary bituminous mix design should also
be done along with tests on subgrade soils, borrow materials, deflection tests on
pavements etc so that cost of bitumen also can be correctly estimated. . The seven combinations presented in the guidelines are as under: 1. Bituminous Top,Granular Base and Granular Subbase. 2. Bituminous Top, Cementitious Base and Cementitious Subbase with aggregate interlayer above the cementitious base. 3. Bituminous Top, Cementitious base and cementitious subbase with SAMI at the interface of base and the bituminous layer. 4. Bituminous Top, Foamed bitumen/bitumen emulsion treated RAP over Cementitious subbase 5. Bituminous Top, Cementitious base and granular subbase with crack relief layer of aggregate layer above the cementitious base. 6. Bituminous Top, WMM and Cementitious subbase ( Only design examples without plates) 7. High Modulus Bituminous Mixes ( Example given in Annexure II. . Annexure II gives additional examples of design of pavements including long life pavements, pavements. Note: For traffic < 2 msa IRC: SP:72-2015 should be referred to. City roads should be designed for a minimum design traffic of 2 msa . Illustrating design of various combinations are given below 10.1 Granular Base and Granular Subbase
Granular base
Critical location for vertical subgrade strain Fig. 10.1 Bituminous Surfacing with Granular Base and Granular Sub-base.
Fig.10.1 shows the cross section of a bituminous pavement with granular base and subbase. It is considered as a three layer elastic structure consisting of bituminous top, granular base and subbase
44
and the subgrade. The granular layers are treated as a single layer. Strain and stresses are required only for three layer elastic system. The critical strain locations are shown in the figure. For traffic greater than 10 msa and less or equal to 20 msa, VG30 or VG40 can be used. For traffic > 20 msa, o n l y VG40 bitumen or binder of equivalent stiffness i s r e c o m m e n d e d for DBM for plains in India. GSB consists of a separation/filter in the bottom (gradation I,II,VII of MORTH) and the grading III and IV GSB of cl 401.1 of MORTH may be used as drainage layers on top of the filter layer where drainage measure is necessary. The Plates 1,2 and 3 gives approximate thicknesses of bituminous layer, WMM and granular subbase for a subgrade CBR of 5. These are merely for illustration.
Plate 1 CBR 5% Pavement Thickness,mm
800 700 600 500
30 60
400 300
225
40 70
40
40
120
150
170
250
250
250
40
40 80
100
250
220
40
225
WMM/WBM GSB
200 100
BC/SDBC/SMA/ GGRB DBM
225
250
300
300
300
300
300
0 1
5
2
10
3
20
4
30
5
50
Design traffic, msa
6
100
7
150
45
Plate 2 CBR 10% Pavement Thickness,mm
700 600 500 400
300
30 50
40 50
40 75
250
250
250
40
40
40
95
120
140
BC/SDBC/SMA/ GGRB DBM
250
250
250
WMM/WBM
50 150
GSB
200 100
200
200
200
2
3
200
200
200
200
0 1
5
10
20
4
30
5
50
6
100
7
150
Design traffic, msa
Plate 3 CBR 15% Pavement Thickness,mm
700 600 500 400 300 200
100
50 50
150
40 50
40 75
250
250
40
40
95
120
BC/SDBC/SMA/ GGRB DBM
250
250
WMM/WBM
40 50 150
GSB
150
150
225
225
2
3
225
225
225
225
0 1
5
10
20
4
30
5
50
6
100
7
150
Design traffic, msa
Illustration 1 Design traffic= 5 msa, Effective Subgrade CBR= 5, Consider the following trial thickness:
46
BT=90mm,WMM=225mm,GSB=225mm Esubgrade = 10 CBR=10x5= 50 MPa MR_granular =0.2 x Esubgrade x h0.45, h = thickness of granular layer (GSB + WMM) MR_granular = 0.2 x 50 x 4500.45= 155 MPa. MR_ bituminous layer= 2000 MPa with VG30 bitumen, Consider Va=3.5, Vb=12.5 Allowable tensile strain in the bituminous layer ( Eq.6.1)=491x10–6 Allowable vertical subgrade strain (6.4)
=784x10–6
Computed strains are: Tensile strain in BT= 390x10–6 < 491x10–6
,
vertical subgrade strain= 543x10–6 << 784x10–6 Consider thickness of BT=80mm(DBM= 50mm,BC=30) thickness of GSB=200mm.WMM=200 mm MR_granular = 0.2 x 50 x 400.45= 148 MPa. MR_ bituminous layer= 2000 MPa with VG30 Computed strains are: Tensile strain in BT= 432x10–6 < 491x10–6 hence safe , vertical subgrade strain= 705x10–6 < 784x10–6 Hence safe If the Va= 4.5% and the Vb=11.50%, Allowable tensile strain in BT =411x10–6 > 432x10–6 fails Increase DBM thickness to 60mm, New tensile stain= 403x10–6 < 411x10–6 hence safe, Vertical subgrade strain is safe. Reducing thickness of GSB may lead to lower densityof WMM. Such exercise is to be carried out for pavement design
Illustration 2 Design Traffic=150 msa, effective Subgrade CBR=10, Trial Thickness GSB=200mm,WMM=250, BT=180mm(DBM=140mm, SMA=40mm) Check the adequacy of design? 140mm DBM is to be laid in two layers ( DBM 1=80 mm,DBM2=60mm) Esubgrade = 17.6 x CBR0.64= 77 MPa MR_granular =0.2 x Esubgrade x h0.45, h = thickness of granular layer (GSB + WMM) MR_granular = 0.2 x 77 x 4500.45= 240 MPa. MR_ bituminous layer= 3000 MPa For the given composition of Pavement thicknesses, 90% Reliability is adopted i.e., Eq. 6.2 and Eq. 6.5 are used Allowable Horizontal Tensile Strain in Bituminous Layeris145x10–6 for VG40 Mixes for Va=3% and Vb=11.5% Allowable Vertical Compressive Strain on Subgrade is 292x10–6 Computation I Horizontal Tensile Strain in Bituminous Layer is 143 x10-6 < 145 x10–6
47
II. Vertical Compressive Strain on Subgrade is 233 x 10–6< 291 x 10–6 Hence the Pavement Composition is Safe. If DBM 2 is provided in two layers, the bottom layer is considered to have an air void of 3% and volume of bitumen may reach about 12.5%. Users can develop the design following the above procedure. . 10.2 Bituminous Pavements with Cemented Base and Cemented Subbase with Crack Relief Interlayer of Aggregate Critical tensile strain location
Fig. 10.2 Bituminous Surfacing, Cement Treated Base and Cement Treated Sub-Base with Aggregate Interlayer
Fig. 10.2 shows a five layer elastic structure consisting of bituminous surfacing, aggregate interlayer layer, cemented base, cemented subbase and the subgrade. Material properties such as modulus and poisson’s ratio are the input parameters apart from loads and geometry of the pavement for the IITPAVE software. For traffic > 20 msa, VG 40 bitumen is used for limiting rutting. DBM has a design air void of 3% and less than 4.5% after rolling. Cracking of cemented base is taken as the design life of a pavement. For traffic greater than 30 msa, minimum thickness of bituminous layer consisting of DBM and BC layers is taken as 100 mm (AASHTO-1993) even though the thickness requirement may be less from structural consideration. Residual life of the bituminous layer against fatigue cracking is not considered since it cracks faster after the fracture of the cemented base. In high rainfall area, upper100 mm of the cemented subbase (D) having the gradation 4,5,6 of the Table V1 of Annexure V is porous acting as the drainage layer over lower cemented subbase (F). The gradings III and IV of Table 400.1 of MORTH with fines less than 2 % containing about 2-3% cement can also be used, In such cases where CTB is used, it is necessary to carry out fatigue damage analysis for the safety of CTB against premature fracture by the spectrum of axle loads for design traffic ≥ 10 msa. Thickness may have to be increased. One can also assume a pavement composition considering the constructability and carry out fatigue damage analysis without considering standard axle loads. This is illustrated in Annexure II.
48
Plate 4 CBR 5% Pavement Thickness,mm
700 600
40 75
300
40 60
40 60
40
50
100
100
100
100
100
100
100
100
110
120
130
140
250
250
250
250
250
250
500 400
40 60
40 60
100 200 100
BC/SDBC/SMA /GGRB DBM AIL CTB
200
CT_GSB
0 1
2
5
3
10
4
20
5
30
6
50
7
100
150
Design traffic, msa
Plate 5 CBR 10% Pavement Thickness,mm
600
500 400 300
40
40
50
100
100
100
100
50
40 60
40 60
40 60
100
100
100
100
100
100
110
120
75 100
AIL
200 100
BC/SDBC/SMA/ GGRB DBM
225
200
250
225
250
250
250
CTB CT_GSB
0 1
5
2
10
3
20
4
30
5
50
Design traffic, msa
6
100
7
150
49
Plate 6 CBR 15% 600
Pavement Thickness,mm
500
400
40 75
40 100
300 100
100
40
40
60
60
40
40
40 50
100
100
100
100
100
BC/SDBC/SMA/ GGRB
100
100
100
110
120
DBM AIL
200 100
200
200
225
225
225
225
225
CTB CT_GSB
0 1
5
2
10
3
20
4
30
5
50
6
100
7
150
Design traffic, msa Illustration Traffic 150 msa Subgrade CBR = 10percent, MR subgrade = 17.6*CBR0.64 = 77MPa,MR bituminousl layer = 3000 MPa Consider SMA/GGRB + DBM = 100 mm, Aggregate interlayer = 100 mm (MR = 450 MPa), Cemented base = 120 mm (E = 5000 MPa), cemented subbase = 225 mm(600 MPa Va =3.5 % and Vb= 12.5% for the DBM2 laid in a single layer (PLATE NO 5 for CBR=10 ,design traffic 150 msa)
Equations 6.2, 6.5 and 6.7 are used as design criteria. I.
Allowable Horizontal Tensile Strain in Bituminous Layer is 140 x 10–6.
II.
Allowable Vertical Compressive Strain on Subgrade is 292 x 10–6.
III.
Allowable Tensile Strain in Cementitious Layer is 64.77x10–6. From IITPAVE Software the computed strains are
IV.
Horizontal Tensile Strain in Bituminous Layer is 129.5x10-6< 140 x 10–6.
V.
Vertical Compressive Strain on Subgrade is 201 x 10–6 <292 x 10–6
VI.
Tensile Strain in Cementitious Layer is 51.0 x 10–6 <64.77x10–6
Hence the design is safe from the consideration of standard axle repetitions. The plates above give an approximate thickness quickly. However cumulative fatigue damage analysis and its safety from the construction traffic are to be examined as explained in Annex II.3.2. If 28 day bending strength is less than 1.4 MPa, the thickness of CTB will be higher. An example of the check for the construction traffic is given below.
50
Check for Construction traffic Crack relief layer of WMM is generally laid after seven day curing of CTB. The CTB must be safe against the construction traffic. There may be about 70 trips of dumpers equivalent to 140 repetitions of each axle of the tandem axle of the dumper for laying two km of 100mm thick WMM by paver. Flexural strength of CTB after 7 days= about 1.00 MPa ( 20% of 5 MPa strength at 7 days). Dumpers carry about 10 cum of WMM weighing about 210kN. Approximate gross weight of three axle dumper=320 kN. Consider 120 kN load on a single axle load of the 240 kN tandem axle load. Neglecting interference of tandem axle for the thin CTB, Take tyre pressure=0.80 MPa. Load on each wheel= 30,000N, allowable flexural stress for 140 repetitios0.795 MPa (Eq.6.7), Bending stress= 0.96 MPa> 0.795 MPa, increase the thickness to 150mm Bending stress=0.84 Mpa > 0.795 MPa hence unsafe. Increase the thickness to 165mm Bending stress=0.781 Mpa < 0.795 MPa hence safe. Such an exercise has to be carried. .10.3 Cemented Base and Cemented Subbase with Sami at the Interface of Cemented Base and the Bituminous Layer
SAMI layer
Critical location for vertical strain Fig. 10.3 Bituminous Surfacing with Cemented Granular Base and Cemented Granular Sub-base with Stress Absorbing Membrane Interlayer (SAMI)
Fig.10.3 shows a four layer pavement consisting of bituminous surfacing, cemented base, Cemented subbase and the subgrade. For traffic > 20 msa, VG40 bitumen is used. The DBM mix is designed for air void of 3 percent and compacted to an air void content less than 4.5% after rolling Cracking of cemented base is taken as the life of pavement. Minimum thickness of bituminous layer for major highways is recommended as 100 mm as per the AASHTO guidelines. Stress on the
51
underside of the bituminous layer over un-cracked cemented layer is compressive In such cases where CTB is used, it is necessary to carry out fatigue damage analysis for the safety of CTB against premature fracture by the spectrum of axle loads. Thickness may have to be increased.
Plate 7 CBR 5% Pavement Thickness,mm
600 500 400 300
40
40
50
40 50
150
160
170
150
40 60
40 60
40 60
150
160
170 CTB CT_GSB
200 100
BC/SDBC/SMA/ GGRB DBM
250
250
250
2
3
250
250
250
250
0 1
5
10
20
4
30
5
50
Design traffic, msa
6
100
7
150
52
Plate 8 CBR 10% 600
Pavement Thickness,mm
SAMI above CTB 500 400
40
40
50
300
130
140
140
50
40 50
40 60
40 60
160
140
150
160
CTB CT_GSB
200 100
BC/SDBC/SMA/ GGRB DBM
250
250
250
250
250
250
250
0 1 5
2 10
3 20
4 30
5 50
6 100
7150
Design traffic, msa
Plate 9 CBR 15% Pavement Thickness,mm
600 500 400
40
40
50
300
120
130
140
40 50
40 60
40 60
40 60
140
130
140
150 CTB CT_GSB
200 100
BC/SDBC/SMA /SDBC DBM
250
250
250
2
3
250
250
250
250
0 1
5
10
20
4
30
5
50
6
100
7
150
Design traffic, msa Illustration Traffic 150 msa Subgrade CBR = 10%, MRsubgrade= 17.6*CBR0.64= 77 MPa, MRbituminous layer= 3000 MPa E of cemented base = 5000 MPa, E of cemented subbase = 600 MPa From PLATE 15, SMA/GGRB + DBM =100 mm, Cemented base=160mm,Cemented subbase= 250 mm SAMI is provided on the top of cemented base.
53
For the given traffic, 90 % Reliability is adopted i.e., Equations 6.2, 6.5 and 6.7 are used. I. Allowable Horizontal Tensile Strain in Bituminous Layer is 140 x 10–6. II. Allowable Vertical Compressive Strain on Subgrade is 291 x 10–6. III. Allowable Tensile Strain in Cementitious Layer is 64.77 x. 10–6. From IITPAVE Software the computed strains are I. Horizontal Tensile Strain in Bituminous Layer is –4.137 x 10–6 (Compressive). II. Vertical Compressive Strain on Subgrade is 191 x 10–6. III. Tensile Strain in Cementitious Layer is 58 x. 10–6. The Pavement composition is safe. A minimum thickness of 100 mm has been adopted for SMA/GGRB and DBM even though there is no tensile stress at the bottom. The reduction in thickness of the cemented base increases the bending stresses considerably because it is inversely proportional to the square of the thickness. It is necessary to carry out fatigue damage analysis for the safety of CTB against fracture by the spectrum of axle loads. Structural safety of the CTB should be examined for the construction traffic also as explained earlier. 10.4 Foamed Bitumen/Bitumen Emulsion Treated Rap/Aggregates Over Cemented Subbase
Fig. 10.4 Bituminous Surfacing with RAP and Cemented Sub-base
Fig. 10.4 shows a four layer pavement consisting of bituminous surfacing, recycled layer Reclaimed asphalt pavement, cemented subbase and the subgrade. VG 40 is used for traffic > 20 msa. Even
54
bitumen emulsion/foamed bitumen treated fresh aggregates can be used to obtain stronger base of flexible pavements as per the international practice. DBM has a design air void of 3 percent compacted to less than 4.5% air void after rolling Fatigue failure of the bituminous layer is the end of pavement life. Cemented subbase is similar to that in clause 9.3.
Plate 10 CBR 5% Pavement Thickness,mm
600 500 400 300
40
40
40
40
130
150
170
100
200
200
200
200
2
3
40 60
40 60
140
170
250
250
40 60
200
Foamed/Em_bi t CT_GSB
200 100
BC/SDBC/SMA/ GGRB DBM
250
0 1
5
10
20
4
30
5
50
6
100
7
150
Design traffic, msa
Plate 11 CBR 10% Pavement Thickness,mm
600 500 40 50
400 300
40
40
40
40
100
100
110
120
200
200
200
200
2
3
40 60
40 60
100
130
160
250
250
250
Foamed/Em_bi t CT_GSB
200 100 0
1
5
10
20
4
30
5
50
Design traffic, msa
6
100
Bc/SDBC/SMA/ GGRB DBM
7
150
55
Plate 12 CBR 15% Pavement Thickness,mm
600 500 400 300
40
40
40
50
100
100
100
100
200
200
200
200
2
3
40 60
40 60
40 60
100
110
140
250
250
250
200 100
BC/SDBC/SMA/ GGRB DBM Foamed/Em_bi t CT_GSB
0 1
5
10
20
4
30
5
50
6
100
7
150
Design traffic, msa Illustration Traffic 150 msa Subgrade CBR = 10 percent, Esubgrade= 17.6*CBR0.64 = 77 MPa, MR bituminous layer = 3000 MPa MRof foamed bitumen treated RAP = 800 MPa, E of cemented subbase = 600 MPa Pavement thicknesses for CBR 10 percent (PLATE 11), SMA/GGRB + DBM = 100 mm, RAP = 160 mm, cemented subbase = 250 mm Design traffic = 150 msa 90 percent Reliability is adopted i.e., Equations 6.2 and Equation 6.5 are used. I. Allowable Horizontal Tensile Strain in Bituminous Layer is 140 x 10–6. For Va=3.5%,Vb=12.5% II. Allowable Vertical Compressive Strain on Subgrade is 291 x 10–6. From IITPAVE Software the computed strains are I. Horizontal Tensile Strain in Bituminous Layer is 106 x 10–6. II. Vertical Compressive Strain on Subgrade is 260 x 10–6. Minimum thickness of 100 mm for the DBM+ SMA/GGRB has been adopted for traffic greater than 30 msa as per AAHSTO 93 Guidelines since the wearing course of thickness 40mm may have to be milled and replaced at regular intervals. The Pavement Composition is Safe.. A safe design is
56
necessary for RAP mixes due to variability. Users may optimize the design further. 10.5 Cemented Base and Granular Subbase with Crack Relief Layer of Aggregate Interlayer Above the Cemented Base
Fig. 10.5 Bituminous Surfacing, Cement Treated Base and Granular Sub-base with Aggregate Interlayer
Some highways may need total rehabilitation due to complete damage of bituminous layers because repeated overlays do not work. Users may have a choice of bituminous surface, aggregate interlayer, cemented base while retaining the existing granular subbase. If the problem is due to subgrade problem, the entire pavement has to be milled and the materials are to be stock piled nearby for reuse after the appropriate treatment of subgrade with lime or cement. The drainage layer in GSB is required to be restored in area where rainfall may damage the pavements. It is modeled as a five layer elastic structure in IITPAVE software. In a two layer construction of bituminous layer consisting of BC and DBM, the bottom layer should have a design air void of 3 percent and less than 4.5% air void after the compaction This would also resist stripping due to water percolating from the top to the bottom of the bituminous layer or rising from below. The aggregate interlayer acting as a crack relief layer should meet the specifications of Wet Mix Macadam (WMM) and compacted to 100% density to lower the air void. If the WMM layer is likely to be used by construction traffic for some time. it
57
may be treated with 2 to 3 percent bitumen emulsion in the wet mix plant itself for stability under traffic. A reliability of 80 % is used for traffic upto 20 msa and 90 percent for traffic > 20 msa. VG30 or V40 bitumen can be used for traffic up to 20 msa and VG 40 for traffic > 20msa. For colder area, recommendation of IRC: 111-2009 shall be followed. The different catalogue of thicknesses are given as
Plate 13 CBR 5% 700
Pavement Thickness,mm
600 500
40 75
40
50
50
100
100
100
400 210
200
230
220
40 60
40 60
40 60
100
100
100
210
200
190
BC/SDBC/SMA/ GGRB DBM
300 AIL 200 100
250
250
250
250
250
250
250
CTB GSB
0 1
5
2
10
3
20
4
30
5
Design traffic, msa
50
6
100
7
150
58
Plate 14 CBR 10% 700
Pavement Thickness,mm
600 500
40
50
40 75
40 100
100
100
180
190
40 60
40 60
40 60
100
100
100
170
180
400 300
210
200
190
BC/SDBC/SMA /GGRB DBM AIL
200 100
CTB 250
250
250
250
250
250
250 GSB
0 1
2
5
3
10
4
20
5
30
50
6
100
7
150
Design traffic, msa
Plate 15 CBR 15% Pavement Thickness,mm
700 600 500
40 75
40
50
100
100
160
170
180
50 100
40 60
40 60
40 60
100
100
100
150
160
170
400 300
190
AIL
200 100
CTB 250
250
250
250
250
250
250
0 1
5
2
10
3
20
4
30
Design traffic, msa Illustration Traffic 150 msa
Bc/SDBC/SMA /GGRB DBM
5
50
6
100
7
150
GSB
59
Subgrade CBR = 10 percent, MRsubgrade= 17.6*CBR0.64 = 77 MPa, MR of bituminous layer = 3000 MPa MR of Aggregate crack relief layer = 450 MPa, E of cemented base = 5000 MPa, E of granular subbase = 77 x (0.2) x (250)0.45 = 185 MPa From PLATE 14, Pavement composition SMA/GGRB+DBM = 100 mm, Aggregate interlayer = 100 mm, cemented base = 190 mm, granular subbase = 250 mm For the given composition of Pavement thicknesses, 90 percent Reliability is adopted i.e., Equation 6.2, Equation 6.5 and Equation 6.7 are used. Allowable Horizontal Tensile Strain in Bituminous Layer is 140 x 10–6,Va=3.5%,Vb=12.5% Allowable Vertical Compressive Strain on Subgrade is 291 x 10–6. Allowable Tensile Strain in Cementitious Layer is 64.77 x. 10–6. From IITPAVE Software the computed strains are I. Horizontal Tensile Strain in Bituminous Layer is 127 x. 10–6. II. Vertical Compressive Strain on Subgrade is 157 x 10–6. III. Tensile Strain in Cementitious Layer is 58.12 x 10–6. Hence the Pavement Composition is safe for repetitions of equivalent standard axles Check should be made for the cumulative fatigue damage and construction traffic 10.6 Bituminous pavements with Cementitious subbase: A strong subgrade and a strong cementitious subbase can provide a good performing pavement. Though PLATES for thickness combinations are not provided, an example is presented below illustrating how to work out thickness of pavements composed of cemented subbase, Wet Mix Macadam base and bituminous surfacing. NH-8 from Ahmedabad to Jodhpur and some others built long back have cemented layer below the Wet Mix Macadam. Fig. 10.6 shows a combination of bituminous pavements with WMM base and cemented granular subbase with a 7 day strength from 1.5 MPa to 3.0 MPa. Cement bound granular subbase Grading IV of IRC: SP- 89 having strength in the range 0.75-1.5 MPa is not recommended for major highways as per South African practice but it can be used for others if the design traffic less than 10 msa.
60
Critical location for tensile strain
Bituminous layer Wet mix macadam
Subgrade
Fig.10.6BituminousSurfacingwithWetMixMacadamBaseandCementedSub-base
Illustration Traffic = 100 msa, Subgrade CBR = 8 percent Trial Pavement Composition: Bituminous layer= (SMA/GGRB + DBM) = 120 mm, WMM base = 150 mm, Cemented subbase (1.5-3.0 MPa strength) = 300 mm. For 8 percent CBR, modulus of subgrade =17.6*80.64 = 66 MPa, MR bituminous layer =3000 MPa, Ecementedsubbase= 600 MPa. Mr of the WMM layer is stress dependent and it can be determined from trials as illustrated below (Annex_VIII ). 𝜃
𝑀𝑟𝑔𝑟𝑎𝑛 = 𝐾1 ( )𝐾2 × [ 𝜌
𝜏𝑜𝑐𝑡 𝐾 ] 3 𝜌
1
………….
𝜏𝑜𝑐𝑡 = 𝑂𝑐𝑡𝑎ℎ𝑒𝑑𝑟𝑎𝑙 𝑠𝑡𝑟𝑒𝑠𝑠𝑒𝑠 = √3 √(𝜎2 − 𝜎3 )2 + (𝜎3 − 𝜎1 )2 + (𝜎1 − 𝜎2 )2 in kPa
10.1 10.2
𝜃 = Sum of principal stresses, kPa 𝜎1 ,𝜎2 , 𝜎3 are principal stresses in granular layer
𝜌 =atmospheric pressure=100kPa, K1, K2 and K3 are experimental test constants of 220, 1.05and -0.2 respectively (Annex VIII) As a first trial, assume Mr (WMM) = 350 MPa and carry out stress analysis and compute the stresses due to a standard axle load of 80000 N at the mid depth of 195mm(120+75) of the WMM layer at a radial distance of 155mm, 𝜎𝑧 = 0.1186 MPa=118.6 kPa
61
𝜎𝑡 = 0.0147 Mpa= 14.70 kPa 𝜎𝑟 =0.0369 Mpa =36.90 kPa
σz, σt
and
σr due to the overburden pressure( depth 195 mm) are also to be computed
σz,=195x 2300x9.8/1000=4395.30 Pa=4.395 kPa ( average density=2300kg/m3 assumed) 𝜇
σt=, σr = 1−𝜇 𝜎𝑧 =2.366 kPa assuming a Poisson’s ratio of 0.35 Total of principal stresses are 𝜎𝑧 = 122.995 kPa 𝜎𝑡 = 17.066 kPa 𝜎𝑟 =39.266 kPa
𝜃(𝜎𝑧 + 𝜎𝑡 + 𝜎𝑟 )=179.50 kPaa
𝜏𝑜𝑐𝑡 = 78.90 kPa from Eqs 10.1 and 10.2
For above mentioned experimental test constants MR (WMM) = 426.5 Mpa from Eq.6.1. Mr value is higher than the assumed. Another iteration is needed. The Mr value after the 2nd iteration is 402 MPa. Additional iterations will give values close to the above. The Mr value of 350 MPa may be retained for a safer pavement For 90 per cent reliability, Allowable Tensile strain on the bottom of Bituminous layer (εt) = 161 * 10-6 Allowable Compressive strain on the top of subgrade (εz) = 318.9 * 10-6 For the above thickness, the strains at the critical locations calculated by IITPAVE software are: Tensile strain at the bottom of Bituminous Layer (εt )= 158 * 10–6 Compressive strain on the top of subgrade (εz) = 259 * 10–6 Hence design is safe. A little higher thickness of CT subbase or the bituminous layer will give much lower tensile strain in the bituminous layer. 10.7 : High Modulus Bituminous Mixes (HMBM) is a standard practice in France and many European countries(77,78,79) and numerous trials in South Africa, Australia and USA have given very good results. The only novelty is use of low penetration refinery bitumen obtained without air blowing. Some countries add some polymers during dry mixing to obtain high modulus bituminous
62
mixes. It is found to be economical with better performance. Bitumen of penetration 10-25 is generally used in the production of mixes. A high Modulus mix made up of Dense Bitumen Macadam using 10-20 bitumen was used in United Kingdom ( 81) and it had a very similar property to that of the commonly used French High Modulus Mix. Such a mix is highly rut resistant and thickness becomes lower resulting in use of lower thickness of pavements requiring less amount of aggregates. The modulus of the HMBM at 150C is greater than 14000 MPa at 10 Hz frequency. Nominal Maximum aggregate size used in such mixes is 14 to 18 mm so that higher amount of binder is introduced. DBM also attained a high modulus like HMBM with 35-50 penetration bitumen in UK with a filler content between 7 and 11% (81) and appropriate compaction. IIT Kharagpur obtained an unmodified hard asphalt residue from Haldia refinery before it was blended with lighter extracts for production of viscosity grade of binders and developed a high modulus mix both with DBM 1 and DBM 2. . Some other refineries in India also adopt similar method for the production of conventional bitumen from hard asphalt residue The hard asphalt had a penetration of about 22 which is within the specifications limits of binder for high modulus mixes (77,78,79). Bituminous mixes were cast with the hard binder from Haldia and a modulus as high as 7000 MPar was obtained at 350 C for the DBM 1. A commercially available chemical additive to hot aggregates with VG30 bitumen also resulted in a modulus of over 5500 MPa. Warm mix additives allow mixing and compaction at lower temperatures. Pavement design using a hard bitumen is illustrated Annex.II, and a comparison of pavement design using both hard asphalt and VG40 mix is also given for a traffic of 300 msa.
63
11. INTERNALDRAINAGE IN PAVEMENT 11.1 The performance of a pavement can be seriously affected for lack of drainage measures to drain out the water if it is allowed to enter into the pavement. Major highways have paved shoulders and thick bituminous layers, and there is little chance of entry of water into the pavement through the impermeable pavement surface made up of thick bituminous mixes such as BC/SDBC/SMA and DBM built in layers. But the water that can enter into the pavement from the median should be taken out through the drainage layer with suitable arrangement. Water infiltrating through the median may carry fines that may choke the gsb layer which extend across the median also in many caes. Hence a nonwoven synthetic geotextile of 200 gsm may be provided at the gsb level in the median as shown in Figure 11.5. If the median is sealed with clay soil, very little water will infiltrate through it because of low permeability of clay. After the traffic operation and ageing of the bituminous layers with time, thinner bituminous layers may get cracked allowing water to enter into its body during the rains. Some of the measures to guard against poor drainage conditions are maintenance of proper camber so as to facilitate quick run-off of surface water and provision of appropriate surface and sub-surface drains where necessary. Drainage measures are especially important when the road is in cutting or built on low permeability soil or situated in heavy rainfall/snow fall area. On curves, the rain water should be intercepted in the median and taken out through pipes below the pavement rather than allowing it flow over the pavement on the other side through cuts in the median. The medians get chocked frequently and stagnating water together with moving traffic may cause damage to the bituminous layer 11.2 On new roads, the aim should be to construct the pavement above the water table as economically practicable. The difference between the top of subgrade level and the level of water table (i) for non-flood zones-should not be less than 1.0 m and (ii) for flood zones and for water logged areas, it should not be less than 1.5m. In the zone of capillary saturation, consideration should be given to the installation of suitable capillary cut off as per IRC-34 at the appropriate level under the pavement. 11.3 When the traditional granular construction is provided on a relatively low permeability subgrade, the granular sub-base should be extended over the entire formation width in order to drain the pavement structural section. Care should be exercised to ensure that its exposed ends do not get covered by the embankment soil when the eroded slope is repaired after the monsoon. The trench type section without drainage measures should not be adopted in any case as it would lead to the entrapment of water in the pavement structure. 11.4 If the granular sub-base is of softer variety which may undergo crushing during rolling leading to denser gradation and low permeability, the top 100 to 150 mm thickness should consist of an open graded crushed stone layer of Los Angeles abrasion value not exceeding 40 to ensure proper drainage. The filter layer must have enough permeability to prevent development of undesirable pore water pressure and it should rain away any free water that enters into it all be it at much lower rate as compared to the drainage layer.
64
The filter/separation layer should satisfy the following criteria: D15 D15
of filter layer ≥5 of subgrade
D15 of filter D85 layer of subgrade
… 11.1
≤5
… 11.2
To prevent entry of soil particles into the drainage layer And
D50 of filter D50 layer of subgrade
≤ 25
… 11.3
65
D85 means the size of sieve that allows 85 percent by weight of the material to pass through it. Similar is the meaning of D50 and D15. The permeable sub-base when placed on the erodible subgrade soil should be underlain by a layer of filter material to prevent the intrusion of soil fines into the drainage layer (Fig.11.2). Non-woven geosynthetic also can be provided to act as a filter/separation layer. Some typical drainage system is illustrated in Figs. 11.1, 11.2, 11.3 and 11.4. 11.5 When large in flows are to be taken care of, an adequately designed sub-surface drainage system consisting of an open graded drainage layer with collection and outlet pipes should be provided. The system should be designed on a rational basis using seepage principles to estimate the inflow quantities and the outflow conductivity of the drainage system. It should be ensured that the outflow capabilities of the system are at least equal to the total inflow so that no free water accumulates in the pavement structural section. If granular sub-base is not required because of strong subgrade, commercially available geo-composite can be used for drainage and separation. A design example is given in Appendix V. 11.6 Very often, water enters the base, sub-base or the subgrade at the junction of the shoulder and the bituminous surfacing. To counteract the harmful effects of this water, it is recommended that the shoulders should be well-shaped and if possible, constructed of impermeable material. Major highways have paved shoulder to keep away water from the subgrade. For other roads also with design traffic of 20 msa or less; the base should be constructed 300-450 mm wider than the required bituminous surfacing so that the run-off water disperses harmlessly well clear off the main carriageway. 11.7 Shoulders should be accorded special attention during subsequent maintenance operation too. They should be dressed periodically so that they always conform to the requisite cross-fall and are not higher than the level of carriageway at any time. Drainage layer Carriageway
Filter layer
Shoulder Seal seal
Water flow
Water Table Drainage material
Water table
Synthetic fabric
Perforated pipe Fig 11.1 Pavement along a slope
66
Embankment Geotextile filter if necessary Natural ground
Flow of water Fill Drainage Layer
1:1
1.25:1 Outlet drain Pipes Fig 11.2 Drainage below fill on a slope
Pavement Layers
End 0.5m stabilized with 2.5% emulsion or 2%cement
shoulder
Sub grade
Drainage layer
Filter layer
Fig.11.3 pavement with filter and drainage layers
67
Fig. 11.4 Longitudinal Pipe at the Edge of the Drainage Layer with Outlet Pipe
Figure 11.5 Nonwoven 200gsm synthetic geotextile below the median tocapture the fines infiltrating with water
11.8 Drainage Requirement Heavy axle loads commonly ply on major roads in India and therefore, it should be ensured that the unbound layers do not undergo unacceptable permanent deformation under repeated loading. The subgrade and the granular layers with entrapped water would be subjected to large pore water pressure under heavy loads causing erosion of the unbound layer. It is necessary to provide a drainage layer to drain away the water entering into the pavement. The grading III and IV of granular sub-
68
bases in Table 400.1 of MORTH (62) have the necessary permeability of well over 300 m/day (81) duly verified at IIT Kharagpur. If open graded aggregates are found be a little unstable during rolling and operation of construction traffic, 2% cement or 2 to 2,5% bitumen emulsion can be used to make them stable. Field test by Ridgeway in USA indicated that it is the duration of low intensity sustained rainfall rather than high intensity rainfall that is critical for infiltration of water into the pavement. It was found that the infiltration rate through the joints/cracks was 0.223m3/day/m and this value can be used for design of drainage layer in the absence of field data. The infiltration rate per unit area qi in m3/day/m2 can be expressed as: 𝑞𝑐 = 𝐼𝑐 [
𝑁𝑐 𝑊𝑝
+
𝑊𝑐 𝐶𝑠 𝑊𝑝
] + 𝐾𝑝
10.4
Ic= Crack infiltration rate (0.223 m3/day/m) Nc= Number of longitudinal cracks (i.e., number of lanes plus one) Wp= Width of pavement subjected to infiltration Wc= Length of the transvers cracks or joints (equal to the width of the pavement) Cs= Spacing of transverse cracks taken as 12 m for bituminous pavement Kp=Rate of infiltration through un cracked area (assumed zero for thick bituminous pavements) An example for the design of drainage layer is given in Appendix V.
12 DESIGN IN FROSTAFFECTEDAREAS 12.1 In areas susceptible to frost action, the design will have to be related to actual depth of penetration and severity of the frost. At the subgrade level, fine grained clayey and silty soils are more susceptible to ice formation, but freezing conditions could also develop within the pavement structure if water has a chance of ingress from above. 12.2 One remedy against frost attack is to increase the depth of construction to correspond to the depth of frost penetration, but this may not always be economically practicable. As a general rule, it would be inadvisable to provide total thickness less than 450 mm even when the CBR value of the subgrade warrants a smaller thickness .In addition, the materials used for building up the crust should be frost resistant. 12.3 Another precaution against frost attack is that water should not be allowed to collect at the subgrade level which may happen on account of infiltration through the pavement surface or verges or due to capillary rise from a high water table. Whereas capillary rise can be prevented by sub soil drainage measures and cut-offs, infiltrating water can be checked only by providing a suitable wearing surface.
IRC: 37-2012
13. SUMMARYOF PAVEMENT DESIGN PROCEDURE AND USE OF IITPAVE SOFTWARE The analysis and design of pavement may be carried out by the following approach: A. For traffic lower than 2 msa, recommendation of IRC: SP: 72-2015 may be used. B. IITPAVE software can be used for the analysis of stresses in pavements. It is a multilayer elastic analysis program. This software is included along with the Pavement Design Guidelines in a CD. Details and method of data inputs are self-explanatory. The necessary steps required to use this software are: i. Open the folder IRC_37_IITPAVE. ii. Double-click IITPAVE_EX file in the IRC_37_IITPAVE folder. This is an executable jar file. A home screen will appear. iii. From the Home screen, user can manually give input through input window by clicking on ‘Design New Pavement Section’. User can also give input through properly formatted input file by clicking on ‘Edit Existing File’ option then browsing and opening the input file. iv. Next an input window will come. All the inputs required have to be given through the input window. v. First, number of layers to be selected from drop down menu to fix up input boxes for different layer inputs. vi. Next, Resilient modulus/ Elastic modulus (E) values of the various layers in MPa, Poisson’s ratio and thickness of various layers in mm excluding the subgrade thickness are to be provided. vii. Single wheel load and the tyre pressure are to be provided (20000 N single wheel load and a tyre pressure of 0.56 MPa has been used for calibration of the fatigue and rutting equations and the same pressure is to be used for stress analysis). Change of pressure even up to 0.80 MPa has a small effect upon stress values in deeper layers. For the cumulative fatigue damage analysis of the cemented base, tyre prsssure is taken as 0.8 MPa. viii. The number of points for stress computations is to be given through the drop down menu for analysis points. ix. Corresponding to different points, the values of depth Z in mm and the corresponding value of radial distance from wheel centre (r) in mm are to be given. x. Provide whether analysis is for single wheel load or dual wheel load by selecting 1 or 2 from drop down menu beside “Wheel Set”. Only dual wheel load may be needed in most cases. xi. The output of the program will provide stresses, strains and deflections at the desired points. Next check if the computed strains are less than the permissible strain. If not then run the program with a new thickness combinations till the permissible strain values are achieved. epT, epR and epZ will be the outputs that will be of interest. For cemented base tensile stress below the cemented layer SigmaT/SigmaR are needed for cumulative fatigue damage analysis. xii. In most cases the tensile strain at the bottom of the bituminous layer is higher in the longitudinal direction (epT) rather than in radial direction (epR). If tensile strain in the bituminous layer is high, increase the thickness of the bituminous layer. xiii. Tensile strains in the cementitious bases also are to be computed for design. If the tensile
IRC: 37-2012 xiv. xv.
strain/stress in the cemented layer is higher, increase the thickness of the cemented layer. Vertical subgrade strain (epZ) should be less than the permissible value for the design traffic. If the vertical subgrade strain is higher, increase the thickness of sub-base layer. Stress values can also be easily computed by changing directly the input file which is to be written in a format as illustrated in the manual and browse the input file by clicking ‘Edit Existing File’ on home screen of IITPAVE.
The above approach can be used to arrive at the pavement thickness satisfying the limiting strains. C.
Design plates showing approximate thickness of materials of different compositions are provided in the text of the guidelines only for illustration. Users are required to design pavements starting with initial thickness given in different plates and develop an optimum design that is sustainable. The software can help in finding the thickness for any traffic other than that given in the design catalogue.
IRC: 37-2012
ANNEX-I Considerations in Design of Bituminous Pavements I-1 The IITPAVE software computes stresses in a multi-layer elastic system with rough interfaces. Vertical subgrade strain, Єv, is an index of bearing capacity of the subgrade and it needs to be kept below a certain value for a given traffic to limit rutting in the subgrade and granular layers of a wellconstructed pavement. Research findings indicate that the plastic vertical strain, Єp, in pavement materials depends upon the magnitude of elastic vertical strain, Єv, given as Єp/Єv= K x (N)c
… (I-1)
Where Єp and Єv are plastic and elastic strains respectively, N=number of repetition of axle loads, K and c are constants If the computed Єv on the subgrade for a given wheel load is low, the vertical elastic strain in the upper granular layers also is low. Hence limiting the subgrade strain controls the rutting in the subgrade as well as in the granular layers. Bituminous layer must have a rut resistant mix. The elastic tensile strains, Єt, at the bottom and at the top of the bituminous layer should not exceed a certain limit for a given design traffic to control development of cracks during the design period. Surface crack may develop on either side of the wheel path as shown in Fig. I-1(b). Tensile stains near the edge of the tyres can initiate cracking at the surface when the top layer gets oxidized causing brittleness, and surface cracks may develop much earlier than those at the bottom particularly at higher temperatures. A number of cases of Top Down Cracking followed by raveling and pothole formation within a year or two of opening to traffic has been found at many locations (41, 55,74,75) in the BC layer in India. Bleeding and rutting (51, 53 and 63) on the surface have also been very common because of secondary compaction owing to the use of softer bitumen for the hot climate and heavy traffic. Transverse surface cracks may also develop due to horizontal shear stresses applied by wheels of heavy commercial vehicles during acceleration and braking if the mix does not have sufficient tensile strength. The surface cracks responsible for top down cracking shown in Fig. I-1(b) can be minimized to a great extent by using a gap graded wearing course containing higher amount of high viscosity binder such as polymer/Crumb rubber modified binder. Low temperature transverse cracking associated with stiffer binders is not a problem in plains of India due to higher temperatures. Bitumen of lower viscosity controls transverse surface cracking in colder regions as per international experience. In the light of the above, the approach discussed in the following is recommended for design of bituminous pavements.
IRC: 37-2012
BT
Granular base
z
Fig. I-1 Cracks at Bottom (a) and Top (b)
I-2 Rutting in Subgrade and Granular Layer A large amount of field data for rutting in flexible pavements was collected and analyzed during several research projects of MORTH ( 56,59,60) and a relationship between the computed vertical elastic subgrade strain and repetitions of standard axle loads was developed for 20 mm of rutting and this criteria was subsequently adopted in IRC-37. The bituminous layers were not very thick in India in eighties and nineties when rutting data was collected and most of the rutting took place in the subgrade and the granular layers only. The equations for rutting for 80 per cent and 90 per cent reliability respectively are given (46, 54) as N = 4.1656 x 10-08 [1/εv]^4.5337
… (I-2)
N = 1.41x 10-88 x [1/εv]^4.5337
… (I-3)
Equations I-2 and I-3 were used as the rutting criterion in IRC: 37-2012 for controlling rutting in the Subgrade and granular layers for a reliability levels of 80% and 90% respectively, and the rutting in the bituminous layer was to be taken care of by selecting binder of appropriate hardness and mix design. A more correct approach would be to estimate permanent deformation in different layers by spectrum of axle loads in different seasons adopted in mechanistic-empirical pavement design method as recommended in MEPDG (3) but this would require massive coordinated research effort in the laboratory as well as in the field in different regions of the country so that laboratory equations can be calibrated and validated from field performance. Data from other countries can not directly be used due to different types of traffic and climate. Providing large thickness of granular layer does not cause any marked reduction in the thickness of bituminous layer from fatigue considerations. The thickness of the sub-base should be sufficient (i) to stand the construction traffic (ii) to offer a stiffer support for the compaction of WMM and that of the granular base also should be thick enough so as not to cause the damage to untreated GSB. These factors were considered in developing the design charts of IRC: 37-2012. Field performance data of pavements designed as per IRC: 37-2012 is very encouraging. Delay in its implementation and continued use of softer binder has been causing unacceptable rutting in the bituminous layer rather than in the granular layer within a year or two of construction of highways (51, 53,74,75 ) because of heavy axle loads. The use of stiff binder is now re-emphasized for the bituminous mixes to limit rutting. I-3 Rutting in the Bituminous Layer
IRC: 37-2012 Thickness of bituminous layers in India for flexible pavements with granular bases can be close to 200 mm or higher for high volume roads. The rutting in the bituminous layer is to be controlled by selecting a mix using binder of appropriate viscosity considering the traffic, climate and field experience. Rut resistant mixes for high volume traffic with pavement temperatures of 60°C and higher can be obtained by using high viscosity binders. Laboratory rut tests using IITKGP RUT TESTER on mixes with VG40 bitumen, and some modified binders indicated almost equal performance at 50°C. VG30 bitumen gave much higher rutting in the rut tester, a phenomenon observed in the field also though it meets the Superpave specification requirement (5, 40) of 1 kPa for G*/sinδ at 64°C using Dynamic Shear Rheometer. Even when modified bitumen is used in the wearing course, the deep rutting had occurred in the DBM layer made with VG 30 bitumen in many projects (53) and a stiffer binder is necessary for both wearing course and DBM layer under heavy traffic conditions for the maximum pavement temperatures of 60ºC and higher, very common on plains of India. IRC: 111-2009 (29) recommends use of VG 40 bitumen in BC as well as DBM layer if the pavement carries over 2000 commercial vehicles per lane per day. Wearing courses with Stone Matrix Asphalt (IRC: SP: 79-2008) (32) and Gap Graded mix with rubberized bitumen ( IRC:SP:1072015 are known to be rut resistant. Annex-VII gives guidelines for the selection of binder. I.4 Fatigue Resistant Bituminous Layer Laboratory tests and field performance indicates (3, 22, 28, 40 and 42) that fatigue life of a bituminous layer depends to a great extent on the bitumen content for a given mix. A bituminous layer with higher modulus develops lower tensile strain by a wheel load at the bottom of the layer. To ensure that the bottom Dense Bituminous Macadam has a higher fatigue life, it should contain higher bitumen content. Softer grade bitumen such as VG 30 may give rise to an unstable mix with higher bitumen content if exposed to heavy traffic in the hot climate. For a bituminous layer thickness of 150 mm and higher, the temperature of the bottom DBM is lower than the top and there is little chance of rutting in the bottom layer if the air void is close to 2.5 to 3 percent. Higher bitumen content in the bottom layer makes the mix resistant to stripping as well and it also provides a strong barrier for entry of moisture into the upper bituminous layer (22). Computations further shows that the tensile strain in the wearing course near the edge of the tyre can be high (3, 52 and 67) and therefore, the wearing course also must be age and fatigue resistant in addition to being rut resistant. In a single layer DBM, the DBM mix designed for 3% air void content is compacted to an air void less than 4.5% so that the air void after the secondary compaction is close to 3 per cent. Mixes with Polymer and Crumb Rubber Modified binders have fatigue lives which can be three to ten times higher than the mixes with neat bitumen (40, 46 and 69) depending upon the binder content and designers can utilize this property in designing high fatigue life bituminous pavements after carrying out laboratory tests both on normal and modified bituminous mixes for the evaluation of their relative fatigue lives. 1.5 Fatigue resistant wearing course: Bituminous Concrete wearing course with neat bitumen oxidizes and becomes brittle faster due to strong sun light and high temperatures in hot climates, and heavy traffic initiates fatigue cracking in surface layer. Raveling and pothole formation follow as soon as the monsoon begins. Repair during the rains is difficult. It is, therefore, recommended that a fatigue and age resistant mix is used for wearing course to protect underlying DBM. I-6 Mix Design and Fatigue Life
IRC: 37-2012 Aggregates for acceptable for the bituminous mixes can have wide range of specific gravities ranging from 2.6 to as high as 2.95 with water absorption up to 2% in different regions of country and the binder content for a dense graded aggregates can very widely. The bitumen content of the DBM mixes used in major highways in India is expected to have an air void content from 3 to 5 per cent (64) and the mix design is carried out for an air void of 4.0 per cent. The wearing course of BC is considered to have a life of 5 years and the DBM is assumed to last for the full design period of 15 or 20 years. While the BC with neat bitumen does not last more than three years under heavy traffic in hot weather, DBM also has suffered distress when exposed to traffic for about six months due to logistic reasons. It is, therefore, necessary redesign DBM mix to have a long fatigue life. This can be achieved by increasing its bitumen content so that it does not crack extensively during the design life of the pavement. Asphalt Institute (8), Shell (62) and MEPDG (3) have given equations for determination of fatigue life of bituminous mixtures for different values of volume of bitumen (Vb) and the air voids (Va) and same basic approach was used to modify the calibrated fatigue equations to obtain high fatigue mixes. The fatigue Equations I-4 and I-5 having a reliability of 80 and 90 percent are modified to include the mix design variables such as air void and volume of bitumen as given below, Nf= 1.6064 * C * 10–04x [1/εt] 3.89* [1/MR]0.854 For 80% reliability Nf= 0.5161 * C * 10-04x [1/εt]3.89* [1/MR]0.854 For 90 % reliability C = 10M, and
(I-4) … (I-5)
𝑉𝑏
𝑀 = 4.84 (𝑉𝑎+𝑉𝑏 − 0.69)
Va= percent volume of air void and Vb=percent volume of bitumen in a given volume of bituminous mix. Nf= fatigue life, єt= maximum tensile strain at the bottom of DBM. MR= Resilient modulus of bituminous mix. Equations I-4 and I-5 give fatigue lives for 20 percent cracked area of the bituminous layer at a reliability level of 80 and 90 per cent respectively at the end of the design period. In other words, only ten percent of the area may have 20 per cent cracks if 90 per cent reliability is used for high volume highways. To avoid frequent maintenance, a higher reliability is recommended for highways having a design traffic exceeding 20 msa. Since most places on plains of India have maximum pavement temperatures approaching 60ºC or higher, VG 40 bitumen is recommended for heavy traffic. The thickness requirement is less when stiffer binder is used. Mix design and pavement design should be integrated to get an optimum design. Using the principle outlined above, a pavement can be designed so that the bottom rich bituminous layer has a long fracture life and only the wearing course is subjected to distress such as surface cracks, rutting, raveling, pot holes etc due to traffic and ageing and it would need replacement from time to time. Fig.I-1 shows (40) the results of beam fatigue tests at 4.7% and 5.2% binder contents on dense graded mixes. Optimum binder content is close to 4.7%. It can be seen that for all the mixes
IRC: 37-2012 VG30,VG40 and PMB40, increasing the binder content increases the fatigue life considerably. A PMB mix with 5.2 binder content has life which is about six times higher than the conventional VG40 mix with a binder content of 4.7%. Fig. I-2 illustrates the effect of mix design variables on fatigue life. It can be seen that for a mix modulus of 3000 MPa, if Va is decreased from 5 percent to 3 percent and Vb is increased from 10 percent to 13percent, the fatigue life at the tensile strain of 200E-04 increases from 107 to 5x107. Similarly, the fatigue life of the mix with a modulus of 1700 MPa increases from 1.7x107 to 8.0X 107 for identical change of mix variable. Mix design thus plays a very important role in design of bituminous pavements. It can also be seen that the fatigue life of the softer mix (Mr=1700MPa) is higher than that of the stiffer mix (Mr=3000MPa) but thickness also will be greater for the less stiff mix. Designers can develop better thickness combinations than what are given in different PLATES using innovative design and the life cycle cost can be optimized considering the response of constructed pavements in different climate. Higher binder content ensures durability and provides resistance to ageing. 200000 180000 160000
Binder content
Fatigue life
140000 120000 100000 5.2
80000
4.7
5.2
60000 5.2
40000 20000
4.7
5.2
4.7
4.7
0
VG30
VG40
PMB40 with SBS
Fig.I-1 Bituminous mixes with different types of binders
IRC: 37-2012
Fatigue life Fig. I-2 Effect of Air Void and Volume of Bitumen on Fatigue Life Bituminous Layer
I-8 Flexural Fatigue of Thin Wearing Course
Tensile stress
When a thin wearing course of bituminous layer of 50mm or less is provided over a granular layer, a wheel load causes compressive bending strain at the bottom of the bituminous layer in most cases . The bending compressive stress decreases with increasing thickness and becomes tensile with higher thickness of the bituminous layer as seen from Fig.I-3. Hence a fatigue analysis of thin bituminous layer over granular bases is not necessary. Mr of the supporting layer affects the value of the tensile strain in the bituminous layer.If the support is soft tensile stress in bituminous layer may be high and layer may crack if stiff binders are used
I-3 Tensile Strain in Thin Bituminous Wearing Course
A thin bituminous wearing course over a granular may suffer from punching failure if the base is not well compacted.
IRC: 37-2012 I-9 Resilient Modulus and Poisson’s Ratio of Bituminous Mixes Indicative values of Resilient Modulus(Mr) shown in Table7.1 were obtained by in house fabricated repeated four point beam bending test at IIT Kharagpur on the same principle as ASTM D 7460-10 (13) under constant strain mode at 10 Hz frequency. Comparable values of modulus were obtained by indirect tensile test method as per ASTM-D-4123 with a loading time of 0.1 seconds and rest period of 0.9 seconds. The Mr values vary greatly because of wide range of viscosity of VG-30 and VG-40, and the modulus for the pavement design should be determined in the laboratory or estimated from regression equations. These moduli were used in calibration of fatigue equation. Though ASTM D 4123 was withdrawn and replaced with ASTM D 7369-09 (12), equipment using both the ASTM methods are commercially available, former being more popular. European standard EN 1269726(2012) for the indirect tensile test similar to ASTM D 4123 is also available commercially. Poison’s ratio of a dense graded bituminous mix varies to a great deal upon load application. Under tension, it is lower and the value may exceed even 0.50 under compression indicating increase in volume. Most commonly used values are from 0.3 to 0.4. A value of 0.35 has been used in the guidelines for the computation of strains and stresses. Effect of Poisson’s ratio on stresses in pavement is presented in Annex.II. I-10 Temperature of Bituminous Layer for Pavement Design Fatigue behavior and resilient modulus of a bituminous material depend upon its temperature and its variation within it. Hourly temperature data for consecutive three years were collected from Indian Meteorological Department for Mumbai, Guwahati, Hyderabad, New Delhi, Chennai and Kharagpur and Average Annual Air Temperatures were computed (54) respectively as 27.10°C, 23.87°C, 26.16°C, 24.53°C, 27.97°C and 28.16°C. A number of correlations have been developed relating Average Monthly Pavement Temperature (AMPT) and Average Monthly Air Temperature (AMAT) (69, 19) as well as Average Annual Pavement Temperature (AAPT) and Average Annual Air Temperature (AAAT) (18). Equations given by Witczak and Brunton et al. are given respectively as AMPT (oC)= (1.05 xAMAT(oC))+ 5.0 (Witczak) AMPT (oC)= (1.15 xAMAT(oC))+ 3.17 (Brunton et al.)
… I-9 … I-10
In the absence of any coordinated study in India, the AAAT values for different locations in India as mentioned above are substituted in Equations I-9 and I-10 and Average Annual Pavement Temperature comes to about 35°C recognizing that the above equations may not be strictly valid for Indian conditions. Hence resilient modulus of bituminous layer at 35°C has been considered in the guide lines for pavement analysis. This needs updating through a well-coordinated pavement performance study in India. High summer temperatures dictate selection of stiffer binders. Similar approach can be adopted for temperature of the bituminous layer for pavement design in some snow bound regions in Himachal Pradesh, Uttarakhand, Jammu and Kashmir and Arunachal Pradesh. A more scientific approach is to recognize that there is a variation of temperature from the surface to the bottom of the bituminous layer in different seasons and this factor should be considered in
IRC: 37-2012 prediction of fatigue cracking, rutting and roughness in terms of IRI. I-11 Design charts Based on the fatigue and rutting equations, bituminous pavements can be designed for different subgrade modulus (CBR values). Design is to be optimized considering various options such as availability of aggregates, use of foamed bitumen/bitumen emulsion/ cement modified reclaimed asphalt pavements, foamed bitumen, cementitious base and cementitious sub-base. Reclaimed Asphalt Pavements (RAP) contains best quality aggregates and their use in GSB and WMM amounts to its under-utilization. The top 40 mm to 50 mm of the bituminous layer would suffer damage due to traffic and faster ageing and it would require renewal from time to time. Hot recycling can also be an alternative to conserve the materials. Since every region is unique from the consideration of traffic, climate, cost and availability of materials, engineers have to develop pavement design to suit the respective regions.In case of heavy rainfall area, proper internal drainage should be provided to increase the pavement life. A few examples of pavement designs are illustrated for CBR values of 5 per cent to 15 per cent and design traffic ranging from 5 msa to 150 msa. The plates give an approximate thickness of the bituminous layer. City streets should be designed for the minimum design traffic of 2 msa because of heavily loaded construction traffic from time to time.
IRC: 37-2012
ANNEX-II II.1 Analytical Method of Evaluation of Effective CBR of subgrade Problem: If the CBR of the embankment soil is 8% and CBR of the 500mm of the compacted borrow material over the embankment is 20%, What is the effective CBR for design of a flexible pavement? Solution E of the embankment soil
= 17.6 *8^0.64 =66.6 MPa
E of the select borrow material=17.6*20^0.64=119.7 MPa Consider a two layer elastic system consisting of 500mm of select borrow soil of modulus 119.7 MPa and the embankment soil of modulus 66.6 MPa as shown in Fig.II(I) Apply a tyre pressure of 0.56 MPa over a radius of 106.5 mm (the load is 20000 N) Pressure=0.56 MPa, radius of loaded area=106.5 mm
119.6 MPa
500m m
Mr=?
66.6 MPa
Fig II (1)
Fig II (2)
The maximum deflection (𝛿) due to a single wheel applying a pressure of 0.56 MPa over a radius of 106.5mm (Fig II (1)) as per IITPAVE is 0.963 mm The effective modulus of the single subgrade layer which gives the same deflection as the two layer is given as Mr (effective resilient modulus) = 2(1−𝜇2)p𝑎/ 𝛿 = 2(1-0.352)*0.56*106.5/0.963=108.69 MPa Where p is tyre pressure, a is radius of loaded area. Only Mr is used in stress analysis Effective CBR= 17.2 for Mr 108.69 MPa, maximum allowable CBR=15 If µ=0.5, CBR= 16.8 Hence Poison’s ratio has little effect on the effective CBR. II.2 Minimum thickness of granular subbase Minimum thickness of the subbase is necessary (i) to limit permanent deformation in the subgrade due to dumpers and other construction vehicles (ii)to provide a strong platform for the compaction of Wet Mix Macadam base and (iii) to act as a separation ( minimum compacted thickness 100mm) and a drainage( minimum compacted thickness 100mm) layers. Minimum thickness of any compacted granular layer should be at least 2.5 times the maximum nominal size of aggregates.
IRC: 37-2012 Example: An embankment soil has a CBR of 6%, and the borrow soil that will form the top 500mm of the subgrade has a CBR of 8% (minimum acceptable). Examine the adequacy of the thickness of the GSB for embankment soils for the construction traffic used for laying of WMM so that the subgrade does not suffer excessive permanent deformation. An approximate method is presented below Solution: Select a trial thickness of 150 mm for the granular subbase Effective subgrade CBR= 7 (say) and its modulus= 60 MPa Mr of the GSB=60 x 0.2 x (150)0.45 =114 MPa A three axle dumper carries at least 10 cum of granular material for delivery at site.There may be about 200 repetitions of dumpers for laying 250mm thick WMM over the compacted GSB of thickness 150mm over a single lane of length 2 km stretch. Dumpers can have an average gross weight of 320 kN. Considering that the tandem and steering axles carrying loads of 240 kN and 80 kN respectively and assuming no overlap of stresses in GSB layers due to tandem axle: VDF of dumper= 2x(120/80) 4 + (80/65)4 =12.41 Total standard axle load repetitions during the construction=200x12.41=2483 standard axles Allowable vertical subgrade strain (Eq.6.5) = 3308 x 10^-6 Computed vertical subgrade strain for 150 mm GSB as per IITPAVE=3603 x 10^-6 >3308 x 10^-6 Hence the thickness of 150 mm GSB is deficient. Increase the thickness to 200mm Mr of the GSB= 130.2 MPa Computed vertical subgrade strain=2481x 10^-6<3308 x 10^-6 Hence the minimum thickness of GSB = 200mm . If heavier dumpers are operating, computations can be done accordingly. Load equivalency factors are not strictly valid for gsb over subgrade. High load may cause more damage. Hence the above method for the minimum thickness of GSB is only a rough guide The field data about the construction traffic is necessary. If subgrade CBR is higher, thickness of granular subbase layer also will be less subject to the minimum required for filter and drainage layer particularly in high rainfall area. Higher subgrade CBR will give lower thickness of GSB. This is broadly reflected in the various plates as suggested by experienced members of the H-2 committee . WMM also needs a good support for its compaction to a higher density.
II.3 Work-out Examples Illustrating the Design Method Example - l: Design the pavement for construction of a new flexible pavement
IRC: 37-2012 Use the following data: (i) (ii)
Four lane divided carriageway Initial traffic in the year of completion of construction = 5000 CV/day (Sum of both directions) (iii) Percentage of Single, Tandem and Tridem axles are 45 percent, 45 percent and 10 percent Respectively (iv) Traffic growth rate per annum = 6.0 per cent (v) Design life = 20 years (vi) Vehicle damage factor = 5.2 (Based on axle load survey) (vii) CBR of soil below the 500 mm of the subgrade
= 3 per cent
(viii) CBR of the top 500 mm of the subgrade from borrow pits
= 10 per cent
DESIGN CALCULATIONS (i)
Lane Distribution factor = 0.75 in each direction
(ii)
Initial traffic = 2500 CVPD assuming 50 per cent in each direction.
(iii)
Vehicle Damage Factor (VDF) computed for the traffic = 5.2.
(iv)
Cumulative number of standard axles to be catered for in the design 2500×365×((1+0.06)20 −1)
N= (v)
0.06
× 0.75 × 5.2 = 131 msa
CBR of the embankment material = 3 %, CBR of 500 mm of subgrade = 10 %, Effective CBR of the subgrade
=7%
Design resilient modulus of the compacted subgrade = 17.6 (7)0.64= 62 MPa II.3.1 Bituminous pavements with untreated granular layer:
Consider design (i) with bitumen as well as modified bitumen and (ii) bituminous mixes with different Poisson’s ratio II.3.1.1 Pavement wearing course of SMA/GGRB and DBM with VG40 bitumen Trial composition: BT=190mm,WMM = 250 mm, GSB = 230 mm, total thickness of granular layer =
480mm
Resilient modulus of the granular layer = 0.2 x (480)0.45x 62 = 200 Mpa., reliability=90% Design traffic=131 msa Allowable tensile strain in BT with VG 40 bitumen with bottom DBM layer having air void of 3 per cent ( mix design for 2.5% ) with Volume of bitumen as 11.5 % =150µ€, Allowable Vertical subgrade strain=301 µ€
IRC: 37-2012
Computed tensile and vertical strains are 146µ€<150 µ€ and 244 µ€<301 µ€ respectively If DBM II is provided in the bottom with an air void of 3% ( mix design for 2.5% ) in place of DBM I, the Vb=12.5% and the Allowable tensile strain in the bituminous layer= 156 µ€ Consider a thickness of 180 mm of SMA/GGRB+DBM, tensile strain by IITPAVE= 154 µ€. A 10mm reduction in thickness of DBM I is possible if DBM II is used as the lower layer. II.3.1.2 Bituminous mixes with elastomeric PMB for pavement design PMB mixes may have a fatigue life which may range from three to ten times despite a lower resilient modulus. It is affected less by moisture and ageing . Consider the following: Resilient Modulus of the PMB mix=2000 MPa at 350C , Poisson’s ratio=0.35 The fatigue life is three times of the conventional bituminous mixes i.e. the coefficient of Eq.6.2 is multiplied by 3. Considering WMM and GSB as 250mm and 230mm and the design traffic=131 msa Allowable tensile strain in the bottom PMB layer=207 µ€ (Va=3 , Vb= 12.5 ) Consider a thickness of DBM and SMA/GGRB both with PMB= 160mm Computed tensile strain in the bituminous layer= 214 µ€ > 207 µ€ Computed vertical subgrade strain=309 µ€>301 µ€ due to lower modulus of PMB mix. Redesign is needed New thickness of BT = 170 mm and GSB=250 mm Rule: If subgrade strain is too high, increase GSB thickness, if tensile strain in the bituminous layer is too high, increase the thickness of the bituminous layer. GSB=250mm, WMM=250mm,Total thickness of granular layer = 500mm Modulus of the granular layer= 62x0.2x(500)^0.45=203 MPa Computed tensile strain in the bituminous layer= 200 µ€ < 207 µ€ Computed vertical subgrade strain=282 µ€<301 µ€ hence safe Final section: DBM+SMA/GGRB with PMB=170 mm,WMM=250mm,GSB=250mm Conventional pavement with VG40 with Vb for lower DBM=11.5%, DBM+SMA/GGRB=190mm, WMM=250mm, GSB=230mm. Thickness of PMB mix is lower by 20mm and that of GSB is higher by 20mm. If the PMB mix has higher modulus, the thickness of BT will be lower.PMB mixes may last longer due to lower ageing rate but recyclability of PMB mixes is to be established from sustainability considerations. II.3.1.3 Effect of variation in Poisson’s Ratio on thickness of bituminous layers Poisson’s Ratio of a bituminous mix can vary with temperature and higher temperatures give Poisson’s ratio approaching 0.50 Consider the above design with VG40 for traffic=131 msa For thickness of DBM+SMA/GGRB=190mm, WMM+GSB=480 mm, µ=0.35, Maximum tensile strain in the bituminous layer= 146 µ€, Maximum subgrade strain=231 µ€ If µ of the bituminous layer=0.5, the two strains are found as 137 µ€ <150 µ€ and 234 µ€<301 µ€ Hence µ=0.35 gives a more conservative design for the bituminous layer even if µ is higher Effect on subgrade strain is marginal. II.3.1.4 Effect of different Mr values of bituminous layers on pavement thickness Viscosity of VG40 bitumen varies from 3200 Poise to 4600 Poise and accordingly, the resilient modulus of the bituminous mixes also may vary over a range. Consider modulus of bituminous layer as 2500MPa, 3000MPa and 3500MPa
IRC: 37-2012 Design traffic as above =131 msa, Thickness of granular layers as above: WMM=250 mm, GSB = 230 mm If Modulus of bituminous layer is 2500 MPa Allowable tensile strain in Bituminous layer with VG 40 bitumen with bottom DBM layer having air void of 3 per cent with Volume of bitumen as 11.5 % =156µ€, Allowable Vertical subgrade strain=301 µ€ Thickness of SMA/GGRB and DBM over WMM and GSB = 200 mm at reliability of 90 per cent. Computed strains are 153 µ€< 156 µ€ and 244 µ€< 301 µ€ respectively, hence safe. . If Modulus of bituminous layer is 3000 MPa Allowable tensile strain in Bituminous layer with VG 40 bitumen with bottom DBM layer having air void of 3 per cent with Volume of bitumen as 11.5 % =150µ€, Vertical subgrade strain=301 µ€ Thickness of SMA/GGRB and DBM over WMM and GSB = 190 mm at reliability of 90 per cent. Computed strains are 146µ€<150 µ€ and 231<301 µ€ µ€ respectively, hence safe. If Modulus of bituminous layer is 3500 MPa Allowable tensile strain in Bituminous layer with VG 40 bitumen with bottom DBM layer having air void of 3 per cent with Volume of bitumen as 11.5 % =145µ€, Vertical subgrade strain=301 µ€ Thickness of BC and DBM over WMM and GSB = 180 mm at reliability of 90 per cent. Computed strains are 141 µ€<145 µ€ and 246 µ€<301 µ€ respectively, hence hence safe. It can be seen that there is variation of 5mm of thickness per 100mm of the bituminous layer for a change of 500 MPa in the Mr value. Variation of bituminous thickness with modulus for same traffic Resilient modulus of BT (Mpa) 2500 3000 3500
Thickness of BT (mm) 200 190 180
II.3.1.5 Stage Construction and Economy: In this type of construction, thickness of the WMM and GSB is provided for the full design period and the thickness of the bituminous is then determined for a shorter design period. Consider a design period of ten years for the example -1 Design traffic = 46.9 msa for ten years (131 msa for 20 years) WMM=250mm,GSB=230mm, Trial thicknress of BT=150mm, considering an air void of 3% and volume of bitumen of 11.5% Allowable tensile strain in DBM layer=195 µ€ Allowable vertical subgrade strain =377 µ€ Computed tensile strain in DBM =175 µ€< 195 µ€ safe against fatigue cracking Computed vertical subgrade strain =287 µ€< 377 µ€ safe against rutting After the design period of ten years for the stage construction, the pavement may suffer structural damage in the form of full depth cracking and overlay may not lead to a durable wearing course.
IRC: 37-2012 Rehabilitation by hot or cold recycling may be needed followed by application of wearing course after ten years. This is avoided if the design is done for the full design period. Thicker pavements do not suffer damage at the bottom. For a design period of 20 years, about 40mm of additional thickness of BT is needed. Life cycle cost may be much lower for pavements designed for 20 years rather than 10 years for the stage construction. II.3.2 Bituminous pavement with cemented base and cemented sub-base with aggregate Interlayer of 100 mm Design traffic as above 131 msa. BT = 100 mm. Aggregate interlayer= 100 mm. Cement treated base = 120 mm Cemented sub-base layer = 250 mm. Checking of the safety of cemented base due to spectrum of axle loads resulting in msa of 131. Since there are plenty of single, tandem and tridem axle loads which are far higher than standard axle load, thickness of cement layer must be checked for sudden fracture due to higher axle loads using cumulative damage principle. One tandem axle is taken as two single axles and one tridem axle is taken as three axles carrying equal load on each axle since the interference of stresses at the cemented base is negligible due to axles being about 1.30 m to 1.40 m apart from each other. All multiple axle vehicles are combination of single, tandem and tridem axles. The axle load data can be classified or grouped in such a manner that all tandem and tridem axles can be converted into single axle repetition for stress analysis.The axle load spectrum of the traffic data is as follows. Other convenient format on axle sheet can be used. If single axle load is 200 kN, the input for the wheel load is 50000 N at tyre pressure of 0.80 MPa.Tandem and tridem axle loads of 400 kN and 600kN respectively may be considered as two single axles and three single axles each carrying a load of 200 kN with a tyre pressure of 0.80 MPa. SingleAxle Loads Axle Load Class (kN)
Percentage ofAxles
TandemAxle Loads
TridemAxle Loads
Axle Load Class (kN)
Axle Load Class (kN)
Percentage of Axles
Percentage ofAxles
185-195
0.64
390-410
1.85
585-615
1.4
175-185
0.8
370-390
2.03
555-585
1.6
165-175
0.8
350-370
2.03
525-555
1.60
155-165
2.58
330-350
2.08
495-525
1.80
145-155
2.58
310-330
2.08
465-495
1.80
135-145
5.8
290-310
4.17
435-465
4.4
125-135
5.8
270-290
4.17
405-435
4.4
IRC: 37-2012 SingleAxle Loads Axle Load Class (kN)
Percentage ofAxles
TandemAxle Loads
TridemAxle Loads
Axle Load Class (kN)
Axle Load Class (kN)
Percentage of Axles
Percentage ofAxles
115-125
11.82
250-270
12.67
375-405
13.10
105-115
11.82
230-250
12.67
345-375
13.10
95-105
12.9
210-230
10.45
315-345
10.90
85-95
12.16
190-210
10.45
285-315
10.4
<85
32.3
170-190
7.05
255-285
7.15
<170
28.28
<255
28.33
100
Total
100
Total
100
Total
Cumulative fatigue damage analysis is computed as follows for Single, Tandem and Tridem Axle respectively considering flexural strength of cemented base as 1.4 MPa. Cummulative fatigue damage analysis for SingleAxle Modulus of Rupture of the cementitious base = 1.4 MPa Axle Load in kN
Expected Repitations
Stress in MPa
Stress Ratio
Fatigue Life
Fatigue life Consumed
190
72504
0.70
0.50
5.37E + 05
0.14
180
90631
0.66
0.47
1.12E + 06
0.08
170
90631
0.63
0.45
2.33E + 06
0.04
160
292283
0.59
0.42
4.85E + 06
0.06
150
292283
0.55
0.39
1.01E + 07
0.03
140
657071
0.52
0.37
2.10E + 07
0.03
130
657071
0.48
0.34
4.38E + 07
0.02
120
1339066
0.44
0.32
9.11E + 07
0.01
110
1339066
0.40
0.29
1.90E + 08
0.01
100
1461417
0.37
0.26
3.95E + 08
0.00
90
1377584
0.33
0.24
8.22E + 08
0.00
85
3659206
0.31
0.22
1.19E + 09
0.00
cum. Damage Cumulative fatigue damage analysis for Tandem Axle
0.42
IRC: 37-2012 Modulus of Rupture of the cementitious base = 1.4 MPa Axle Load in kN
Expected Repitations
Stress in MPa
Stress Ratio
Fatigue Life
Fatigue life consumed
400
419166
0.74
0.53
2.58E + 05
1.626
380
459950
0.70
0.50
5.37E + 05
0.857
360
459950
0.66
0.47
1.12E + 06
340
471279
0.63
0.45
2.33E + 06
0.202
320
471279
0.59
0.42
4.85E + 06
0.097
300
944823
0.55
0.39
1.01E + 07
0.094
280
944823
0.52
0.37
2.10E + 07
0.045
260
2870721
0.48
0.34
4.38E + 07
0.066
240
2870721
0.44
0.32
9.11E + 07
0.032
220
2367722
0.40
0.29
1.90E + 08
0.012
200
2367722
0.37
0.26
3.95E + 08
0.006
180
1597363
0.33
0.24
8.22E + 08
0.002
170
6407576
0.31
0.22
1.19E + 09
0.005
0.411
cumm. Damage
3.46
Cummulative fatigue damage analysis for TridemAxle Modulus of Rupture of the cementitious base = 1.4 MPa Axle Load in kN
Expected Repitations
Stress in MPa
Stress Ratio
Fatigue Life
Fatigue life consumed
600
105736
0.74
0.53
2.58E+05
0.410
570
120841
0.70
0.50
5.37E+05
0.225
540
120841
0.66
0.47
1.12E+06
0.108
510
135946
0.63
0.45
2.33E+06
0.058
480
135946
0.59
0.42
4.85E+06
0.028
450
332312
0.55
0.39
1.01E+07
0.033
IRC: 37-2012 420
332312
0.52
0.37
2.10E+07
0.016
390
989383
0.48
0.34
4.38E+07
0.023
360
989383
0.44
0.32
9.11E+07
330
823227
0.40
0.29
1.90E+08
0.004
300
785464
0.37
0.26
3.95E+08
0.002
270
540007
0.33
0.24
8.22E+08
0.001
255
2139635
0.31
0.22
1.19E+09
0.002
cumm. Damage
0.011
0.92
It can be seen that total fatigue damage due to single,tandem and tridem is 3.46+0.42+0.92 = 4.8 which is greater than 1. Hence the pavement is unsafe and cemented layer will crack prematurely. It can also be noticed that the Tandem axle weighing 400 kN causes maximum fatigue damage followed by Tandem axle of 380 kN. It can also be seen that stress variation is linear due to linear elastic stress analysis. If the load is halved, stress also is halved. If the heavy axle loads are not allowed, the pavement would be safe. If the cemented layer thickness is increased from 120 mm to 160 mm total Cumulative Fatigue Damage (CFD) is computed as 0.92 and for 165 mm CFD is 0.89 and hence the pavement is safe. Hence minimum thickness of CTB=160mm If the modulus of rupture is 1.20 MPa, a thickness of 220 mm is needed. Such checks are to be made on cemented layer because of heavy loading over the highways in India. Tensile strain in the bituminous layer and the vertical subgrade strain are very low because of stiff CTB and cement treated SB Check for the construction traffic: A dumper carries 10 m3 of WMM weighing about 21 ton for the paver for laying 100mm thick crack relief layer. The gross weight of the three axle dumper may about 320 kN. The tandem axle may carry a load of about 240kN with steering axle about 80kN. If the minimum 28 day flexural strength of CTB is 1.4 MPa, the 7 day strength is likely to be about 1.0 MPa (at least 70%). There will be about 70 trips of dumpers for laying two km of 100mm thick one lane wide WMM. Each axle of tandem axle will weigh about 120kN and there will be 70 repetitions of each axle of the three axle dumpers. Allowable flexural stress for about 140 repetitions( two tandem axles) of dumpers (Eq.6.7)= 0.795 MPa. The computed stress for 120mm CTB=0.917 MPa and hence unsafe Increase thickness to 160mm,CTB, the maximum flexural stress due to 12 ton single axle load =0.767MPa and hence the CTB of 160mm is safe for construction traffic after seven days of curing. If cement treated subbase is severely cracked by the construction traffic the thickness of CTB may be increased to obtain a safer pavement by considering a modulus of 400 MPa for the cement treated GSB layer. II.3.3 Bituminous Pavement with Cemented Base and Cemented Sub-base with Sami Layer over Cemented Base
IRC: 37-2012 Design traffic = 131 msa. Bituminous layer with VG 40 (BC + DBM) = 100 mm. Take Cemented base= 165 mm for carrying out cumulative fatigue damage analysis Cemented sub-base layer = 250 m. Checks must be done for cumulative fatigue damage and the construction traffic as explained in the previous example. II.3.4 Bituminous Pavement with Reclaimed Asphalt Pavement (RAP) Treated with Foamed Bitumen/Bitumen Emulsion and Cemented Sub-base Following composition is satisfies all the strain requirement Design traffic = 131 msa. BT with VG 40 = 100 mm. Consider thickness of treated aggregates RAP = 180 mm, The design is found to be safe from strain consideration. Cemented sub-base layer = 250 m. II.3.6 Bituminous Pavement with Cemented Base and Granular Sub-base with 100 mm WMM Layer over Cemented Base Design traffic = 131msa BT = 100mm WMM = 100 mm, Cemented base = 195 mm, GSB = 250 mm. Tensile Strain in CTB=64.45< 65.5. Other values are well below the allowable limits. A second level check on CTB using cumulative damage principle and safety against construction traffic have to be done. In such cases thickness of CTB may approach 250 mm or higher after cumulative fatigue damage analysis and further check for the construction traffic. II.3.7 Design of Flexible Pavement for 300 msa with (i) VG40 mix and with (ii) high modulus mix Many highways have heavy volume of traffic and the design traffic at the end of concession period can be as high as 300 msa. The pavement design and mix design can be integrated and a thicker bituminous layer may need milling of the top 40 mm to 50 mm after ten yeras and replaced with a fresh bituminous wearing course or a hot in-situ recycled mix to remove the cracked and deformed spots and restore the riding quality. Assumed Design Parameters Resilient modulus of subgrade = 77 MPa (corresponding to Effective subgrade CBR of 10 %). Bituminous layer should be bottom rich, that is, the bottom layer should have an air-void of 2.5 percent compacted to 4% air void(air void for pavement design =3%). This can be established from
IRC: 37-2012 the laboratory tests and a field trial. (i) Pavement with VG40 Resilient Modulus of Bituminous layer = 3000 MPa Consider a thickness of BT VG40 = 235mm, granular layer=350mm thick Strains in different layers for a design traffic of 300 msa, Modulus of subgrade=77 MPa ,Modulus of granular layer=210 MPa Allowable tensile strain in the bitumen rich bottom DBM I layer with air void of 3% and volume of bitumen of 11.5% for DBM I= 121 µЄ , Allowable Vertical subgrade strain= 250 µЄ Computed (i) maximum tensile strain = 113 µЄ<121 µЄ and (ii) Vertical subgrade strain=221 µЄ<250 µЄ, hence safe. (ii)
Pavement with High modulus bituminous mix It has been possible to get a minimum modulus of 5500 MPa at 350Cwith hard bitumen ( not air blown) from Haldia refinery before blending for the manufacture of VG30 and VG40. Even VG40 modified with Eva and commercially available additives has much higher modulus.
Resilient modulus of High Modulus mix = 5500 MPa Consider thickness of granular=350mm , Thickness of High modulus mix= 180 mm Allowable tensile strain HMA layer=106 µЄ,Allowable vertical subgrade strain = 250 µЄ. Computed maximum vertical subgrade strain = 242 µЄ<250 µЄ Computed tensile strain in HMA layer=107 µЄ>106 µЄ Increase thickness to 190mm Computed tensile strain in HMA layer=100 µЄ<106 µЄ Safe It can be seen that there is a 45mm reduction in thickness if high modulus bituminous mix in place of mix with VG40. For long lasting pavement, the wearing courses such as SMA or Gap graded Rubberised bitumen mix(GGRB) should be used to give a trouble free service for a minimum period of ten years. II.3.8 Long Life Pavements The pavements which have a life of 50 years or longer are termed as long life pavements also termed as perpetual pavements. If the tensile strain caused by the traffic in the bituminous layer is less than 70 micro strains ( 81 µЄ at 350C) as per tests in laboratories at 200C in US, the endurance limit of the material, the bituminous layer never cracks (Asphalt Institute, MS-4, 7th edition 2007). Similarly if vertical subgrade strain is less than 200 micro strain, rutting in subgrade will be negligible (Fig.II-1). Design of such a pavement is illustrated in the guideline.
IRC: 37-2012
0 Endurance limit=81 µЄ at 35 C
Limiting tensile strain at 350C
Endurance limit=200 µЄ
µmore or less more or less Limiting vertical subgrade strain
Fig. II-1 Limiting Design Strain for a Long Life Pavement
Design Example of Long Life Pavement Solution 1 Pavement composition: BT, GSB,subgrade Allowable subgrade strain = 200 µЄ. Allowable tensile strain in the bituminous layer = 81 µЄ (for 350C). Modulus of the Bituminous layer = 3000 MPa (VG 40 Bitumen). Elastic Modulus of subgrade = 62 MPa (corresponding to CBR of 7 %). Granular subbase = 200 mm( without WMM). Modulus of 200mm of GSB = 135 MPa Modulus of BT =3000 MPa Trial thickness of the BT with VG40 = 310 mm(say). The computed strain in the bituminous layer is 92 µЄ> 81 µЄ and the vertical subgrade strain is obtained as 225 µЄ >200 µЄ. (The analysis is done by IITPAVE software). The pavement is unsafe . Add WMM=250mm above GSB A strong support of WMM is necessary for the compaction of BT layer Combined modulus of GSB and WMM= 194 MPa The computed strain in the bituminous layer is 80 µЄ< 81 µЄ and the vertical subgrade strain is obtained as 156µЄ <200 µЄ. (The analysis is done by IITPAVE software). The pavement is safe. Further fine tuning can be done Final thickness BT=310mm.WMM=250mm,GSB=200 mm over a subgrade CBR of 7. Solution 2 Consider cement treated granular subbase of thickness 300mm, WMM layer=150mm and a trial thickness of the bituminous layer=250mm of modulus=3000MPa, Mr of WMM layer=350 MPa, the value can be computed from Eq. VIII.1 given in Annex.VIII. A conservative value of 350 MPa is considered, E of CTGSB= 600 MPA. Computed strains: Tensile strain in bituminous layer79.2 µЄ<81 µЄ, Vertical subgrade strain = 157 µЄ< 200 µЄ Solution 3
IRC: 37-2012 Use of high Modulus binder Consider a thickness of 190 mm for the high modulus mix Modulus of BT= 5500 MPa, WMM= 150mm of modulus 350 MPa, CTGSB=300mm of modulus 600 MPa Allowable tensile strain in BT=81 µЄ Maximum computed tensile strain in BT=78 µЄ<81 µЄ safe Vertical subgrade strain is 172 µЄ<< 200 µЄ .Safe
Conventional design for 150 msa Consider the same CBR =7 WMM=250mm. GSB=250mm Consider a BT of 200mm with VG40 and modulus 3000 MPa. Computed tensile strain in BT=136 µЄ<145 µЄ Vertical subgrade strain is 224 µЄ<< 292 µЄ .Safe Table below summarizes the different long life pavements with conventional pavements
Comparison of design of long life and conventional pavements subgrade CBR = 7 Solution 1 Long life pavement BT-310mm with VG40 WMM=250mm GSB=200mm
Solution 2 Long life pavement BT-250mm with VG40 WMM=150mm CTGSB=300mm
Solution 3 Long life pavement BT-190mm with hard bitumen WMM=150mm CTGSB=300mm
Conventional For a design life of 150 msa BT=200mm with VG40 WMM=250mm GSB=250mm
A long life pavement with WMM and GSB needs 310mm of BT with VG40 binder while that with Cement Treated subbase needs 250mm of BT. A thickness of 190mm is needed for the high modulus mix while 200mm of BT is needed for 150 msa which may be attained in 15 to 20 years of life or even earlier. It can be seen that solution 3 can be most economical. Practitioners of pavement design should examine optimization of pavement design discussed above for the efficient use of materials for a long life pavement. Low strength cementitious GSB permits use of marginal aggregates. II.3.9 Additional thickness of bottom DBM Considering the maximum undulation of 10mm permitted in WMM, the thickness of the bottom DBM may be increased by 10mm to ensure that there is no deficiency of the DBM thickness anywhere.
IRC: 37-2012
ANNEX-III Equivalence of Thickness of Bituminous Mixes of Different Moduli Two elastic layers are approximately of equal flexural stiffness if their moduli, Poisson’s ratios and thickness are related as 𝐸1 𝐻13 𝐸2 𝐻23 = 12(1 − 𝜇12 ) 12(1 − 𝜇22 ) Where E1, H1, µ1 and E2, H2, µ2 are the Elastic Modulus, Thickness and Poisson’s Ratio of the two mixes. Similar layers can be combined into less number of layers for computation if necessary. Example Convert 180 mm of DBM with VG 30 with modulus 2000 MPa into that with VG 40 with modulus 3000 MPa 180 mm DBM with VG 30 180 x (2000/3000)0.333= 157 mm DBM with VG 40 bitumen Though bending behavior may be comparable from the above equivalence, rutting in the bituminous layer will be reduced with stiffer bitumen. Sometimes, several layers may have to be combined in one layer for analysis since too many layers cause problems in elastic layered analysis. This can be done as follows If there are three similar layers of thicknesses H1, H2, H3 having the moduli E1, E2, E3 with same Poisson’s ratios, the modulus (Eq) of a single layer of thickness H1+ H2 + H3 is given as
𝑬𝟏 𝑯𝟑𝟏 + 𝑬𝟐 𝑯𝟑𝟐 + 𝑬𝟑 𝑯𝟐𝟑 𝟎.𝟑𝟑𝟑 𝑬𝒒 = ( ) 𝑯𝟏 + 𝑯 𝟐 + 𝑯𝟑 The above concept may be needed if several layers of similar materials are to be analysed. IITPAVE considers only five layers
ANNEX-IV Selection of Subgrade CBR for Pavement Design Specimen preparation for the CBR test shall be done as per BIS 2720-8 and BIS 2720-16. For design traffic less than 5 msa, standard compaction as per BIS 2720-7 shall be used
IRC: 37-2012 Sample Preparation 1. The test specimens should be prepared by static compaction to obtain the desired field or laboratory density at the required moisture content for the CBR test. This method of compaction requires considerable pressure and there is a possibility of the actual density varying with depth though the mean density may be the one desired. Dynamic method gives uniform density across the depth. Indian standards mentioned above give detailed procedures for determination of optimum moisture content, dry density and CBR of materials Selection of Subgrade CBR for Pavement Design The CBR values of the subgrade soil varies along a highway alignment even on a homogeneous section. 90 percentile CBR is recommended in the guidelines. Method of determination of the 90th percentile is given below. The following example illustrates the procedure for finding the Design. 16 CBR values for a highway alignment are as follows: 3.5, 5.2, 8.0, 6.8, 8.8, 4.2, 6.4, 4.6, 9.0, 5.7, 8.4, 8.2, 7.3, 8.6, 8.9, 7.6 Arrange the above 16 values in ascending order 3.5, 4.2, 4.6, 5.2, 5.7, 6.4, 6.8, 7.3, 7.6, 8.0, 8.2, 8.4, 8.6, 8.8, 8.9, 9.0 Now calculate the percentage greater than equal to each of the values as follows: For CBR of 3.5, percentage of values greater than equal to 3.5 = (16/16) * 100 = 100 For CBR of 4.2, percentage of values greater than equal to 4.2= (15/16)*100=93.75and so on. Now a plot is made between percentages of values greater than equal and the CBR values versus the CBR as follows.
IRC: 37-2012 Fig. IV.1 Evaluation of Subgrade CBR for Pavement Design th
th
The 90 percentile CBR value = 4.7, and 80 percentile CBR = 5.7 in. Asphalt Institute of USA (6) recommends 87.5 percentile subgrade modulus for design traffic greater than one msa. NOTE: If the data is very large, the CBR values can be grouped for homogenous sections and the Same procedure followed. Special Points Relating to Design of Pavement on Expansive Soils Potentially expansive soils, such as, black cotton soils are montmorillonite clays and are characterized by their extreme hardness and deep cracks when dry and with a tendency for heaving during the process of wetting. Road beds made up of such soils when subjected to changes in moisture content due to seasonal wetting and drying or due to any other reason undergo volumetric changes leading to pavement distortion, cracking and general unevenness. In semi-arid climatic conditions, pronounced short wet and long dry conditions occur, which aggravate the problem of swelling and shrinkage. Due recognition of these problems at the design stage itself is required so that counter measures could be devised and incorporated in the pavement structure. A proper design incooperating the following measures may considerably minimize the problems associated with expansive soils. The amount of volume change that occurs when an expansive soil road bed is exposed to additional moisture depends on the following: (a)
The dry density of the compacted soil.
(b)
The moisture content.
(c)
Structure of soil and method of compaction.
Expansive soils swell very little when compacted at low densities and high moisture but swell greatly when compacted at high densities and low moisture. Hence, where the probability of moisture variation in the subgrade is high, it is expedient to compact the soil slightly wet of the field optimum moisture content (+2 per cent) determined on the basis of a field trial. Experience shows that generally, it is not practicable to compact expansive soils at OMC determined by Laboratory Proctor Test. It is, therefore, necessary to study its field moisture density relationship through compacting the soil at different moisture contents and under the same number of roller passes. A minimum density corresponding to 95 percent of the standard proctor density should be attained in the field and recommended moisture content should be. 1-2 per cent wet of optimum moisture content. Buffer Layer There is a definite gain in placing the pavement on a non-expansive impermeable soil cushion of 0.61.0 m thickness. It prevents ingress of water in the underlying expansive soil layer, counteracts swelling and secondly even if the underlying expansive soil heaves, the movement will be more uniform and consequently more tolerable. However, where provision of non-expansive buffer layer is not economically feasible, a blanket course of suitable impermeable material and thickness as discussed below must be provided.
IRC: 37-2012 Flyash- Soil blend Plenty of Flyash is available in many locations and it can be blended with soil to obtain 9i) lower plasticity index (ii) lower swelling and shrinkage ands (iii) higher CBR. Such a method was adopted in some projects in Maharashtra Blanket Course An impermeable blanket course of at least 225 mm thickness and composed of coarse/ medium sand or non-plastic moorum having PI less than five should be provided on the expansive soil subgrade as a blanklet course to serve as an effective intrusion barrier. It must have very low permeability. The blanket course should extend over the entire formation width. Spray of Bitumen emulsion Spray of bitumen emulsion over the compacted clay layers prevents entry of moisture into the clayee soil and thereby, swelling, shrinkage and loss of strength is arrested. Alternatively, lime-stabilized black cotton sub-base extending over the entire formation width may be provided together with measures for efficient drainage of the pavement section.
IRC: 37-2012
ANNEXURE-V Drainage Layer A good drainage can significantly reduce the magnitude of damage to pavements. Special attention should, therefore, be given to provision of good drainage measures. The desirable requirements are: a. Provision must be made for the lateral drainage of the pavement. The Granular sub-base/base should accordingly be extended across the full width of shoulders b. No standing water should be allowed on either side of the road shoulders. c. In case of existing road having no history of being over topped,a minimum height of 1 m between the top of the subgrade level and the highest water level should be ensured. Example of Design of a Drainage Layer Design a granular drainage layer for a four lane heavy duty divided highway for an annual precipitation of 1200 mm. Longitudinal slope = 3 per cent, Camber = 2.5 per cent. Crack Infiltration Method Water travels along AD (Fig.V-1) due to camber and the longitudinal slope
cracks)=12m for flexible pavement
Depth of drainage layer = 450 mm (Assuming BT= 200 mm and WMM = 250 mm) Width of the drainage layer (one side of the median only) =8.5m+1.0m+2x0.45=10.4m with 1.0 m wide Unpaved shoulder Fig. V-1, AB = 10.4 m, AC = 10.4 x 0.03/0.025 = 12.48 m.
Fig. V-1 Direction of Flow of Water
AD = (10.42+ 12.482)0.5= 16.24 m Elevation drop along AC = 12 .48 x 0.03 = 0.374 m Elevation drop along CD = 10.4 x 0.025 = 0.26 m Total drop along AD = 0.634 m. Hydraulic gradient = (Elevation drop)/length = I = .634/16.24 =0.039 qI= IC [Nc/Wp+ WC/ (WpCs)],Ic=0.223 m3/day/m, Nc(center line and two edges)=3,Wp(paved width + shoulder)=10.4 m, Wc (crack width)=Wp,Cs (assumed intervals of transverse
IRC: 37-2012 qi= 0.083 m3/day/m2. Amount of water infiltration in pavement per meter width along the flow path AD = 0.0.083 x 1 x 16.24 = 1.35 m3/day= Q Gradient = 0.039 Q = KAI, KA=1.35/0.039=34.62, Where, A = Area of Cross Section = Thickness of drainage Layer, m x 1.0 m, K = coefficient of Permeability, m/day, If depth of drainage layer is 100 mm A=1x0.1=0.1m2, K=34.62/0.1 =346.62 m/day If the thickness of drainage layer is doubled, the permeability requirement is half. The above approach can be adopted if there is a fair chance of water infiltrating through the surface, joints and edges of the bituminous layer. Major highways with paved shoulders have thick bituminous layer and there is little possibility of water entering into the lower layers. Table V-1 Permeability of Untreated GSB as per MORTH(13) and AASHTO (1993)
% passing,Sieve size,mm
Grading I of MORTH
Grading II of MORTH
Grading III of MORTH
75 100 100 53 80-100 100 55-75 26.5 55-90 70-100 12.5 9.5 35-65 50-80 10-30 4.75 25-55 40-55 2.36 20-40 30-50 0.85 10-15 10-15 0.425 <5 0.075 <5 <5 Co eff. of permeability 100* 96* 588* (m/day) * reference no 80, **finer than 25mm sieve
Grading IV of MORTH
100 50-80 15-35
Grading V MORTH
Grading VI
100
100 75-100 -
95-100 25-60**
55-75 30-55 10-25 0-8 0-3
0-10 0-5 -
107*
614*
80-100
55-90 35-65 25-50 10-20 2-10
0-5 <5 329*
-
100*
AASHTO57
MORTH
Rain water may however enter through the pervious median if not designed properly and infiltrating
IRC: 37-2012 water may carry fines which may choke the GSB.A nonwoven synthetic geotextile may be provided at the level of GSB to arrest all the fines. Table V-1 gives the permeability of the six gradations of the GSB of MORTH and the AASHTO 57 gradation for comparison. It is seen that all the MORTH gradations have the permeability close to about 100m/day or higher. They can be used for most of the heavy duty pavements in which only limited amount water can enter due to thick bituminous surfacing up to shoulder. The gradations III and IV have very high permeability and can be used where drainage requirement for a pavement is large. Permeability values of grading III and AASHTO 57 are comparable but they may be unstable under construction traffic and they may be treated with 2% cement or 3% bitumen emulsion for stability during the pavement construction. Alternatively. SS2 bitumen emulsion diluted 50:50 with water can be sprayed on the surface at the rate of 0.2 kg/m2 of residual bitumen followed by light application of stone dust to ensure stability under the construction traffic.
Annexure-VI Recommendation for Bituminous Wearing Courses for Flexible Pavements VI-1: India is a large country with vastly different climate in different areas, and many regional agencies have developed specifications to suite the local conditions. Pavement temperatures in plains are generally high and the bituminous wearing course ages faster and becomes brittle. In high rainfall area, the wearing course may suffer moisture damage leading to raveling, pothole formation, and the moisture may infiltrate into the lower layers. Hence a wearing course must be resistant to ageing, moisture damage and rutting at high temperatures. VI-2: The wearing course of premix carpet (PMC) and seal coat has given mixed performance. Built up area has generally poor drainage leading to early damage of PMC under adverse moisture condition and heavy traffic while it has performed reasonably well in well drained residential areas and rural roads., PMC may last for about five to six years on low volume roads with good coating of aggregates with bitumen and proper sealing of voids with sand- bitumen. Deficiency of binder leads to early failure. Applying seal coat is a manual operation and has to be done very carefully by experienced personnel for proper sealing. Mixed Seal surfacing also has performed well on low volume roads in high rainfall area of West Bengal when appropriate amount of binder was added in the aggregates.. Local experience is the best guide. VI-3: Two coat surface dressing (SD) has been used with success in South Africa (24) with traffic up to 5 msa even in wet region. The bitumen film in SD is thick and makes the pavement completely impervious to ingress of water. Thick bitumen film resists ageing for a longer period and does not undergo fatigue cracking. Even for high volume roads single coat surface dressing can rejuvenate the pavements, restore skid resistance and improve the riding quality. In wet area such as North-East where clear days are fewer, bitumen emulsion may be convenient to use since the aggregates need not be dry and even windy conditions will help in evaporation of water from the emulsion VI-4. The UK practice for wearing course for heavy traffic has been to use Hot Rolled Asphalt (RA) containing gap graded aggregates with 30/45 penetration bitumen having binder content of about 7%.
IRC: 37-2012 The surface is made skid resistance by applying coated chipping and rolling while the mix is still hot. High binder content gives a durable mix with lower rate of ageing and a stiff binder prevents rutting. VI-5. It is recommended that the SDBC and BC wearing courses with unmodified bitumen should be limited to a design traffic of 10 msa. SDBC has a higher voids in mineral aggregates (VMA) and needs greater amount of binder for 4% air void rather than 4.5% used in practice.. Lime should be used as a filler since it acts as an antioxidant and prevents moisture damage apart from stiffening the mix. Low volume of traffic may not cause large scale damage even after the wearing course has become somewhat oxidised and the expected life can be a minimum of five years for a properly designed mix. It should be ensured that a minimum of two hours of curing at the mixing temperature should be done to allow for full absorption of binder into the aggregates before the evaluation of the compacted density and theoretical density for air void, VMA and Vfb computation. VI-6. For design traffic between 10 msa and 20 msa, BC with with PMB-40/CRMB-60 should be preferred to avoid frequent pothole repair, raveling and patching. For traffic exceeding 20 msa, Gap graded(GG) mixes such as Stone Marix Asphalt (SMA) with PMB 40 or a gap graded mix with rubberised bitumen(GGRB) containing at least 20% crumb rubber are recommended. Both SMA ( IRC:SP:79-2008) and GGRB (IRC:SP-107-2015) are very popular on heavily trafficked roads in Europe and in some states of USA. Key to a durable wearing course is to use a stiff binder in greater amount leading to thicker films around the aggregates. Crumb rubber modified binder has carbon black which resist ageing. But it should be ensured that CRMB has uniform distribution of the crumb rubber in entire volume without any settlement. PMB mixes also oxidises slowly as compared to neat bitumen. VI-7: Bituminous wearing layers over Water Bound Macadam (WBM) base have performed well in South Africa. WBM bases consist of coarse single-sized aggregates together with fines added during the construction to fill the voids. Such bases are recommended by South Africa (68) for pavements carrying heavy traffic under wet condition. Tests in South Africa by Heavy Vehicle Simulator (50) found that WBM resists deformation better than the graded crushed stone base courses even under wet condition because of better interlocking of larger aggregate size of WBM. The work is labour intensive, but it can be one of the options in areas where there is serious problem with construction equipment. VI-8 Mastic Asphalt (MA) wearing course 25mm thick is very popular in West Bengal and in many places in India because of its durability even under heavy rainfall during the monsoon. MA is very hard and it does not suffer moisture damage because of its voidless mass. But laying process is slow and the aggregates embedded over the surface for providing skid resistance generate noise in the beginning and roughness is high initially. After a hot summer, the aggregates get embedded into the MA making the surface glassy with poor skid resistance. But its durability is unsurpassable. The skid resistance can be restored by surface dressing with smaller size aggregates with reduced noise. It can be considered as a part of the structural layer since bonding with lower bituminous layer is ensured by tack coat.
IRC: 37-2012 ANNEX-VII Binders and bituminous mixes VII-1 Binder must be stiff enough so that bituminous mixes do not undergo unacceptable permanent deformation along the wheel path in the prevailing climate. It should not become brittle at low temperatures. Hence its selection should be performance based. Volume of Commercial traffic and temperature should determine the selection of a binder in a given region. IRC: 111-2009 (29) recommends VG-40 bitumen if commercial vehicle exceeds 2000 per lane per day for plain terrain. VII-2 Three types of bitumen namely VG-10,VG-30 and VG-40 are being produced in Indian refineries though VG 20 also is included in the BIS specifications. VG10 is used for low volume roads and VG30 and VG40 are being used for highway and other roads. When the binders were examined as per the performance grading (PG) scale of USA, the average high temperature grading for VG 30 and VG40 corresponds to PG 64 and PG70 respectively. There is considerable variability in the results since viscosity range (VR) for bitumen is quite large. The VR of VG40 is from 3200 to 4800 poise while that of VG30 is 2400 to 3600 poise measured at 600C, 3200 to 3600 range being the overlap . If temperature alone is the consideration, VG30 and VG40 become the choice for pavement temperatures up to 640C and 700C respectively. It may be mentioned that viscosity grading was found to be deficient in USA and it was replaced with PG grading about twenty years back while UK and Europe continue to use penetration grading in conjunction with softening point. VII-3: Summer temperatures are much lower in UK and European countries, average air temperature being 190C in UK in the hottest month while hottest month’s air temperatures exceed 400C in India in many regions. For heavy traffic, the British practice (23) is to use mix with stiffer bitumen of penetration 35 to 45 to obtain high strength bases which allows 10-15 percent reduction in thickness for equivalent performance or an extended life for the same thickness of softer binder. Much harder grade of bitumen (15, 25 and 35 penetration) is used in bases for very heavily trafficked highways in UK and other European countries in spite of very low temperatures. VII-4: SUPERPAVE Mix design recommends a binder based on temperature only if the traffic is less or equal to 10 msa. For traffic greater than 10 msa and less than 30, it recommends a binder which is one grade higher than that needed for 10 msa. If the traffic is greater than 30 msa, selection of binder which is of one grade higher is strongly advised. As per Superpave guidelines. If VG30 bitumen is to be selected for pavement temperature up to up to 640 C for traffic less or equal to 10 msa . VG40 should be preferred to VG30 for traffic from 10 msa to 30 msa and it must be used for traffic greater than 30 msa. For areas on intersection and traffic signals, binder grade is bumped by two grades (say PG76) which imply use of modified binder of grade PMB40. VII-5 AASHTO-TP70-13 has brought out a new criterion in 2013 for the selection of binders based on multiple stress creep and recovery test for assessing rutting potential of bituminous binders. The test evaluates the recoverable and permanent deformations in binders under repeated shear load at the temperature of interest (maximum pavement temperature). Based on the field trials, Federal Highway
IRC: 37-2012 Administration of USA has given the following recommendations for the selection of binders
Table VII-1: Binder selection criteria as per AASHTO TP70-13 Binder classification Standard Grade (Based on temperature only) Heavy Grade Very Heavy Grade Extreme Grade
Traffic based on 20 years projection < 3 msa > 3 msa > 10 msa > 30 msa
Almost all our major roads fall under extreme grade. VII-6: Pavement temperatures for the Selection of Grade of Binder in different regions Selection of binder and mix design are very important for a good performing bituminous layer. Harder binders are both moisture and rut resistant. Both temperature and traffic determine the binder grade. Maximum pavement temperature at a location can be determined (5) from the following widely acceptable equation (Asphalt Institute, 2001) T20mm = [(Tair - .00618Lat2 + 0.2289Lat + 42.2) * (0.9545)] – 17.78… Where T20mm= Pavement temperature at a depth of 20mm Lat = Latitude of the place Tair = Average of maximum temperatures of seven days
VII-1
The above equation was found to be closer to pavement temperatures observed at Kharagpur. For maximum air temperatures of 380C and 470C, the corresponding pavement temperatures are 610C and 690C respectively. For traffic exceeding 20 msa, the maximum air temperature should be determined at 98% reliability given as Maximum air temperature= Mean of maximum 7 day temperatures + 2 standard deviation. The selection of the binder shall be done on the basis of design traffic and regional temperatures. The design traffic for the selection of binder shall be computed on the basis of 15/20 years of traffic projection. VII-7 Selection of Binders Considering the practice around the world and the Indian experience, the recommendation for the selection of a binder is given in Table VII-2.
IRC: 37-2012
Table VII-2 Selection of Binder for Bituminous Mixes
Traffic
Design traffic ≤ 10 msa
Design traffic > 10 msa but ≤ 20 msa
Binder type for plains in India
Select VG30 for maximum pavement temperature less than 640C , VG30/VG40 for pavement temperature less than 640C . VG40 may be preferred
Design traffic > 20 msa
VG40 shall be used
Traffic intersection
Polymer modified binder/Mastic asphalt
Binder for other locations and snow bound mountainous region VG 10 is recommended for traffic up to 20 msa when the maximum pavement temperature is expected to be <300C. VG30 binder is recommended. Local experience shall prevail Polymer modified binder/Mastic asphalt
Remarks
For snow bound and other areas with large difference between maximum and minimum pavement temperatures , modified binders to stand both high and low temperatures are recommended .
VII-8: Evaluation of bituminous Mix parameters: Air voids (Va),Volume of bitumen (Vb), Voids in mineral aggregates (VMA), theoretical density (Gmm), Bulk Density (Gmb) and Resilient Modulus of Bituminous Mixes are the mix parameters that must be evaluated for all the bituminous mixes for pavement design and construction.For aggregates with water absorption less than 2%, the aggregates and bitumen should be mixed at the mixing temperature of the bitumen and the mixture shall be kept in the oven at the mixing temperature for about two hours in order that the bitumen is absorbed fully into the aggregate pores (MS2). Gmm should be determined on the cured mixes and Gmb shall be done on the compacted mixes. The curing is done to simulate absorption of bitumen during the mixing, storage in silo, transportation, laying and compaction. The mix remains hot for about two hours. Laboratory curing for two hours is essential to ensure that effective bitumen content and air voids are computed correctly and there is no deficiency of bitumen in the mix. For Resilient Modulus test the mixed sample shall be cured for four hours at 1350C and brought to the compaction temperature before compaction by a Marshall hammer to prepare test specimens. This method gives modulus after the pavement has undergone some initial ageing for a few months.It is strongly recommended that 2% lime should be added in the bituminous mix (i) eliminate moisture damage (ii) slow down ageing process and (iii) stiffen the mix further. Lime is also a safeguard against any clay contamination in the aggregates. VII-9 If the DBM laid in a single layer, it shall be designed for 3% air void and compacted to air void of 4.5% or less. For two layer DBM, the bottom layer shall be designed for 2.5% air void and
IRC: 37-2012 compacted to an air void of 4% or less. Such mixes may last till the design period due to higher resistance to fatigue and moisture damage. DBM-I contains larger size aggregates and it should be ensured that there is no segregation by taking appropriate measures. Higher size aggregates in DBM display strong interlocking and have a better load spreading behavior due to higher modulus. ANNEX-VIII Resilient Modulus of Granular Materials The guide lines for the resilient modulus of granular materials over subgrade recommend use of widely used Shell’s equation given by Equation 7.1, which gives a conservative value. There is now an increasing trend of using a more scientific approach for determination of resilient modulus, Mrgrn, by triaxial test since it is dependent upon the sum of the principal stresses as well as the octahedral shear stress as given below (14). 𝜃
𝑀𝑟𝑔𝑟𝑎𝑛 = 𝐾1 ( )𝐾2 × [ 𝜌
𝜏𝑜𝑐𝑡 𝐾 ] 3 𝜌
𝜏𝑜𝑐𝑡 = 𝑂𝑐𝑡𝑎ℎ𝑒𝑑𝑟𝑎𝑙 𝑠𝑡𝑟𝑒𝑠𝑠𝑒𝑠 =
…… 1 √3
VIII-1
√(𝜎2 − 𝜎3 )2 + (𝜎3 − 𝜎1 )2 + (𝜎1 − 𝜎2 )2
𝜃 = 𝑠𝑢𝑚 𝑜𝑓 𝑡ℎ𝑒 𝑝𝑟𝑖𝑛𝑐𝑖𝑝𝑎𝑙 𝑠𝑡𝑟𝑒𝑠𝑠𝑒𝑠
𝜎1 , 𝜎2 𝑎𝑛𝑑 𝜎3
𝑎𝑟𝑒 𝑡ℎ𝑒 𝑝𝑟𝑖𝑐𝑖𝑝𝑎𝑙 𝑠𝑡𝑟𝑒𝑠𝑠𝑒𝑠 𝑖𝑛 𝑡ℎ𝑒 𝑔𝑟𝑎𝑛𝑢𝑙𝑎𝑟 𝑙𝑎𝑦𝑒𝑟 𝑑𝑢𝑒 𝑡𝑜 𝑡ℎ𝑒 𝑡𝑟𝑎𝑓𝑓𝑖𝑐 𝑙𝑜𝑎𝑑 𝑎𝑛𝑑 𝑜𝑣𝑒𝑟 𝑏𝑢𝑟𝑑𝑒𝑛 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒
𝜌 =atmospheric pressure=100kPa, K1,K2 and K3 are experimental test constant. The constants of EquationVIII-1 are to be evaluated from repeated triaxial test as per the AASHTO –T307-99.The default values of the constants k1,k2and k3 may be taken as 220,1.05and -0.2 respectively (16) for crushed rocks meeting the gradation of Wet Mix Macadam as per MORTH specifications. A typical modulus value of 450MPa over a strong cemented base may be taken for design (67). A strong base leads to better packing of aggregates and confinement between two strong layers leads to higher modulus than given by Shell’s equation Eq 7.1 When WMM is laid over a cementitious subbase, its resilient modulus(MR) will not be high due to lower strength of support and the MR can be evaluated by using Eq.VIII-1. An example is given in cl. 10.6 to illustrate the use of the Eq.VIII-.1A modulus value of 350 MPa is appropriate in most cases..
104
ANNEX-IX Reclaimed Asphalt Pavement and Mix Design Recycling of Reclaimed Asphalt Pavements (RAP) must be used for technical, economical, and environmental reasons. Use of RAP has been favored all over the world over virgin materials in the light of the increasing cost of bitumen, the scarcity of quality aggregates, and the pressing need to preserve the environment. The use of RAP also decreases the amount of waste produced and help store solve the disposal problems of highway construction materials. Reclaimed Asphalt Pavements contain best quality aggregates and they can be effectively improved with foamed asphalt/bitumen emulsion along with/without fresh aggregates and crusher dust to impart necessary strength for durable pavements. If only the surface layer is weathered or damaged, hot recycling can be an attractive proposition. Several recycling techniques, such as hot mix plant recycling, hot in-place recycling, cold mix plant recycling, cold in-place recycling, and full depth reclamation, have evolved over the past 35 years. In-place recycling not only reduces the use of new materials but also reduces emissions, traffic, and energy associated with the transport and production of these materials. Hot Mix Recycling is the most common method of recycling asphalt pavements in developed countries. It involves combining RAP with new or “virgin” aggregate, new asphalt binder, and recycling agents in a central hot mix plant to produce a recycled mix. The amount of RAP allowed in a recycled hot bituminous mix as per different guidelines varies from agencies to agencies. Some routinely allow 15 per cent or less RAP while others permit larger amounts of RAP . Higher RAP concentrations require adjustments in mix design and binder selection. 20% RAP in DBM mixes has successfully has been used in India different projects. International guidelines such as those given by the Asphalt Institute may be used for design of bituminous mixes with RAP. For hot mix design using RAP or otherwise also moisture sensitivity test needing vacuum saturation as per AASHTO-T283 (MORTH) must be carried out to ensure a durable mix. A minimum of six test specimens are compacted to an air void of 7±0.5 %.Three samples are conditioned by vacuum saturation. A minimum tensile strength ratio of 80% for the conditioned and unconditioned specimens. Research by all concern is needed to develop technology to use 100% RAP in pavement construction. Cold Mix Plant Recycling is a method of recycling where RAP and emulsified bitumen or foamed bitumen are mixed cold in a centrally located cold mix plant. Many old road alignment having thick bituminous layers are being abandoned in four and six laning projects and the entire RAP can be salvaged by milling machine and reused in new construction. Even cement treated aggregates have been milled and reused in USA, South Africa and China. Since the components of a cold mix plant are fairly portable, it can be assembled in satellite locations close to a project site. Cold recycled mix is hauled to the job site with conventional dump trucks or belly dump trucks. Placement and compaction of cold recycled mixes are done with the same conventional pavers and rollers used for hot mix asphalt construction. Cold recycled mixes are normally overlaid with hot mix asphalt or surface dressing (chip seal) depending on the anticipated traffic level for the finished pavement.
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Cold in place recycling (CIPR): It involves (21) rehabilitation of the existing asphalt or granular road surface. The existing surface is pulverized and the material is mixed on the site with foamed bitumen or bitumen emulsion. The process of in-situ recycling of distressed pavement using cold- mix technology is referred to as cold in-place recycling (CIPR). CIPR thus is a pavement rehabilitation measure that typically consists of the following operations, Often all are carried out in one-pass of a recycling machine and a badly distressed pavement is transformed into a stronger good looking pavement 1. 2.
Milling the existing pavement layers up to a depth of 300 mm; Treatment with bitumen emulsion or foamed bitumen, often in combination with addition of crusher dust, fresh aggregates if required, and a small percentage of active filler such as cement; 3. Adding compaction water; and 4.
Repaving the mix.
5.
Compaction with a pad foot roller when the compacted thickness exceeds 150mm
In a CIPR process as described above, the top bituminous layer (Reclaimed asphalt pavement) as well as a part or whole of the granular or stabilized base layer are recycled. The residual binder content added to the mineral aggregates in the process of CIPR is generally lower (<4 per cent) in comparison to hot bituminous mixtures. The recycled product is not used as final surfacing layer but used as base or sub-base layer. CIPR is an attractive alternative for highway rehabilitation operations because of its economic and environmental advantages. Major economic advantages involve the recycling of existing road surface aggregates and reduced haul requirements for incorporating new aggregates. In India, there are many regions where aggregates resources are limited or will be depleted in the near future. Aggregate haul in these regions is quite expensive. In addition, impacts on adjacent haul roads are minimized or eliminated because of reduced new aggregate requirements. A major environmental advantage involved in the use of cold in-place recycling is that there is no requirement for heat during construction work. CIPR is an energy efficient process that does not produce harmful emissions and does not require the bituminous mixtures to be transported to an off-site plant. In addition, transportation of large amounts of aggregate are reduced and hence it is fuel efficient also. A number of roads rehabilitated with CIPR have been performing well. Design for Bitumen Emulsion RAP Mixes Gradations of Aggregates The aggregates from RAP may not have the required gradation for a good mix. RAP alone has poor internal friction and its CBR may be as low as 30 though a close graded crushed aggregates may have a CBR as high as 200. Addition of crusher dust containing particle size from 6 mm to 0.075 mm and fines passing 0.075 mm adds to angle of internal friction as well as some cohesion to the RAP mixes. The crusher dust requirement can be 15 to 30 percent and 1 per cent cement or lime or both by weight of dry aggregates helps in dispersion of the bitumen emulsion in the mix. Lime modifies the clay that may have contaminated the RAP. The RAP may need re-crushing if they have lumped up during storage. If milled aggregates are from those of Bituminous Macadam, it may be open graded and some additional fresh aggregates may be necessary for the adjustment of gradation. The grading
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of the blend of RAP/fresh aggregates and crusher dust should meet the requirement shown in TableIX-1 adopted from the South African Standard ‘TG2 (64) CSIR Built Environment, Pretoria. The grading has been slightly adjusted to correspond to the sieve size designation in MORTH. Table IX-1 Gradation of RAP Mixes
Sieve size, mm
per cent passing
45
100
37.5
87-100
26.6
77-100
19
66-99
13.2
67-87
4.74
33-50
2.36
25-47
0.60
12-27
0.3
8-21
0.075
2-9
Some RAP may be contaminated with clay which might have risen from the subgrade during the wet weather. Addition of 2 percent lime would modify the clay and the mix becomes suitable for use. Use of Bitumen Emulsion for recycling Since the blend of RAP and crusher dust consists of plenty of fine particles, only slow setting emulsion (SS2) with a minimum residual bitumen content of 60 per cent is recommended to prevent the emulsion from breaking during the mixing and construction. Residual bitumen content(RBC) should be determined for each lot of the emulsion so that the RBC is same all the RAP mixes. Determination of Optimum Fluid Content Mix design of cold mix can be done using published standards such as Manual 14 ‘The design and Use of Granular Emulsion Mixes’ Published by South African Bitumen and Tar association (SABITA) (7) and TG-2 of South Africa (64), Asphalt cold Mix Manual (MS-14)Published by Asphalt Institute, USA and IRC: IRC SP 100 - 2014 – ‘Use of Cold Mix Technology in Construction and Maintenance of Roads Using Bitumen Emulsion’. A RAP bitumen emulsion mix has to be compacted to a maximum dry density only at optimum fluid content. Marshall method of compaction is done at different fluid content to arrive at the optimum fluid content. A simple method as per Sourh African standard ( Manual 14,Sabita) is in the following. Step 1 Prepare a 50:50 blend of bitumen emulsion and water by volume. Water is to be added to bitumen emulsion and not the emulsion to water for dilution to prevent breaking. Compatibility for
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dilution may be checked. Step2 Actual water content of the blend of RAP, crusher dust and filler should be determined by oven drying and fluid content starting with 4% by weight of the blend may be aqdded to the dry RAP mix increment of 1 per cent by weight of the blend may be added and thoroughly mixed. The mix is transferred to a standard 100 mm diameter Marshall mould and compacted by 75 blows on each face at ambient temperature. A minimum of three samples are cast at the fluid content.More samples are cast at a fluid increment of 1%. Step 3 Dry density should be computed at each fluid content as per the following Ddd= Dbulk 1+FC
… IX1
Where, Ddd = Dry density in kg/m3, Dbulk= Bulk density in kg/m2, FC = Fluid content by dry weight of aggregates in decimal. A plot is made for dry density vs fluid content as shown below and maximum dry density and the corresponding optimum fluid content is determined. Optimum fluid content is necessary for the compaction of the RAP mixes to the maximum density.
Fig. IX-1: Dry Density-Fluid Content Relation for Blended Rap Mix
Step 4 Marshall Samples are prepared at different emulsion content starting from 3 per cent to 4 percent by weight of the total mix in increment of 0.5 percent. Additional water is added first and mixed then the bitumen emulsion is added and mixed again. The total fluid should be close to the optimum. The sample is compacted in a Marshall mould by applying 75 blows on each face. Six samples are prepared at each fluid content.
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Step5 The samples should be left in the mould for 24 hours and then placed in an oven for 72 hours at 40°C for curing after the extraction. The sample should be kept on a tray. Most of the moisture will be lost through evaporation. Step6 Laboratory tests: Indirect tensile strength tests are to be carried out on dry samples in a standard Marshall loading frame which applies load at rate of nearly 50 mm per minute(50.8 mm per minute) and the maximum load is determined at 25°C.Three samples are also Tested at 25°C after 24 hours of soaking in water.
12.7mm Fig. IX–2 Marshall Samples of RAP100 mm Diameter and 63 mm Long Under Indirect Tensile Test
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The loading strip is 12.7mm wide and about 70 mm long to test samples of height up to 70 mm. A modified version of Marshall Test apparatus as per Fig.IX-2 can be fabricated in a workshop. It is available commercially also. Indirect tensile strength, ITS (kPa) =
2000P
πdh
… IX-2
110
Where, P = the maximum load in Newton, d= diameter of the sample, mm (may be close to 100 mm), h = height (thickness) of the sample, mm Table IX-2 Minimum Strength Requirement of RAP Mixes
Strength Test
Specimen diameter
Minimum strength, KPa
ITSdry,25°C
100 mm
> 225 kPa
ITSwet25°C
100 mm
> 100 kPa
If ITSdry is greater than 400kPa and ITSwet is less than 50 percent of the dry value, it is indicative of contamination with clay and 1 to 2 percent lime may be necessary for modifying the plasticity of the clay (64). For 100 mm diameter samples, aggregates passing 26.5 mm sieve should be used for mix design. Effect of higher aggregate size is to increase the strength of the mixes. The bitumen emulsion content satisfying the minimum strength requirement given in Table: IX 2 can be used for the mixing. Additional 1.5 to 2.5 percent water may be added to the RAP mixes during the construction due to rapid evaporation of water from the mix in the hot weather since optimum fluid content is necessary for maximum compaction. Within a few hours of laying of RAP mix, the top layer is able to stand the construction traffic due to loss of water on hot sunny days. For small scale work, even WMM plant can be used for the preparation of Bitumen Emulsion-Rap mixes after crushing of the RAP and adjusting the gradation with new aggregates. Design of Foamed Bitumen RAP Mixes Foamed bitumen is produced by injecting water into hot bitumen resulting in instantaneous foaming. The injected water turns into steam which is enclosed by finely divided bitumen bubbles. The foamed bitumen is injected into the cold RAP mixes or even fresh aggregates containing optimum amount of moisture for compaction Such operation is highly specialized and agency’s expertise may be used for the production and construction of RAP mixes with foamed bitumen. The gradation of aggregates can be same as that given in Table: IX-1. The properties such as maximum expansion ratio (ERm), halflife (HL), type of binder used for foaming, viscosity of bitumen etc play important role in quality of mix production. TG2(66) may be referred for details. Mixing Moisture Content: The Mixing Moisture Content (MMC) of a foamed bitumen mix is moisture content in the RAP mix when foamed bitumen is injected which is practically same as Compaction Moisture Content (CMC) required for maximum compaction. A few trials are needed to determine the MMC. Methods of sample preparation and curing are same as that of the bitumen emulsion mixes. Mixes are prepared with different amount of foamed bitumen and that which meets the minimum strength requirement in dry and wet condition is adopted for production of foamed mixes. Most commonly used bitumen content for foaming is about 2.5%% by weight of the dry RAP-dust mixture. 75 blow compaction on each face of 100mm diameter Marshall Samples is adopted for sample
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preparation. Test procedures and strength requirement are also identical as given earlier for bitumen emulsion mixes. Foamed asphalt mixes need less water and cure faster but in hot weather prevalent in most parts of India, emulsion mixes also may cure faster due to migration of moisture from high vapour pressure region in the interior to low vapour pressure region on the surface. Recommendation of TG-2 published by Asphalt Academy, South Africa may be used. One of the advantages of the cold mixes are that they can be stock piled and used later when needed. Wet weather damage of RAP due to lack of an impermeable wearing course as well as due to poor drainage can be a serious problem. Table IX- Resilient modulus of RAP Mixes Table-IX-1: Recommended Values of Resilient Modulus and Poisson Ratio o
Material Material
Foamed bitumen(FB) RAP Bitumen emulsion(BE) RAP
Recommended Resilient Modulus (MPa) and Poisson’s Ratio 800 for FB
Range of Moduli (MPa)
Remarks
600-1200
Higher modulus of RAP is not sustainable due to 800 for BE heavy load. Wet water damage can be a serious problem hence higher modulus is not recommended berevisedbasedonFWDtestonintreated aggregates A modulus of 800 MPa with foamed bitumen is recommended for RAP mixes for pavement service pavements design considering the variability of the mixes though laboratory tests under controlled conditions give modulus values ranging from 600 – 1200 MPa under a haversine loading pulse of duration 0.1 seconds with a rest period of 0.9 seconds. FWD tests on in-service pavements having RAP mixes have given Modulus as high as 1600 MPa which may decrease due to heavy axle loads as well as due to moisture damage. Hence the value of modulus mentioned above is more appropriate. RAP mixes keep on gaining strength with loss of moisture but high modulus determined in the laboratory is not sustainable due to high tensile stresses caused by heavy traffic on highway in India. Fines of the RAP mixes captures most of the bitumen of foamed bitumen mixes and particulate nature of the RAP is maintained. Its behavior over granular and cemented sub-bases is different due to different level of confinement from the top and the bottom.
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Compaction of Cold mix: For compacted thickness from 150mm to 300mm of the bitumen emulsion or foamed bitumen mixes in a single pass, it is necessary to use heavy sheep foot /pad foot roller in vibration mode, getting the necessary profile by graders followed by compaction with smooth roller after.
ANNEX-X Pavement Layers with Chemical Stabilized Materials X-1 Chemically stabilized soils and aggregates may include all kinds of stabilization such as cement, lime, lime-flyash, or their combination, proprietary chemical stabilizers, enzymes, polymers and any other stabilizer provided these meet the strength and durability requirements. While cement, lime, lime- flyash stabilized materials are well known for their strength, the performance and durability of the commercially produced stabilizers should meet the additional requirements of leachability and concentration of heavy metals. Where stabilized materials are used in the pavement, only mechanized method of construction for laying and compaction should be used for major highways. The equipment capable of administering the design doses of stabilizer and quantity of water and producing a uniform and homogeneous mix should be preferred. Such materials are also termed as cemented or cementitious materials.Properties of cementitious bases and subbases are given in Annex- XI X-2 IRC:SP: 89-2010 Guidelines for soil and granular material stabilization using cement, lime & fly ash’ has very comprehensively described the entire process of stabilization. High strength chemically stabilized layer having a 7-day unconfined strength of 6-12 MPa given Table 8 of IRC:SP-89-2010 is not recommended because of possible wide shrinkage and thermal cracks that may occur during the service. Cementitious materials having lower strength can be used in bases and sub-bases. Since they are required to act as important load bearing layers, a minimum 7 day strength of 4.5 MPa for bases of cement treated aggregate is necessary for long term durability as measured by Wetting and drying/freezing and thawing tests (BIS: 4332 (Part IV) - 1968). Specimens having a strength of 5 MPa and above are generally stable under durability tests. Cement stabilized aggregate specimens should be stored in a moist curing room/ curing chamber undisturbed for seven days before tests. Modulus of rupture of cementitious bases may be taken as 20 per cent of the 28 day UCS (MEPDG) for flexure strength evaluation in the absence of laboratory tests though the Low strength cementitious materials give flexural strength which may approach 30% of the unconfined strength (16) X-3: Lime-Soil and Lime-fly ash-aggregate mixes develop strength at a low rate and strength for their acceptability should be determined at 28days. These low setting stabilizers develop fine cracks unlike cement treated materials in which the rate of strength gain is high. These binders need less water for compaction which indirectly reduces shrinkage also. Long term strength of lime–soil or lime-fly ash-soil mixtures can be determined by curing sealed samples at 400 C (as per ASTM D5102-09 ‘Standard Test Methods for Unconfined Compressive Strength of Compacted Soil – Lime Mixtures’. Accelerated curing may be used to provide a correlation between normal and accelerated
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curing strengths for the material-binder combination. Three day curing of lime of lime –fly ash soil at 500 C is found to be equivalent to about 33 to 38 days of moist curing at ambient temperature of about 300 C (16). Some typical values of unconfined compressive strength and modulus of rupture of lime - fly ash concrete suitable for cemented bases extracted from IRC: SP: 20-2002 ’Rural road Manual’ are given below: Table X-1: Expected Strength of Lime-Flyash Concrete Mixes*
S.No
Proportion of Lime: Fly Ash and Water Content, percent 28 day strength in MPa Coarse Aggregate by Weight by Weight of Mix UCS MR
1
1:2.0:2.5:5.25
10.0
6.9
1.48
2
1:2.0:2.7:6.3
11.0
7.5
1.48
3
1:1.5:2.25:5.25
9.7
7.5
1.48
*Extracted from IRC: SP: 20-2002 ’Rural Road Manual’ The proportions may vary depending upon the quality of materials, and laboratory tests are required to be done prior to construction to ensure that the materials have the minimum strength. Different trials are necessary to arrive at a good mix proportion for a base and a sub-base. Construction procedure is explained in IRC: SP: 20-2002. Table X–1 indicates that even for lime-fly ash stabilized materials, flexural strength can be about 20 per cent of the UCS. Published literature also (16) shows that flexural strength may be as high as 35 to 40 percent of the UCS of lime or lime-fly ash stabilized soil. The recommendation of MEPDG (3) for assigning the flexural strength as 20 percent of the UCS is very reasonable if laboratory data is not available. Long term strength gain for slow setting stabilizers can be considered in design. X-4 Laboratory tests: A number of laboratory tests such as flexure tests, direct tension test, longitudinal resonant frequency test, indirect tensile strength test, direct compression test etc can be used to measure elastic modulus of cementitious material. Unconfined compression tests give high values of modulus. AUSTROADS recommend flexure load test since this is considered to closer to stress/strain in the cemented base layer caused by traffic loading. Relation between UCS and elastic modulus was recommended (14) as E (cemented base) = k x UCS
… 9-I
UCS = Unconfined strength at 28 days, MPa; k = 1000 to 1250. E value of the cemented bases containing 4 to 6 per cent Ordinary Portland, slag or Pozzolanic cements is recommended as 5000 MPa. If field evaluation by FWD indicates higher modulus, fresh estimate of pavement life can be made. It is the longf term modulus rather than initial modulus that is important for a durable mix. Poisson’s ratio of the cemented layer may vary from 0.2 to 0.25. A value of 0.25 may be adopted. Stresses are not very sensitive to Poisson’s ratio.
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Cemented granular sub-base may have cement from 2 to 4 per cent to get a 7-day strength of 1.5 to 3.0 MPa. Its modulus as determined in laboratory on cylindrical specimen with extensometer may range from 2000 MPa to 3000MPa. since it forms the platform for the construction traffic, it cracks and cannot retain the initial modulus. A value of 600 MPa is recommended and its fatigue behavior is not considered because of cracks. If the stabilized soil sub-bases have 7-day UCS values in the range 0.75 to 1.5 MPa, the recommended E value for design is 400MPa. Field tests by FWD should be routinely done to collect data for obtaining pavement design parameters. Cement requirement for a given strength is much higher for soils than for granular materials. For the commercially available propriety cementing materials, the binder contents have to be determined from laboratory tests to meet the strength requirement. X-5 Laboratory strength and field strength: While a laboratory test is done in a controlled condition, there is a variability such as gradation, cement content, moisture content and curing in the field during the construction. Hence a higher strength should be aimed at in the laboratory to ensure a minimum strength in the field. MORTH recommends laboratory values to be 1. 5 times the required field strength. X-6 Cemented bases should be compacted in a single layer to a maximum compacted thickness of 200 mm. With heavy Pad feet compactors in vibration mode, it is now possible to compact even 300mm of thickness. Necessary shaping by graders is needed before the final compaction by smooth wheel rollers. Cementitious layers laid at different times may not have the strength because of lack of interface bond unless special care is taken. X-7 After the construction, curing as recommended in IRC: SP: 89-2010 must be done immediately to aid in development of strength and prevent drying shrinkage. Spraying of bitumen emulsion immediately after the construction is a very effective method of curing. Exposing the compacted layer to sun damages the stabilized layer and it does not develop strengths as intended. A minimum of 7 day curing is necessary for the cementitious base and 3 days for the cementitious subbase. X-8 Quality Control in field: The density has to be checked by sand jar apparatus in a single layer for thickness less than 200mm. for thicker construction, density is to be checked both in top half followed by the bottom half. A minimum of 97% density must be attained. Cubes should be cast during the paving operation in the field itself by vibratory hammer used for the mix design in the laboratory and strength test should be done after seven days of curing to ensure that the stabilisedatsrial has the necessary strength. The cement mixed material shall not be allowed to dry in hot son and water may be sprayed during rolling if the surface is dry during the hot windy condition. Curing shall begin immediately by wet Jut mat or by spraying of bitumen emulsion. X-9 Cemented layers normally develop transverse and longitudinal cracks due to shrinkage and thermal stresses during hydration and during the service life. Hence a layer of Stress Absorbing Membrane Inter layer (SAMI) of elastomeric modified binder (AUSTROADS2012) is to be provided over the cemented base to resist reflection cracking. The rate of spread of the binder is about 2 litres/m2 followed by light coating of aggregates of size 10 mm to prevent pick up of the binder by the wheels of construction machinery. Geotextile seal and many other commercially available synthetic products available commercially have the promise to retard crack propagation in the bituminous layer. SAMI is not very effective if the crack opening is more than 3 mm.
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X-10 Another very effective and widely tested method of arresting the cracks from propagating to the upper bituminous layer is to provide an interlayer of good quality dense graded aggregates between the bituminous layer and the cemented base. The aggregate layer should extend beyond the cemented base by about 0.5 m so that moisture, if any, travels down to the porous sub-base. Wet Mix Macadam (WMM) meeting the IRC/MORTH specification can form a good aggregate interlayer. WMM should be compacted to 100% or higher of the laboratory density. Being sandwiched between two stiff layers; the aggregates behaves as layer of high modulus under heavy load while its modulus is lower when lighter loads act. Priming and tack coating are required before laying of the bituminous layer. Use of 1 to 2 percent bitumen emulsion in WMM is needed only when the construction traffic is likely to disturb the compacted WMM requiring reshaping. Thickness of 100 mm to 150 mm has been used as the interlayer by different organisations. The Guidelines proposes a thickness of 100 mm. X-11 Behaviour of cemented base after traffic associated cracking: The cemented layers may develop numerous cracks due to fatigue cracking and its modulus may be reduced drastically from 5000 MPa to 500 MPa. Falling Weight Deflectometer can be a good tool to examine the condition of the cemented base at any time during its service life. The bituminous layer has low tensile strain before cracking of the cemented base but the tensile strain value becomes very high after the cracking of the base and fatigue failure of the bituminous layer also is imminent. If the thickness of the bituminous layer is higher 175 mm (14), the bituminous layer also has a considerable remaining life and the total life of the pavement is the sum of the fatigue life of the cemented layer and that of the bituminous layer. If the sum of the thickness of bituminous layer and 75 percent of the thickness of aggregate interlayer is greater than 175 mm (14), the pavement life is again equal to the fatigue lives of the cemented and that of the bituminous layers. X-12 Maximum nominal size of the aggregate of the granular base and sub-base should be about 40% of the compacted thickness of the layer for obtaining a homogeneous mass in mechanized construction. The granular sub-bases having grading I,II,V anv VI of MORTH can be used in the construction of cemented bases and sub-bases while grading III and IV of the granular sub-base can be used as a porous cemented drainage layer laid above the coarse graded cemented sub-base where necessary. X-13 Procedure for the mix design method for cementitious granular bases and sub-bases is same as adopted for the Dry Lean Concrete (MORTH) except that the cementitious stabilizer content is much lower due to low strength requirement. Optimum moisture content has to be determined by trial. Since maximum size of aggregates can be 53 mm, 150 mm cubes have to be made at different moisture content and compacted by vibratory hammer with square plates. Details are given in MORTH Specifications for determination of optimum moisture content, method of curing and evaluation of strength. The properties of cementitious bases and sub-bases are given in the Table in Annexure X1. X-14 For stabilized soils, even 50 mm diameter, 100 mm high cylindrical samples can be used for UCS after curing. Beam size can be 50 mm x 50 mm x 300 mm for flexure tests after curing X-14 If the soil is modified by addition of small percentage of lime/cement/other stabilizers, CBR
116
tests can be done to evaluate the quality of the modified soil. If the cement content is 2 percent or higher, unconfined compression strength should be determined to determine the strength of the stabilized soil.
117
ANNEX-XI Properties of Cementitious Bases and Sub-bases: S.N o
Descripti on
UCS(MPa)
Modulus of
Elastic
Range of
rupture
modulus, E
modulus
Remarks
(MPa) and Poisson ratio 1
Granular
4.5-7.0 at 7days for
determined by
E = 1000 *
Initial Modulus Material must
material for
cement
third point
UCS (MPa) at
can be in the
pass durability
base layer
(28 days for lime-
bending test at 28
28 days
range
test as
flyash), determined
days, as per IS-
(Austroads)
3000-14000
prescribed
by test on 150mm
516
Adopted value MPa(67).
in IRC: SP: 89:
cube
except that the
E=5000,
Austroads
2010 for long
(IRC:SP:89:2010)
sample should be
µ=0.25
recommends
term durability.
Cube shall be made
compacted by the
flexural
Loss of Weight
as per
vibratory hammer
modulus of
in 12 cycles of
IS-516 except that
with rectangular
5000MPa for
wetting-drying
they shall me
foot at OMC,
pavement
or freeze thaw
compacted by the
(20 per cent
design
not more than
vibratory
of the UCS at
assuming
14 per cent
hammer at OMC as
28days may be
28day UCS
per MORTH
taken as default
value of
specifications for
value for
5MPa
DLC
Modulus of Rupture)
118 2
—
Granular
1.5-3.0 at 7days for
material for
cement
E = 600,
Initial Modulus
µ = 0.25
can be in the
sub-base
(28 days for lime-
range
flyash), determined
2000-10000
by test on 150mm
MPa, it may be
cube
reduced to 500-
(IRC:SP:89:2010)
800
—
MPa due to cracking by the construction traffic(49)
3
—
Granular
0.75-1.5 at
E = 400, µ = 0.25
Initial Modulus
material for sub-base
7days for cement (28 days for lime-fly
(Design
ash), determined by
500-7000
traffic <10
test on 150 mm cube
MPa, it may be
msa)
(IRC:SP:89:2010)
reduced to 400-
can be in the range
600 MPa due to cracking by the construction traffic (67)
Not recommended for traffic> 10 msa
119 4
—
Chemically
UCS: 0.75 to 1.5
E = 400 MPa
the range of the Quantity of
stabilized
MPa at 7days for
moduli
chemical
soil for
cement (28 days for
depends on the
stabilizer
sub-base
lime flyash) UCS is
soil type
requirement
to be done
is much larger
cylindrical specimen
than that of
50
The granular
mm dia 100 mm
material for the
height. (ASTM
same strength.
D-5102-09)
Generally not recommended for heavy traffic in wet area
5
Chemically
UCS: 4.5 to 7
Mr
=
0.7
MPa E=3500
stabilized
MPa at 7days for
(MEPDG) for UCS MPa is
soil for base
cement (28 days for
value of
recommend by
lime flyash) UCS is
5.2 MPa for soil
MEPDG for
to be done on
cement.
soil cement
cylindrical specimen ASTM-D1635-06
base value is
50 mm dia 100mm
Standard Test
to be
height.
Method for
determined
Flexural Strength
from the
of Soil-Cement
flexural test
Using Simple
By measuring
Beam with Third-
load and
Point Loading
deflection after28days Of curing
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