Heavy Du ty Indus nd ustr trii al Pav em en t Des i g n Gu i d e Revision Revisi on 1.035 1.035 19 March 2007
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Contents Foreword
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Introduction and Background
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Scope of the Guide.........................................................................................................................5 Guide.........................................................................................................................5 Background: Design Methods .................................................................................... ........................................................................................................7 ....................7
Pavement Pavement Design Principles Princi ples - General General
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Overview of Pavement Design System..........................................................................................9 Input Variables ..................................................................................... .....................................................................................................................9 ................................9 Structural Analysis Analysis .................................................................................. ...............................................................................................................9 .............................9 Key Performance Indicators -– Level of Service (LOS) ...............................................................13
Pavement Pavement Materials
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Asphalt ................................................................................................. .........................................................................................................................................16 ........................................16 Function – wearing surface................................................................................................16 Function – structural ..................................................................................... ..........................................................................................................16 .....................16 Volumetric analysis ........................................................................................ ............................................................................................................17 ....................17 Other Issues.......................................................................................................................20 Issues.......................................................................................................................20 Composite/ Resin Modified Asphalt Asphalt .............................................................................................2 .............................................................................................21 1 Granular Material..................................... Material................................................................................................................. .....................................................................................22 .........22 Stabilised Material ................................................................................................ ........................................................................................................................23 ........................23 Subgrade......................................................................................................................................25
Traffic
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Vehicle Types....................................................................................................................... Types...............................................................................................................................28 ........28 Unequal Axle Loads ...................................................................................... ...........................................................................................................29 .....................29 Equal Axle Loads ............................................................................................ ...............................................................................................................29 ...................29 Coordinate System for Vehicles...................................................................................................3 Vehicles...................................................................................................30 0 Vehicle Wander ....................................................................................... ............................................................................................................................32 .....................................32 Payload Distribution Distribution ............................................................................................. .....................................................................................................................33 ........................33 Traffic Growth....................................................................................................................... Growth...............................................................................................................................35 ........35 Dynamic and Static Structural Loading........................................................................................36 Modelling of Multiple Multiple Wheels and Axle Groups ...........................................................................38 Nature of Damage Pulses ..................................................................................... ..................................................................................................39 .............39 Design Traffic Loading ........................................................................................ .................................................................................................................40 .........................40
New Pavement Pavement Design
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Design Period.................................................................................................................. Period...............................................................................................................................42 .............42 Material Properties and Performance Models..............................................................................43 Models..............................................................................43 Subgrade Properties and Performance Models.................................................................43 Models.................................................................43 Unbound Granular Material Properties ..............................................................................45 Asphalt Properties Properties and Performance Performance Models Models ....................................................................46
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Contents
Cement Stabilised Material Performance Models..............................................................50 Models..............................................................50
Environment
52
Drainage (surface and subsurface)..............................................................................................5 subsurface)..............................................................................................53 3 Subgrade Volume Change......................................................................................................... Change...........................................................................................................54 ..54 Weathering / ageing ...................................................................................... .....................................................................................................................55 ...............................55
Construction Implications
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General.........................................................................................................................................58 Compaction, Workability and Layer Bonding ...............................................................................5 ...............................................................................59 9 Curing...........................................................................................................................................61 Opening to Traffic................................................................................................................ Traffic.........................................................................................................................62 .........62
Pavement Pavement Maintenance
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Routine Maintenance Maintenance ........................................................................................... ...................................................................................................................64 ........................64 Major Maintenance.............................................................................................................. Maintenance.......................................................................................................................65 .........65 In-Service Monitoring Monitoring ............................................................................................ ...................................................................................................................66 .......................66
Pavement Pavement Rehabilitatio n
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Site Investigation Investigation .................................................................................................. ..........................................................................................................................68 ........................68 Functional and Structural Condition Assessment Assessment ........................................................................69 Treatment Types ........................................................................................ ..........................................................................................................................70 ..................................70 Functional Rehabilitation.......................................................................................... Rehabilitation....................................................................................................70 ..........70 Structural Rehabilitation.....................................................................................................71 Rehabilitation.....................................................................................................71
Caveats
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Life Cycle Costing
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Analysis Period – Service Life......................................................................................................76 Life......................................................................................................76 Present Worth Analysis ...................................................................................... ................................................................................................................77 ..........................77
Case Studies
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Case Study 1................................................................................................................................8 1................................................................................................................................80 0 Loading ............................................................................. ..............................................................................................................................80 .................................................80 Pavement Model ....................................................................................... ................................................................................................................80 .........................80 Results ........................................................................................ ...............................................................................................................................81 .......................................81
Appen Ap pendi dices ces
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Material failure mode and implication......................................................................................... implication...........................................................................................86 ..86 Improved asphalt material characterisation characterisation ................................................................................ .................................................................................87 .87
References
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Foreword
Foreword The purpose of this Guide is to assist pavement designers designers and managers with the planning, design, construction, maintenance and rehabilitation of heavy duty flexible pavements. Although the principles principles can be applied to various various types of heavy duty pavements, this guide is primarily directed at port and container terminal pavements. The Guide covers the assessment assessment of input parameters needed needed for design. Material properties, traffic factors, environmental considerations, considerations, pavement design methods, maintenance and rehabilitation rehabilitation treatments and life cycle costing are also discussed. At the end of the guide guide a few case studies are presented. The Guide is a collaborative effort currently involving:
Dr. Leigh Wardle of Mincad Systems (Melbourne, Australia); Australia);
Ian Rickards (Pioneer Road Services Pty Ltd, Melbourne, Australia)
John Lancaster (formerly Pioneer Road Services)
Dr. Susan Tighe (Dept. Civil and Environmental Engineering, Engineering, University of Waterloo, Canada)
The Guide presents the authors’ attempt to reflect best practice in the design, construction and rehabilitation rehabilitation of heavy duty flexible flexible pavements. The Guide will steer the designer through all necessary design considerations and suggests external sources for research updates. updates. It is intended to be supplementary supplementary to other published design guides guides with a focus on industrial industrial pavements. pavements. The primary tool used in this guide to carry out the pavement design analysis is a program called HIPAVE that has been specifically developed for heavy duty flexible pavements. The Guide is a ‘living document’ that will be regularly updated to reflect advances in pavement technology technology and made freely available available via the Internet at no charge. It is the author’s goal to preserve the relevance and currency of the Guide by in-house research and development and continuous liaison with international experts in pavement technology.
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Introduction and Background
Introduct ntro duction ion and and Backgr Background ound
Introduction and Background
Scope of the t he Gui Guide de This Guide addresses design of heavy duty flexible pavements for ports and container terminal terminal pavements. The Guide focuses on the structural design design of pavements rather than structural structural detailing or design detailing. detailing. The primary tool used in this design guide to reinforce the concepts is a program called HIPAVE, developed by Mincad Systems, which has been specifically developed developed for port and container terminal pavements. pavements. However, this is a stand alone document document which can serve as a useful tool for highlighting key elements to the design, construction, maintenance and rehabilitation rehabilitation of heavy duty flexible pavements. The Guide covers the assessment of input parameters needed for design, design methods for flexible pavements and gives guidance on life cycle costing, construction, maintenance maintenance and rehabilitation rehabilitation issues. The guide is grouped into sections as briefly described herein. A brief overview of pavement design design including the the input variables variables and structural analysis is presented, followed by a brief discussion on key performance indicators, including the concept concept of level of service. Overall a pavement pavement design system is presented in this section to assist assist with heavy duty flexible pavement pavement design. The core of the design system is mechanistic structural analysis software such as layered elastic analysis. The next few sections of the Guide contain a detailed discussion of subgrade evaluation, pavement materials evaluation, analysis of traffic loading and structural design in addition to other factors f actors relevant to pavement design. Various issues associated with construction of heavy duty f lexible pavements are presented including compaction, workability and layer bonding, curing requirements, and the ability to open to traffic. Pavement maintenance maintenance in terms of typical routine maintenance and and major maintenance maintenance are presented. Pavement rehabilitation rehabilitation including site investigation, condition assessment assessment in terms of functional and structural considerations considerations and the various typical treatment treatment types are presented. The next section presents the concept concept of life cycle costing. The analysis period, period, service life and the present present worth analysis are described described in this section. section. The last section includes case studies. The procedures in this Guide are intended for the design of pavements for which the primary distress mode mode is load associated. associated. If other modes of stress, stress, for example environmental environmental distress, have a significant effect on pavement performance, their effect should be separately assessed. It is emphasized that this document should be used as a guide only; it should not be referred to as a design specification. specification. The designer must exercise exercise judgment in choice of values for the t he parameters that are incorporated into particular designs. Pavement design is just one aspect associated with the achievement of sound pavement performance. performance. Pavement performance performance also depends on other other factors such as sound material quality control, adequate drainage, construction tolerances and pavement maintenance. maintenance.
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Introduction and Background
Although, this guide guide is written with emphasis emphasis on Australian Australian practices, it does does have relevance to the design and construction of port and terminal container pavements around the world.
Introduction and Background
Background: Backgroun d: Design Design Methods thod s Many aspects of the design methods for highway/road pavements such as those presented in the new Austroads Pavement Design Guide (2004) are not appropriate for designing heavy duty flexible pavements for applications such as ports and container terminals. Traditionally, Traditionally, port pavements have been designed using chart-based, empirical processes such as the British Ports Association method (British Ports Association, 1996). In more recent times, designers designers have combined the full range of vehicles vehicles and shipping containers into a single number of repetitions of an ‘equivalent standard axle’. This equivalent axle would would be applied in layered layered elastic design using tools such as CIRCLY (Wardle, 2004) and APSDS (Airport Pavement Structural Design System, Wardle, 1999). Alternatively, many many designers prefer prefer to use the actual wheel layouts of of the vehicles and these can be used directly in CIRCLY and APSDS. While CIRCLY and APSDS have been used very successfully for the design of heavy duty industrial pavements, unwieldy data input makes it very difficult to model more than one or two payloads per vehicle. HIPAVE (Heavy Industrial PAVEment design), an outgrowth of CIRCLY and APSDS, was released in late 2005. 2005. HIPAVE has been designed designed to conveniently handle handle comprehensive comprehensive details of the freight handling vehicles and the characteristics of the payload distribution for each vehicle. In recent years the ASCE have been developing a Port and Intermodal Yard Pavement Design Design Guide. Smallridge and Jacob Jacob (2001) give an outline of of the Guide. At the time of writing, the Guide is close to becoming becoming available available in draft form (Jacob, (Jacob, 2006).
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Pavement Pavement Design Princip les - General General
Pavement Design si gn Pri rinc ncip iples les - General General The goal of pavement design is to select the pavement design which is cost effective and provides a high level of service for the given traffic and environmental conditions. conditions. The designer must have sufficient knowledge of the available materials, the expected traffic loading, the local environment environment and their interactions. interactions. Ultimately all of these factors must be examined in order to predict the performance of a candidate pavement design. design. Furthermore the designer designer must have an understanding understanding of the level of performance and pavement condition considered satisfactory for the operational conditions of the project. A systematic approach approach to pavement pavement design is required required as there are many many variables and interactions which which influence the outcome. outcome. HIPAVE facilitates the rapid evaluation of the variables and the user should, systematically, use this capability to examine “what if” scenarios to try and identify the level of risk associated with the various pavement options as illustrated for instance in case study 2.
Introduction and Background
Overview of o f Pa Pavement vement Desig Designn System Input Variables Design Design Traffic Traffi c The wheel layout, load distribution, loading rate (speed) and t yre pressures can all have a significant influence influence on pavement performance. performance. In addition to the current traffic, attention need to be given to future traffic, including the change in volume, mass and compositio composition n during the design period. period. Detailed consideration consideration of traffic is presented in the next section. The static load under stacked containers while considerable considerable is not generally a structural pavement design issue as the magnitude of the load is generally less than under heavy vehicles vehicles and the loads are relatively relatively widespread. The extreme stress at the surface under the container corner castings is however critical to the selection of the surfacing material.
Subgrade and Pavement Materials Details of the materials in the pavement structure should include:
strength/stiffness measurements measurements which can be used to quantify their load carrying properties;
estimates of typical variations in material properties associated with changes in moisture, temperature, aging, shrinkage during the curing stage
details on how pavement materials deteriorate due to fatigue under repeated loading and
performance criteria including limiting value(s) value(s) of stresses or strains at which a given degree of distress will occur.
Structural Analysi Analysiss The aim of structural analysis is to predict the critical strains and/or stresses which are induced by the traffic loading in the trial trial pavement design. Several trial pavement pavement configurations or designs are analyzed and the most appropriate design is selected at the end of the analysis based on the technical and economic constraints. constraints. The traffic loading can be more generic ( I’m unsure what this means ) or it can include the details of each combination of vehicle model and payload.
Distress Prediction
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Pavement Pavement Design Principl es - General General
The structural analysis is used to estimate the allowable loading and associated distress of the trial pavement pavement design. The performance criteria, criteria, in this case pavement distress prediction, prediction, assigned to pavement pavement materials, and to the subgrade, are typically relationships between the strain induced by the single application of a load and the number of such applications which will result in the condition of the material, or the pavement, pavement, reaching an allowable allowable limit. The allowable limit limit is related to a maximum distress or level of service. Generally most performance models may be represented graphically by a plot of tolerable strain versus load repetitions repetitions (generally by a straight line of 'best fit' on a log-log plot). Equation 1 below, below, shows the typical typical model format
⎡ k ⎤ N = ⎢⎣ ε ⎥⎦
b
[1] where N
is the predicted life (repetitions)
k
is a material constant
b
is the damage exponent of the material
ε
is the induced strain (dimensionless (dimensionless strain)
Log-log relationships relationships can be readily converted converted to the above form. For some material types the appropriate performance relationship may be in a different functional form but, the concept and intent is the same. A pavement structure structure consists of a variety variety of materials which which have different distress distress modes. For example, a granular pavement surfaced with asphalt will have an allowable loading loading determined by the “weakest “weakest link”. The weakest link is the layer that has the highest Cumulative Damage Damage Factor (CDF), that is the t he one for which the allowable loading loading is the first to be exceeded by the design traffic loading. If all loads applied to the pavement are of identical type and magnitude, then the number of repetitions to “failure” can be obtained directly from the limiting strain versus repetitions criteria. criteria. The service life is then determined determined as the amount of time (usually in years) during which the number of repetitions is just sufficient to cause failure.
Cumulative Damage Factor In reality the pavement is subjected to a range of loadings, and each magnitude of load produces its own level of strain and stress in the pavement. Determining the service service life in these circumstances circumstances is more involved. There are two conventional ways of handling this issue. The first is to convert the numbers of loads of different magnitude to an equivalent number of loads of a standard magnitude – equivalent in the sense that they will cause the same amount amount of pavement damage. This involves estimating estimating the approximate passes of different vehicle loads to passes of an ‘equivalent’ standard load or "design vehicle". This methodology is no longer necessary now that computer software such as layered elastic analysis is available.
Introduction and Background
The second method used to deal deal with loads of different magnitudes magnitudes (i.e. actual traffic) is to use the concept of cumulative damage. The system explicitly accumulates the contribution from each loading in the traffic spectrum at each analysis analysis point by using Miner's hypothesis. hypothesis. The damage factor for the i-th loading is defined as the number of repetitions (n i) of a given response parameter divided by the ‘allowable’ ‘allowable’ repetitions (N i) of the response parameter that would cause failure. failure. The Cumulative Damage Damage Factor (CDF) for the parameter parameter is given by summing the damage factors over all the loadings in the traffic spectrum as shown in equation 2 below:
Cumulative Damage Factor = Σ ni / Ni
[2]
The system is presumed to have reached its design life when the t he cumulative damage reaches 1.0. If the cumulative damage damage is less than 1.0 1.0 the system has excess excess capacity or remaining life and the cumulative damage represents represents the proportion of life consumed. If the cumulative damage is greater greater than 1.0 the system is predicted to ‘fail’ before all of the design traffic has been applied. The procedure takes account of:
the design repetitions of each vehicle/load condition; condition; and
the material performance properties used in the design model.
This approach allows analyses to be conducted by directly using a mix of vehicle or axle types. It is not necessary to approximate passes passes of different vehicles or axles to passes of an ‘equivalent’ standard load. In this method, the proportion of damage caused by loads of a given magnitude is equal to the ratio of the number of such loads in the design period to the number of such loads which will cause failure as derived from the performance criteria. The sum of these ratios for all load magnitudes indicates the total distress which will occur. If this sum is less than or equal to 1.0, then the pavement configuration configuration being analyzed is assumed assumed to be adequate. Conversely, if this is not the case, then the trial pavement configuration is deemed to be unacceptable and must be modified in the next trial so that the deficiency is overcome. overcome. The next trial will focus on the inadequacy and and will adjust accordingly. accordingly. For example, this might mean an increase increase in pavement thickness thickness or a modification modification to stiffness. The process is repeated repeated until a satisfactory result in achieved. The results of the mechanistic analysis are readily assessed by a number of graphical formats. formats. For example, Figure 1 is a sample cumulative cumulative damage plot produced by the HIPAVE program.
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Pavement Pavement Design Principl es - General General
Figure 1: HIPAVE graph - Subgrade Damage Damage Factor vs. container load. Note that on this “Spectral “ Spectral Damage Graph” Graph ” there is a data point for f or each combination of vehicle model and payload – in this example the container weight distribution was specified at an interval of one tonne. HIPAVE can also generate graphs that show the variation of the damage factor across the pavement, as shown by:
Figure 2: HIPAVE cumulative damage graph - Damage Factor vs. lateral position
Introduction and Background
Key Perfo Performance rmance Indicators Indic ators -– Level of o f Service Servi ce (LOS) (LOS) The deterioration of a given pavement under traffic loading and environmental distress mechanisms can be characterized in terms of a number of distress modes such as rutting, cracking and roughness. roughness. Furthermore the progressive progressive deterioration over the life cycle of the pavement can be quantified in terms of various parameters such as maximum rut depth, cracking and various measures of rideability and roughness. These indicators are commonly commonly called Key Performance Performance Indicators (KPIs) or Key Performance Measures (KPMs). From a pavement design viewpoint the choice of acceptable values of the KPIs will influence the the selection of the relevant relevant damage model model or transfer transfer function. The designer should should understand the KPI’s on which the damage damage models are based. For instance the rut depth limit assumed in the Corp of Engineers subgrade strain criteria is 25 mm. If the designer considers a lesser lesser value e.g. 15 mm is appropriate then the model must be modified. The damage model or transfer function, i.e. the relationship between the calculated stress/strain and life is a critical element in the design process and the designer should examine the background research used in the development of the models to ensure confidence in the outcomes.
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Pavement Materials
Pavement Pa vement Materi Materials als The following sections detail typical pavement materials that are used in the various layers of the pavement structure and is directed to the design of heavy duty flexible pavements for ports and terminal terminal container areas. For additional information, information, please refer to Chapter 6 of Austroads 2004, for a treatise of pavement materials or the appropriate local material pavement design practices. For more detailed information on the material properties and performance models to be used in the design process refer to the “New Pavement Design’ section.
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Pavement Materials
Ass p h alt A al t The following additional considerations considerations should be taken into account, for heavy duty pavement design:
Function – wearing surface The wearing course or surface layer is generally subjected to much greater forces in heavy duty pavement conditions, compared with traditional highway or road design. Typically the pavement located at a port or container terminal is subjected to highly channelised (straddle (straddle carriers especially) especially) and extreme wheel loads. loads. Vehicles execute tight turns and there is a tendency t endency toward mechanical abrasion abrasion and indentation damage damage to the surface. The wearing surface design design objective is therefore to maximise deformation deformation resistance. With these loading conditions, conditions, it is necessary to design the wearing surface so it has the ability to provide both fatigue resistance and deformation resistance under industrial load conditions. Notwithstanding Notwithstanding the extreme wheel loads, the empirical evidence in Australia suggests the use of conventional asphalt asphalt mixes, designed to meet heavy road traffic road traffic stress has given good performance in the context of heavy duty pavements, the exception being under highly channelised loading by straddle carriers and container corner castings. castings. To put this this into perspective, perspective, the significantly greater magnitude magnitude of loads in industrial pavements is to some extent balanced by significantly lower passages of load relative relative to many highway facilities facilities with extensive truck traffic. The relatively higher higher stiffness of the heavy duty duty pavement, pavement, provided in in order to protect protect the subgrade, results results in greater support support for the wearing surface. surface. It is possible to enhance the functional performance of the wearing surface using polymer modified binder (PMB) (PMB) or Multigrade bitumen (refer Austroads AP-T41/06), stiffer bitumen such as Class 600 (refer Australian Standard AS 2008, Standards Australia, 1997) or Gilsonite modified bitumen . Modern methods of asphalt characterisation (see appendix…) appendix…) provide a rational measure of the benefit of mix modification to facilitate the selection of optimum mix components. It is evident however that under the extreme stress of container corner castings, some punching shear shear deformation and crushing crushing is inevitable. This will adversely impact the performance of thin surfacing layers enabling water penetration and weakening base materials. Overall, special special attention must be given given to the design of of the asphalt layer. layer. A minimum asphalt thickness is necessary to ensure there is structural integrity and a bond with the underlayer. underlayer. This is especially recommended recommended in areas where where heavy vehicles perform tight turning manoeuvres, where it is advisable to ensure a 50 mm minimum asphalt thickness for highway vehicles and 100 mm for heavy container handling equipment, always with a prime coat to ensure a good bond.
Function uncti on – structural stru ctural
Pavement Materials
Asphalt base and and subbase layers layers will contribute significantly significantly to the structural structural adequacy of the heavy duty pavement pavement design. The design objectives objectives are to provide high stiffness and load spreading, spreading, and control fatigue cracking. cracking. Fundamentally Fundamentally both of these objectives can be met by selecting harder grades of bitumen, and increasing the bitumen content to improve fatigue f atigue performance (taking into account the support provided by base and foundation foundation layers) The optimisation of the binder binder content is discussed in the following section. Research (Rickards, (Rickards, et al 2006) has shown that the selection of mix gradation, gradation, which is slightly fine of the theoretical theoretical maximum density, yields the highest stiffness, together with a higher filler content (material passing the 75 micron sieve) to stiffen the mortar. Experience has shown shown that while the selection of large stone stone mixes (e.g. > 20 mm nominal mix mix size) in theory yields higher stiffness, stiffness, workability issues and the tendency to segregate will often jeopardize field performance reducing reducing stiffness and a resulting in a propensity to moisture damage due to higher relative permeability. permeability. It is suggested that for practical purposes, purposes, a 20 mm nominal maximum aggregate size is used for these types of pavements. Historically, larger size mix has been used when thick asphalt layers were required. Conversely, French practice suggests that for a 14 mm nominal mix the layer thickness should be between 70 mm and 120 mm (5 – 8 times nominal mix size). A caution is provided about the potential loss of shape in the compaction of a layer at the maximum thickness but in multi-layer structures any loss of shape may be corrected by subsequent subsequent layers. For all practical purposes purposes individual layers > 120 mm thick will not be required hence hence a 14 mm mix is a practical upper size. Certainly this mix will demand more binder than a larger stone mix but it is this factor that will benefit field performance both at a theoretical level (better fatigue performance) and practical level (improved homogeneity workability and impermeability). impermeability).
Volumetric analysis It is critical to understand the importance of optimising the bitumen content to achieve optimum air void content content in mix design. It is a fundamental requirement requirement that the binder content be optimised at the in service mix density i.e. the design binder content must achieve the target air voids at a level of compaction in the laboratory that faithfully represents the level of compaction in the field. The consequence of optimisation at incorrect laboratory density is shown in Table 1. Table 1: Impact of Laboratory Density on Field Performance Laboratory density c/f in service density
Resulting bitumen content
Consequence fatigue performance
Consequence deformation resistance
Other Potential Issues
Lab >> in service density
Too low
Significant reduction
Minimal
Less durable; prone to moisture damage
Lab << in
Too high
Minimal
High risk of
Minimal
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Pavement Materials
service density
deformation
The as constructed mix density is strongly affected by construction practice which in turn is strongly influenced by construction conditions (layer thickness, temperature / weather conditions) conditions) . Subsequently secondary secondary compaction occurs occurs under traffic, to an extent influenced by loading conditions, initial relative density, layer location and climatic conditions (e.g. hot versus temperate locations), especially for unmodified bitumens. The possible consequence consequence of significant secondary secondary compaction is loss of texture and rutting. Existing empirical mix design methods, such as the Marshall method, must be carefully evaluated evaluated prior to use in the heavy duty pavement pavement design. Empirical evidence from Australian port facilities suggest 75 blow Marshall mixes have performed well in asphalt base layers but are prone to deformation in wearing course layers under channelised traffic. As a general guide, guide, the in service air air voids should be be greater than 3%. 3%. Research has shown that the strength of the aggregate skeleton is lost due to lack of void space and subsequent development of pore pressure effects at voids <3%. Furthermore, deformation will occur under conditions of heavy traffic in hot weather. If the in service air voids are greater than about about 7% for a fine mix (slightly less in coarse graded mix) the mix will be more permeable to air and moisture and that will adversely impact durability. The following laboratory tests can be useful for volumetric analysis in the mix design: •
British Standard Refusal Density (BS RD)
•
Marshall Compaction Compaction (@ 75 / 75 blows subsequently referred to simply as Marshall)
It could also be useful to characterize any existing asphalt that has performed satisfactorily at the site, under known traffic conditions and subsequently evaluate its suitability for use in similar applications. applications. (e.g. if construction records are unavailable, unavailable, then take representative cores and determine bulk density, modulus, maximum theoretical density , PSD, binder content, binder viscosity. The BS RD (BS 598 Part 104) provides a benchmark density value i.e. the practical maximum density density of any mix. For practical mix design purposes purposes for an industrial pavement, subjected to heavy channelised traffic, it can be assumed the in service mix density will approach the maximum density (especially unmodified bitumen mixes). Mix optimisation then is achieved achieved by determining determining the binder content to give the target air voids (V a) 3% at BS RD. For wearing course applications other than under channelised traffic (including heavy front loaders) 75 blow Marshall Marshall mixes have a history of good performance. performance. It is speculated that deformation resistance of the Marshall mixes under these loading conditions is adequate because even at low field voids, deformation at the surface is “ironed out” or rectified by the random traffic path. The comparison of Marshall and BS RD density is useful and may provide interim guidance for mix targets. As a suggestion, Table 2 is designed to provide information on laboratory optimisation conditions, subject to subsequent verification by in service measures.
Pavement Materials
Table 2: Suggested mix design target air voids (Va) relative to design conditions
Traffic condition
Wearing course
Basecourse asphalt (> 75 mm cover)
Subbase asphalt (> 150 mm cover)
Heavy channelised traffic
Va = 3% @ BS RD
Va = 2% @ BS RD
Va = 1% @ BS RD or Va = 4% @ 75 Blow Marshall
Heavy random traffic
Va = 1% @ BS RD or Va = 5% @ 75 Blow Marshall
Va 4% *@ 75 Blow Marshall
Va 3% *@ 75 Blow Marshall
The BS RD has its origins in compaction compliance testing for subbase asphalt with a minimum requirement requirement of 96% BS RD for acceptance (on layers > 75 mm thick approximately). approximately). In the preceding table table this would ensure ensure ≤ 5% voids at construction – a desirable target. target. Further the evidence evidence of good performance performance of 75 blow blow Marshall mixes suggests subsequent traffic compaction does not reduce voids t o critical levels. It has been observed that well-compacted mixes containing thermoplastic rubber polymer binders binders do not compact significantly under under traffic. Therefore target air voids could be reduced by approximately 1% when these materials are used in the asphalt. Note, these values are provided as a general guide and have had limited empirical verification. The user is advised to verify the design assumptions against field experience wherever possible. Complete laboratory testing on asphalt mixes should always be carried out and combined with field data whenever possible.
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Pavement Materials
Other Issues Asphalt manufactured manufactured with conventional conventional bitumen bitumen or SBS based PMB PMB can be prone to degradation on exposure to hydraulic fluid and fuel leaks. In short, these materials can soften the binder resulting in a significantly reduced resistance to deformation and mechanical damage. Other polymers may resist the softening effect and suppliers should be consulted. The Shell “FuelSafe” binder has exhibited substantially improved resistance to damage by hydrocarbon hydrocarbon spills. The PRS Rigiphalte product product referred to in the following provides significant resistance to both chemical and mechanical damage.
Pavement Materials
Composite/ Resin Modified Ass p h alt A al t A number of composite composite products – known known generically generically as Resin Modified Modified Asphalt (RMA) – offer enhanced toughness toughness which can make it a desirable wearing course for heavy duty pavements. pavements. These RMA materials materials consist of an asphalt carrier mix with high air voids. An extremely low viscosity viscosity highly modified cementitious cementitious grout is then pumped into the voids and vibrated vibrated to remove air pockets. This composite material material has improved resistance to mechanical and chemical damage while the bituminous carrier mix has the ability to absorb shrinkage strains and inhibit cracking. Its high crushing strength makes it ideal for use in container stack areas to resist deformation under the highly channelised straddle traffic and to resist crushing under container corner castings. The RMA materials have higher stiffness relative to asphalt and may fatigue under repeated flexure. Their performance parameters (modulus and fatigue) can be entered into HIPAVE and evaluated evaluated as part of the design analyses. analyses. In the longer term it may also be prudent to conduct pavement deflection testing (see below) to establish tolerable limits to confirm the adequacy of the pavement foundation support to avoid premature fatigue failure of the RMA. A comparison of of the dynamic modulus modulus of the PRS Rigiphalte Rigiphalte and a typical typical asphalt surfacing at a slow loading loading frequency (1Hz) is given in Figure Figure 8 on page 47. It is observed that the Rigiphalte product has significantly higher modulus and elastic performance parameters over the temperature spectrum.
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Pavement Materials
Granular Material The depth and quality of unbound granular material is a critical parameter in the heavy duty pavement pavement design process. This layer assists in providing providing adequate support for the surfacing materials and also provides resistance to rutting in the subgrade due to shear failure. failure. The properties required required in granular layers are a function of the applied traffic stress level and load frequency over the design period. The required depth of selected layers will vary with subgrade strength. The strength of granular materials varies with applied load stress which sets up mechanical interlock within the granular matrix and higher stress results in higher stiffness in the aggregate matrix. The stiffness of an an unbound granular layer layer is also dependent on the stiffness of support layers and this diminishes with depth in the pavement. Hence, it is important to utilize unbound granular materials of quality appropriate to the position position in the structure. Well compacted high high strength aggregates are required required for high stress locations close to the surface. At lower levels levels in the pavement, lesser quality aggregates may be used, provided they are of sufficient quality to mobilise the assigned layer stiffness. Examination of the stress distribution throughout the granular layer (e.g. by inspecting HIPAVE outputs), enables determination of material property needs (strength) throughout the pavement structure. A good starting point point is to examine applicability of of local State Road Agency Agency specifications for highway pavements, for use in heavy duty off-road pavements. The specifications relate relate to material quality and compaction compaction requirements. requirements. Attention must be paid to layer thickness, in relation to maximum particle size and density requirements. requirements. Close attention must also be given given to ensure high construction construction standards as discussed in some detail in section…and experience experience has taught that premature failure is most often related to poor construction practice and less to material selection.
Pavement Materials
Stabil Stabilis ised ed Material Material Unbound pavement materials can be stabilized by either chemical and/or mechanical processes. Chemical stabilization involves mixing additives such as bitumen or cement in quantities and to layer depths as determined by the pavement design requirements. Granular materials materials treated with bitumen or hydraulic binders (such as cement) are generally referred to as “stabilised” if they are to act as a bound layer or “modified” if they are to act as an unbound layer with improved properties properties such as reduced plasticity. plasticity. Engineering judgment judgment needs to be exercised exercised in modeling the resulting material. .A suggested delimiter between “stabilised” and “modified” conditions, is a UCS (7 day cured) of 0.8 MPa. Definition or determination determination of the degree of stabilisation is important, since a stiff, “stabilised” material will be prone to flexural fatigue and hence needs to be considered in the design. The material can be produced in a mixing plant or in-situ, using special equipment. The plant produced product, in general, should be of better quality due to enhanced product control in terms of uniformity of raw material and mixing. Conversely, the quality / variation of in-situ stabilised material may not be fully known, as it is a function of the random sampling sampling regime. Refer to Austroads (2006b) (2006b) for further reading on additives. Stabilised materials are usually described as ‘modified’ if only a relatively low level of binder is added (such as up to about 2% by mass). The addition of low quantities of lime or cement may serve to reduce the plasticity and improve marginal granular material such that that it doesn’t act as a bound bound layer. If high quantities of of cement are used (e.g. > 2% by mass) shrinkage cracking may ensue, which may reflect through to the surface. Experience in highway applications suggests that shrinkage cracks from cement treated subbase layers is substantially retarded when there is at least 175 mm cover. However the caution is noted that the rate of reflection may be related to the magnitude of vehicle loading. The type and quantity of stabilant affects the assigned modulus for the layer which should be determined by laboratory testing. The curing conditions and compaction in the field can have a significant affect on the modulus and fatigue performance performance of bound layers. Prudence also needs to be exercised in the adoption of the fatigue performance parameters especially for variable materials. Ideally, some laboratory fatigue characterization characterization should be done, to gauge the material performance and check the validity of any assumed fatigue fatigue performance relationship. relationship. In the conduct of the flexure test an appropriate density must be replicated recognizing recognizing the effect of compaction density gradient and potential reduction at the bottom of the bound layer. Generally the Unconfined Compressive Compressive Strength (UCS) is used as a specification parameter. A number of empirical UCS UCS modulus relationships relationships exist (e.g. modulus equals 1000 UCS (MPa)) and the pavement designer should be aware of the substantial range in the scale of factors.
23
24
Pavement Materials
The fatigue performance of stabilised material is a problematic design issue because of the change in material performance with time (curing),the effects of f luctuation in density and moisture content and the effect of shrinkage cracking. Great care needs to be taken in the pavement design especially where the stabilised material is a significant determinant determinant of overall pavement design life. Refer to section 6 below, for further discussion. Materials can also be mechanically stabilised, by blending components without necessarily the need for binding binding agents (chemical (chemical additives). In such cases, the components are blended in proportions to achieve a target PSD and Atterberg Limits and ideally the product product strength should then be assessed, assessed, using CBR &/or Repeated Load Triaxial (RLT) testing, which may also be valid for “modified” materials.
Pavement Materials
Subgrade The determination of an appropriate modulus of the subgrade layer for heavy duty pavements is similar similar to highway and road pavement pavement structures. Designers are advised to refer to Chapters Chapters 4 and 5 of of Austroads (2004), (2004), or the usual local local standard, for advice on characterizing subgrade materials. However industrial pavements pavements are often located in areas of extremely complex and very weak geological conditions with for instance extremely thick layers of saturated estuarine silts. The designer is cautioned cautioned that particularly in the case of extremely weak or saturated subgrade conditions the need for detailed and competent geotechnical exploration exploration is essential to ensure a complete understanding of the conditions and the associated risks (refer Rollings and Rollings, 2005 and ASCE, 2001). While pavement pavement thickness design may ensure the subgrade subgrade is adequately adequately protected to limit deformation by shear failure, geotechnical advice is essential to prevent the potential for substantially greater loss of shape due to differential consolidation. It is noted that the subgrade stress distribution in heavy duty pavements pavements is significantly different than that occurring normally in road pavements, due to the higher magnitude magnitude of loading and load duration. duration. It is important, therefore to recognize that subgrade performance models used routinely for highway pavement design are generally not applicable for pavements subjected to loading by much heavier vehicles that impart far higher stresses in the pavement and with greater areas (depths) of influence on material material behaviour. Refer to Section: Subgrade Properties Properties and Performance Models on page 43 below for further details.
25
Traffic
Traffic The following sections detail typical heavy duty traffic considerations considerations for the design of heavy duty flexible pavements for ports and terminal container areas.
27
28
Traffic
Vehicle Types In order to design a heavy duty pavement, it is important to have detailed information information on the types of vehicles vehicles that will operate operate on the site. It is possible that both off-road off-road and heavy road-use commercial vehicles, such as semi-trailers, may traffic the site. Initial contact should therefore be made with the facility operator, to obtain details of the type of vehicles using the site, including their load configurations and paths through the site. A wide range of of vehicle types are used used at intermodal/container intermodal/container terminals terminals such as straddle carriers, forklifts, gantry cranes, and semi-trailers. For mechanistic pavement design, it is important to know what the typical wheel loads are for any given payload payload on the vehicle. Theoretically these loads loads can be calculated from the geometry geometry and mass of the vehicle. A more practical approach approach is to use axle load values given in specifications provided by equipment manufacturers. This approach is used in HIPAVE. Container handling equipment equipment can be broadly sub-divided into two categories according to the load transfer characteristics: •
unequal loads on each axle; and
•
equal loads on each axle.
Traffic
Unequal Unequal Axle Axl e Loads At ports and container container terminals, there are many vehicles vehicles that have unequal unequal axle loads. Examples of these vehicles are Fork Lifts Lifts and Reach Stackers. Stackers. In this case, the vehicle loading characteristics are specified in terms of two load cases that express the axle loads as a function of of Container Weight. For example this could be the Unladen case together with one specific Container Weight. Figure 3 below illustrates illustrates the concept of unequal axle loads. loads. Axle loads for other container weights are obtained automatically by linear interpolation.
Figure 3: Load Distribution and Position of an Unequal Axle Load Using HIPAVE
Equal Axle Axl e Loads Vehicles such as straddle straddle carriers are assumed to have equal equal loads on each axle. In this case the vehicle loading characteristics are specified in terms of the unladen and laden weights of the vehicle, the number of axle rows (i.e. the number of axles seen from one side of the vehicle), the total number of wheels on the vehicle and the tyre pressure. The traffic analysis should therefore therefore consider the number of trips in the “design area” by both laden and unladen vehicles.
29
30
Traffic
Coordinate Coordi nate System for Vehi Vehicles cles In the evaluation of vehicles, the X axis is t aken as the direction transverse to the lane as shown in Figure 4. To ensure consistency between between results for different vehicle types it is recommended recommended that X = 0 correspond to the lane centreline. centreline. Usually all vehicles are assumed to have their centrelines at X=0.
Figure 4: Example of Coordinate Positioning Figure 5 illustrates the convention convention used to define the wheel locations. locations. This example is for a Hyster Fork Lift -Model H40.00-16CH. HIPAVE will normally model the two axle loadings as separate components, components, with the front axle (assumed to be on Y=0) as component 1 and the rear axle as component component 2. Modelling the two axles axles as separate components means that the two axles are modelled as two separate load cases, i.e. there is (assumed to be ?) no interaction between axle loads. In practice, it is usually only necessary to model the wheels on one side (X ≥ 0) of the vehicle, however, it may be prudent to model the whole axle, to verify whether there is interaction between the wheels . It may also be prudent to model all axle groups in one load case, where the distance between axles is similar to the width of the vehicle.
Traffic
Figure 5: Wheel Load Location for a Hyster Fork Lift – Model H40.00-16CH
31
32
Traffic
Vehi Ve hicl clee Wand Wander er Vehicle Wander is the design parameter representing the directional tracking width of the vehicle, which usually can be represented by a normal distribution, or wander width, around a notional centre-line along the vehicle path. It is important for the pavement designer to recognize that vehicles at ports and container terminals may not always travel along the confined wheel-paths wheel-paths due to the scale of the site and nature of the operations. Thus, the facility owner should be consulted about details of typical vehicle movements, including apparent wander width, which the designer can then use in the design model. model. One of the unique features of HIPAVE is that it is able to model vehicle wander, enabling economical pavement design It should be noted that vehicle wander is not normally considered in routine road pavement design, due to the narrow lane width, hindering any significant wander. However, in the design of heavy duty pavements, it should be considered as it can have a significant impact on long term performance of the pavement structure and hence, pavement pavement construction cost. For example at ports, gantry crane areas areas may result in manouevres that are heavily channelised while in other areas where vehicles are not as restricted, there might be extensive wander.
Traffic
Payload Distr Distribut ibution ion Estimating the payload distribution is a critical component of the pavement design process. The relative proportions proportions of each container weight in the overall overall spectrum are important for economical economical pavement design. design. A relatively small number number of heavy loads may be more damaging than a higher number of smaller loads. It is also important to account for the fact that each vehicle will handle a range of container weights or payloads. Ideally, the designer should be able to to specify the detailed container weight distributions. For example, the British Ports Association Association Guide (1996) includes includes information on container weight frequency spectrum, based on data provided by United Kingdom (UK) (UK) ports. Figure 6 shows the container container weight distribution for 40 foot containers. HIPAVE, in contrast to other other existing techniques, techniques, does not force force the designer to use a single design container weight, or to convert all vehicle characteristics to repetitions repetitions of an “equivalent” design design vehicle or load. HIPAVE allows the designer to input detailed container weight distributions which ultimately provides a more realistic impact of payload distribution on the pavement structure.
33
34
Traffic
Figure 6: Container weight distribution for 40 foot containers at UK ports (British Ports Association 1996). Care should be taken to ensure that the container load spectrum is reasonably up to date. For example, the summary summary data provided by the British Ports Association Association rd Guide (3 edn, 1996), is the same as used in the second edition (1986) – so is at least 20 years old. Data provided by some major major Australian port terminals terminals suggest that the peak loads may be 4-5 tonnes higher than the BPA data.
Traffic
35
Traffic Growth rowt h The compound growth of traffic volume is commonly specified as a percentage increase in annual annual traffic volumes. volumes. If compound growth growth is constant throughout throughout the design period, the cumulative growth factor over the design period can be calculated as shown in Equation 3. =
Cumulative Growth Factor (CGF) =
(1 + 0.01R) P − 1 0.01R
for R > 0
P
for R = 0
[3]
where R = Annual Growth Rate (%), and P = Design Period (years). Table 3 below provides values of CGF for a representative range of design periods and annual growth rates, P and R respectively. Table 3: Cumulative Growth Factor (CGF) Design Period (P)
Ann ual Gro wt h Rate Rat e (R) (%)
(years)
0
1
2
3
4
6
8
10
5
5
5.1
5.2
5.3
5.4
5.6
5.9
6.1
10
10
10.5
10.9
11.5
12.0
13.2
14.5
15.9
15
15
16.1
17.3
18.6
20.0
23.3
27.2
31.8
20
20
22.0
24.3
26.9
29.8
36.8
45.8
57.3
25
25
28.2
32.0
36.5
41.6
54.9
73.1
98.3
30
30
34.8
40.6
47.6
56.1
79.1
113.3
164.5
35
35
41.7
50.0
60.5
73.7
111.4
172.3
271.0
40
40
48.9
60.4
75.4
95.0
154.8
259.1
442.6
36
Traffic
Dynamic and Static Structural Struct ural Loading The pavement designer also needs to have an accurate representation of the structural loading on the pavement. This data can often be obtained by the facility owner or from the vehicle equipment equipment suppliers. Information on the typical range range of possible loadings on the vehicles should also be incorporated into the design. Special attention should be given to high stress areas such as situations where vehicles conduct tight turns/ cornering manoeuvres, braking/acceleration, braking/acceleration, or in areas where the dynamic effects associated with rough surfacings can be of critical importance. In short, these areas result in higher stress and it is appropriate appropriate in those situations to apply a load multiplication multiplication factor. Table 4 provides some guidance guidance on how to address this situation and is based on British Port Association (1986, 1996).
Traffic
Table 4: Suggested Load Factors to Address High Stress Areas Load Factor* Vehicle Brak Brakin ing g
Corn Corner erin ing g
Acce Accele lera rati tion on Unev Uneven en Surface
Front Lift Truck
1.3
1.4
1.1
1.2
Straddle Carrier
1.5
1.6
1.1
1.2
Side Lift Truck
1.2
1.3
1.1
1.2
Tractor & Trailer
1.1
1.3
1.1
1.2
*Note: where conditions conditions apply simultaneously, simultaneously, the factors should be multiplied together. The values provided in Table 4 are provided as guidance. However, engineering judgement / experience experience should be exercised in adopting adopting load factors. factors. For example: 1) In an area where a front lift truck is exposed to an uneven surface, a load load factor of 1.2 could be applied. 2) In an area where the side lift truck is is accelerating and also exposed exposed to a corner, the load factor could be 1.1 *1.3 = 1.43 The pavement loading is usually represented in the design model, as circular loading, at constant tyre stress, as applied by the t yre ‘footprints’. It is likely, in reality, that the tyre footprint is more of an elliptical contact area, with non-uniform contact stress, but a circular contact contact area is is adopted to simplify calculations. calculations. Furthermore, the design of thin asphalt surfaced pavements, under heavy point loads, may be considered problematic due to the size of the load footprint and magnitude of the load, in relation to the layer thickness thickness (refer to further discussion discussion on Asphalt Asphalt Fatigue … page #). At this point in time, a simple simple approach approach to modeling the various effects is a reasonable assumption. assumption. However, it is important important to note there are are some shortcomings with current knowledge about the interaction of closely spaced axle groups and subsequent recommendations (Wardle et al, 1999), particularly in relation to assessing stresses & strains in the subgrade area.
37
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Traffic
Modelling of Multiple Multipl e Whee Wheels ls and Axle A xle Group Groupss HIPAVE lets you model the pavement with the actual wheel wheel layouts of the vehicles that operate on the pavement. pavement. Care needs to be taken to select which wheels wheels to include in the model. Extensive research gives gives us guidance on choosing the "right" "right" combination of wheels wheels to include in the model. Using more wheels wheels can lead to inaccurate model model predictions (Wardle et al, 1999). 1999). Further details are given in the HIPAVE User Manual (Wardle, 2005). The recommended model for base/sub-base materials and subgrade performance relationship recommended recommended for heavy duty loads is described below (Material Properties and Performance Performance Models on page 43). 43). This methodology methodology was derived from full-scale aircraft pavement tests conducted by the US Army Corps of Engineers at their Waterways Experiment Station (WES). These 'WES' tests were essentially conducted using single single gear assemblies. assemblies. No tests were carried out to investigate the increased damage that might result due to interaction effects of adjacent gear assemblies. Considerable Considerable uncertainty exists with respect respect to prediction of damage for aircrafts that have main gears in close close proximity. APSDS has been used to study multiple gear interaction effects for a Boeing 747 and 777 aircraft using a range of alternative damage models (Rodway 1995a, Wardle and Rodway 1998, Rodway, Wardle and Wickham 1999). 1999). Results from these studies show that that the successful calibration of simplified design models against the full-scale test data does not create a capability to confidently confidently extrapolate beyond the limits limits of the test data. The studies showed that simple damage models give unrealistic predictions for the damage caused by all sixteen main wheels of the aircraft when compared to that computed for a single isolated 4-wheel 4-wheel gear. Three different performance performance models, each of which gave a similar 'goodness of fit' to the full-scale test data, gave greatly different predictions of the damage caused by the interactions of the sixteen main wheels. The differences between the alternative predictions increased with increasing depth to subgrade. Given the above comments, as a general rule only groups of wheels that are within two metres of each other should be modeled modeled as a single load case. For example, the most appropriate way of modeling a Fork Lift is described in the section Coordinate System for Vehicles.
Traffic
Nature of Damage Pulses The WES tests were performed performed on relatively relatively thin pavements. pavements. In most of the test sections the elastic models predict a distinct strain pulse at subgrade level for each axle of a two-axle gear. For deep pavements (say (say 1.5 m or more) the models predict a single combined pulse pulse resulting from the entire gear. gear. In other words, a two-axle gear produces two strain pulses per pass for shallow subgrades and one strain pulse, of significantly different different shape, for deep subgrades. subgrades. HIPAVE uses strain repetitions as the basis for damage damage predictions, not passes or coverages. coverages. Pulse counts and pulse shapes shapes both change with pavement pavement thickness. There is significant uncertainty in the design of thick pavements because data must be extrapolated from thinner test pavements which have narrower pulses than those expected for the deeper subgrades. subgrades. There is still no experimental experimental data to show to what extent pavement damage depends on the transverse and longitudinal widths of the load pulse. The designer is referred to the US research at the National Airport Pavement Test Facility (NAPTF). This facility has conducted full scale scale pavement test loading under under simulated B747 and B777 load gear hence loading and pavement configurations are of a similar dimension to heavy heavy industrial pavements. pavements. Numerous researchers researchers are analysing the performance of the test pavements and this will lead to improvements improvements in the design models.
39
40
Traffic
Design Traffic Loading L oading Pavement damage is a function of the cumulative damage induced in the pavement and subgrade materials, materials, by the traffic, over the design period. The duration of the design period affects the pavement composition and hence, construction (materials) cost. The predictive capacity capacity of the design, is related related to the accuracy accuracy of the design data, therefore, the facility operator should be consulted to advise on the planned traffic usage of the site, in terms of the number and types of vehicles, throughout the design period.
New Pavement Design
New Pavement Pavement Desi Design gn
41
42
New Pavement Design
Desig Designn Perio Periodd The purpose of the pavement design, is to ensure with a high degree of confidence, that the pavement is structurally adequate to ensure it remains in serviceable condition, without significant maintenance expense, throughout the designated design period. Some suggested design periods are as follows : •
rehabilitation of existing in-service in-service pavement : 10 – 15 years
•
new pavement construction or major pavement rehabilitation : 15 – 25 years
The design period refers refers to the serviceable life of the pavement structure. structure. It can also be considered as the time when pavement distress, sufficient to render the facility practically dysfunctional, dysfunctional, occurs over a significant proportion of the area. A distinction exists exists between the structural structural and functional functional performance parameters parameters This differentiation is important and the designer must ensure the owner understands that that the surfacing may require cyclical rehabilitation to remedy deterioration of the functional performance parameters, i.e. roughness and rutting, within the design period. The mechanistic pavement pavement design method is outlined outlined in Austroads 2004. HIPAVE takes into account the effects of vehicle wander that is a much more prominent design consideration consideration than with roads. Depending on the operational operational logistics the industrial pavement may have highly channelised traffic in tight lane configurations or more random and wider traffic paths in roadways. Industrial pavements could be significantly over or under designed if vehicle wander is ignored. Additionally Additionally HIPAVE enables the estimation of damage over the design container weight spectrum, to again avoid costly over or under design.
New Pavement Design
Material Material Properties Prop erties and Perfo Pe rformance rmance Models A discussion of the factors that contribute to the the performance of flexible flexible pavement pavement structures is given in the t he Appendix: Material failure mode and implication on page 86. The mode and consequence of failure of the different pavement materials will impact the selection of pavement pavement components. components. By this process the intention intention of this discussion is to focus designers’ attention on the factors that may impact on the realisation of the properties used in the design. Experience has shown that poor pavement design methodology may reduce the performance of the pavement, but poor construction quality will devastate the performance. Each passage of a vehicle over a unit of pavement area (and indeed within an effective vicinity) causes causes damage to pavement material material layers. The damage accumulates with each vehicle pass, resulting eventually in ultimate pavement failure. The mechanistic pavement pavement design method attempts attempts to determine the design life of the pavement, in terms of number of load passes until a defined failure of the pavement has occurred. In the mechanistic empirical design method two separate modes of failure are assumed; deformation due to subgrade shear failure, or fatigue cracking of layer(s) of bound pavement materials (which will ultimately result in subgrade shear failure due to consequent loss of load spreading). spreading). In reality both modes are a simplification simplification of a complex environment. Deformation may occur as a consequence of consolidation consolidat ion and shear failure in pavement layers; cracking and loss of strength may occur in some pavement materials not as a consequence of fatigue. Care needs to be taken to ensure that realistic data is input and that material performance models used in design are valid for the load case under consideration. There currently is no widely accepted purely mechanistic pavement design undertaken and a semi-empirical / mechanistic approach is usually adopted. Subgrade performance models are currently empirically based on field trials, correlating axle group passes to deformation (usually 20 mm deep rut) in various subgrade soil conditions (strengths). The asphalt component is designed to resist premature deformation and premature flexural fatigue induced cracking, with the former catered for in the mix design process and the latter in t he pavement thickness design process.
Subgrade Prop Properties erties and Perf Perform ormance ance Models Models In general practice in Australia the stiffness or modulus of the subgrade E SG (MPa) is related to the CRB in the general relationship ESG (MPa) = 10.0 CBR
43
44
New Pavement Design
It must be understood that this is but one of numerous modulus/CBR modulus/CBR relationships that have been derived by various researchers. researchers . However the value of assigned subgrade modulus modulus is probably less critical to the outcome than the accuracy of the damage models models used in the design. In the derivation of the following subgrade subgrade deformation model model the above relationship was was used. It must be understood that if a different relationship were used, a different damage model would be derived. Caution should be exercised before adopting road-based models for design in offroad situations, such as airports and ports, because of the much greater magnitude of loading with the latter cases and non-linearity non-linearity of subgrade subgrade behavior. In general, the subgrade strain relationship, can be expressed as follows: N Where
N
=
= (k / ε)b
[4]
predicted design life (at strain level ε)
k
=
material (subgrade) constant
b
=
material damage exponent
ε
=
load induced strain in the material
Wardle et al (2001) report on the US Army Corps of Engineers CBR method (Method S77-1), for design of flexible aircraft pavements, which has yielded generally satisfactory pavement performance, performance, when used for design of pavements over a range of subgrade strengths strengths and vehicle loadings. loadings. Wardle et al (2001) report the results of back-analysis pavements on subgrades from CBR 3 – 15%, for aircraft masses ranging from 40 – 397 tonnes, using APSDS to derive derive the performance constants ‘k’ and ‘b’ (see above), above), below. Accordingly, the following following subgrade performance model is suggested for aircraft between 40 – 400 tonnes (tyre pressures listed in Wardle et al (2001)), for subgrade design CBR ranging from 3 – 15 %, for 10,000 to 100,000 vehicle passes during the design period: k
=
(1.64 x 10-9 x E3) – (4.31 x 10-7 xE2) + (2.18 x 10-5 x E) + 0.00289
b
=
(-2.12 x 10-7 x E3) + (8.38 x 10-4 x E2) – (0.0274 x E) + 9.57
E
=
subgrade modulus (MPa; usually expressed as 10 x CBR )
The above performance relationships may be used with prudence, for design of pavements supporting heavy off-road vehicles, such as at ports / container terminals. More recently analyses of the performance data from the full scale trials at the National Airport Pavement Test Facility (NAPTF) has been carried out (Lancaster 2006) in an attempt to improve the empirical empirical verification of our design design models. The findings from those analyses were inconclusive because of the various failure mechanisms observed however the analyses did not indicate a need to change the current modeling practices.
New Pavement Design
Unbound Granul Granular ar Mate Material rial Properties Prop erties The performance of the unbound granular layers is governed by the material specifications rather rather than thickness design. design. In other words it is assumed assumed that the specification ensures that the materials used will resist crushing and shear failure under the applied stress. stress. The adequacy of this approach approach is demonstrated empirically empirically in the generally good performance of these materials in heavy duty industrial and airport pavements pavements internationally. Historically failures failures in well specified granular granular base layers have been due to the effects of poor construction practice, excess moisture and other factors rather than inherent material inadequacy. inadequacy. The derivation of the subgrade performance performance model (previous section) resulted from analyses using the Barker-Brabston method derived at the US Corp of Engineers has been calculated and is shown in Figure 7 below. 1000
US CORP OF ENGINEERS (AR MY TM 5-825-2-1) 5-825-2-1) SUBL AYERING AYERING FOR UNBOUND GRANULAR MATERIALS E2 500 MPa
) a P M ( R E Y A L R E P P U F O S U L U D O M
E2 320 MPa
E1 150 MPa
t = 200 mm Base t = 150 mm Base 100
t = 100 mm Base t = 200 mm Subbase t = 150 mm Subbase t = 100 mm Subbase
10 10
100 MODULUS OF SUPPORT LAYER (MPa)
1000
Figure 7: Sublayering of Unbound Granular Layers (after Barker and Brabston, 1975)
The stiffness of unbound granular layers varies with; 1
the quality quality of the aggregate aggregate (base (base or subbase material) soundness, durability, particle size size distribution, distribution, angularity, angularity, etc.
2
the thickness of the layer, and
3
the stiffness of the supporting layer
4
moisture content (or saturation ratio) and PI
5
stress-state
6
relative density
45
46
New Pavement Design
The Barker-Brabston model and the derived subgrade damage model assumes the granular layers to be isotropic. isotropic. Preliminary analyses analyses of the NAPTF trial data is being evaluated to test this model and results to date do not indicate a need for change. It is assumed that the specification limits of strength and durability will ensure the preservation of the layer stiffness. stiffness. Empirical evidence evidence suggests this is the case and there is no evidence of failures attributed to aggregate breakdown in compliant materials. To ensure the mobilisation of the Barker-Brabston base layer moduli the contract documents must specify compaction to be ≥100% modified compaction and dry back to <70% Degree of Saturation (DOS). To ensure compaction achievement and to minimise the effects of interface conditions construction layer thickness for granular layers should be between 100 mm and 150 mm. Thin layers are potentially potentially at greater risk than thick layers and delamination delamination at the t he top of the base layer (caused by rework and over-watering) must be avoided – catastrophic failure of the wearing surface may be the consequence. Some natural gravels can also provide satisfactory performance, whilst possibly not conforming totally with standard specifications for quarry produced crushed rock however, engineering judgement should be exercised when analyzing properties of the natural gravel (e.g. PSD, PI) and ideally additional testing such as soaked CBR and Repeated Load Triaxial Testing should be done to evaluate the material performance, for ranking against standard materials. On major projects the examination of the source rock by geotechnical engineers engineers is prudent to ensure the granular materials will exhibit adequate durability in the project climatic and hydrological environment.
Asphalt Asp halt Properties and Pe Perfo rformance rmance Models Models The measurement of asphalt materials modulus is now routinely carried out in Australia and elsewhere elsewhere using using the Indirect Tensile Test (ITT) method. Likewise asphalt fatigue testing is routinely carried out in Australia using the beam flexure f lexure test method (Austroads, 2006a) to derive flexural modulus and fatigue performance parameters. The stiffness of asphalt and corresponding response to load is significantly affected by the pavement temperature temperature in service. Figure 8 and the predictive models models illustrate the moduli for dense graded asphalt over the range of operating conditions and is compared against against the composite resin modified asphalt asphalt product Rigiphalte. Rigiphalte. It is observed as would be expected the latter is both stiffer and less affected by temperature increase.
New Pavement Design
Table 5: Typical Modulus (MPa) of Asphalt Traffic speed (km/h) Temperature (°C)
0-5 km/hr
10-20 km/hr
50 km/hr
10
12,500
15,000
16,300
15
9,600
12,600
14,000
20
6,900
9,900
11,200
25
4,600
7,300
8,600
30
2,800
5,500
6,200
35
1,700
3,4005
4,300
The designer should be aware of the range in operating conditions and then select representative values for design considering as well the typical environmental variations. Refer to Dickinson (1981) (1981) for further details of observed temperature temperature fluctuations by season and depth in asphalt in Australia. The weighted Mean Annual Pavement Temperature (wMAPT) approach has proven to be reasonable and the following relationship to Mean Annual Air Temperature (MAAT) is derived from the Austroads Austroads pavement design guide. guide. Essentially the wMAPT is the notional pavement temperature at which the design traffic causes the same damage as the segmented traffic over the temperature spectrum. wMAPT = 1.3 MAAT + 5 (oC) DYNAMIC DYNAMIC MODULUS E* V TEMPERATURE FREQUENCY 10 Hz 100000
) a P M ( * E S U L U D O M C I M A N Y D
10000
Rigiphalte AC 14 MTD MTD
1000 10
15
20
25 30 35 40 PAVEMENT TEMPERATURE TEMPERATURE (oC )
45
50
Figure 8: Comparison of the unconfined dynamic dynamic modulus of asphalt and Rigiphalte over the typical operational temperature range. ......
47
48
New Pavement Design
With the improved dynamic modulus characterisation available available from the SPT a more rigorous analysis analysis is possible. On conclusion of the HIPAVE spectral spectral damage analysis the user may reduce the traffic to a selected number of passages of a single extreme load case and determine determine damage. The dynamic modulus modulus of the asphalt and traffic spectrum can then be manually input to represent the full temperature and traffic spectrum. The damage at each temperature temperature and traffic spectrum is estimated and the cumulative damage summed and compared with the damage calculated at wMAPT. In future development development of HIPAVE the spectral spectral damage related to temperature may be automated. While the fatigue data is not used as a specification parameter parameter it’s application over many years has established confidence in conservative nature of the asphalt fatigue models used in desig design n practice (refer to the Shell method following). following). The flexure test is of most value in the evaluation of alternative binders, binders, with the limitation being that the relationship between field and laboratory performance is uncertain and has not had substantial empirical validation in the Australian environment. It is known that the fatigue performance of asphalt in the field is considerably greater than in the laboratory (at given tensile tensile strain). This is thought to be primarily due to the effects of the healing of micro-cracks in the bitumen binder in warm conditions during rest periods between between loads. The Strategic Highways Highways Research Program (SHRP) from their comparison of laboratory (N LAB) and field (N FIELD) asphalt fatigue suggests Shift Factor (SF) of 10 to 14 for 85% and 50% design reliability i.e. (NFIELD) = SF. (NLAB) One of the limitations of the laboratory asphalt fatigue test is that it is a continuous cyclical test at a low temperature in order to complete testing within a reasonable timeframe. These test conditions conditions do not allow healing healing of the micro-cracks. micro-cracks. Consequently Consequently caution is advised in the interpretation of fatigue in mixes with Polymer Modified Binder (PMB) because research suggests the healing of binder may be inhibited by the polymer components. Considering Considering the magnitude of many industrial pavement projects the cost of specific materials characterisation characterisation is warranted although it must be understood that the relationship between between the laboratory and field f ield performance data is not yet well calibrated. Notwithstanding, Notwithstanding, it is valuable to use the laboratory test data as a point of verification of the input parameters parameters used in the design process. In time, these data bases will be established and will provide valuable insight into performance. At the initial pavement pavement design stage the use of predictive models for stiffness and and fatigue performance performance are considered adequate. adequate. A number of approaches approaches of greater or lesser complexity are available and their use is preferable to simply adopting typical values. The application of the predictive predictive methods gives the designer designer a better feel for the critical mix parameters. parameters. One such method based on on the Shell Pavement Design Design Guide is available in an Microsoft Excel spreadsheet that may be downloaded from www.mincad.com.au/hdipdg . www.mincad.com.au/hdipdg .
New Pavement Design
Internationally the trend is to use the triaxial test methods such as the US Simple Performance Test (SPT) to determine the dynamic modulus of asphalt over the range of temperature and load frequency frequency (refer to appendix appendix X) expected in the field. For each material a master curve is developed to enable the designer to input asphalt properties that enable the estimation of damage across the full climatic spectrum. This facilitates the move away from the simplifying weighted Mean Annual Pavement Temperature (wMAPT) (wMAPT) approach often used for road pavement pavement design. While this approach has served us well over decades and seems to be appropriate for conventional binders binders it does not adequately treat modified binders because of the consequent changes in temperature sensitivity. Interestingly for airport applications (where similar load magnitude to ports are applied) the US Department of Army & Air Force Technical Manuals (Nov. 1989) TM 5-825-8-1 and AFM 88-6, respectively, state that 75 – 125 mm asphalt thickness generally suffices, over a thick granular pavement, provided that: “it must be assumed that if the minimum thickness of asphalt is used as specified in TM 5-825-2 / AFM 88-6 Chapter Chapter 2, then fatigue cracking cracking will not be considered. considered. Thus, for a conventional pavement, pavement, the design problem is one of determining the thickness of pavement required to protect the subgrade, with adequate controls in place for the granular components (i.e. material, quality, density, susbsurface moisture control etc). This compares with the empirical performance observation of Australian ports where 150 mm asphalt on unbound granular base materials has given good performance over decades and fatigue cracking in the wheelpaths has generally not been observed The empirical evidence evidence suggests the pavement thickness thickness required to protect the subgrade provides sufficiently strong support to protect the asphalt from fatigue, with adequate controls in place for the granular components (i.e. material, quality, density, density, subsurface moisture control control etc). The designer is cautioned about the reliability of the analysis of thin layers – particularly wearing wearing surfaces. In the design models it is assumed the layers layers are homogeneous, homogeneous, the tyre contact stress is uniform uniform and normal normal to the surface. In practice it is difficult to compact thin asphalt layers so their properties will be different to similar materials placed at greater depth; tyre stress is far from uniform and often has a considerable shear force component due to the tyre properties and acceleration. It is suggested that the analysis of layers of thickness < 50% of the model tyre contact radius be treated with caution. caution. In highway conditions conditions this relates to layer thickness < 40 mm; in heavy duty applications 80 mm is probably more appropriate.
49
50
New Pavement Design
Cement Cement Stabilised Stabil ised Material Material Performance Perfor mance Models Only limited research has been conducted into the performance of cement stabilised pavement materials in Australia and a number of research needs have been identified by ARRB (Jameson, (Jameson, 1995). The range of exponents used used by various agencies for the fatigue performance relationships (e.g. 8 t o 18), is considered testament to the significant variation in performance expectation of these materials. The uncertainty is exacerbated by the varying gravel properties and cement content, the effects of curing prior to traffic exposure (including construction construction traffic) and potential loss of performance because of debonding of cement treated layers. Shrinkage cracking during curing and cracking at construction joint appear to be inevitable. It is also apparent that density profiles can be expected in deeper lift construction and may further complicate material performance prediction. In Australia there is no routine testing to measure the modulus or fatigue performance parameters parameters used in design analyses. analyses. Typically the material is specified specified by a minimum Unconfined Compressive Strength (UCS) requirement. In the AustRoads PDG PDG an empirical relationship relationship is give give to derive modulus modulus from the UCS parameter but research has shown this relationship to have a substantial variation. Thus the designer is faced with the adoption of rather arbitrary modulus values in the analysis to determine critical critical stress/strain magnitude. magnitude. The relevance of the design process is then further constrained by the paucity of current research into the damage model i.e. what is the critical tensor (stress or strain) and what are t he material constants and damage exponent? The designer is directed to South African (SA) research on cement stabilised pavements. The extensive SA research on the topic and and full scale trials using the Heavy Vehicle Simulator Simulator (HVS) would imply imply that this approach is state of the art. In short, in SA, it involves a three phase damage model for CTB; crack initiation, crack propagation and finally crushing reverting to the properties of the unbound granular component Caution is given rega regarding rding the relative magnitude magnitude of loading in the research and the stress sensitivity of cement bound materials The cement treated layers are not usually placed in the upper pavement layers, to avoid reflection cracking in the surface layer. There is empirical evidence of good performance of cement stabilised subbase layers when placed on hydraulically hydraulical ly placed sand. The use of an unbound granular base (typically about 250 mm thick) in this case appeared to inhibit reflection cracking in the asphalt wearing wearing surface. In other facilities, when placed on fine grained estuarine silty clay, the cement treated materials appeared to suffer significant erosion erosion at the cracks resulting in poor functional performance. performance. The use of geofabric materials or other filter media should be explored.
52
Environment
Environment
New Pavement Design
Drainage (su (surf rface ace and subsurface) The designer must consider the moisture regime within the total pavement structure and conduct the design accordingly. Consideration therefore must be given to both surface and subsurface drainage requirements and particularly the location of the Water Table in relation to the Finished Surface Level of the pavement. It is also important to consider the relative permeability of the pavement layers, to avoid the potential for development of excessive pore water pressures within the pavement, especially for unbound granular materials.
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54
Environment
Subgrade Volume Volu me Change The degree of pavement surface smoothness is an important performance parameter which can affect the safety and ride quality of transport vehicles in heavy duty pavements. Pavement Pavement smoothness may be affected by pavement failure (e.g. deformation) and/or volume changes in expansive clay subgrades, which may have an influence extending extending beyond one metre above above the reactive material. Another cause of subgrade volume change can be related to the consolidation settlement particularly in saturated silty subgrades. The designer is again cautioned about need for expert geotechnical advice – particularly in locations with weak and/or saturated subgrade conditions.
New Pavement Design
Weathering Weathering / ageing ageing Bituminous surfacing materials are exposed to the extremes of the weather as well as loading. The bitumen in the mix will suffer oxidative oxidative hardening in the event event high air voids exist due to poor mix design or construction construction practices. The ageing of the wearing surface is slow if the insitu air voids are reduced to ≤ 5% by construction and traffic compaction. The soundness of the aggregate aggregate component must be defined in the specification.
55
Construction Implications Implications
Construction Implications
57
58
Construction Implications
General As stated previously previously deficiencies in in established design design methods and and practices may have a minor impact on pavement performance whereas poor construction quality can devastate performance. The specifier must ensure the contractor has quality assurance procedures in place during the construction process, and conduct audits to monitor compliance with design standards. This may include materials performance testing to verify t he assumed values of the pavement design components have been realised. The use of deflection testing during during construction is recommended. recommended. The recording of deflection data at key steps in the construction sequence, e.g. at the completion of the construction platform be it a capping layer or compacted subgrade; and at the completion of the granular basecourse, may be compared with t he deflection calculated using the analytical model and thereby confirm (or otherwise) the input parameters. This is considered benchmark data and subsequent pavement performance monitoring over the long term will enable the fine tuning of critical benchmark deflection limits to substantially reduce the risk of failure in heavy duty pavement facilities. In recent times lightweight hand held falling weight deflection devices have become available and comparative testing with the larger FWD’s has shown reasonable results with some of the alternatives.
Construction Implications Implications
Compaction, ompaction , W Workabili orkability ty and Layer Layer Bonding Bondi ng The proper compaction of all component layers in the pavement structure is vital for good performance. performance. An industrial pavement pavement is subjected to substantial wheel wheel loads and the compaction equipment used must generate similar compactive effort. Compaction testing is a routine and essential construction process and is generally used as a surrogate measure for the more fundamental layer stiffness parameter. Critical features of the design should be highlighted for assurance testing. Standard road pavement specifications specifications may not fully suffice, but can form a good basis (to modify to suit). Over weak subgrade materials (CBR <5) a capping layer or construction platform comprising select material is an essential ingredient to provide access to construction traffic without significant shear deformation and to provide an anvil to enable the compaction of subsequent subsequent layers. The consistency of the select capping capping material and it’s strength under the anticipated conditions of moisture and stress must be assured. The determination of OMC and density density should be based on standard standard compaction energy recognizing recognizing the possibility of low support stiffness. For granular subbase materials the determination of OMC and density should be based on modified compaction energy and the target density should exceed 97% of modified compaction density For granular base materials the determination of OMC and density should be based on modified compaction energy and the target density should exceed 100% of modified compaction density. On completion of granular base layers they must be allowed to dry back to about 70% Degree Of Saturation (DOS) in order to mobilise maximum stiffness. In recent times a number of catastrophic failures in heavy duty pavement applications have been primarily ascribed to the neglect of this fundamental construction requirement. requirement. The base must be primed to toughen the interface and facilitate the bond with the asphalt surfacing. The base must be thoroughly swept with a stiff broom to remove fines and dust and present a solid granular matrix.
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60
Construction Implications
If a cement treated base is utilized, uniform compaction is essential and a density gradient must must be avoided. avoided. Density at the the bottom of the the layer is vital to stiffness stiffness and fatigue performance. performance. A strong construction anvil and careful careful quality control is essential for this to be achieved. achieved. In heavy duty applications applications the required thickness of cement treated layer (to control fatigue) may require multi layer applications, depending upon upon its location within the overall overall pavement. Where multiple layers layers of CTB are specified specified specific treatments to ensure ensure a bond at the interface are vital to ongoing performance. performance. Layered elastic analysis analysis and field test results from the Accelerated Load Load Facility (ALF) testing testing showing dramatic dramatic reductions in performance where poor bond was was achieved It is strongly recommended that that placing and compaction trials be conducted conducted in order to verify compliance compliance The finished CTB surface must be primed or a curing membrane applied to assist curing and assist bonding if asphalt is to be placed. The workability of asphalt has a significant influence on compaction achievement. achievement. Research (Rickards et al 2006) has shown that the gradation of the aggregate component has a significant influence and a gradation fine of maximum density provides best workability and the highest modulus. A maximum 20 mm nominal mix size is recommended recommended and 14 mm is preferred. A minimum asphalt layer layer thickness five times nominal mix size is recommended to assist compaction achievement, and up to seven times is preferred to creating another another layer interface. The selection of the recommended gradation and mix size will facilitate the establishment establishment of a complete bond at interfaces. interfaces. Ideally asphalt should should be placed on a primed surface, or a primer sealed surface ( ≤7 mm) if construction construction traffic is to use the pavement. A uniform tack coat (that resists tracking) should be applied to the primer seal and to the prime if dusty.
Construction Implications Implications
Curing Cement treated base materials will require curing prior to trafficking to ensure the achievement achievement of the design strength over a period of time that may vary depending on design aims. Resin Modified Asphalt (RMA) must be cured according to the manufacturer’s directions.
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62
Construction Implications
Opening to Traffic Subject only the preceding requirements for curing and surfacing, other pavement components should should normally be able to be opened to traffic on completion. completion. It is noted that newly placed asphalt may be relatively tender in periods of hot weather and the surface will be scuffed by turning and sliding tyres (for instance tridem axle groups). This is generally superficial superficial and aesthetic damage if the asphalt asphalt is placed at the recommended layer thickness and density.
Pavement Mainten ance
Pavement Pa vement Main Maintenanc tenancee Pavement maintenance maintenance is usually related to the pavement type, design period and pavement failure mode(s). mode(s). Routine maintenance maintenance costs may be expected to increase increase towards the end of the design period, unless proactive rehabilitation treatments (i.e. major maintenance) are conducted, to extend the pavement life. Ideally, it would be decided in the planning phase, as to what would be tolerable delays to the facility operation, which subsequently subsequently may influence the pavement design.
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64
Pavement Mainten ance
Routine Routin e Main Maintenance tenance These activities are minor in nature and are influenced by the pavement design. The defects are normally due due to environmental factors. Typical examples examples of routine maintenance would include:
•
Crack sealing and pothole repair
•
Surface Drainage Repairs such as providing minimum surface slope / crossfall; provision of pits, kerb and channel, etc. as per usual stormwater drainage design; refer to port design standards
•
Subsurface Drainage Repairs Repairs such as regular inspection of outlets and pipes flushed at least annually
Pavement Mainten ance
Major Maintenance Major maintenance should not normally be required, until the pavement reaches the end of the design period, unless there are unanticipated conditions such as: •
Change in facility operating conditions (traffic)
•
Shortcomings with the construction
•
Unexpected changes in environmental conditions
•
Shortcomings in design
Container corner castings and trailer legs impart high contact stresses, which may cause localised pavement distress, potentially resulting in an unserviceable pavement condition unless maintained. Ideally, a Pavement Management System (PMS) would be implemented, involving monitoring of key pavement performance parameters, which would enable timely pavement treatments, with respect to both service level and budgetary considerations.
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66
Pavement Mainten ance
In-Service Monitoring Ideally, the pavement inspection protocol would be described in the Pavement Management Management System document. The frequency of inspections may need to increase in the latter part of the design period, to enable timely intervention to address signs of any unanticipated unanticipated pavement defects. Records of maintenance maintenance activities would be an integral part of the PMS. The surveys may be done by trained trained inspectors using manual methods based on visual condition rating and/or with the assistance of automated data collection devices such as Laser Profilometers (with video imaging if needed). For further guidance on implementation implementation / operation of of PMS refer to Haas et al (1994).
Pavement Pavement Rehabilit Rehabilit ation
Pavement Rehabil Rehabilititaation ti on
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Pavement Pavement Rehabilit Rehabilit ation
Site Investigation A thorough geotechnical geotechnical and pavement pavement investigation investigation is an essential essential component component to the design of rehabilitation rehabilitation treatments. The existing pavement pavement condition and composition including layer types and thicknesses, subgrade type and existing drainage conditions should be examined prior to rehabilitation. rehabilitat ion. The designer must be confident that the cause of the observed pavement defects is understood before proceeding with rehabilitation rehabilitation design. design. It is usual that subgrade conditions would would be investigated to a much greater depth than typically done for road pavement design. As mentioned above, above, pavement pavement deflection testing may may be a useful (vital) (vital) component, component, to identify pavement uniformity and strengthening needs.
Pavement Pavement Rehabilit Rehabilit ation
Function unct ionaal and Structura Structur al Condition Assessment Poor drainage is the most common cause of pavement distress and every pavement condition assessment must include assessment of the condition and effectiveness of the surface and subsurface subsurface drainage drainage system / requirements. requirements. If it is found to be defective it is prudent to remedy the drainage problems first (or address as part of the design, i.e. wrt wrt inished Surface Surface levels, level level control, tied into drainage drainage design design etc) and perhaps defer other assessments until the effect of the change can be measured (i.e. in the event that drainage deemed to be the problem, hence retro-fit and monitor for strength improvement, improvement, if still a serviceable serviceable pav’t condition) Deflection testing will enable the designer to assess the overall pavement strength and variability. If visual survey and deflection testing testing indicate relatively isolated isolated weakness attention to drainage and/or pavement patching may be an appropriate interim step to achieve a more consistent condition, depending on cause of defects. If the weakness is shown to be consistent throughout the site, then a design for full rehabilitation is appropriate. By the back-calculation of the Falling Weight Deflectometer (FWD) deflection bowl, it is possible to derive reasonable estimates of t he strength of the existing pavement profile (however, ideally supplemented by knowledge of pavement makeup, material properties, moisture % and and subgrade strength, to refine assumptions.) assumptions.) The back calculation process may be facilitated by a number of software packages or done manually by conducting analyses iteratively modifying layer stiffness values (which may be aided by laboratory & field testing) until a reasonable match is achieved between the calculated calculated and measured deflection deflection bowl. Once the estimate of the residual strength of the pavement layers is determined, the design of the appropriate rehabilitation treatment treatment continues as for a new construction using the derived values for the remnant pavement.
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Pavement Pavement Rehabilit Rehabilit ation
Treatm Treatment ent Types Functional Rehabilitation An asphalt overlay, overlay, in conjunction conjunction with selective patching patching is generally generally the most effective and efficient treatment to address functional deficiencies such as roughness, rutting and cracking (depending on design life, extent of defects). Often rutting is confined to shear failure within the asphalt layer as a consequence of poor mix design or selection. selection. This can be proven by cutting a trench and checking checking the profile of the base layer - if the base is sound and not deformed it may be assumed the failure is confined to the asphalt layer and it may simply be milled and replaced with more appropriate appropriate material. Core sampling and testing testing of the failed asphalt is suggested to confirm the probable cause of the deformation. The presence of isolated minor longitudinal cracking may not warrant deflection testing and routine maintenance and crack sealing would be the first treatment. If fatigue cracking is evident (with or without rutting) it is an indicator of inadequate structural capacity and deflection testing should be conducted and a rehabilitation treatment determined determined by design analyses. Armed with the deflection data data and the knowledge of the pavement composition it is possible to iteratively modify the pavement layer stiffness parameters until a reasonable match is achieved between the measured and calculated calculated deflection bowls. bowls. The designer must understand understand the back calculation process is an inexact science but may highlight where in the structure the main deficiencies deficiencies exist. These are often in the upper layers due to high stress and moisture ingress in which case the lower foundation layers may be preserved. It is prudent to supplement the design projections projections with further field sampling and material testing. The repair of asphalt damage at corner castings using asphalt is generally only a temporary fix. Consideration Consideration should be given to the use of RMA provided provided the strength of the base can be assured.
Pavement Pavement Rehabilit Rehabilit ation
Structural Rehabilitation Rehabilitation Subject to the satisfactory evaluation of the existing remnant pavement and the conduct of design analyses an asphalt overlay is generally the most expedient means of improving the bearing bearing capacity of the pavement. pavement. Obviously there will will be an attendant elevation elevation of the surface levels. If this is not tolerable, some of the existing pavement must be removed and replaced, with the associated modification of the foundation design parameters. Reprocessing Reprocessing the excavated base materials through an asphalt plant has been shown to be an effective solution. solution. Generically known known as a Bitumen Treated Base (BTB) the material can be designed and treated to achieve performance properties close to that of virgin asphalt materials. materials. BTB materials evaluation evaluation prior to the design enables the designer to input the relevant properties into the design analyses. Another option is is to in-situ stabilize (from memory, large stabilizers can readily readily pulverize 100 mm AC and blend with say 150 mm granular + bitumen and cement).
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Caveats
Caveats Pavement design outputs outputs are essentially dependent dependent on the input values. As noted in this guide, there are a number of factors, including the accuracy of input material properties and the constraints of the layered elastic model, that will influence the reliability of design predictions. predictions. The design values chosen chosen for material properties are likely to be gross simplifications of the complex and variable properties of the pavement and subgrade subgrade materials. This should flag to the designer the importance importance of empirical benchmarking and the need to revisit projects to monitor performance against predictions to aid the verification and calibration of the design assumptions. Although design design software can produce produce apparently apparently accurate solutions solutions to problems, the predictions cannot be any more reliable than the degree to which the calibrated performance relations fit the original empirical data such as full scale trafficking tests. Thus continuous long term evaluation of material, design, construction and maintenance practices is important. Care must be taken to ensure that the sophistication of the analysis method is consistent with the quality of the input data. Otherwise so many assumptions must be made about the uncertain parameters that the model predictions will be meaningless.
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Life Cycle Costing
Life Lif e Cycle ycl e Costin os tingg
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Life Cycle Costing
Ann aly A al y s i s Per Perii o d – Serv Ser v i c e Li Life In all premium pavements the owner / user incurs substantial costs as a consequence consequence of facility downtime for rehabilitation / pavement repairs or operational traffic speed reduction due to roughness plant operator health and safety Preliminary cost estimates estimates can assist the selection of an optimum design design period. The annualised construction construction cost trends downward downward as the design period increases. increases. The annualised maintenance maintenance cost – which should include the cost to the owner for facility disruption – can exceed the initial initial construction cost. Typically the annualized annualized cost for repair and rehabilitation rehabilitation may exhibit a minimum minimum value. This reflects the fact that the lighter construction for a short design period is more prone to damage by the unplanned overload or mechanical damage. As the design period projects too far into the future t he cumulative cost of regular maintenance interventions mounts. The designer must also specifically address each component of t he pavement structure when considering considering an appropriate design period. Conventional wearing wearing surface materials (asphalt, pavers) will suffer damage by container corner castings and will require regular regular cyclical repair and replacement. replacement. This is primarily mechanical mechanical and is slightly influenced influenced by structural issues. Innovative materials such such as the generic resin modified asphalt (e.g. PRS Rigiphalte™) provide longer service life under high stress conditions but will require good structural support.
Life Cycle Costing
Present resent Worth Analysis The economic impact of the pavement design and long term performance can be calculated by the port port or terminal container manager. manager. Initial construction costs costs (INC) and rehabilitation construction costs (RHC) are the costs associated to build the initial heavy duty pavement. The calculation for these two types of costs is based on material quantities and the unit cost. Material quantities come from the volume of each layered material in the designed pavement structure, and the unit cost should relate to current available prices (Tighe 2001). The future rehabilitation construction cost needs to be discounted to the present time with a discount rate “r”, as expressed in the following equation:
PWRHC = ∑ i
where:PWRHC
RHC i (1 + r)i
=
present worth of total rehabilitation costs
RHCi
=
rehabilitation cost at Year i
r
=
discount rate, specified by the user
i
=
number of years to each rehabilitation
[6]
Maintenance costs (MC) includes the yearly maintenance cost which increases at a certain rate, and scheduled one-time maintenance costs for any specified year(s). The present worth of total maintenance cost is the summation of yearly maintenance cost:
PWMC = ∑ i
where: PWMC
MC i (1 + r) i
=
present worth of total maintenance cost;
MCi
=
maintenance cost at Year i
r
=
discount rate
i
=
number of years to each maintenance.
[7]
The residual cost in a life cycle analysis refers to the salvage values and the terminal value. The salvage return percent of each layer material is specified by the designer as an input. The terminal value is determined based on the remaining serviceability of the pavement at the end of analysis period.
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Case Studi es
Case Studies Stud ies
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Case Studi es
Case Case Study Stud y 1 Loading The only vehicle used for the design was a Kalmar ESC340 (front cabin) straddle carrier with an unladen weight of 62 tonne and a tyre pressure of 0.56 MPa. The following design vehicle movements were used: •
900,000 loaded straddle movements
•
900,000 unloaded straddle movements
Table 6 gives the container weight distribution that was used. Table 6: Case Study 1: Container Weight Distribution
Container Weight Range
Containers Containers at thi s Range
(tonne)
(%)
0–5
15%
5 – 10
15%
10 – 15
10%
15 – 20
15%
20 – 25
25%
25– 30
20%
For each container weight range the heaviest container weight in the range r ange was assumed for all containers in that range.
Pavement Pa vement Model Mod el Figure 9 shows the Pavement Structure used for Case Study 1.
Case Studi es
Thickness (mm)
Modulus Modulus , E (MPa)
Poisson's Ratio
100 mm 200 mm
2800 MPa ?
0.4 0.3
As p h alt Asp Base Course
700 mm
?
0.3
Subbase Course
60 MPa (CBR=6)
0.4
Subgrade
Figure 9: Pavement Structure for Case Study 1.
????(comment ????(comment about fatigue properties, vb=11% for asphalt, Wardle et. al 2001 for subgrade.) Barker-Brabston for Base and subbase.)
Results Table 7 summarizes the maximum CDF for each layer.
Table 7: Case Case Study 1: Results Summary Summary
Figure 10 is the Asphalt Damage Damage Factor "profile" across the pavement. pavement. Note that X = 0 corresponds to the centreline of each vehicle.
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Case Studi es
Figure 10: Asphalt Damage Factor vs. lateral offset. Figure 11 is the Subgrade Damage Factor profile across the pavement.
Figure 11 Subgrade Damage Factor vs. lateral offset. Figure 12 is the Spectral Damage Graph showing the Asphalt Damage Factor contribution from each container load.
Case Studi es
Figure 12: Asphalt Damage Factor vs. container load. Figure 13 is the Spectral Damage Graph showing the Subgrade Damage Factor contribution from each container load
Figure 13: Subgrade Damage Factor vs. container load. It is interesting to compare the Spectral Damage Graphs for the asphalt and subgrade layers (Figure 12 and Figure 13). For the subgrade (Figure 13), the t he greatest damage contribution is due to the heaviest container weight weight (30 tonne). For the asphalt layer (Figure 12), the greatest greatest damage contribution is due to the unladen machines.
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Append Ap pend ices ic es
App p end A en d i c es
85
86
Appendices
Material Material failure fail ure mode mo de and and implication In selecting the composition of the pavement the mode of failure of the candidate materials needs to be carefully carefully considered. For instance well designed designed and constructed unbound granular granular layers deform slowly as a result of the applied stress but generally still retain the original original layer stiffness. Indeed evidence shows shows that provided the granular materials are not overstressed the layer stiffness will increase with time i.e. the structural strength of the foundation foundation is retained or enhanced. enhanced. Over a similar period the condition of the wearing surface is likely to deteriorate and deform and it is likely that resurfacing will will be required. In the likely event that the structural strength of the foundation is retained the resurfacing activity will ‘reset’ both the functional and structural requirements. If the strength of the pavement foundation relies relies on the structural contribution of bound materials repeated applications applications of load stress result in the deterioration of both the structural and functional requirements, and resurfacing alone will not reset the structural requirement. requirement. In this case an assessment assessment of the integrity integrity of the key structural layer is required with careful evaluation and design analysis.
Append Ap pend ices ic es
Improved Improv ed asph asphalt alt material material characterisation The following is extracted from the NCHRP report 465. The Superpave volumetric mix design procedure developed in the Asphalt Research Program (1987–1993) (1987–1993) of the Strategic Highway Research Program (SHRP) does not include a simple, mechanical “proof” test analogous to the Marshall stability and flow tests or the Hveem stabilometer method. Instead, the original Superpave method relied on strict conformance to the t he material specifications and volumetric mix criteria to ensure satisfactory performance of mix designs intended for low-traffic-volume low-traffic-volume 6 situations (defined as no more than 10 equivalent single axle loads [ESALs] applied over the service life of the pavement). For higher trafficked projects, the original SHRP Superpave mix analysis procedures required a check for tertiary creep behaviour with the repeated shear at constant stress ratio test (AASHTO TP7) and a rigorous evaluation of the mix design’s potential for permanent deformation, deformation, fatigue cracking, and low-temperature cracking using several other complex test methods in AASHTO TP7 and TP9. TP9. User experience with the Superpave mix design and analysis method, combined with the long-standing problems associated with the original SHRP Superpave performance models supporting supporting what was then termed “Level 2 and 3” analyses, demonstrated the need for such simple performance tests (SPTs). In 1996, work sponsored by FHWA began at the University of Maryland at College Park (UMCP) to identify and validate SPTs for permanent deformation, deformation, fatigue cracking, and lowtemperature cracking to complement and support the Superpave volumetric mix design method. In 1999, this effort was transferred to Task C of NCHRP Project 9-19, “Superpave Support and Performance Models Management,” with the major portion of the task conducted by a research team headed by UMCP subcontractor Arizona State University (ASU). The research team was directed to evaluate as potential SPTs only existing test Methods measuring hot mix asphalt (HMA) response characteristics. The principal evaluation criteria were (1) accuracy (i.e., good correlation of the HMA-response characteristic to actual field performance); (2) reliability (i.e., a minimum number of false negatives and positives); (3) ease of use; and (4) reasonable equipment cost. The research team conducted a comprehensive laboratory testing program to statistically correlate the actual performance of HMA materials from t he MnRoad, Wes-Track, and FHWA Accelerated Loading Facility (ALF) experiments with the measured responses of specimens prepared from original materials for 33 promising test method–test parameter combinations.
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88
Appendices
Based on the results of this testing program, the research team recommends three test-parameter combinations combinations for further f ield validation as an SPT for permanent deformation: (1) the dynamic modulus term, E*/sin φ, (determined from the triaxial t riaxial dynamic modulus test; (2) the flow time, Ft, determined from the triaxial static creep test; and (3) the flow number, Fn, determined from the triaxial repeated load test. All combinations combinations exhibit a coefficient of determination, R 2, of 0.9 or greater for the combined correlation of the laboratory test results with performance in the MnRoad, Wes-Track, and FHWA ALF experiments. For fatigue cracking, the experimental results are far less conclusive. The research team recommends the dynamic modulus, E*, measured at low test temperatures; the modulus offers a fair correlation with field performance data and provides some consistency with one of the tests recommended for permanent deformation. For low temperature cracking, the team recommends the creep compliance measured by the indirect tensile creep test at long loading times and low temperatures; this recommendation recommendation is based solely on work carried out for SHRP and C-SHRP and recently confirmed in NCHRP Project 1-37A, “Development of the 2002 Guide for the Design of New and Rehabilitated Pavement Structures.” The NCHRP report 465 includes a detailed description of the experimental program, a discussion of the research results and the basis for selection of t he candidate SPTs, a description of the future field validation effort, and five supporting appendixes presenting presenting test methods for the candidate SPTs: In Australian practice the dynamic modulus E* master curve is developed from testing at 4 temperatures (5o; 20o;35o & 50oC) and 6 load frequencies (0.1; 0.5; 1; 5; 10 & 25 Hz) using time temperature superposition superposition principles. principles. From this testing the response to load performance of candidate asphalt materials can be measured over the extremes of temperature and load load duration. This data is then able to be used in HIPAVE to calculate damage over the full temperature spectrum. The dynamic modulus master curve clearly distinguishes the benefits of modified binders by quantifying the improvement in stiffness and elastic response at high temperature and/or and/or slow loading conditions. conditions. This is particularly advantageous advantageous because historical modulus measurements at a single temperature (typically 20 o or 25oC) often fail to discriminate between conventional and modified binders. Further research into the effect of confinement confinement in the field is needed. Intuitively the significant increase in dynamic modulus and elasticity (reduction in phase angle) observed in the triaxial cell cell with confining pressure is likely likely in the field. Early work by Marchionna et al supports this intuition by the observation that deflections on thick asphalt pavement structures did not appear to increase with temperature. In applications in t he industrial pavement environment the deformation relationships between Dynamic modulus (E*) and elasticity (Sine phase angle) will require calibration. In the interim the empirical evidence evidence suggest adhering adhering to the fundamentals will yield good performance i.e. using all crushed aggregate; dense gradation; hard binder grades in hot environs; binder content optimization at appropriate laboratory laboratory compaction effort. effort. Wheel-track testing may provide provide a reasonable ranking of deformation resistance in the laboratory.
Append Ap pend ices ic es
Fatigue testing is routinely carried out in Australia (4 point flexure) and serves to rank the performance of different mix gradations gradations and binder binder types. At this stage of development development we tend to use the laboratory fatigue test more to verify the predictive fatigue models developed developed by Shell and implemented implemented by Austroads. Austroads. As more performance evidence is gained the apparently conservative predictive models will be recalibrated. Of value is the use of the fatigue test to develop appropriate damage models for innovative materials. materials. Figure 14 below compares compares the fatigue performance of conventional conventional asphalt against the resin modified asphalt PRS Rigiphalte. Figure 14: Fatigue performance performance of conventional asphalt asphalt against the resin modified asphalt PRS Rigiphalte. Compariso n of fatigu e properties Rigiph alte and and AC14 C320 C320 o
Constant strain; 20 C; 10 Hz 1000
) n i a r t s o r c i m ( n 100 i a r t s e l i s n e T
Typical Typical AC14 C320 k = 3050; b = 5 Rigiphalte lab data k = 390; b = 11.1 Rigiphalte design k = 300; b = 10
10 1E+04
1E+05
1E+06
1E+07
1E+08
Cycles to failure fai lure
The Dynamic Shear Rheometer (DSR) is another laboratory tool to enhance the selection of the best bitumen and filler filler combination to enhance enhance mix properties. In common with the SPT the t he DSR provides the material characterisation over the full combination of temperature temperature and loading frequency. frequency. The DSR can test bitumen and the bitumen filler mastic to develop complex shear modulus master curves, and to measure the elastic and viscous viscous component of the binder. binder. These latter parameters are considered to be significant in both fatigue and deformation resistance potential. In application available binders and fillers would first be characterised and then the binder exhibiting the most potential would be incorporated in asphalt samples t o determine the (more arduous) dynamic modulus master curve evaluation.
89
References
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