Lec 16 Highway Engineering - Flexible Pavemen Design

Highway Eng.

Design of Flexible Pavements

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Structural Design of Flexible Pavements In this lecture; A- Types of Pavements B- Design of HMA Pavements C- AASHTO 1993 Method. 1- Loading 2- Materials & Soil 3- Enviroment Information listed in this lecture is mainly taken from Traffic and Highway Engineering (Garber, 2009), Asphalt Pavements (Lavin, 2003),Pavement Analysis and Design(Huang, 2004), http://www.pavementinteractive.org (Accessed on 2015) and Highways (O’Flaherty, 2007).

A- Types of Pavements Generally, hard surfaced pavements are typically categorized into flexible and rigid pavements: Flexible pavements. Those which are surfaced with bituminous (or asphalt) materials. These types of pavements are called "flexible" since the total pavement structure "bends" or "deflects" due to traffic loads. A flexible pavement structure is generally composed of several layers of materials which can accommodate this "flexing". Flexible pavements comprise about 94 percent of U.S. paved roads.

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Rigid pavements. Those which are surfaced with portland cement concrete (PCC). These types of pavements are called "rigid" because they are much stiffer than flexible pavements due to PCC's high stiffness. Rigid pavements comprise 6 percent of U.S. paved roads.

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B- Structural Design of HMA Pavements As shown above, the flexible pavement structure is typically composed of several layers of material each of which receives the loads from the above layer, spreads them out, then passes them on to the layer below. Thus, the further down in the pavement structure a particular layer is, the less load (in terms of force per area) it must carry (see Figure in P. 195). B-1 Basic Structural Elements Material layers are usually arranged within a pavement structure in order of descending load bearing capacity with the highest load bearing capacity material (and most expensive) on the top and the lowest load bearing capacity material (and least expensive) on the bottom. A typical flexible pavement structure (Figure 2) consists of: Lecture 16

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Surface Course. The layer in contact with traffic loads. It provides characteristics such as friction, smoothness, noise control, rut resistance and drainage. In addition, it

prevents

entrance

of

surface

water

into

the

underlying base, sub

base and subgrade . This top structural layer of material is sometimes subdivided into two layers: the wearing course (top) and binder course (bottom). Surface courses are most often constructed from hot-mix asphalt HMA. Base Course. The layer immediately beneath the surface course. It provides additional load distribution and contributes to drainage. Base courses are usually constructed out of crushed aggregate or HMA (stabilised). Subbase Course. The layer between the base course and subgrade. It functions primarily as structural support but it can also minimize the intrusion of fines from the subgrade into the pavement structure and improve drainage. The subbase generally consists of lower quality materials than the base course but better than the subgrade soils. A subbase course is not always needed or used. Subbase courses are generally constructed out of crushed aggregate or suitable fill.

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B-1 Methods of Design The goal of structural design is to determine the number, material composition and thickness of the different layers within a pavement structure required to accommodate a given loading regime ‫نظام‬. This includes the surface course as well as any underlying base or subbase layers. Calculations are chiefly concerned with traffic loading stresses. The principal methods of structural design in use today are (from simplest to most complex) design catalogs, empirical and mechanistic-empirical. Design Catalogs The simplest approach to HMA pavement structural design involves selecting a predetermined design from a catalog. Typically, design catalogs contain a listing of common loading, environmental and service regimes and the corresponding recommended pavement structures. State and local agencies often include them in their design manuals. Empirical Design Many pavement structural design procedures use an empirical approach. This means that the relationships between design inputs (e.g., loads, materials, layer configurations and environment) and pavement failure were determined using experience, experimentation or a combination of both. Although the scientific basis for these relationships is not firmly established, they can be used with confidence as long as the limitations with such an approach are recognized. Specifically, it is not wise to use an empirically derived relationship to describe phenomena that occur outside the range of the original data used to develop the relationship. Examples of these methods is 1993 AASHTO method.

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Mechanistic-Empirical Design The most advanced pavement structural design uses a mechanistic-empirical approach. Unlike an empirical approach, a mechanistic approach seeks to explain phenomena only by reference to physical causes. In pavement design, the phenomena are the stresses, strains and deflections within a pavement structure, and the physical causes are the loads and material properties of the pavement structure. The relationship between these phenomena and their physical causes is typically described using various mathematical models. AASHTO has a full procedure and software for conducting mechanistic-empirical pavement design.

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C- 1993 AASHTO Empirical Design Method for Flexible Pavements The 1993 AASHTO Guide for the Design of Pavement Structures is the basis for the AASHTO method of flexible pavement design. Design Considerations The factors considered in the AASHTO procedure for the design of flexible pavement as presented in the 1993 guide are: • Pavement performance • Traffic • Roadbed soils (subgrade material) • Materials of construction • Environment • Drainage • Reliability Pavement Performance. The primary factors considered under pavement performance are the structural and functional performance of the pavement. Structural performance is related to the physical condition of the pavement with respect to factors that have a negative impact on the capability of the pavement to carry the traffic load. These factors include cracking, faulting, raveling, and so forth. Functional performance is an indication of how effectively the pavement serves the user. The main factor considered under functional performance is riding comfort. To quantify pavement performance, a concept known as the serviceability performance was developed. Under this concept, a procedure was developed to Lecture 16

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determine the present serviceability index (PSI) of the pavement, based on its roughness and distress. The scale of PSI ranges from 0 to 5, where 0 is the lowest PSI and 5 is the highest. Two serviceability indices are used in the design procedure: the initial serviceability index (pi), which is the serviceability index immediately after the construction of the pavement; and the terminal serviceability index (pt), which is the minimum acceptable value before resurfacing or reconstruction is necessary. Recommended values for the terminal serviceability index are 2.5 or 3.0 for major highways and 2.0 for highways with a lower classification. ΔPSI = Po - Pt

Traffic Load. In the AASHTO design method, the traffic load is determined in terms of the number of repetitions of an 18,000-lb (80 kilonewtons (kN)) single-axle load applied to the pavement on two sets of dual tires. This is usually referred to as the equivalent single-axle load (ESAL).

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The equivalence factors used in this case are based on the terminal serviceability index to be used in the design and the structural number (SN) (see definition of SN in Page ). The Tables (1a & 1b) below give traffic equivalence factors for pt of 2.5 for single and tandem axles respectively.

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To determine the ESAL, the number of different types of vehicles such as cars, buses, single-unit trucks, and multiple-unit trucks expected to use the facility during its lifetime must be known. These can then be converted to equivalent 18,000-lb loads using the equivalency factors given in the two Tables above. The total ESAL applied on the highway during its design period can be determined only after the design period and traffic growth factors are known. The design period is the number of years the pavement will effectively continue to carry the traffic load without requiring an overlay. Flexible highway pavements are usually designed for a 20-year period. Since traffic volume does not remain constant over the design period of the pavement, it is essential that the rate of traffic growth be determined and applied when calculating the total ESAL. Annual growth rates can be obtained from regional planning agencies or from state highway departments. These usually are based on traffic volume counts over several years. The overall growth rate in the United States is between 3 and 10 percent per year. The growth factors (Grn) for different growth rates and design periods can be obtained from Equation below:

where r = i / 100 and is not zero. If annual growth is zero, growth factor = design period. i = growth rate. n = design life, yrs. The Table below shows calculated growth factors (Grn) for different growth rates (r) and design periods (n) which can be used to determine the total ESAL over the design period.

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The portion of the total ESAL acting on the design lane (fd) is used in the determination of pavement thickness. Either lane of a two-lane highway can be considered as the design lane whereas for multilane highways, the outside lane is considered. The identification of the design lane is important because in some cases more trucks will travel in one direction than in the other or trucks may travel heavily loaded in one direction and empty in the other direction. Thus, it is necessary to determine the relevant proportion of trucks on the design lane. A general equation for the accumulated ESAL for each category of axle load is obtained as

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Materials of Construction. The materials used for construction can be classified under three general groups: those used for subbase construction (a3), those used for base construction (a2), and those used for surface construction (a1). The quality of the material used is determined in terms of the layer coefficient, a3, a2 and a1, which are used to convert the actual thickness of the subbase, base and surface to an equivalent SN respectively. Figures below used to find layer coefficients.

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Reliability: The AASHTO guide incorporates in the design a reliability factor R% to account for uncertainties in traffic prediction and pavement performance. R% indicates the probability that the pavement designed will not reach the terminal serviceability level before the end of the design period.

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Drainage requirements: Criteria on ability of various drainage methods to remove moisture from the pavement are depend on engineer and the drainage quality which depend on the time that water removed from pavement granular materials within. Drainage quality effect represent in pavement thickness by sample of m. taken from AASHTO recommendation that depend on the selected quality of drainage and percent of time pavement structure is exposed to moisture level approaching to saturated during a year.

Thickness Requirements: Using the input parameters described in the preceding sections, the total pavement thickness requirement is obtained from the monograph in terms of structural number SN. SN is an index number equal to the weighted sum of pavement layer thicknesses, as follows:

SN= a1D1 + a2D2m2 + a3D3m3 Where: a1, a2, and a3 are numbers known as layer coefficients can be find from layer modules(Mr) by using AASHTO specific charts or default formula; Lecture 16

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D1, D2, and D3 are layer thicknesses; and m2 and m3 are layer drainage coefficients used for granular layers. The values of D1, D2 and D3 have to meet certain minimum practical thicknesses as shown inTable 2.

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R-value : soil resistance value Chart below for base courses.

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Structural No. (SN): An index number derived from an analysis of traffic, roadbed, soil conditions, and reliability. That may be converted to thickness of several of flexible pavement layers: SN1 ≤ a1 D1

therefore D1min ≥ SN1/ a1

SN2 ≤ a1 D1 + a2 D2 m2

therefore D2min ≥ (SN2 - a1 D1)/a2m2

SN3≤ a1 D1 + a2 D2 m2 + a3 D3 m3

therefore D3min ≥ (SN3 - a1 D1 - a2 D2 m2) /a3m3

In other words: SN3 = a1 D1 + a2 D2 m2 + a3 D3 m3 In general, a1 taken as 0.44 for plant asphalt mix high stability, a2 taken as 0.14 for crushed stone base course, a3 taken as 0.11 for sandy gravel subbase course. AASHTO asphalt pavement design procedure In the AASHTO design procedure for asphalt pavements, the basic design equation (or design chart) and the structural number SN are the key focus of the procedure. The following steps summarize the procedure: 1 Determine the required reliability R% and overall standard deviation So for the pavement. 2 Determine the total accumulated ESALs (w18) for the design life of the pavement and annual growth rate. 3 Determine the subgrade soil resilient modulus, MR. 4 Determine the design serviceability loss, PSI. 5 Using the four values selected above and the AASHTO design nomograph (chart), determine the required structural number (SN) for the asphalt pavement. 6 The selected structural number and SN equation and its required values are then computed to determine the thickness of each layer. Lecture 16

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Example: Design the pavement for an expressway consisting of an asphalt concrete surface, a crushed-stone base,and a granular subbase using the 1993 AASHTO design chart. The cumulative ESAL in thedesign lane for a design period of 15 years is (7*106). The area has good quality drainage with 10% of thetime the moisture level is approaching saturation. The effective roadbed soil resilient modulus is 7 ksi, the subbase has a CBR value of 80, the resilient modulus of the base is 40 Lb, and the resilient modulus ofasphalt concrete is 4.5 * 105 psi. Assume a reliability level = 95%, So = 0.45, Po = 4.6 and Pt = 3.0. Solution Step 1: Reliability (R) = 95% and overall standard deviation (So)= 0:45 (Given) Step 2: Step 3: W18= 7 * 106 (Given) Step 3: Effective road-bed soil resilient modulus = 7 ksi (Given); Resilient modulus of subbase = 20 ksi (Figure); Resilient modulus of base = 40 ksi (Given) and Resilient modulus of asphalt concrete surface = 450 ksi (Given) Step 4: ∆PSI = Po – Pt = 4.6 - 3.0 = 1.6 Step 5: SN3 = 5.2 ( design chart; subgrade MR of 7 ksi) SN2= 3.5 (design chart; subbase MR of 20 ksi) SN1= 2.7 (design chart; base MR of 40 ksi) Step 6: a3 = 0.14 (Figure); a2= 0.17 (Figure); a1= 0.44 (Figure) ; Drainage coefficients = m2= m3= 1.1 (Table) SN Equation ---- > SN1 = a1 D1 ------ > 2.7 = 0.44 D1 D1=6.1 in. (Round to 6.5 in.) SN Equation ---- > SN2 = a1 D1 + a2 D2 m2 ----- > 3.5 = 0.44 * 6.5 + 0.17 *D2 * 1.1 Lecture 16

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D2=3.4 in. (Use a minimum value of 6 in.) (Table) SN Equation --- > SN3 = a1 D1 + a2 D2 m2 + a3 D3 m3 ---- > 5.2 = 0.44 * 6.5 + 0.17 * 6 * 1.1+ 0.14 * D3 * 1.1 D3=7.9 in. (Round to 8 in.) Hence. For design use ----- > D1=6.5 in.; D2=6 in. and D3=8 in. ===========================================

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Lec 16 Highway Engineering - Flexible Pavemen Design

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