AASHTO Design Method

Fig. Typical cross section of a Flexible Pavement

Subgrade (Prepared Road Bed)The subgrade is usually the natural material located along the horizontal alignment of the pavement and serves as the foundation of the pavement structure. It also may consist of a layer of selected borrow materials, well compacted to prescribed specifications. It may be necessary to treat the subgrade material to achieve certain strength properties required for the type of pavement being constructed. Subbase CourseLocated immediately above the subgrade, the subbase component consists of material of a superior quality to that which is generally used for subgrade construction. The requirements for subbase materials usually are given in terms of the gradation, plastic characteristics, and strength. When the quality of the subgrade material meets the requirements of the subbase material, the subbase component may be omitted. In cases where suitable subbase material is not readily available, the available material can be treated with other materials to achieve the necessary properties. This process of treating soils to improve their engineering properties is known as stabilization.

Base CourseThe base course lies immediately above the subbase. It is placed immediately above the subgrade if a subbase course is not used. This course usually consists of granular materials such as crushed stone, crushed or uncrushed slag, crushed or uncrushed gravel, and sand. The specifications for base course materials usually include more strict requirements than those for subbase materials, particularly with respect to their plasticity, gradation, and strength. Materials that do not have the required properties can be used as base materials if they are properly stabilized with Portland cement, asphalt, or lime. In some cases, high-quality base course materials also may be treated with asphalt or Portland cement to improve the stiffness characteristics of heavy-duty pavements.

Surface CourseThe surface course is the upper course of the road pavement and is constructed immediately above the base course. The surface course in flexible pavements usually consists of a mixture of mineral aggregates and asphalt. It should be capable of withstanding high tire pressures, resisting abrasive forces due to traffic, providing a skid resistant driving surface, and preventing the penetration of surface water into the underlying layers. The thickness of the wearing surface can vary from 3 in. to more than 6 in., depending on the expected traffic on the pavement. The quality of the surface course of a flexible pavement depends on the mix design of the asphalt concrete used.

Stress Distribution within a Flexible Pavement

SOIL STABILIZATIONSoil stabilization is the treatment of natural soil to improve its engineering properties. Soil stabilization methods can be divided into two categories, namely, mechanical and chemical. Mechanical stabilization is the blending of different grades of soils to obtain a required grade. Chemical stabilization is the blending of the natural soil with chemical agents. Several blending agents have been used to obtain different effects. The most commonly used agents are Portland cement, asphalt binders, and lime.

Cement StabilizationCement stabilization of soils usually involves the addition of 5 to 14 percent Portland cement by volume of the compacted mixture to the soil being stabilized. This type of stabilizare tbe used as base course materials. Although the best results have been obtained when well-graded granular materials were stabilized with cement, the Portland Cement Association has indicated that nearly all types of soil can be stabilized with cement. The procedure for stabilizing soils with cement involves:• Pulverizing the soil• Mixing the required quantity of cement with the pulverized soil• Compacting the soil cement mixture• Curing the compacted layeration is used mainly to obtain the required engineering properties of soils that

Asphalt StabilizationAsphalt stabilization is carried out to achieve one or both of the following:• Waterproofing of natural materials• Binding of natural materialsWaterproofing the natural material through asphalt stabilization aids in maintaining the water content at a required level by providing a membrane that impedes the penetration of water, thereby reducing the effect of any surface water that may enter the soil when it is used as a base course. In addition, surface water is prevented from seeping into the subgrade, which protects the subgrade from failing due to increase in moisture content.Binding improves the durability characteristics of the natural soil by providing an adhesive characteristic, whereby the soil particles adhere to each other, increasing cohesion.Several types of soil can be stabilized with asphalt, although it is generally required that less than 25 percent of the material passes the No. 200 sieve. This is necessary because the smaller soil particles tend to have extremely large surface areas per unit volume and require a large amount of bituminous material for the soil surfaces to be adequately coated. It is also necessary to use soils that have a plasticity index (PI) of less than 10, because difficulty may be encountered in mixing soils with a high PI, which may result in the plastic fines swelling on contact with water and thereby losing strength.

Prime CoatsPrime coats are obtained by spraying asphalt binder materials onto non-asphalt base courses. Prime coats are used mainly to: • Provide a waterproof surface on the base• Fill capillary voids in the base• Facilitate the bonding of loose mineral particles• Facilitate the adhesion of the surface treatment to the baseMedium-curing cutbacks normally are used for prime coating with MC-30 recommended for priming a dense flexible base and MC-70 for more granular-type base materials. The rate of spray is usually between 0.2 and 0.35 gal/yd2 for the MC-30 and between 0.3 and 0.6 gal/yd2 for the MC-70. The amount of asphalt binder used, however, should be the maximum that can be absorbed completely by the base within 24 hours of application under favorable weather conditions. The base course must contain a nominal amount of water to facilitate the penetration of the asphalt materialinto the base. It is therefore necessary to lightly spray the surface of the base course with water just before the application of the prime coat if its surface has become dry and dusty.

Tack CoatsA tack coat is a thin layer of asphalt material sprayed over an old pavement to facilitate the bonding of the old pavement and a new course which is to be placed over the old pavement. In this case, the rate of application of the asphalt material should be limited, since none of this material is expected to penetrate the old pavement. Asphalt emulsions such as SS-1, SS-1H, CSS-1, and CSS-1H normally are used for tack coats after they have been thinned with an equal amount of water. The rate of application varies from 0.05 to 0.15 gal/yd2 of the thinned material. Rapid-curing cutback asphalts such as RC-70 also may be used as tack coats.

SUPERPAVE SYSTEMS This system is known as Superpave, which is a shortened form for superior performing asphalt pavements. The research leading to the development of this new system was initiated because prior to Superpave, it was difficult to relate the results obtained from laboratory analysis in the existing systems to the performance of the pavement without field experience. Selection of materials includes the selection of asphalt binder and suitable mineral aggregates.Selection of Asphalt Binder. The binder selected for a given project is based on the range of temperatures through which the pavement will be exposed and the traffic to be carried during its lifetime. The binders are classified with respect to the range of temperatures at which their physical property requirements must be met. For example, a binder that is classified as PG52-34 must satisfy all the high-temperature physical property requirements at temperatures up to at least 52oC and all the low temperature physical requirements down to at least 34oC. The high-pavement design temperature is defined at a depth of 20 mm below the pavement surface and the low pavement design temperature at the surface of the pavement.Selection of Mineral Aggregate. The aggregate properties include the angularity of the coarse aggregates, the angularity of the fine aggregates, the amount of flat and elongated particles in the coarse aggregates, and the clay content.

Empirical equations are used to relate observed or measurable phenomena (pavement characteristics) with outcomes (pavement performance).  This article presents the 1993 AASHTO Guide basic design equation for flexible pavements.  This empirical equation is widely used and has the following form:

(these variables will be further explained in the Inputs section)

where: W18 = predicted number of 80 kN (18,000 lb.) ESALs

ZR = standard normal deviate

So = combined standard error of the traffic prediction and

performance prediction

SN = Structural Number (an index that is indicative of the total pavement thickness required)

= a1D1 + a2D2m2 + a3D3m3+…

ai = ithlayer coefficient Di = ithlayer thickness (inches)

mi = ith layer drainage coe fficient

ΔPSI = difference between the initial design serviceability index, po, and the design terminal serviceability index, pt

MR = subgrade resilient modulus (in psi)

This equation is not the only empirical equation available but it does give a good sense of what an empirical equation looks like, what factors it considers and how empirical observations are incorporated into an empirical equation

InputsThe 1993 AASHTO Guide equation requires a number of inputs related to loads, pavement structure and subgrade support.  These inputs are:

The predicted loading .  The predicted loading is simply the predicted number of 80 kN (18,000 lb.) ESALs that the pavement will experience over its design lifetime.

Reliability .  The reliability of the pavement design-performance process is the probability that a pavement section designed using the process will perform satisfactorily over the traffic and environmental conditions for the design period .  In other words, there must be some assurance that a pavement will perform as intended given variability in such things as construction, environment and materials.  The ZR and So variables account for reliability.

Pavement structure.  The pavement structure is characterized by the Structural Number (SN).  The Structural Number is an abstract number expressing the structural strength of a pavement required for given combinations of soil support (MR), total traffic expressed in ESALs, terminal serviceability and environment.  The Structural Number is converted to actual layer thicknesses (e.g., 150 mm (6 inches) of HMA) using a layer coefficient (a) that represents the relative strength of the construction materials in that layer.  Additionally, all layers below the HMA layer are assigned a drainage coefficient (m) that represents the relative loss of strength in a layer due to its drainage characteristics and the total time it is exposed to near-saturation moisture conditions.  Generally, quick-draining layers that almost never become saturated can have coefficients as high as 1.4 while slow-draining layers that are often saturated can have drainage coefficients as low as 0.40.  Keep in mind that a drainage coefficient is basically a way of making a specific layer thicker.  If a fundamental drainage problem is suspected, thicker layers may only be of marginal benefit – a better solution is to address the actual drainage problem by using very dense layers (to minimize water infiltration) or designing a

drainage system.  Because of the peril associated with its use, often times the drainage coefficient is neglected (i.e., set as m = 1.0).

Serviceable life.  The difference in present serviceability index (PSI) between construction and end-of-life is the serviceability life.  The equation compares this to default values of 4.2 for the immediately-after-construction value and 1.5 for end-of-life (terminal serviceability).  Typical values used now are:

Post-construction: 4.0 – 5.0 depending upon construction quality, smoothness, etc. End-of-life (called “terminal serviceability”): 1.5 – 3.0 depending upon road use (e.g., interstate highway, urban arterial, residential)

Subgrade support.  Subgrade support is characterized by the subgrade’s resilient modulus(MR).  Intuitively, the amount of structural support offered by the subgrade should be a large factor in determining the required pavement structure.

OutputsThe 1993 AASHTO Guide equation can be solved for any one of the variables as long as all the others are supplied.  Typically, the output is either total ESALs or the required Structural Number (or the associated pavement layer depths).  To be most accurate, the flexible pavement equation described in this chapter should be solved simultaneously with the flexible pavement ESAL equation.  This solution method is an iterative process that solves for ESALs in both equations by varying the Structural Number.  It is iterative because the Structural Number (SN) has two key influences:

1. The Structural Number determines the total number of ESALs that a particular pavement can support. This is evident in the flexible pavement design equation presented in this section.

2. The Structural Number also determines what the 80 kN (18,000 lb.) ESAL is for a given load.

Therefore, the Structural Number is required to determine the number of ESALs to design for before the pavement is ever designed.  The iterative design process usually proceeds as follows:

1. Determine and gather flexible pavement design inputs (ZR, So, ΔPSI and MR).2. Determine and gather flexible pavement ESAL equation inputs (Lx, L2x, G).3. Assume a Structural Number (SN).4. Determine the equivalency factor for each load type by solving the ESAL equation using the

assumed SN for each load type.5. Estimate the traffic count for each load type for the entire design life of the pavement and multiply

it by the calculated ESAL to obtain the total number of ESALs expected over the design life of the pavement.

6. Insert the assumed SN into the design equation and calculate the total number of ESALs that the pavement will support over its design life.

7. Compare the ESAL values in #5 and #6. If they are reasonably close (say within 5 percent) use the assumed SN. If they are not reasonably close, assume a different SN, go to step #4 and repeat the process.

In practice, the flexible pavement design equation is usually solved independently of the ESAL equation by using an ESAL value that is assumed independent of structural number.  Although this assumption is not true, pavement structure depths calculated using it are reasonably accurate.  This design process usually proceeds as follows:

1. Assume a structural number (SN) for ESAL calculations.  Although often not overtly stated, a structural number must be assumed in order to calculate ESALs.

2. Determine the load equivalency factor (LEF) for each load type by solving the ESAL equation using the assumed SN for each load type. Typically, a standard set of load types is used (e.g., single unit trucks, tractor-trailer trucks and buses).

3. Estimate the traffic count for each load type for the entire design life of the pavement and multiply it by the calculated LEF to obtain the total number of ESALs expected over the design life of the pavement.

4. Determine and gather flexible pavement design inputs (ZR, So, ΔPSI and MR).5. Solve the design equation for SN.6. Check to see that the computed SN value is reasonably close to that assumed for ESAL

calculations.  This step of often neglected.

Design ConsiderationsThe 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

A general equation for the accumulated ESAL for each category of axle load is obtained asESALi= fd x Grn x AADTi x 365 x Ni x FEi whereESALi= equivalent accumulated 18,000-lb (80 kN) single-axle load for the axle category ifd = design lane factorGrn = growth factor for a given growth rate r and design period nAADTi= first year annual average daily traffic for axle category iNi = number of axles on each vehicle in category iFEi = load equivalency factor for axle category iThe growth factors (Grn) for different growth rates and design periods can be obtained as: Grn = [(1 + r)n –1]/r

r = i / 100 and is not zero. If annual growth is zero, growth factor _ design periodi = growth raten = design life, years

The equivalent 18,000-lb load can also be determined from the vehicle type, if the axle load is unknown, by using a truck factor for that vehicle type. The truck factor is defined as the number of 18,000-lb single-load applications caused by a single passage of a vehicle. These have been determined for each class of vehicle from the expression : Σ (number of axles x load equivalency factor)Truck factor = ----------------------------------------------------------- number of vehicles

Example :Computing Accumulated Equivalent Single-Axle Load for a Proposed Eight-LaneHighway Using Load Equivalency Factors. An eight-lane divided highway is to be constructed on a new alignment. Traffic volume forecasts indicate that the average annual daily traffic (AADT) in both directions during the first year of operation will be 12,000 vpd with the following vehicle mix and axle loads.Passenger cars (1000 lb/axle) = 50%2-axle single-unit trucks (6000 lb/axle) = 33%3-axle single-unit trucks (10,000 lb/axle) = 17%The vehicle mix is expected to remain the same throughout the design life of the pavement. If the expected annual traffic growth rate is 4% for all vehicles, determine the design ESAL, given a design period of 20 years. The percent of traffic on the design lane is 45%, and the pavement has a terminal serviceability index (pt) of 2.5 and SN of 5.The following data apply:Growth factor = 29.78 Percent truck volume on design lane = 45Load equivalency factors Passenger cars (1000 lb/axle) = 0.00002 (negligible)2-axle single-unit trucks (6000 lb/axle) = 0.0103-axle single-unit trucks (10,000 lb/axle) = 0.0877Solution: The ESAL for each class of vehicle is computed from Eq.:ESAL = fd x Gjt x AADT x 365 x Ni x FEi

3-axle single-unit trucks = 0.45 x 29.78 x 12,000 x 0.17 x 365 x 3 x 0.0877 = 2.6343 x 106

2-axle single-unit trucks = 0.45 x 29.78 x 12,000 x 0.33 x 365 x 2 x 0.010 = 0.3874 x 106

Total ESAL = 3.0217 x 106

It can be seen that the contribution of passenger cars to the ESAL is negligible. Passengercars are therefore omitted when computing ESAL values. This example illustratesthe conversion of axle loads to ESAL using axle load equivalency factors.

Quality of Drainage Water Removed WithinExcellent 2 hoursGood 1 dayFair 1 weekPoor 1 monthVery poor (water will not drain)

The reliability factor FR is given as:

log10FR = - ZRSo

So = estimated overall standard deviation( 0.4-0.5)ZR = standard normal variate for a given reliability (R%)

Note : Mr (lb/in2) = 1500 CBR (for fine-grain soils with soaked CBR of 10 or less)

Variation in Granular Subbase Layer Coefficient, a3, with Various Subbase Strength Parameters

Variation in Granular Base Layer Coefficient, a2, with Various Subbase StrengthParameters

Prob. The predicted traffic mix of a proposed four-lane urban non-interstate freeway:Passenger cars= 69%Single-unit trucks:2-axle, 5000 lb/axle = 20%2-axle, 9000 lb/axle = 5%3-axle or more 23,000 lb/axle = 4%Tractor semitrailers and combinations:3-axle 20,000 lb/axle= 2%The projected AADT during the first year of operation is 3500 (both directions). Determine the truck factor for this section of highway. Assume SN = 4 and pt = 2.5.

Prob. A six-lane divided highway is to be designed to replace an existing highway. The present AADT (both directions) of 6000 vehicles is expected to grow at 5% per annum. Assume SN= 4 and pt =2.5. The percent of traffic on the design lane is 45%. Determine the design ESAL if the design life is 20 years and the vehicle mix is: Passenger cars (1000 lb/axle) = 60%2-axle single-unit trucks (5000 lb/axle) = 30%3-axle single-unit trucks (7000 lb/axle) = 10%

Prob.

The traffic on the design lane of a proposed four-lane rural interstate highway consists of 40% trucks. If classification studies have shown that the truck factor can be taken as 0.45, design a suitable flexible pavement using the 1993 AASHTO procedure if the AADT on the design lane during the first year of operation is 1000, and pi = 4.2 and pt= 2.5.Growth rate = 4% Design life = 20 years Reliability level = 95% Standard deviation = 0.45.The pavement structure will be exposed to moisture levels approaching saturation 20% of the time, and it will take about one week for drainage of water. Effective CBR of the subgrade material is 7. CBR of the base and subbase are 70 and 22, respectively, and Mr for the asphalt mixture, 450,000 lb/in2.