MPC - Selection of Subgrade Modulus for Pavement Overlay Design Procedures (MPC-94-34)

Pavement engineers continuously look for ways to improve pavement service life and performance. Historically, pavement design procedures were empirical. In many cases, relationships were based on factors such as traffic loading and volumes, materials, layer configurations and the environment (Mahoney et al., 1991). During the last decade, however, traditional pavement design procedures have been changed to incorporate elastic and/or viscoelastic theories as well as experience and various empirical tests. These new mechanistic-empirical procedures address two different aspects of pavement design. The mechanistic element allows engineers to examine the stresses, strains, and deflections in the pavement structure. The empirical element, on the other hand, tries to establish a relationship between these mechanistic responses and the performance of the pavement structure.

Most newly developed pavement and overlay design procedures also require the characterization of materials. This requirement resulted in the development of several laboratory tests to simulate actual field conditions in the laboratory. One of these tests is the resilient modulus test for subgrade soils. It is believed that the adoption of this new testing procedure will result in more reliable and cost-effective designs of pavement structures. This chapter presents a background of the tests traditionally used for roadbed soil characterization and the latest test, resilient modulus. This discussion includes three different procedures for determining the resilient modulus value: laboratory testing, back calculation, and correlation studies. Finally, the chapter describes how the methods used for material characterization fit into the latest AASHTO overlay design procedure.

Traditional Subgrade Testing Procedures

Over the years, several testing procedures have been developed by state highway agencies to characterize roadbed soils. Two of the most common tests include the California Bearing Ratio (CBR) and Resistance Value (R-value). Both of these tests estimate the "strength" of the subgrade for use in the pavement design procedures.

California Bearing Ratio (CBR)

The CBR test was first developed by the California Division of Highways around 1930 (Asphalt Institute, 1978). During World War II, the U.S. Army Corps of Engineers modified the original procedure in order to incorporate the test into their flexible pavement design method for airport runways. Later, this test was adopted by the American Society of Testing Materials (ASTM) in 1961 and by the American Association of State Highway and Transportation Officials (AASHTO) in 1972. Both organizations, however, adopted procedures with minor modifications to the test used by the Corps (Asphalt Institute, 1978).

The CBR is a shear strength test based on penetration that can be completed on the soil in the field (ASTM D 4429) or on "undisturbed" or disturbed samples in the laboratory (ASTM D 1883, AASHTO T 193). In order to properly design a pavement structure based on the CBR value, the test is completed using samples at or near saturated soil conditions to represent the worst subgrade strength. Therefore, the field testing procedure is primarily used for evaluating the properties on existing pavement sections while laboratory testing is completed on saturated soil samples.

Laboratory testing for the CBR value, using disturbed samples, involves several steps. First, the subgrade soil is compacted in molds 152-mm (6-in.) in diameter and 152 to 178-mm (6 to 7-in.) in height. In order to simulate field conditions, samples should be prepared using the expected moisture content, density, and method of compaction. After preparing the samples, a dead weight is applied to the sample to simulate the loading of the overlying pavement structure (base and pavement layers). Next, the assembly (soil, mold, and dead weight) is submerged in water for 4 days. This step allows the sample to become saturated and, therefore, allows the test to be completed on the worst subgrade strength. After removing the sample and draining it for 15 minutes, loading is applied to the assembly with a piston having an area of 1,935-mm² (3-in.²). This rod penetrates through the soil at a rate of 1.3-mm (0.05-in.) per minute and the load is recorded at the following penetrations: 2.5, 5.0, 7.5, 10.0, and 12.5-mm (0.1, 0.2, 0.3, 0.4, 0.5-in., respectively). A graph of load versus penetration is then constructed using the above results. The resulting plot is often not linear because of surface irregularities and consolidation during testing and must be corrected by re-zeroing the load-penetration curve. Finally, the following equation is used to determine the CBR value by substituting the corrected value of the unit load at 2.5-mm (0.1-in.) penetration:

The value in the denominator corresponds to the pressure required to reach the amount of penetration in a standard crushed rock. For example, it takes 6.9-MPa (1000-psi) to obtain 2.5-mm (0.1-in.) penetration in crushed rock. Each level of penetration has a corresponding pressure. Typically, the CBR value decreases as penetration increases. As a result, the ratio at 2.5-mm (0.1-in.) of penetration is frequently used to determine the CBR value for pavement design (Wright & Paquette,1987). The CBR values range from 0 to 100, characterizing a roadbed soil as bad to excellent, respectively.

Resistance Value (R-Value)

The R-value is also used to evaluate roadbed soil for highways. This test was originally developed at the California Division of Highways by F. N. Hveem and R. M. Carmany in 1948. It is a closed-system triaxial test that measures the internal friction or "resistance" of the soil in a stabilometer. Figure 2.1 presents a basic schematic diagram of the stabilometer test.

This test is usually completed on disturbed samples in the laboratory (ASTM D 2844, AASHTO T 190). First, a sample, 102-mm (4-in.) in diameter and 62 to 65-mm (2.45 to 2.55-in.) in height, is prepared using a mechanical kneading compactor which simulates field compaction techniques. Next, this sample is placed into the stabilometer between a testing head and a bottom plunger. A vertical pressure of 1.1-MPa (160-psi) is then applied to the testing head, creating a horizontal pressure on the fluid within the rubber membrane that surrounds the sample (refer to Figure 2.1). This horizontal pressure is measured and recorded as p_h. Next, the applied vertical pressure is reduced to 0.55-MPa (80-psi) and the horizontal pressure reduced to 35-kPa (5-psi) with the stabilometer pump handle. After zeroing the displacement dial indicator on the stabilometer, the calibrated pump handle is turned to increase the horizontal pressure to 690-kPa (100-psi). The number of revolutions is recorded as D₂. The following formula is then used to determine the R-value:

Hveem (1949) explained that the applied vertical pressure of 1.1-MPa (160-psi) was chosen arbitrarily and this value is not a critical matter in the R-value test. He supports this statement from laboratory testing that showed no effect on the ratio of p_v/p_h where the applied vertical pressure varied from 0.7 to 2.8-MPa (100 to 400-psi). Because of this observation, some states use a different vertical pressure in their R-value testing to ensure that the sample is saturated. California uses an exudation pressure of 1.7-MPa (240-psi) while Washington uses 2.1-MPa (300-psi) (Huang, 1993). The R-values also range from 0 to 100, but characterize a roadbed soil as a liquid (p_h = p_v) to a rigid sample (p_h = 0), respectively.

Development of the Resilient Modulus Test

Overall, the traditional soil tests listed above do not fully simulate actual loading conditions in the field. Instead, they measure different soil properties related to the strength of the soil. As a result, the resilient modulus test was developed by Seed et al. (1963) to reflect several observations in the field and from research projects.

One important idea came from the American Association of State Highway Officials (AASHO) Road Test which was conducted from October 15, 1958 to November 30, 1960 in Ottawa, Illinois. Researchers concluded that when a load is applied to the pavement surface the resulting deflection is a strong indicator of pavement performance (HRB, 1962). A majority of the surface deflection can be accounted for by the load-induced strain within the subgrade. Approximately 60 to 80 percent of the measured surface deflection was found to develop in the subgrade at the AASHO Road Test (HRB, 1962). Therefore, the resilient modulus test for subgrade soils models an important part of flexible pavement performance.

Another important observation contributing to the development of the M_R test is the stress in the pavement structure resulting from loading. The stress at a given point in the pavement structure is zero when the wheel load is at a considerable distance away. However, when this load is directly above the point, the stress is at its maximum value. In many cases, it is reasonable to assume the stress pulse to be a haversine or triangular loading even though the duration of the pulse depends on the vehicle speed and the depth of the point below the pavement surface (Huang, 1993). Because the vehicle speed varies a great deal and the depth of the material may not be known during design, the AASHTO specifications recommend a haversine load wave with a duration of 0.1 second and a rest period of 0.9 second (AASHTO, 1992). As a result, the M_R test accounts for the type and duration of loading expected in the field.

A third important observation is the fact that most paving materials experience some permanent deformation after each load application (Huang, 1993). Figure 2.2 shows how the amount of strain under repeated loading in a material changes over time. In the beginning, the material shows a considerable increase in the amount of permanent deformation (accumulated plastic strain). However, as the number of loads increases, the accumulated plastic strain levels off and the material is essentially elastic (recoverable strain).

This phenomenon usually occurs after 100 to 200 load applications. Because the applied load is smaller than the material's strength, the M_R test can be completed on the same sample for several different loadings and environmental conditions (Huang, 1993).

In 1986, the resilient modulus test became the basis for the AASHTO Guide for Design of Pavement Structures. According to AASHTO (1993), the M_R value has three important advantages over the soil support value used in the previous editions:

With the above observations and advantages, it is clear that resilient modulus testing can directly measure the strength of the subgrade soil and provide information which reflect field conditions.

Resilient Modulus Laboratory Testing

The Interim Method of Test for Resilient Modulus of Unbound Granular Base/Subbase Materials and Subgrade Soils - SHRP Protocol P46 (AASHTO: T 294-92 I) outlines the latest testing procedure (refer to Appendix A). This specification separates subgrade material into two different categories: Type I (granular) and Type II (cohesive). Each type of soil has a different conditioning cycle and fifteen loading sequences, varying in confining and deviator stresses. Overall, Type I soils undergo higher stresses, both confining and deviator, because of their higher resistance to deformation. The loading sequence for Type II soils is presented in Table 2.1. The amount of deformation in the soil sample is recorded using two linear variable differential transducers (LVDT's) outside of the testing chamber.

Table 2.1 Testing Sequence for Type II Soils (AASHTO, 1992)
Sequence No.	Confining Pressure S₃,psi	Deviator Pressure S_d,psi	Number of Load Repetitions
0*	6	4	1000
1	6	2	100
2	6	4	100
3	6	6	100
4	6	8	100
5	6	10	100
6	3	2	100
7	3	4	100
8	3	6	100
9	3	8	100
10	3	10	100
11	0	2	100
12	0	4	100
13	0	6	100
14	0	8	100
15	0	10	100

* preconditioning

However, the original AASHTO T-274 specifications required 2 LVDT's on rings within the test chamber. These LVDT's are normally placed at a specified gage length depending on the size of the sample. Figure 2.3 shows both of these LVDT locations.

Back Calculation of Resilient Modulus

The laboratory resilient modulus test is relatively complex and it requires obtaining field samples. As a result, several agencies have looked into non-destructive back calculation procedures to estimate the strength of the soils in-place. The back calculation procedures involve collecting surface deflection data in the field on existing pavement sections through non-destructive testing and then plugging these values into a computer program to obtain the M_R values. Surface deflection measurements provide pavement engineers with a rapid, relatively inexpensive, and non-destructive method of examining the basic response of the pavement structure to applied loads (Ali & Khosla, 1987). Because this analysis is normally performed on existing highway sections, the back calculation of resilient modulus values is primarily used in designing pavement overlays.

Non-Destructive Testing Equipment

Several different types of testing equipment were developed to examine the in-situ characteristics of a pavement structure. Non-destructive testing (NDT) equipment can be divided into four general categories: static deflection, steady-state deflection, impulse load deflection, and wave propagation. However, only the first three categories provide deflection measurements.

Static deflection devices measure the pavement's response to loads applied with a slow moving vehicle or a stationary loading frame (Stoffels & Lytton, 1987). Three common NDT devices in this category include: Benkelman beam, California traveling deflectometer, and LaCroix deflectometer. Figure 2.4 shows a picture of the Benkelman beam which was widely used by highway agencies. The measurement probe on the beam is placed between the rear dual tires of a 80-kN (18-kip) single-axle load truck. As the truck slowly moves away from the support (reference) beam, the rebound deflection of the probe is measured at specific distances, creating a deflection basin. Overall, this measuring device is easy to use, but it is a slow process and has several other disadvantages. Because the support beam must be an immovable reference point, the use of this device is limited to flexible pavements. In addition, the loads used to measure the surface deflection do not represent actual field conditions, impulse loads. Therefore, empirical correlations must be developed in order to use the results in any mechanistic pavement design procedure (Huang, 1993).

Steady state deflection systems, on the other hand, measure the pavement's response to loads applied by a vibratory device. Research has shown that the deflection at any specific driving frequency is approximately proportional to the amplitude of the load. However, at low frequencies, this factor approaches the value of the static pavement stiffness (Stoffels & Lytton, 1987). Therefore, the vibratory device must apply a compressive force of varying magnitude, a dynamic force superimposed over a static force, in order to account for these effects. Two of the most common systems in this category include the Dynaflect and the Road Rater. Both devices use inertial motion sensors (geophones), placed at specific distances away from the point of loading, to record the surface deflection. This type of NDT device does not require a reference point like the static equipment. It is also a rapid method of analyzing a section's structural adequacy. Some of the disadvantages of this testing procedure include the inability to apply the actual loads in the form of steady-state vibration and the effect some large static loads may have on stress sensitive materials (Huang, 1993).

The third system, impulse load deflection, applies a transient force impulse to the pavement surface and records its response. This impulse is created by selecting a weight and dropping it a certain height. This type of NDT equipment is commonly called a Falling Weight Deflectometer (FWD). Three commonly used FWDs include: Dynatest, KUAB, and Phoenix. Figures 2.5 and 2.6 show pictures of the Wyoming DOT KUAB deflectometer. These testing devices allow another method of rapidly analyzing a section's structural adequacy for use in a mechanistic pavement design procedure. Overall, most pavement engineers agree that the FWD provides an accurate method of modeling actual moving loads in both magnitude and duration (Huang, 1993). This device also uses a relatively small static load compared to the impulse loading. However, these devices have some disadvantages. In many cases, it is difficult to obtain reliable results from the inertial motion sensors in the low frequency range. It is also difficult to produce force impulses that have a short duration to reliably measure the deflections in the significant frequency range of the pavement section (Stoffels & Lytton, 1987).

Back Calculation Computer Programs

There are several computer programs that can use deflection data to back calculate the strength of the different layers in a pavement structure. Some of the most widely used back calculation programs include: MODULUS, EVERCALC, and BOUSDEF. All of these programs compare the deflection basins from field data to theoretical basins to determine back calculated M_R values. However, each program computes these moduli by using different methodologies and assumptions. The first program, MODULUS, was developed at Texas A & M University. MODULUS determines M_R values based upon a layered elastic code called WES5. This code creates a large database of theoretical deflection basins and matches, through interpolation, the best basin to the field data. The second program, EVERCALC, was developed at the University of Washington. In this program, theoretical deflections are based on CHEVRON, another layered elastic code. The third program, BOUSDEF, was developed at Oregon State University. This program uses the method of equivalent thicknesses, assuming one thick, uniform layer of material, and the Boussinesq theory to determine theoretical basins. Overall, by matching the deflection basin measured in the field, a M_R value is calculated for the surface, base, and subgrade layer.

Even though these computer programs provide pavement engineers with a quick method of obtaining M_R values, the following problems associated with back calculation procedures must be taken into consideration (Uddin, 1984):

In addition, three factors can influence the deflection measurements used in these computer programs: loading, climate and pavement condition. Loading should simulate the conditions used in the design process, typically, a 40-kN (9000-lbs.) wheel load. Climate factors such as temperature and moisture can also affect pavement deflections. These conditions should be recorded so that corrections can be made to the deflection measurements before using them in a computer program. Finally, pavement conditions influence the deflection measurements. During testing, careful selection of test sections should be made in order to avoid testing over a distress such as cracking or rutting (Huang, 1993).

M_R Determination from Correlation Studies

In many cases, agencies lack the large capital required for the laboratory M_R equipment and/or their pavement engineers are unfamiliar with this new subgrade soil property. As a result, correlation charts and equations have been created to convert values from some of the commonly used soil tests to resilient modulus values. Figure 2.7 presents a correlation chart for most common soil tests. This chart was developed using data from the AASHO Road Test and several design curves from California, Washington, and Kentucky (Van Til et al., 1972).

The soil support scale, on the far left, has values ranging from 1 to 10 and was developed using AASHO Road Test data. A 3.0 on the scale represents the silty clay roadbed soil while a 10.0 represents the crushed rock base material. In order to use this scale, highway agencies developed relationships between their commonly used material characterization test and the soil support scale. As a result, each state usually adopted a different test which caused variations in selecting subgrade strength. This problem contributed to the adoption of the M_R value as the material property used to design pavement structures. Through several research projects on the AASHO roadbed soil, it was shown that the soil support value (S) of 3.0 had a M_R value of 20,684-kPa (3000-psi). The rest of the correlations for converting soil support values to M_R values were based on this relationship.

Figure 2.7 Correlation Chart for Common Soil Tests

Source: Van Til et al. (1972)

Besides these correlations, two well known equations have also been developed through research to convert values from the strength tests to resilient modulus values. Heukelom and Klomp (1972) developed the following equation to convert CBR values to M_R values:

On the other hand, the Asphalt Institute (1982) developed the following equation to calculate resilient modulus from R-values:

Other equations have also been developed by state highway agencies. One example is Nebraska. Woolstrum (1990) reported a method to reliably determine the resilient modulus value based on the Nebraska Group Index (NGI). This index is similar to the group index developed by AASHTO because it uses the percent retained on the No. 200 sieve, the liquid limit, and the plasticity index. However, the NGI allows negative values for granular materials. Through a regression analysis, fourth-order equations were developed under three moisture conditions: optimum, wet, and dry. These equations correlated well with M_R values obtained in the laboratory. Even though the use of the correlation charts and equations to obtain resilient modulus values is acceptable, AASHTO (1993) recommends that "user agencies acquire the necessary equipment to measure M_R."

Selection of a Design M_R Value

Because of the importance of material characterization, several factors must be taken into consideration when selecting a M_R value for pavement design. According to Darter et al. (1992), "regardless of the method used, the design subgrade M_R value must be consistent with the value used in the design performance equation for the AASHO Road Test subgrade." The 1993 AASHTO guide uses a value of 20,684-kPa (3000-psi), but does not justify its selection. This value is one of the underlying assumptions of the flexible pavement performance model. Based on a study by Thompson and Robnett (1976), this value is appropriate when the AASHO soil is about 1% wet of optimum and subjected to a deviator stress of about 41.4-kPa (6-psi) or more. In addition, these results were based on laboratory tests using zero confining pressure, and they reported little effect when testing the samples using a confining pressure of 20.7 to 34.5-kPa (3 to 5-psi). Therefore, when selecting a M_R value from laboratory testing, a zero confining pressure and a 41.4-kPa (6-psi) deviator stress is suggested (Elliott, 1992).

Besides the above considerations, other factors such as water content, soil type, and sample condition must be accounted for when selecting an M_R value from the laboratory testing. First, water content is important because of its effects on M_R values obtained either above or below the optimum value. In 1989, Elfino and Davidson reported variations in the resilient modulus value of 7-41% from soils at different water contents. Second, whether the sample is undisturbed or disturbed will influence the M_R. Third, soil type may influence the M_R because of the differences in quality and soil strength. Overall, by considering these variations, an appropriate M_R value will be selected to represent the design field conditions.

The above observations also play an important part when determining a back calculated M_R value. In order to make a non-destructive testing value consistent with the 20,684-kPa (3000-psi) value, the calculated M_R value is multiplied by a correction factor. The need for a correction factor resulted from the fact that most NDT programs assume the measure deflection, at a certain distance away from the loading plate is attributable solely to the subgrade. In many cases, the amount of stress at this point is less than 41.4-kPa (6-psi), giving a higher resilient modulus value. Therefore, by reducing the back calculated resilient modulus value, one of the underlying assumptions in the flexible pavement performance model is satisfied.

Utilization of Soil M_R in the AASHTO Overlay Design Procedures

Over the years, several highway agencies developed their own overlay design procedures. In addition, AASHTO recently released the 1993 AASHTO Guide for Design of Pavement Structures. In the AASHTO guide, the determination of the subgrade resilient modulus value is essential for designing both new pavements and overlay thicknesses. If the design resilient modulus value is too high, the thickness of the pavement layer will be insufficient. If the design resilient modulus value is too low, the thickness will be conservative and not cost-effective. The implications of selecting resilient modulus values in designing new pavements will not be discussed here since the objective of this research project is to evaluate the new AASHTO overlay design procedure for asphalt pavements.

The 1993 AASHTO Overlay Design Procedure

The 1993 AASHTO Guide for Design of Pavement Structures outlines an eight step procedure for determining the overlay thickness. These steps include evaluating the existing pavement design and construction, traffic analysis, condition surveys, deflection testing, coring and materials testing, determination of required structural number for future traffic (SN_f), determination of effective structural number (SN_eff) of the existing pavement, and determination of the overlay thickness (D_ol). Each of these steps provides valuable information to determine an appropriate overlay design.

In the first step, evaluating the existing pavement design and construction, thicknesses of each layer and material types and characterization should be determined. Next, in the traffic analysis, the past cumulative 80-kN (18-kip) equivalent single-axle loads (ESALs) (N_p) and the future 80-kN (18-kip) ESALs (N_f) should be estimated. This traffic information is important in determining the SN_f value and overlay thickness. Third, pavement condition surveys provide information needed to determine the structural coefficients for each pavement layer. Fourth, deflection testing provides the basic information needed in the AASHTO overlay design procedures. Some type of NDT device, usually a FWD, provides this type of data. The AASHTO guide recommends using the following formula for determining the resilient modulus value of the subgrade soil based on the deflection measurements:

Fifth, coring and materials testing provides additional information to confirm the values obtained from reviewing construction records. Laboratory testing of the subgrade soil is recommended if deflection testing is not completed on a pavement section. In addition, the thicknesses of all the layers in the pavement structure can be confirmed by coring. Sixth, the SN_f value is determined by using several pieces of information. These items include: the effective design subgrade resilient modulus, design present serviceability index (PSI) loss, overlay design reliability (R), and the overall standard deviation (S_o) for flexible pavement. Seventh, the SN_eff value is determined using one or more of the following three methods: non-destructive testing (NDT), pavement condition surveys (PCS), and remaining life (RL). Finally, the overlay thickness (D_ol) is determined by taking the difference between the SN_f and SN_eff values and dividing this quantity by the layer coefficient for new asphalt pavement (D_ol = (SN_f - Sn_eff)/a_ol).

Determining the Need for an Overlay

Structural deterioration is any condition that reduces the load-carrying capacity of the pavement (Darter et al., 1992). As time and the number of loads applied (traffic) to a pavement section increase, the structural capacity (SC) of the section decreases from its initial state, SC_o, as shown in Figure 2.8. When an evaluation for an overlay is conducted, the section's structural capacity is evaluated and denoted by SC_eff. In order to repair the section and return it to its original or higher capacity, SC_f, an overlay is placed with a value of SC_ol (Note: SC_f = SC_eff + SC_ol). This method of evaluation is known as the structural deficiency approach.

Figure 2.8 Structural Capacity Loss Over Time and with Traffic

Source: Darter et al. (1992)

In order to obtain the "correct" thickness of the overlay, the evaluation of the effective structural capacity must be accurate by examining the existing pavement conditions and determining how the pavement materials will behave in the future. However, this is very difficult since the declining relationship is not well defined. It is often assumed by many agencies that a section's structural capacity is linear in order to simplify calculations and provide a conservative measurement. As a result, the AASHTO guide uses three different methods to determine a section's asphalt overlay thickness: non-destructive testing (NDT), pavement condition surveys (PCS), and remaining life (RL).

The first method, NDT, involves determining the effective structural capacity, expressed as the effective structural number (SN_eff) for flexible pavements, based on non-destructive deflection measurements. This data is often obtained using a Falling Weight Deflectometer (FWD). The SN_eff is determined with the following formula as a function of the total thickness and overall stiffness of a section:

The E_p value is based on a back calculation procedure for resilient modulus described in the AASHTO guide.

The second method involves using pavement condition surveys. This type of visual survey determines the SN_eff value based on the distress conditions observed in the field, drainage surveys, and maintenance history. For flexible pavements, the following distress types should be examined: alligator cracking, rutting, transverse and longitudinal cracks, and localized failing areas. Each distress type is converted to a layer coefficient based on the percentage of the surface condition. The following formula is then used to determine the SN_eff value:

Remaining life is the third procedure to determine the effective structural capacity. This method determines the SN_eff value based on fatigue damage from traffic. As the name implies, the amount of load-carrying capacity remaining in the pavement section is determined. This procedure requires the knowledge of past traffic (N_p) and estimates the total traffic the pavement could be expected to carry to "failure" (N_1.5). This failure is often assumed to be 1.5 on the Present Serviceability Index (PSI). In general, a new pavement has a PSI between 4 and 5, and repair is usually needed when the PSI is between 1.5 and 2.5. The following formula determines the remaining life:

The RL value is then converted to a condition factor (CF) ranging from 0.5 to 1.0 using a graph of CF versus RL. The SN_eff value is then computed with the following formula:

Darter et al. (1992) cites the following four major sources of error in this procedure: the predictive capability of the AASHO Road Test equations, the large variations in performance typically observed even among pavements of seemingly identical designs, estimation of the past 18-kip ESALs, and the inability to account for the amount of preoverlay repair to the pavement. Overall, this evaluation procedure should only be used for pavement sections which have very little visible deterioration and no previous overlays.

Chapter Summary

Material characterization is important in designing pavement sections. The traditional methods for examining the characteristics of the subgrade, CBR and R-value, do not provide information that directly represent field conditions. However, the resilient modulus test measures a subgrade's ability to recover after loading. Therefore, this value is expected to improve the modeling of actual field conditions and to provide a better basis for pavement designs. A soil's M_R value may be measured by using the following three techniques: laboratory testing, back calculation, and correlation charts/equations. Once a M_R value is determined for a section, this value can be used to calculate an overlay design thickness.

Chapter 2. Literature Review

Introduction