Chapter 3. Design of Experiment

Introduction

In this research project, extensive data were collected in the field and laboratory to fulfill the objectives of the study presented in Chapter 1. Figure 3.1 shows the seven basic steps performed in this research. These steps were: site selection, data collection, resilient modulus determinations, data base preparation, data analysis, effect of M_R on overlay thicknesses, and conclusions. In this chapter, each one of the above evaluation strategies will be discussed.

Site Selection

Nine pavement test sections were selected in the State of Wyoming. These sections represent typical cohesive subgrade soil conditions throughout the state (refer to Figure 3.2). Overall, a typical cross-section of the pavement structure included an asphalt concrete layer, a granular or treated base (asphalt or cement), and the underlying subgrade soil. Because of the relatively low traffic volumes in Wyoming, pavement structures do not normally have a subbase layer.

Data Collection

In the summer of 1992 and spring of 1993, extensive field data were collected on all test sections included in the experiment. This field evaluation included pavement and subgrade coring, deflection measurements, and condition surveys. At each site, three pavement cores and three Shelby tubes of subgrade soil were obtained. Table 3.1 summarizes the locations of the nine test sections.

The Wyoming DOT's KUAB 2m-Falling Weight Deflectometer was used to take deflection measurements at each site using three different levels of loading: 26.7, 40.0, 53.4-kN (6000, 9000, and 12000-lbs.). Figure 3.3 shows the locations of the sensors used to take the deflection measurements in this research.

Figure 3.3 Layout of FWD Sensors

Other important data, such as pavement and air temperatures, were recorded for later use in correcting the temperature to the standard value of 21° Celsius (70° Fahrenheit). Pavement condition surveys were completed on each test section to examine pavement surface conditions.

Laboratory Testing and Resilient Modulus Determination

After obtaining the soil samples from the field, several laboratory tests were initially conducted to determine the soil classification of the subgrade at each test section. These preliminary tests included: sieve analysis, Atterburg Limits, water content determinations, and R-values. The AASHTO Soil Classification system was later used to determine the soil type at each test section. The equation below, occasionally used by the Wyoming DOT, was used in estimating the optimum water content for each sample:

where:

• ω = optimum water content (%),
• LL = liquid limit

All laboratory tests were conducted in accordance with their respective AASHTO specification. Table 3.2 summarizes these testing specifications.

Resilient modulus values were then determined for each test section from: laboratory testing based on 41.4-kPa (6-psi) deviator stress, laboratory testing based on actual field stress conditions, and from deflection measurements.

Laboratory Testing for Resilient Modulus

Laboratory soil resilient modulus tests were performed on the Wyoming DOT machine manufactured by the Interlaken Technology Corporation. The system has a Series 3300 98-kN (22-kip) capacity test frame, a Series 3230, 16 channel data acquisition system, and a Series 3200 controller. This device is located in the Materials Branch at the Wyoming Department of Transportation. Figures 3.4 and 3.5 show the resilient modulus testing device. All samples tested were 71-mm (2.8-in.) in diameter and 152-mm (6-in.) in height. These measurements were selected in accordance with the specifications, a height not less than two times the diameter and a minimum diameter of 71-mm (2.8-in.) or five times the nominal particle size (AASHTO, 1992). In this research project, deformation readings were recorded at two different locations during laboratory testing. First, from 2 LVDT's located outside of the triaxial cell on the loading piston (referred to as the actuator in this report) and second, from three LVDT's located on the rings inside of the testing chamber. Even though some testing programs available for M_R testing automatically average the signals from the LVDT's, individual measurements were saved in a computer file in this research project. This procedure was used to identify and eliminate inconsistent deformation measurements coming from the LVDT's. All applied load and deformation readings were also stored in a computer file for later analysis.

Figure 3.4 Interlaken Resilient Modulus Testing Equipment

Figure 3.5 Resilient Modulus Testing Chamber

Subgrade cores obtained from the summer of 1992 were tested in two conditions, undisturbed and disturbed. Due to the fact that the soils were left in the Shelby tubes for several months prior to testing, normal extraction could not be performed without disturbing the samples. The solution to this problem was to freeze the tubes, extract the soil samples, and let the cores thaw for twenty-four hours prior to testing. Overall, this procedure was successful and allowed soil samples to be removed from the tubes with minimal disturbance. Resilient modulus testing was performed on some of the frozen samples, but reasonable M_R values were difficult to obtain because of the low stresses applied to the samples and the high stiffness of the frozen cores. As a result, the frozen condition was not considered in the analysis. After testing was completed for the undisturbed samples, the disturbed (remolded) samples were prepared by crumbling the sample and re-compacting it to 152-mm (6-in.) using five lifts and static compaction. This procedure is fully described in the specifications (refer to Appendix A).

Subgrade cores from the spring of 1993 were tested for resilient modulus shortly after obtaining them from the field. Again, two conditions were considered, undisturbed and disturbed (remolded). Unlike the first set of subgrade cores, samples were easily removed from the Shelby tubes and, therefore, they did not require freezing prior to extraction. LVDT measurements during M_R testing were also taken outside and inside the testing chamber.

After the laboratory M_R testing was completed, deformation and applied load readings from the last five cycles of loading condition were retrieved from the data files created during the tests. Several spreadsheets were developed to accept these data as well as the length and diameter of each sample. By entering this information, the resilient modulus values for all nine test sections were calculated automatically for each testing condition. Plots were then constructed by using the log₁₀(resilient modulus) versus the deviator stress. Simple regression analysis was then performed to estimate resilient modulus based on deviator stresses. As suggested in the literature, a design M_R value was determined by substituting a deviator stress of 41.4-kPa (6-psi) into this resulting equation.

Besides using this suggested deviator stress, the actual stresses in the subgrade were computed by using the computer program BISAR. This computer program, developed by De Jong et al. (1973), computes the stresses in a n-layer pavement structure by considering the vertical and horizontal loads. Information obtained in the field evaluation, specifically the thicknesses of each pavement layer, and certain material properties were entered into this program. Table 3.3 summarizes the values for the material properties commonly used by the Wyoming DOT. These computed deviator stresses were then substituted into the linear equations developed in the laboratory testing to determine another design resilient modulus value based on actual field stress conditions.

Back Calculation of M_R

In addition to the laboratory analysis, the deflection data collected were used to determine subgrade M_R values with the following three back calculation programs: MODULUS, EVERCALC, and BOUSDEF. All of these programs compare the deflection basins based on field data to theoretical basins in order to determine the back calculated resilient modulus values. Deflection measurements used in these three computer programs were corrected to a standard temperature of 21° Celsius (70° Fahrenheit) with a computer program called TAFFY. This program was developed by the Colorado Department of Transportation (1988) to determine temperature adjustment factors. The average air temperature, the surface temperature, and the mean pavement temperature all affect this adjustment factor.

Data Base Preparation and Data Analysis

All field and laboratory data were summarized in a computerized data base. Statistical analyses were then performed to determine how fundamental soil properties, linear variable differential transducer (LVDT) placement during M_R testing, and sample condition influence resilient modulus values. Further analyses were completed to examine the relationship between laboratory and back calculated M_R values. These results and analyses will be presented in Chapter 4.

Evaluating the Effect of M_R Selection on Overlay Thicknesses

Three different sets of M_R values were used to complete this analysis. In addition to the values calculated using a 41.4-kPa (6-psi) deviator stress and the actual field deviator stress, the AASHTO equation based on deflection measurements was used to determine the third set of M_R values. Overlay thicknesses were then determined by using the 1993 AASHTO NDT overlay design procedure for asphalt overlay on asphalt pavements. Finally, several analyses were performed to determine the effects of selected M_R values on the resulting overlay thicknesses. These results and analyses will be presented and discussed further in Chapter 5.

Chapter Summary

This chapter presented the data collection and overall evaluation strategies followed in this research project. Site selection, data collection, resilient modulus calculations, data base preparation, data analysis, effect of M_R on overlay thickness, and conclusions were the seven steps.  Overall, each step provided a way to thoroughly satisfy the objectives of this research project.