Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
8 3. METHODOLOGY AND EXPERIMENTAL PROGRAM 3.1. Review of Test Equipment Specifications In preparation for the upcoming tasks, a critical review was conducted of the test capabilities, specifications, and similarities and differences of available Hamburg test equipment in the US. A comparative review of some of the critical technical aspects of the representative equipment in the US was carried out and presented to what is required by AASTHO T 324. It is worth noting that results of this task, which are presented subsequently in this report, identified four major manufacturers of HWT in the US. These vendors are referred to as vendors A, B, C, and D to protect their anonymity. 3.2. Nationwide Survey A nationwide survey was conducted to collect information from state agencies on the use of HWTs. The survey was posted online and was distributed through various LISTSERVs; it was also announced at related TRB committees. The research team complemented statesâ responses with a review of state specifications available online as well as through email communications, which allowed a 100% response rate. A copy of the survey is provided as well as the contact information of survey respondents are presented in Appendix A. The prepared survey consisted of 13 questions, which are listed below: 1. What type of LWT do you use? (Please choose one or more manufacturers) 2. Does your machine have a single wheel or two wheels? 3. Which specification do you use? (Please choose one) 4. How often do you calibrate your LWT (months)? 5. What does the calibration include? 6. Is your laboratory AMRL certified for AASHTO T-324? 7. What test temperature(s) do you use? (°C) 8. What is the acceptance criteria used in your state? Please attach a copy of your specifications. 9. What type of specimens do you use? 10. Does you agency specify requirements for the Hamburg test specimen fabrication? 11. Do you have test data that you can share? (Please choose one) 12. How is the result of the Hamburg test reported? 13. How do you use the data you obtain from the machine? 3.3. Experimental Program The objective of the experimental program was to identify potential issues with different aspects of AASHTO T 324 standard procedure, mainly on its specifications of what needs to be measured, and the needed accuracy and resolution of the measurements. As these critical points are identified, the research team evaluated the capability of the existing equipment to accurately measure, control, and maintain the desired test conditions. Finally, the minimum equipment capabilities, components, and design features to ensure the consistency and accuracy of the test were presented. The experimental program concentrated on the following items of the current AASHTO T 324: ⢠Wheel position waveform, frequency, and maximum speed;
9 ⢠Impression measurement system; ⢠Temperature measurement and control system; ⢠Wheel dimensions; ⢠Wheel loads; ⢠Specimen and track length; and ⢠Data collection and reporting. Other factors within the current standard were also analyzed to accommodate any changes and new recommendations as needed. The following sections present the experimental program conducted to evaluate the aforementioned factors for the different Hamburg equipment available in the US market. 3.3.1 Wheel Position Waveform, Frequency, and Maximum Speed Section 5.1 of AASHTO T 324 specifies the movement of the wheel over the specimen. The wheel is required to reciprocate over the specimen such that its position varies sinusoidally over time. The frequency of this movement is specified to be 52 ± 2 passes per minute. Additionally, the maximum speed is specified to be 0.305 m/s (1 ft/s) and is expected to be reached at the midpoint of the specimen. An extensive evaluation of the HWTs identified in the project was undertaken to assess compliance with the specifications of section 5.1. HWTs from Vendors A, B, C, and D were evaluated in this study. Two approaches were considered to record the position of the HWT wheel as a function of time. The first approach studied the feasibility of using an accelerometer to measure acceleration of the sliding mechanism. The acceleration could subsequently be integrated with respect to time to obtain velocity and integrated once more to yield distance or position. However, this approach required acquisition of correctly sized accelerometers and signal conditioning equipment. The second approach studied the possibility of using a video camera to capture images at a high rate and performing image analysis to obtain the position of the wheel as a function of time. The second approach was selected as the equipment and accessories needed to perform the experiment were available in-house. A GoPro camera was used to capture the video of the HWT wheel during its travel. This camera was attached to the moving loading arm using an adhesive mount. Figure 1 shows the camera set up with an adhesive mount. Aluminum slab specimens were fabricated and used to minimize vibrations during video recording. A ruler was affixed to the top of the slab and the camera was focused on the ruler. As the loading arm moves along its track, the attached camera focuses on different parts of the ruler along the slab specimen. The ruler reading coinciding with the center of each frame of video was recorded during post-processing. This information was combined with time data (obtained from a video recording rate of 240 frames/second) to obtain a distance versus time graph. Two types of rulers were evaluated by the research team and are presented in Appendix B. Various camera mounting systems (gooseneck/clamp and adhesive mount), camera-to-specimen distances, and lighting sources were evaluated to obtain an accurate video. The best video quality was obtained with a non-reflective paper ruler (1/16 in. subdivision), an adhesive mount, a focus distance of 5 in., and a professional lighting source (Lowel DP).
10 (a) Overall setup (b) Camera and ruler Figure 1 Experimental setup for wheel position analysis The HWT was allowed to reciprocate for a few cycles before triggering the GoPro camera to capture video at 240 frames/second. The GoPro camera setup and control were achieved using the GoPro app on the iPhone. The video data file was further processed as follows:
11 1. The video file was split into individual image frames. Each picture frame obtained was 1280 pixels wide and 720 pixels high. 2. MATLAB software was used to add a vertical red line in the middle of each frame (i.e., to change the color of column number 640 to red). 3. The images were re-assembled back to a video file. 4. The video was analyzed frame-by-frame and the position on the ruler coinciding with the red line was noted. The corresponding frame number was also recorded. It should be noted that the time increment from one frame to the next is 1/240 second. Figure 2 presents the image of a frame after the red line addition in MATLAB software. In this frame, the red line coincides with the 2.75 in. mark on the upper scale of the ruler. Figure 2 Image frame after MATLAB processing Figure 3 shows a typical plot of the recorded ruler readings or the wheel position as a function of time, obtained with the aforementioned post-processing procedure.
12 Figure 3 Wheel position as a function of time 3.3.2 Impression Measurement System Section 5.3 of the AASHTO T 324 specification requires that an LVDT be used to measure the impression of the wheel as it tracks over the specimen. It further specifies that the LVDT have a minimum range of 20 mm (0.8 in.) with an accuracy of 0.15 mm (0.006 in.). Additionally, this system should be capable of measuring the impression at least every 400 passes, recording the number of passes applied, and be able to collect the measurements without stopping the test. The current equipment verification procedure detailed in the Appendix of AASHTO T 324 requires that the LVDT be checked in accordance with the applicable ASTM D 6027 procedure or per manufacturerâs recommendations. However, it does not require verification that the measurements be recorded at specific locations along the track. These locations, currently set by the vendors, should be standardized to enable test result comparisons between different vendors. As a first step, the calibration of the LVDTs was verified for all the HWTs evaluated. Additionally, an aluminum specimen with a curvature mimicking a rutted specimen was designed and fabricated to enable verification that the impression readings were being recorded at the locations specified by the vendors. Figure 4 presents the picture of the fabricated specimen and the engineering drawing of the specimen is presented in Appendix C. Since the curvature or ârutâ of this specimen is machined per the drawing in the appendix, the depression at any location along the track is precisely known. The maximum depression of the manufactured specimen is 19.05 mm (0.75 in.) and is located at the midpoint of the track. The aluminum specimen allows for verification of LVDT readings and confirms if the readings are being recorded at the locations specified by the vendors.
13 Figure 4 Metal specimen for verifying locations of deformation readings During the course of the study, the research team fabricated a new metal specimen with a longer curved track length to avoid the problem of the wheel âclimbing outâ of the track. The machine drawing of this metal specimen and the analytical solution of the wheel and metal- specimen interaction is presented in Appendix C. Figure 5 Modified metal specimen for verifying locations of deformation readings
14 The calibration of the impression measurement systems from the vendors were verified as described in the instruction manuals provided with the machines. Next, the aluminum specimen was installed in each of the machines to verify that the readings were being recorded at exactly the locations specified by the vendors. The steps of this procedure are described as follows: 1. The aluminum specimen was flipped upside down to enable the machine to obtain the âzeroâ readings. Figure 6(a) presents the picture of the flipped specimen, showing the flat surface for the âzeroâ readings. 2. The HWT was allowed to reciprocate for 80 cycles to enable the machine to record âzeroâ readings. 3. The aluminum specimen was flipped again to allow the wheel to track over the curved machine surface. This is shown in Figure 6(b). The aluminum specimen was also centered along the track of the wheel. 4. The HWT was allowed to reciprocate for 80 additional cycles to ensure that readings of the curved surface of the aluminum specimen were recorded. (a) Flat surface for obtaining zero readings
15 (b) Recording deformation readings along the curvature (ruler shown for scale) Figure 6 Procedure for verifying locations of deformation readings It is also noted that the impression of the curvature of the metal specimen was recorded by connecting the electrical output of the machine LVDT to a data acquisition system. As shown in Figure 7, data were collected at a frequency of 100 Hz. Impression measurement system readings obtained from all the machines were compared to this reference profile.
16 Figure 7 Curvature of the metal specimen recorded by machine LVDT connected to data acquisition system 3.3.3 Temperature Control System The temperature control system comprises a water tank, heater(s), temperature sensor(s), circulating pump, and controller. AASHTO T 324 specifies that the temperature control system in the HWT be capable of maintaining the set temperature in the water tank to within ±1.0°C over a range of 25 to 70°C (77 to 158°F). Additionally, it specifies that the water be mechanically circulated in the tank to reduce the temperature gradient. The following section describes the experimental setup used for this purpose. The current AASHTO T-324 specification requires verification of the temperature in the bath at four locations. The procedure also specifies the preconditioning time for temperature stabilization to be 30 minutes. In order to quantify the temperature gradient across the specimen and to verify that the preconditioning duration is adequate, instrumented hot mix asphalt (HMA) specimens were used. Four Resistance Temperature Detectors (RTDs) were used with each SGC specimen, with two RTDs on top and two on the bottom of the slab specimen. Figure 8 shows the locations of the RTDs in the SGC specimen. Details of the specimen preparation and instrumentation are presented in Appendix D. The RTDs were connected to a DATAQ DI- 718Bx data acquisition system for monitoring. The data was collected at a sampling rate of 1/8 (or 0.125) Hz.
17 Figure 8 Temperature sensor locations 3.3.4 Wheel Dimensions The thickness and diameter of the wheels from the different manufacturers were measured. A digital caliper was used for this task. The measurements were taken diametrically and along the thickness of the wheels at four different locations as indicated in Figure 9. Figure 9 Measuring details: The geometry of the steel wheel (Vendor A)
18 3.3.5 Wheel Loads Section 5.1 of AASHTO T-324 specifies the load on the wheel is 703 ± 4.5 N (158 ± 1.0 lbs.). The load on the wheel is measured by a calibrated load cell. For vendor A, a spacer is placed to ensure the wheel is horizontally levelled; see Figure 10. Figure 10 Measuring details: The process of using load cell (Vendor A) 3.3.6 Specimen and Track Length AASHTO T 324 specifies that two High Density Polyethylene (HDPE) molds be used to secure the specimen in the testing tray of the machine. The schematic from the specification is shown in Figure 11. As can be seen, the specification allows some of the dimensions to be decided by the manufacturer. The wheel track lengths of the evaluated machines were obtained by analysis of the GoPro data. The ruler reading from the video frames corresponding to the ends of the track were noted to compute the track length.
19 Figure 11 Specimen mold (reproduced from AASHTO T 324) 3.3.7 Free Circulating Water on Mounting System Sections 5.5 and 5.6 of AASHTO T-324 requires that the specimen mounting system (slab or cylinder) must suspend the specimen and provide a minimum of 20 mm (0.8 in.) of free circulating water on all sides. After filling the water tank and inserting the specimens, the water depths on all sides were measured using a ruler (Figure 12(a) and (b)). For each tray, six sides of free water circulating were measured (Figure 12(c)). For each side, three measurements were conducted and the average results were reported.
20 (a) Measurement procedure (b) Measurement procedure (c) layout of the measurement location Figure 12 Free water circulating on the mounting system 3.3.8 Data Collection and Reporting AASHTO T 324 requires five parameters to be collected and reported to quantify the performance of a mixture to rutting and moisture susceptibility: number of passes at maximum impression, maximum impression, creep slope, strip slope, and Stripping Inflection Point (SIP). In this analysis, the data collection schemes adopted by the vendors were reviewed and evaluated. Specifically, the number of data points collected and the spacing between the data points were identified and summarized. In addition, the calculation schemes for the five performance indicators were reviewed and analyzed. It should be noted that the current AASHTO T 324 specification only requires data collection at the center (± 1/2 in.) of the track. However, state agencies utilize different collection schemes in the calculation of the rut depth.
