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66 5. CONCLUSIONS AND RECOMMENDATIONS After performing a comprehensive evaluation of the machines conforming to AASHTO T 324, it is concluded that there are differences between commercially available HWT machines in the US market. Furthermore, available HWT machines do not meet all the requirements set forth in AASHTO T 324 including requirements for the waveform, the temperature range, and the reporting parameters. One should acknowledge, however, that some of the observed differences are due to the ambiguity of the specification and the lack of detailed requirements for every aspect of the test method. The following represents a summary of the main shortcomings identified during the testing program. Waveform: Results of the experimental program showed that not all the machines available in the market are able to produce a sinusoidal wave. AASHTO T 324 specifies that the wheel be required to reciprocate over the specimen such that its position varies sinusoidally over time. In the case of a pure-sinusoidal machine, the wheel spends equal amounts of time on the front and back halves of the track. However, in the case of the non-sinusoidal machine, the wheel spends more time on the back half of the track as compared to the front half. Temperature control system: AASHTO T 324 specifies the use of a water bath capable of controlling the temperature within ±1.0°C over a range of 25 to 70°C (34°F over a range of 77°F to 158°F). Since the majority of the HWT machines do not have a cooling system, obtaining 25°C in the bath is highly dependent on the incoming water temperature and was not possible when the water temperature was warmer than 25°C. When tested at 50°C, even though the average temperatures at the end of 30 minutes of conditioning were within the specification limit of 50 ± 1°C, some locations in the HMA specimen were not within the specified range. Therefore, a longer pre-conditioning time is deemed necessary. When tested at 70°C, all machines were able to heat the specimen; however, some locations in the HMA specimen were not within the required range. The upper range of 70°C is too high and is not encountered in any region of the US. Based on the results of the survey, the highest test temperature used by the states was 56°C. Deformation measurements: AASHTO T 324 does not currently specify the locations of the deformation readings or the number of deformation readings. Current specification has resulted in major discrepancies among manufacturers, as some machines record deformations at only five locations while others record deformations at 227 locations. Results also suggest that the deformation readings are sometimes not being recorded at the pre-determined locations along the track. Furthermore, the researchers located the center of wheel travel for each of the machines before performing the evaluations, it is important that the specimen molds be centered with respect to the travel of the wheel. Therefore, the vendors should mark the center of travel on the machines to allow users to line up the molds with that mark. Data collection and reporting: AASHTO T 324 requires five parameters to be collected and reported to quantify the performance of a mix to rutting and moisture susceptibility: number of passes at maximum impression, maximum impression, creep slope, strip slope, and Stripping Inflection Point (SIP). Upon review of the current requirements detailed in AASHTO T 324, one may note that not enough specifics are provided to allow for consistent analysis and reporting of the five aforementioned performance indicators. For example, AASHTO T 324 does not define how to find the âsteady-state portionâ to plot the creep slope.
67 At least seven computer programs, developed by four manufacturers and two state DOTs, were identified for analyzing HWT test data and reporting the necessary parameters. Two mixes, which were tested using the HWT manufactured by Vendor A, were selected for analysis by the various methods. Mix 1 was a poor performing mix that stripped during testing and Mix 2 was a good performing mix that did not strip during testing. For Mix 1, substantial differences were observed between the different analysis methods especially in the reporting of the SIP. Furthermore, some of the available methods do not report the five performance parameters specified by AASHTO T 324. For Mix 2, only two of the seven methods successfully identified this mix as a non-stripping mix. In addition to these discrepancies, the approach adopted by Iowa DOT can only analyze HWT results obtained from the machine manufactured by Vendor A. 5.1. Recommendations Based on the results of the experimental program, revisions to AASHTO T 324 and to the configurations of the available HWT machines are necessary. Modifications were proposed to address equipment capabilities, components, or design features in order to ensure proper testing and accurate, reproducible results. The key elements of AASHTO T 324 specifications to conduct the Hamburg Wheel Track (HWT) test were identified to be the loading mechanism, temperature measurement and control system, impression measurement system, test specimen size, and data collection and reporting sections. Proposed modifications are discussed in this report to ensure repeatable measurements and that the results from different manufacturers are comparable. These modifications include change to temperature measurement and range, impression measurement system, data collection, and data analysis and reporting. In addition to the proposed modifications to the AASHTO T 324 specifications, the vendors are expected to modify their equipment to meet the new requirements. Based on the findings of the experimental program, it is concluded that there are differences between commercially available HWT machines in the US market. After addressing the proposed modifications to the equipment configurations and to the specifications, a laboratory experimental program shall be conducted in order to compare the results obtained with HWT devices from various manufacturers when testing the same asphalt mixture. The experimental program recommended testing a range of contrasting asphalt mixtures using the four main types of Hamburg test equipment available in the US market and to compare the five performance parameters statistically according to ASTM E1169, Standard Guide for Conducting Ruggedness Tests.
