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3 1. SUMMARY The objective of this study was to evaluate the capability of the Hamburg Wheel Tracking (HWT) devices available in the US market and to identify potential issues with different aspects of AASHTO T 324 standard procedure in order to ensure proper testing and accurate, reproducible results. Based on the results of this study, researchers were tasked to provide proposed revisions with commentary to AASHTO T 324 to enable the use of a performance type specification for Hamburg test equipment. A comprehensive experimental program was conducted to evaluate the capability of five commercially available HWT equipment as well as their ability to accurately measure, control, and maintain the desired test conditions as specified in AASHTO T 324. The experimental program concentrated on the following items of the current AASHTO T 324- 14: ⢠Wheel position waveform, frequency, and maximum speed; ⢠Impression measurement system; ⢠Temperature measurement and control system; ⢠Free circulating water on mounting system; ⢠Wheel dimensions; ⢠Wheel loads; ⢠Specimen and track length; and ⢠Data collection and reporting. 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 reason for some of the observed differences is due to the ambiguity of the specification and the lack of detailed requirements for the different aspects of the test method. The following represents a summary of the main findings of the experimental program. Waveform: Results of the experimental program showed that two of the four machines available in the market were able to produce a sinusoidal wave (Vendors B and D). AASHTO T 324 specifies that the wheel be required to reciprocate over the specimen such that its position varies sinusoidally over time. Temperature control system: Since the majority of the HWT machines do not have a cooling system, obtaining 25°C (77°F) in the bath is dependent on the incoming water temperature and was not possible when the water temperature was warmer than 25°C. When testing 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 recommended. On the other hand, the upper range of 70°C specified by AASHTO T 324 is too high and is not encountered in any regions of the US based on the 50% reliability 7-day
4 average maximum high pavement temperatures computed using the LTPPBind software. 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 along the track length. Results also suggest that the deformation readings are sometimes not being recorded at the pre-determined locations along the track. 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). 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. At least seven methods, developed by four manufacturers and two state DOTs, were identified for analyzing HWT test data and reporting the performance parameters. Two mixtures (Mix 1 and Mix 2), which were tested using the HWT manufactured by Vendor A, were selected for analysis by the various methods. Mix 1 was a poor performing mixture that stripped during testing and Mix 2 was a good performing mixture that did not strip during testing. For Mix 1, substantial differences were observed amongst 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. Based on the results of the experimental program, revisions to AASHTO T 324 and to the configurations of the available HWT machines are recommended. Modifications are proposed to address equipment capabilities, components, or design features in order to ensure proper testing and accurate, reproducible results. 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 may need to modify their equipment to meet the new specification requirements. 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 recommends testing contrasting asphalt mixtures using the four main types of Hamburg test equipment available in the US market and comparing the results statistically in accordance with ASTM E 1169 Standard Practice for Conducting Ruggedness Tests.
