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Abstract
The Hamburg Wheel Tracking (HWT) laboratory test uses loaded wheel(s) to apply a moving load on asphalt mixture specimens to simulate traffic loading on asphalt pavements. Given that different machines with various degrees of compliance with the current AASHTO T 324 requirements are used by highway agencies, this study aimed to assess the capabilities of commercially available and representative HWT equipment, and to evaluate different analysis and reporting methods for rutting and stripping performance assessment. After performing a comprehensive evaluation of devices from different vendors, considerable discrepancies on equipment capabilities and configurations were identified. The machines did not meet all the requirements set forth in AASHTO T 324 including those for the wheel position waveform, the temperature range, and the reporting parameters. Evaluation of existing analysis methods revealed significant inconsistencies among different methods and deficiencies in the specification. Recommended modifications to the machines and test method were provided.
Acknowledgement
This study is based on the results of NCHRP 20-07 (Task 361), Hamburg Wheel Track Test Equipment Requirements and Improvements to AASHTO T 324. The authors would like to acknowledge the assistance of the Technical Review Panel for NCHRP Project 20-07 (Task 361), Dr. Ed Harrigan, and the Louisiana Department of Transportation and Development. The assistance of vendors in providing support to the researchers and the staff of the Louisiana Transportation Research Center (LTRC) is greatly appreciated.
Discussion
DAVID ANDERSON: Interesting work, Louay. How did you establish the tolerances, the levels for the tolerances? The tolerances, I would expect, reflect the interpretation of the data or their significance to the data or are they simply the ability to accomplish the test? What are the criteria for establishing the tolerances?
LOUAY MOHAMMAD: We established the tolerances basically in how good the best machine that could reproduce the known profile within that framework, and that was established as the tolerance because we didn’t want to eliminate …
DAVID ANDERSON: Okay, so it’s sort of an ad hoc way to establish the tolerances.
LOUAY MOHAMMAD:: Again, this is a dynamic process. We wanted to establish something. As we have more data, this probably could change; however, at this point, it’s a good approach in an initial assessment in that you are comparing those machine operational requirements to a known expectation.
DAVID ANDERSON: But you haven’t done it in a ruggedness scenario to see how these different tolerances affect the output from the machine or the interpretation of the data.
LOUAY MOHAMMAD: That’s well understood, and the charge to us from the study was we are not addressing the effect of those on the test results.
DAVID ANDERSON: I just wanted some background.
LOUAY MOHAMMAD: Yeah, that was exactly … Remember what I said and I qualified it. Everything we’ve done is on the operation of the equipment, and how those operational differences in the equipment affect the performance of an asphalt mixture is the next step.
DAVID ANDERSON: Thank you.
LOUAY MOHAMMAD: And if you would like to fund the study, Dave, I’ll welcome you in.
GAYLE KING: Louay, long overdue but a fine piece of work. Thank you.
LOUAY MOHAMMAD: Thank you.
GAYLE KING: I’ve had the chance to work with several different Hamburg units through the years, starting in 1990. Now, 27 years after the instrument arrived in the US, finally you’re doing the ruggedness testing that should have done from the beginning. Our results always seemed consistent and reliable though, with one puzzling exception. For a while, the two wheels in the same unit were yielding significantly different results. We finally determined that the wheel bearing would start to fail, causing the wheel to drag rather than rolling freely. Drag obviously made the test more severe and the mix rutted more. How would your protocol catch that kind of problem before bad data is reported?
LOUAY MOHAMMAD: Well, you know, it’s like anything else we do. In any piece of equipment, we must calibrate, and this particular equipment, there was no calibration procedure done. And so, this study offered a methodology to calibrate every component that you want to check. So, if there is a problem with wearing on the bearing, then you would catch it basically if you do that calibration.
GEOFFREY ROWE: Louay, I’m Geoff Rowe over here. I don’t think you see me because there’s a big bright light behind me!
LOUAY MOHAMMAD: Yes, I can see you now.
GEOFFREY ROWE: Very nice piece of work, Louay. I’m just going to make a fun historical comment. For those of us who enjoy looking at old asphalt things! If you go back into the early 1930s, you’ll see that immersion wheel tracking started in the USA with a circular test track originally developed by US Bureau of Public Roads (AAPT, 1933 – pages 54 to 62) and the Standard Oil Company Development Company also used it in New Jersey (ASTM, 1937) back in that time frame. Later folks in Europe referenced this work in the development of devices used in the UK. Finally, Esso further developed this device in Hamburg and it became known as the HWT Test. The use of this device then came back to the US, and we’re using it now here in the USA probably more than anyone else in the world. But from a historical point of view, go back in the old AAPTs and there it is. Immersion wheel tracking started and was first published in AAPT round about 1930 (see 1933 volume).
LOUAY MOHAMMAD: Thank you for the historical news.