Friday, December 16, 2011

Hamburg Wheel Test Overview

There have been varying opinions of the Tensile Strength Ratio (TSR) test and how well it really reflects moisture sensitivity.  Because of these concerns, Caltrans is currently moving towards the replacement of the TSR test with the Hamburg Wheel Tracking Test which originated in Germany in the mid 1970s.  The test examines the susceptibility of the HMA to rutting and moisture damage.
Hamburg Wheel Tracking Device (HWTD) from (Stuart and Youtcheff, 2002).
The Hamburg Wheel Tracking Test uses a steel wheel with weight that rolls over the sample in a heated water bath.  A designated number of passes are performed on the sample.  I'm not sure what number Caltrans will settle on but in the United States, 20,000 passes seems to be used very commonly.  The rut depth is measured by the machine periodically, usually every 20, 50, 100 or 200 passes.  20,000 passes can take around 6.5 hours whereas the entire test can take around 3 days.  As you know, this is not much better than the TSR test so it doesn't look like we'll be shortening our mix design turn around time in the near future.

Several analytics are examined with the Hamburg Wheel Tracking Test including post-compaction consolidation, creep slope, stripping inflection point, and stripping slope.  The chart below illustrates these pretty well:

APA Samples showing rutting after 8,000 load cycles.
*This chart's data is for the Asphalt Pavement Analyzer, not the Hamburg Device but illustrates the  terminology well.

This type of information can tell us not only at what point in a roads' life it will become moisture sensitive but also how much the moisture will affect the road once it hits that point.  Studies have found that there is good correlation between the Hamburg test and field performance but it has also been determined that the test can fail to differentiate between some mixtures.  If you are interested in some of the more nitty gritty details there is a great writeup about the test by the Federal Highway Administration here and an evaluation of the Hamburg test for Caltrans by UC Davis here.

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