...Designing Mixtures That Perform
Background
The Hamburg Wheel Tracking Device is known as a Spurbildungsgerat in Germany, where it has been used as a specification tool since the mid 1970’s. Since Hamburg is the major seaport for Germany, the roads are subjected to a large number of heavily loaded, slow moving trucks. The Road Authority uses the Wheel Tracking Test as a specification requirement for their most severely stressed pavements. The Hamburg Wheel Tracking Device is currently being used by several laboratories, including the SemMaterials lab, to evaluate rutting and stripping characteristics of pavement samples from projects throughout North America. This is a “Proof Test” to complement mix design procedures, and is not presented as a competing technology to SUPERPAVE™.
The Testing Procedure
The test consists of two separate steel wheels moving back and forth on asphalt concrete specimens. The device can be used for testing 10” road cores, but is more often run simultaneously on two laboratory compacted asphalt concrete slabs, which are mounted and placed in a temperature controlled water bath. The slabs are compacted using the Linear Kneading Compactor. The kneading action of the compactor achieves the desired density without fracturing aggregates.
After duplicate specimens are prepared and pre-conditioned and placed in the device, the wheels are set in motion and automatic data recording starts. This data, collected per wheel pass, includes rut depth and bath temperature. The test continues for 20,000 cycles or 20 mm of deformation, whichever is reached first.
Data Interpretation
Colorado DOT has acquired several testing devices used in Europe, including the Hamburg Wheeltracking Device. A 1993 CDOT study comparing several test methods found that the Hamburg test results gave the best correlation to field rutting caused by moisture sensitivity (Report CDOT-DTD-R-93-19). As a result, Colorado now uses the Hamburg Device for an Incentive Payment Special Provision. For SemMaterials mixture evaluation and research projects, the Wheel Tracking Test data is analyzed beyond the reporting of a final result at 20,000 cycles. Four indices are measured from the graph (see below) of Permanent Deformation versus Wheel Passes:
The Post Compaction Consolidation is the amount of deformation which rapidly occurs during the first few minutes of the test. The steel wheel has some compacting effects on the mixes. A point of inflection occurs after this initial consolidation is completed.
The Inverse Creep Slope is reported in passes per mm. The higher this value the more resistant the mix is to permanent deformation.
The Stripping Inflection Point is determined where the Creep Slope and Stripping Slope intersect. It is defined as the number of passes at which moisture damage begins to adversely affect the mixture. The curve of Permanent Deformations vs. Wheel Passes abruptly turns downward. The Stripping Inflection Point is related to the amount of mechanical energy required to produce stripping, under the test conditions. A higher Stripping Inflection Point would mean that a pavement would be less likely to strip.
The Inverse Stripping Slope is similar to the Inverse Creep Slope in calculation. However, the slope is calculated after the Stripping Inflection Point. The lower the Inverse Stripping Slope the more severe the moisture damage.
We want to be your solutions provider. The Hamburg Wheel Tracking Device is just one tool we use to help you build, maintain and repair structurally sound pavements. For more information on help with your paving problems, contact your local SemMaterials representative.