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Bituminous Mixtures Laboratory (BML)

Equipment Hamburg Wheel Tracking Device

Figure 1: Close-up of wheel and slab (without water) photo
Figure1: Close-up of wheel and slab (without water)

Measures the rutting and moisture susceptibility of an asphalt paving mixture by rolling a steel wheel across the surface of an asphalt concrete slab that is immersed in hot water (generally held at 50°C.) Susceptibilities to rutting and moisture are based on pass/fail criteria.

The Hamburg Wheel-Tracking Device measures the combined effects of rutting and moisture damage by rolling a steel wheel across the surface of an asphalt concrete slab that is immersed in hot water. The device was developed in the 1970's by Esso A.G. of Hamburg, Germany, based on a similar British device that had a rubber tire. The machine was originally called the Esso Wheel-Tracking Device. The City of Hamburg finalized the test method and developed a pass/fail criterion to guarantee that mixtures that pass the test have a very low susceptibility to rutting.(1) This device costs $60,000 and is shown in figures 1 and 2.

The device was originally used by the City of Hamburg to measure rutting susceptibility. The test was performed for 9,540 wheel passes at either 40 or 50°C. Water was used to obtain the required test temperature instead of an environmental air chamber. The City of Hamburg later increased the number of wheel passes to 19,200 and found that some mixtures began to deteriorate from moisture damage. Greater than 10,000 wheel passes was generally needed to show the effects of moisture damage.

The machine tests slabs that typically have a length of 320 mm, a width of 260 mm, and a thickness of either 40, 80, or 120 mm. Thicknesses up to 150 mm can be tested. The thickness of the slab is specified to be a minimum of three times the nominal maximum aggregate size.(A) The mass of a slab having a thickness of 80 mm is approximately 15 kg. Pavement cores having a minimum diameter of 250 mm can also be tested.

The required air-void level for laboratory-prepared specimens is not given by the City of Hamburg procedure. The Federal Highway Administration at the Turner-Fairbank Highway Research Center is using 7 ±1 percent air voids for dense-graded hot-mix asphalts, and 5.5 ±0.5 percent for stone matrix asphalts. The Colorado Department of Transportation (CDOT) also uses 7 ±1 percent air voids for dense-graded hot-mix asphalts.(2)

Figure 2: Rut depth vs. number of wheel passes graph
Figure 2: Rut depth vs. number of wheel passes

Specimens are secured in reusable steel containers using plaster of Paris. Each specimen is placed into a container so that its surface is level with the top edge of the container. This allows the full range of the rut depth measurement system to be utilized. Containers are manufactured in heights of 40, 80, and 120 mm. Steel spacers can be placed under cores and pavement slabs if needed. The container with the specimen is then placed into the wheel-tracking device. The container rests on steel; this provides a rigid, load-bearing base for the specimen.

The temperature of the water bath can be set from 25 to 70°C. The most commonly used test temperature in Hamburg is 50°C, although 40°C has been used when testing certain base mixtures. A water temperature of 50°C is reached within 45 min. Specimens are conditioned at the test temperature for a minimum of 30 min. Heat is provided by heated coils in the water. The temperature of the water is then maintained by these heating coils and by introducing cold water from a faucet.(B)

The device tests two slabs simultaneously using two reciprocating solid steel wheels. The wheels have a diameter of 203.5 mm and a width of 47.0 mm. The load is fixed at 685 N and the average contact stress given by the manufacturer is 0.73 MPa. This assumes an average contact area of 970 m2, which is based on the 47.0-mm wheel width and an average contact length of 20.6 mm in the direction of travel. However, the contact area increases with rut depth, and thus the contact stress is variable. The manufacturer states that a contact stress of 0.73 MPa approximates the stress produced by one rear tire of a double-axle truck. The average speed of each wheel is approximately 1.1 km/h; each wheel travels approximately 230 mm before reversing direction, and the device operates at approximately 53 ±2 wheel passes/min.

The number of wheel passes being used in the United States is 20,000, although up to 100,000 wheel passes can be applied. CDOT recommends maximum allowable rut depths of 4 mm at 10,000 wheel passes and 10 mm at 20,000 wheel passes, based on correlations between the test results and moisture damage in dense-graded hot-mix asphalt pavements.(3) The City of Hamburg uses a maximum allowable rut depth of 4 mm at 19,200 wheel passes. The rut depth in each slab is measured automatically and continuously by a linear variable differential transformer that has an accuracy of 0.01 mm. A printout of the data can be obtained at every 20, 50,100, or 200 wheel passes. Approximately 6.5 h are needed to apply 20,000 wheel passes; however, the device will automatically stop if the rut depth in one of the slabs exceeds 30 mm. The total time to perform a test from start to finish, including specimen fabrication, is 3 days.

The post-compaction consolidation, creep slope, stripping inflection point, and stripping slope, shown in figure 3, can also be analyzed.(4) The post-compaction consolidation is the deformation (mm) at 1,000 wheel passes. It is called post-compaction consolidation because it is assumed that the wheel is densifying the mixture within the first 1,000 wheel passes.

The creep slope is used to measure rutting susceptibility. It measures the accumulation of permanent deformation primarily due to mechanisms other than moisture damage. It is the inverse of the rate of deformation (wheel passes per 1-mm rut depth) in the linear region of the plot between the post-compaction consolidation and the stripping inflection point. Creep slopes have been used to evaluate rutting susceptibility instead of rut depths because the number of wheel passes at which moisture damage starts to affect performance varies widely from mixture to mixture. Furthermore, the rut depths often exceed the maximum measurable rut depth of 25 to 30 mm, even if there is no moisture damage.

The stripping inflection point and the stripping slope are used to measure moisture damage. The stripping inflection point is the number of wheel passes at the intersection of the creep slope and the stripping slope. This is the number of wheel passes at which moisture damage starts to dominate performance. CDOT reports that an inflection point below 10,000 wheel passes indicates moisture susceptibility.(3) The stripping slope measures the accumulation of permanent deformation primarily due to moisture damage. It is the inverse of the rate of deformation (wheel passes per 1-mm rut depth) after the stripping inflection point.

Inverse slopes are used for both the creep slope and the stripping slope so that these slopes can be reported in terms of wheel passes along with the number of wheel passes at the stripping inflection point. Higher creep slopes, stripping inflection points, and stripping slopes indicate less damage.(4)

The shape of the curve in figure 1 is the same as typical permanent deformation curves provided by creep and repeated load tests. The curves from these tests are also broken down into three regions. The final region, called the tertiary region, is where the specimen is rapidly failing. Based on the examination of many slabs and pavement cores, the tertiary regions of the curves produced by the Hamburg Wheel-Tracking Device appear to be primarily related to moisture damage, rather than to other mechanisms that cause permanent deformation, such as viscous flow. Mixtures that are susceptible to moisture damage also tend to start losing fine aggregates around the stripping inflection point, and coarse aggregate particles may become dislodged. However, there is no method for separating the deformation due to viscous flow from the deformation due to moisture damage, because dry specimens cannot be tested. There is also no method for determining the amount of deformation and the amount of fine particles generated if any of the aggregate particles are crushed by the steel wheel.(C)

Additional disadvantages are that the data cannot be used in mechanistic pavement analyses and cannot be used to determine the modulus of the mixture or layer coefficients used by American Association of State Highway and Transportation Officials thickness design procedures. This is due to the complex and unknown state of stress in the slab.

FOOTNOTES

A. The effect of thickness on the test results has not been determined.

B. There may be some variability in the data resulting from the use of tap water effectiveness of some antistripping additives. Distilled water is specified in most test methods used to determine the moisture susceptibility of asphalt mixtures in order to reduce the between-laboratory testing variability.

C. Correlating the test data to field performance is difficult since the test combines two distress modes and the steel wheel can crush some aggregates.

REFERENCES

1. Tracking Test, Determination of the Track Depth of High-Stability Binding Layers. Construction Bureau, Civil Engineering Office, Department of City Traffic, Hamburg, Germany, 1991.

2. Aschenbrener, T. "Evaluation of the Hamburg Wheel-Tracking Device to Predict Moisture Damage in Hot-Mix Asphalt." Transportation Research Record 1492, Transportation Research Board, Washington, DC, 1995, pp. 193-201.

3. Aschenbrener, T., R. Terrel, and R. Zamora. Comparison of the Hamburg Wheel-Tracking Device and the Environmental Conditioning System to Pavements of Known Stripping Performance (CDOT-DTD-R-94-1) Colorado Department of Transportation, Denver, CO, January 1994.

4. Hines, M. "The Hamburg Wheel-Tracking Device." Proceedings of the Twenty-Eighth Paving and Transportation Conference. Civil Engineering Department, The University of New Mexico, Albuquerque, NM, 1991.

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Updated: 04/07/2011
  
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