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Asphalt mixture performance tests help agencies, contractors, and researchers
improve asphalt performance and prolong service life of flexible pavements.
This video is part of a series on asphalt performance testing
and demonstrates the Hamburg Wheel-Track Test, or HWTT.
This test determines the rutting and moisture susceptibility of asphalt mixtures using a reciprocating steel wheel testing device.
The Hamburg Wheel-Track Test is based on AASHTO T 324-19: Standard Method of Test for
Hamburg Wheel-Track Testing of Compacted Asphalt Mixtures.
All AASHTO standards mentioned in this video are
voluntary standards and are not required under Federal law.
Equipment and supplies needed for the Hamburg Wheel-Track Test include:
Hamburg Wheel Tracking Device;
Temperature control system; Wheel counter;
Specimen mounting system; Impression measurement system;
150-millimeter diameter SGC samples or field cores.
For laboratory prepared asphalt mixtures,
prepare and short-term oven condition the mixture according to AASHTO R 30.
Compact four 150-millimeter diameter samples to a target height and air void content
using a Superpave Gyratory Compactor, or SGC, as suggested in AASHTO T 312.
Specimen thickness should be at least twice the nominal maximum aggregate size,
generally yielding a specimen 38 to 100 millimeter thick.
Allow compacted specimens to cool on a clean, flat surface until cool to the touch.
Field cores are tested at the air void content
and thickness at which they are obtained or cut or trimmed to evaluate the layer of interest.
If the field core specimen is too short for the mounting tray, plaster of Paris,
aluminum or HDPE shims, or other suitable material can be used to fill the mounting tray.
Determine the bulk specific gravity of the test specimens as suggested in AASHTO T 166.
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Cut the specimens using a wet saw along equal secant lines, or chords,
such that when joined together in the molds, there is no space between the cut edges.
The amount of material sawed from the SGC cylindrical specimens may vary to achieve a gap width
no greater than 7.5 millimeters between the molds.
Consistent cutting may involve the use of a jig.
Secure the test specimens in the mounting tray in such a way that
the top of the specimen is level with the top surface of the HDPE molds.
Power on the testing device and all components and start the testing device software.
Enter pertinent project information and testing configuration parameters.
Select test temperature,
allowable rut depth, and maximum number of passes based on applicable specifications.
Fill the machine’s water bath with water using the valves, in the manual mode,
or automatically if the machine can operate in automatic mode.
Place the mounting trays with the test specimens into the Hamburg Wheel Tracking Device.
Adjust the height of the specimen tray following manufacturer recommendations,
and secure by hand-tightening the bolts.
Adjust the LDT height to obtain the height readings.
For manual mode, loosen the two screws on the LDT mount and slide the LDT up or down to the desired height.
Tighten the screws.
Ensure the micro-control unit's LDT reads between 10 and 18 millimeter.
For automatic mode, follow the manufacturer’s recommendations.
Enter a start delay of 45 minutes to condition the specimens to the selected test temperature.
Including the conditioning time, the specimens should not be submerged
longer than a total of 60 minutes plus or minus 5 minutes before starting the test.
After preconditioning, lower the wheels onto the edge of the specimens.
For machines that start automatically after the preconditioning cycle,
you may lower the wheels onto the edge of the test specimens
such that a majority of the wheel is in contact with the HDPE molds in the mounting tray.
For manual mode, click “start” to begin the test.
Run the test for either the specified number of cycles- typically 20,000 - or threshold rut depth.
The test should operate with a smooth movement across the test specimen.
At approximately 52 cycles per minute a 20,000 cycle test will last about 7 hours.
The Wheel Tracking device will stop at 20,000 passes or when the test has reached the maximum rutting depth.
The device software automatically saves the test data file.
After the test is complete, raise the wheels.
You may take an image of the specimens in the water after the test is completed.
The cloudiness of the water can often give valuable information to the data to determine stripping potential.
Drain the water bath
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and remove the specimen mounting trays and rutted specimens.
Clean the heating element, tray, and water bath according to the manufacturer’s recommendations.
Turn the wheels after each test, so the same section of the wheel surface
is not in contact with the test specimen from test to test.
Analyze the test data for number of passes to failure
and stripping inflection points as described in the standard or methods specified by the agency.
The rut depth can be taken as an average value of the five middle deformation locations.
For a typical Hamburg Wheel Track Test result report,
two sets or pairs of SGC specimens are tested, using a total of four SGC specimens.
A Hamburg Wheel Track Test data report may include the following parameters:
Asphalt mixture type; Compaction method if applicable;
Number of passes at maximum impression;
Maximum rut depth; Test temperature;
Specimen(s) air voids;
Type and amount of anti-stripping additive used;
Creep slope;
Stripping slope;
Stripping inflection point;
and device manufacturer and model.
For more information about asphalt testing procedures or other pavement testing resources,
visit fhwa.dot.gov.