Asphalt concrete mixture is the most frequently used material for building roads. Asphalt concrete is a mixture of asphalt binder and aggregates. The mixture is designed by adjusting the quantity of asphalt binder to balance two common distresses: rutting and cracking. Rutting is the formation of depressions in the pavement in the direction of the traffic flow caused by repeated wheel loadings. If the percentage of asphalt binder is too low, the mixture is prone to cracking. If the percentage of asphalt binder is too high, the mixture is prone to rutting.
Loaded wheel tracking devices have been used in the asphalt industry to determine pavement mixture design rutting performance in the laboratory by simulating a vehicle wheel rolling over a sample of asphalt mixture. These devices apply a weight to a wheel—either by dead weight or applied weight using a pneumatic system—that travels back and forth on a sample of compacted asphalt mixture. If the mixture is not designed properly with the right combination of aggregates, aggregate gradation, and binder content, the samples will rut prematurely. The wheel tracking test allows designers and practitioners to adjust the mixture to meet the requirements of the pavement conditions, including the environment and traffic load.
One of wheel-tracking tests is described in the AASHTO T324 standard and is known in the industry as the Hamburg Wheel-Tracking (HWT) test. Referring to FIG. 1, in a typical HWT test, two cylindrical asphalt mixture samples 12 are cut to form a continuous surface. The samples 12 are placed in a tray 10 and surrounded by plastic molds 14 to maintain their circular shape during testing and then are placed in 50° C. water. The samples 12 and the molds 14 are held within a sample compartment 16. The samples 12 are loaded with a wheel load of 703 N (158 lbf). More specifically, the samples 12 are loaded into a Hamburg Wheel Tracker device such as the Hamburg Wheel Tracker device 50 illustrated in FIG. 2. The Hamburg Wheel Tracker device 50 includes one or more (typically two) load arm assemblies 52. Each load arm assembly 52 includes a wheel 54 as shown in FIG. 3. The wheel 54 passes back and forth over the samples 12 as indicated by the arrow 56 in FIG. 3. The test is performed to a maximum of 20,000 passes of the wheel over the specimens or until a vertical deformation of 12.5 mm is recorded Hamburg Wheel Tracker devices and HWT tests are described in more detail in U.S. Patent Application Publication No. 2015/0292989, the disclosure of which is incorporated by reference in its entirety.
Hamburg Wheel Trackers currently used in the industry have four distinct actions that are verified to meet the equipment requirements of the standard: the load applied to the sample, the measurement of the vertical displacement of the wheel as the sample ruts, the temperature of the sample conditioning environment, and the horizontal displacement, velocity, and waveform pattern of the wheel along the wheel path. Because asphalt mixture is sensitive to load and temperature, these values must be accurately measured.
Load, vertical displacement, and temperature can be measured using equipment such as calibrated load cells, height blocks, and temperature devices, respectively, but such equipment requires considerable effort such as lifting weights over 100 lbs to place the load cell beneath the wheel as well as removing the displacement measurement devices from the Hamburg Wheel Tracker device.
Perhaps the most difficult part of complying with the test standard is verifying the correct horizontal motion of the wheel because this is a dynamic process and cannot be performed with the wheel in one position like the load and vertical displacement measurements. The verifications include the goodness of fit of horizontal wheel movement to a sinusoidal shape, the speed of the wheel at the center of motion, the period of motion, and the amplitude of the motion or the distance traveled. The horizontal motion is important to ensure even loading of the samples or specimens so results can be accurately compared between laboratories. Also, the center of motion of the load should be symmetrical over the two specimens being loaded. The interface or joint between the two specimens can be a weakened area that generally ruts more than other areas. Therefore, the vertical displacement system should be aligned with this interface to accurately measure the displacement at this critical position.