# Laboratory Validation of Surface-Initiated Transverse Cracking of Asphalt Pavement
## 2 Methodology  
The Hamburg wheel-tracking device (HWTD) is used in this study to evaluate the cracking potential of the material or structure under various conditions. The HWTD was firstly developed in Germany and is now extensively used by many US state Departments of Transportation (DOTs) and industries to evaluate the rutting and moisture damage potential of asphalt mixture. It applies repeated loading on asphalt mixture specimens submerged in water at relatively high temperature (typically at 50 \({}^{\circ}\)C) and uses rut depth and a stripping inflection point at a specific loading cycle to describe the rutting and moisture resistance of the mixture. As specified in AASHTO T324 [9], Hamburg Wheel-Track Testing of Compacted Hot Mix Asphalt (HMA), the standard load on the wheel is around 705 N (158 lb). The wheel rolls on the specimen at 52 \(\pm\) 2 passes per minute, with the position varying sinusoidally over time. The wheel-tracking device shall shut off when 20,000 passes have occurred or when the test has achieved the maximum rut depth (i.e., 0.5 inches). Research studies [10; 11] indicate that the rut depth from the HWTD can correlate well with field rutting and moisture damage performance; thus, it has become popular equipment at both research institutes and industry laboratories.  
As a repeated loading device that simulates cyclic traffic loading in the field, it is hypothesized that HWTD can also be utilized for evaluating the cracking resistance of the pavement. However, considering the fundamental difference in failure mechanism between rutting and cracking, the HWTD needs to be modified and improved in the following four areas in order to make it potentially a cracking evaluation device as well. They include:  
* Test conditions need to be changed to ensure that cracking instead of permanent deformation is the dominant failure mechanism. Typically, asphalt mixture reacts as viscoelastic-plastic material under standard HWTD loading condition, since the test is performed at a relative high temperature (i.e., 50 \({}^{\circ}\)C) and low loading rate (52 passes per minute). To obtain asphalt material that responds as more elastic-brittle solid, the test temperature should be decreased to room level (i.e., 20-25 \({}^{\circ}\)C) or even lower (close to zero degrees), or the wheel load speed needs to be increased to a higher value that can simulate a fast-moving vehicle.
* Long-term aging of the specimen should be included, since it is a critical factor that affects both fatigue cracking and low-temperature thermal cracking [3; 12; 13]. The aging of asphalt mixture increases the stiffness of the material, reduces its ductility, and could negatively affect its resistance to fatigue-induced cracking [14; 15]. It is also found that aging decreases the low-temperature cracking resistance of asphalt concrete pavement and increases the crack frequency [16].
* The base (or support) stiffness of the test specimen should be adjustable to match with the field pavement condition. Such adjustment can be achieved by placing rubber with different stiffness under the specimen. Typically, the potential for fatigue cracking initiation increases with the decrease of the base modulus [17].  
Based on these considerations, the equipment from the _James Cox & Sons, Inc._ company is selected, and some modifications were made to their standard Hamburg wheel-tracking device (HWTD) so that the cracking properties of the material can be evaluated. These changes include:  
* An extra chiller was connected to the equipment to maintain test temperature at around 20 \({}^{\circ}\)C.
* Wider loading rate range: maximum 70 passes per minute, which is faster than the standard wheel load speed (52 \(\pm\) 2).
* Capable of including rubber base to adjust the support conditions.  
Figure 1 shows a picture of HWDT from _James Cox & Sons, Inc._ A linear variable differential transducer (LVDT) was used to measure the rut depth along the length of the wheel's path over a range of 0 to 20 mm (0-0.8 inches).