@article{shan_tan_underwood_kim_2010, title={Application of Thixotropy to Analyze Fatigue and Healing Characteristics of Asphalt Binder}, volume={2179}, ISSN={0361-1981 2169-4052}, url={http://dx.doi.org/10.3141/2179-10}, DOI={10.3141/2179-10}, abstractNote={ The fatigue performance of asphalt binder is critical to understanding the fatigue performance of asphalt mixtures, especially the effects of healing. Although research into the fatigue and healing characteristics of asphalt binder is found in numerous references, an efficient technique to evaluate these characteristics still does not exist. Thixotropy is a concept that may help explain the material behavior and provide an efficient evaluation technique. This property is related to the breakdown and buildup of microstructure that may cause the changes observed during fatigue and healing tests. Thus, tracking the thixotropy of asphalt binders may be a good method to study fatigue and healing. For this study, experiments were performed to characterize the fatigue and healing characteristics of three typical asphalt binders. Then a common thixotropic model was characterized with a relatively simple stepped-flow test and oscillation experiment. The resulting model shows good correlation with the measured fatigue and healing tests. This finding, though based on a limited number of binders, suggests that thixotropy may play a role in the fatigue and healing characteristics of asphalt binder. }, number={1}, journal={Transportation Research Record: Journal of the Transportation Research Board}, publisher={SAGE Publications}, author={Shan, Liyan and Tan, Yiqiu and Underwood, Shane and Kim, Y. Richard}, year={2010}, month={Jan}, pages={85–92} }
 @article{underwood_heidari_guddati_kim_2005, title={Experimental Investigation of Anisotropy in Asphalt Concrete}, volume={1929}, ISBN={["0-309-09403-8"]}, ISSN={0361-1981 2169-4052}, url={http://dx.doi.org/10.1177/0361198105192900128}, DOI={10.3141/1929-28}, abstractNote={Accurate multiaxial characterization of asphalt concrete requires a thorough understanding of its anisotropic behavior. For that purpose a study has been conducted to examine the anisotropic properties of asphalt concrete in the linear viscoelastic range, with growing damage, and during volumetric deformation. Tests were conducted on specimens cored in the vertical and horizontal directions from gyratory-compacted specimens. Anisotropy was found to have no effect on the linear viscoelastic properties of the material. This finding is supported by subsequent results from monotonic constant crosshead rate uniaxial tension and uniaxial compression tests. It was also found that anisotropy contributes greatly to the behavior of asphalt concrete in compression, but shows little, if any, effect on tensile properties. In addition, the strong dependence of anisotropy on temperature and strain rate is presented. Finally, promising results from a simplified method of extracting the anisotropic behavior of asphalt concrete with the use of the hydrostatic test are also introduced.}, number={1}, journal={Transportation Research Record: Journal of the Transportation Research Board}, publisher={SAGE Publications}, author={Underwood, Shane and Heidari, A. Homayoun and Guddati, Murthy and Kim, Y. Richard}, year={2005}, month={Jan}, pages={238–247} }
 @article{underwood_kim_2003, title={Determination of Depth of Surface Cracks in Asphalt Pavements}, volume={1853}, ISSN={0361-1981 2169-4052}, url={http://dx.doi.org/10.3141/1853-16}, DOI={10.3141/1853-16}, abstractNote={ Nondestructive measurement of crack depths of asphalt pavements in situ could be a valuable tool for engineers in rehabilitation planning. Such measurements currently must be made by first coring or trenching a pavement and then measuring the crack by hand. Two methods for performing this task nondestructively are presented. The two methods, surface wave and ultrasonic, use the slowing effect that a crack has on a wave. Two signal-processing techniques were used to analyze the surface wave method—the fast Fourier transform (FFT) and the short kernel method (SKM). The FFT method provided a frequency spectrum that was used to find the energy carried by specific frequencies. The percent energy reduction (PER) was computed and plotted at each crack depth; this plot revealed that PER values increase as crack depth increases. The SKM method showed the wave velocity to decrease as the crack depth in creased. By comparing the wave velocity of the cracked pavement with that of the undamaged pavement, a phase velocity ratio plot was developed and was shown to be adequate for predicting crack depth. Ultrasonic testing proved to be a simpler and more direct method than surface wave testing. It was not necessary to know the wave properties of an undamaged pavement with this method, and a quantitative prediction of crack depth was obtained. While encouraging results were observed with both methods, ultrasonic testing showed the most promise for application because of the commercial availability of ultrasonic meters and the direct prediction of crack depth. }, number={1}, journal={Transportation Research Record: Journal of the Transportation Research Board}, publisher={SAGE Publications}, author={Underwood, Shane and Kim, Y. Richard}, year={2003}, month={Jan}, pages={143–149} }