Thermally Induced Fracture Performance of Asphalt Mixtures

Abstract: A major distress mode in asphalt pavements is low temperature cracking, which results from the contraction and expansion of the asphalt pavement under extreme temperature changes. The potential for low temperature cracking is an interplay between the environment, the road structure and importantly the properties of the asphalt mixture. The thermal cracking performance of asphalt concrete mixtures can be evaluated by conducting thermal stress restrained specimen tests (TSRST) which is known to be correlated well with the fracture temperatures observed in the field. Although TSRST provides a good estimation of the field performance, it may be unrealistic to implement the obtained results in a design framework. On the other hand, recent studies showed Superpave indirect tension tests can be used to evaluate fracture performance (fatigue, moisture damage, low temperature cracking, etc.) of the asphalt concrete  mixtures. In addition, the obtained elastic and viscoelastic parameters from the Superpave IDT tests can be used as an input parameter to establish a design framework. The study presented in this thesis has a main objective to develop a framework using Superpave IDT test results as input parameters in order to evaluate the low temperature cracking performance of asphalt concrete mixtures. Moreover, the study aims to investigate micro-mechanically the low temperature cracking behavior of bitumen using atomic force microscopy (AFM) as a tool.The numerical model has been developed by integrating fracture energy threshold into an asphalt concrete thermal fracture model, considering non-linear thermal contraction coefficients. Based on the asphalt concrete mixture viscoelastic properties, this integrated model can predict thermally induced stresses and fracture temperatures. The elastic, viscoelastic and fracture energy input parameters of the model were measured by conducting indirect tension tests and the thermal contraction coefficients were measured experimentally. The proposed model has been validated by comparing the predicted fracture temperatures with the results obtained from TSRST tests. It was found that, while there is a quantitative discrepancy, the predicted ranking was correct. In the measurement of the thermal contraction coefficients it was observed that the thermal contraction coefficient in asphalt concrete is non-linear in the temperature range of interest for low temperature cracking. The implications of having non-linear thermal contraction coefficient were investigated numerically.In an effort to understand the effect of bitumen properties on low temperature fatigue cracking, AFM was used to characterize the morphology of bitumen. The AFM topographic and phase contrast image confirmed the existence of bee-shaped microstructure and different phases. The bitumen samples were subjected to both environmental and mechanical loading and after loading, micro-cracks appeared in the interfaces of the bitumen surface, confirming bitumen itself may also crack. It was also found that the presence of wax and wax crystallization plays a vital role in low temperature cracking performance of bitumen.