Effects of viscoelastic stress redistribution on the cracking performance of asphalt pavements
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Although the Rheological behavior of hot mix asphalt (HMA) mixtures has been well recognized, it has rarely been taken into account in predicting the pavement response and evaluating its cracking performance. In this paper, a detailed analysis of the rheological behavior of asphalt concrete (AC) and its effect on the pavement response is carried out using a viscoelastic boundary element method developed recently by the authors. The research findings show that the stress distribution in flexible pavements is continuously changing as the pavement is loaded, because viscoelastic bituminous materials relax stresses. Generally, both compressive stresses at the top and tensile stresses at the bottom of the AC layer reduce with increased loading time or number of repeated loads. Continuous repeated loads may lead to accumulation of residual tensile stresses at the surface of the pavement and residual compressive stresses at the bottom of the AC layer. The presence of significant tensile stresses at the top of the pavement layer results in an accumulation of dissipated creep strain energy (DCSE) over time (or with number of loads) and may eventually lead to the formation of a crack. As time increases, the tension at the bottom of AC layer reduces significantly and may finally turn into compression. At the same time, a considerable level of tension builds up at the surface of the AC layer through loading and unloading cycles. This could possibly explain why bottom-up cracking does not appear as frequently as top-down cracking. Interestingly, these findings also appear to explain why top-down cracks have not been observed in parking lots. In summary, this paper clearly shows that the rheological behavior of the HMA mixture results in load-induced stress redistributions that may dominate the failure mode of pavement structure. The results presented imply a wide range of consequences for pavement engineers. The nature of the viscoelastic stress redistributions depends on the rheology of the binder, mixtures, pavement structure, and the loads applied. Therefore, successful optimization of a pavement structure and mixtures for enhanced cracking performance may require the inclusion of viscoelastic effects. The research findings reported in this paper open a door for the evaluation of the cracking performance of asphalt pavements considering the rheology of asphalt mixtures and for the subsequent adjustment of the current criteria for selection and design of mixtures and binder types.