Viscoelastic response of flexible pavements under moving dynamic loads

This study proposes a mechanistic design model that analyzes the structural performance of flexible pavements under moving dynamic loads generated by heavy vehicles. It assumes a linear viscoelastic behavior of flexible pavements. The model expands the framework formulated by the Federal Highway Administration (FHWA) for the mechanistic design program called VESYS. It applies Boltzman's superposition principle to translate the pavement response from a stationary load to pavement response from a moving load of time-dependent amplitude. Experimental dynamic load data obtained using the instrumented vehicle developed by the National Research Council of Canada (NRCC) are used as input in the model. -- The variable dynamic responses along the longitudinal axis of the pavement are translated into fatigue cracking and rutting by assuming spatial repeatability of the dynamic loads. The 90th percentile of the cumulative damage values is assumed to represent the overall damage of pavement section. This damage value is then compared with the damage caused by moving loads of constant magnitude (static) through a relationship called Pavement Life Ratio (PLR). This is defined as the ratio of the accumulated number of load repetitions to failure due to static load divided by repetitions to failure due to dynamic loads. -- Furthermore, the model is used to investigate the performance of two suspension types (Air and Rubber) on three types of flexible pavements with different structural strengths. The effects of other vehicle characteristics on pavement damage is also investigated. -- Results from the analysis show that, moving dynamic loads are more crucial to pavement damage than static loads and the magnitude of the damage is dependent on the suspension type. It was found that, rubber suspension causes greater pavement damage than air suspension and the difference can be as much as 43%. Moreover pavement surface roughness and vehicle parameters like speed, axle spacing and axle configuration substantially influence the suspension type to increase pavement damage. For multiple axles, unequal load-sharing between the individual axles further increase pavement damage.