An optimum pavement structure design requires characterization of materials under traffic loading. Investigation of stress-strain relationship of materials, under traffic loading is advised to determine the appropriate stiffness modulus. Since stiffness modulus is extensively dependent to loading dynamics, the loading parameters in the laboratory testing condition should simulate the field condition as close as possible. In this paper, the importance of accurate determination of stiffness modulus was discussed. Significant loading parameters including loading waveform, loading time, and rest time were expressed in subgrade layer. It was demonstrated that for subgrade layer; haversine loading waveform can better present what practically occurs in the field compared to the square waveform. Furthermore, for this layer, the effect of loading time is intensified due to the increase in depth and decrease in the quality of materials. In addition, because of elasto-plastic nature of subgrade material, the rest period should be considered in determination of stiffness modulus.

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