Numerical evaluation of surface-initiated cracking in flexible pavement overlays with field observations

Transverse cracking observed in the field overlay pavements is usually categorised into two types, (1) thermal cracking initiated at the surface of pavement and (2) reflective cracking initiated at the pavement layer above the existing pavement cracks or joints. Recently, another transverse cracking phenomenon was noticed in some field investigations but was less studied. Crack was observed from both the surface and the bottom of field cores, but cannot be visually observed from the middle layer. In addition, the surface and the bottom cracks lined up well showing the tendency of meeting each other. This study aimed to evaluate the causes of such transverse cracking phenomenon and identify the critical factors that could contribute to such cracking based on field pavement conditions. Three-dimensional (3-D) Finite-element Method models were developed to evaluate the critical pavement responses under different conditions. Both traffic load and thermal load were considered in the model. Field core data were used to validate the modelling results for specific influencing factors. Results showed that overlay thickness above the existing crack/joint as well as the stiffness of the overlay material can affect the location and amplitude of the critical responses. Potential mechanisms that could contribute to the surface-initiated cracking in the pavement overlay with a underneath crack/joint were: with thick pavement layer above existing crack/joint and stiff/aged overlay material, the existing pavement with crack/joint is protected from movement; hence the critical stress could be transferred from the bottom of pavement overlay to pavement surface by thermal loads. In other words, such observed cracking is essentially thermal cracking; with the joints/cracks underneath, the thermal cracks are localised to the surface above the joints/cracks.