Top-down cracking is a type of cracking that rivals the severity and prevalence of reflective cracking. It significantly reduces the pavement's quality service life. Yet the nature of top-down cracking has not been completely understood. Recent studies of the causes of top-down cracking have focused on identifying the mechanisms that induce tensile stresses at the surface by applying different combinations of surface tractions and the finite element method. Asphalt concrete is treated as a uniform linear elastic material. A new and different approach is presented for investigating the causes of top-down cracking by means of micromechanics. In this approach, asphalt concrete is viewed as a bonded granular material, and the microstructure, including aggregate particle configuration and mastic stiffness, is considered. Theories that predict the existence of tensile stress under compressive loading were reviewed. Both qualitative and quantitative experimental methods were developed to observe the location of top-down cracking and to measure the tensile strains in the pavement. The experimental results indicate the following: (a) top-down cracking may initiate not only at the pavement surface but also at some distance down from the surface; (b) both tensile-type and shear-type cracking could initiate top-down cracking; and (c) top-down cracking may most likely initiate when the mastic is weaker or the pavement temperature is higher. Therefore, a mix sensitive to rutting may also be sensitive to cracking.
Abstract