# Initiation and Propagation of Top-Down Cracking in Asphalt Pavement
## 1 Introduction  
Cracking alongside the wheel path is one of the common distress types seen on asphalt pavements, especially on heavy-duty ones. Different from the traditional bottom-up fatigue cracking, this type of longitudinal cracking, observed within two to six years after opening to traffic [1; 2; 3; 4], initiates at the top part of asphalt layer and then propagates downward, and is often called top-down cracking (TDC). There are two different views on TDC mechanisms. Some researchers believe that TDC is caused by the tensile stress/strain initiated at the surface of the pavement. For example, [PERSON] [5] used Australian CIRCLY(a computer program for mechanistic pavement design and analysis) elastic analysis program to analyze the effects of the lateral and vertical contact stresses within the pavement and stresses of the tyre on the pavement within the asphalt layers, and concluded that the surface tensile stress at the tyre edge was the cause for top-down cracking. Similar findings were later published by [PERSON] [4] and [PERSON] [6], respectively. By using a finite element (FE) program CAPA (FE program developed at Delft University of Technology in the Netherlands), [PERSON] [7] analyzed the pavement structure under the measured non-uniform tyre-pavement contact load with the South African Vehicle-Road Pressure Transducer Array (VRSPTA). He found that higher tensile stress at the surface of the layer isassociated with non-uniform tyre-pavement contact pressure and aging of the asphalt mixture, which ultimately led to TDC. [PERSON] [8] suggested that longitudinal TDC cracks were caused by principal tension stress due to radial ply tyres. [PERSON] and [PERSON] et al. [9] later developed the models for TDC initiation and propagation based on the \"tension cracking mechanism assumption\".  
The second view on TDC is that it is caused by shear stress/strain. By using the FE method to analyze the pavement stress/strain distributions under tyre loads, [PERSON] [10] found that traffic-induced shear strain on the vertical plane at tyre edge was much larger than the lateral tensile strain at same position. [PERSON] [11] computed the stress distributions using the discrete element method (DEM) and bimodal mechanics. The results indicated that the crack was initiated at a depth of 25 mm, and obvious plastic deformation was observed below the cracking zone. He believed that the crack was caused by the shear stress. Based on the calculated octahedral shear strain for three pavement structures, [PERSON] et al. [12; 13] found that the maximum shear strain location is at the inner side of tyre edge, which indicated that the top-down crack was more likely to occur in the summer time. In recent years, [PERSON] and [PERSON] have shifted their research focuses from the \"tension cracking mechanism assumption\" to the \"shear cracking mechanism assumption\" and proposed that TDC was shear-induced or shear-dominant cracking.  
This paper, based on the latest findings from TDC-oriented accelerated pavement testing (APT) and laboratory uniaxial repeated penetration tests, develops and validates new TDC initiation and propagation mechanisms. Details are presented in the following sections.