ABSTRACT
Top-down cracking is a well recognized pavement deterioration mechanism where cracks initiate at the pavement surface and propagate downwards with time. Top-down cracking is caused by horizontal (surface) tensile strains generated at the edge of the tire. Factors affecting the magnitude of the horizontal tensile strains are mainly the: applied load, tire configuration, stiffness and thickness of the asphalt layers, thickness and material of base/sub-base layers, and bearing capacity of the subgrade. The tire structure also has a variable detrimental effect. This paper, adopting the principal of Foundation Surface Modulus (FSM), determines the thickness of the asphalt layer above which top-down cracking is the only pavement deterioration mechanism in comparison to the bottom up cracking. The study considered FSM ranging from 50 MPa to 400 MPa which covers the flexible and flexible with hydraulically bound base (semi-flexible) type of pavements. It also considered asphalt layer stiffness ranging from 2,000 MPa to 12,000 MPa which covers low, medium, or high stiffness asphalts and indirectly asphalt’s stiffness variation due to air temperature or oxidization of asphalts with time. As for the type of axle loads and tire structure, single axle-dual tire loads of 80 kN and 130 kN, single axle-single tire load of 40 kN and super single axle-singe tire of 140 kN, all with smooth tire surface, were considered. For the above-mentioned axle loads the findings concluded that when the thickness of the asphalt layers is greater than, approximately, 210 mm, 250 mm, 220 mm and 330 mm, respectively, the flexible or semi-flexible pavement will fail only due to top-down cracking and not due to bottom up cracking regardless of the magnitude of FSM and asphalt stiffness. However, top down cracking also occurs at lesser thicknesses of asphalt layers depending on the magnitude of FSM and asphalt stiffness.
