Asphalt Layers Thickness Determination for Top Down Cracking Initiation and a Case Study

Top-down cracking (TDC) is a deterioration mechanism where cracks initiate at the pavement surface and propagate downwards with time. It is caused by horizontal tensile strains generated at the tire-pavement contact area. This paper presents a methodology for determining the thickness of the asphalt layer above which TDC will be the predominant deterioration mechanism in comparison to the bottom up cracking. The methodology covers two scenarios: one when all asphalt layers have the same stiffness modulus and the other when the asphalt layers have two different stiffness moduli. i.e. stiffness of surface layer (40 mm thick), S1, and stiffness of the rest of asphalt layers, S2. The findings concluded that in flexible and semi-flexible pavements with uniform asphalt stiffness layers, when the thickness of the asphalt layers is above a value within the range 100–210 mm (for ESAL80 axle loading), the pavement will most probably fail due to top-down cracking and not due to bottom up cracking. The exact thickness above which TDC will occur depends on the magnitude of Foundation Surface Modulus (FSM) and the asphalt stiffness. Similar range of thicknesses was found (100–230 mm) above which TDC will occur in flexible pavements with non-uniform asphalt stiffness values (S1/S2 is up to 1.13). A case study along a motorway section of 16 km long confirms the findings of the methodology developed.