A program to design asphalt concrete overlays to mitigate reflection cracking

Abstract

One of the major concerns for pavement engineers is that many pavements considered structurally sound after being overlaid exhibit a cracking pattern similar to the existing pavement. The new overlay cracks because of its inability to withstand the shear and tensile stresses due to the movement of the underlying pavement. This movement is due to traffic loading and thermal contraction. This propagation of an existing cracking pattern from the old pavement into and through a new overlay is called "Reflection Crack". Reinforcing grids reduce the amount of water that enters the sublayers of a pavement by reinforcing the overlay. The appearance of a reflection crack is delayed, and reinforcing the pavement reduces the width of the crack that develops. Various engineering reinforcing grids have been used in recent years to mitigate the occurrence and propagation of reflection cracking. Reinforcing grids made of fiberglass or polypropylene has been used for this purpose. The main objectives of the research are to understand the occurrence and behavior of reflection cracking and for devising ways of mitigating them and to put together an effective and complete package of computer programs to design asphalt concrete overlays. Another primary objective is to evaluate the performance of glass grids and the Stress Absorbing Membrane Interlayer (SAMI) in preventing or delaying the occurrence of reflective cracks and to develop design equations using fracture mechanics criteria for flexible overlays of both flexible and rigid pavements with the SAMI and reinforcing grids. The overlay procedure is mechanical-empirical in concept. The existing pavement is represented as a beam on an elastic foundation in the equations. Along with fracture mechanics, basic asphalt concrete properties were used to calculate the rate at which the cracks propagate through the overlay due to traffic and thermal action. The stress intensity factors (Ks) from finite element analysis for different combinations of layer thickness and moduli were interpolated to get the stress intensity factor at the tip of the crack. The rate of crack growth, the number of loads and the number of days for crack to propagate 0.05 inches were calculated. The Reflection Cracking Design program was formulated. The reinforcing glass grid and SAMI properties were included and evaluated. The results are evaluated for pavements with and without reinforcing grids. The results showed that the pavement performs better when it is reinforced with a fiberglass grid. The propagation of the reflection cracks is considerably stalled. Hence, through this thesis, it can be concluded that the asphalt concrete overlays with reinforcing grids are the best solution to mitigate the growth of reflection cracks.