METHODOLOGY FOR PREDICTING THE REFLECTION CRACKING LIFE OF ASPHALT CONCRETE OVERLAYS

An experimental testing procedure has been established in the report, for quantitative analysis of overlays designed to reduce reflection cracking. From this testing procedure the overlay scheme that shows the most resistance to cracking can be chosen. The procedure uses crack propagation tests on overlay samples which are conducted on a machine called the "overlay testor." This machine was specifically built to stimulate the displacements resulting from temperature changes in the cracked or jointed pavement or base materials that are beneath an overlay. Various types of overlay samples were investigated on the overlay tester. These include samples with fabric, such as "Petromat," and samples composed of different grades of asphalt, AC-5, AC-10, and AC-20; various gradations, open graded, dense graded, and hot sand mixes; and different overlay thickness of one, two, and three inches. In order to design an overlay, determining its proper thickness and reinforcing as well as the quantitites of the constituents used in the mix, its reflection cracking life must be predicted so that the proposed overlay's resistance to cracking can be easily seen. One way to determine the fatigue life is to perform fatigue tests on overlay samples using the overlay tester. These tests can be time consuming and costly. Another method to find the fatigue life is to use a method of stress analysis that considers the concentration of stresses near the crack tip, known as "fracture mechanics."