SOME FATIGUE CONSIDERATIONS IN THE DESIGN OF ASPHALT CONCRETE PAVEMENTS

Abstract

Fatigue failure on a road is examined, and fatigue concepts are used to simulate failure. In the development of this fatigue simulation, design factors such as truck traffic, documentation of failure, materials characterization, environmental conditions, and development of a fatigue computer program are considered in detail. A design procedure using these concepts is presented. Details are given of the location and history of the highway that was investigated and of the field study that was made to identify the method of failure. A weigh-scale site was located for checking and enforcing limits of 18,000 lbs single axle loads on this highway section. This provided information on vehicle gross weights, weight of individual axles and direction of traffic. The characterization of subgrade materials is discussed. The resilient modulus was determined by repetitive loading triaxial tests. Materials characterization parameters for asphalt concrete, base and subbase, and the Benkelman Beam deflections were used as input to the conputer program. This program with interation was used to solve for the subgrade modulus. The characterization of granular materials is reviewed. Density, moisture conditions, and lateral and axial strains are discussed and laboratory test results on base and subbase are tabulated. The material properties determined by laboratory methods were used to calculate deflections which were then compared to Benkelman Beam deflections. Good agreement was noted. The analytical study as well as laboratory determined strength properties were then used to simulate fatigue. The procedure used to do this is briefly outlined. A fatigue design procedure is detailed which consists of six steps: assumption of a pavement structure; calculation of pavement temperatures at bottom of asphalt mat; calculation of stresses and strains at bottom of asphalt mat; prediction of traffic loading; coupling of temperature, strains and traffic loading; and calculation of fatigue life and temperature, strains and traffic loading; and calculation of fatigue life and revision of pavement structure as required.