STRUCTURAL OVERLAYS FOR PAVEMENT REHABILITATION

Research in California on the use of deflection measurements for asphalt concrete overlay design resulted in a revision to the California overlay design method presented at the Western Summer Meeting of the Highway Research Board in August 1970. Deflection reduction characteristics and tolerable deflection levels of asphalt concrete were revised based on the performance of highway projects under study since 1960. A higher initial deflection level will result in a slightly greater percentage of reduction for a given overlay thickness. An evaluation of the design method compares predicted versus measured deflections on 69 reconstructed highways. The deflection level after reconstruction can be predicted within plus or minus 0.008 in. (plus or minus 0.2 mm) with a 95 percent confidence limit by using the deflection reduction guide. Pavement deflections are measured by using the California traveling deflectometer, which provides a dynamic type of measurement while traveling along the roadway at 1/2 mph (0.8 km/h). An asphalt concrete overlay design guide was developed that simplifies the procedure for determining overlay thicknesses, but other factors such as the condition of the structural section and reflection cracking potential may alter the design. Since 1960, the overlay method has been used on approximately 450 different roadways. /Author/