Validation of Backcalculated Modulus Values from Falling Weight Deflectometer through Instrumentation on I-35 in Oklahoma

Several state departments of transportation evaluate the modulus of pavement layers from nondestructive deflection tests such as the falling weight deflectometer (FWD). Although the FWD is a widely used nondestructive testing device, it still does not fully represent the loading conditions generated by moving vehicular traffic. Therefore, the modulus values backcalculated from FWD data may be different from those values backcalculated from vehicular traffic loading data. A comparison is presented of modulus values backcalculated from FWD and strain gauge measurements collected on a 1,000-ft-long instrumented section of I-35 in central Oklahoma. The FWD data were captured at different temperatures with different loading configurations. Meanwhile, longitudinal and transverse strain data were captured from May 30, 2008, through October 28, 2009, over a wide range of middepth pavement temperatures (40°F to 110°F) under actual vehicular traffic loading. The influence of different backcalculated modulus values is demonstrated through the prediction of remaining life in terms of number of equivalent single-axle loads of the existing pavement. Also presented is a description of the testing program, data collection efforts, and subsequent analysis for the I-35 instrumented section. Overall, modulus values backcalculated from the FWD and from longitudinal strain gauges showed close agreement at temperatures above 50°F.