The Falling Weight Deflectometer (FWD) has become a popular device for evaluating both airfield and highway pavements. Typically, deflection tests with the FWD or HWD (Heavy Weight Deflectometer) are performed at predetermined load levels up to approximately 120 kN (27,000 Ibf) for the FWD and 240 kN (54,000 Ibf) for the HWD. This paper addresses the choice of load level for pavements exhibiting stress-sensitive behavior and the effect both on backcalculated moduli and the resulting rehabilitation designs for a number of pavement evaluation projects performed over the last few years.
It is well known that many unbound materials used in pavements exhibit non-linear behavior in laboratory tests, as discussed in the paper. The effect of such non-linear response in pavement structures suggests that deflection testing should be performed using load levels at or near the expected design loads, or in a manner which allows reasonable determination of material parameters defining the non-linear behavior.
Several pavement studies have been selected that exhibit non-linear response using various HWD test load levels, from approximately 60 kN (13,000 Ibf) to the full 240 kN (54,000 Ibf), with all tests on a given project performed on the same series of test points, at the same time. Results of backcalculation and evaluation analyses for these pavements are presented in the paper.
Analysis of deflection basins measured at the different load levels show significantly different backcalculated moduli which appear to be consistent with the stress-stiffening or stress-softening behavior exhibited by similar materials in laboratory tests. As a result, evaluation of structural capacity for a given (expected) traffic loading is significantly affected by these moduli values, and required overlay thicknesses may vary by a factor of two or more for different deflection test load levels, at a given pavement section. The analyses presented in the paper clearly illustrate the necessity for adequately defining actual material response in evaluating the pavement system response. Asphalt surfaced pavements tend to be more prone to these non-linear response characteristics, due to the significant structural contribution of the unbound layers often found in these pavements.