Backcalculation and Sensitivity of Non-Destructive Tests to Temperature Variations in Flexible Pavements

Abstract

<p>The Light Weight Deflectometer (LWD) is a smaller version of the Falling Weight Deflectometer (FWD) non-destructive test for determining the properties of the pavement-subgrade systems. Unlike the FWD, normally having up to 9 sensors measuring load-deflections histories, the LWD records the data using a central geophone located right under the loading plate. Up to two sensors can be attached to the device though. Therefore, the limited information obtained from the LWD creates some difficulties for the backcalculation.</p> <p>Elastostatic method of backcalculation layer elastic moduli of pavement structures does not take into account the dynamic nature of tests. Another limitation of the elastostatic approach is that it gives only one piece of information from each loaddeflection history recorded by a geophone. The number of unknowns typically exceeds two (at least pavement and subgrade elastic moduli for a two-layer approximation) for the real pavement-subgrade structure.</p> <p>The dynamic impedance function for the Mindlin plate idealization supported by elastic half space is calculated and related to the impedance of an equivalent single degree of freedom (SDOF) oscillator approximation. Unlike elastostatic, dynamic backcalculation provides one with two pieces of information (real and imaginary components of the dynamic impedance) allowing one to back-calculate an additional pavement-subgrade system property. It is shown, that elastostatic backcalculation gives the elastostatic stiffness value which, combined with the dynamic impedance of the Mindlin plate model, allows one to estimate an effective sub grade modulus E_s. and elastic modulus of pavement E_p or an equivalent asphalt thickness h_a. The presence of shallow bedrock indicated, however, the backcalculation with the simplified model can be problematic.</p> <p>The back-calculation approach suggested in this study is applied to sets of in-situ and simulated data. A frequency domain analysis showed some limitations for backcalculation of pavement-subgrade properties for real LWD data.</p> <p>The low frequency LWD and FWD devices (up to 50-80 Hz), as well as the higher frequency IE test (20 kHz), are compared in terms of temperature sensitivity of the respective responses. Non-isothermal computer simulations were performed and comparison was made. The results demonstrate that the LWD is more sensitive to temperature changes in the pavement layer than the FWD. Back-calculated elastic moduli of the sub grade did not show any sensitivity to temperature fluctuations. The impact echo test response showed high sensitivity of the response to temperature changes in thin pavements (up to 100 mm thick).</p>