Enhancing Backcalculation Procedures Through Consideration of Thickness Variability

Abstract

Pavement structural performance and in situ structural capacity are evaluated by performing deflection testing and backcalculation analysis. Layer thickness information obtained from as-built data and core-bore logs is an essential input for this analysis and has significant impact on the backcalculated layer moduli. However, as-built records are not always available, and they provide general layer thickness information. Also, the number of cores and bores is limited because of their destructive nature. The current practice of backcalculation uses the common assumption that a pavement section has constant layer thickness, with allowable tolerance ignored, and that all variability in the measured deflection is related to variability in material stiffness. A pavement section is not expected to have identical thickness along its length. Also, pavement materials are undeniably nonhomogeneous and have variable stiffness along section length. However, this variability is expected to be within reasonable limits, especially for relatively new pavements constructed in accordance with stringent construction specifications and material properties and which exhibit similar distresses and deflections. A procedure was developed to account for the normal variability in layer thickness (allowable tolerance). This procedure is outlined and an implementation example is provided. The impact of ignoring the allowed variability in layer thickness on the backcalculation results, as well as on the design pavement moduli that are used in the rehabilitation analysis and design, is discussed.