A rapid approach for considering nonlinear response of flexible pavements under FWD and estimation of remaining lives of pavements
Most mechanistic empirical pavement analyses are based on the layered elastic models where the stress sensitivity of geomaterials and their nonlinearity are either ignored or accounted for indirectly. Advanced numerical models, such as finite element models, are required to consider the nonlinear nature of the geomaterials. Existing pavements are often tested with the Falling Weight Deflectometer (FWD) to estimate the input moduli to the layered elastic models in order to estimate remaining life and structural capacity of pavements. It has been more than 30 years that FWD tests are being performed to measure structural adequacy of pavements. Backcalculation is the method of evaluation of pavement surface deflections caused by specific pavement deflection devices in order to determine the moduli of pavement layers. Even though the deflection bowls may be influenced by the load-induced nonlinearity during the FWD testing, the extraction of the layer moduli is carried out using a layered elastic forward model. For being consistent in evaluation of the readings, efforts to conduct backcalculation using the nonlinear algorithms are of great academic interest. Because of time consuming characteristic of nonlinear algorithm, this method is not practical for day-to-day implementation. This thesis puts forward a methodology used to relate deflections obtained from nonlinear analysis to those of linear elastic under FWD loading condition at conventional locations. This thesis has two significant contributions. Firstly, the conversion of linear elastic deflections to nonlinear deflections enables one to take full advantage of fast layered elastic analysis of pavements while the geomaterials’ stress dependency behavior is accounted for. Secondly, it will be shown in the relevant section that backcalculation based on linear elastic deflections at conventional locations results in a more realistic computation of layer moduli rather than simply using nonlinear field deflections. In addition, the estimation of critical pavement responses using structural parameters of pavements and FWD readings is also discussed. The conventional approach to determine remaining lives of pavements consists of the following steps. FWD is applied on the pavement to backcalculate layer moduli. Then the pavement becomes subject to design vehicle to calculate critical pavement strains using linear elastic software programs. Critical responses are then converted to allowable number of load application to reach failure. FWD loading is applied on various pavements while nonlinear material properties are present and deflections at conventional locations are measured using finite element (FE) analysis. FWD nonlinear field representative deflection bowls are simulated in the program. Then, equivalent single axle load (ESAL) dual tire is applied to the same pavement structures under linear elastic assumptions to follow the conventional procedure. Critical pavement responses include tensile strains at the bottom of HMA and compressive strains on the top of subgrade, are determined. ANNs are then developed to estimate these responses based on FWD readings and structural properties of the system. Backcalculation process oftentimes provides nonunique layer moduli. The usage of developed ANN provides the elimination of backcalculation process to estimate remaining life. Hence, by using ANN models one can predict remaining lives of pavements while there will be no need to go through backcalculation. ^
Shirazi, Shahabaldin, "A rapid approach for considering nonlinear response of flexible pavements under FWD and estimation of remaining lives of pavements" (2015). ETD Collection for University of Texas, El Paso. AAI10000816.