How does the deflection basin shape obtained from FWD testing inform decisions regarding pavement rehabilitation strategies?

The Falling Weight Deflectometer (FWD) test applies a controlled load to the pavement surface, and sensors measure the resulting deflections at various distances from the load. These deflection measurements create a "deflection basin," which is a graphical representation of how the pavement deflects under the load. The shape of this basin provides key information about the structural condition of the pavement and the properties of its underlying layers. A narrow, deep basin indicates a relatively stiff pavement structure with a weak subgrade, meaning the load is concentrated near the loading point and the subgrade is deflecting significantly. This suggests that rehabilitation strategies should focus on improving the subgrade's support, possibly through stabilization techniques or drainage improvements. A wide, shallow basin suggests a weaker pavement structure but a stronger subgrade, indicating that the pavement layers themselves are contributing to the majority of the deflection. In this case, rehabilitation should focus on strengthening the pavement layers, such as through overlays or reconstruction. The overall magnitude of the deflections is also important; larger deflections generally indicate a weaker pavement structure. By analyzing the deflection basin, engineers can backcalculate the elastic moduli of the pavement layers and the subgrade. Elastic modulus is a measure of a material's stiffness. This allows for a more accurate assessment of the pavement's structural capacity and helps determine the most appropriate and cost-effective rehabilitation strategy. For example, if backcalculation shows a weak asphalt layer, an asphalt overlay might be selected. If the base layer is weak, full-depth reclamation might be needed. Therefore, the deflection basin provides a crucial fingerprint of the pavement's structural condition, guiding the selection of rehabilitation strategies tailored to address the specific weaknesses identified.