Guidelines for the Structural Evaluation of Rigid Pavement by Falling Weight Deflectometer (without CD)
2014 Edition

Calibration of the Falling Weight Deflectometer (FWD) is vital to ensure reliable and consistent deflection measurements. According to IRC:115-2014 Clause 5.5, the process involves verifying the accuracy of load sensors by comparing readings against known weights, calibrating deflection sensors using standard reference plates or devices, performing repeatability tests by conducting multiple drops at identical locations to confirm measurement consistency, and considering environmental factors similar to field conditions. These steps guarantee precise data collection essential for structural evaluation.

IRC 117 advises periodic inspections every 3 to 5 years or upon the appearance of cracks to detect voids beneath pavements. Detection methods include using Falling Weight Deflectometer (FWD) testing and Ground Penetrating Radar (GPR). Voids typically form due to permanent settlement of subgrade or granular layers under heavy traffic loads. Repair involves cementitious grouting injected under pressure through drilled holes to fill the voids, followed by curing before reopening to traffic. Additionally, retrofitting with dowel or tie bars may be employed to restore load transfer efficiency, as detailed in Appendix I of the standard.

Load Transfer Efficiency (LTE) is evaluated primarily through Falling Weight Deflectometer (FWD) testing by measuring deflections on both the loaded and unloaded sides of a pavement joint. The LTE is calculated as the ratio of the unloaded slab deflection to the loaded slab deflection, multiplied by 100%. For sensor spacing of 300 mm, a correction factor is applied. High LTE values near 100% indicate efficient load transfer, whereas values below 50% for transverse joints or 40% for longitudinal joints signify deterioration requiring retrofitting with dowel or tie bars. Regular FWD assessments every 3 to 5 years help monitor joint conditions effectively.

From FWD measurements, deflections at fixed distances (0, 300, 600, and 900 mm) from the load center are recorded. The deflection basin area parameter is computed as six times the square of the sum of the central deflection plus twice the sum of the other three deflections. Using this parameter, the radius of relative stiffness is determined from charts or computational tools. Subsequently, the subgrade modulus (k) is calculated considering the concrete elastic modulus (Ec), slab thickness, radius of relative stiffness, and Poisson's ratio. The elastic modulus of concrete is then computed from the subgrade modulus and geometric parameters. For design, the subgrade modulus is typically taken as half the FWD-derived value to account for static loading conditions.

IRC 117 recommends that structural assessments using Falling Weight Deflectometer (FWD) be carried out at intervals of 3 to 5 years. This frequency allows for timely identification of pavement distress, such as void formation or joint deterioration, enabling prompt maintenance actions. Testing should focus on critical locations including slab interiors, corners, and joints, and be conducted under favorable temperature conditions to minimize the influence of thermal gradients on deflection readings.