Guidelines for Structural Evaluation and Strengthening of Flexible Road Pavements Using Falling Weight Deflectometer (FWD) Technique
2014 Edition

The recommended process for conducting FWD deflection tests involves: marking the test location, positioning the calibrated FWD load plate ensuring no water underneath, verifying pavement slopes do not exceed 10%, placing displacement sensors (geophones) on the surface, setting the falling mass to achieve a target load of 40 kN, performing a seating load drop without recording data, executing at least three load drops while recording peak loads and deflections at multiple radial distances, adjusting the load if deflections are not within expected ranges, and recording ambient air and pavement temperatures periodically. This approach ensures reliable and repeatable deflection measurements essential for pavement structural assessment.

Pavement layer moduli are backcalculated from FWD deflection measurements normalized to a 40 kN load. Using inputs such as deflection values, radial sensor distances, layer thicknesses, Poisson's ratios, applied load, and load plate radius, an iterative optimization algorithm adjusts assumed layer stiffness values to minimize differences between measured and computed deflections. The KGPBACK software, developed by IIT Kharagpur, is specifically recommended in IRC 115 for this purpose. After backcalculation, bituminous layer moduli are temperature-corrected to 35°C to standardize design conditions.

Bituminous layer moduli obtained from FWD tests require correction to a standard temperature of 35°C to reflect typical Indian tropical conditions. The correction uses a logarithmic formula where the corrected modulus equals the measured modulus raised to a temperature-dependent exponent, valid for test temperatures between 20°C and 45°C. Thin bituminous layers under 40 mm or sections classified as 'Poor' do not require correction. In cold or high-altitude regions with prolonged low temperatures, moduli are measured only when pavement temperature exceeds 20°C, negating the need for adjustment.

IRC 115 prescribes a mechanistic-empirical overlay design process using FWD data by first measuring and normalizing pavement surface deflections, collecting layer thickness and type data, and backcalculating layer moduli with software assuming a three-layer pavement system. Moduli are adjusted for temperature and moisture, and the 15th percentile values are selected for analysis. Critical strains at the bituminous layer bottom and subgrade top are computed using linear elastic layered theory. Remaining pavement life is estimated using fatigue and rutting models per IRC:37-2012. Overlay thickness trials are conducted to ensure strain values remain below allowable limits for the calculated design traffic, ensuring structural adequacy and durability.

Test pits should be excavated measuring approximately 0.6 m by 0.6 m at intervals of about 1 km or greater when pavement uniformity is established. These pits are located along the outer lanes starting from the edge of the pavement into the shoulder. The number of pavement layers, their material descriptions, any visible distress, and thicknesses are carefully documented. Samples may be collected for laboratory testing to assess material degradation and stiffness. After recording, pits must be refilled and compacted promptly, with safety barricades and night visibility measures implemented until backfilling is complete. This procedure complements other non-destructive methods to ensure accurate pavement structural evaluation.