Falling Weight Deflectometer (FWD) is a nondestructive method used for measuring the physical properties of pavement. The FWD generates a load pulse that simulates a moving wheel and measures the vertical deflection at the point of loading and at various offsets from the load. This load can be as low as 5,000 pounds when used for quality control of subgrade or aggregate base compaction, and as high as 29,000 pounds when used for airfield pavement evaluation. Deflections are measured by an array of 7 high-precision geophones, which are automatically lowered to the pavement surface at the beginning of the test cycle. Infrasense calibrates its FWD annually in accordance with the AASHTO R32 procedure, and monthly in accordance with the Long-Term Pavement Performance Program relative calibration procedure.
The FWD testing is carried out according to ASTM D4694 using a Dynatest Model 8002 trailer-mounted system. This system is typically equipped with 7 deflection sensors (geophones) at offsets 0″, 8″, 12″, 18″, 24″, 36″, and 60″ from the load cell, as well as an integrated distance encoder and GPS unit for location referencing. FWD tests are carried out at a specified interval along a project section or at target locations (e.g., joints in PCC pavements). The testing sequence typically includes a single seating drop, followed by multiple drops at increasing target loads (e.g., 6, 9, 12, and 16 kips). FWD data and corresponding project station, GPS coordinates, air temperature, and pavement temperature data are saved to the data collection laptop in a MS Access database file for subsequent processing.
FWD testing can be used for:
- Back calculation of pavement layer moduli
- Estimation of pavement remaining lift
- QA of newly constructed pavements
- Structural benchmarking of pavements
- Load transfer efficiency (LTE) across concrete joints