The Falling Weight Deflectometer is a non-destructive testing device that has been widely used in pavement engineering to evaluate pavement structural condition. Though the FWD has been in use in Europe for over 40 years, and is fairly common on the East Coast, this technology is only beginning to gain popularity in Western North America. WCRT owns two FWD units, one trailer-mounted, one truck-mounted.
How the FWD works
The FWD is designed to apply dynamic loads to a pavement surface, which simulates the load produced by a rolling wheel. Different loads can be applied, based on pavement strength and projected traffic. The pavement response (deflection) is measured by 7 to 9 geophone sensors, spaced at variable distances from the loading plate.
Superior to the Benkleman Beam test, which generates only a deflection result, the FWD generates a deflection curve that provides an accurate account of the pavement stiffness. The shape of the deflection curve, rather than the maximum deflection, is actually more important for calculating pavement life cycle. If the thickness of pavement layers is known (see GPR), the strength of each layer can be established using back-calculation or forward calculation. With a projected traffic load, we are able to estimate the length of life before pavement rehabilitation is needed.
Features of our FWD trailer
– Hydraulic system for lifting of weights directly driven by a gasoline engine, allowing fast operation
– 3 configurations of rubber springs for different load ranges
– Load cell recording actual load in kN
– Split load plate for a better contact between the load plate and pavement
– Set of 7 to 9 sensors (velocity transducers) registering pavement deflections spaced from the point of loading to 1.8 m off the centre of the load
– Strength of subgrade (subgrade modulus Mr) can be calculated from the sensor located at 1.8 m distance from the load
– 2 additional sensors can be added at the other side of the load plate for a load transfer between concrete slabs on rigid pavements
– Nu-Metrics NS60 distance measuring instrument (DMI)
– Automatic recording of pavement and ambient temperature at each test location
– Video system allowing monitoring of pavement surface at test locations
Features of our FWD/HWD truck
– FWD components are truck-mounted with folding sensor bar at the rear
– Heavier weights, larger capacity load cell and the springs configuration allow testing of heavy-loaded pavement structures at airports and major highways
– Truck can carry GPR antenna at the front, allowing simultaneous FWD and GPR testing