The Falling Weight Deflectometer (FWD) is a non-destructive testing device that has been widely used to evaluate pavement structural condition and strength of subgrade.
A big advantage of FWD testing is that it is fast and safe. The driver/operator runs equipment from a laptop computer without the need of stepping out of the vehicle. The following data is collected at each test point:
- deflection at each sensor location
- distance at each sensor location
- pavement surface and ambient temperatures
- GPS coordinates
FWD measures deflections of pavement at specified distances from the load point.
The FWD applies dynamic loads to a pavement surface. Loading pulse duration and characteristics simulate effect of a wheel tire. 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 load plate.
Superior to the Benkelman Beam test, which generates only one deflection point, 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, draws more accurate picture of pavement mechanistics. Knowing the thickness of pavement layers (see GPR), the elastic modulus of each layer can be established using back-calculation or forward calculation. With projected traffic loading, pavement life-span with maintenance and rehabilitation cycles can be estimated.
WCRT currently owns two machines: A Sprinter Van loaded with an FWD, and a trailer mounted HWD (Heavy Weight Deflectometer).
- All FWD electronic components are weather-protected inside the van
- Quiet operation. The Mercedes diesel engine is very quiet compared to the Ford diesel engines.
- Improved FWD operation – the latest upgrades installed by JILS. They include a precision mass lifting system eliminating need of force calibrating, rollers guiding mass perpendicularly to the ground on sloped roads, more advanced rubber springs, reliable USB connection instead of the data acquisition card and a new computer with Windows 10
- Hydraulic system run by the hydraulic pump driven by belt from the van engine results in a quiet operation
- Load cell records actual load in kN
- Split load plate provides a better contact with pavement surface
- Set of 7 to 9 sensors (velocity transducers) registers pavement deflections spaced from the point of loading to 1.8 m off the center of the load plate
- Nu-Metrics NS60 distance measuring instrument (DMI)
- Trimble Ag132 GPS
- Automatic recording of pavement surface and ambient temperature at each test location
- Video camera allows monitoring of pavement surface at each test locations
- Sensor bar is easily detachable for transport