Falling weight deflectometer
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A falling weight deflectometer, FWD or F for short, is a testing device used by civil engineers to evaluate the physical properties of a pavement. This could include (but is not limited to) highways, local roads, and airport runways. The machine is usually contained within a trailer that is towed to a location by another vehicle.
The FWD is designed to impart a load pulse to the pavement surface which simulates the load produced by a rolling vehicle wheel. The load is produced by dropping a large weight, and transmitted to the pavement through a circular load plate. A load cell mounted on top of the load plate measures the load imparted to the pavement surface. Deflection sensors (most FWDs use geophones, force-balance seismometers are also used) mounted radially from the center of the load plate measure the deformation of the pavement in response to the load.
FWD data is most often used to calculate stiffness-related parameters of a pavement structure. The process of calculating the elastic moduli of individual layers in a multi-layer system (e.g. asphalt concrete on top of a base course on top of the subgrade) based on surface deflections is known as "backcalculation", as there is no closed-form solution. Instead, initial moduli are assumed, surface deflections calculated, and then the moduli are adjusted in an iterative fashion to converge on the measured deflections. This process is computationally intensive, highly error-prone and requires an experienced analyst. Instead, many analysts use simplified methods to calculate related parameters that are empirical in nature.
FWD data can also be used to calculate the degree of load transfer between adjacent concrete slabs, and to detect voids under slabs.
