Falling weight deflectometer

A falling weight deflectometer, FWD, is a testing device used by civil engineers to evaluate the physical properties of pavement. FWD data is primary used to estimate pavement structural capacity for 1) overlay design and 2) to determine if a pavement is being overloaded. Use includes (but is not limited to) highways, local roads, airport pavements, and railway tracks. The machine is usually contained within a trailer that can be either towed to a location by another vehicle or, when used on railway tracks, placed on a hand trolley and pushed to the location.

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 - typically 300mm diameter. 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. Some typical offsets are 0mm, 200mm, 300mm, 450mm, 600mm, 900mm, 1200mm 1500mm. The deflections measured at these geophones are termed D0, D200, D300 etc.

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 although quick on modern computers. It can give quite misleading results and requires an experienced analyst.

Instead, many analysts use simplified methods to calculate related parameters that are empirical in nature. The most common is maximum deflection under the centre of the load plate (D0) which is related to empirical measures such as the Benkelman Beam deflection (after minor adjustment for differences in the two devices). Historically some used the radius of curvature (D0-D200) but this is out of favour now because it is clear that the steel loading plate of 300mm diameter affects the shape of the deflection bowl between the centre (D0) and the D200 sensor at 200mm. However this means that a lot of useful information about the shape of the deflected bowl is wasted. Horak and Emery have published indices that use this information: BLI=D0-D300 and gives an indication of the basecourse performance, MLI = D300-D600 and gives an indication of the subbase performance, and LLI=D600-D900 and gives an indication of subgrade performance. These and other similar indices are known as shape factors. The FWD data can also be very useful in helping the engineer divide the length of the pavement into homogeneous sections.

FWD data can also be used to calculate the degree of load transfer between adjacent concrete slabs, and to detect voids under slabs.

A Light Weight Deflectometer (LWD) is a portable falling weight deflectometer. It is used primarily to test insitu base and subgrade moduli during construction.

A Heavy Weight Deflectometer (HWD) is a falling weight deflectometer that uses higher loads, used primarily for testing airport pavements.

A Rolling Weight Deflectometer (RWD) is a deflectometer that can gather data at a much higher speed (as high as 55 mph) than the FWD. It is a specially designed tractor-trailer with laser measuring devices mounted on a beam under the trailer. Another advantage of the RWD over the FWD is that it can gather continuous deflection data as opposed to discrete deflection data collected by the FWD.

The test materials are described in ASTM D 4694, and the test method is defined in ASTM D 4695.