.jpg)
 | This article needs additional citations for verification. (February 2013) |
A falling weight deflectometer (FWD) is a testing device used by civil engineers to evaluate the physical properties of pavement in highways, local roads, airport pavements, harbor areas, railway tracks and elsewhere. The data acquired from FWDs is primarily used to estimate pavement structural capacity, to facilitate overlay design or determine if a pavement is being overloaded. Depending on its design, a FWD may be contained within a towable trailer or it may be built into a self-propelled vehicle such as a truck or van. Comprehensive road survey vehicles typically consist of a FWD mounted on a heavy truck together with a ground-penetrating radar and impact attenuator.
During testing, a FWD subjects the pavement surface to a load pulse which simulates the load produced by a rolling vehicle wheel. The load pulse is produced by dropping a large weight onto a "buffer" which shapes the pulse, and then transmitted to the pavement through a circular load plate. Data are acquired from various sensors for use in post-test analysis of pavement properties. Deflection sensors are used to measure the deformation of the pavement in response to the load pulse. In some FWDs the magnitude of the applied load pulse is an assumed constant value determined by system design; in others the force is measured by load cells.
The load plate may be solid or segmented. Segmented load plates adapt to the shape of the pavement to more evenly distribute the load on uneven surfaces. The load plate diameter is typically 300 mm diameter on roads and 450 mm on airports, and the load for road testing is about 40 kN, producing about 567 kPa pressure under the load plate (50 kN / 707 kPa according to European standard).