Falling Weight Deflectometer (FWD)
The FWD is a non-destructive technique used to assess the structural capacity and performance of pavements by measuring their deflection under a load. The FWD is mainly used on highways, airports, ports and hardstanding areas, but can also be applied to subgrades and sub-bases during construction. The FWD is trailer mounted and is operated via a laptop from within a towing vehicle. Each individual test takes approximately one minute to conduct and distances between test points vary depending on survey requirements.
The basic principle of an FWD test is to determine the deflection (i.e. the vertical movement) of the pavement under a dynamic load. The load is created by dropping weights onto a series of buffers that transfer the force to a loading plate that sits on the pavement surface. The resulting downward deflection of the pavement is recorded by geophones positioned at known distances away from the loading plate which can measure deflections to an accuracy of ± 1 micron (1 thousandths of a millimetre).The measurement recorded at each geophone can then be used to measure an accurate deflection bowl of the whole pavement rather than just a central deflection. This then allows for an accurate picture of the entire pavement construction, from the bound layers to the condition of the underlying subgrade, to be calculated.
