Backcalculation

A falling weight deflectometer (FWD) is a testing device used by civil engineers to evaluate the physical properties of pavement. FWD data is primarily 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, harbor areas and railway tracks. The machine is usually contained within a trailer that can be towed to a location by another vehicle. It can also be built on a pickup truck, inside a mini van or on a heavy truck together with a TMA protection.

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 300 mm diameter on roads and 450 mm on airports. A load cell mounted on top of the load plate measures the load imparted to the pavement surface. The load plate can be solid or segmented. The advantage of a segmented load plate is that it adopts to the shape of the pavement, giving an even distribution of the load on uneven surfaces. Typically, the load for road testing is about 50 kN giving about 700 kPa pressure under the load plate.

There are two different types of load impact systems; single-mass (e.g. Dynatest, Carl Bro, PaveTesting) and double-mass (KUAB). In a single-mass system, a weight is dropped onto a single buffer connected to a load plate, which rests on the surface being tested. The load force is transferred through the plate, and the plate creates a deflection that simulates a wheel load. In the double-mass system, the weight drops onto a double-buffer system, which includes a first buffer, a second weight, and a second buffer. The double-mass system essentially produces a longer loading duration that more precisely represents a wheel load. The double-mass system has higher reproducibility and gives a more accurate result on pavements built on soft soils. The single-mass system will seriously overestimate the capacity of pavements built on soft soils. However, single-mass FWDs are smaller, cheaper and faster. Low-cost FWD:s for the Indian market are currently (2015) being developed independently by Geotran, PaveTesting and KUAB.

Deflection sensors (geophones; force-balance seismometers) 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 sensors are termed D0, D200, D300 etc. The advantages of seismometers compared to geophones are built-in calibration devices and higher range (5 mm vs 2 mm). Geophones are more sensitive to disturbance immediately before the impact since the initial error is integrated. Geophones however are much cheaper than seismometers. Dynatest, Carl Bro, Jils and PaveTesting use geophones while KUAB have seismometers in their standard FWD's and geophones in their low-cost models.

...
Wikipedia