In all systems, the load pulse shape and rise time is important because it can affect the peak values of center deflection by as much as 10% to 20%. In unlocked mode the FWD works as a double mass system giving 50 kN long pulse. There are also combined single/double mass systems where the falling weight and middle weight can be locked together giving 150 kN short pulse. This produces a longer loading duration that more precisely simulates a wheel load, and yields higher reproducibility and gives a more accurate result on pavements built on soft soils. In a double-mass system, the weight drops onto an assembly consisting of a first buffer, a second weight, and a second buffer.

Single-mass FWDs are typically smaller, faster and less expensive but, when used on soft soils, may overestimate the capacity of pavements due to the mass inertia of the pavement material. In a single-mass system, a weight is dropped onto a single buffer connected to a load plate, which in turn rests on the surface being tested. There are two different types of load impact systems single-mass and double-mass. 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). Segmented load plates adapt to the shape of the pavement to more evenly distribute the load on uneven surfaces. The load plate may be solid or segmented. 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. Deflection sensors are used to measure the deformation of the pavement in response to the load pulse. Data are acquired from various sensors for use in post-test analysis of pavement properties. 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. Comprehensive road survey vehicles typically consist of a FWD mounted on a heavy truck together with a ground-penetrating radar and impact attenuator.ĭuring testing, a FWD subjects the pavement surface to a load pulse which simulates the load produced by a rolling vehicle wheel. 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. 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. A falling weight deflectometer, towed by a truckĪ 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.

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