FWDs - impulse devices

The FWD was initially developed in France in 1963 (Bretonniere 1963) and then evolved in Denmark (Ullidtz 1987) as an alternative non-destructive device for structural evaluation of pavements. Today, the following FWDs exist on the market Dynatest (Dynatest International A/S 2014), KUAB (ERI2014), PRIMAX (Grontmij A/S) and JILS (JILS) FWD. 

An FWD has the ability to provide information for the structural condition of all pavement layers, including the subgrade. In particular, with FWD deflection measurements, the following may be obtained (a) estimation of modulus of elasticity or stiffness modulus of each layer, (b) estimation of remaining life of pavement, (c) detection of weak points in the pavement, (d) estimation of the severity of potential cracks in stabilised layers, (e) determination of the effectiveness of the load transfer at the joints of rigid pavements and (f) detection of cracks on rigid pavements. 

The determination of the overlay thickness is usually performed by analytical methods similar to those applied in the analytical pavement design methodologies. However, by converting deflection measurements obtained by FWD to Benkelman beam, or to deflectograph deflection, the established methodologies described in Section 15.17, may also be used. 

FWD in most cases is mounted on a trailer, rather than on a vehicle. The load is applied through a 300 mm diameter circular metal plate for road pavements or through a 450 mm 

The resulting deflections are measured at the centre of the applied load (through a hole in the centre of the loading plate) and at various distances away from the load with sensors. The sensors may be displacement transducers, velocity transducers, accelerometers, geophones or even seismometers. 

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