Falling Weight Deflectometer
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What is it?
The FWD has been developed from the 'deflectmetre a boulet' originally devised by Bretonniere in 1963. A force pulse is applied to the road surface by a specially designed loading system which represents the dynamic short-term loading of a heavy wheel load (1 ESA). This produces an impact load of 25-30ms duration, and a peak force of up to 120kN (adjustable). The deflection bowl response of the pavement is measured with a set of 7 precision geophones at a range of distances from the loading plate.
It is concluded that if the deflection bowl is measured under an FWD test, and the theory of elasticity is then used to determine the moduli of the individual layers that would produce the same deflection bowl, then the resulting layer moduli will be representative of the pavement materials under moving traffic loads.
Because no reference point (or support) is needed for the FWD deflection bowl measurement, the deflections can be measured with high accuracy. A high accuracy is necessary because the subgrade modulus must often be determined from deflections of only 2-3mm. The accuracy of the FWD deflections is further ensured by carrying out measurements two to three times at each point to assess repeatability. This will allow the effects of different loadings to be evaluated and identify any external factors such as passing vehicles which may have affected results.
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