Assessment of the performance of the structural properties of concrete pavements by means of the falling mass deflectometer

The paper describes the survey and assessment of the structural properties of concrete pavements with the aid of in-situ and non-destructiveloading tests performed with the Dynatest 8000 falling-weight deflectometer (FWD). The following parameters for carriageway slabs with joints are considered: (I) modulus of elasticity of the concrete slabs; (2) modulus of subgrade reaction (K) (3) degree of load transfer at the joints; (4) modulus of subgrade reaction under joints and slab corners. The FWD applies a dynamic load 105 kN within a period of 25 - 30 milliseconds, which corresponds to the load due to a heavy goods vehicle in motion. On each slab to be measured the load applied and the deflections due to it are measured in the middle of the slab and at a joint. The measured data are automatically stored by a microcomputer. for subsequent analysis. The above-mentioned parameters are calculated from the deflection curves obtained from each set of measurements ( seven deflections at different distances from the loading plate). Also, the nonlinear stress behaviour of the subgrade is determined; this value is important in connection with determining the correct moduli for the concrete and subgrade. The structural condition under the joint and the edge of the slab is analysed by means of Westergaard's latest equations. The degree of load transfer and its effect on the value of K at the joint is also calculated. The condition of the sub-base in the vicinity of the joint is determined from the ratio of the moduli (values of K) under the middle of the slab and under the joint. When the fundamental structural properties have been evaluated, the critical compressive and tensile stresses are determined. Up to this point the properties of the pavement can be calculated by methods of engineering mechanics. The final step is the prediction of the behaviour of the pavement (evenness and cracking). This constitutes an empirical step, and the relevant criteria have to be adjusted to the local conditions. Worked examples based on cases encountered in actual practice are given.