to ensure optimum performance from a road structure it is necessary that the specified thicknesses of material are laid. a simple method of checking thickness is required to ensure that the pavement-layer thicknesses comply with the specification. this report describes performance tests carried out on a non-destructive method of thickness measurement that has been developed and used in germany. in this method a probe is placed on the layer surface directly over an aluminium foil placed under the layer. the reduction in the self-inductance of the measuring coil in the probe due to the foil is related to the distance between the probe and the foil. the performance of the instrument was examined in laboratory-scale tests, in controlled-pavement-construction experiments and under site conditions. the tests, which compared the thickness measured by the instrument with core and line measurements showed that the instrument was accurate to within 2 percent over the 250 mm range specified for the instrument. further tests, which examined the influence of hot layers on the thickness measurements, showed that stable and accurate measurements could be obtained only if the surface temperature of the material was less than 60 deg c. tests made with electrically conducting paint as a substitute for aluminium foil showed that the impedance of conducting paints at present available is too high for use in thickness measurements.(a)

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