The paper describes the non-destructive, surface wave propagation method of testing about a 40-ft length of road at a time. In this technique, vibrations are generated in the road surface at a number of frequencies within the range 28 to 24000 c/sec, ant the wavelength and velocity of the vibrations are determined. The results can be made to give the elastic module of the various layers in the road and the thickness of the surfacing's and bases. Laboratory test to measure the elastic module and the strength of specimens made from the same materials unable the results obtained on cemented bases or road slabs to be converted into strength. By making measurements at only one selected frequencies it was possible to test selected lengths of cemented bases at rates of up to 1000 ft/day. The method located weak areas in the cemented granular bases which were confirmed by subsequent excavations. In adequate compaction at the bottom of concrete road slabs has also been located by the surface wave propagation method. From the results, an estimate has been made of the equivalent thickness of concrete having the same elastic properties as the material at the surface of the slab. Changes in the properties of the base materials resulting form traffic have also been studied by the surface wave propagation method. The method has detected. (a) increases in compaction of granular bases and sub-bases, (b) Stripping of bituminous bases, and (c) development of cracks in cement-bound bases.
Abstract