Summary
A method has been devised for determining the thickness of concrete road slabs by measuring, at the surface of the slab, the velocity of propagation of vibrations at audio and ultrasonic frequencies.
A vibration generator acting vertically on the surface of the slab produces vibrations of known frequency which are detected by a pick-up. The pick-up is moved progressively away from the generator and positions are noted at which the vibrations at the generator and pick-up are in phase. The distance between successive positions gives the wavelength (λ) of the vibrations; the velocity of propagation is determined from the product of the wavelength and frequency.
In the frequency range from 1,000 c/s to 4,000 c/s, the vibrations are identified asffexural waves and their velocity (V) depends upon the frequency, the thickness (T), and the elastic constants of the concrete slab. At frequencies above 50 kc/s, the velocity is constant, and the propagation becomes sensibly independent of the frequency and the thickness of the slab, i.e. it is then the Rayleigh wave velocity (VR) of surface waves in the concrete. From the theoretical analysis, a graph is prepared giving the variation of V/VR with the quantity πT/λ, and this enables the thickness of the slab to be calculated from the experimental results.
Tests have been made on slabs of thicknesses varying from 4 to 9 in., some slabs being laid directly on the subgrade and some on prepared bases of soil cement and hoggin. The results have generally been satisfactory and, in many cases, the dimensions of cores drilled from the slabs have agreed with the calculated values of thickness to within ±10 per cent.
