Quality control of concrete tunnel shells using impact echo methods

摘要

Mined tunnels usually consist of a layer of reinforced shotcrete, plastic sealings against water pressure, if necessary, and load-carrying reinforced concrete shell. For concreting the shell, closed steel moulds are used. To protect the sealing against leaking, the surfaces of the shotcrete and the concrete should be plain. However, the surface of the shotcrete is visible after spraying, but the surface of the concrete is not. A great deal of experience is necessary to ensure that at the ridge the concrete is close to the plastic sealing, because this region is not inspectable during concreting. Using impact echo techniques, one may measure the shell thickness in a non-destructive way and check whether gravel nests or complete cavities exist. In this paper, a measuring technique will be shown which allows simple and cost-effective thickness evaluation. Having identified defects in the shell, it is very simple to fill the cavities by injection with cement pastes. An example, where the described method has been used, is presented in this paper.