Location of a defect in a concrete block by a non-destructive technique

This paper describes a method to estimate the position of a crack in a concrete block using several vibration pick-ups. Since the concrete block investigated in our project is very large, we uses a low frequency vibration signal instead of a ultra sonic. Conventional method using the low frequency vibration signal drives the concrete block with a vibration force, and estimates the resonant frequency of the concrete block, by which the position of the crack is estimated. However, this method only works well in the cases: (1) the crack is just like a sheet and the sheet is parallel to the surface of the block (standing wave is generated in this case), or (2) the crack is big so that it causes the shift of the resonant frequency. We have developed another method to estimate the position of a sheet-like crack in a concrete block using several vibration pick-ups. In our method the position of the first reflection wave from the crack due to the vibration force is used to find the position of the crack instead of the resonant frequency. The positions of cracks can be estimated by our method when the concrete block has more than one crack, while it is very difficult to estimate the location by the conventional method. However, the method is still insufficient since the effect of the surface wave cannot be deleted correctly. Therefore, this paper proposes a new method to achieve a better location of a crack. Some experiments were carried out, and good results were obtained, for example a slant crack could be detected.