Detecting flaws in concrete blocks using the impact-echo method

The objectives of this paper are to investigate the impact response of concrete blocks and to study the feasibility of using the impact-echo method for detection of flaws in concrete blocks. Numerical studies were carried out to acquire the transient responses of intact concrete blocks subjected to impact. In addition, the impact responses of concrete blocks containing flaws were explored. Numerical results were verified by experimental studies on concrete blocks with/without flaws. It is shown that the impact response of a concrete block is composed of frequencies corresponding to the modes of vibration of the block. Among these frequencies, there is a predominant frequency and its value depends on the geometry and dimensions of the block for a given P-wave speed in concrete. It is also shown that the presence of a flaw disrupts the modes of vibration. A shift of the predominant frequency to a lower value is a key indication of the presence of the flaw. In addition, multiple wave reflections between the impact surface and the surface of the flaw produce a large amplitude peak in the spectrum at the frequency corresponding to the depth of the flaw.