Use of the normalized impact-echo spectrum to monitor the setting process of mortar

The objective of this paper is to introduce a newly developed nondestructive test technique called the normalized impact-echo spectrum and its application to monitoring the stiffening process of mortar. The impact-echo method is an effective and nondestructive technique for locating flaws in concrete. The depth of an air–concrete interface is determined by identifying the large amplitude peak in the spectrum at the dominant frequency corresponding to multiple wave reflections from the interface. However, the measure of the peak amplitude does not make sense because it changes with different impact forces. To make the peak amplitude meaningful, there is a need to normalize the spectrum on impact force. After normalization, the amplitudes in impact-echo spectra can provide useful information on the intensity of wave reflections at the interface. In this paper, the normalized impact-echo spectra were used to monitor the setting process of mortar. Experimental results demonstrate that the normalization of the impact-echo spectra can make the peak amplitude steady for various impacts and the coefficient of variation is less than 1.5%. The variation in the amplitude of the normalized impact-echo spectrum with the mortar age can indeed reflect the stiffening process of mortar. The results show that the proposed technique is suitable to monitor the setting process of mortar.