Ultrasound Diffusion for Crack Depth Determination in Concrete

Abstract

Concrete structures are continually subject to effects that degrade their structural integrity. Near surface damage in concrete may take the form of distributed microcracks or large distinct cracks that extend to the surface of the structure. Much of the damage in concrete develops at length scales that are smaller than the size of the majority of the aggregate [7]. It has been shown in [8] that diffuse (incoherent) ultrasonic methods, which make use of the scattered fields, may be viable for assessing damage in concrete. Several recent research papers [3] [6] report efforts to non-destructively determine the depth of surface breaking cracks in concrete using the elastic stress wave time-of-flight (wave velocity) techniques [5]. In these papers, the crack depth is determined if the velocity of wave propagation in the sound concrete is known and a particular wave path and wave pulse-crack interaction is assumed. The frequency in which these experiments were conducted ranges from 0 to 100 kHz. A frequency of 100 kHz corresponds to a wavelength on the order of several centimeters in concrete. In the impact-echo method [6], the user must interpret the recorded echo signals. Quality interpretation requires a user experienced in waveform recognition. In [3] the frequencies used were in the range of 0 to 70 kHz. The wave source is generated by an impact of the steel ball on the specimen surface. It is difficult to generate a high frequency waves without damaging the surface. The sensitivity to crack depths of the surface wave methods is one wavelength and hence it becomes insensitive and cannot detect deep cracks.