A review of ultrasonic Coda Wave Interferometry in concrete

• CWI evaluates relative velocity changes (dV/V) of ultrasonic sound speed.
• CWI zoom in the tiny changes of concrete acousto-elasticity properties
• CWI allows to track thermal, mechanical and chemical damage.
• CWI has an incredible sensitivity as low as 2.10− 5in dV/V.
• dV/V greater than 2.10− 3/MPa indicates a level of significant damage.

Concrete is a multicomposite material with heterogeneities ranging in size from micrometers to centimeters. At low frequency (below 50 kHz), ultrasound propagates through concrete without suffering scattering and absorption. On the contrary, above ≈ 100 kHz, the waves strongly interact with all the heterogeneities and enter a multiple scattering regime. This regime induces 1) substantial attenuation of coherent waves (direct waves and ballistic echoes), a feature that disables most conventional imaging techniques; and 2) the onset of late arrival signals that form the ultrasonic coda. An important feature of coda waves is their very high sensitivity to weak changes in the medium. Over the last years Coda Wave Interferometry (CWI) applied to ultrasound in concrete has been widely adopted in the non-destructive testing community. This article reviews several applications based on the precise processing of ultrasonic coda, in the case of thermal and/or stress and/or damage changes.