Crack depth measurement in concrete using diffuse ultrasound

Abstract

Cracking in concrete structures is problematic because these cracks can significantly influence the stability of a concrete structure and compromise its durability. The first step to evaluate the serviceability of an in-field concrete structure is to have accurate information on existing crack depth. It is thus of paramount importance to be able to accurately determine the depth of cracks in these concrete structures. This research employs a diffusive ultrasonic technique to measure the depth of surface cracks in concrete. Ultrasonic measurements on a 25.4 × 33 × 60.96 cm3 concrete block containing an artificial crack with varying depths from 2.54 to 10.16 cm are conducted. Contact transducers with one transmitting and the other receiving the ultrasonic signals are mounted on the concrete surface on opposite sides of the crack. A pulse signal with the duration of 2μs is transmitted. In this frequency regime, wavelengths are sufficiently short (comparable with the aggregate size) so that a diffuse ultrasonic signal is detected. The arrival of the diffuse ultrasonic energy at the receiver is delayed by the existence of the crack. This lag-time and the diffusivity of the concrete sample are measured, and a finite element model is employed to solve the inverse problem to determine the crack depth from these measured diffuse ultrasonic parameters.