Non destructive evaluation of degraded concrete cover using high-frequency ultrasound

In this paper, the feasibility of detecting and characterizing concrete cover degradation by ultrasonic testing is investigated. The on-site nondestructive evaluation of concrete cover is vital to monitor the integrity of concrete structures and prevent irreversible damage. Given the thickness of the degraded layer - which can be as small as a few millimeters - the sensitivity of ultrasound propagation to cover degradation is expected to increase with frequency. Therefore, high-frequency ultrasound (0.5 to 1 MHz) is used. In order to assess the feasibility of detecting concrete cover degradation, velocity and attenuation measurements were made on both halves of a mortar slab. One half was chemically degraded, while the other half remained sound. The pulse velocity and the attenuation coefficient were measured on both halves for compression, shear and surface waves. After 45 days of degradation, up to 24% velocity decrease was observed, while attenuation increased up to 10 times. High-frequency ultrasound is thus able to detect changes in the microstructure of the concrete cover even at an early stage of degradation.