Applicability of ultrasonic testing to neutralization depth measurement and corrosion detection of reinforcing steel bars in concrete structures

Reinforced concrete (RC) structures are widely used in a variety of infrastructures, such as power plants and so on, and rebars in RC structures are protected from corrosion by the highly alkaline environment of concrete. However, the rebar will be placed in a corrosive environment if neutralization progresses to the rebar. When rebar corrodes, expansion pressure causes cracking and delamination. Corrosion of steel bars is considered to be the most serious age-related deterioration of RC structures, and it is important to accurately determine the depth of neutralization and the state of corrosion of steel bars in order to properly maintain and manage RC structures. In this report, we tried to evaluate the applicability of ultrasonic testing as a nondestructive testing method for measuring the depth of neutralization and determining the corrosion state of rebars in RC structures. We conducted accelerated neutralization tests to prepare mortar specimens with different depths of neutralization and measured the sound velocity of ultrasonic waves propagating through the neutralized and non-neutralized sections, respectively. Experiment results showed that the sound velocity in the neutralized section increased in the surface layer of the specimens, while no significant change was observed in the interior. It is assumed that the faster sound velocity at the neutralized zone in the surface layer is not due to the neutralization phenomenon but to the organizational changes under the environment of the accelerated neutralization test. These results indicate that it is difficult to identify the presence or absence of neutralization using ultrasonic sound velocity changes. We immersed mortar specimens containing round bar-shaped rebars in salt water (salinity of approximately 3.4%) and energized to corrode the rebars. Then, we measured ultrasonic signals from the vicinity of the rebar during the process of rebar corrosion. As a result, when cracks were observed, echoes from the rebar and echoes from the cracks overlapped. In addition, what appeared to be echoes from micro-cracks appeared as the corrosion progressed. These results indicate that the corrosion state of the rebar can be evaluated by UT. (author)