Monitoring of Damage in Reinforcement in Concrete

Abstract

One of the major causes of catastrophic failures in reinforced concrete structures such as bridges, dams and buildings is the deterioration of reinforcing bars due to corrosion. If the degradation remains unnoticed inside concrete, it further accelerates and causes huge loss of life and property. Removal of concrete to visually inspect the reinforcement is detrimental to the structure. Hence, it is imperative to develop a non-intrusive health monitoring technique for early detection of corrosion related damages in steel embedded in concrete. Guided ultrasonic waves offer a potentially attractive solution. In the present study, bars embedded in concrete are subjected to corrosion and monitored ultrasonically using longitudinal guided waves. A mode with minimal attenuation and mode shape that is suitable for detection of a particular type of damage is chosen from dispersion curves. The reinforcements have been excited at one end with an ultrasonic pulse using piezoelectric transducers and are received at the other end (pulse transmission technique). In pulse echo technique same transducer is used as transmitter and receiver. Ultrasonic testing is carried out on bars in concrete by simulating corrosion in the form of area loss (notches) and delamination (tape wrapped on embedded bar).The developed methodology is applied to reinforced concrete beams specimens undergoing accelerated corrosion. Two corrosion environments- i.e. presence of chlorides (referred to as chloride corrosion, CC) and absence of chlorides (referred to as oxide corrosion, OC) are chosen. To investigate the ability of ultrasonics in predicting the level of deterioration, a correlation is attempted between ultrasonic voltages and destructive tests such as mass loss, pull out strength and tensile strength at different stages and ages of corrosion It is observed that corrosion of reinforcing bars in concrete is discernible using ultrasonic guided waves. Through a judicious selection of different ultrasonic modes, not only the corrosion phenomenon can be monitored but also different types of corrosion occurring in different environments can be successfully identified. From the limited experimental data approximate relations between ultrasonic signals and physical conditions of the corroded bars have been developed.