Corrosion Monitoring of FRP Protected RC Structure Using Ultrasonic Guided Wave and Acoustic Emission Technique

Abstract

Non-destructive testing is needed for localizing and characterizing the growing damage in existing structures and also for the quality control of new structures. Visual inspection is a commonly used method; however it is not very suitable for early detection of damage and is very much dependent on the experience of the inspector. Ultrasonic guided wave monitoring and acoustic emission technique are the methods that can be used effectively for the structural health diagnosis. In the present study, a RC Cylinders, of dimensions 150mm diameter and 300mm length has an embedded steel bar of dia. 25 mm and 600mm in length, is subjected to accelerated induced corrosion. Ultrasonic guided wave monitoring is done periodically after every 24 hours for 28 days along with this regular AE monitoring 24 hours a day for 28days was done.. A mode with minimum attenuation and mode shape that is sensitive to particular type of damage due to corrosion is chosen from dispersion curves. For this purpose, Pulse-echo investigation is used using peizo-electric transducers. Also, AE sensors detecting acoustic emission signals are mounted on the RC Cylinder specimen before it is subjected to accelerated corrosion. AE set up simultaneously records the events and hits continuously for 28 days relating to crack initiation and progression due to corrosion in RC beams as the exposure to corrosion increases. It is observed that corrosion of reinforcing bars in concrete is discernible using ultrasonic guided wave and acoustic emission technique. Ultrasonic guided wave monitoring is done using surface seeking mode and core seeking mode for detecting corrosion. These modes are able to explain the delamination as well as the pitting of the rebar in the RC beam due to corrosion. From acoustic emission monitoring it is observed that there is an immediate increase in cumulative hits and events which corresponds to the development of cracks in the RC beam due to corrosion. Comparing the results from both ultrasonic monitoring and acoustic emission monitoring it is observed that acoustic emission is very effective in detecting the initiation of the corrosion during the initial stages whereas ultrasonic guided waves monitoring gives very clear data during the later age of corrosion propagation.