Ultrasonic Guided Wave and Acoustic Emission Techniques for monitoring corrosion in Reinforced Concrete Structures

Abstract

Corrosion of reinforcement is one of the principal causes of premature failure of Reinforced Concrete (RC) structures. In India, the annual loss due to corrosion has been estimated about 4% of the country’s gross domestic product (GDP) i.e., about USD 40 billion in a year in both infrastructure and industry segments. Exposure to extreme environments and the continuous ingress of chlorides in concrete from various sources neutralises the protective passive layer of concrete which is alkaline with a pH ranging from 12 to 13. The presence of moisture and oxygen leads to the formation of oxides and the initiation of reinforcement corrosion takes place. Corrosion of reinforcement in concrete affects the durability of the RC structures in two ways: the formation of rust products with larger volume than steel leads to spalling and the cracking of the concrete cover; and the area of cross-section of the steel bars reduces drastically due to dissolution of the steel leading to pit formation and hence, loss in the tensile strength and load carrying capacity of the structure. Therefore, an early prognosis and diagnosis of corrosion-induced damage in RC structures could facilitate the engineers to adopt correct repair measures. A large variety of non-destructive techniques have been reported by researchers worldwide for detecting corrosion-induced damage in RC structures like visual inspection, electrochemical, Optical methods, PZT and piezo ceramic patches etc. but they suffer from practical applications owing to large civil infrastructure. Recently, the advent of wave propagation technologies for damage detection in RC structures have given a new dimension to damage monitoring capabilities. In this work, an integrated approach using active Ultrasonic Guided Wave (UGW) and passive Acoustic Emission (AE) technique for corrosion monitoring in RC structures has been attempted. Active UGW monitors the deterioration of reinforcing bar while corrosion takes place in RC structure whereas the AE technique listens to the cracks and records the damage in the surrounding concrete. Initiation of corrosion in RC structures before it can be observed visually is done using the two techniques. For this, AE sensors were mounted on the concrete surface and typical guided wave modes were propagated through the embedded rebar. The NDT results were co-related with the already established electrochemical techniques. Active monitoring using UGW was found to have limited significance for tracking the initiation of corrosion when only the outer surface of the reinforcing bar is affected. Surface modifications on the embedded rebar during the initial stages of corrosion is clearly indicated by the L(0,1) mode. On the other hand, passive monitoring using AE clearly identified the onset and the initiation of corrosion during the early stages. The origin and the location of micro-cracks were well identified with the help of AE event maps. Further, the techniques were used to investigate the progression of corrosion-induced damage in RC structures. It was found that UGW clearly differentiated the surface corrosion from the pitting corrosion with the help of specific guided wave modes. On the other hand, AE parameters like AE hits and cumulative signal strength (CSS) successfully picked up the regions of initiation of steel depassivation and progression of corrosion in the form of micro- and macro-cracking in the surrounding concrete. From the AE event maps, the entire damage progression from initiation of corrosion damage to damage progression in the later stages could be clearly observed. Further, the effect of varying rates and different corrosion environments were investigated using the two wave propagation techniques. It was confirmed that corrosion mechanism is unaltered by varying rates of accelerated corrosion though the damage progression is delayed and picked up by both UGW and AE. In case of RC beams subjected to accelerated corrosion in the absence of chlorides simulating slow corrosion, UGW confirmed that pitting is insignificant due to slow steel-concrete bond deterioration, by picking up only delamination. On the other hand, AE clearly indicated the corrosion initiation as well as progression stages by cumulative AE hits. The efficacy of UGW and AE techniques was also investigated to monitor the corrosion impediment offered using Fiber Reinforced Polymer (FRP) wraps on corroding RC cylinders. UGW monitoring with surface-seeking and core-seeking modes established that pulse-echo method is suitable to assess corrosion delay in FRP wrapped samples. The recorded AE hits exhibited significant decrease after the wrapping was done indicating success in corrosion impediment. This was confirmed by drop in the values of amplitude of AE hits from 100 dB to 65 dB after wrapping. No peak/knee was observed in the CSS curve, indicating that FRP wraps effectively delayed the corrosion progression. It is observed that corrosion-induced damage in RC structures can effectively be tracked right from initiation to progression to concrete cracking using an integrated health monitoring approach involving wave propagation technologies of active ultrasonic guided wave and passive acoustic emission techniques along with benchmark electrochemical measurements. This would go a long way in non-destructive evaluation of residual capacities of reinforcements, which would help a great deal in devising a post-corrosion maintenance strategy of RC structures.