Multimodal Monitoring of Corrosion in Reinforced Concrete for Effective Lifecycle Management of Built Facilities

Monitoring the corrosion of steel rebars is paramount to ensuring the safety and serviceability of reinforced concrete (RC) structures. Conventional electro-chemical techniques can provide an overall estimate of the extent of corrosion. However, a detailed account of the extent of corrosion would help in understanding the residual strength of corroding RC structures. A passive wave-based technique such as acoustic emissions can identify the location of corrosion but always requires the presence of transducers on the structure. In active wave-based techniques, the structure is excited through a pulse excitation and their subsequent response to this excitation is measured. Thus, for active techniques, the transducers need not always be present in the structure. In guided wave ultrasonics, the excitation pulse is imparted through a waveguide to determine the state of corrosion. This technique relies on parameters such as time of flight or attenuation of the incident signal to predict the state of corrosion. These parameters can be susceptible to uncertainties in the transducer of ultrasonic coupling. In the present study, concrete specimens with embedded steel bars have been subjected to accelerated corrosion. They have been monitored with a combination of active and passive techniques. The received signals are analyzed through a modified S-Transform-based time-frequency approach to obtain a range of modes that propagate through the specimen. The changes in the modal composition of the guided wave signals due to corrosion are parameterized and correlated to various stages of corrosion. A holistic understanding of the stages of corrosion is developed by the inclusion of acoustic emission hits to guided wave parameters. Based on the Guided Wave Ultrasonics and acoustic emission parameters, corrosion has been classified into Initiation, Intermediate, and Advanced. Subsequently, destructive tests have been performed to measure the residual strength of the corroded bars. Thus, this paper presents a novel proof of concept study for monitoring corrosion with Guided Wave Ultrasonics and acoustic emissions.