RESEARCH ARTICLES CURRENT SCIENCE, VOL. 108, NO. 10, 25 MAY 2015 1890 *For correspondence. (e-mail: sbhalla@civil.iitd.ac.in) Non-destructive assessment of rebar corrosion based on equivalent structural parameters using peizo-transducers V. Talakokula 1 , S. Bhalla 2, *, B. Bhattacharjee 2 and A. Gupta 2 1 Department of Civil Engineering, ABES Engineering College, Ghaziabad 201 009, India 2 Department of Civil Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110 016, India Occurrence of corrosion in rebars of reinforced con- crete (RC) structures is a common problem faced by the ageing infrastructure across the world. This article presents a newly developed approach for detecting and quantifying corrosion of steel bars util- izing a piezoelectric ceramic (PZT) patch surface- bonded on the rebars employing equivalent structural parameters using the electro-mechanical impedance (EMI) technique. This technique utilizes the electro- mechanical coupling property of piezoelectric materi- als for a damage diagnosis. Through tests on three steel rebars, empherical relations are derived to relate the corrosion-induced mass and stiffness loss to the loss in the equivalent mass and stiffness identified by the PZT patch. The equivalent mass loss and stiffness loss correlate well with the actual mass loss and stiff- ness loss and, provide an alternative corrosion as- sessment paradigm suitable for diagnosing corrosion in steel rebars. The model-based corrosion assessment presented can be utilized for real-life steel structures. Keywords: Electro-mechanical impedance technique, piezoelectric ceramic sensors, reinforced concrete, steel. CORROSION is defined as the undesirable deterioration of a metal or an alloy caused by its interaction with the envi- ronment that adversely affects the properties of the metal/ alloy, which should otherwise be preserved. Interest of the scientific community in corrosion has been increasing because of the wastage of precious metal, apart from en- dangering the structural performance and inducing fail- ure. The present study deals with the corrosion in steel rebars, which has been identified as the most common cause of deterioration and premature failure of reinforced concrete (RC) structures 1 . Reinforcement corrosion in- duced structural failure does not necessarily imply struc- tural collapse, but in most cases manifests as the loss of structural serviceability, characterized by concrete spal- ling and the excessive deflection of the affected RC members 2 . Practical experience and experimental obser- vations suggest that corrosion-affected RC structures deteriorate at different rates as measured by strength and serviceability, with the latter deteriorating at a much fast- er rate. The reason for this is attributed to the fact that the corrosion products exert an expansive pressure on the concrete. Due to the low tensile strength of concrete, this expansive pressure leads to concrete cracking, spalling and debonding between the reinforcement and the sur- rounding concrete; all these effects soon become promi- nent once corrosion actively propagates in the structure. As a consequence, the stiffness of the structure reduces and deflection increases 3,4 . This article extends the application of a new approach developed for assessment of corrosion in steel rebars. While our previous studies were on the corrosion assess- ment in RC structures and comparison of sensing abilities of the surface bonded with that of embedded sensor for corrosion assessment in RC structures, the present article focuses on bare steel bars using surface-bonded sensors. The results obtained can be a basis for assessing the cor- rosion in real-life steel structures. The article first pre- sents a brief description of the electro-mechanical impedance (EMI) technique, followed by details of the specific experiments conducted on steel rebar specimens and the development of empirical equivalent models based on the measured electromechanical data. Electro-mechanical impedance technique The piezoelectric ceramic (PZT) patch has a unique pro- perty of generating surface charges on application of mechanical stress known as direct effect. Conversely it undergoes mechanical deformation when subjected to electric fields. These direct and the converse effects are illustrated in Figure 1. The EMI technique using PZT patches is a relatively new technique for structural health monitoring (SHM). The technique was invented by Liang et al. 5 and further developed by several research groups 6–13 . The EMI tech- nique has been experimentally found to be powerful in detecting localized incipient damage in a variety of struc- tures 14–20 . In this technique, a PZT patch is surface-bon- ded to the monitored structure as shown in Figure 2 and electrically excited by means of an impedance analyser/ LCR meter. Under external field excitation, the bonded