Assessment of Damage in Corroded and FRP Repaired Corroded Beams Using Advanced NDT Techniques

Abstract

Corrosion of steel in concrete is inevitable and is a worldwide problem which causes heavy losses to the economy and industry. Concrete due to its alkaline nature (pH > 13.5) provides a passive protective layer of oxides around the reinforcing steel initially but when exposed to severe environmental conditions (like marine exposure) corrosion in steel bar is initiated due to depassivation of this layer. Corrosion product formed has a tendency to exert tensile pressure on surrounding concrete as rust product has volume about 6-10 times that of parent steel leading to cracking and spalling of surrounding concrete. Bond deterioration at steel concrete interface and reduction in steel area due to corrosion which ultimately reduces the load carrying capacity of Reinforced Concrete (RC) structure and can be catastrophic. In countries like India with diverse environmental condition, losses due to corrosion has been estimated USD 40 billion per year for industrial and infrastructural segments. Due to this, most of the structures need repair with 10-15 years of their construction. Therefore, it becomes very important to monitor corrosion in RC structures alongwith sustained loading so that proper timely remedial measures can be adopted before degradation leads to catastrophic failures. Numerous studies have been reported by researchers in last few decades for monitoring corrosion in RC structures like visual inspection, electrochemical, optical methods, measurement of linear polarization resistance and concrete resistivity, X-ray, gamma rays, etc. But these techniques only provide qualitative information on the presence of corrosion but do not indicate the rate or extent of corrosion. In this study, an effort has been made to monitor various stages and aspects of real time corrosion in RC structures using a judicious combination of various nondestructive monitoring tools of active Ultrasonic Guided Wave (UGW), passive Acoustic Emission (AE) technique and optical Infra-Red Thermographic (IRT) technique. Bars with simulated corrosion damage representing pitting and delamination in the bars prepared and were tested in air and concrete using UGW and IRT. Further RC beams (80 x 80 x 400 mm) were subjected to real time actual accelerated impressed current corrosion and were simultaneously monitored using advanced NDT techniques of UGW, IRT and AE. As the corrosion progresses in RC beams a significant drop in transmitted signal strength of a specific core seeking guided wave mode is observed representing efficiency of UGW in picking progression of corrosion but UGW is not very effective in picking up the initiation of corrosion. It was well corroborated by AE testing which successfully picks up the initiation of corrosion along with its progression in the surrounding concrete. It is well depicted by various micro-cracking and macrocracking phases at different stages of corrosion in the corroding beam as picked up by increase in number of cumulative AE hits and their amplitudes with progressive corrosion. AE X-Y event plots clearly demonstrate the initiation and progression of cracks in pictorial representation. But localisation of corrosion damage still remains a challenge and is explored in this study. IRT effectively picks up corrosion in rebars in concrete as the rust products produced get heated at a fast rate than the parent steel and the same is represented as differential temperature profiles in the IRT. Hence, it can be concluded from the detailed qualitative as well as quantitative NDT monitoring of corrosion that initiation is effectively picked up by AE, well supported by UGW for picking up progression and localisation by thermographic images in IRT. It is important to mention that UGW picks up deterioration only in embedded rebars while AE picks up the effect of corrosion on the surrounding concrete. Further in this study, effect of varying levels of corrosion alongwith sustained loading on the overall performance of real size RC beams (127 x 227 x 4100 mm) was investigated using a combination of global vibration diagnostics and local Acoustic Emission technique. With the increase in corrosion level in RC beams, a shift and drop in fundamental frequency and drop in Frequency Response Function (FRF) amplitudes is observed as vibration signal attenuates due to increasing corrosion cracks. An attempt has been made to establish a correlation between damage index (based on FRF amplitude) and ultimate load of corroded as well as Glass Fibre Reinforced Polymer (GFRP) repaired corroded beams. During flexural testing of corroded beams there is a drop in the ultimate load carrying capacity and mid span deflection. For severely corroded samples, ultimate failure behavior changed from ductile to brittle which is alarming. AE monitoring during flexural testing of corroding beams indicate variation in a number of AE hits and their corresponding amplitudes with an increase in corrosion level. In healthy beams, specific micro-cracking (Phase I) and macro-cracking (Phase III) phases of damage progression are depicted by AE monitoring but with the increase in corrosion levels, a significant drop in cumulative AE hits along with depletion of different AE cracking phases is observed due to the attenuation of the captured AE signals caused by pre-existing cracks as compared to healthy beam. Hence, AE monitoring can serve as a non-destructive tool to monitor initiation and progression of damage and cracks much before they are observed on the surface and also pick up effect of increasing corrosion and sustained loading marked by depletion of AE phases. Further the beams corroded to different levels were repaired using GFRP wraps to investigate the efficacy of FRP wrapping for strengthening corroded structures. The performance of GFRP repair was assessed using global vibration and local AE monitoring during flexural loading. Micro-concrete and GFRP repair of corroded RC beams leads to a significant improvement in dynamic characteristics of beams both in terms of frequency as well as FRF amplitudes. Also as a result of repair, AE phases reappeared with an increase in the number of AE hits pointing towards improved integrity of beams after GFRP repair. AE X-Y event plots give a pictorial representation of actual cracking inside the concrete much before it is observed on the surface. All the cracking events right from initiation to the progression of micro-cracks into macrocracks is well presented by AE X-Y event plots. AE also aids in the visualization of the repaired concrete beneath the FRP wrap as the surface of structure is not exposed for visual observation. Also a correlation is established between non-destructive parameters of cumulative AE hits and ultimate loads in both corroded and GFRP repaired corroded beams to facilitate non-destructive evaluation of RC structures. Hence it can be concluded that global vibration monitoring and local AE monitoring of RC structures can serve as a potential tool for non-destructive evaluation of effect of corrosion with loading and its efficacy post repair of corroded RC structures which would help in deriving a post-corrosion maintenance strategy for RC structures.