Studies on the Mechanical Behavior of FRP Reinforced Concrete Beams Using Acoustic Emission and Digital Image Correlation

Abstract

Corrosion of steel reinforcement in Reinforced Concrete (RC) structures is one of the major challenges faced by the construction industry which severely limits the service life of structures. It is primarily caused by the penetration of aggressive ions like chloride and sulphates, which are either contributed from the concrete ingredients or by external agencies like de-icing salts, marine water, and atmosphere, etc. Several types of protective coatings like epoxy-coatings, biofilms, and galvanization, etc. have been suggested to protect the RC bars from corrosion. But these techniques only delay the onset of corrosion and do not prevent it completely. The use of non-corroding Fibre Reinforced Polymer (FRP) reinforcing bars in place of steel reinforcing bars in concrete offers a potentially good alternative in extreme environmental applications. Glass Fibre Reinforced Polymer (GFRP) composites have extensive applications in various fields such as the aerospace and automotive engineering sectors. During the last two decades, there have been more novel applications for GFRP in the construction sector. GFRP composites have emerged as promising material that can be used in lieu of steel for Reinforced Concrete (RC) structures due to their high longitudinal tensile strength, excellent corrosion and fatigue resistant properties; and high stiffness to weight ratio. Furthermore, relevant design codes and guidelines have also been developed for the use of GFRP bars in RC structures. The behavior of structural members reinforced with GFRP bars is different than that of steel-reinforced concrete members due to the lower modulus of elasticity, brittle behavior, and sudden failure. The sudden brittle failure in GFRP RC structures necessitates the implementation of an effective and real-time health monitoring framework for assessing the structure's integrity, structural performance and crack evolution before the damage becomes catastrophic and disastrous. In this research effort, to develop a damage monitoring methodology for GFRP reinforced concrete beams, their mechanical performance and structural behavior under flexure are first compared with the steel-reinforced beams. The load-deflection plot of steel reinforced and GFRP reinforced concrete beams show contrasting profiles. With increasing reinforcement ratio, steel-reinforced beams typically show an increase in ultimate load-carrying capacity, shrinking plastic zone with reduced ductility, and failure taking place at the much lower strains by steel yielding followed by concrete crushing. On the contrary, GFRP reinforced beams exhibit bi-linear load-deflection response up to the failure without any yielding and exhibit higher ultimate load-carrying capacities and deflections due to their low elastic modulus as compared to steel reinforced beams indicating enhanced ductility and ultimate strength. During flexural loading of steel RC and GFRP RC beams, they are simultaneously subjected to Acoustic Emission (AE) and Digital Image Correlation (DIC) monitoring to investigate the initiation and progression of damage in these differently reinforced beams. AE accurately determines the onset of cracking and monitors the development of micro-and-macro cracks in differently reinforced concrete beams using AE parameters like cumulative AE-hits and their amplitudes and Cumulative Signal Strength (CSS). GFRP reinforced concrete beams exhibit a larger number of cumulative AE hits of higher amplitudes as compared to steel reinforced beams due to their low elastic modulus and different bond characteristics. It is also well supported by the larger number of Knees (steep rise) in the CSS plots. AE parameter of Average Frequency (AF) and Rise Angle (RA) plots can be used to predict the type of cracking in the RC beam. High AF and low RA values indicate pure bending cracks in steel-reinforced beams at the microcracking level, whereas low AF and high RA values point towards shear cracks in GFRP reinforced beams initially. At the macro cracking level, a marginal increase in RA and the drop of AF indicates flexure-failure by steel yielding followed by concrete crushing in steel-reinforced beams. On the other hand, in the GFRP-reinforced beams, a significant increase in AF value and a decrease in RA value indicate sudden shear failure followed by concrete crushing. Similarly, Average Frequency (AF) and Rise Time (RT) signal values are averaged to distinguish the fracture behavior in RC beams. At the early stage of fracture analysis, high AF and shorter RT signal values indicate pure flexural cracks in steel RC beams, whereas low AF and longer RT signal values point towards shear cracks in GFRP RC beams. At the final stage of fracture analysis, a substantial shift from short to slightly longer RT signal indicates flexure-failure by steel yielding followed by concrete crushing in steel RC beams. On the contrary, a strong shift from longer to short RT signal and lower to high AF was observed in GFRP RC beams points towards sudden shear failure followed by concrete crushing. Further, AE parameters of Average frequency (AF) and Duration (μs) with time show a significant drop in AF, and a jump in AE duration at the same time indicates critical failure points in steel RC and GFRP RC beams. These changes are better indicative of damage than the load-deflection plots. It is also observed that the moving average line of AF was high and the duration line was low in the GFRP reinforced beams in comparison to the steel-reinforced beams. This is due to the fact that GFRP bars have lower elastic modulus and different bond characteristics and deflect more than steel bars, causing AE activity to accumulate leading to a high frequency of hits. Along with AE monitoring which serves as an “ear”, DIC provides an “eye” to the damage monitoring strategy proposed in this work. DIC has the potential to serve as an online crack mapping tool in the form of surface strains and displacements and gives an indication much before the actual cracking is visible to the naked eye. It provides a mean for the exact localization of crack on the surface specimen using longitudinal surface strain (εxx) profiles. AE technique offers a qualitative insight into the damage process in concrete by studying variation in various AE parameters hits. The AE XY-events maps and DIC surface strain (εxx) profiles at different stages of damage in the steel and GFRP RC beams closely match with the actual cracked patterns observed in these differently RC beams.