Behaviour of Shear-Critical Reinforced Concrete Beams Strengthened with Fiber Reinforced Cementitious Mortar

Abstract

Extensive research has been conducted on strengthening of shear-critical reinforced concrete (RC) beams, particularly using fiber reinforced polymer (FRP) strengthening systems. This previous research has helped to better understand the behaviour of shear strengthening systems and has improved the performance of existing shear strengthening systems. However, there is still a potential to further improve upon the performance of existing shear strengthening systems. A cement-based composite system is an innovative strengthening system that has similar benefits (such as light weight, ease of installation and non-corroding) to FRP systems, but overcomes some of the draw backs (such as poor compatibility with concrete substrate, lack of vapour permeability and fire resistance) of using epoxy as bonding agent in FRP systems. A cement-based composite replaces the epoxy with cementitious mortar and the fiber sheets with fabric or grids. The current study presents the results of an experimental study conducted to investigate the effectiveness of cement-based composite systems in comparison to an existing epoxy-based system (carbon fiber reinforced polymer, CFRP) to strengthen shear-critical RC beams. Two types of cement-based systems were investigated in this study: carbon fiber reinforced polymer (CFRP) grid embedded in mortar (CGM) and carbon fabric reinforced cementitious mortar (CFRCM). The experimental study consisted of two phases. Phase I focused on flexural testing of seven medium-scale shear-critical reinforced concrete (RC) beams. The objective of this phase was to evaluate the potential of FRCM shear strengthening. The test variables included the type of FRCM (carbon FRCM or CFRCM and glass FRCM or GFRCM) and the strengthening scheme (side bonded vs. U-wrapped). Phase II was designed based on results of Phase I study, and it consisted of flexural testing of twenty (20) large-scale shear-critical RC beams strengthened with cement-based systems. The objective of this phase was to evaluate the effectiveness of the two types of cement-based strengthening systems in comparison to the existing epoxy-based FRP system. The test variables included: the shear span to depth ratio (slender and deep beams), amount of internal transverse steel reinforcement and type of strengthening system (CFRCM, CGM and CFRP). The results showed that the cement-based systems (CFRP grid in mortar and CFRCM) performed better compared to the epoxy-based system (CFRP sheet) in terms of the increase in shear capacity relative to the ultimate strength of the strengthening systems. The results also showed that the bond of cement–based system with the concrete substrate was sufficient that u-wrapping may not be required; the studied side-bonded systems did not exhibit signs of premature debonding. This is in contrast to most FRP fabric strengthening systems were u-wrapping is required for adequate bond. In addition, cement-based systems exhibited a better ability to control diagonal (shear) crack widths compared to the epoxy-based system tested, providing a greater reduction in diagonal crack width despite the relative lower ultimate strength and stiffness of the cement-based systems. Shear strengthening resulted in reduced shear strength contribution from stirrups. The strengthened beams with stirrups exhibited steeper shear cracks compared to control unstrengthened beams with stirrups. Similarly, the presence of stirrups reduces the shear strength contribution from strengthening. Again, the addition of stirrups results in steeper shear cracks which intersect fewer fibers tows in the strengthening system which results in a reduced shear strength contribution from strengthening layer. Lastly, the existing models to predict the ultimate load of strengthened shear-critical RC beams were evaluated and modifications to these methods were proposed.