Multilayer versus Monolayer Externally Bonded CFRP Sheets for Shear Strengthening of Concrete T-Beams

Abstract

The use of externally bonded (EB) carbon fiber–reinforced polymer (CFRP) fabrics for strengthening existing structures has received worldwide acceptance since the late 1990s. The composites produced at the time were of low density and have established the experimental database used by codes and guidelines to develop their analytical design models. As for projects requiring higher densities, the design guidelines recommend the use of several plies (multilayers) of CFRP fabrics. Use of a multilayer fabric leads to higher installation costs. Therefore, the need for high-density CFRPs has been recognized over the last few years by manufacturers. However, outstanding issues remain to be investigated, such as (1) the maximum number of EB-CFRP layers that can be installed without exhibiting slippage or debonding, particularly on vertical surfaces where strengthening in shear is required; and (2) the effectiveness of shear strengthening with multilayer CFRP fabrics, taking into account premature debonding failure and the effect of CFRP thickness. This study aims to evaluate and compare experimentally the behavior of 18 full-size reinforced concrete (RC) T-beams strengthened in shear with multilayer CFRP sheets to that of beams with monolayer strengthening fabric of equivalent density. The influence of internal transverse steel reinforcement (steel stirrups) on EB-CFRP fabric performance is also examined. The test results revealed that the increase in CFRP weight enables an increase in shear resistance, but not in a proportional way, up to an optimal value (300 g/m2 in this study). In addition, strains in CFRP featured lower values as the number of layers increased and were more significant in monolayers than multilayers of equivalent weight. Finally, predictions of current design guidelines were closer to experimental results for beams without steel stirrups because they do not yet consider the interaction between EB-FRP and transverse steel.