Behaviour and modelling of RC beams strengthened in flexure with near-surface mounted FRP strips

Abstract

The near-surface mounted (NSM) FRP method has attracted increasing attention worldwide as one of the most promising techniques for structural strengthening and as an effective alternative to the externally bonded FRP method. In the NSM FRP method, grooves are cut into the concrete cover of a concrete member for the embedding of FRP bars using an adhesive. Compared to the externally bonded FRP method, the NSM FRP method has a number of advantages including a reduced risk of debonding failure and better protection of the FRP reinforcement. This thesis presents a theoretical study carried out by the candidate over the past few years on the behaviour and modelling of RC beams flexurally-strengthened with NSM CFRP strips (i.e. FRP bars of narrow rectangular section). CFRP strips were focused on because they possess a larger perimeter-to-cross-sectional area ratio than bars of other shapes and thus offer the strongest bond with the concrete member being strengthened. Of the six main chapters of the thesis, three deal with the bond behaviour of CFRP strips near-surface mounted to concrete while the other three are concerned with debonding failure at the ends of CFRP strips. A 3-D meso-scale finite element (FE) model for NSM CFRP strip-to-concrete bonded joints, in which very small elements (of the order of 1mm) were employed, was developed to study the bond behaviour between NSM CFRP strips and concrete, including the failure process, the local bond stress distribution and the bond-slip relationship. This 3-D meso-scale FE model was verified through comparisons with test results and was then adopted to carry out a parametric study. Based on results from this parametric study, empirical equations for the maximum bond stress and the interfacial fracture energy were formulated, and a bond-slip model was proposed. A bond strength model for NSM CFRP strip-to-concrete bonded joints based on the interfacial fracture energy approach was established. The influence of the bond length of CFRP strips on the bond strength of NSM CFRP strips bonded to concrete were investigated based on the proposed bond-slip model. Apart from investigations into the bond behaviour of NSM CFRP strips, this thesis also presents investigations into a common debonding failure mode which occurs at the ends of NSM strips and involves the detachment of the FRP reinforcement and the concrete cover (i.e. bar-end cover separation). An analytical solution is first presented for the interfacial interaction forces between the FRP reinforcement and the concrete in RC beams strengthened with NSM FRP; this solution was extended from an existing interfacial stress solution previously derived for RC beams strengthened with an externally bonded FRP plate. The analytical solution was verified with results from a 3-D linear elastic FE model. Numerical results obtained from the solution show that high interaction forces exist near the cut-off points of NSM CFRP strips, which explains why bar-end debonding failure commonly occurs. Following the study on interfacial interaction forces, a 2-D nonlinear FE model to predict bar-end cover separation failure of RC beams flexurally-strengthened with NSM FRP is presented. Some key factors, which were not addressed or were not simultaneously addressed in existing FE models, were considered in developing this FE model; in particular, the NSM FRP-to-concrete interface was modelled using the bond-slip model presented earlier in the thesis. Based on findings obtained using this 2-D FE model, a simplified FE model for predicting bar-end cover separation failure was next proposed. This simplified FE model contains only an isolated segment between two adjacent cracks of an RC beam nearest to an end of the FRP reinforcement. Results from the parametric study using this simplified FE model were used to establish an expression for the failure strain in the FRP, based on which a method for predicting bar-end cover separation failure was formulated.