Strengthening reinforced concrete structures with FRP composites

Sammanfattning: Civil infrastructures made of reinforced concrete (RC) play an important role in the economic activities and services of society. However, signs of deterioration and functional deficiency are commonly found in existing RC structures. Thus, there is a great demand for upgrading the capacity and performance of existing concrete structures. Fibre reinforced polymer (FRP) composites have been widely used as externally bonded reinforcement (EBR) for strengthening RC structures since the 1990s. The work of this thesis aimed to investigate robust and efficient FRP-strengthening systems for structural strengthening of existing RC structures with a focus on RC bridge superstructures. Three different strengthening techniques were investigated. A recently developed technique, namely the stepwise prestressing method, was studied in the current work to eliminate the need for mechanical anchors when using prestressed carbon-FRP (CFRP) plate as EBR for strengthening RC beams. Experiments showed that this method could realise the self-anchorage of prestressed CFRP plates on the surface of concrete beams given prestressing levels of 25-30% (of the CFRP tensile capacity). Despite no installation of mechanical anchors, the self-anchored prestressed plates were demonstrated to be efficient in reducing crack widths and improving the flexural capacity of the strengthened RC beams. At the debonding of the CFRP plates, the utilization ratios were in the range of 81-86% (of the CFRP tensile capacity) indicating significantly improved utilisation of the plates compared with equivalent non-prestressed plates. A practical modelling strategy was also developed to enable nonlinear FEA of the CFRP-strengthened RC beam. Using the FEA, parametric studies on the self-anchored plates indicated an optimal prestressing level of 40% (specifically for the investigated specimen), above which both load-carrying and deflection capacities of the strengthened beam would decrease due to CFRP debonding before yielding of steel reinforcement. A hybrid FRP system for strengthening RC beams with a T-shaped cross-section, representing the deck and girder system of RC bridge superstructures, was also investigated. The hybrid system included self-anchored prestressed CFRP plates applied to the soffit of the T-beams and prefabricated glass-FRP (GFRP) panels installed on the top of the T-beam flanges. In the strengthened RC T-beams subjected to bending, the CFRP plate acted as tensile reinforcement and the GFRP panel took most of the compressive force. Flexural tests showed that the applied hybrid FRP strengthening system was robust and efficient in improving the flexural stiffness and capacity. The tests also highlighted substantial residual capacity after the CFRP debonding, as the compressive zone shifted to the GFRP panel and concrete crushing at the top of the T-beam was prevented. The current work also investigated effective FRP strengthening systems for deteriorated concrete beams with highly corroded steel reinforcement. The system included externally bonded FRP reinforcement on the beam soffit and CFRP U-jackets along the span. Flexural tests showed that the system was efficient in upgrading the flexural capacity of deteriorated concrete beams, despite local corrosion levels of steel reinforcement up to 57% and unrepaired concrete cover with up to 2 mm wide corrosion-induced cracks. The U-jackets effectively suppressed spalling of the concrete cover and thus enabled improved utilisation of the bonded FRP reinforcement on the beam soffits, with a utilisation ratio of CFRP plates up to 64% and even rupture of GFRP laminates. In summary, the FRP-strengthening systems investigated in the current work were demonstrated to be robust and efficient in strengthening RC members subjected to bending.