Flexural Strengthening of Reinforced Concrete Beams Using Externally Bonded CFRP--An Innovative Method for the Application of Prestressed CFRP Laminates

Sammanfattning: A large number of existing bridges in European countries are structurally deficient and functionally obsolete due to the deterioration of aging bridges and the lack of structural and geometric capacity to accommodate the increasing traffic volume and load levels. To meet the demand for holistic and effective strengthening techniques the SUREBRIDGE solution is proposed for the refurbishment of concrete bridges, which includes the technique of using prestressed carbon fiber reinforced polymer (CFRP) laminate as externally bonded reinforcement for flexural strengthening. This thesis studied an innovative method, adopted in the SUREBRIDGE solution, for the application of prestressed CFRP laminates, which aimed to realize the self-anchorage of the prestressed laminates on RC members without the need for conventional mechanical anchorage. The method, named the stepwise prestressing method, was implemented in the laboratory to apply a prestressed CFRP laminate to an RC beam for flexural strengthening. The implementation showed that the CFRP laminate prestressed with a force up to 100 kN was self-anchored to the beam without installing anchors at the laminate ends. Finite element (FE) analyses were performed to further study the interfacial stresses in the CFRP-concrete adhesive joint. The FE results revealed that, owing to the use of the stepwise prestressing method, the interfacial shear stresses were significantly reduced, which yielded a sufficient margin for the safe self-anchorage of the prestressed laminate. The experiment program of four-point bending tests was carried out to investigate the effectiveness of the method for the flexural strengthening of the RC beam with the self-anchored prestressed laminate in both serviceability limit state (SLS) and ultimate limit state (ULS). The performance of this prestressed beam was evaluated and further compared with an un-strengthened beam and a beam strengthened with an unstressed CFRP laminate. The comparison of experimental results showed that, even though no end anchorage was used, the self-anchored prestressed laminate effectively improved the performance of the strengthened beam regarding bending stiffness after cracking stage, widths of crack openings, ultimate capacity, and the utilization ratio of CFRP laminates at failure. Nonlinear finite element (NLFE) analyses of the beams subjected to the bending tests were conducted to perform parametric studies on prestressing levels and the elastic modulus of CFRP laminates. The parametric studies delivered optimization recommendations for the application of prestressed laminates with a consideration of bending response, failure model, ultimate capacity, and ductility of strengthened beams.