FRP strengthening of concrete walls with openings

Abstract: The thesis deals with the axial strength of axially and eccentrically loaded concrete walls with cut-out openings strengthened by fiber-reinforced polymers (FRPs). Background: Functional modifications of concrete structures are common because existing structures must often be adapted to comply with current living standards. Such modifications may include the addition of new windows or doors and paths for ventilation and heating systems, all of which require openings to be cut into structural walls. These openings are a source of weakness and can size dependently reduce the structures’ stiffness and load-bearing capacity, thus, requiring the element to be repaired. Aim and objectives: The main aim of this project was to develop a toolbox containing solutions for strengthening concrete walls with existing or newly created openings using FRP materials. The two immediate objectives sought are: (1) An assessment of the research level on concrete walls with and without openings; (2) An experimental and numerical investigation of the structural behavior of the FRP strengthened walls with openings. Methods of investigation: The experimental program was defined by reviewing the relevant tests performed to date. The literature review revealed research gaps that the current study aims to fill. Moreover, preliminary nonlinear finite element analyses were performed prior to the experimental program in order to gain insight into the structural behavior of these elements. Nine specimens designed to represent typical wall panels in residential buildings, at half-scale, were constructed for testing to failure. The two types of openings examined comprised symmetric halfscaled single door-type openings, and symmetric half scaled double door-type openings. The test matrix was divided into three stages, namely: (1) Reference specimens, (2) Pre-cracked specimens strengthened by FRP and (3) Un-cracked specimens strengthened by FRP. The strengthening method used was FRP confinement with the aid of mechanical anchorages. Results: The results indicate that the 25% and 50% reductions in cross-sectional area of the solid wall caused by introducing the small opening and large opening reduced its load carrying capacity by nearly 36% and 50%, respectively. The application of the FRP confinement increases the capacity and the stiffness of the specimens with cut-out openings. The axial strengths were between 85-94.8% and 56.5-63.4% for specimens having a small and large opening, respectively, of that of a solid wall. Conclusion: The FRP-confinement together with the mechanical anchorages was able to partly restore the capacity of a solid wall. Better results might have been possible if longitudinal FRP strips or bi-directional fibers were used. The effects of steel anchorages were not investigated and it is believed that they might have had positive influences. However, the optimal distance between the anchors should be further investigated. Moreover, the influence of the prestressing force of the anchorages may also be an important parameterthat has led to an increase in capacity. Keywords: Strengthening, Fiber-reinforced polymers, Concrete walls, Openings, Axial load, Eccentricity, Out-of-plane behavior, Two-Way