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Abstract
Use of fiber reinforced polymers, FRP in strengthening reinforced concrete elements, has increased rapidly in the last three decades. FRP is made of high-tensile-strength fibers such as carbon (CFRP), glass (GFRP), aramid (AFRP). FRP laminates (sheets and strips) are used widely to strengthen and repair different RC elements. FRP has an outstanding characteristic since it is corrosion free. This can increase significantly the service life of structures by reducing concrete deterioration. Other advantages of FRP laminates of particular interest in the field of structural applications are its high strength-to-weight ratio, good fatigue behavior, low relaxation, and easy handling and installation. The drawbacks of FRP reinforcement include its high cost, brittle failure, low shear strength, lack of ductility due its linear stress-strain behavior up to failure, and low tensile strain at ultimate.
Although the FRP has very high tensile strength, this is often not fully utilized due to the possible debonding between the FRP and the concrete surface under relatively lower loads. Debonding is one of the main problems regarding the use of externally bonded FRP strips. Several types of debonding may occur when strengthening concrete beams with FRP strips. These types can be generally divided into two main groups: i) plate end debonding; where the debonding occurs at the end of the strip, and ii) intermediate crack debonding; where the debonding occurs under the flexural or flexural shear cracks. This thesis aims to investigate the bond characteristics between high strength concrete beams and carbon fiber reinforced polymers, CFRP. The intermediate crack debonding is the major debonding failure mode.
An experimental program was conducted at the Reinforced Concrete Research Unit at the Faculty of Engineering, Ain Shams University to study the behavior of RC beams strengthened by CFRP strips. The program consists of testing eight simply supported beams with overall dimensions of 150 mm width, 300 mm depth and 3000 mm length, divided into four groups depending on different parameters. The first group consisted of three beams varying in compressive strength (specimens B1 to B3). The second group consisted of three beams varying in bond length (specimens B3, B4 and B6). The third group consisted of three beams varying in wrapping system (specimens B6 to B8). The fourth group consisted of two beams varying in concrete cover (specimens B3 and B5). The analytical phase of this study includes a rational analysis to predict the behavior of RC beams strengthened with CFRP strips at the tension side of the beam and GFRP sheets used as U-wraps. An assessment regarding some of the existing codes provisions and models from the literature to predict intermediate crack debonding was performed. Based on the experimental results of the tested beams and the analytical work, new parameters were proposed to predict the intermediate crack debonding failure for simply supported beams strengthened with CFRP strips.