||External bonding of fiber reinforced plastic (FRP) strips to reinforced concrete (RC) beams has been widely accepted as an efficient and effective method for strengthening and upgrading members. With more and more attention being paid to the strengthening area for RC structures using FRP strips, there was been an exponential growth of research. The objective of the present thesis is to study the enhancement of the mechanical performance of strengthened virgin/precracked RC beams through optimization of design variables (e.g. taper ended FRP strips) based on experimental investigation, accompanied with confirmation by numerical analysis and fracture toughness measurement. This thesis also develops reliable theoretical models for proper evaluation of strengthened beam capacity, identifies the tendency of occurrence of various failure modes and establishes practical guidelines for optimized design criteria and material selection. To predict the load carrying capacity of a strengthened RC beam when concrete cover separation takes place, a simple and accurate design methodology is built. The analytical expression is developed taking into account the stress concentrations in concrete near the tension rebar closest to the cut off point of the FRP strip. The predictions based on the present analytical model are compared to 58 experimental samples from the literature and good agreement has been obtained. A failure diagram is established to show the relationship among different failure modes for RC beams strengthened with FRP strips, and how failure modes change with the relative FRP strip thickness and the distance from the end of the FRP strip to the support. The idea behind the failure diagram is that the failure mode associated with the lowest strain in FRP or concrete by comparison is likely to occur. The predictions based on the present failure diagram are compared to experimental data and good agreement on failure mode and load carrying capacity has been obtained. Therefore, a design guideline can be developed based on failure diagram. The experimental study was carried out using a four point bending test. Several bonding parameters are investigated, such as the FRP strip thickness and end tapering of FRP strips. It is shown that increasing the thickness of FRP strips can give rise to a transition of failure mechanisms from rupture of FRP strips, delamination of FRP strips to concrete cover separation. FRP strips with tapered ends can significantly improve the strengthening performance, including increasing both the load carrying capacity and the deflection at failure for both undamaged and ageing/damaged beams. To prove the experimental results, the finite element method was used to analyze the interlaminar principal, shear and normal stress distributions along the FRP strip-concrete interface, with a special focus on stress concentration at the end of FRP strips. The numerical study indicates that all stress concentrations at the FRP strip ends are reduced with the tapered ends, which in turn leads to a larger load carrying capacity. In addition, the effects of other taper parameters are studied. A simple guideline is proposed for optimal design of tapered FRP strips for given RC beam properties and dimensions. The solutions are formulated based on the determination of critical transition conditions of FRP strips that can give rise to the maximum strengthening performance. The effect of taper configuration of FRP strips on interlaminar fracture behavior is studied through the asymmetric double cantilever beam (ADCB) test. It is found that the taper configuration can significantly affect the load-displacement behavior as well as the crack growth resistance curves. In the sample with tapered FRP strips, there were apparently separate stages of crack propagation corresponding to different FRP strip thicknesses.