Reliability-based Analysis And Design On Shear Capacity Of Concrete Beams Strengthened With Cfrp Laminates

Strengthening and rehabilitation of existing reinforced concrete structures can fully tap the potential of the structure, meet the different requirements of using function of structure, and bring enormous economic and social benefits as well. Due to their advantages of high tensile strength, low mass density, corrosion resistance, fatigue endurance, nonmagnetic, easy treatment during construction, etc, carbon fiber reinforced polymer (CFRP) laminates have been widely applied to strengthening reinforced concrete components. Their shear capacities would be therefore improved and the brittle failure model could also be avoided, effectively. So far, the computational models of the shear capacity of concrete beams strengthened with FRP are experienmentally based on those for conventional unstrengthened reinforced concrete beams. The draft of design guideline'Technical Specification for Engineering Application of Fiber Reinforced Polymers'is coming soon. Some design parameters of relevant provisions in this guideline are short of theorectial basis and empirical to some extent. To make a comprehensive and reasonable understanding of those design parameters, those design provisions were assessed from a probabilistic point of view. After this, the effect of environmental factor on the performance of components was also taken into account, which is meaninigful for developing a durability-based reliability design method for FRP strengthened reinforced concrete structures.The Rackwitz-Fiessler method was used to study the reliability of the shear design method of FRP reinforced concrete beams in the'Technical Specification for Engineering Application with Fiber Reinforced Polymer'(Draft). The research results showed that the application of FRP laminates to the beams might lead to a decrease in reliability level compared with the unstrengthened ones, no matter which mode was adopted. The depressed scopes of reliability index of the three modes were 19.2%～28.5%,7.4%～19.9% and 17.1%-31.9%, respectively. The effects of the load effect factor, concrete strength grade, stirrup reinforcement ratio and FRP reinforcement ratio on reliability were great.In order to make a further investigation on the resistance factor from the probabilistic point of view, a so-called reliability strengthening ratio was introduced, which was defined as the ratio of average reliability indexes of beams after and before strengthening. By calculating the reliability indexes of unstrenghted beams and strenghted ones using U jacketing, side bonding and complete wrapping strengthening modes, the relationships between resistance factor and reliability strengthening ratio of three strengthening modes, which characterized the structural performance applying probabilistic approach, were drawn. So different resistance factor could be used according to different reliability strengthening ratio. The performance-based research work, which was based on probability theory, was explored.The time-dependent ultimate state functions of beams before and after FRP shear strengthening were established on the basis of the time-dependent resistances and time variant load effects, under the chlorine-circumstance. The time-dependent shear capacity analysis of pre-and post-strengthened reinforced concrete beams was carried out. The Monte Carlo-Rackwitz-Fiessler method was used to study the time-dependent reliability with an emphasis on chloride-ingress corrosion in reinforcement. The research results showed that the stirrups, with smaller concrete covers, were prone to be corroded. Significant degradation of shear capacity and reliability of the beams exposed to chloride environmental conditions were observed. When it is approaching the end of 50-year service life and the effect of corrosion of both stirrups and longitudinal bars to shear capacity was considered, the shear capacity dropped to 40.6%, and the scope of reduced reliability indexes was 15.6%-32.2%, compared with the initial moment. CFRP laminates were effective to enhance shear capacity, decrease the decay rate of reliability index, and prolong the service life of components. The effects of concrete cover and corrosion current on time-dependent reliability were notable.