Structural Performance Degrading And Seismic Evaluation Of Existing Reinforced Concrete Structures

Corrosion of steel bar imbedded in concrete stands out increasingly, which makes structural performance of reinforced concrete buildings degrading, so evaluating seismic performance of existing structures with corrosion damage is of great importance to minimize losses of earthquake disasters. In this paper, test studies combined with finite element simulations and theoretical analyses are used to systematically investigate deterioration models of structural behaviors of corroded reinforced concrete members and the evaluation method for seismic performance of existing damaged structures.Corrosion of steel bar leads to a reduction in bar confinement provided by concrete cover and changes at the steel-concrete interface. An analytical method is proposed to evaluate the corrosion pressure due to rust expansion and the maximum residual confinement of concrete cover. Based on test results, the friction coefficient and the composite action of adhesion and interlock at the interface between corroded steel and concrete are obtained. The mechanism of mechanical interactions between corroded bar and concrete is investigated, and an analytical model of the ultimate bond strength is established, which takes all factors into account, such as corrosion degree, thickness of concrete cover, diameter of steel bar, concrete strength, and expansive rate of rust. The model is used to analyze bond strength of some pullout members, and theoretical results agree with test results well.Simulation test and finite element method are used to study behaviors of corroded unbonded reinforced concrete beams. Factors that influence the stress of steel bar in heavily corroded reinforced concrete members are investigated in detail. Results indicate that reinforcement index can reflect effects of both concrete strength and reinforcement ratio on the steel stress. Then a formula of utilization coefficient of steel strength depending on reinforcement index of unbonded reinforced concrete beams is offered. On the basis of these studies, degrading mechanism of load capacity for ordinary corroded reinforced concrete beams is analyzed, and a calculation method of flexural capacity, explicit in concept and wide in applied range, is proposed, which considers both corrosion degree and reinforcement index. The method is used to analyze flexural capacity of large numbers of tested members, and reasonably excellent agreement between calculated results and experimental results is obtained.Factors affecting flexural stiffness of corroded reinforced concrete beams are investigated, and a conclusion is reached that main factors affecting the establishment of calculation model are steel strain between cracks tending to uniformly distributed and steel strain lagging behind concrete strain for bond degrading due to steel corrosion. Throwing off conventional plane hypothesis, the section stiffness of flexural members with deteriorated bond strength is analyzed by means of stiffness analytic method. A comprehensive strain coefficient is adopted to reflect effects of both steel strain tending to uniformly distributed and lagging behind, and a calculation model of flexural stiffness of reinforced concrete beams with corroded bars is established. By means of finite element analysis, the relationship between comprehensive strain coefficient and reduced coefficient of bond strength is built. The formula is used to analyze flexural stiffness of some tested members, and calculated results coincide with test results well.As the re-service term of structures is usually less than its design working life, the principle of equal earthquake risks is suggested to determine the seismic fortification criterion for evaluating seismic performance of existing structures. According to the fortification level of Code for Seismic Design of Buildings (GB50011-2001), earthquake intensities, accelerations of ground motion and earthquake influence coefficient for seismic evaluation are given for different re-service term. In order to analyze stochastic responses of existing structures under earthquakes, a method is also proposed to determine parameters of stochastic model of ground motion.Since the uncertainty of corrosion of steel bar is very high and its probabilistic model is difficult to be built, the interval analysis is introduced into the evaluation of seismic reliability of existing corrosion-damaged structures. Modeling uncertainties of corrosion damage as interval numbers, an interval model is presented to predict structural corrosion damage. Then an interval analysis method of seismic reliability of existing corrosion-damaged structures is put forward. Considering influences of both structural performance degrading and re-service term shortening on results of evaluation, a seismic performance evaluation method based on the capacity spectrum method is put forward for existing damaged structures.