Numerical Simulation Of Axially Compressvie Mechanical Behavior Of Preloaded Circular Concrete Columns With Cfrp Confinement

Fiber reinforced polymer (FRP) has been more and more widely used to rehabilitate and strengthen concrete structures for its advantages, such as lightweight, high-strength, fatigue enduring capacity, easy construction etc., over conventional building materials. Many domestic and foreign researchers have carried out plenty of experimental studies and theoretical analysis, and proposed some mature mechanical models. However most of these are centered on the behavior of unpreloaded columns. In practice, concrete columns are preloaded before strengthened. And maybe in some situations, the differences of loading paths have a significant effect. Studies on the strengthening preloaded concrete columns with FRP are just underway. Tests results related to this question are not sufficient due to the limit of testers and some other test conditions, and investigations on some important parameters carried out were insufficient. Even that the conclusions are different from each other. The incremental form of nonlinear elasticity theory of concrete proposed by Elwi and Murray and the relationship between transverse strain and axial strain model of CFRP confined concrete are combined to simulate the axially compressive mechanical behavior of unpreloaded and preloaded circular concrete columns with CFRP confinement.First, the simulation of the mechanical behavior of unpreloaded concrete columns with CFRP confinement is carried out, and some analysis programs are compiled. The comparison between the predicted results and corresponding tests results indicate that the proposed numerical procedures are reasonable and accurate. On the basis, the numerical simulation of preloaded concrete columns strengthened with CFRP is conducted. The comparison between predicted results and corresponding tests results indicates that the proposed numerical procedures are also applicable to analyze the preloaded concrete columns strengthened with CFRP. Then the effects of main parameters such as preloading stress level, ultimate confining ratio, elastic modulus, layer number of CFRP, lateral reinforcement ratio and longitudinal reinforcement ratio are evaluated. It's concluded that preloading stress level has an evident influence on the load capacity of strengthened concrete columns while ultimate confining ratio is small, and the higher the preloading stress level is, the lower the load capacity of strengthened concrete columns with CFRP is; and preloading stress level has little effect while ultimate confining ratio is big enough. However, the value of load capacity falls with preloading stress level when preloading stress level exceeds a certain extent. The volumetric lateral reinforcement ratio and longitudinal reinforcement ratio have positive linear relation with the axial capacity of columns. Preloading stress level influencing factors are formulated for preloaded plain concrete columns and reinforced concrete columns respectively. After contrast, it's obtained that lateral reinforcement reduces the effect of preloading stress level on the load capacity of concrete columns strengthened with CFRP.In practice, most concrete columns are under long term sustained load before being strengthened, so it's necessary to take the time-dependent effect into consideration. On the basis of forementioned analysis, concrete shrinkage and creep predicting models, time-dependent strength models and damage predicting models are combined to numerically simulate the axially compressive mechanical behavior of preloaded concrete columns with CFRP confinement and investigate the effects of concrete shrinkage, creep, variety of concrete strength with time and concrete damage. The following conclusions are obtained: it's better to strengthen concrete columns within short term after axial load is applied; preloading stress level has significant effect on the load capacity, and the higher is the preloading stress level, the lower is the load capacity of preloaded columns with CFRP confinement; relative humidity influences the load capacity of preloaded columns with CFRP confinement evidently too; the load capacity of preloaded columns strengthened with CFRP descends rapidly first, and then changes gently with the increase of load-sustaining time before strengthening; it's better to unload sufficiently before strengthening to improve the strengthening effect.