Load-bearing Performance Of Prestressed Confined Concrete Considering Creep

With the progress of industrialization in infrastructure, segmental assembling concrete structure developed rapidly in recent years. Sufficient prestress is usually applied in segmental assembling concrete structure to fix the prefabricated components, which results in a higher stress level during the daily service, then conspicuous phenomena of concrete creep and prestress relaxation may come into being. To further improve structural performance, confined concrete has been more and more frequently used as major load-bearing components in this structure system. However, the study of prestressed confined concrete considering the coupling effect of creep and relaxation is far from enough.In this context, we conducted systematic research on concrete-filled steel tube(CFST)and FRP-confined concrete, respectively. Theoretical models were carefully established and analysis tools were extensively explored, then several conclusions and contributions have been achieved as follows.(1) The UserMat secondary development based on the OpenSees finite element software was completely explored, which can effectively embed the uniaxial material constitutive models proposed in this study into the OpenSees material library. A detailed flowchart of the UserMat secondary development and associated source code in C++ language were illustrated. Besides, a general fiber element model was established for concrete segmental bridge piers, which can be further used in efficient numerical modeling of various segmental assembling concrete structure.(2) The constitutive models of steel-confined concrete and FRP-confined concrete were investigated, respectively. First, an exponential descending branch was proposed for steel-confined concrete, which can effectively depict the softening behavior of concrete with a concise format and clear physical parameters. Then based on an experimental database including 976 specimens, a modified FRP-confined concrete constitutive model was suggested to improve its applicability on different FRP types and high-strength concrete. These two new material models were successfully embedded into OpenSees material library, and its accuracy was verified by a series of experiments under different load cases.(3) An analytical model was established for highly efficient estimation of long-term deformation in prestressed CFST at any designated target time considering creep and relaxation. This model incorporated the combined effects of creep behavior, geometrical compatibility and stress redistribution between concrete and steel tube. This new model can improve the efficiency up to 10-15 times compared with the classical model proposed by Naguib and Mirmiran. Then the mechanism underlying the coupling effect of creep and relaxation on mechanical performance of confined concrete components were elaborated. Subsequently, a constitutive model considering creep and relaxation was proposed based on the calculated stress-strain status. According to the parametric study, creep and relaxation effects can lead up to 25%-30% prestressing loss and be negative to the lateral load-resisting capacity associated with serviceability limit state.(4) Considering the material characteristic of FRP, an analytical model of long-term deformation in prestressed FRP-confined concrete was established. This model can be used to efficiently predict the stress-strain status of FRP-confined concrete considering creep and relaxation at any designated target time. Then a constitutive model considering creep and relaxation was proposed, and the mechanism underlying the coupling effect of FRP creep, concrete creep and prestress relaxation on mechanical performance were elaborated. Numerical study indicated that predicted long-term deformation and mechanical performance were in good agreement with available experimental results.