Experimental And Theoretical Research On Hybrid FRP And Seismic Performance For Strengthening Corroded RC Columns

Reinforcement corrosion is one of the major worldwide deterioration problems for RC structures, and Fiber reinforced polymers (FRPs) are becoming popular for seismic strengthening of corroded RC structures, due to their corrosion resistance, high strength-to-weight ratios, ease of installation and so on, but the deformation capacity of single FRP composites is insufficient, which restrict severely FRP composites to further using widely in field. However, hybrid technique is an effective method for solving this problem. Therefore, the research on seismic strengthening of corroded RC columns with hybrid FRP (HFRP) composites with excellent comprehensive properties has important engineering and theoretical value. This paper researches systematically on hybrid FRP composites and seismic performance for strengthening corroded RC columns in three aspects including experiments, theory and design method, main contents are listed as follow:(1) In order to make up the HFRP composites with excellent comprehensive properties, a hybrid idea of Carbon FRP (CFRP), aramid FRP (AFRP) and Glass FRP (GFRP) is firstly proposed. this paper researches experimentally uniaxial tensile property of eleven HFRP composites specimens, hybrid parameters are fiber species, relative volume percent of carbon fiber, laminated mode, and the effects of HFRP composites on strengthening RC structures are verified in further by flexural experiments of eleven RC beams. Based on the experimental study of material properties, a simple and practical strength model for HFRP composites is established. The results show the CFRP/AFRP/GFRP hybrid FRP composites have good hybrid effect and strengthening effect for RC structures, the hybrid effect coefficient is 0.647.(2) Because the stress-strain model of concrete confined by HFRP composites is the premise of seismic strengthening design and strengthening effect evaluation for concrete structures strengthened with HFRP composites, based on the experimental data and the analysis on the research production, a stress-strain model is developed for concrete confined with HFRP composites. The feature of the model is that it includes the effects of HFRP confinement on the concrete microstructure by evaluating the internal concrete damage using the axial, lateral and volumetric strains, which introduces that the model is simple and has a physical and fundamental interpretation with the mechanical behavior of concrete. Meantime, the secant concrete modulus is used in the model and expressed as a function of the secant modulus softening coefficient.(3) Proper evaluation of the bond characteristics of corroded reinforcement in FRP-wrapped concrete is prime importance in FRP repair of corrosion-damaged RC members. Pull-out tests of 96 specimens with different reinforcement types, cover depths, corrosion degrees, with or without strengthening using FRP are carried out to study the bond behavior between corroded reinforcement and concrete. Meantime, a simple bond-slip model for corroded reinforcement is proposed. The results show that when the corrosion degree is below 5%, the main influencing factors for the peak bond strength are reinforcement type and cover depth, FRP confinement plays a less important role, but FRP confinement can effectively slow down the declining speed of bond stress.(4) The seismic experiments of ten corroded RC columns strengthened with CFRP/AFRP/GFRP laminated hybrid FRP sheets under low frequency cyclic loading are carried out, bearing capacity, stiffness, ductility, energy dissipation and damage evolution are analyzed, the effects of the reinforcement corrosion, axial compression ratio and stirrup reinforcement ratio on the seismic behavior are discussed, and the failure mechanism of corroded RC columns strengthened with HFRP composites under low frequency cyclic loading is revealed. Results show that the CFRP/AFRP/GFRP laminated hybrid FRP sheets can improve significantly the seismic performance of corroded RC columns, the accumulated energy dissipation can be increased by 12.5 times.(5) According to the experimental results of corroded RC columns strengthened with CFRP/AFRP/GFRP laminated hybrid FRP sheets, the restoring force model of RC corroded columns strengthened with HFRP composites is presented based on cross-section analysis for undamaged RC element and Clough hysteresis rule, the bond-slip model is firstly introduced to consider the effect of corrosion on bond characteristic in the model. And displacement-based seismic design method of corroded RC columns strengthened with HFRP composites is presented, this method is simple, practical and accurate, which will play a guiding role in seismic evaluation and design of corroded RC columns strengthened with HFRP composites.The main originalities of this paper lie in:(1) Stress-strain model of concrete confined by HFRP composites is developed. The model includes the effects of HFRP confinement on the concrete microstructure by evaluating the internal concrete damage using the axial, lateral and volumetric strains, which introduces that the model is simple and has a physical and fundamental interpretation with the mechanical behavior of concrete. Meantime, the secant concrete modulus is used in the model.(2) The seismic experiments of ten corroded RC columns strengthened with hybrid FRP sheets under low frequency cyclic loading are systematically carried out. The seismic performance of corroded RC columns strengthened with hybrid fiber sheets of CFRP, AFRP and GFRP is firstly researched, the hybrid fiber sheet has good ductility. Bearing capacity, stiffness, ductility, energy dissipation and damage evolution of ten corroded RC columns strengthened with hybrid FRP sheets under low frequency cyclic loading are analyzed. Meantime, the bond characteristic of corroded reinforcement in FRP-wrapped concrete is researched by pull-out tests, which provides experimental data for theoretical analysis.(3) Restoring force model and seismic design method of RC corroded columns strengthened with HFRP composites are presented. Based on cross-section analysis for undamaged RC element and Clough hysteresis rule, the restoring force model of RC corroded columns strengthened with HFRP composites is presented, the bond-slip model is firstly introduced to consider the effect of corrosion on bond characteristic in the model. And displacement-based seismic design method of corroded RC columns strengthened with HFRP composites is developed.In conclusion, the seismic strengthening technique of using CFRP/AFRP/GFRP laminated hybrid FRP sheets for corroded RC columns is developed, and the seismic performance of corroded RC columns can be improved significantly. The construction technology is simple and convenience, so that it has a wide application prospect in seismic strengthening field.