Investigation On Axial Compression Behavior Of CFRP-steel-confined Undamaged And/or Damaged Concrete In Square Columns And Seismic Performance Of CFRP-repaired Damaged Square RC Columns

Lessons from earthquake disasters show that buildings constructed according to early seismic design codes generally have deficiencies in both the seismic conceptual design and details,and there is an urgent need for seismic strengthening of these old buildings.Meanwhile,buildings designed in accordance with modern seismic design codes may need to be upgraded for enhanced seismic performance due to the change of function.On the other hand,structural members moderately or seriously damaged in an earthquake should be repaired reliably.Consequently,it is of great practical significance to systematically investigate the seismic performance of fiber reinforced polymer(FRP)-confined undamaged and/or damaged reinforced concrete(RC)members.However,most of the existing researches related are experimental investigations on the seismic performance of FRP-confined undamaged and/or damaged RC columns.Only a few experiments have been conducted to explore the axial compression behavior of FRP-confined undamaged reinforced concrete at material level,and the resulting constitutive models for FRPconfined undamaged reinforced concrete under compression were mainly based on the test results of small-sized FRP-confined RC columns with low level of volumetric ratio of transverse steel reinforcement(TSR),without taking the effect of variation of concrete grade into account.In addition,although some researchers have studied the compression behavior of FRP-repaired damaged plain concrete and established its stress-strain model,experimental and theoretical investigation specifically devoted to the axial compression behavior of FRP-repaired damaged reinforced concrete has rarely been reported in the literature so far.In view of the problems and deficiencies in the current research,axial compression tests on 74 carbon FRP(CFRP)-confined undamaged and/or damaged square RC columns were conducted first in this study for understanding the axial compressive behavior of CFRPconfined undamaged and/or damaged reinforced concrete at material level.On this basis,a pseudo-static test on 6 CFRP-repaired damaged square RC columns was carried out to investigate the seismic performance of CFRP-repaired damaged square RC columns at component level.The main contents are as follows:(1)The axial compressive behavior of CFRP-confined undamaged reinforced concrete with medium or high level of volumetric ratio of TSR under monotonic loading,including the ultimate stress and ultimate strain,the peak stress and corresponding strain,the intercept stress and stress-strain relationship,was investigated by changing the concrete strength,the amount of internal stirrups and the number of layers of CFRP wraps.A stress-strain model for CFRPconfined undamaged concrete in square RC columns under monotonic compression,which is applicable to any level of volumertric ratio of TSR,was proposed based on the present test results,as well as the available compression test results on square RC columns with low level of volumetric ratio of TSR.(2)The axial compressive behavior of CFRP-repaired damaged reinforced concrete(CRDRC)under monotonic loading,including the ultimate stress and ultimate strain,the intercept stress and initial elastic modulus,was investigated by changing the level of predamage,the concrete strength,the quantities of internal stirrups and of the CFRP sheets.A stress-strain model for CRDRC in square columns under monotonic compression was proposed based on the results of regression analysis.(3)The axial compressive behavior of CRDRC under cyclic loading,including the residual strain,unloading modulus,stress deterioration ratio and equations for unloading and reloading paths,was investigated by changing the level of pre-damage,the concrete strength,the quantities of internal stirrups and of the CFRP sheets.A stress-strain model for CRDRC in square columns under cyclic compression was then proposed.(4)With the axial compression ratio and the level of pre-damage as test variables,the seismic performance of CFRP-repaired damaged square RC columns was investigated,including the displacement ductility ratio,the degradation of strength and stiffness,as well as the accumulative dissipated energy.(5)Based on the Open Sees finite element platform,the proposed stress-strain model for CFRP-confined undamaged and/or damaged concrete in square RC columns was embedded into Open Sees uniaxial material library.Fiber elements were then used to simulate the seismic performance of CFRP-confined undamaged and/or damaged square RC columns.The close agreement between the prediction and the test results obtained from the current experiment and other researches verifies the applicability and precision of the constitutive model proposed for CFRP-confined undamaged and/or damaged reinforced concrete in square columns.The innovations of this research are as follows:(1)Unlike the existing constitutive models which are only suitable for the case of low level of volumetric ratio of TSR,the stress-strain model for CFRP-confined undamaged concrete in square RC columns proposed herein is applicable to any level of volumetric ratio of TSR,with the impact of concrete grade on the ultimate strain of CFRP-steel-confined concrete being taken into account.(2)The constitutive model for CRDRC in square columns under monotonic and cyclic compression was proposed for the first time.(3)The proposed constitutive model for CFRP-confined undamaged and/or damaged reinforced concrete in square columns was incorporated into the Open Sees uniaxial material library,which has laid a solid foundation for the accurate numerical simulation of seismic performance of CFRP-confned undamaged and/or damaged square RC columns.