Research On Seismic Performance Of Reinforced Low-strength Concrete Columns Confined By Basalt Fiber-reinforced Polymer

The reinforced concrete columns in existing buildings structure generally have low-strength concrete and low web reinforcement ratio,which cannot meet the requirements of current seismic design code.In order to ensure the safety,the seismic strengthening is usually needed.Due to the advantages of good mechanical properties and low production price,basalt fiber reinforced polymer(BFRP)is preferable for strengthening structures.Compared with CFRP(Carbon Fiber Reinforced Polymer),BFRP has slightly lower tensile strength and better elongation percentage.Currently,the calculation models and design method of the seismic performance of FRP confined reinforced concrete columns are usually derived from the research about CFRP,which lead to the relatively low accuracy when applied with BFRP.The specific knowledge about BFRP strengthened concrete structures is insufficient.Therefore,more loading experiments and corresponding theory research should be completed,which could accelerate the application and popularization in practical engineering and promote the integration into the strengthening code system.The thesis started from the test of the mechanical behavior of BFRP sheets,then the constitutive model of BFRP confined concrete was established.Afterwards,the seismic behavior confined reinforced low-strength concrete columns were investigated by experiment,finite element method and theoretical analysis.The main contents are shown as follows.(1)Tensile performance test were done for BFRP sheets,which were divided to seventeen groups for different types.The influence of parameters including weave architecture and mass per unit area on the test results was investigated.Moreover,by using the scanning electron microscope on tensile failure specimens,the influence of parameters such as fiber-resin matrix interface on the macroscopic mechanical properties was also analyzed.The results showed that the mechanical properties and stability of unidirectional sheets were better than the bidirectional sheets.The standard tensile strengths of bidirectional sheets were 25%～30% lower than those of unidirectional sheets,but the former had much larger standard deviation and variable coefficient.The peak loads of the specimens with strong interface between fiber and matrix were higher than those with weak interface.The design values of strengths can be calculated based on the standard tensile strengths of different types of BFRP sheets.(2)Five groups of the BFRP confined concrete columns were tested to study the compressive behavior,and the influence of section shapes,BFRP types and number of layers on compressive behavior of columns was explored.The test results indicated that the compressive strengths of confined specimens were increased by 20% to 71% for circular columns and by 23% to 41% for square columns.Similarly,the ultimate strains were improved by 49% to 296% for circular specimens and by 45% to 145% for square specimens.The 2-layer unidirectional BFRP jacket had the best confinement effect,whereas the confining effectiveness of bidirectional BFRP wrapping was relatively lower than that of unidirectional BFRP wrapping.Finite element numerical models were also established by using the software ABAQUS,and then the stress distributions of BFRP jackets and concrete were analyzed.Basing on the existing experimental database,strength and ultimate strain models were developed for BFRP confined circular and square columns,which were proved of higher precision after verified with existing models.Subsequently,the constitutive model of BFRP confined circular concrete column was also presented.(3)Twenty-four full-scale reinforced concrete columns were tested subjected to low cyclic lateral load.The influence of parameters such as concrete strength,axial compression ratio and FRP types on the test results was investigated,and the seismic behavior of the reinforced concrete columns before and after confinement was evaluated by means of hysteretic curve,ductility factor,energy dissipation and degeneration of stiffness and strength,etc.The differences of seismic performance between BFRP and CFRP under same lateral confining stress were focused to compare.The test results showed that after confinement,the failure modes of columns changed into flexural failure.The strength,ductility and energy dissipation capacity of columns were improved significantly,and BFRP can provide better confinement especially for columns with high axial compression ratio and low-strength concrete.With same lateral confining stress,the peak load of BFRP and CFRP confined columns were similar.But for columns with low-strength concrete,the former had better ductility and energy dissipation capacity than the latter,which indicated a better seismic strengthening effect.(4)Finite element numerical models of BFRP confined reinforced concrete columns were established by using ABAQUS,and the influence of concrete strength on the strain distributions of concrete and BFRP jackets were investigated.Analysis results found that the strains of concrete and BFRP jackets were both relatively larger,but the high strain zones of BFRP jackets became smaller.Meanwhile,parameter analysis was conducted considering the influence of strengthening method,shear span ratio,number of FRP layers,longitudinal and web reinforcement ratio,which can provide data support to subsequent theoretical analysis.(5)Based on the result of the tests and simulation,the regression analysis for the data of FRP confined concrete columns with low-strength concrete was done,and the modified model of the load-displacement skeleton curve for confined columns was explored.Then,the formula of unloading stiffness was given,and the load-displacement restoring force model of confined columns was set up.The expressions of the BFRP strengthening amounts on the basis of strength and displacement were recommended,and the seismic design method and its example for BFRP strengthened existing columns were proposed.