Experimental And Theoretical Analysis Of Axial Compressive Behavior Of Pre-damaged Concrete Prisms Confined With Basalt Fiber-reinforced Polymer

The seismic fortification target required by the seismic code of China is “no damage under minor earthquake,repairable under moderate earthquake,and no collapse under large earthquake”.As for the actual earthquake-damaged structures,except for those with severe damage,and the structures with unrepairable excessive residual displacement need to be demolished and reconstructed,a large number of structures with minor,moderate or relatively severe damage can be repaired/strengthened in stead of demolition and reconstruction,which are neither economical nor beneficial for the reconstruction and economic recovery of the disaster area.Therefore,it is an inevitable choice to repair and strengthen these structures.In recent years,fiber reinforced polymer(FRP)composites have become an important material in civil engineering due to its excellent physical and mechanical properties and chemical properties,especially in the seismic retrofitting/strengthening of columns.The axial compressive behavior of pre-damaged concrete confined with BFRP is the theory basis for the retrofitting of earthquake-damaged structure.Domestic and foreign researchers have carried out a large number of experimental and theoretical studies on FRP-confined concrete,however there are few studies on the mechanical properties of FRP-confined pre-damaged concrete.At present,some reports on FRP-confined concrete cylinders have been fund.However,the columns in practical engineering are mainly square shape.The research on FRP-confined concrete prisms with initial damage is quite limited.In view of the research gap of the FRP-confined pre-damaged concrete prisms,the experimental study on the compressive behavior of pre-damaged prisms confined with BFRP was carried out.The constitutive relationship of the axial compressive behavior of the BFRP-confined pre-damaged prisms was derived based on the experimental results,and a simplified calculation formula for the skeleton curve of FRPstrengthened damaged reinforced concrete(RC)columns was proposed.The main contents of the study are as follows:(1)In order to investigate the influences of the number of BFRP layers and predamage levels on the axial compressive mechanical properties of BFRP-confined concrete prisms,a total of 37(including 34 BFRP-confined specimens and 3 plain concrete specimens)specimens with a length of 150 mm,a height of 300 mm and a chamfer radius of 23 mm were subjected to a monotonic axial compression test.Based on the test results,a strength model,strain model and stress-strain relationship model of BFRP-confined undamaged concrete prisms were proposed.(2)Based on three kinds of damage evaluation indexes,the damage evolution parameters were introduced.A strength model,strain model and stress-strain relationship model of BFRP-confined prisms under monotonic axial compression loading were proposed with considering the effect of pre-damage.(3)Cyclic compression tests were carried out on 16 BFRP-confined concrete prisms with a side length of 150 mm,a height of 300 mm and a chamfer radius of 23 mm.The influences of the number of BFRP layers and the initial damage levels on the cyclic compressive behavior of BFRP-confined prism were studied.Based on the test results,the equations of the plastic strain and the unloading/reloading paths of BFRPconfined prism were proposed.On the basic of the damage evaluation indexes,equations of the plastic strain and the unloading/reloading paths of BFRP-confined prisms with considering the influence of initial damage levels were also developed.Combining the established monotonic stress-strain model with the unloading/reloading paths,the constitutive relationship model of BFRP-confined undamaged and damaged prisms under cyclic compression loading was proposed.(4)Based on the constitutive relationship of BFRP-confined pre-damaged concrete prisms under monotonic axial compression loading,a simplified calculation process for the skeleton curve of RC columns retrofitted by BFRP with considering the influence of initial damage is derived.The calculated results showed a good agreement with the experimental results.