Improvement Method And Design Theory For Fire Resistance Of RC Members Strengthened With NSM FRP Bars

The technology of near-surface-mounted（NSM）strengthening with fiber-reinforced polymer（FRP）bars has received wide attention due to its outstanding advantages.However,in engineering applications,there is still problem of insufficient fire resistance with this strengthening technology.One point is FRP bars are made of resin and fibers.These two components are easily damaged in high temperature or fire,thus reducing the contribution of FRP bars to structural fire resistance.The other point is if the research focuses on the fire protection,it will be inevitable to deviate from the meaning of structural design.Therefore,starting from two aspects of materials and structure,this research was conducted using a combination of experiment,numerical simulation and theoretical calculation.The improvement and design method of fire resistance of flexural RC members strengthened with NSM FRP bars were studied.The specific research contents and results are as follows:（1）A phenolic resin-based basalt fiber polymer bar（P-BFRP）with excellent high-temperature performance and mechanical properties has been developed.The thermos-physical and thermomechanical properties of phenolic resin-based,vinyl resin-based and epoxy resin-based basalt FRP bars（abbreviated as P-BFRP,V-BFRP,and E-BFRP）at high temperatures were investigated.The results of thermos-physical properties showed that the residual weight of P-BFRP bars was the largest at high temperature,and the decomposition temperature of P-BFRP bars was significantly higher than that of V-and E-BFRP bars.These three types of BFRP bars showed similar specific heat capacity and thermal conductivity.The glass transition temperature（T_g）of the P-BFRP bar was 375℃,which is significantly higher than the other two BFRP bars;the P-BFRP bars exhibited stronger bonding at fiber-resin interface and thus almost no interfacial bonding damage occurred from ambient temperature to 300℃.The results of thermomechanical properties showed that compared with V-and E-BFRP bars,P-BFRP bars exhibited higher percentage of residual tensile strength at high temperature,and higher T_g delayed the fiber-resin bonding damage inside the bars;the residual tensile strength of P-BFRP bars did not decrease below 300℃ and still provided higher values above 425℃.In high temperature,the alternating and progressive failure of fibers and fiber bundles led to the reduction in elastic modulus of BFRP bars,and three types of BFRP bars exhibited similar degradation.Using the hyperbolic tangent function as the basic model,a constitutive model of high-temperature mechanical properties suitable for the three BFRP bars were obtained.（2）An experimental investigation on the fire resistance of flexural RC beams reinforced with NSM BFRP bars was carried out.The failure modes,temperature distribution,deflection changes and fire resistance of the beams were analyzed.The results showed that under the loading of four-point bending,all beams strengthened with BFRP bars exhibited similar failure modes when they finally reached the fire resistance.There were pure bending cracks and bending-shear cracks on the sides of the beam,and the cracks at the bottom of the beam was relatively fine.No falling of grouting material occurred in the grooves.Although the fire protection board at the end of beam cracked,only a small part fell off,and the glue under the fire protection board had all been carbonized.The BFRP bars have all failed:the resin decomposing and the bars cracking.When the strengthening scheme in this research（epoxy glue in the anchoring section,grouting material in the middle section）was used and fire protection was only carried out in the anchoring section,the beams strengthened with NSM BFRP bars can reach the fire resistance of at least 1.5 hours under a load ratio of 0.5,meeting the 2^nd-class fire resistance in the specification.（3）A numerical simulation of the temperature research of RC beams strengthened with NSM FRP bars was carried out,including model verification and parameter analysis.The research showed that the thermal properties selected（or tested）of the material were reasonable,and the simulation results was in good agreement with the experimental results.The parameter studies found that when fire environment is the same,the temperature fields of the CFRP-strengthened beam and the BFRP-reinforced beam were similar,and only the cross-section temperature of the FRP bars was different.In the fire,the temperature on the cross section of both CFRP bars and BFRP bars in the strengthened beams was distributed in a gradient,and the isotherms were a straight line and a curve with the opening facing the inside of the beams,respectively.The temperature transfer rate inside BFRP bars is significantly lower than that of CFRP bars,and the thermal conductivities of carbon fiber and basalt fiber were different,which has a direct effect on the transverse thermal conductivity of FRP bars;When the 8mm×3-～8mm×5-thick fire protection is arranged along the whole length of strengthened beams,no decline in mechanical properties of P-BFRP bars occurred within 2-hours fire,while the mechanical properties of V-BFRP,E-BFRP and CFRP bars had started to degrade at different time.（4）A numerical simulation of the thermal-mechanical coupling research of RC beams strengthened with NSM CFRP bars was carried out.Firstly,the established model was verified based on the experiment.The results showed that the thermal constitutive relation of the material selected in this research was reasonable,and the simulation results were in good agreement with the experimental results.Then the fire resistance of NSM-FRP bars strengthening RC beam with fire protection along the whole length of the beam was evaluated by simulation.The results showed that the stress level of BFRP bars in strengthened beams was generally higher than that of CFRP bars;after FRP bars failed in the later stage of fire,the steel bars took over the tensile stress at the bottom of the beams,which effectively avoided the brittle failure of beams caused by the failure of FRP bars.Compared with V-BFRP bars,E-BFRP bars and CFRP bars,P-BFRP bars can contribute the highest fire resistance to strengthened beams when the thickness of fire protection is same,followed by CFRP bars.Higher residual tensile strength can effectively delay the failure time and thus allow the strengthened beam to achieve a higher fire resistance.（5）Combined with research on experiments and finite-element simulation,the improvement technology and design method of fire resistance of flexural RC beams strengthened with NSM FRP bars were proposed.Based on the existing calculation methods and specification recommendations,the temperature and fire resistance of the strengthened beams in the experiments of this research were calculated and verified.Among them,the 500℃ isotherm method was more conducive to engineering application when determining the effective section of concrete in fire.Based on research on experiments and finite-element simulation,the strength design method was proposed.The fire resistance of the strengthened beam was predicted through different strength efficiency of FRP bars,and the table regarding parameter design was given according to different fire resistance rating.Based on the traditional passive fire protection technology and active fire protection technology,an improvement system of fire resistance was proposed,including two aspects of technologies:materials and structures.In terms of material,it was proposed to use FRP bars with higher residual mechanical properties and to configure reasonable fire protection;in terms of structures,it was proposed to focus on anti-slip design at the anchoring section.