Bond Behavior Of CFRP Grids Reinforcement At Elevated Temperatures And Fire Resistance Of CFRP Grids Strengthened RC Beams

The carbon-fiber-reinforced polymer(CFRP)grids strengthening technology of concrete structures is to fix the CFRP grids to the roughed concrete surface first,and then spray or plaster polymer mortar to integrate CFRP grids,polymer mortar and concrete into a single structure to work together.Understanding the fire resistance of reinforced concrete beams strengthened with CFRP grids is an important precondition for using this technology into structural strengthening of buildings,and is also a problem not yet solved.In this thesis,firstly the behavior of CFRP grids and polymer mortar strengthening system at elevated temperature were determined from the material level.Secondly,based on the constitutive models obtained from the material-level research,the fire resistance of reinforced concrete beams strengthened with CFRP grids was investigated deeply from the structural member level.In the aspect of material-level research,firstly based on 24 mechanical tests at elevated temperature and the existing test data in the literature,it was discovered that the lower of the fiber content of CFRP,the greater degradation of the mechanical properties(tensile strength and elastic modulus)at elevated temperature of CFRP.The available Bisby's degradation model was modified,and a new mechanical degradation model at elevated temperature of CFRP,which considered the effect of fiber content,was eatablished.The new degradation model agreed better with the experimental results.Secondly based on 18 archorage tests at room temperature and 26 bond-slip tests at room temperature,the bond behavior between CFRP grids and polymer mortar at room temperature was analysised.The cementation force and friction force provided by the longitudinal bars with smooth surface of CFRP grids are relatively weak,and the force transfer behavior depends to a great extent on the mechanical archorage provided by the transverse bars of CFRP grids.The bond stress-slip constitutive model at room temperature was established,and the constitutive model agreed well with the experimental results.At last based on 24 bond-slip tests at elavated temperature,the bond behavior between CFRP grids and polymer mortar at elevated temperature was analysised.When the temperature exceeded the glass transition temperature T_g of the basic resin,the characteristic values of the bond stress-slip curves degradated rapidly.On the basis of the constitutive model at room temperature,the bond stress-slip constitutive model at elevated temperature was established,and the constitutive model agreed well with the experimental results.In the aspect of structural-member-level research,firstly based on the standard fire tests and the after-cooling static tests of full-scale beams,the fire endurance behavior of reinforced concrete beams strengthened with CFRP grids was obtained,including the heat transfer behavior and the mechanical degradation behavior.And reinforced concrete beams strengthened with CFRP grids had better after-cooling mechanical behavior than the reinforced concrete beams strengthened with externally bonded FRP or near-surface mounted FRP.Secondly based on the theotetical models from the material-level research,the sequentially coupled thermal-stress analysis finite element model considered accurately of the thermal properties and the mechanical behaviors at elevated temperatues,and used spring element to consider the bond behaviors of CFRP grids to polymer mortar and steel rebar to concrete at elevated temperatue.The transient temperature field,the transient mid-span deflection increasement and the transient stress distribution of the fire test beams were simulated accurately.At last using the accurate finite element model,the fire resistance periods of 276 parametric specimens were calculated.Through the regression analysis of parametric studies,the simplified calculation method of fire resistance period was established.Based on the simple method for predicting temperature and the normal section analysis at elevated temperature,the capacity-based calculation method of fire resistance period was established.On the basis of the two fire resistance period calculation methods,the structural fire resistance design procedure of reinforced concrete beams strengthened with CFRP grids was proposed.