Experimental Research On Shear Bearing Capacity Of Ultra High Performance Concrete Beams With Stirrups Under Fire Conditions

Ultra-High Performance Concrete(UHPC)is a new type of cement-based composite material with ultra-high strength,toughness and durability.The design and configuration of high-strength reinforcement in its matrix can give full play to the performance of each material.Compared with ordinary concrete,UHPC has a great difference in the bearing mechanism of inclined section.The main performance is that the coarse aggregate is removed from the UHPC component,and the occlusal effect of aggregate on both sides of shear oblique crack is weak.The bridge action of steel fiber and the high bonding property between UHPC and steel bar have great influence on the bearing capacity of the inclined section of beam.UHPC has high coefficient of thermal conductivity and low heat,under high temperature,the fire will aggravate UHPC beam inclined section bearing performance of recession and eventually lead to shear failure occurs,UHPC beam damage risk significantly greater than ordinary concrete beams under fire,at present,about the fire under configuration stirrup UHPC beam inclined section bearing performance study is less,The design method of shear bearing capacity of inclined section of UHPC beam still needs the verification of fire test data and the support of fire resistance design theory.Therefore,in view of the above key scientific and technological problems,this thesis carries out bearing performance tests on the inclined section of UHPC simply supported beam under fire.The main work is as follows:(1)Three UHPC simply supported beams were designed and made,and fire resistance tests of inclined section were carried out under the coupling of ISO834 standard temperature rising and dead load.With the hoop ratio,shear span ratio and load level as the test parameters,the hoop ratio is 0.27%,0.20% and 0.16%,the shear span ratio is 2.5 and 3.5,and the load level is 0.25,0.35 and 0.45.The temperature field,deflection deformation and fire resistance limit are obtained in the test.The crack development,burst and failure characteristics of the test beam are observed.The shear span ratio and load level have significant influence on the shear performance of the inclined section of UHPC beam under fire,while the stirrup ratio has little influence on the shear performance of the inclined section of UHPC beam,but it can delay the crack development.With the increase of shear span ratio and load level,the deformation at the loading point of the test beam increases and the fire resistance limit decreases.(2)Based on the test data,the temperature field,deflection-time curve,load-displacement curve after fire and other data were studied and analyzed.The safety of bearing capacity and failure mode of UHPC beam under fire were compared and analyzed from three aspects: high temperature burst of UHPC beam under high heating rate,high thermal conductivity coefficient and nonlinear temperature field would lead to attenuation of resistance of UHPC beam under inclined section,and low fire protection performance of UHPC beam under inclined section shear force transfer mechanism.The results show that the temperature gradient inside the beam is obvious when the beam is exposed to fire.The loading point displacement of UHPC beam under fire can be divided into slow growth stage,fast growth stage and failure stage.The final failure mode of the test beam under loading is similar to that of ordinary concrete,which shows the characteristics of shear compression failure and cable-tension failure.(3)Synthetically-analyze the influence of parameters such as shear-span ratio,hoop-ratio and load ratio on the bearing performance of inclined section of UHPC simply supported beam at high temperature,and the influence degree of different parameter changes on the fire resistance limit,displacement deformation and ultimate load of UHPC simply supported beam,and compare the difference of influence law of each parameter on UHPC beam and ordinary concrete beam.The fire safety control methods,such as 150℃～220℃ high temperature dry heat curing and recommended comprehensive stirrup ratio,are put forward.