Experimental Study On Shear Performance Of Hybrid Fiber Reinforced Self-compacting Concrete Beams After Exposure To High Temperature

As the main load-bearing member of the tunnel structure,the shield segment has a long design reference period and high earth pressure,so the quality of the segment is extremely high.The traditional reinforced concrete tunnel segment has dense reinforcement and is relatively bulky,which brings great difficulties to the production,transportation and construction of the segment.In addition,with the increase in the number of tunnels,various types of tunnel-related fire accidents have also increased significantly.Due to the fierce temperature rise of the tunnel fire,large-scale bursting and collapse of the support layer will occur,and the bearing capacity of the components will decrease,resulting in huge social and economic losses.Fiber self-compacting concrete(HyFRSCC)has the advantages of fiber reinforced concrete and self-compacting concrete.Good workability combined with high strength and toughness can ensure the structural compactness during pouring construction,improve the construction quality,and give full play to the resistance of the fiber Cracking,strengthening and toughening.Moreover,the melting point of polypropylene fiber is low,and the high temperature melting can effectively release the internal vapor pressure of the concrete and alleviate the high temperature burst phenomenon of high performance concrete.Based on the above analysis and in conjunction with the project supported by the National Natural Science Foundation of China,"Hybrid fiber impact on high-performance concrete tunnel segment performance after fire and improvement mechanism"(51678100),this topic takes fiber dosage,coupling ratio and shear span ratio as Variables,systematically studied the morphology and shear capacity of HyFRSCC beams before and after high temperature.The main research contents and conclusions are as follows:(1)C50 self-compacting concrete that meets workability and strength requirements was prepared,and the cube test block was subjected to compressive strength test before and after high temperature and microscopic test.The research shows that the mixing of steel fiber and polypropylene fiber can effectively improve the compressive strength of concrete before and after high temperature Shows good fire resistance at high temperature,ensuring the integrity of the test piece(2)By studying the temperature field of HyFRSCC simply supported beam at high temperature,the temperature field and temperature gradient of the cross section of the beam with different fiber content were systematically analyzed.After comparing the numerical simulation with the actual measurement results,after determining that the numerical simulation can effectively evaluate the concrete temperature field The T500 critical temperature isotherm in the HyFRSCC beam is delineated.(3)Through the experimental research on the shear resistance of the HyFRSCC symmetric dip beam before and after high temperature,the effects of fiber content,coupling ratio and shear span ratio on the failure shape and shear capacity of the test beam were analyzed.The results show that the failure shape and bearing capacity of the test beam are significantly different before and after high temperature.After high temperature,the great deterioration of the concrete strength will change the test beam f:rom shear failure to super-reinforcement failure.The influence of the beam bearing capacity is not as significant as that of the coupling rate on the bearing capacity of the test beam.(4)Through the experimental study on the shear performance of HyFRSCC simply supported beams before and after high temperature,the effect of fiber content on the failure shape and shear capacity of the test beam was analyzed and compared with the inclined beam specimen.The test results show that the introduction of axial force can increase the peak load of the test beam,limit the crack propagation,and reduce the mid-span displacement of the test beam.And after the high temperature,the strength of the concrete deteriorates seriously,and the introduction of axial force will even change the failure shape of the test beam.(5)Combined with the danage pattern and shear bearing capacity of HyFRSCC beams before and after high temperature,it is proposed that the optimal steel fiber content to improve the shear performance of the structure is 40kg/m3,and the optimal fiber content combination for structural fire resistance design is the hybrid fiber HF401 group.According to the T500 method in"European Structural Fire Resistance Design Code",the calculation method of the shear capacity of HyFRSCC beams after high temperature is proposed.