Study On Preparation And Mechanical Properties Of Recycled Brick Powder Aluminate Cement Based Ultra High Performance Concrete After Elevated Temperatures

Ultra high performance concrete(UHPC)has been widely applied to longspan bridges,high-rise buildings,nuclear reactor containment,military protection and other engineering structures because of its excellent high strength,high toughness and ultra-high durability.However,the traditional UHPC made of Portland cement as cementing material and ordinary carbon steel fiber or polymer fiber as reinforcing material is prone to burst and performance degradation at high temperature in fire,which restricts its engineering application range.Therefore,a new type of fireresistant UHPC was prepared with aluminate cement(AC)as cementing material,recycled brick powder(RBP)as auxiliary cementing material and fire-resistant steel fiber as reinforcing material in this paper.The mixture ratio of green aluminate cement-based cementing material was optimized by testing the compressive strength of hardened paste with different replacement rates of RBP at different simulated fire temperatures.The UHPC mixture was prepared with the optimized cementitious material,standard sand,ordinary steel fiber and fire-resistant steel fiber with different dosages.The effects of steel fiber types and dosages on the bursting resistance,mechanical properties and microstructure of aluminate cement-based UHPC after different simulated fire temperatures were systematically studied.The main results are summarized as follows:(1)The workability of aluminate cement-based cementitious materials with different amounts of RBP and the flexural strength and compressive strength of hardened cementitious materials slurry after different simulated fire temperatures were studied.The results show that the workability of freshly mixed slurry gradually becomes worse with the increase of recycled brick powder content.When the content of RBP is constant,the flexural strength of hardened paste specimen decreases with the increase of simulated fire temperature,while the compressive strength increases at first and then decreases with the increase of temperature,reaching the maximum compressive strength at 200℃.Based on the test results,empirical calculation models of flexural strength and compressive strength considering the influence of RBP content and simulated fire temperature are established.(2)The effects of different dosage of ordinary steel fiber and fire-resistant steel fiber on splitting tensile properties of aluminate cement-based UHPC after different simulated fire temperatures were studied by 147 cube specimens.The test results show that the splitting tensile strength of the specimen and the area under the load-displacement curve first increase and then decrease with the increase of simulated fire temperature,reaching the maximum at 400℃.At the same temperature,the splitting tensile strength increases with the increase of steel fiber content,the load-displacement curve becomes fuller,and the toughness gradually increases.The reinforcement effect of fire-resistant steel fiber on splitting tensile property is better than that of ordinary steel fiber.Based on the test results,an empirical calculation model of splitting tensile strength considering the comprehensive influence of fireresistant steel fiber content and simulated fire temperature is proposed.(3)The influence of different content of fire-resistant steel fiber on the compressive strength of aluminate cement-based UHPC cubes after different simulated fire temperatures was studied by 84 cubes specimens.The results show that,the cube compressive strength first increases and then decreases with the increase of simulated fire temperature,and the critical temperature is 400℃.When the content of fire-resistant steel fiber is 3%,the cube compressive strength reaches the maximum value of 198.55MPa;At the same temperature,the cube compressive strength shows an increasing trend with the increase of steel fiber content.Based on the test results,an empirical calculation model of cubic compressive strength considering the comprehensive influence of simulated fire temperature and the content of fireresistant steel fiber is proposed.(4)The influence of the content of fire-resistant steel fiber on the uniaxial compression performance of aluminate cement-based UHPC under different simulated fire temperatures was studied by 84 prism specimens.The test results show that the axial compressive strength first increases and then decreases with the increase of simulated fire temperature,and the axial compressive strength reaches the maximum after the high temperature of 400℃.Regardless of the content of fireresistant steel fiber,the peak point of the stress-strain curve gradually moves upward to the right,and the area under the curve gradually increases when the simulated fire temperature does not exceed 400℃.When the simulated fire temperature further rises to 1000℃,the stress-strain curve tends to flatten gradually,and the peak point moves downward to the right,with the area under the curve gradually decreasing and the toughness gradually losing.When the simulated fire temperature reaches 1200℃,the peak point of the stress-strain curve of the specimen without steel fiber continues to move downward to the right,but the peak point of the curve of the specimen with steel fiber moves downward to the left.Based on the test results,the calculation models of axial compressive strength,peak strain and elastic modulus considering the influence of simulated fire temperature are put forward,and the constitutive equation of stress-strain curve considering the influence of simulated fire temperature is established.(5)The microstructure,mineral composition and phase evolution of aluminate cement-based UHPC and its cementitious materials after different temperatures were studied by means of SEM and XRD.The test results shown that the hydration reaction of cementitious materials was gradually improved,the UHPC matrix structure was more compact,and the steel fiber was tightly bonded to the matrix when the temperature did not exceed 400℃.the internal pore cracks of UHPC gradually became obvious,and the aggregate gradually separated from the UHPC matrix with the gradual increase of exposure temperature.When the simulated fire temperature reached 1000℃,ordinary steel fiber was oxidized to residue,and fireresistant steel fiber was still intact.