Study On The Mechanical Properties Of Ultra-High Performance Concrete With Recycled Brick Powder After Elevated Temperature

Ultra-High Performance Concrete(UHPC)is gradually applied in various fields of civil engineering for its excellent mechanical properties and durability.However,the disadvantages and hazards of UHPC,such as high cement content and the explosive cracking caused by the high compactness of microstructure,are urgent problems to be solved.Therefore,based on the previous research,this paper systematically discusses the mechanical properties and microstructure of cementitious materials with recycled brick powder(RBP)and straw ash(SA)replacing part of cement,and further studies the mechanical properties and microstructure of UHPC with recycled brick powder and steel fiber.The main research contents are as follows:(1)The workability,mass loss at elevated temperature,apparent change,strength at different ages and after elevated temperature of cementitious materials mixed with RBP and SA were studied,and the mechanism was analyzed from the perspective of phase composition and micro-morphology.The results show that the expansion of cement paste decreases with the addition of RBP and SA,especially when SA is added;with the increase of the content of RBP and SA,the surface crack of the specimen becomes fine after elevated temperature,especially when the content of RBP is more than 15%,there is no obvious crack on the surface of the specimen,and the addition of RBP can effectively restrain the elevated temperature crack of the specimen.With the increase of age,the compressive strength of specimens mixed with RBP tends to increase,which indicates that RBP inhibits the thermal damage of steam curing;with the increase of the target temperature,the compressive strength of all specimens increase at first and then decrease,reaching the maximum compressive strength at 300℃,and the compressive strength of RBP is better than that of SA.Through microscopic analysis,it is found that elevated temperature before 400℃ can accelerate the hydration of cement and improve the activity of mineral admixture.RBP and SA have certain pozzolanic activity,and the activity of RBP is better than that of SA.(2)Based on the cementitious material of 25%recycled brick powder,the effects of the content and type of steel fiber on the compressive strength and splitting tensile properties of UHPC specimens after elevated temperature were studied,and explained by the observation of microstructure.The test results show that the compressive strength and splitting tensile strength of UHPC specimens increase at first and then decrease after elevated temperature,reaching the maximum strength at 300℃.At 300℃,2%ordinary steel fiber and 1%fire-resistant steel fiber have better effect on compressive strength,and the splitting tensile strength increases significantly with the increase of steel fiber content.Before 800℃,the strength of UHPC specimens increase by ordinary steel fiber more than that of fire-resistant steel fiber,but it is opposite after 800℃.At any temperature,the total energy dissipation capacity of fire-resistant steel fiber UHPC is better than that of ordinary steel fiber UHPC.From the SEM micrograph,it can be seen that the concrete matrix is dense at 300℃,and the steel fiber is tightly bonded with the matrix.When the temperature is higher than 800℃,the ordinary steel fiber begins to oxidize and become rough,losing the bonding property with the concrete matrix.At 1000℃,it is completely oxidized,forming channels in the matrix,while the fire-resistant steel fiber can still keep its basic appearance.(3)Through the axial compressive strength test,the effects of temperature and steel fiber on the axial compressive properties are discussed.The results show that the geometric shape of the UHPC stress-strain curve at 200℃ and 300 ℃ is similar to that at 20℃.The shape of the UHPC stress-strain curve at 400℃～800℃ is flattened gradually with the increase of temperature,and the peak point gradually moves to the right and down,while the UHPC stress-strain curve at 800℃～1000 ℃ is further flattened,and the peak point moves to the lower left,that is,with the increase of the target temperature,the axial compressive strength increases at first and then decreases.300℃ is the critical temperature;the elastic modulus increase at first and then decrease;the peak strain remains unchanged at first,then increases,and begins to decrease after 800℃.Based on the analysis of the test results,the formulas for calculating the axial compressive strength and elastic modulus of UHPC after elevated temperature are established.According to the geometric characteristics of the stress-strain curve of the test,the axial compression constitutive equation of UHPC after elevated temperature is established,in which the rising section is quintic polynomial and the descending section is rational,and the variation equations of parameters of ascending section and descending section with temperature are determined,which provides a theoretical reference for the evaluation of bearing capacity of UHPC after elevated temperature.