Experimental Study On Mechanical And Shrinkage Properties Of Recycled Brick Powder UHPC

China is the largest cement production country in the world,cement production ranks first in the world for many years.The ’carbon peak action plan before 2030 ’issued by the State Council clearly puts forward ’strengthening the development and application of new cementitious materials,low carbon concrete,wood and bamboo building materials and other low carbon building materials products ’,the development and application of low-carbon concrete has undoubtedly become an important part of social low-carbon development.The recycled brick powder is obtained by a series of treatment of waste bricks in construction waste,which is a fine powder with particle size less than 75pm and certain activity.Ultra-high performance concrete(UHPC)has the advantages of high strength and durability,but its application and development are restricted to some extent by the problems of large amount of cementitious materials,high cost and large energy consumption.The application of recycled brick powder in ultra-high performance concrete is one of the ways to reduce the energy consumption of UHPC and realize the resource utilization of construction waste.In this context,the recycled brick powder was used to replace cement to prepare Ultra-high performance concrete.The effects of recycled brick powder replacement rate on the fluidity,mechanical properties and shrinkage of Ultra-high performance concrete were studied and analyzed.The main conclusions are as follows:(1)The fluidity of UHPC slurry can be improved by replacing cement with recycled brick powder.With the increase of the replacement rate of recycled brick powder,the fluidity of each group of slurry increases first and then decreases,but the increase and decrease are less than 5%.When the replacement rate of recycled brick powder is 30%,the fluidity of UHPC reaches 200mm,and the fluidity is the best.(2)The compressive strength of UHPC decreases with the increase of the replacement rate of recycled brick powder,but the 28d compressive strength of each group of specimens is greater than 120MPa.The recycled brick powder has a certain weakening effect on the strength development of UHPC.(3)The variation of flexural toughness and tensile strain of recycled brick powder UHPC is basically the same,which increases first and then decreases with the increase of brick powder replacement rate.When the replacement rate is 40%,the flexural toughness of UHPC is optimal.When the replacement rate is 30%,the tensile strength of UHPC reaches the maximum value of 8.87MPa,and the material exhibits good strain hardening characteristics.(4)Adding recycled brick powder as cementitious material into UHPC can improve the bonding ability between steel fiber and matrix,and enhance the energy dissipation capacity of materials.With the increase of the replacement rate of recycled brick powder,the ultimate bond strength of the specimen increases first and then decreases.The work at the maximum load increases continuously when the fiber is pulled out.When the replacement rate is 40%,the ultimate bond strength reaches the maximum value of 10.9MPa.(5)The particle size of recycled brick powder particles is smaller than that of cement particles,and it has porous characteristics.It not only fills the pores as micro-aggregates,but also participates in the secondary hydration reaction,and further optimizes the pore structure of UHPC.But the brick powder content should not exceed 40%.(6)The shringkage of recycled brick powder UHPC is mainly autogenous shrinkage,drying shrinkage accounts for 15%～26%of the total shrinkage.Recycled brick powder reduced the autogenous shrinkage and drying shrinkage of UHPC.The autogenous shrinkage mainly occurred in the first 7d age,and the drying shrinkage mainly occurred in the first 28d age.The shrinkage prediction model obtained by fitting the experimental data can better predict the development law of autogenous shrinkage and drying shrinkage of UHPC.