Study On Workability And Mechanical Properties And Microstructure Of Ultra High Performance Concrete

Ultra-High Performance Concrete(UHPC)is a new type of cement-based composite material.Compared with ordinary concrete,it has the characteristics of high strength,high toughness and high durability.Generally,UHPC has a very low water-to-binder ratio and a large amount of cementitious materials.Quartz sand is selected as the aggregate instead of coarse aggregate,and the steam pressure curing method is mainly used in the preparation process,which result in high cost,high energy consumption,easy shrinkage and cracking,and the workability is affected to some extent,limiting its application in the actual project.In order to prepare UHPC with good workability,the standard curing methods were adopted to prepare it with an expansion degree of not less than 400 mm and compressive strength of not less than 100 MPa and study its workability and mechanical properties and microstructure.The main research content and results include:(1)Through the expansion test,the effects of water-binder ratio,steel fiber type and content,different types of coarse and fine aggregates were studied on the fluidity of UHPC.The results show that based on the UHPC design mix ratio in this paper,the expansion of the mixture was basically more than 400 mm,and there was no bleeding and segregation.(2)The effects of water-binder ratio,steel fiber content and curing system on the compressive strength,flexural strength and splitting tensile strength of UHPC were studied.The results showed that with the decrease of water binder ratio and the increase of steel fiber content,the basic mechanical properties of UHPC were improved.The early strength of UHPC were improved significantly through the steam curing method,but after curing to the specified age,the strength of UHPC had no significant change with the standard curing method ?(3)The effects of aggregate type,content and steel fiber type on the compressive strength and flexural strength of UHPC were studied.Based on the test results,the expressions of the relationship between UHPC compression ratio,tensile compression ratio and steel fiber content were proposed.(4)Through the X-ray diffraction,scanning electron microscope and pore structure test of UHPC without coarse aggregate,the effect of water-binder ratio on the phase and microscopic morphology of UHPC hydration products was studied and the bond degree between steel fiber and mortar was observed.The results show that the matrix of UHPC paste was dense,C-S-H were filled between the steel fiber and the mortar,and the density of the interface transition zone was improved.As the water-binder ratio decreases and the steel fiber content increases,the porosity of UHPC decreases and the mechanical properties increased.