Experimental Study On The Basic Performance Of Ultra-high Performance Concrete Flexural Performance Of Slabs

In recent years,Ultra-high performance concrete（UHPC）has become a hot research topic because of its high strength,high toughness and high durability.In this thesis,the mix proportion of ultra-high performance concrete was optimized and designed to solve the problems of large amount of cement,single aggregate,poor fluidity of the mixture and complex maintenance methods in traditional UHPC.The shrinkage performance,mechanical properties and microscopic characteristics of the optimized mix proportion under different maintenance systems were studied.A 30 mm thin plate was made to study the flexural bearing capacity of the structure,and further verify the good toughness of the optimized UHPC material.The work and conclusions of this thesis are as follows:（1）The effects of water-binder ratio,sand-binder ratio,aeolian sand replacement rate,fly ash content,silica fume content,microsphere content and steel fiber content on the compressive strength,flexural strength and fluidity of UHPC were studied by single factor experiment.The main influencing factors were finally determined:water-binder ratio,fly ash content,silica fume content and steel fiber content.（2）Through orthogonal test analysis,taking fluidity,compressive strength and flexural strength as technical indicators,the UHPC mix ratio was determined:water-cement ratio0.18,sand-cement ratio 1.25,silica fume content 15%,fly ash content 15%,bead content5%,aeolian sand replacement rate 10%,steel fiber content（1.5%,2.0%,2.5%）.（3）By studying the early self-shrinkage law of UHPC with three kinds of steel fiber content after optimization,the results show that the early self-shrinkage rate of UHPC after optimization is lower than 600×10^-6.By studying the drying shrinkage variation of the optimized UHPC ratio under natural curing,standard curing and hot water curing conditions,the results showed that natural curing>standard curing>hot water curing,and the larger the steel fiber content was,the smaller the shrinkage rate was（4）By studying the strength variation（cube compressive strength,flexural strength and splitting tensile strength）of UHPC with three kinds of steel fiber content under natural curing,standard curing and hot water curing after optimization,the results show that hot water curing>standard curing>natural curing;by studying the 28d axial tensile strength test of different steel fiber content,the results show that the greater the steel fiber content,the higher the tensile strength of the matrix.（5）The change rule of stress-strain curve was obtained by UHPC axial compression test.The results show that the greater the steel fiber content,the more gentle the failure descending section;through the preparation of UHPC thin plate,the load-deflection change was studied.The results show that the greater the steel fiber content is,the greater the ultimate bending strength and deflection of the thin plate are,and the stronger the toughness is.（6）Through the analysis of bubble spacing coefficient of hardened concrete and nuclear magnetic resonance pore structure test,the results show that the overall porosity of UHPC is low,and the harmless pores are mainly in the cementitious pores.SEM scanning electron microscope and XRD phase analysis showed that a large number of hydration products such as C-S-H gel,CH,AFt and AFm were formed in UHPC,which could effectively improve the microstructure of UHPC.