Study On The Rheological Properties Of Ultra-High Performance Concrete And Its Influence On The Distribution Characteristics Of Steel Fiber

Ultra-high performance concrete（UHPC）is a new type of building material based on the principle of compact packing and reinforced by multiple powder particles and fibers,which has broad application prospects.However,due to the composition characteristics of UHPC such as low water to cement ratio and high superplasticizer dosage,UHPC has a high plastic viscosity,which seriously affects construction and application of UHPC.In addition,the rheological properties of paste would also affect distribution of fibers in matrix,and the distribution of fibers in UHPC would have a significant impact on fiber reinforcement efficiency and performance of UHPC.Therefore,rheological properties of UHPC are controlled and influence of rheological properties on distribution of steel fibers are studied in this paper.The main research results are as follows:（1）The relationship between UHPC rheological properties and paste microstructure.The paper measured the time-dependent plastic viscosity and shear modulus of paste under different water to cement ratios and superplasticizer dosages.Based on the T₂relaxation time,the content and distribution of free water in the paste were analyzed,and the number of early hydration products and ion concentration in the interstitial solution were tested.The results indicate that the rheological properties of paste are closely related to the spacing between solid particles,which continuously decreases over time,leading to an increase in plastic viscosity.The lower the water to cement ratio,the smaller the gap between particles.A decrease in the amount of superplasticizer will lead to a decrease in the number of hydration products,and the shrinkage rate of particle gaps will slow down over time,thereby reducing the plastic viscosity over time.（2）The effect of mineral admixtures on the rheological properties of UHPC.To reduce the viscosity of UHPC,ultrafine fly ash,silica fume,and two different particle sizes of limestone powder were used to replace some cement to regulate the plastic viscosity,and the particle water film parameters were calculated.The results indicate that replacing cement with20%fly ash and 10%silica fume can reduce the plastic viscosity of UHPC.The effect of limestone powder on plastic viscosity is related to particle size.Limestone powder with a coarse particle size（average particle size 4.1μm）can reduce plastic viscosity.When the particle size of limestone powder is less than 1μm,limestone powder will reduce the thickness of the particle water film,thereby increasing the UHPC plastic viscosity.（3）The influence of UHPC rheological properties on the distribution characteristics of steel fibers.When concrete flows,steel fibers can be arranged in a directional manner along the flow direction,but the flow directional effect is affected by the rheological parameters of the mixture.Results show that when the plastic viscosity of the mixture decreases from 98.71Pa·s to 15.57 Pa·s,the dispersion coefficient of steel fibers first increases and then decreases,and the directional coefficient of steel fibers at the same flow distance position first increases and then decreases.It can ensure good dispersion and non-segregation of steel fibers,and the optimal plastic viscosity range for flow orientation is 22.50 Pa·s～37.92 Pa·s.