Study On Rheological Characteristics Of Ultra-High Performance Fibre Reinforced Concrete And Its Regulation

Ultra-high performance concrete(UHPC)has ultra-high strength,ultra-high toughness and ultra-high durability.With the development of research and technology,UHPC is widely used in the field of engineering and construction with its excellent performance.Steel fibres are usually used in strengthening and toughening the mechanical properties of UHPC,and its orientation and distribution state has a great impact on the performance of UHPC.In the UHPC studies on steel fibre orientation and distribution,however,there are still deficiencies regarding the effects of slurry rheological properties on steel fibre orientation and distribution and kinetic processes.This involves the lack of in-depth investigation of rheological parameters(static yield stress,dynamic yield stress,plastic viscosity,and thixotropy)concerning fibre orientation and distribution;and the lack of systematic studies of the dynamics of steel fibre distribution based on rheological properties.This involves a lack of in-depth studies on the effects of rheological parameters(static yield stress,dynamic yield stress,plastic viscosity,and thixotropy)on fibre orientation and distribution;and a lack of systematic studies on the dynamics of steel fibre distribution based on rheological properties.Based on the above issues,this study carried out related technical research on UHPC,mainly including:(1)The UHPC skeleton is rationally designed based on the Modified Andreasen and Andersen(MAA)model.To obtain the adjustment of the rheological properties of UHPC,bentonite is employed instead of stone powder to prepare UHPC with excellent performance;(2)The relationship between rheological properties and steel fibre orientation and distribution is studied to realize the efficient utilization of steel fibre in UHPC.(3)The high-efficiency simulation method of the high-precision rheological model is used to analyze the influence of rheological parameters,vibration state,and initial fibre position on the settlement dynamics and mechanism of UHPC steel fibre.The main research contents and conclusions of this paper are as follows:(1)The bentonite-UHPC is designed and prepared by a modified MAA model,and the effects of bentonite on rheological properties,mechanical properties,hydration process,and microstructure of UHPC are investigated.The obtained results reveal that the addition of the bentonite can effectively regulate the rheological properties of UHPC.More exactly,When the bentonite content rises from 0% to 15.0%,the flowability of the UHPC slurry decreases by 55.71%,while the static yield stress,dynamic yield stress,plastic viscosity and thixotropy increase by 1705.29%,557.90%,116.14% and 3.94%,respectively.In addition,the thermodynamic study found that the maximum exothermic peak and cumulative heat release of UHPC could be reduced with the increase of bentonite from 0% to 10.0%.Additionally,based on the microstructure analysis results,there is optimal bentonite content,which is beneficial for obtaining a much denser structure of UHPC composite.(2)The orientation and distribution of steel fibre in UHPC is investigated by using bentonite as the rheology controller.The functional relationship between the distribution and orientation of steel fibre and the rheological parameters of fresh UHPC is established.The obtained results show that based on an optimized rheology control,the orientation and distribution of steel fibres in UHPC can be well improved,which simultaneously brings about 60% enhancement for flexural strength development in this study.Additionally,considering the functional correlation between plastic viscosity and flexural strength,a prediction model for flexural strength of UHPC is established here,in which the error in prediction is less than 15.0%.(3)The rheological parameters of the UHPC manure are calculated based on the H-B model,and the dynamic process of settling the steel fibre is simulated by SPH based on rheological parameters.The effects of the rheological parameters,the vibratory state,and the initial position of the steel fibre on the process of settling the steel fibre are studied.The results indicate that the settlement process of steel fibre can be split into three stages.The vibration accelerates the process of steel fibre but does not change the settlement law of steel fibre.Furthermore,the properties of shear thinning and shear thickening of non-Newtonian fluids have a significant effect on the pre-vibration period but have little effect on the post-vibration period.Furthermore,the dynamic processes of steel fibres at different initial positions are significantly different at the start of the vibration.When the rheological properties of the slurry are constant,the nearer the initial position of the steel fibres is to the bottom of the slurry,the slower the initial settling velocity and the smaller the settling degree.In this paper,the development law and the influence mechanism of bentonite are studied on the rheological properties of UHPC.The main influencing factors of steel fibre orientation and distribution and the kinetics process of steel fibre settlement are clarified under different rheological parameters.It provides a theoretical reference for the performance development of UHPC based on rheological properties.