Study On The Influence Of Lubrication Improver On The Workability Of Ultra High Performance Concrete

Ultra high performance concrete（UHPC）has ultra high strength,ultra high toughness and excellent durability.However,the poor workability of UHPC makes it difficult to be used in site casting engineering in practical applications.The main reason is that its water-binder ratio is very low,the water content is very small,the water film thickness is extremely thin,the viscosity is high,and the workability is poor.In this paper,based on the basic theory of tribology,by studying the water film thickness and the contact condition of solid particles in UHPC,it is found that there are two friction forms name as fluid friction and boundary friction in the flow process of fresh UHPC,and the boundary friction is more dominant than the fluid friction.Thus,it is proposed that the internal friction of UHPC can be reduced by adding lubrication improver into UHPC to reduce the friction of solid particles,so as to achieve the purpose of reducing the viscosity and improving the workability.Combined with the analysis of the influence of lubrication improver on the mix design,the mix design of UHPC containing lubrication improver was optimized,and the influence of lubrication improver on the workability and other related properties of UHPC was studied.The main research achievements are as follows:（1）Based on the analysis and calculation on mix proportion,it is found that due to the difference in particle size,the addition of lubrication improver can reduce the total internal surface area of UHPC and increase the water film thickness,and the higher the dosage,the greater the water film thickness.However,the difference in particle size also caused the difference of the experimental group from the blank group to increase with the increase of the dosage.Due to the difference in density,the bulk density of the experimental group decreased with the increase of the dosage,in which,the experimental groups with fly ash microbeads and glass microbeads had small decrease of bulk density and the effect of lubrication improver on the strength of UHPC is not obvious.（2）Based on the research results about the influence of lubrication improver on the mechanical properties of UHPC,it is found that fly ash microbeads and glass microbeads have little effect on strength of UHPC.Polymethyl methacrylate（PMMA）powder have significant adverse effect on strength.When the PMMA powder content is15%,the strength drops from 183.2MPa of the blank group to 148.6MPa.（3）Based on the research results about the influence of lubrication improver on the workability of UHPC,it is found that the two kinds of rigid surface lubrication improver improved the workability obviously.When the content of fly ash microbeads is 20%,the slump flow is 115mm higher than that of the blank group,the height-diameter ratio of the slumped test specimen is 61.6%lower.When the content of glass microbeads is100%,the slump flow is 165mm higher than that of the blank group,the height-diameter ratio of the slumped test specimen is 54.7%lower.It shows that the two kinds of rigid lubrication improver can improve the friction environment,reduce the internal friction and reduce the viscosity.The group with the best effect when mixing is15FAB+100GB.PMMA powder is used as a flexible surface lubrication improver.When the content is 5%,the workability of UHPC is best improved,and the height-diameter ratio of the slumped test specimen decreases by 53.7%.（4）The analysis and comparison of the data obtained in the experiments of this paper shows that slump and slump flow are not sensitive to the evaluation of UHPC viscosity when compared with the high-diameter ratio of the slumped test specimen which is more suitable for showing the change of viscosity,and the trend is consistent with T₅₀₀.