Study On The Effect Of Steel Fibre Distribution On The Performance Of Ultra High Performance Concrete And Its Regulation

Ultra-high performance concrete(UHPC)is a new cement-based material,which is characterized by ultra-high strength,high toughness and excellent durability.It is a promising material in the use of super high-rise buildings,bridge structures,marine,nuclear power and underground engineering facilities.As the most commonly used fiber material in the production of UHPC,steel fibre added in the UHPC will improve its strength and toughness.However,the distribution of steel fibers inside UHPC(including the fiber orientation distribution on the two-dimensional scale and the steel fiber network structure on the three-dimensional scale)will have a significant impact on the fiber reinforcement efficiency and the performance of UHPC.Therefore,in order to make the material designed and performance controlled scientific,reasonable and effective,the study on the influence and control of steel fibre distribution on UHPC performance is carried out.The research contents and main results are as follows:(1)UHPC with high fluidity and various rheology are the research basis of steel fibre orientation and distribution.Therefore,based on the modified Andreasen&Andersen model,a self-compacting ultra-high performance concrete with metakaolin is produced,and its properties are studied.The results show that on the one hand,metakaolin compared to silica fume is more conductive to the improvement of UHPC toughness.On the other hand,the physical structure of metakaolin will increase the viscosity of UHPC,which is not conducive to the discharge of internal bubbles,resulting in the generation of macro defects in UHPC.Therefore,based on the physical and chemical synergistic effect of metakaolin partially replacing silica fume on UHPC,UHPC with ultra-fine mineral admixture composed of 60% silica fume and 40% metakaolin has excellent workability and mechanical properties.(2)Based on the influence of UHPC flowing on the steel fibre movement,the orientation and distribution of steel fiber in UHPC are studied,and the dynamic movement process of steel fiber under UHPC flowing is also analyzed.Besides,the influence of flow parameters(casting height,casting length and viscosity)on the steel fibre orientation and distribution are also studied.The results show that the orientation and distribution of steel fiber in UHPC will change regularly under the UHPC flowing.There will be three regions about steel fibre orientation and distribution appeared along the flow direction of UHPC,which is disorder region,stable region and re-disorder.However,when the content of steel fiber in UHPC is relatively larger(>2 vol.%),the efficiency of steel fibre orientation and distribution optimization with UHPC flowing will be reduced due to the motion interference between fibers.In addition,an appropriate flow parameters are conducive to the optimization efficiency of steel fibre orientation and distribution improved and the stable region extended under the action of UHPC flow.(3)Through the analysis of the fibre network structure in the UHPC with different steel fibre content or hybrid fibre,the formation and optimization of steel fiber network structure in UHPC are studied,and the relationship between its performance and steel fiber network structure is discussed.The results show that with the increase of steel fiber content,the possibility of contact between steel fibers in UHPC increases.When the content of steel fiber is larger than 2.0 vol.%,a large number of steel fibers are overlapped to form a dense steel fiber network structure in UHPC.It will cause that the performance of UHPC with relatively high steel fibre content is changed significantly,especially the resistance of UHPC,which is a potential danger to the electrochemical stability of UHPC in electrolyte environment.In addition,based on the size effect of long steel fibre and short steel fibre hybrid,and the the separate effect of Polyvinyl alcohol fiber and steel fiber hybrid,the steel fiber network structure of UHPC can be optimized and the performance of UHPC will be improved.(4)Through the analysis of mechanical properties of UHPC with hybrid fibre,the influence of hybrid steel fiber on the mechanical properties of UHPC is explored.In addition,it establishes the relationship between steel fiber network structure and mechanical properties,and reveals the reinforcement mechanism of hybrid steel fiber on mechanical properties of UHPC.The results show that although the hooked-end steel fiber compared to straight steel fibre has more stronger bonding ability with the matrix,hooked-end steel fiber is easier to cause the dense steel fiber network structure formed in UHPC,which cause the reinforcement efficiency of steel fiber to UHPC reducing.Therefore,based on the synergistic effect of hybrid hooked-end steel fiber and straight steel fiber on the mechanical properties of UHPC,an UHPC with excellent mechanical properties can be produced by the 0.5% straight steel fiber and1.5% hooked-end steel fiber hybrid.(5)The durability of UHPC is studied by a marine environment climate simulation system.Through the characterization of UHPC performance in different marine environment and the analysis of steel fiber network structure in UHPC with different steel fiber content,the relationship between steel fiber network structure and durability of UHPC is established.The results show that UHPC has excellent durability and will not be damaged by ion erosion in conventional seawater.However,the temperature will increase the diffusion of ions,which leads to the corrosion of UHPC in high temperature seawater.In addition,when the connection network of steel fiber inside UHPC is formed,the micro defects on UHPC surface will easily cause UHPC electrochemical corrosion appeared in electrolyte environment,which also causes the corrosion damage of UHPC with high content of steel fiber in dry and wet circulating seawater environment.