Research On Influence Of Steel-polypropylene Blended Fiber On Performance Of Ultra-high Performance Concrete

Ultra High Performance Concrete(UHPC)is a new type of concrete material with excellent performance index.However,its matrix still retains the shortcomings of high brittleness and poor toughness of common concrete,which can be compensated by adding fibers.At present,the most widely used fiber is steel fiber,which can enhance and toughen the matrix from brittle failure to plastic failure.However,due to the disadvantages of steel fiber,such as high cost,easy rust,high density and easy sinking,UHPC can not be widely used in actual projects at present.Therefore,it is necessary to explore a relatively inexpensive fiber to replace a part of expensive steel fibers and exert the synergy between the two fibers so that they can meet the performance and reduce the cost appropriately.Polypropylene fiber is one of the most common synthetic fibers on the market at present.It is inexpensive and has good ductility.It has unique advantages in restraining early cracking and improving material toughness.However,there are relatively few studies on polypropylene fibers used for UHPC at present,and related literatures point out that there are objections to the best mixing proportion and content of mechanical properties of steel fibers and polypropylene fibers in UHPC matrix,and further research is required.Based on this,through the expansion test,strength test(compression,flexure,splitting tensile,axial compression)and toughness test(impact toughness,bending toughness),the specific influence laws of single-doping and mutual-doping of steel fiber and polypropylene fiber on the working performance,strength and toughness of UHPC were explored,and the strength-size effect laws of single-doped fiber were also studied.The following conclusions are drawn:(1)Both mono-doped and blended fibers reduce the fluidity of UHPC,but the reduction of blended fibers is greater.Single-doped steel fiber can significantly improve various strength indexes,while single-doped polypropylene fiber can slightly improve the flexural strength,but it has little or even reduced influence on other strength indexes.Steel-polypropylene fiber blending can significantly improve the strength of UHPC matrix,and when 1.0% steel fiber is mixed with 0.05%,0.1% and 0.15% polypropylene fiber respectively,it will show positive mixing effect on various strength indexes.(2)For single-doped fiber system,the strength-size effect of single-doped steel fiber is more significant than that of single-doped polypropylene fiber.For the blended fiber system,the size effect is obvious generally.The size effect of compressive strength is most significant when 2.0% steel fiber is mixed with 0.05% polypropylene fiber,the size effect of flexural strength is most significant when 2.0% steel fiber is mixed with 0.1%polypropylene fiber,and the size effect of splitting tensile strength is most significant when 2.5% steel fiber is mixed with 0.2% polypropylene fiber.(3)Single-doped steel fiber can significantly improve the impact toughness and bending toughness of UHPC.Single-doped polypropylene fiber has little influence on the impact toughness and bending toughness of UHPC and even has negative effect.The impact toughness and bending toughness of steel-polypropylene fibers can also be significantly improved by blending them.With the increase of the content of polypropylene fibers,the impact toughness and bending toughness first decrease and then increase.