Study On Physical And Mechanical Properties Of Fully Recycled Coarse Aggregate Concrete Reinforced With Fiber

Since the reform and opening up,China’s urbanization has been rapidly promoted under the premise of rapid development of national economy.As the basic building materials of urbanization,natural coarse aggregate has been exploited in large quantities.The large-scale exploitation of natural coarse aggregate leads to the shortage of resources,the destruction of ecological balance,and a large number of construction waste and environmental pollution.It is urgent to find a new type of aggregate to replace the natural coarse aggregate,so it is an inevitable choice to recycle a large number of waste concrete.Compared with natural coarse aggregate,recycled coarse aggregate obtained by crushing,screening and cleaning of waste concrete has the advantages of wide source of raw materials,green environmental protection,price economy and national policy support.Fiber has the effect of toughening and crack resistance,which can solve the shortcomings of high brittleness of recycled concrete,and also solve the problems of low tensile strength and easy cracking of concrete.At present,the research on fiber reinforced concrete mainly focuses on the combination of fiber and ordinary concrete,while the research on fiber reinforced recycled concrete is rare.Based on this,in this paper,firstly,the physical and mechanical properties of single fiber reinforced fully recycled coarse aggregate concrete are studied by taking the type and volume content of single fiber as the variable parameters.Secondly,the effects of different fiber content on the physical and mechanical properties of fully recycled coarse aggregate concrete are studied under the condition of fixed the ratio of a kind of fiber.In addition,the conversion of strength index of different fiber recycled coarse aggregate concrete and the analysis of the optimal content are also carried out.The main conclusions are as follows:(1)The influence of mixing effect of different fiber and steel fiber on the slump of concrete mixture is: Basalt Fiber < glass fiber < polypropylene fiber.The decrease of slump without steel fiber is greater than that with steel fiber,and the slump reduction of glass fiber and polypropylene fiber is greater than basalt fiber and polypropylene fiber.In addition,the slump reduction of basalt fiber and glass fiber is smaller than that of polypropylene fiber and glass fiber.(2)The overall performance of the influence of different single fiber content on the cube compressive strength is as follows: with the increase of fiber content,the cube compressive strength first increases and then slowly decreases.But in general,the increase of fiber content will increase the cube compressive strength of recycled concrete,the overall increase effect is: polypropylene fiber< glass fiber < basalt fiber < steel fiber.The effect of different fiber content on the flexural strength is that the flexural strength increases significantly with the increase of fiber content,and the overall increasing effect is as follows: glass fiber < polypropylene fiber < basalt fiber < steel fiber.(3)For the steel-polypropylene fiber reinforced fully recycled coarse aggregate concrete,with the increase of the volume of steel fiber,the compressive strength of concrete cube is improved,but when the steel fiber content is 1.5%,the addition of polypropylene fiber has a negative effect on the cube compressive strength.On the other hand,with the increase of steel fiber volume content,the flexural strength of recycled concrete increases as a whole.Also,with the increase of polypropylene fiber volume content,the flexural strength of recycled concrete decreases as a whole.For the polypropylene-basalt fiber reinforced fully recycled coarse aggregate concrete,with the increase of the volume of polypropylene fiber,the compressive strength of concrete cube decreases as a whole,but with the increase of basalt fiber content,the cube compressive strength of concrete cube increase as a whole.On the other hand,the results show that the mixing of polypropylene fiber and basalt fiber has negative mixing effect on the axial compression strain ductility and axial compression toughness ratio,and the flexural strength increases with the increase of the volume of polypropylene fiber or basalt fiber.(4)According to the code for design of concrete structure(GB50010-2010)and the formula of static elastic modulus of recycled concrete of Zhou Jinghai,the calculation formula and method of static elastic modulus of recycled coarse aggregate concrete with single fiber are fitted.In addition,the relationship among cube compressive strength,cylinder compressive strength and flexural strength was fitted by Origin.For the mixed fiber,considering the influence of fiber type and fiber content on the compressive strength,the concept of hybrid influence coefficient of two kinds of fiber on the cube compressive strength is proposed.The relationship between the static elastic modulus and the characteristic value of the two kinds of fiber content of the concrete is fitted.Also,the calculation formulas of cube compressive strength,cylinder compressive strength and flexural strength of two kinds of fiber reinforced recycled coarse aggregate concrete are fitted by Origin.(5)For single fiber,in order to enhance the physical and mechanical properties of recycled coarse aggregate concrete,the recommended volume content of steel fiber,polypropylene fiber,basalt fiber and glass fiber is 1.5%,0.2%,0.3% and 0.1%,respectively.For the specimens with steel fiber,there are three volume contents of the four kinds of fiber.Within the test range,when the volume content of steel fiber is 1%,the volume content of polypropylene fiber,basalt fiber and glass fiber are 0.15%,0.2% and 0.15%,respectively,the performance of recycled coarse aggregate concrete is the best.(6)Through a series of studies,the physical and mechanical properties,strength index conversion formula and optimal content of different single or mixed fiber reinforced full recycled coarse aggregate concrete are obtained.The research results can provide reference for the study of mechanical properties of fiber reinforced full recycled coarse aggregate concrete and its practical application.