| Concrete toughness refers to the ability of concrete to absorb energy in the process of deformation and fracture,which is the comprehensive performance of measuring material strength and deformation.China’s evaluation of concrete bending toughness is mainly based on CECS13:2009,and the main research object of this standard is steel fiber(SF)concrete.With the increasing application of fiber concrete,it isparticularly important to explore the methods of evaluating and improving the bending toughness of fiber concrete.In this paper,Polypropylene fiber(PPF)concrete,Polyformaldehyde fiber(POMF)concrete,Polyacrylonitrile fiber(PANF)concrete,Polyvinylalcohol fiber(PVAF)concrete,SF concrete and Glass fiber concrete are taken as the research objects.Meanwhile,combined with the compilation of the national standard " Synthetic fiber for cement concrete and mortar"(GB/T21120-2018)and the national standard "Alkali resistant glass fibers for cement concrete and mortar"(GB/T3 8143-2019),the evaluation method of bending toughness of fiber concrete,the bending toughness of fiber concrete and microstructure evolution at room temperature and high temperature are studied.The main works are as follows:(1)Based on the analysis of the characteristics of four common evaluation systems of fiber concrete bending toughness,i.e.European standard RLIEM TC 162-TDF,American Standard ASTM-C1609,Japanese standard JSCE-SF4 and Chinese standard CECS13:2009,the evaluation methods of bending toughness of synthetic fiber concrete and glass fiber concrete in national standard "Synthetic fiber for cement concrete and mortar"(GB/T21120-2018)and national standard "Alkali resistant glass fibers for cement concrete and mortar"(GB/T38143-2019)are established.(2)The reasonable fiber contents of bending toughness of PPF concrete,PVAF concrete,POMF concrete,PANF concrete,SF concrete and GF concrete were studied by the method of notched beam test.Then,based on the load-displacement curve obtained from the test,the relationship between the mid-span deflection of the notched beam(δ)and the crack opening displacement(CMOD)was explored.The results show that the reasonable fiber content range of PVAF、POMF、PPF to improve the concrete bending toughness is 1.2 kg/m3～1.5 kg/m3;PANF is 1.5kg/m3-1.8kg/m3;SF is about 1%;GF is about 1%.After comprehensive consideration,the order of the six kinds of fibers to improve the bending toughness of concrete is:SF>GF>PVAF>POMF>PPF>PANF.Moreover,there is a linear relationship betweenδ and CMOD,so CMOD can be used to predict the mid-span deflection δ.(3)The PVAF concrete,PANF concrete,POMF concrete,PPF concrete and plain concrete(JZ)were heated at 50℃,100℃,200℃ and 400℃ in a high temperature furnace so as to explore the change laws of bending toughness of synthetic fiber concrete at high temperature.The results show that the equivalent bending strength of fiber concrete(feq1)increases first and then decreases as the temperature increases.The development trend of the equivalent bending strength(feq2)of different fiber concrete varies at different temperatures,but the value offeq1+feq2 of each group reaches the maximum value at 50℃ and decreases gradually with the increase of temperature.(4)Using the real-time micro loading test system of environmental scanning electron microscope(ESEM),and stretching the fiber cement mortar test piece after high temperature,then recording the load-displacement curve,and finally the microstructure evolution process of the test piece during the tensile process should be observed simultaneously.The results show that when the temperature is between 20 and 200℃,the tensile strength of test pieces PVAF and PPF increases with the rising temperature;when 200℃,there is a rebound stage in the strength of the JZ,PVAF and PPF test pieces,and the toughening effect of the fibers are gradually lost at this stage,and the areas covered by the load-displacement curves are reduced.When the temperature rises to 500℃,the tensile strength of the test pieces tends to decrease,and the originally dense structures gradually become loose and the strength decreases. |
