The Critical Failure Phenomenon And Information Identification Of Steel Fiber Reinforced Concrete

Steel fiber reinforced concrete(SFRC)has high tensile strength,flexural strength and good fracture toughness,which can meet the needs of some special complex projects.This paper aims at studying the mechanical properties of steel fiber reinforced concrete and the precursory characteristics of structural instability,based on basic damage of concrete in practical engineering structure deformation form,through uniaxial compression and splitting tensile strength and fracture toughness test,with the help of resistivity method,acoustic emission technology and digital image correlation method to characterize the damage evolution of the steel fiber reinforced concrete is studied and the precursory signal characteristics of instability and failure of steel fiber reinforced concrete are obtained.The main research contents and conclusions are as follows:(1)The resistivity of SFRC in early age under uniaxial compression is monitored.The results showed that the electrical resistivity signal can reflect the occurrence,development,and penetration of micro-cracks in concrete,which is closely related to the volume fractions of steel fibers and the age.At the age of 28 days,the resistivity of plain concrete goes through a state of decrease abruptly,decrease slowly and increase suddenly,corresponding to the compaction stage,elastic-plastic stage and failure stage under uniaxial compression respectively.The resistivity of SFRC has experienced sudden decrease,equilibrium and sudden increase.The steady decreasing stage of resistivity can be used as the failure precursor of plain concrete,and the steady rising stage of resistivity can be used as the failure precursor of steel fiber reinforced concrete.(2)The fracture toughness tests were conducted on concrete with four volume fraction of steel fibers,and fracture load,stress intensity factors and fracture energy were obtained,AE as well as DIC were applied to analyze the AE energy and full-field strain of concrete during the fracture damage.The results showed that the fracture property of concrete is significantly improved by the incorporation of steel fiber.The fracture process of SFRC consists of elastic stage,crack stable extension stage,crack unstable extension stage and damage stage.The plain concrete exhibits obvious brittle failure,when the peak stress is reached,strong and dense acoustic emission signals are generated.However,the AE signal of SFRC is still weak when it reaches the peak load.After the load drops slowly for a certain time,strong AE signal is generated due to the fiber/matrix debinding and fiber breakage.The results of DIC showed that the Von-Mises equivalent strain(Evm)gradually increases,and a high strain band appears along the loading direction near the peak load,the width and length of the crack increase gradually.(3)The fracture toughness tests were carried out on SFRC with different crack height ratios(0.1,0.2 and 0.3).The results showed that with the increase of the initial crack-depth ratio,the peak load,stress intensity factor and the subcritical crack propagation of concrete are decreased.This is due to the boundary constraint effect leads to the decrease of the length of the fracture process zone(FPZ),the crack propagation is insufficient.With the increase of crack-depth ratio,concrete is closer to brittle failure.When the crack-depth ratio is 0.3,the characteristic of AE energy of SFRC is similar to that of plain concrete,after the peak load,the AE energy is weak.(4)The splitting tensile strength of concrete with four different steel fiber contents was tested,the results showed that the addition of steel fibers significantly improved the splitting tensile strength of concrete.With the increase in steel fiber content,the splitting tensile strength of concrete increased first and then decreased.The splitting tensile process of SFRC mainly included the following four stages:(I)slow accumulation of elastic energy;(II)rapid accumulation of elastic energy;(III)rapid accumulation of dissipated energy;(IV)slow decrease of elastic energy and slow increase of dissipated energy.In stage III,the strong AE energy was generated,the occurrence of this stage could be regarded as a precursor to the failure of the specimen.