Study On Acoustoelastic Coefficient And Ultrasonic Stress Detection Of Steel Fiber Reinforced Concrete

Steel fiber reinforced concrete(SFRC)is widely used in civil engineering.Due to long-term service,many SFRC structures have many problems such as reduced bearing capacity,decreased stiffness and weakened durability.In order to ensure the safety of engineering structures,it is necessary to diagnose the working stress of SFRC structures in service.At present,the problem of working stress diagnosis of SFRC structures in service has not been solved well.The research objective of this paper is to obtain a practical ultrasonic stress diagnosis method for SFRC.The research method combining theory and test is adopted,and the main work is as follows:(1)The relationship between the acoustoelastic coefficient of SFRC and the volume content of steel fiber is studied experimentally.In this study,four specimens with different steel fiber content are made,and the volume content is 0,0.6%,1.2% and1.8% respectively.Wave velocity measurement tests are carried out under uniaxial compression stress in the elastic stage,and the acoustoelastic coefficient of SFRC with different content is analyzed.The results show that the acoustoelastic coefficient does not always decrease with the increase of steel fiber volume content.Among all specimens,the SFRC with 0.6% volume content has the lowest acoustoelastic coefficient.The variation trend of the acoustoelastic coefficient along the loading direction with the volume content of steel fiber is similar to that perpendicular to the loading direction.The conclusion is that the acoustoelastic coefficient is affected by the volume content of steel fiber,and the acoustoelastic coefficient can be used as an index to judge the best content of steel fiber.(2)The correlation between the damage of SFRC and ultrasonic wave velocity is studied experimentally.In this study,seven specimens with different steel fiber content are made,and the volume content of 0,0.3%,0.6%,0.9%,1.2%,1.5%,1.8% are used respectively.The wave velocity and strain measurement tests are carried out under uniaxial compression until failure,and the relationship between the damage value of SFRC with different content and the ultrasonic wave velocity is analyzed.The results show that the relationship between damage and wave velocity can be divided into three stages: elastic stage,damage accumulation stage and failure stage.The addition of steel fiber increases the stress threshold value of damage in plain concrete,but the stress threshold value does not always increase with the increase of steel fiber volume content.Among the above specimens,the stress threshold value of damage in concrete with0.6% steel fiber volume content is the highest.The conclusion is that the addition of steel fiber can improve the anti-damage ability of plain concrete,the damage condition of SFRC can be judged by using the ultrasonic wave velocity,and the damage stress threshold value can be considered as an index to judge the optimal amount of steel fiber.(3)The ultrasonic stress diagnosis method of SFRC members is discussed.A method of working stress calculating by using ultrasonic wave velocity is proposed,and7 control specimens with the same mix ratio as those in work(2)are used to verify the method.The verification results show that the error of the proposed method is less than13.1%.The conclusion is that it is feasible to use ultrasonic wave velocity to diagnose the working stress of SFRC axial compression members.