Research On Internal Force Diagnosis Of Pure Bending Beam By Ultrasonic Wave Velocity-crack Synthesis Method

In this paper,the method of diagnosing the internal force of reinforced concrete pure bending beam by using the ultrasonic wave velocity in the compression zone and the maximum crack width at the tensile edge is studied.Firstly,based on the crack width and wave velocity,the bending moment formula of pure bending beam is derived.Secondly,the loading experiment of reinforced concrete beam is designed to study the relationship between ultrasonic wave velocity and concrete stress in compression zone,and the relationship between the maximum crack width of tensile edge and the stress of tensile reinforcement.Finally,the feasibility of ultrasonic wave velocity crack synthesis method for diagnosing the internal force of pure bending beam is discussed,and the relevant calculation formula is given.The main work and achievements are as follows:(1)Through the loading experiment of pure bending beam and the uniaxial compression experiment of plain concrete,the acoustoelastic coefficients in bending and compression are measured.After comparing and analyzing the results of two different methods,the methods to obtain the acoustoelastic coefficients of pure bending beam under compression are summarized,according to the acoustoelastic theory and the measured data,the feasibility of ultrasonic wave velocity used to diagnose the concrete stress in the compression zone of pure bending beam is studied.The results show that:(1)The minimum difference between the bending and compression acoustoelastic coefficients is 10.5%,and it is shown that the acoustoelastic coefficient of concrete in the compression zone of pure bending beam can be determined by plane measurement method and uniaxial compression test;(2)The concrete stress in compression zone of pure bending beam can be deduced by ultrasonic wave velocity.(2)Based on the loading and unloading experiments of reinforced concrete beams,the variation of the maximum crack width under the load is measured.Combined with the existing crack formula,the modified calculation method of the maximum crack width considering the unloading residual crack width is proposed.Finally,based on the experimental data,the feasibility of calculating the tensile reinforcement stress from the maximum crack width at the tensile edge of the specimen is discussed.The results show that:(1)There is a linear relationship between the maximum crack width increment and the tensile reinforcement stress increment,and the residual crack width caused by the collapse of brittle materials can not be ignored;(2)According to the maximum crack width of the tensile edge of the specimen,the tensile reinforcement stress can be estimated.(3)The bending moment calculation formula is obtained by combining the first two research results,and the ultrasonic wave velocity measured in the compression zone and the maximum crack width of the tensile edge of the specimen are substituted into the formula to estimated the bending moment,which is compared with the actual loading value.The results show that:(1)The method of deriving the bending moment of pure bending beam only by the wave velocity needs to be further improved;(2)The actual loading value of bending moment of pure bending beam can not be estimated only by the crack;(3)The minimum difference between the calculated value and the actual loading value is 4%.This method can be used to diagnose the bending moment of pure bending beam.