Study On Stress Test Of Reinforced Concrete T Beam Under Simple Stress Field

At present,the research object of the stress ultrasonic coda wave test of concrete structures has been gradually expanded from the standard cube or prism test block to the meter-level test piece,and its stress field has also extended from the uniform tensile and compressive stress field to the stress that is closer to the engineering application.This paper studies the content of the "Experimental Test of Relationship between Time-domain Signals and Changes of Three-point Bending Stress Field" in the National Natural Science Foundation of China(Grant No.51478072),and takes a reinforced concrete T-beam as the research object,with reference to the research results of ultrasonic coda wave testing of concrete stress at home and abroad,combines coda wave interferometry test of stress field of concrete beams completed in the earlier stage of the research group,proposes the research idea of using the first-order signal parameters to characterize the stress field of the concrete beam.On the basis of establishing the time-domain measurement model of coda wave for specific stress field increment,designed and implemented a three-point bending-stress ultrasonic field test for a concrete beam,test data of 22 load levels were collected.Based on the statistical analysis of the data,takes the zero-level load as the origin,Taylor expansion using ultrasonic coda as a function vector with load level as an independent variable,analyzes the proportion of energy of each order(first-order to fifth-order)signal,removes the influence of high-order signals,extracts first-order signal and establishes a linear relationship between first-order signal parameters and the load levels.The main research content is as follows.(1)Based on the results of literature research,the research route of this paper is put forward.Focusing on the research achievements of ultrasonic coda wave in stress testing of concrete structures,and collects,summaries and analyses the principles,methods,test schemes,and the results of related research at home and abroad.Based on the results of coda wave interferometry and amplitude analysis of the concrete beam stress field completed in the earlier stage of the research group,the technical route of using the first-order signal parameters to characterize the three-point bending stress field of concrete beams is put forward,the three-point bending test scheme,the parameter index of ultrasonic instrument and the load grade of three point bending load are preliminarily defined.(2)To establish a time-domain measurement model of ultrasonic coda wave for stress increment field coefficient based on Taylor expansion.Taking the load of a specific(three-point bending)stress field as the independent variable,the ultrasonic coda at each load level is used as a function vector,Taylor expansion is performed at the zero-level load,and the unfolding order is determined according to the relative energy magnitude of each order signal.Based on this,a measurement model that characterizes the load level using a first-order signal(projection value in the first-order change direction of the signal vector caused by a specific stress field)is proposed.(3)Experimental study on time-domain measurement model parameters of ultrasonic coda waves in the three-point bending stress field of a T-beam.The ultrasonic coda test program for the three-point bending stress field of a reinforced concrete T-beam was designed and implemented,test data of 22 load levels were collected.The MATLAB calculation program prepared in advance by the research group was used to extract the first-order signal(linear part)from the original ultrasonic coda waves under various load levels,the linear relationship between first-order signal parameters and the load levels is obtained.Through comparative analysis of the relationship curves of multiple tests,the influence of temperature on the relationship curve was studied.