Experimental Study On The Correlation Between Accoustic Emissions And Bonding Stresses In Pull-out Specimens

In reinforced concrete(RC)structures,the bonding between the steel bar and the concrete is a basic premise to ensure the common force and coordinated deformation of the two kinds of materials.The interaction between the two materials is mainly achieved by the bonding stresses at the interface between the steel bars and the concrete.Due to the numerous influential factors,complex bonding failure mechanism and limited measuring techniques,the problem of accurate measurement of the bonding stress distrubution remains to be further studied.This problem also seriously delays the development of scientific research and engineering inovations in civil engineering.As a non-destructive testing technology,acoustic emission(AE)can sense the subtle changes in materials and structures real-timely and dynamically.It has great potential on exploring structural responses and their changes under the load(such as the distribution of bonding stresses in pull-out specimens).Based on the work accomplished previously,this paper conducted an experimental study on the bonding stress distribution and the distribution of AE signals in concrete pull-out specimens,with particular attention paid to the correlation between the two distributions along the length of steel bars.The outcomes of this research will lay a foundation for the study on the bonding stress distribution in RC structural components by using AE technology.Based on established research,this paper summarized the progress in AE technology and RC bonding stresses which has made so far,and then further explored these topics from the theoretical and the experimental aspects.Firstly,by combing the existing theory,the distribution law of the bonding stresses along the length of the steel bar in a pull-out specimen was summarized.Secondly,pull-out experiment was carried out,and steel bar strains and AE signals were collected.Finally,the distribution of bonding stresses and AE signal were studied in both qualitative and quantitative perspectives.The correlation between the distributions was analyzed qualitatively and quantitatively.The major topics discussed in this paper are as follows:1.The summary on the bonding stress distribution in pull-out specimensBased on related literature,the influence factors,the characteristics and shape of the bonding stress distribution between the steel bar and concrete in pull-out specimens were investigated,and it lays a foundation for the future research on the correlation between AE signal distribution and the bonding stress distribution.2.The qualitative analysis on the correlation between AE signal distribution and the bonding stress distribution in pull-out specimensAccording to the physical nature and characteristics of AE phenomena,the intrinsic relationship between AE signals and stresses was qualitatively analyzed.Based on the theories of concrete random damage mechanics,several typical quantitative models of AE parameters and stresses were analyzed..The work in these two areas has formed a theoretical foundation for subsequent research.3.Experiment on the correlation between the bonding stress distibution and the AE signal distributionFirst,bare stell bar pulling tests were performed to establish the strains vs.the tension force relationship for the steel bars which would be embedded into concrete pullout specimens.Secondly,the steel bars were used to make concrete specimens,and then a pull-out experiment was performed.During the loading process,the strain data of the steel bars were collected to estimate the average bond stress between the steel bars and the concrete;AE signals were collected to establish the distribution of some AE parameters along the length of the steel bars.4.Establishment of a model to characterize the AE parameter seriesThe relation between AE parameters and the bonding stress distributions were qualitatively investigated by analyzing the bonding stress diagram and that of the AE signals.The method used to construct the AE parameter series was studied based on the constant increment rule of the Time Series theory.The approach of decomposing the AE parameter series into two components(i.e.a trend term and a residual series)was investigated according to a theorem in Time Series field.The mathematic function was constructed using several curve-fitting methods.The residual series was analyzed based on the variation in its autocorrelation coefficient.On the ground of the above work,the following important conclusions were drawn.Firstly,there is a correlation between the AE parameter distribution and the bonding stress distribution in pull-out specimens with deformed steel bars.Specifically,the AE activity was greater in the interval where the bonding stresses were larger,and there was a nonlinear and non-determinstic quantitative relationship between the two kinds of physical quantities.