Research On Damaged Fractal Dimensional Cracks And Acoustic Emission Characteristics Of Web-free Reinforced Concrete Beams

Concrete beams without web reinforcement,whose shear bearing capacity mainly provided by the concrete,is a kind of component in large-scale building structure of common application at present.The shear failure of concrete with great harm belongs to brittle failure.From the early 20 th century,the shear failure mechanism of concrete beams without web reinforcement has been studied extensively among scholars from all over the world,and while the shear failure from the perspective of internal damage is relative lack of analysis.Therefore,it is of great significance to explore the shear mechanism of concrete beam without web reinforcement and analyze its damage evolution mechanism and damage basis from the perspective of damage mechanics.In this paper,the damage evolution of concrete beams without web reinforcement was studied with shear span ratio and longitudinal reinforcement ratio as variables.The shear failure experiment of concrete beams without web reinforcement under the action of two variables were carried out to investigate the influence of shear span ratio and longitudinal reinforcement ratio on the basic mechanical properties of concrete beams without web reinforcement and the evolution law of surface crack damage of the experimental beams.At the same time,the acoustic emission instrument was used to collect the acoustic emission signal of the whole process,according to which the damage process of the shear failure of the concrete beam without web reinforcement was described and its acoustic emission damage evolution model was established.Moreover,the GM(1,1)prediction model of acoustic emission was established based on the grey theory to predict the acoustic emission parameters of the failure process of concrete beams without web reinforcement,and to realize the prediction of the damage process.The main research contents are as follows:(1)A four-point loading shear experiment was carried out on nine groups of concrete beams without web reinforcement with shear span ratios λ of 1.5,2,2.5,and longitudinal reinforcement ratios of 1.28%,1.62%,and 1.99%,respectively for the acquisition of the mechanical properties such as load,strain and deflection in order to investigate the influence of shear span ratio and longitudinal reinforcement ratio on mechanical properties.The acoustic emission signal was detected synchronously by acoustic emission instrument during the whole process of the experiment.(2)Based on the development and distribution of the cracks,the damage cracks on the surface of the experimental beam were analyzed by using the fractal geometry theory to verify the fractal effect in the shear process of the concrete beam without web reinforcement.The box counting method was conducted to calculate the fractal dimension D of the surface crack of the beam under the action of the graded load and the ultimate load.The relationship between the fractal dimension D of the beam surface and the ultimate load,the graded load and the mid span deflection,and the development law affected by the shear span ratio and the longitudinal reinforcement ratio was discussed for further judgment of the damage degree of concrete beams without web reinforcement and the statistics of the average orientation and angle of cracks on the surface of experimental beams.(3)According to the acoustic emission characteristic parameters such as ring count and energy,the damage evolution process of concrete beams without web reinforcement was qualitatively analyzed,and the acoustic emission energy was quantitatively calculated by the mean and peak value of acoustic emission energy.The activity coefficient Act was introduced to characterize the full-shear activity of the concrete beam without web reinforcement.The relationship between the fractal dimension and the mean value of acoustic emission energy,which combined macro crack and micro damage was established to explore qualitatively the damage evolution law of the concrete beam without web reinforcement.The AE damage evolution model of the concrete beam without web reinforcement was established based on the cumulative ring count of acoustic emission.(4)In view of the problem that the prediction value of the traditional prediction model is generally too large,the modified coefficient N was introduced to modify the GM(1,1)traditional prediction model based on the grey theory.Three methods were used to define the correction coefficient,and the correction results were compared with the experiment results,so as to realize the quantitative prediction of acoustic emission characteristic parameters in the damage process of concrete beams without web reinforcement.