Analysis of concrete meso damage based on CT

CT is one of the effective means to analyze concrete meso-damage. Based on CT image under uniaxial compression, this paper used Amira—three-dimensional reconstruction software to segment the images, extract pores, count and calculate the pore volume .This paper analyzed the corresponding relation between pressure —pore volume diagram and pressure—displacement diagram under cyclic loading. The results showed that: under uniaxial loading condition, pore volume change can be used to describe the change of the meso pore of the concrete specimen, including compaction, initiation, extension and perforation.


Introduction
Concrete is a kind of complex artificial composite materi als.The long-term practice [1][2][3] showed that the macrosc opic failure of concrete under external loads included por e initiation, propagation and coalescence.Therefore, if yo u want to find the relationship of concrete meso-damage and macro mechanical properties, the first thing was how to descry the damage process of concrete with feature of concrete meso.Computer tomography technology also kn own as the CT, CT experiments on concrete material nondestructive detection and real-time observation of change s of concrete crack had unique advantage.Academician C hen Hou-qun [4] completed real-time scanned observatio n on concrete meso-damage under uniaxial compression using CT, and obtained the initiation, propagation and co alescence of the whole process of CT image of internal m icro cracks of concrete; Wu Li-qin [5] put CT number cro ss section as the characteristics of representative failure o f materials, from the angle of CT number to described th e failure process of concrete internal.Liu Han-kun [6] did 3D reconstruction of CT images by using MIMICS softw are, a three-dimensional geometric model of concrete, an d used Abaqus finite element software to complete the nu merical simulation analysis, finally achieved good results.Liang Xinyu, Dang FA ning [7] based on changes of the average CT number of CT image in concrete meso dama ge process, put forward the concrete damage variable bas ed on CT number, which showed that concrete specimens under uniaxial compression experienced elastic compre ssion, CT scale crack initiation, crack propagation and co alescence, straight to the macroscopic failure of concret e. Zhou Huoming, Yang Yu and other [8] using acoustic emission location technology and CT image, based on ac oustic emission positioning and section CT number variat ions of rock under uniaxial compression , crack of rock fa ilure process was studied.Yu Aiping, Zhao Yanlin, Feng Yifeng [9] studied the failure process of bonding steel rei nforced concrete by using acoustic emission instrument t o real-time locate tracking corrosion in the pull-out test.L iu Jinghong [10] studied the fractal dimension of acoustic emission parameters of coal rock under uniaxial compres sion by using the fractal theory, and the correlation dimen sion of the acoustic emission energy decreasing continuo usly is seen as the precursor to instability failure of coal a nd rock.Raymond Lam, Li Shulinand other [11] accordin g to the number of acoustic emission and the stress leve l of damage variable, the probability density function abo ut acoustic emission number was proposed, and fitted of the linear relationship between the number of damage var iable and acoustic emission number through the curve.A mira [12] is a 3D modeling software system, and it not onl y can achieve reconstructed using CT image, but also it c an precisely statistic each composition material volume.t his paper, based on the Amira software, extracted the crac k of concrete test block under different loading stages, co unted the crack volume, and got the change curve about p ore volume and pressure, finally discussed the concrete m eso-damage process.

CT experiment and pore volume statist ics 2.1 The test specimen preparation
In the test of concrete specimens with information such as shown in table 1.

Scanning device
CT detection system in the test is ACTIS300-320/225X from the State Key laboratory in China University of Mining & Technology (Beijing) [13], and the image size in pixels is 500*500.The scanning thickness is 0.2mm.Moreover, the specimen specification is a cylinder with a radius of 50mm and a high of 190mm.The specimens were carried out in 6 stages of scanning, including the initial phase, as shown in Table 2. Schematic diagram of experimental system was as shown in Figure 1.

The result of the experiment
The specimen loading pressure-time curve was as shown in Figure 2.
998 CT images were obtained.Select one section under different stress phase as shown in Figure 3.

Fig.3 CT scanning image of the failure process of specimen und er uniaxial compression
There is no obvious changes and crackle in the first stages until the peak pressure.
The Amira software extracted the crack porosity as shown in Figure 4.
Concrete pore and crack is a green in figure 3, and pore crack changed more clearly than CT original images through the Amira software extracted pore crack.Based on the Amira software, the pore volume and the center coordinates of pore mass were counted in in different loading stages, as shown in table 3.
Table 3 :The pore volume and center coordinate of pore crack mass in different loading stages The ordinate of the concrete test block ranged from (0 , 0 , 0 ) to ( 125 , 125 , 91 ) .From the table above, the center of mass coordinates for the initial pore in the concrete test block was (62.4,60.6, 43.4) when the pressure was 0 KN.Thus, we can conclude that the initial center of mass coordinate in the concrete test block is uniform.After the destruction of the concrete test block, the center of mass coordinate in the concrete test block was (83.  the Y and Z coordinates almost didn't change, and the change of the X coordinate was more obvious.Generally speaking, the two showed some correlation.There was almost no change for the coordinate Z of the center coordinate form the beginning to the final failure for instability, thus, a conclusion can be made that the failure section is through, and this is the same with the spatial shape of the crack in the concrete test block from the CT images.As for the X , Y coordinates , with the enlargement of the external load , the center of mass coordinate in the concrete test block increased at the same time , furthermore , it expanse toward , and this agree with the idea that the two main cracks appear at the edge of the specimens .Draw a cure of "pore volume-stress" for the specimens based on the pore volumes in different stress stages from the above table, as figure 5.
Due to the high porosity of the specimen, namely, the severity of the initial damage, on the phrase of 30% ultimate stress, there was no obvious elastic compression for the test blocks on the whole, instead, the volume increased slightly.This is because of the severity of the damage, then the test blocks find a faster energy release methods, and it makes the elastic compression phrase shorter than before, accelerates the specimen into the stage of stable crack extension, and finally the pore volume increase slowly.When approach the limit stress, the test blocks enter the crack unable development stage, and the pore volume increases suddenly, finally the blocks fails for the instability.Compared the damage localization maps on different phrase with the corresponding CT images, obvious cracks can be found only in the final failure stage with CT images.Compared with CT technology , the Acoustic emission instrument is more sensitive than former in damage localization , for instance , on the phrase of 23% ultimate press , one main crack appeared ,which indicates that the Acoustic emission instrument can track the damage localization of the concrete earlier , at the same time ,it approves the conclusion that the test blocks find a faster energy release methods , and it makes the elastic phrase shorter than before for the reason of its' severe initial damage .Through a comprehensive analysis from the damage maps with Acoustic emission instrument , load-pore volume diagram and load-time curve ,we can conclude that : c in the early stage that the load was lower than the ultimate stress , the damage localization was disordered , the damage variable of the concrete is relative small , and the initial pore volume increases slighted ; in the cyclic load-time curve , the second loading curve deviates from the first loading curve , indicating that the blocks has experienced pore compression ,and step into the phrase of stable crack extension in advance ; part of the damage localizations overlap with the distribution of the initial pore , the discreteness of the damage localization is relevant with the randomness of initial damage ; d when the load pass the 30% of the ultimate load , load-time curve increase slowly , with the incensement of the load , damage localization distribute around the final failure surface gradually and orderly , and the damage variable of concrete increase gradually ; load-pore volume curve increase slowly , from which we can see that pore distribution showed no obvious changes , and the center of mass coordinate in the concrete test block didn't change significantly , finally ,the blocks was in the phrase of stable crack extension ; e when the load approaches the ultimate load ---134KN , from the loading curve , the stress keep almost constant , while the strain increase sharply ; The damage localization points increase sharply in the final failure surface , and damage variable of concrete showed a significant incensement as well as the volume of pore crack ; the center of mass coordinate in the concrete test block changed obviously , and the blocks stepped into the phrase of instable crack extension , then the two main cracks appeared , which connected the failure surface caused by parts of the original pore , finally , the blocks failed in a instable way .

Conclusions
From the analyses above, we can conclude that: Based on CT images, utilize Amira 3D visualization soft ware to complete pore segmentation, and count the pore v olume and the center of mass coordinate in the concrete t est block under different stress phrase.The conclusion sh ows that the changes of the former reflect the concrete tes t blocks experience three main deformation stages: the ela stic pore compression phrase, the phrase of stable crack e xtension, the phrase of instable crack extension.The cent er of mass coordinate in the concrete test block under diff erent phrase have certain correlation with the destruction process of the concrete test blocks.After the the destructi on of the concrete test block , the center of mass coordina te in the concrete test block is close to the initial center of mass coordinate in the concrete test block , this provide a theoretical basis for the pore structure of concrete in dete ction of weak surface and in reinforcement in the future .As for whether the distribution of pore in concrete have a decisive influence on the failure of concrete under uniaxi al compression, it still calls for plenty of subsequent expe riment to prove.
But by the limit of software, so image segmentation h ad not been a unified standard, and this paper also has the following shortcomings to be improved: A) Automatic threshold segmentation has high requirem ents on the picture, but the manual segmentation is m ore subjectivity.the segmentation effect by using the method of combination of human and computer is i mproved, and the overall regularity is no problem, bu t the data accuracy will be affected, which is not con ducive to the next quantitative analysis B) amount of initial crack is differ from the statistical va lues, there may being reasons: measurement error; th e pore distribution may also have an impact, because the pore volume of statistics is owned to the local blo ck; the main reason is that Amira statistical softwar e requirement for the volume of pore sealing, so the s urrounding open pores or not closed pores in the test block will not be statistics, resulting that pore statisti cs is not comprehensive enough; Some micro pore w as not extracted in the process.

Figure 1 .
Figure 1.schematic diagram of experimental system

Figure 2 pressure displacement diagramFigure 4 :
Figure 2 pressure displacement diagram DS2 series full information of acoustic emission signal a nalyzed to complete the AE data acquisition of the whol e loading process.The relevant acoustic emission parame ter setting, threshold values of 100dB, PDT: 150us, HDT: 300us, HLT: 500us.The related technical indicators: 8 c hannel 3MHZ, sampling rate, data collection methods: 4 channel synchronous data acquisition, RS-35C integrate d front sensor: amplifier gain: 100 times.

Figure 5 :
Figure 5: concrete specimens AE damage location map under the loading process of different stage of stress

Table 1 .
The mix of concrete' in the test material Amount of per cubic concrete Cement/kg Water/kg Sand/kg Gravel/kg DOI: 10.1051/ C Owned by the authors, published by EDP Sciences, 2015

Table 2 .
The load corresponding to the 6 scanning time of the specimen 5, 61.8, 40.5).Compared with the center of mass coordinate for the initial pore in the concrete test block,