Issue 46

J. Liu et alii, Frattura ed Integrità Strutturale, 46 (2018) 352-360; DOI: 10.3221/IGF-ESIS.46.32 353 I NTRODUCTION oncrete is a kind of complex multi-phase composite material, initial defects such as micro cracks and pores generate in concrete structure in the process of pouring for the reason of hydration heat release, shrink, etc. The micro cracks expand to macro-cracks under external load or environmental factors, then lead to the decline of the strength, stiffness and eventually cause stability damage to engineering. The relationship between the macro performance indexes and failure characteristic parameters has been a technical problem in various fields [1]. The current research of concrete is based on the conventional mechanics test method, that is speculate the mechanical character of structural materials through comparative analysis of test data instead of using the change of the internal meso structure directly to explain the damage evolution process under the load [2]. Computerized tomography technology (CT technology) can reach dynamic nondestructive observation on the change of internal meso structure in concrete material [3-6], and CT images are visual basic information to establish the meso damage model of concrete [7, 8]. Degree of damage in concrete can be judged according to the tests, and this is of great significance to evaluate the concrete performance [9, 10]. The key of the research on concrete meso damage is to identify the existence of meso damage and the expand process of the damage effectively. CT is short as Computerized Tomography, compared with other methods, CT test has its incomparable advantage on nondestructive detection and real time observation in concrete material under various loading condition. CT images can be obtained by scanning the sections of concrete with industrial CT scanning. Both of the meso structure of concrete and its changing process can be analyzed through CT images, then the law of evolution and expansion for damage in concrete failure process is the key to the CT images analysis. Therefore, CT test is an effective mean to detect the meso pore structure, crack initiation, propagation and coalescence with no damage. In view of this, quite a lot of research work, from different views with different research methods, was done by scholars at home and abroad. Foreign researches started quantitative study on concrete CT detection in 1980s. Morgan [11] is the first to use the medical CT to run CT scanning on concrete specimens, and concrete section images for aggregate, mortar, crack was obtained clearly. Chotard [12] got pretty effect on the change of the internal structure for cement while hydrating. Lawer [13] analyzed failure mode of concrete surface with digital image correlation technique, and the research conclusion could reflect the internal structural change during cracks evolution process, besides, the influence of aggregate shape, crack shape on concrete strength, toughness was discussed based on CT images after concrete cracking. Wong [ 14] conducted uniaxial compression CT test to normal and high strength concrete cylinder specimens, then the evolution process of pore under different stress state was studied. Meanwhile, a large number of concrete CT tests were carried out in China, and abundant research results were obtained: academician Chen Houqun using CT real time scanning to observe the meso failure process of concrete under the uniaxial compression, then the whole process CT images of internal meso cracks from initiation, expansion to coalescence was got, finally, quantitative analysis methods based on concrete CT images was established in the cracks area. Ding [15] used medical CT machine and special loading equipment to explore morphology change characteristics of micro cracks under the condition of different loading rate by images analyses and the distribution of CT number. Wu [16] described the damage property of concrete materials, and the failure process in concrete was revealed in meso view, that are Initial crack stress and crack propagation process. Liu [17] used CT technique to scan concrete specimens, then reconstructed the images with Mimics software, finally the 3D geometric model of concrete specimens can be got. The plastic damage model of concrete 3D numerical specimens was introduced to simulate the uniaxial tension and compression loading process, then the rationality of 3D numerical model was identified. Although researchers at homeland and abroad have done much research work based on CT test [18-22], it is rare to see do quantitative analysis on crack initiation and propagation mechanism of concrete by using acoustic emission and CT combined. CT scanning have obvious advantage on real-time and nondestructive detection to internal structural change process, but its disadvantage is for both of the equipment and scanning cost are much higher. The previous real-time monitoring tests are carried out CT scanning after stopping loading when the loading reaches a specific stress stage. The test results may not ideal or the cost may increase if the stresses when scanning are not properly selected. As the unique passive detection technology in nondestructive testing, acoustic emission detection technology can monitor and evaluate the whole concrete structure in real-time, acoustic emission signal can reflect the change situation of the internal structural damage accurately except for its defects that can’t be observed evolution of pore and damage in structures directly. Considering their advantages in detection, tests on combination of acoustic emission and CT can be conducted. Stop loading when acoustic emission signal prompts there are new cracks generated or the old cracks expanded, then explore the relationship between CT scanning images and acoustic emission by scanning concrete with CT. In this thesis, the meso structure of concrete and its changing process was analyzed with the feature of CT images, then the order of damage evolution in concrete is described by damage variable based on CT images. C