Issue 46

J. Liu et alii, Frattura ed Integrità Strutturale, 46 (2018) 352-360; DOI: 10.3221/IGF-ESIS.46.32 358 D AMAGE ANALYSIS DURING THE CONCRETE LOADING PROCESS he concrete’s effective area is calculated by determining the undamaged area of the center in concrete’s CT image, shown as D 2 in Fig. 9. The remaining part is considered to be a damaged component and is designated as D 1 . The definition of damage variable is H = D 1 /D 2 . The load damage curves of concrete specimens are shown in Fig. 10. Figure 10: Loading-damage curve of concrete specimen. D 1 and D 2 are determined by analyzing the crack and damage distribution on the cross sections of concrete’s CT image. A concentric circle is drawn on the cross section of 50 mm. The damage was detected by Acoustic Emission and verified by CT image. The degree of damage is measured by the density of the damage points. D1 is determined if a damage point in a circle is less than a threshold. If the crack passes through the center of the section, this method is invalid, but this situation is very rare. As shown in Fig. 10, the damage points, based on the damage variable of CT images and acoustic emission damage location, in the main cracks area after destruction were compared with the damage points in other area, then the result showed the order of the concrete test specimens. Before 30% of the peak load, the damage variable is less than 1 which indicates the damage location is in disorder; When the load reach 80%~100% of the peak load, the damage curve rises steeply, then the cracks appears, finally, the test specimens instability fails. The loading-damage curve reflects the whole process from disorder to order, and it keeps in pace with press-porosity volume cure. Though the comprehensive analysis on damage location map with acoustic emission, press-porosity volume cure and the load-time curve, that fact can be get as follows. Before 30% of the peak load, the damage points are disordered, and the damage variable in concrete is relatively small, then the volume of the pores in material increase slightly. After the load is steadily increase to 30% of the peak load, the damage points appear in order around the macro failure surface, then the damage variable start to grow, so do the volume of the pores. The change of the pores distribution and the center coordinate are also not obvious, and all of this phenomena indicate that the test specimens are destroying steadily. When the load reaches the peak, the stress of concrete test specimen is almost unchanged, while the damage points around the macro failure surface increase rapidly. The damage variable and the volume of the pores show a marked trend of growth. C ONCLUSION (1) The CT reconstruction images, rapid increase of concrete pore volume and rapid growth of damage variable are used comprehensively to prognosist the evolution and localization of cracks in concrete provides a new method for the concrete analyses on the evolution process of cracks and damage location. (2) The damage variable is established based on CT images and CT images’ damage points; then the set of the 3D meso model supplement the traditional stochastic mathematical model in concrete. The 3D model set reflects the relationship between meso structure in concrete and macro characteristics, and this lay foundation to the concrete research on damage mechanism. T