基于CT图像的再生混凝土细观破坏裂纹分形特征

为深入研究再生混凝土的破坏形态和内部裂纹扩展情况与普通混凝土之间的差异，以不同再生粗骨料(RCA)取代率的再生混凝土为研究对象，利用Phoenix v | tome | x s240微焦点工业CT获取再生混凝土加载到90%预估破坏荷载的二维扫描图像，借助Photoshop CS6图像处理软件，对材料内部破坏裂纹进行提取，进而基于分形几何理论，以分形维数及多重分形谱表征裂纹的分形扩展规律，建立分形维数和多重分形谱特征参数与RCA取代率和再生混凝土抗压强度的关系。结果表明:再生混凝土的细观受力破坏模式与普通混凝土不同，其受力破坏形态不仅取决于粗骨料与水泥浆体的界面黏结强度，还取决于RCA自身性能，当裂纹发展至天然粗骨料或强度较高的RCA时会绕过骨料表面继续发展，发展至强度较低的RCA时会贯穿骨料；分形维数可定量描述混凝土材料内部细观裂纹的整体扩展情况，即裂纹越丰富，分形维数越大；多重分形谱可反映从局部到整体不同层次的细观裂纹特征，裂纹分形维数和多重分形谱特征参数均与RCA取代率呈线性下降关系，与抗压强度呈线性增长关系；本研究可为再生混凝土在大型结构工程中的广泛应用奠定理论和实验基础。 

Fractal characteristics of meso-failure crack in recycled coarse aggregate concrete based on CT image

The work aims to study the difference between recycled coarse aggregate concrete and normal concrete in failure mode and internal crack propagation. The recycled coarse aggregate concretes with different recycled coarse aggregate (RCA) replacement rates were used as the research object. Using the micro-focus industrial CT (Phoenix v | tome | x s240), 2D scanning images of recycled coarse aggregate concrete under 90% predicted failure load were obtained. With Photoshop CS6 image processing software, the failure crack in material was extracted. After that, based on the fractal geometry theory, the fractal dimension and multi-fractal spectrum were used to characterize the fractal propagation law of crack. Finally, the relationship between fractal dimension, multi-fractal spectrum characteristic parameters and RCA replacement rate and compressive strength was established. The results show that the meso-failure mode of recycled coarse aggregate concrete is different from that of normal concrete. The failure form of recycled coarse aggregate concrete depends on the interfacial transition zone (ITZ) between the coarse aggregate and the cement paste and the properties of RCA itself. When the crack develops to the natural coarse aggregate (NCA) or the RCA with higher strength, it will continue to develop around the surface of the aggregate, and it will penetrate through the aggregate when it is developed to the RCA with lower strength; The fractal dimension can quantitatively describe the overall propagation of meso-crack inside the concrete materials, that is, the richer the crack, the larger the fractal dimension; Multi-fractal spectrum can reflect the characteristics of meso-crack at different levels from local to global; The fractal dimension and multi-fractal spectrum characteristic parameters of crack have a linear negative correlation with the RCA replacement rate, and have a linear positive correlation with the compressive strength; This study can lay a theoretical and experimental basis for the wide application of recycled concrete in large-scale structural engineering.