The purpose of this study is to visualize cracks that occur within the concrete which expands due to permeating water. This fracture may be induced when large content of sulfate of cement and high temperature surrounding environment is encountered. Although concrete is not immediately affected after heating under dry condition, it will be deteriorated when exposed water and humid conditions. The concrete expands and fractures under that conditions. This phenomenon was investigated on precast concretes. However, the mechanism of this phenomenon within the concrete is not made clear until now. Therefore, the X-ray CT method was used for visualizing change in internal structure during expansion fracture. The mechanism of the phenomenon was then clarified.
Concrete cylinders with a diameter of 50mm and length of 100mm were prepared as specimens. In the laboratory, the expansion occurred and converged almost in one week's time. The X-ray CT images of specimens were taken at several expansion stages of the specimen. Using the obtained CT images, cracks were visualized on two types of scales, the whole specimen and 1 cm cube. To visualize micro size cracks, the 3DMA (3 Dimensional Medial axis Analysis) was applied to the 3D CT image to analyze the state of fracture initiation and propagation. As a result, it was clear that fractures initiate near specimen surface and progress to its core and the cracks occurred within mortar and boundary between mortar and aggregate.
When concrete is exposed to high temperatures such as fire, it is damaged due to thermal stress. On the other hand, even if concrete is not affected by thermal stress at low temperature, it may deteriorate due to the supply of water after heating.
In general, when the concrete is heated above 70°C, the internal ettringite decompose and sodium sulfate is released. After being exposed to water, water reacts with calcium silicate hydrate or calcium hydroxide to produce gypsum. Furthermore, due to the reaction of gypsum and aluminate more ettringite is produced, hence the concrete expands and deteriorates. It cannot be said that the change of the internal structure of concrete when the concrete deteriorates is sufficiently made clear.
In this study, the change of the internal structure of concrete specimen was visualized by using the X-ray CT which is one of the non-destructive inspection methods. The crack propagation behavior during expansion process, such as above phenomenon, was made clear by using the parameter indicating the amount of void inside the concrete.
