Hirokazu MACHIDA

Elucidation of Crack Generation Mechanism by Early Concrete Volume Change of Super Quick Hardening Concrete

Takumi SHIMOMURA

In this study, we planned and verified the hypothesis of the crack generation mechanism for the purpose of elucidating the crack generation mechanism of Super Quick Hardening Concrete, and examined the effective crack suppression method.
First, we summarized the cases of cracks in Super Quick Hardening Concrete that occur during the replacement work of the expansion and contraction device, and summarized the problems and past research focusing on the current hardening properties of Super Quick Hardening Concrete.
Next, we conducted a crack reproduction experiment that imitated an expansion joint. As a result, it was clarified that the cracks that occurred directly above the guide plate were due to the increase in the contraction displacement difference of the upper surface depending on the presence or absence of the guide plate, and that they occurred in about 1 hour.
Based on these reproduction experiments and previous studies, the cracks that occur in ultrafast hard concrete are those of concrete in which shear deformation near the guide plate due to the difference in the volume change of concrete depending on the presence or absence of the guide plate is occurring at that time. We devised a hypothesis of the crack generation mechanism that cracks occur when the deformation capacity is exceeded.
Furthermore, we conducted a verification experiment to quantitatively test the hypothesis. As a result, it became clear that cracks occurred when the contraction displacement difference on the upper surface exceeded a certain level, demonstrating that the hypothesis of this study was valid.
Finally, from the test results of this study, the required performance of the expansion material, which is effective in suppressing cracks in Super Quick Hardening Concrete, was considered from the viewpoint of the expansion duration of the expansion material.