As Japan's territorial oceans and exclusive economic zone (EEZ) are around 12 times larger than its territorial land area, the country's ability to utilize its deep-sea resources is expected to radically improve its global competitive advantage. While the types of concrete being used to build marine infrastructure in shallow sea areas could be also used in the deep sea, there is a risk that conventional concrete might deteriorate faster in the deep sea.

To address this issue, in this project we conducted experimental studies focusing on two points, the physical damage caused by hydraulic pressure and the chemical deterioration caused by low-temperature sea water, in order to evaluate the physiochemical stability of concrete in the deep sea.

As concrete is a porous material, it is susceptible to the movement of liquid water through microscopic pores and resulting changes in the stress distribution, which could cause physical damage. Therefore, we developed new equipment that could withstand high hydraulic pressure (maximum 20 MPa at a water depth of 2,000 m) and capture images of the internal structure of specimens using micro-X-ray CT technology (Figure 1). The physical damage caused to the specimens by the water pressure was evaluated, based on the image analysis.

Cement paste was immersed in low-temperature sea water and then the progress of its chemical deterioration was observed. It was revealed that rapid deterioration of the cement paste was found at low-temperature sea water, and the mechanism of rapid deterioration was estimated on the basis of thermodynamic phase equilibrium calculations.