Action of Hydraulic Pressure on Portland Cement Mortars - Current Understanding and Related Progress of the First-Ever In-Situ Deep Sea Tests at a 3515 m Depth

Abstract

In order to realize the utilization of cement-based materials in the special extreme environment, the deep sea, the authors have launched a project targeted at creating a technology platform with in-situ methods and systems for monitoring and evaluating cement-based materials located at deep ocean bottom sites. The first in-situ test in the world with a view to investigating the time-dependence of the volumetric stability and microstructure of Portland cement mortar following its long-term exposure to deep-sea conditions is currently underway at a 3515-m depth in the Nankai Trough. This paper reviews previous studies about the influences of deep-sea hydraulic pressure on cement-based materials, verifies the action of short-term hydraulic pressure using Portland cement mortars on a laboratory scale, and introduces the ongoing progress of in-situ deep-sea tests. Results from laboratory tests indicate that dimensional changes were provoked by liquid water infiltration and confinement while under short-term hydraulic pressure, however, time-dependent behavior under stresses such as creep has not appeared. Weight gain, changes in pore-size distribution, compressive strength and bending strength of the cement mortar were monitored after pressurization and depressurization processes.

In-Situ Deep-Sea Monitoring of Cement Mortar Specimen at a Depth of 3515 m and Changes in Mechanical Properties after Exposure to Deep Sea Condition

“…Prior to testing, the pressure gauge was embedded in a mortar with a diameter of 80 mm and a height of 100 mm, which was sufficiently larger than the pressure gauge, and cured for 28 days. A high-pressure vessel with an inner diameter of 150 mm and a height of 250 mm was used for pressurization (Takahashi et al 2021).…”

“…Then, the differences between the results presented in Fig. 10 and the results of the preliminary lab test shown in Takahashi et al (2021) using the same size mortar specimen with a hyperbaric chamber are discussed. In Fig.…”

“…This damage was not caused by the pressure release during the ascent of the specimen. In a previous study, the authors reported that cracks were initiated at the corners of the specimen when hydraulic pressure was applied for a few hours (Takahashi et al 2021). Here, the disintegration of the microstructure could be enhanced due to ion dissolution and the formation of ettringite, thaumasite, and Mg-based hydrates (Kobayashi et al 2021).…”

“…The apparatus designed for in-situ measurements are described in our previous study (Takahashi et al 2021).…”

Study on Deformation Behavior of Sediments and Applicability of Sealants in Seabed Mining

“…Cement-based materials are also adopted to construct offshore structures using underwater concreting. Several kinds of research investigated the properties of cement-based materials in deep-sea conditions and showed the results of the deterioration mechanism due to the microstructural changes with pressurization [18,19] The authors invented an environment-friendly underwater mining method that aims to control the dispersion of seabed surface sediments and fill the extracted areas by sealing a submerged mine site with anti-washout cement-based sealants. That is to say, sealants can restrain suspension by covering the mining area in advance.…”