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

Abstract

The results of the first-ever in-situ monitoring of a large mortar specimen at a depth of 3515 m in the Nankai Trough are presented in this study targeted at creating a technology platform for in-situ monitoring and evaluation of cement-based materials at the seabed to realize deep-sea infrastructures. We successfully monitored in situ the development of strain and hydraulic pressure in the specimen. In addition, the short-term behavior of the specimen can be explained by hydraulic confinement and stress relaxation due to water infiltration. Some contraction strain remained in the specimen even after approximately an exposure to the deep sea condition for one year, causing microstructural damage. The pore entry volume was enhanced toward the center of the specimen, and a decrease in compressive strength and Young's modulus were observed in the specimen after exposure due to the microstructural damage. Further improvement of the in-situ measurements is required to ensure the waterproofing and pressure resistance of the strain and pressure gauges.

Utilization of Cement and Concrete for Deep Sea Infrastructure

“…In a real deep-sea field, PC mortar specimens were severely disintegrated only after 1.5 years exposure [1]. As there could be a size effect on seawater attack, a large size specimen was also exposed [2,9]. Figure 1 shows 350 Φ mm × 400 mm-sized PC mortar specimen (water to cement ratio = 0.6, which was the same with [1]) exposed on the deep seafloor with a depth of 3515 m for 309 days.…”

“…Decalcification of C-S-H, formation of thaumasite and M-S-H may occur at the surface as reported in [1,11], and these could cause disintegration of the specimen. Lots of pinhole-like popouts [5], where brucite, calcite and ettringite were detected in Fig. 2, were observed at the surface.…”

“…Therefore, the balance of the above two mechanisms; hydraulic confinement and stress relaxation must be considered to assess the risk of internal damage of cementitious materials under high hydraulic pressure conditions. Takahashi et al [9] reported that the changes of strain and hydraulic pressure in a mortar specimen was successfully monitored in situ during a long-term exposure to the deep seafloor with a depth of 3,515 m. The long-term in-situ monitoring demonstrated that stress relaxation due to water infiltration was delayed from the specimen surface to the core. When the specimen was saturated with seawater, strain reached a plateau.…”

“…This paper reviews the current research on the use of cementitious materials in deep sea environments and highlights the potential for using new compositions to enhance its properties and applicability. Firstly, a series of laboratory and deep-sea field experiments as well as thermodynamic calculation have been conducted to evaluate the properties of conventional Portland cement (PC)-based materials under deep sea conditions [4,5]. The performance of different types of binder have also been evaluated by comparing the changes in physicochemical properties [6][7][8].…”

Impact of Hydrostatic Pressure on Molecular Structure and Dynamics of the Sodium and Chloride Ions in Portlandite Nanopores

“…Wang et al reported that water penetration during pressurization may cause microstructural damage to concrete [23]. Following 309 days of exposure at a depth of 3515 m, a specimen's compressive strength decreased by 27.7%, as assessed by Takahashi et al [24]. These investigations primarily focused on the impact of low temperature and hydraulic pressure on the mechanical characteristics and microstructure of cement-based materials.…”