Subsea shield tunnels serve as critical passages connecting two sides of sea areas and play a vital role in supporting the development of a maritime nation and the coordinated integration of land and marine resources. In recent years, the construction of subsea shield tunnels has been rapidly developed, with representative projects including the Lion Rock Subsea Tunnel of the Guangzhou-Shenzhen-Hong Kong High-Speed Railway, the Qingdao Jiaozhou Bay Second Subsea Tunnel, and the Jintang Subsea Tunnel of the Ningbo-Zhoushan Railway, China. However, during both the construction and service phases of shield tunnels, some factors such as construction practices, aggressive environmental conditions, and uneven settlement can lead to cracking, water leakage, and even corrosion of reinforcement in segment concrete. Insufficient toughness and crack resistance of shield segment concrete can compromise the long-term operational safety of shield tunnels. During service, shield segments are subjected to high water pressure and strongly corrosive ionic environments. To enhance the durability of concrete, erosion-inhibiting materials like coatings, migrating corrosion inhibitors, and internally incorporated nanomaterials are often used during prefabrication. These materials significantly improve the durability of segment concrete, and their effect on crack resistance and toughness remains insufficiently explored.

This study was to investigate the enhancement of fracture toughness in shield segment concrete through the use of erosion-inhibiting materials, including silica nanoparticles, migrating corrosion inhibitors, and nano-sized precursors. In addition, the mechanism by which these materials could affect the microstructure of concrete was also analyzed from the perspective of interfacial transition zone optimization.

In this study, the fracture behavior of concrete was investigated by a three-point bending test method. During the loading process, the displacement of the specimens was monitored using digital image correlation (DIC). Also, the interfacial transition zone of the concrete was analyzed via backscattered electron imaging analysis. The fracture toughness of the concrete microstructure was characterized by nano-indentation scratching technique.

Based on the implementation of erosion inhibition measures, the fracture toughness of concrete is notably enhanced. In concrete specimens treated with silica nanoparticles, migrating corrosion inhibitors, and nano-precursors at various dosages, the crack initiation toughness is increased by 22.6%-48.9%, the instability toughness is improved by 18.8%-49.8%, and the fracture energy is elevated by 0.50-1.59 times. The improvement in fracture toughness is closely associated with the physical and chemical interactions of these materials within the concrete. The silica nanoparicles coating penetrates into the cement matrix, promotes a further hydration, and provides additional nucleation sites. Its reaction products effectively fill pores and reduce the number of initial defects and microcracks, resulting in a denser cement matrix with improved strength and deformation properties. The active molecules in the migrating corrosion inhibitor form coordination bonds with calcium ions in cement hydration products, anchoring themselves within the hydration phases. The main components migrate to the steel-concrete interface, forming a protective adsorbed layer that delays reinforcement corrosion. The nano-precursor material undergoes an ion-exchange reaction during cement hydration, in-situ generating calcium carboxylate compounds. These hydrophobic substances are uniformly dispersed in nanoscale within the C-S-H gel network, significantly enhancing the interfacial wetting behavior via reducing the surface energy of pore channels. Furthermore, the nanoparticles released from the precursor decomposition fill internal pores, block connected voids, optimize pore size distribution, and increase matrix density.

The application of erosion-inhibiting materials significantly enhanced the fracture toughness of the concrete. In shield segment concrete treated with nano-silica coating, migrating corrosion inhibitor, and nano-precursors at varying dosages, the peak load was increased by 16.8%-37.1%, crack initiation toughness was improved by 22.6%-48.9%, unstable fracture toughness was increased by 18.8%-49.8%, and fracture energy was enhanced by 0.50-1.59 times, compared to the reference group. Before peak load, the migrating corrosion inhibitor and nano-silica coating both contributed to a reduction in the Crack Tip Opening Displacement (CTOD), while the nano-precursor was effective in lowering CTOD values at all loading stages. During the post-peak phase (specifically at 30% beyond peak stress), the CTOD was decreased by 31.8%-47.2% with nano-precursor incorporation. The erosion-inhibiting materials also improved the properties of the Interfacial Transition Zone (ITZ). The ITZ thickness of the materials treated with silica nanoparticles coating and migrating corrosion inhibitor could be reduced by 7.1% and 6.5%, respectively.