The bond interface is the weakest part of the repair system, and its performance is a key factor impacting the repair effectiveness of damaged concrete constructions. However, the research on the damage law and the mechanism of repair of the bonded interface in the cold region marine environment is not in-depth. In this study, the influence of polyvinyl alcohol (PVA) fibers and crystalline admixtures (CAs) on the mechanical properties and volumetric deformation performance of cementitious repair materials was researched. Furthermore, the deterioration patterns of the bond strength and chloride ion diffusion characteristics of the repair interface under the coupling of seawater-freeze-thaw cycles were investigated. Combined with the composition, micro-morphology, and micro-hardness of hydration products before and after erosion, the damage mechanism of the repaired bonding interface was revealed. The results indicate that the synergistic use of PVA fibers and CAs can significantly improve the compressive strength, bond strength and volume stability of the repair materials. The compressive strength and 40° shear strength of S0.6CA at 28 d were 101.7 MPa and 45.95 MPa, respectively. Under the seawater-freeze-thaw cycle action, the relationship between the contents of free and bound chloride ions in the bonded interface can be better fitted by the Langmuir equation. The deterioration process of the bonding interface and the penetration rate of chloride ions can be effectively delayed by PVA fiber and CAs. After 700 seawater-freeze-thaw cycles, the loss rates of bond strength and chloride diffusion coefficient of S0.6CA were reduced by 26.34% and 52.5%, respectively, compared with S0.

Tunnel Boring Machine (TBM) excavation materials were recycled by sieving and separating particles into sizes 5–10 mm (coarse aggregates) and below 5 mm (manufactured sand) to explore their potential as aggregates in shotcrete production, with the aim of reducing environmental harm from waste disposal. Mix proportion experiments were conducted to evaluate the mechanical properties—including failure patterns, compressive strength, flexural strength, and deflection—of the shotcrete specimens through cubic axial compression and four-point bending tests; furthermore, rebound tests were conducted on shotcrete mixed with the recycled TBM aggregates in foundation pit engineering. These tests assessed the effects of key parameters (water–binder ratio, sand ratio, fly ash content, synthetic fibers, and liquid alkali-free accelerator) on shotcrete composed of recycled TBM sand and gravel. The results indicated that crushing and grading flaky TBM-excavated rock fragments, and subsequently blending them with pre-screened fine aggregates in a 4:1 ratio, yielded manufactured sand with an optimized particle gradation and controlled stone powder content (18%). Adjusting the water–binder ratio (0.4–0.5), fly ash dosage (mixed with 0–20%), and sand ratio (0.5–0.6) are feasible steps in preparing shotcrete with a compressive strength of 29.1 MPa to 50.4 MPa and slump of 9 cm to 20 cm. Moreover, the rebound rate of the shotcrete reached 11.3% by applying polyoxymethylene (POM) fibers with a 0.15% volume fraction and a liquid-state alkali-free setting accelerator (8% dosage), demonstrating that the implemented approach enables a decrease in the rebound rate of shotcrete.

The bonding interface between repair materials and concrete substrate is the weakest link in the entire repair structure. If the interface bonding performance is insufficient, the repair material is prone to cracking or falling off, leading to repair failure. The shrinkage of repair materials is one of the primary factors affecting the bonding performance of these interfaces. In this study, sulphoaluminate cement (SAC) was used to improve the repair performance of ordinary Portland cement (OPC)–granulated blast furnace slag (GGBS) composite repair materials. The influence of SAC on the mechanical properties, bonding performance, expansion behavior, impermeability, and hydration heat of OPC-GGBS-SAC composite repair materials was investigated. The results demonstrate that the rapid hydration of SAC significantly improved the early strength and mechanical properties of the composite system at negative temperatures. The hydration products filled the pores within the concrete matrix, thereby enhancing the mechanical meshing effect at the interface. The early expansion effect of SAC formed a pre-stressor at the interface, which not only strengthened the bonding force between repair materials and the substrate, but also effectively inhibited the shrinkage of the composite system and prevented crack formation, thus significantly promoting the long-term reliability of the bonding interface. An appropriate amount of SAC can accelerate the hydration process of OPC-GGBS system, advance the exothermic peak, and promote the development of early strength. However, excessive incorporation will inhibit the later hydration of the composite system due to the way in which the hydration products wrap the cement particles. When the content of SAC was 5–10%, optimal comprehensive properties of the OPC-GGBS-SAC composite system were attained.