Cementitious materials are multiscale and multiphase composites whose frost resistance at the macroscale is closely governed by microstructural characteristics. However, the interfacial transition zone (ITZ) between clinker and hydrates, recognized as the weakest solid phase, plays a decisive role in the initiation and propagation of microcracks under freezing conditions. Understanding the frost damage mechanism of ITZ is therefore essential for improving the durability of concrete in cold regions. The motivation of this study lies in revealing how freezing affects the mechanical integrity and microstructure of ITZ in its early ages, which remains insufficiently understood in existing research. To address this, a nanoscratch technique was employed for its ability to quantify local fracture properties and interfacial adhesion at the submicronscale, providing a direct and high-resolution assessment of ITZ behavior under freeze–thaw action. The ITZ thickness and fracture properties were characterized in unfrozen cement paste and in cement paste frozen at 1 and 7 days of age to elucidate the microscale frost damage mechanism. Moreover, the enhancement effect of nano-silica modification on frozen ITZ was investigated through the combined use of nanoscratch and mercury intrusion porosimetry (MIP). The correlations among clinker particle size, ITZ thickness, and ITZ fracture properties were further established using nanoscratch coupled with scanning electron microscopy (SEM). This study provides a novel micromechanical insight into the frost deterioration of ITZ and demonstrates the innovative application of nanoscratch technology in characterizing freeze-induced damage in cementitious materials, offering theoretical guidance for designing durable concrete for cold environments.

To achieve proactive preventive maintenance of green and low-carbon infrastructure, this study systematically investigated the long-term durability and resistance degradation of prefabricated recycled aggregate concrete (RAC) bridge components. A 80-month multi-field coupled damage experiment under sustained flexural loading, natural atmospheric exposure, and chloride drying-wetting cycles was conducted, and an in-situ physical exposure and multi-source data-driven Support Vector Regression (SVR) dynamic surrogate model was established. Results indicate a prominent time-dependent ebb-and-flow mechanism of degradation drivers: environmental and stress boundaries dominate the early stage, whereas the material replacement rate ($R_r$) surges to become the absolute dominant driving variable (36.8%) in the ultra-long term ($t=80 \mathrm{months}$), proving the cumulative dominance of recycled aggregates. Concurrently, the residual capacity exhibits a distinct two-stage decay characterized by a 10% critical reinforcement mass loss threshold, beyond which the degradation rate of RAC100 accelerates to 1.63 times that of conventional concrete. Driven by the multi-stage injection of in-situ experimental inspection data (surface crack profiling, 2D spatial chloride profiles, and rebar mass loss), the SVR network successfully achieves a collapse-like convergence of the remaining useful life (RUL) confidence interval, precisely locking the RUL of the RAC100 component at 34.5 years within a 1.4-year error margin. This framework provides critical algorithmic support for the life-cycle safety paradigm shift in low-carbon structures.

Irreversible deterioration of internal microstructure in cementitious materials is induced by extreme service environments, which leads to severe degradation of macroscopic fracture properties. In this study, the fracture mechanics behavior of cementitious materials was systematically investigated under benchmark (N), continuous drying (D), extreme high-temperature (H, 400 °C), and rapid freeze–thaw cycle (F25–F100) conditions. By means of mercury intrusion porosimetry (MIP), nanoscratch technology, and backscattered electron (BSE) image segmentation processing, the evolution laws and synergistic damage mechanisms of the porous continuum matrix, aggregate–paste interfacial transition zone (ITZ), and microcrack network were quantitatively revealed. The results show that the macroscopic fracture energy G_f exhibits significant environmental sensitivity and nonlinear decay characteristics; after drying, F100 freeze–thaw, and 400 °C high-temperature exposure, G_f was decreased by 11.1%, 67.2%, and 83.0%, respectively. Microstructural analysis confirms that total porosity P_t is increased and critical pore diameter d_cr is coarsened and right-shifted by environmental deterioration; nanoscratch tests reveal that the ITZ is always the lowest energy consumption channel for crack propagation, and its local fracture toughness $K_C^{\mathrm{ITZ}}$ suffers a catastrophic drop under high-temperature and severe freeze–thaw conditions (down to 0.18 MPa⋅m^1/2); the microcrack area ratio C is sharply increased (up to 4.65%), and the distribution morphology is evolved from local dispersion to three-dimensional network bridging. The macroscopic fracture mechanism is thus degraded from three-dimensional matrix shear and aggregate interlocking energy dissipation to two-dimensional debonding and slip along low-resistance interfaces. Based on micro–meso physical mechanisms, a cross-scale synergistic prediction model for fracture energy considering matrix pore volume loss (P_t), microcrack network segmentation (C), and ITZ shear anti-cracking barrier ($K_C^{\mathrm{ITZ}}$) was established. The prediction desensitization problem of the single porosity index under extreme conditions was accurately eliminated by this model (R² = 0.984, RMSE = 4.12 N/m), and theoretical support is provided for service life assessment and crack-resistance toughening design of cementitious engineering structures under extreme environments.

Macroscopic fracture testing of nano-modified cement paste requires the labor-intensive fabrication of notched beams. This study establishes a micro-to-macro surrogate modeling framework mapping localized nanomechanical scratch signatures to macroscopic fracture resistance. A database was compiled for pure Portland cement paste (w/c = 0.30, 28-day curing) across seven nano-silica (NS) dosages (0.0–3.0%). Across 21 independent physical beam specimens (N_macro = 21) and 470 validated phase-segmented nanoscratch observations (N_micro = 470), localized descriptors were acquired across unhydrated clinker, hydration products, and the clinker–hydrate interfacial transition zone (micro-ITZ). Within a leak-free pipeline, an XGBoost surrogate model was trained using specimen-grouped nested cross-validation (Nested GroupKFold), achieving cross-validated R² values of 0.878 ± 0.024 (95% CI [0.834, 0.919]) for macro-fracture toughness (\(K_{\text{IC}}^{\text{macro}} \)) and 0.865 ± 0.028 (95% CI [0.815, 0.912]) for fracture energy. (Gf). Leave-one-dosage-out validation demonstrated that when provided with the target mix’s in situ nanoscratch fingerprint, the model successfully tracked the non-monotonic fracture evolution, including the peak at 2.0% NS and the agglomeration degradation at 3.0% NS. Bootstrap Tree-SHAP attribution identified interfacial micro-toughness (\(K_{\text{IC},\text{ITZ}}^{\text{micro}} \)) as the primary predictive contributor (42.5% [95% Cl: 37.8%, 47.1%], revealing a model-suggested transition region around 0.80 MPa m^1/2 above which interfacial crack resistance associates with substantial macroscopic toughening. This framework provides an efficient surrogate approach for evaluating macro-fracture properties from micromechanical indicators.