This study elucidated the evolution and catastrophic failure mechanisms of concrete’s mechanical properties under high-temperature and moisture-coupled environments. Specimens underwent hygrothermal shock simulation via constant-temperature drying (100 °C/200 °C, 4 h) followed by water quenching (20 °C, 30 min). Uniaxial compression tests were performed using a uniaxial compression test machine with synchronized multi-scale damage monitoring that integrated digital image correlation (DIC), acoustic emission (AE), and infrared thermography. The results demonstrated that hygrothermal coupling reduced concrete ductility significantly, in which the peak strain decreased from 0.36% (ambient) to 0.25% for both the 100 °C and 200 °C groups, while compressive strength declined to 42.8 MPa (−2.9%) and 40.3 MPa (−8.6%), respectively, with elevated elastic modulus. DIC analysis revealed the temperature-dependent failure mode reconstruction: progressive end cracking (max strain 0.48%) at ambient temperature transitioned to coordinated dual-end cracking with jump-type damage (abrupt principal strain to 0.1%) at 100 °C and degenerated to brittle fracture oriented along a singular path (principal strain band 0.015%) at 200 °C. AE monitoring indicated drastically reduced micro-damage energy barriers at 200 °C, where cumulative energy (4000 mV·ms) plummeted to merely 2% of the ambient group (200,000 mV·ms). Infrared thermography showed that energy aggregation shifted from “centralized” (ambient) to “edge-to-center migration” (200 °C), with intensified thermal shock effects in fracture zones (ΔT ≈ −7.2 °C). The study established that hygrothermal coupling weakens the aggregate-paste interfacial transition zone (ITZ) by concentrating the strain energy along singular weak paths and inducing brittle failure mode degeneration, which thereby provides theoretical foundations for fire-resistant design and catastrophic failure warning systems in concrete structures exposed to coupled environmental stressors.

Water seepage in saline soils can compromise the stability of landfill cover layers and cause ecological pollution. Biochar, recognized as a novel sustainable material for soil improvement, has garnered significant attention due to its large specific surface area and complex pore structure. Its addition can alter the physico-chemical properties of soil, including pore structure characteristics, compaction behavior, and liquid seepage paths. This study therefore focuses on saline soil, investigating the effects of biochar type and addition rate on the soil’s hydraulic conductivity through variable-head permeability tests. The correlation between changes in microscopic structural characteristics and hydraulic conductivity was analyzed using SEM and MIP techniques. The results indicate the following: (1) Both corn straw and wood biochar affect the hydraulic conductivity of the modified soil, with the degree of influence varying based on the presence and dosage of biochar, generally leading to a reduction. (2) The corn straw biochar-modified soil exhibited a more significant decrease in hydraulic conductivity—by approximately one order of magnitude—compared to wood biochar, reaching a minimum value of 1.04 × 10⁻⁶ cm/s at a 10% addition rate. (3) Analysis from SEM and MIP tests reveals that incorporating biochar alters the original pore structure of the soil, causing macropores to transition towards mesopores and micropores, and resulting in the narrowing or clogging of pore channels, which ultimately reduces the hydraulic conductivity. The finding that biochar amendment decreases hydraulic conductivity provides valuable data to support the design of landfill cover layer schemes.