Freeze-thaw cycles coupled with sulfate attack represent one of the most challenging service environments for concrete. This study aims to enhance the durability of concrete materials in environments characterized by sulfate attack and severe freeze-thaw conditions. Specifically, it investigates the deterioration laws and evolution models of mortar materials containing silica fume under both freeze-thaw and coupled freeze-thaw/sulfate attack conditions. Mortar specimens with varying silica fume contents (0%, 6%, 8%, and 10%) were prepared and subjected to single freeze-thaw and coupled freeze-thaw-sulfate attack tests to examine the impact of different silica fume dosages on the durability of mortar materials under these harsh conditions. Additionally, a quantitative assessment model for damage evolution was established using the entropy weight method and Wiener process model. The research findings indicate that silica fume significantly enhances the sulfate resistance and freeze-thaw durability of mortar materials, with an optimal dosage of 10%. Within the scope of this study, higher silica fume content results in a greater number of sulfate attack-freeze-thaw cycles the mortar can endure before damage and failure, thereby extending its service life. Based on the Wiener stochastic process damage model and field data, it is predicted that the service life of mortar containing 10% silica fume increases most notably to 36.6 years, representing a relative improvement of 45.8 % compared to mortar without silica fume. These results provide valuable references and guidance for the design and construction of concrete structures in regions characterized by high-cold temperatures and salt- corrosive soils.

This research investigates a methodology for probabilistic life prediction of buried steel pipelines subjected to external corrosion. A unified methodology is developed considering multiple stages of degradation related to external corrosion (due to soil) and fatigue. These stages include corrosion pit nucleation, pit growth, transition from pit to short crack, short crack growth, transition from short to long crack, stable long crack growth, and unstable fracture. The methodology is useful in obtaining stage-specific forecasts for the fatigue life of buried steel pipelines subjected to external pitting corrosion fatigue. State-of-the-art computational models are used to predict damage initiation and evolution at each stage. The variability in environmental, material, and loading parameters is propagated through these models to obtain a probabilistic estimate of the remaining service life (RSL) of the pipe. Insights from probabilistic RSL prediction highlight the influence of soil type and pipe coating material on corrosion fatigue life. Global sensitivity analysis is then employed to quantify the relative importance of environmental factors (pH, pipe/soil potential, and chloride concentration), material properties (threshold stress intensity factor), and the range of cyclic stress experienced by the pipe.

With the development of urban rail transit, the subway inevitably needs to run through the main channel of the South-to-North Water Diversion. With the subway being put into operation, the deformation during the construction period has been gradually stabilized, and the cyclic vibration load of the train during the operation period has gradually affected the channel structure. Therefore, in order to further investigate the effect of vibration load on the structure of the Middle Route of the South-to-North Water Diversion Project during the operation period, this paper, based on the actual cases of undercrossing projects in different regions, established a tunnel-soil-channel finite element model considering silty clay and fine sand under different burial depths by using ANSYS. The vertical vibration levels of the roadbed, tunnel wall and channel bottom are extracted and compared with the measured vibration acceleration levels. The dynamic displacement and maximum tensile (compressive) stress under different working conditions are analysed, and the fatigue life of the canal concrete structure is predicted using the obtained maximum tensile (compressive) stress. The results show that under the condition of fine sand, 1 times the hole diameter is the most unfavourable condition. In the prediction of concrete fatigue life by S-N equation, the number of trains that can pass under the most unfavourable condition is about 6.68 x 1010, which is far more than the number of trains that can pass within the service life of the tunnel.

Ultra-high performance concrete (UHPC) exposed to the harsh western saline soil environments in western China experience accelerated damage due to the combined effects of dry-wet cycles, corrosive salt ions, extreme temperatures, and freeze-thaw cycles. This study developed a laboratory erosion protocol to simulate these conditions and evaluate the sulfate resistance of UHPC, investigating the degradation mechanisms associated with variations in water-binder ratio, silica fume content, and fiber type. Wiener theory was employed to predict the lifespan of various UHPC mixtures exposed to these conditions. The results indicate that UHPC demonstrates negligible degradation in performance under erosion simulation conditions when the water-to-binder ratio for the UHPC is 0.20, the silica fume content (relative to the total cementitious material content) is 26 %, and steel fibers are used. After 240 days of erosion, the compressive strength, bending strength and equivalent bending toughness of UHPC reinforced with polyvinyl alcohol (PVA) fiber decreased by 7.79%, 35.48% and 42.01 % respectively, with a decrease in the relative dynamic modulus of elasticity to 97.29%. These declines were more pronounced than in specimens with steel fibers. Phase composition and micro-structural analyses identified that the primary products of sulfate attack in UHPC as ettringite and gypsum, alongside the physical crystallization of anhydrous sodium sulfate, which induced expansion and crystallization stress, forming harmful pores and microcracks. A reliability function curve, based on compressive strength, effectively modeled the degradation process of UHPC under these conditions, predicting a potential durability lifespan exceeding 70 years in western saline soil environments.