Rahat, Hasibul Hasan2025-07-192025-07-192025-07-18vt_gsexam:44348https://hdl.handle.net/10919/136865Concrete infrastructure in the Arctic coastal region faces durability challenges from freeze-thaw (F-T) cycles and chloride ingress due to seawater and deicing salts, resulting in surface scaling, spalling, and cracking. Climate change exacerbates these issues, particularly in high-latitude areas like Alaska, where rising temperatures lead to permafrost thaw and increased infrastructure vulnerability. Projections indicate that climate-related infrastructure costs in Alaska could reach $5.5 billion by 2099, highlighting the need for more resilient infrastructure. This dissertation investigates F-T damage mechanisms, explores strategies to enhance cementitious material durability, optimizes repair techniques for long-term performance, and develops a real-time monitoring technique to track pore water movement within the concrete microstructure and its impact on microstructural cracking. First, this dissertation investigated the combined effects of F-T cycles and chloride ingress on conventional (Group A) and polyvinyl alcohol (PVA) fiber-reinforced (Group B) concretes cured for 14 days and 70 days in plain water and in simulated seawater conditions. This dissertation also utilized a non-destructive technique, transmission X-ray microscopy (TXM), to quantify the diffusion coefficient (Dc) as a function of F-T cycles for the first time in the literature. The results emphasize the importance of the air void system in reducing F-T damage and chloride ingress, with longer curing durations enhancing concrete's resistance to these issues. Group A showed superior performance due to its improved air void system, which enhanced F-T resistance by accommodating internal pressure changes and reducing crack propagation. In addition, seawater exposure exacerbated F-T damage, accelerating chloride ingress and deterioration compared to F-T cycles in plain water. The findings suggest that while PVA fibers enhance mechanical properties, they may also disrupt the air-void system, reducing its protective capabilities against chloride ingress and F-T damage. These results emphasize optimizing air void characteristics in fiber-reinforced concretes to balance mechanical performance with long-term durability in harsh environmental conditions. Second, this dissertation explored the role of cellulose nanofiber (CNF) gels in enhancing F-T resistance and reducing chloride ingress in cement paste. CNF suspensions, prepared using a nitro-oxidation process, formed hydrogels in the presence of metal ions (e.g., Na+, Ca2+) within the pore solution, reducing the freezing point of the pore solution of cement paste. It was found that specimens containing CNF suspensions exhibited improved F-T resistance characteristics when compared to specimens without CNF suspensions. Additionally, TXM analysis showed that specimens containing CNF suspensions exhibited significantly lower diffusion coefficients than conventional cement paste. These findings suggest that CNF gels reduce permeability and enhance durability by modifying the microstructure of the cement paste. Third, this dissertation investigated the durability of concrete repair-substrate interfaces under F-T cycling, emphasizing the impact of substrate moisture conditions and water-to-cement (w/c) ratios on the overlay transition zone (OTZ), the weakest region of repair-substrate concrete analogous to the interfacial transition zone between aggregates and paste in concrete microstructure. Mechanical properties, fracture characteristics, ion diffusion, water absorption, and F-T resistance were evaluated to assess repair performance under varying conditions. The results demonstrated that repair overlays with lower w/c ratios applied to saturated surface-dry (SSD) substrates exhibited superior durability, reducing water absorption, ion penetration, and F-T damage compared to higher w/c ratio repairs. TXM analyzed the time-dependent diffusion along the OTZ for the first time in the literature, finding that repair-substrate specimens showed a notable increase in diffusion coefficient after F-T cycles compared to the substrate. The findings indicate that in the absence of surface preparation, a lower w/c ratio is recommended to mitigate OTZ deficiencies and enhance the durability of concrete repairs in cold and F-T environments. Finally, in situ neutron radiography was utilized for the first time in the literature to track real-time pore water movement in concrete macropores under subzero conditions. The results demonstrated that unfrozen water moved away from the freezing front, which is suspected to generate hydraulic pressure and contribute to microcracking. These findings underscore the role of moisture redistribution and full saturation in F-T damage, providing insights for creating more durable concrete in cold regions. In summary, this dissertation integrates advanced experimental techniques to develop a mechanistic understanding of the deterioration of cementitious materials under extreme environmental conditions. By combining TXM, neutron radiography, and comprehensive materials testing, the findings provide new insights into the interplay between F-T damage and chloride ingress. These results inform the design of more resilient cementitious materials, repair strategies, and construction practices for arctic coastal regions, ultimately contributing to infrastructure longevity and sustainability in the face of climate change.ETDenIn CopyrightArcticfreeze-thaw resistancetransmission X-ray microscopypolyvinyl alcohol fibercuring periodchloride ingressdiffusion coefficientsneutron radiographyconcrete repairDissertation