Multiphase Characteristics of Carbon Fiber-Reinforced Cementitious Materials Under Static and Freeze-Thaw Cyclic Loading Conditions

Date

2023-11-24

Authors

Monazami, Maryam

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Abstract

Aging concrete infrastructure, particularly in colder climates like Canada, demands urgent maintenance and renewal due to the severe temperature variations. These conditions lead to issues such as cracking, spalling, and overall deterioration. To ensure the longevity and functionality of infrastructures, it is crucial to use durable and high-performance materials. Adding fibers to the concrete mix can improve its toughness and resistance to cracking. Fiber-reinforced concrete (FRC) can withstand higher tensile stresses and distribute loads more effectively, increasing the overall durability. Among various types of fibers, carbon fibers (CFs) have gained significant popularity due to their unique ability to confer self-deicing properties to cementitious materials. This characteristic holds particular importance in colder climates, where maintaining safe and accessible infrastructure, during harsh winter conditions is paramount. Carbon fiber-reinforced concrete (CFRC) has various advantages over normal concrete, including self-deicing, high strength, durability, and corrosion resistance. CFRC's self-deicing capability is achieved through the electrical conductivity of CFs, which allows an electric current to be applied to generate heat and melt ice or snow. This feature improves safety by preventing icy surface conditions and lowers maintenance costs for snow removal and deicing chemicals. CFRC is also highly durable and strong, making it suitable for infrastructure and architectural construction. Additionally, its resistance to corrosion ensures long lasting performance and extends the lifespan of CFRC structures. Integrating self-deicing CF reinforcement within concrete bus pads offers a practical approach to leverage their inherent self-deicing property, resulting in heightened passenger safety and convenience throughout the winter season. With CFs generating heat to melt accumulated ice and snow, the bus pads retain a snow-free surface, thereby mitigating the potential for slips and accidents among passengers and pedestrians. This endeavor directly fosters a transit environment that is safer and more accessible. While numerous studies have studied self-deicing characteristics of CFRC in colder climates, a notable research gap exists in examining how CFRC responds to the rigorous challenges of freezing and thawing (FT) cycles. Despite the extensive exploration of CFRC's ability to melt ice and snow, the absence of investigations into its deterioration behavior under cyclic freezing and thawing conditions is a critical oversight. This dissertation aims to fill the existing knowledge gap and challenges related to the performance assessment of CFRC under cyclic freezing and thawing loading conditions, as well as introducing an optimized mix design for concrete suitable for colder climates. The research methodology involves a comprehensive investigation that incorporates both destructive and non-destructive testing techniques. It is clear that a multitude of pertinent factors, encompassing factors such as fiber and aggregate type, fiber length, cement paste composition, and different admixture can have significant impacts on the performance of cementitious composites. Within the context of this dissertation, however, the study has meticulously centered its investigative on carbon fiber's physical properties and its concentration. In the pursuit of refining the mix design to attain optimal outcomes, the research engaged in an array of destructive analyses, including compressive strength tests, splitting tensile strength tests, and flexural strength tests. These tests provide insights into the strength and structural behavior of CFRC under FT conditions, allowing for an evaluation of its performance. In conjunction with conventional destructive tests, this research integrated non-destructive testing (NDT) methodologies to appraise the structural integrity and quality of the CFRC specimens. Employing advanced techniques including ultrasonic testing, rebound hammer analysis, and ground-penetrating radar, a comprehensive evaluation was systematically conducted on CFRC samples subjected to an extensive and rigorous regimen of 300 FT cycles. Throughout this demanding exposure, the samples underwent the complete array of non-destructive assessments at regular 30-cycle intervals. This approach was undertaken to meticulously discern and analyze the cumulative deteriorative effects that emanated from the repetitive FT cycles. These insights yielded a profound understanding of the durability performance of CFRC under the persistent challenge of FT conditions. The synergistic integration of both destructive and non-destructive testing methodologies yields a holistic and nuanced comprehension of CFRC performance in areas with colder climate such as Canada. This assimilated knowledge stands as a pivotal cornerstone for the formulation of an intricately optimized mix design, one fortified to effectively withstand the challenges imposed by cyclic FT cycles. The research outcomes have the potential to contribute to the advancement of CFRC technology, enabling its effective use in regions with colder climates and facilitating the construction of durable and resilient infrastructure in such areas. The dissertation is divided into three milestones, each with its own set of objectives and tasks, to systematically address the research questions and challenges related to CFRC. Milestone 1 encompassed a comprehensive evaluation of mechanical properties and physical properties in different carbon fiber types, emphasizing a comparative analysis on commonly used CFs. The research extended to a novel bitumen-based carbon fiber (BBCF) from Alberta, seeking to understand its microstructure and potential for market adaptability. Techniques such as scanning electron microscopy (SEM) and energy dispersive x-ray (EDX) spectroscopy, along with mechanical and electrical tests, were incorporated to assess the behavior of different types of CFs. Milestone 1 also presented a novel method using a supplementary cementitious materials (SCM) fiber coating technology. This breakthrough improved the interfacial transition zone (ITZ) between fibers and the cement matrix, resulting in improved composite performance. The goal of this milestone was to meticulously compare and establish correlations between the diverse properties exhibited by various fiber types. This systematic investigation attempted to identify the best fiber choice for incorporation into cementitious materials, thereby improving the cementitious composite's overall performance. Milestone 2 shifted the focus to investigating the mechanical and fracture behavior of carbon fiber-reinforced cementitious composite (CFRCC) and the interrelationship between materials properties and mechanical performance. A systematic approach for Laboratory testing and structural analysis has been presented in this milestone. Uniaxial tension tests were performed on dog bone-shaped Carbon Fiber Reinforced Mortar (CFRM) to analyze the behavior of samples subjected to axial tensile forces. Flexural characteristic of CFRC samples is key parameter that involves composite behavior under bending loads. While flexural testing often employs beams, it may not effectively represent the performance of fiber reinforced concrete due to considerable differences in cracking behavior of FRC with normal concrete. This discrepancy is particularly noticeable in slab and pavement applications, owing to the substantial variability in flexural behavior observed in Fiber-Reinforced Concrete (FRC) beams. Additionally, the smaller fracture area resulting from a lower count of fibers further compounds this distinction. During this milestone, a thorough and comprehensive analysis was conducted, focusing on the flexural strength of both round panels and beams. The flexural failure observed in round panels closely emulated the behavior seen in structural slabs, aligning with the principles of the yield line theory. The characterization of flexural behavior involved toughness indices and key flexural strength parameters, including bending strength and modulus of elasticity. This analysis process ultimately led to the identification of an optimal mix design. This finding underscores the significance of fiber content in influencing the overall behavior and performance of CFRC composites. Furthermore, to compare the experimental results of CFRC beam and panel flexure behavior, an analysis of variance was conducted. This statistical examination unveiled a notable 41% discrepancy in flexural properties between the two distinct sample geometries. This observation highlights the importance of considering sample geometry when assessing the flexural behavior of CFRC materials. Milestone 2 also involved a meticulous investigation into the FT behavior of the CFRC samples, evaluating their durability under the stress of 300 FT cycles. By studying the FT performance of the CFRC samples, the research aims to gain insights into the durability and resistance of CFRC to the effects of FT cycles, which can include cracking, spalling, and degradation. This information is valuable for assessing the suitability and long-term performance of CFRC in colder climates, where FT cycles are a significant concern. In Milestone 3, a case study was conducted to evaluate the durability of an electrically conductive CFRC bus pad. The case study involved integrating sensors within the bus pad to monitor factors such as strain, temperature, and moisture content. The goal was to assess the performance and behavior of the CFRC bus pad in real-world conditions. The research project aimed to gain insights into the structural integrity and durability of the CFRC bus pad by continuously monitoring its performance using embedded sensors. These sensors provided data on factors such as strain, temperature variations, and moisture content, allowing assessment the material's response to environmental influences. The study also focused on understanding how the CFRC bus pad performed under different operating conditions and evaluate its ability to withstand environmental factors. The primary objective of the research project in this milestone was to conduct a comprehensive performance assessment of CFRC bus pads, encompassing both short-term and long-term evaluations. Through this assessment, the study sought to gain profound insights into the behavior and durability of CFRC bus pads. The findings derived from this significant milestone played an important role in enhancing the design and performance of CFRC materials, ensuring their suitability for practical applications such as bus pads. Moreover, these findings hold the potential to inform and shape future advancements in electrically conductive CFRC materials for various other purposes

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Keywords

Concrete, Carbon finer, Freeze-thaw performance, Non-destructive testing, microstructure, bitumen

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