| Author: | Hao, Zhihao |
| Title: | Durability of FRP reinforced seawater seasand concrete beams and FRP-strengthened reinforced concrete beams |
| Advisors: | Dai, Jian-guo (CEE) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Subject: | Fiber-reinforced concrete Fiber-reinforced plastics Concrete -- Service life Concrete -- Corrosion Concrete construction Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Civil and Environmental Engineering |
| Pages: | xxviii, 351 pages : color illustrations |
| Language: | English |
| Abstract: | The social and economic development of coastal cities is greatly dependent on marine infrastructure, such as harbors, bridges, offshore platforms, and ports. A primary challenge faced by traditional reinforced concrete (RC) structures is steel corrosion, which leads to structural deterioration and subsequent costly repairs and maintenance. To combat the corrosion issue, fiber-reinforced polymer (FRP) composites have gained substantial recognition as a solution with great potential for marine structures due to their excellent corrosion resistance. The use of FRP composites as an alternative to steel reinforcement opens up possibilities for applying seawater and sea sand concrete (SSC) in construction. SSC replaces fresh water and river sand with locally available seawater and sea sand. This not only alleviates environmental concerns associated with traditional concrete (i.e., the wastage of water resources and the over-exploitation of river sand) but also reduces construction costs by using locally available resources. In civil engineering, FRP is commonly utilized in the form of sheets/plates for strengthening existing structures or bars for constructing new structures. Among the various types of FRP, glass FRP (GFRP) and carbon FRP (CFRP) are most popularly used. GFRP offers the advantage of relatively low cost, while CFRP demonstrates excellent mechanical performance. This dissertation is concerned with the long-term durability of both FRP-reinforced SSC beams and FRP-strengthened conventional RC beams. Considering that the service life of FRP-incorporated structures is expected to be much longer than that of conventional RC structures, it is essential to investigate their long-term and life-cycle behavior. Against this background, this dissertation aims to comprehensively investigate the durability performance of FRP-reinforced SSC beams and FRP-strengthened conventional RC beams. Following a thorough review of relevant research, the first part of this dissertation focuses on the durability of GFRP bars. Given the strong link between the long-term performance of GFRP bars and moisture penetration, a comprehensive evaluation of existing FRP moisture diffusion models was conducted, and a new model was proposed to overcome the limitations of current models. Gravimetric experiments were then conducted on GFRP bars with various diameters (i.e., 6, 10, and 14 mm) at different temperatures (i.e., room temperature of about 23 ℃, 40 ℃, and 60 ℃) in water and simulated SSC pore solution. The test data from this study, combined with previously published data, were used to validate the new model. Notably, this model employs three parameters with clear physical meanings, which are more advantageous than other models and can capture the mechanisms of the moisture uptake of GFRP bars in a more realistic way. Consequently, 240 GFRP bars with different diameters (i.e., 6 mm, 10 mm, and 14 mm) were tested in these two solutions (i.e., water and simulated SSC pore solution). The tensile and interlaminar shear properties were evaluated. Results show that the simulated SSC pore solution, due to the presence of alkaline ions, significantly accelerated the degradation of GFRP bars than water. Smaller bars exhibited more substantial degradation in tensile and interlaminar shear strength, with the size effect being more pronounced in the simulated SSC pore solution. Finally, a new model was proposed, incorporating material degradation and diffusion mechanisms. This model demonstrates a good agreement with test results and provides fitting parameters following the Arrhenius relationship. These parameters have clear physical interpretations, offering a deeper understanding of the degradation processes. To enhance usability, this model was simplified, reducing mathematical complexity while retaining underlying mechanisms. Its reliability was confirmed by validation with existing data. The second part of this dissertation investigates the durability of GFRP bar-reinforced SSC beams. Firstly, the durability of the GFRP bar-to-SSC bonds was evaluated through two groups of pullout tests. In group one, GFRP bars were immersed in a simulated SSC pore solution at different temperatures (i.e., 23 ℃, 40 ℃, and 60 ℃) before being embedded in SSC to create bonded joints for bond performance evaluation. In group two, intact GFRP bars were embedded in SSC and subjected to a wet-dry cycling environment for up to 24 months before bond performance assessment. Results show a significant reduction in GFRP bar tensile strength after exposure to the simulated SSC pore solution, leading to a corresponding loss in bond strength. A correlation between bond strength loss and GFRP bar pre-degradation was established. On the other hand, the bond strength of specimens exposed to the wet-dry cycling environment initially increased and then decreased over time. Subsequently, the effects of wet-dry cycling exposure and sustained loads on GFRP bar-reinforced SSC beams were investigated. Results indicate the flexural behaviors of these beams (i.e., load-deflection response, crack patterns, and ductility) exhibited no evident degradation even after two years. Finally, the focus shifts to FRP-strengthened RC beams. 84 bonded joints with various FRP reinforcement and adhesives were used (i.e., CFRP plate/Sika30, CFRP plate/Araldite106, CFRP sheet/SW-3C, and GFRP sheet/Sika330). Their performance was evaluated after up to 110 months of exposure to wet-dry cycling and outdoor environments. Results reveal that the wet-dry cycling environment caused more degradation than the outdoor environment. In the wet-dry cycling environment, CFRP plate/Sika 30 and CFRP sheet/SW-3C bonded joints exhibited superior durability with 97% and 91% bond strength retentions after 110 months, while CFRP/Araldite106 and GFRP sheet/Sika330 bonded joints exhibited larger reductions with 64% and 52% retentions. Following this, the durability of FRP-strengthened RC beams with the same configuration as bonded joints was explored. Results show that CFRP plate/Sika 30 strengthened beams and CFRP sheet/SW-3C strengthened beams showed better durability with 94% and 96% flexural capacity retentions after 110 months of wet-dry cycling exposure. A reduction factor accounting for the decrease in FRP debonding strain due to bond degradation was proposed for practical safety design applications. In summary, this dissertation develops a comprehensive understanding of the long-term performance of FRP and FRP reinforced/strengthened concrete structures in harsh marine environments. This knowledge contributes to a safer and more economical application of FRP in marine concrete infrastructures in Hong Kong and other coastal cities. |
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| Access: | open access |
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