| Author: | Peng, Kaidi |
| Title: | Strengthening of reinforced concrete beam using geopolymer bonded mini CFRP bar |
| Advisors: | Dai, Jian-guo (CEE) |
| Degree: | Ph.D. |
| Year: | 2022 |
| Subject: | Concrete beams Fiber-reinforced concrete Reinforced concrete construction Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Civil and Environmental Engineering |
| Pages: | xxviii, 248 pages : color illustrations |
| Language: | English |
| Abstract: | Fiber-reinforced polymer (FRP) composites have been widely used in civil engineering because of their lightweight, high strength, good corrosion resistance, and convenience for construction. The use of externally bonded (EB) FRP composites for strengthening existing concrete/steel structures has become prevalent. However, this technology still faces some challenges in practical applications, particularly in an environment where moisture or fire is a critical concern. Recently, fiber-reinforced cementitious matrix (FRCM), in which an inorganic cementitious material is used to replace the polymer material (usually epoxy) as the bonding adhesive, has drawn increasing attention. In the existing literature, FRCM is more promising than EB-FRPs in moist environments or under elevated temperatures. This dissertation aims to develop a new FRCM system based on mini carbon FRP (CFRP) bars and inorganic geopolymer binder (so-called FRGM). A comprehensive experimental and analytical program has been carried out on the structural performance of RC members strengthened with the FRGM system and the interface bond between the FRGM layer and the concrete substrate. The first part of this dissertation focuses on the bonding behavior of geopolymer and concrete substrates. A series of experimental tests, including slant shear test, direct tensile pull-off test, flexural test, and micro-scale observations, were conducted to investigate the geopolymer/concrete bond performance and the mechanical and chemical bonding mechanisms at the interfacial transition zone (ITZ). Three geopolymer mortars (using different aluminosilicate precursors) and their secondary hydration mechanisms with the concrete substrate were compared through microscopic analysis. The purpose was to obtain an optimal mixture for geopolymer mortar to achieve a reliable bond performance with concrete substrate. The second part of this dissertation presents a comprehensive experimental program on the bond behavior between mini CFRP bars and geopolymer mortar. An in-depth understanding of the bond behavior between mini CFRP bars and geopolymer is essential for utilizing mini CFRP bar-reinforced geopolymer in strengthening concrete members. In this regard, pull-out tests on 81 geopolymer specimens with an embedded mini CFRP bar in each specimen were conducted. The experimental tests focused on some key factors that govern the bond behavior of mini CFRP bars in geopolymer matrix, including geopolymer type (contents of fly ash and slag), the embedded length of mini CFRP bars (5d, 10d, and 15d), the volume fraction of fibers (0, 1%, and 2%) in the geopolymer matrix and the diameter of CFRP bars (3, 6 and 10 mm). The pull-out failure mechanisms were clarified, and the bond-slip curves of mini CFRP bars in the geopolymer mortar were obtained. It was found that the bond strength of mini CFRP bars was primarily related to the geopolymer strength, embedment length, and bar diameter. The mini CFRP bar with a diameter of 3 mm embedded in high strength geopolymer (i.e., with high content of slag) exhibited the highest bond strength. This finding laid a solid foundation for the strengthening of RC members using the FRGM system in the following chapters. Then, a modified bond strength model of mini CFRP bars in the geopolymer mortar was proposed based on the current experimental results. The third part of this dissertation presents comprehensive experimental and theoretical investigations on the flexural behavior of RC beams strengthened by the FRGM system. This section aimed to evaluate the efficiency of the FRGM system for flexural strengthening of RC beams. A total of 11 beams were tested, including ten strengthened beams and one reference beam. The influences of three major factors on the strengthening performance were investigated, including the bonding method (geopolymer vs. epoxy), fiber reinforcement in the matrix (plain geopolymer vs. fiber- reinforced geopolymer), and the number/diameter of CFRP bars (7Φ3, 2Φ6, and 2Φ10). It was found that the strengthened beams showed significantly higher stiffnesses, yield loads, and ultimate loads than the control beam and the geopolymer-bonded layer showed similar strengthening efficiency with the epoxy-bonded layer. For the FRGM strengthening system, Φ3 mini FRP bars are more suitable due to the larger specific surface area. The use of short fiber in the geopolymer matrix further improved the crack control capacity. An analytical investigation was conducted to predict the yield and ultimate loads of the strengthened beams. Finally, a practical application case was presented to demonstrate the feasibility of this method in strengthening the concrete superstructure at The Port of Zhanjiang (Guangdong, China). The last part of this dissertation presents experimental and theoretical studies on the shear behaviors of RC beams strengthened by the FRGM system. A total of 12 RC beams, including two reference beams and ten strengthened beams, were prepared and tested using three-point loading tests. Five different parameters were considered in the tests, including the bonding method (geopolymer vs. epoxy bonding), fiber reinforcement in the matrix (plain geopolymer vs. fiber-reinforced geopolymer), the alignment direction of mini CFRP bars [90 degrees (vertical) vs. 45 degrees to the longitudinal direction], the configuration of the strengthening layers (single side vs. double sides), and the shear span-to-depth ratio of RC beams (2.43 vs. 3.18). The results indicated that the RC beams with FRGM strengthening layer gained significantly improved ultimate shear capacity compared with the reference beam. The geopolymer-bonded layer showed a similar shear strengthening efficiency with the epoxy-bonded counterpart. The ultimate load of the double-side strengthened RC beams was more than two times that of the single-side strengthened RC beams. Regarding the FRGM strengthening layer, using additional steel fibers in the geopolymer matrix can further restrict the development of shear cracks and thus improve the shear capacity. The mini CFRP bars with 45-degree alignment led to higher flexibility and shear capacity of the RC beam. The theoretical analysis solution was conducted for predicting the shear capacity of RC beams strengthened with the FRGM system. In summary, an innovative strengthening method using geopolymer-bonded mini FRP bars has been proposed and developed. Multiple levels of experimental tests were extensively conducted, including the properties of mini FRP bars, bond properties of mini FRP bar and geopolymer matrix, bond properties of geopolymer matrix and the concrete substrate, FRGM-flexural-strengthened RC beams, and FRGM-shear-strengthened RC beams. The developed strengthening method is particularly suitable for applications in marine concrete structures. A widespread application of the technology will contribute to the improved serviceability, safety, and durability of existing concrete infrastructures, leading to long-term socio-economic benefits for the construction industry. |
| Rights: | All rights reserved |
| Access: | open access |
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