權 錫玟

English

博士学位論文の要旨及び審査結果の要旨 (Summary of Thesis(DR))othe

權, 錫玟

Tohoku University Repository (TOUR) / 東北大学機関リポジトリ

Development of Ultra-High-PerformanceFiber-Reinforced Cementitious Composites usingMulti-Scale Fiber-Reinforcement System著者 權 錫?号 59学位授与機関 Tohoku University学位授与番号 工博第5125号URL http://hdl.handle.net/10097/62794           クォン ソクミン 氏 名          權 錫玟 (Sukmin Kwon) 授 与 学 位          博士（工学） 学 位 授 与 年 月 日          平成 27年 3月 25日 学位授与の根拠法規 学位規則第 4条第 1項 研究科，専攻の名称 東北大学大学院工学研究科（博士課程）都市・建築学専攻 学 位 論 文 題 目          Development of Ultra-High-Performance Fiber-Reinforced Cementitious Composites Using Multi-Scale Fiber-Reinforcement System 指 導 教 員 東北大学准教授 西脇 智哉 論 文 審 査 委 員          主査 東北大学准教授 西脇 智哉 東北大学教授 持田 灯  東北大学教授 前田 匡樹 東北大学名誉教授 三橋 博三 論 文 内 容 要 旨          1. Introduction Nowadays a wide variety of advanced fiber-reinforced cementitious composites (FRCCs) are available. Advanced cementitious composite materials such as ultra-high-performance fiber-reinforced cementitious composites (UHP-FRCCs) are promising materials. UHP-FRCC belongs to the group of high performance fiber-reinforced cementitious composites (HPFRCCs). HPFRCC exhibits pseudo strain-hardening behavior and multiple cracking behavior after the occurrence of the first crack. As a result, it provides an exceptional energy absorption prior to fracture, and undergoes distributed cracking with extremely small crack width prior to crack local ization. Thus, UHP-FRCC can be a viable solution for improving the resistance of buildings and other infrastructures on the basis of their ultra high strength, high ductility, durability and energy absorption capacity compared with normal concrete and conventional FRCC. The motivation for development of UHP-FRCC is the creation of a new versatile material with a large energy-absorption capacity suitable for seismic design applications and with a long service life by virtue of its crack tolerance. The ability to exhibit pseudo-strain hardening in tension is highly dependent on the matrix, the type and the volume fraction of fiber and the bond properties at the interface between the matrix and the fiber. Some researchers have achieved the strain-hardening behavior of UHP-FRCC through various material design techniques. Some of them developed UHP-FRCC with hybrid fiber reinforcing systems where two or more types of fiber were blended. However, they have not succeeded in showing significant improvement of its mechanical performance. Therefore, it is necessary to develop new material design methods that achieve high performance of UHP-FRCC. In addition, the performance of those of UHP-FRCC with conventional design method is highly dependent on fiber volume fraction. On the other hand, increasing fiber volume fraction decreases workability and increases cost of UHP-FRCC. Consequently, with the sufficient performance for solving workability and cost problems, it is required to develop a method that can determine the lowest fiber volume fraction of UHP-FRCC.  UHP-FRCC exhibits multiple cracks to obtain the high mechanical performance. Those cracks have a high possibility to cause deterioration of concrete structures (i.e. decreasing durability of UHP-FRCC due to acceptance of aggressive substance ingress). Therefore, a repair of those cracks generated as a result of requirement of strain hardening behavior is also a very important work to prevent the deterioration of durability of UHP-FRCC. This thesis attempted to develop a new type of UHP-FRCC using multi-scale fiber-reinforcement system. For that purpose, the influence of factors on tensile behaviors of UHP-FRCC is clarified and a material design concept based on a multi-scale fiber-reinforcement system (i.e. fibers of different scales reinforce cracks of the scale corresponding to each fiber) is proposed based on concrete fracture mechanic (i.e. cracks initiate at the micro-scale, and then those cracks propagate) as shown in Fig. 1. Then, the lowest fiber volume fraction to achieve multiple-cracking behavior of the developed UHP-FRCC was determined for reducing cost and increasing workability. Lastly, self-healing capability of UHP-FRCC was investigated to prevent the deterioration of durability even if cracks occur. ? 640 ?2. General literature research In Chapter 2, previous researches were reviewed. Especially, three major subjects (i.e. UHP-FRCC studied by other researchers, analytical model to determine the lowest fiber volume fraction, and self-healing capability of cementitious composites) were introduced. At first, previous researches on some methods to improve tensile performance of UHP-FRCC such as increasing fiber volume fraction, changing geometrical properties of fiber and using hybridization of reinforcing fibers were reviewed. However, those methods did not exhibit sufficient improvement of tensile performance of UHP-FRCC. Therefore, it became obvious that development of a new material design concept to achieve high performance of UHP-FRCC with low fiber volume fraction was required. Secondly, an analytical model to determine the lowest fiber volume fraction based on a cohesive interface analysis was reviewed. A large fiber volume fraction increases the cost and decreases workability of UHP-FRCC. This model might be useful to decrease fiber volume fraction of UHP-FRCC. However, the model was optimized for normal strength FRCC reinforced with monotype fiber. In other words, the model cannot be applied directly to UHP-FRCC reinforced together with micro-, meso- and macrofiber. Therefore, an extension of the analytical model to determine the lowest fiber volume fraction of UHP-FRCC is required. Finally, previous researches of self-healing capability of FRCC were reviewed. Cracks generally cause deterioration of concrete structures. If self-healing capability can be expected for UHP-FRCC, preventing deterioration of durability of UHP-FRCC could be possible even if cracks occur. Therefore, further investigation of self-healing capability of UHP-FRCC is required.  3. Development of multi-scale fiber-reinforcement system In Chapter 3, a new material design concept based on a multi-scale fiber-reinforcement system was proposed. The material design concept was based on fracture mechanics of concrete. The author focused on cracks initiated at the micro-scale level, i.e. on microcracks in the so-called fracture process zone (FPZ). The idea of multi-scale fiber-reinforcement system is given in Fig. 1. Microfibers can bridge microcracks more efficiently, because they are very thin and the number in cementitious composites is much larger than those of meso- and macrofibers. The ubiquitous distribution of microfibers controls the microcrack formation in FPZ. This phenomenon occurs in the first phases of the fracture process under tensile loading and thus the microfiber can increase the tensile performance. As the microcracks grow and combine into mesocracks, the mesofibers become more active in crack bridging. The pullout resistance of mesofibers increases due to additional bond stress caused by snubbing friction and the microfiber helps the increase of the snubbing friction mechanism. As the mesocracks grow and join into larger macrocracks, macrofiber becomes more active in crack bridging. In this way, primarily the ductility and also the tensile strength can be improved. Macrofibers can provide the UHP-FRCC with a stable post-peak response. From a mechanical point of view a material design concept based on multi-scale fiber-reinforcement system will be an effective solution to improve tensile performance including tensile strength and ductility. In this chapter, the author defined the micro-, meso- and macro-scales as being on the orders of 10-1 mm or less, 100 mm, and 101 mm or more, respectively. The first step was to confirm effectiveness of fibers on each scale and their combinations. Then the second one was to experimentally verify that multi-scale fiber-reinforcement system exhibits synergistic effects. Furthermore, the author proposed a method for optimizing the shape and size of fibers corresponding to various crack scales and for improving the tensile properties of UHP-FRCCs by reinforcing them from the micro- to the macro-scale. For that purpose, effectiveness of different types of fiber on the reinforcement and the mechanism by which multi-scale fiber-reinforcement system works were studied. Then, based on the proposed material design concept, an optimum  Fig. 1 Schematic illustration of multi-scale fiber-reinforcement system Fracture process zoneMacrofiberbridgingmacrocrackMesofiberbridgingmesocrackPropagation of crackMicrofiberbridgingmicrocrackMagnified image? 641 ?tensile response of the UHP-FRCC with regard to tensile strength, tensile strain capacity and energy absorption capacity was determined by means of tension tests. As a result, the microfiber was very effective to improve toughness. Mesofiber showed a positive effect to prevent brittle failure and to improve tensile strength. In addition, macrofiber was very effective for improving ductility of UHP-FRCC. Finally, UHP-FRCC made by the proposed material design concept based on the multi-scale fiber-reinforcement system exhibited significant improvement of tensile properties comparing with those by monofiber reinforcement and with previously reported UHP-FRCCs as shown in Figs. 2 and 3, respectively. Therefore, the multi-scale fiber-reinforcement system was finally employed to establish a new design concept.        Fig. 2 Tensile stress–strain curves at the same fiber volume fraction Fig. 3 Influence of the total volume fraction of fibers on tensile strain capacity on the comparison with previously reported UHP-FRCCs  4. Proposal of extended analytical model to determine the lowest fiber volume fraction of UHP-FRCC Chapter 4 was initiated to determine the lowest fiber volume fraction with strain hardening behavior of developed UHP-FRCC. High volume fraction of fiber to obtain ductility reduces workability and increases cost. Therefore, reduction of the fiber volume fraction is one of important works to solve workability and cost problems of UHP-FRCC. As introduced in Chapter 2, analytical model was proposed to determine the lowest volume fraction of fiber. However, this proposed model was optimized for normal strength FRCC reinforced with monofiber. To apply the analytical model to UHP-FRCC, there were two obstacles. One was the fact that UHP-FRCC contains three different types of fibers. The other was that ultra-high strength mortar (UHSM) was too brittle to carry out the uniaxial tension test. For overcoming these obstacles, extended analytical model to determine the lowest fiber volume fraction of UHP-FRCC was proposed. The first obstacle was overcome as follows. Hybrid fiber composites showed a multiscale structure of cracking, in which macrofibers played the main role to prevent the sudden propagation of macro-cracks. Therefore, microfibers and mesofibers can be smeared in the cement-based matrix. Then, a method for determining the cohesive parameter (i.e. kC) of UHSM containing micro- and mesofiber was  Fig. 4 Range of possible crack spacing values in the UHP-FRCC ? 642 ?developed to apply the analytical model to UHP-FRCC. In addition, uniaxial tension test was performed to determine the bond parameter (i.e. kB) on the basis of crack spacing data in the UHP-FRCC. As a result, the lowest fiber volume fraction of UHP-FRCC using multi-scale fiber-reinforcement system was determined as shown in Fig. 4. Proposal of the extended analytical model to determine the lowest fiber volume fraction of UHP-FRCC can be useful to solve the problems due to cost and workability.  5. Experimental study on self-healing capability of cracked UHP-FRCC Chapter 5 was to demonstrate the self-healing capability to prove the durability of cracked UHP-FRCC. For making UHP-FRCC ductile, crack tolerance is essential. There is a common belief that cracks cause deterioration of concrete structures. Therefore, it is necessary to repair for preventing the deterioration of durability of UHP-FRCC when cracks occur. If those cracks are self-healed, decreasing durability of UHP-FRCC might be compensated. For this purpose, an experimental series was performed to investigate the self-healing capability (i.e. filling the crack to prevent deterioration of durability) of the UHP-FRCC. As a result, self-healing of cracks in UHP-FRCC was confirmed as shown in Fig. 5. These results proved that the durability problem of UHP-FRCC can be solved by means of self-healing capability even if cracks occur to obtain ductile performance. In addition, no corrosion of steel fiber was confirmed. Because of the self-healing, UHP-FRCC could possibly protect itself against the ingress of aggressive agents, even if cracks occur.  6. Conclusions In this thesis, a new material design concept for the development of UHP-FRCC using multi-scale fiber-reinforcement system has been made. It has been demonstrated that applying a multi-scale fiber-reinforcement system to UHPC materials is a promising approach in the development of a very strong and ductile UHP-FRCCs. In addition, proposal of the extended analytical model to determine the lowest fiber volume fraction of UHP-FRCC can be useful to decrease cost and to increase workability. Moreover, durability problem of UHP-FRCC can be solved by means of self-healing capability even if cracks occur. Developed UHP-FRCC with an ultra-high compressive strength, tensile strength, tensile strain capacity and energy-absorption capacity are expected to lead to versatile structural applications. This is especially true from a seismic design perspective because of the large energy-absorption capacity. In addition, this thesis will provide engineers with a useful asset in material design method for UHP-FRCC, which is a promising material, for realizing the safety and security in buildings and infrastructures of next generation.  Before self-healing                After self-healing   Fig. 5 Time dependence of self-healing 0.1 mm 0.1 mmCracksSelf-healing products? 643 ?? 644 ?

Development of Ultra-High-Performance Fiber-Reinforced Cementitious Composites using Multi-Scale Fiber-Reinforcement System

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多段階繊維補強による超高強度・高靱性繊維補強セメント複合材料の開発

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