Performance Enhancement of Ultra-High-Performance Fiber-Reinforced Concrete and Model Development for Practical Utilization
저자
발행사항
서울 : 고려대학교 대학원, 2014
학위논문사항
학위논문(박사)-- 고려대학교 대학원 : 건축·사회환경공학과 구조공학전공 2014. 8
발행연도
2014
작성언어
영어
주제어
발행국(도시)
서울
형태사항
627 p ; 26 cm
일반주기명
지도교수: 윤영수
DOI식별코드
소장기관
Until today, extensive efforts have been made to increase the strength of concrete. However, as an inevitable result of brittle failure is accompanied by the increase of strength, the application of high-strength concrete in practice has been limited. The incorporation of fibers into the higher strength concrete can nullify this brittle failure by improving ductility, fracture toughness, and energy absorption capacity via fiber bridging. Especially, the recently developed ultra-high-performance fiber-reinforced concrete (UHPFRC) exhibits superior strength and ductility, as well as exceptional durability by optimizing the granular mixture with a low water-to-binder ratio, which leads to the homogenization of microstructure, and by incorporating a high amount of steel fibers. These advantages result in the substantial decrease in the self-weight of structural members made of UHPFRC by decreasing the cross-sectional area and the increase in the service life of concrete structures. Thus, many civil engineers are nowadays attractive to use this material.
To practically use such a newly developed UHPFRC in the structures, three major properties such as material properties, bond performance between the concrete and the reinforcement, and structural performance should be investigated. Furthermore, adequate analytical and design techniques need to be proposed. Therefore, in this dissertation, the three major properties of UHPFRC were investigated along with the development of several useful models for numerical analysis and structural design.
In addition, to overcome the obstacles for its practical utilization (i.e., high potential of shrinkage cracking and high cost), three variables (i.e., degree of restraint, content of shrinkage reducing admixture (SRA), and rebar type) for reducing the shrinkage cracking potential and two variables (i.e., fiber length and placement method) for reducing the cost by improving the tensile performance were considered. These variables were properly incorporated into the categories of the three major properties mentioned in above.
Firstly, the material properties including shrinkage and mechanical properties were investigated. In the case of shrinkage properties, to improve the restrained shrinkage performance, two different types of restrained shrinkage tests (i.e., ring-test and autogenous shrinkage stress test) were carried out according to the size of ring for degree of restraint, SRA content, and rebar type. Free shrinkage and uniaxial tensile tests were also performed from the very early age to precisely evaluate the various parameters with regard to the restrained shrinkage performance. For the case of mechanical properties, to solve the problems of UHPFRC restricting its application, three main variables were firstly determined, and then the mechanical properties were investigated according to these three variables including the SRA content, fiber length, and placement method. In addition, the optimized tension-softening models for UHPFRC were suggested from the fracture mechanics-based inverse analysis and micromechanics-based modeling considering fiber orientation.
Secondly, the bond performance of three different rebars embedded in UHPFRC was investigated. Since JSCE recommendation (2004) suggested to carefully use the deformed steel rebar with UHPFRC due to its high potential of shrinkage cracking, two alternative reinforcements (i.e., round steel rebar and glass fiber-reinforced polymer (GFRP) rebar) were investigated along with the deformed steel rebar. Based on the test results obtained from the present study and literature, normalized bond strength, development length, and local bond stress-slip models were suggested.
Thirdly, the structural performance of UHPFRC beams reinforced with deformed steel and GFRP rebars was investigated under quasi-static and impact loading conditions. Round steel rebar was not investigated due to its poor bond performance, and SRA was not included due to its poor mechanical performance. For the case of quasi-static loading condition, the flexural behavior of UHPFRC beams reinforced with deformed steel and GFRP rebars and hybrid reinforcement was experimentally investigated and predicted by using two different analytical methods (i.e., multi-layer analysis and finite element analysis) incorporating the suggested tension-softening curves. In particular, the serviceability deflection of GFRP bar reinforced UHPFRC beams was predicted using a newly developed model of effective moment of inertia for strain-hardening UHPFRC. In the case of impact loading condition, the impact response of reinforced UHPFRC beams was evaluated using a drop-weight impact test machine. The deflection-time histories were then predicted using single-degree-of-freedom system incorporating the enhanced resistance-deflection curve obtained from multi-layer analysis.
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