Flexural performance improvement of concrete members by using ultra-rapid-hardening cementitious composites with strain-hardening behavior

다국어 초록 (Multilingual Abstract)

This thesis describes a series of process for developing ultra-rapid strain- hardening cementitious composites (URSHCC). URSHCC means polyethylene fiber and a cement mortar mixture optimized for it. It was developed for the purpose of repair and reinforcement and improved the disadvantages of existing fiber reinforced concretes (FRCs) such as engineered cementitious composites (ECC) and ultra-high-performance fiber-reinforced concrete (UHPFRC) that were widely used. The target compressive strength, tensile strength and tensile strain capacity was 100 MPa, 6 MPa, and 4%, respectively. In addition, it was intended to achieve 70% of the target performance in 4 hours in air without water curing or long-term curing, and ultra-rapid-hardening cement (URHC) was used accordingly. Other binders were adopted from the supplementary cementitious materials (SCMs), such as ground granulated blast furnace slag (GGBFS), cement kiln dust (CKD), and silica fume, etc. After the development, various tests were conducted to verify that the URSHCC can be used as a repair material. Adhesion to existing materials and autogenous recovery from damage were evaluated, and performance under dynamic loading was also identified to prevent sudden collapse caused by earthquakes or terrorism. And finally, concrete beams and reinforced concrete beams with different sizes were repaired with URSHCC and the flexural performance was evaluated in both static and dynamic conditions. After determining the fiber length and volume fraction of URSHCC for the intended purpose, the reinforcement efficiency can be improved through proper pre-treatment of the reinforcing area. If the adhesion with the existing concrete is insufficient, it becomes challenging for URSHCC to demonstrate its effectiveness. Particularly under impact loads, despite having a sufficiently high dynamic increase factor (DIF) of the material itself, the repair efficiency has been poor, and it could even lead to additional risks due to material detachment, so caution should be exercised. The findings of this research can provide valuable information for using fiber- reinforced concrete as a repair and reinforcement material, enabling the provision of safer construction services in the event of disasters.

목차 (Table of Contents)

• CHAPTER 1. INTRODUCTION 1
• 1.1. Research Background 1
• 1.2. Objectives and Scopes 3
• 1.3. Literature review 7
• 1.3.1. Strain-hardening cementitious composites 8
• 1.3.2. Ultra-rapid-hardening cement-based materials 11
• 1.3.3. Flexural behavior of RC beam under impact loading 13
• 1.4. Organization 17
• CHAPTER 2. DEVELOPMENT AND MECHANICAL PERFORMANCE OF URSHCC 21
• 2.1. Test Program 23
• 2.1.1. Material properties 23
• 2.1.2. Mixing sequence and sample fabrication 29
• 2.1.3. Compressive and tensile test setup 31
• 2.1.4. TG/DTG and XRD analyses 32
• 2.1.5. Scanning electron microscopy (SEM) analyses 33
• 2.2. Effects of supplementary cementitious materials 34
• 2.2.1. Determination of materials compatible with OPC 34
• 2.2.2. Optimal mixture design with URH cement 53
• 2.2.3. Tensile behavior 55
• 2.3. Effects of curing time at the early age 72
• 2.4. Shrinkage behavior 78
• 2.4.1. Shrinkage test setup 78
• 2.4.2. Effect of URH cement on the strain and temperature variation 81
• 2.4.3. Evaluation of shrinkage behavior in comparison with UHPFRC and ECC 86
• 2.5. Summary 88
• CHAPTER 3. SELF-HEALING CAPACITY 90
• 3.1. Test program 90
• 3.1.1. Fiber properties 90
• 3.1.2. Mixing sequence and specimen fabrication 91
• 3.1.3. Curing and healing processes with wet-dry cycles 92
• 3.2. Direct tensile behavior of URSHCC 93
• 3.2.1. General behavior without self-healing 96
• 3.2.2. Effect of FRI on post-cracking strength 101
• 3.2.3. Deterioration in tensile performance after self-healing 104
• 3.3. Beneficial effect of self-healing on the tensile performance of URSHCC 109
• 3.3.1. Pre-strain of 0.5% 109
• 3.3.2. Pre-strain of 1.0% 111
• 3.4. Cracking behavior 116
• 3.4.1. Number of cracks 116
• 3.4.2. Crack widths measured before self-healing 119
• 3.4.3 Crack filling effect after self-healing 122
• 3.5. Verification of self-healing products 125
• 3.6. Differences from healing performance of other FRCCs 128
• 3.7. Summary 132
• CHAPTER 4. DYNAMIC MECHANICAL PROPERTIES OF URSHCC 133
• 4.1. Test program . 137
• 4.1.1. Static and dynamic compressive test 137
• 4.1.2. Static flexural test 140
• 4.1.3. Drop-weight impact test 140
• 4.2. Test results and discussion 141
• 4.2.1. Strain-rate effect on the compressive behavior of URSHCC 141
• 4.2.2. Strain-rate effect on the flexural behavior of URSHCC 148
• 4.2.3. Comparative strain-rate sensitivity 159
• 4.3. Summary 165
• CHAPTER 5. FLEXURAL STRENGTHENING EFFECT OF URSHCC ON CONCRETE MEMBERS 167
• 5.1. Strengthening effect of URSHCC on concrete beam 167
• 5.1.1. Test program 168
• 5.1.2. Tensile behavior of URSHCC 175
• 5.1.3. Flexural behavior of concrete beam strengthened with URSHCC 180
• 5.2. Flexural behavior of RC beam repaired with URSHCC 198
• 5.2.1. Test specimens 200
• 5.2.2. Drop-weight impact and static flexural test setup 205
• 5.2.3. Effect of kinetic energy and URSHCC repairing 209
• 5.2.4. Comparison with beams repaired with other FRCs 228
• 5.2.5. Comparison with beams fabricated with other FRCs 234
• 5.2.6. Support rotation 239
• 5.3. Summary 242
• CHAPTER 6. CONCLUSION 244
• 6.1. Major findings of this study 244
• 6.2. Recommendations for further studies 246
• REFERENCES 248
• 국문 초록 262
• ACKNOWLEDGEMENT 264