High strain rate effects on the mechanical resistance of ultra-high-performance fiber-reinforced concrete containing nanoparticles
저자
발행사항
서울 : Graduate School Sejong University, 2023
학위논문사항
학위논문(박사)-- Graduate School Sejong University : 토목환경공학과 ultra-high-performance fiber-reinforced concrete 2023. 8
발행연도
2023
작성언어
영어
주제어
발행국(도시)
서울
형태사항
266 ; 26 cm
일반주기명
지도교수: Dong Joo KIM
UCI식별코드
I804:11042-200000689095
소장기관
The addition of nanoparticles (NPs) to ultra-high-performance fiber-reinforced concrete (UHPFRC) has significantly increased the mechanical properties of UHPFRC at static strain rates. However, there is relatively little information known on the mechanical characteristics of UHPFRC incorporating NPs at high strain rates. Thus, this study is carried out to develop a better understanding of the high strain rate effects on the mechanical resistance of UHPFRC containing NPs. This research includes five main contents, which are as follows:
First, comprehensive information regarding the effect of NPs on the mechanical properties of cement-based materials (CBMs) as well as concrete was presented. An overview of NPs used for concrete and their morphological features was provided. In addition, the effects of various types of NPs on the performance of CBMs and concrete such as interfacial bond strength, compressive strength, tensile strength, and flexural strength, have been summarized. Finally, the optimal contents of several NPs for improving concrete mechanical strengths were provided.
Second, the loading rate effects on the mechanical properties of fiber-matrix zone (FMZ) surrounding steel fibers and matrices were investigated by conducting nanoindentation tests. As the loading rate increased from 1.0 to 8.0 mN/s, ultra-high performance concrete (UHPC) produced higher rate sensitivity for hardness (H) of FMZ in comparison with high-performance concrete (HPC). The dynamic increase factor (DIF) for the H of UHPC and HPC was 1.75 and 1.46, respectively. The higher DIF for H of UHPC in comparison with HPC would be based on the higher content of ultra-high density calcium silicate hydrate (UHD C–S–H) and high density calcium silicate hydrate (HD C–S–H), which were highly sensitive to the loading rates.
Third, the effects of loading rates on the pullout resistance of smooth-steel fibers embedded in UHPC containing NPs at various pullout speeds ranging from 0.0167 to 500 mm/s were investigated. Three types of NPs, namely, nano-CaCO3 (NC), nano-SiO2 (NS), and multi-walled carbon nanotube (MWCNT), were added to UHPC at dosages between 1.0 and 3.0% by weight of cement. The addition of NPs to UHPC notably increased the rate sensitivity of fiber pullout resistance. Among the NPs, the addition of NC 3.0% produced the highest rate sensitivities for the DIFs of both equivalent bond strength \tau_{eq} (DIF = 1.95) and peak bond strength \tau_{peak} (DIF = 2.06) of UHPC, whereas UHPC without NPs obtained the lowest rate sensitivity of \tau_{eq} (DIF = 1.18), and UHPC containing 3.0% MWCNT had the lowest rate sensitivity of \tau_{peak} (DIF = 1.62). Nanoindentation tests, scanning electron microscopy, and energy-dispersive X-ray spectroscopy analysis revealed that the higher amount of calcium silicate hydrate (C–S–H) at FMZ produced higher pullout resistance and rate sensitivity.
Fourth, the effects of NPs on the tensile resistance of UHPFRC containing 1.5 vol.% smooth steel fibers, at both static (0.000167 s–1) and high strain rates (61.86–162.00 s–1), were discovered. Three types of NPs including NC (3.0%), NS (1.0%), and MWCNT (1.0%) were considered. Among these, NC generated the highest tensile strength of UHPFRC at both static and high strain rates, whereas MWCNT exhibited the lowest tensile strength. The post-cracking tensile strength of UHPFRC containing NC was 17.73 and 52.13 MPa at static and high strain rates, respectively, and that of UHPFRC containing MWCNT was 13.99 and 37.61 MPa, respectively. The enhancement in tensile strength of UHPFRC containing NC can be attributed to the enhancement of the C–S–H content with a higher calcium-to-silica ratio in FMZ, which in turn increased the interfacial bond strength of steel fibers embedded in UHPFRC containing NC.
Finally, the fracture resistance of UHPFRC containing various types of NPs under strain rates (0.000333–156.55 s−1) was investigated. Four matrices including UHPFRC without NPs and UHPFRCs containing 3.0% NC, 1.0% MWCNT, or 1.0% NS, were examined. All UHPFRCs containing NPs produced higher rate-sensitive fracture resistance than UHPFRC. The fracture resistance enhancement in matrices containing NPs was attributed to the improvement in the interfacial bond strength of smooth steel fibers embedded in UHPFRCs containing NPs.
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