Quantitative Evaluation on Carbon Nanotube Distribution for Functional Ultra-High Performance Concrete

Abstract

이 논문은 우수한 역학적 성능과 내구성, 작업성을 보유한 초고성능 콘크리트 (Ultra-high performance concrete, UHPC)에 독특한 전기적, 열적 특성과 화학적 안정성을 지닌 탄소나노튜브 (Carbon nanotube, CNT)를 혼입하였다. 이를 통해 보수/보강과 동시에 발열 양생, 융설/융빙, 균열 감지, 교통흐름 감지 등의 기능성을 통합한 차세대 건설 재료의 개발이 가능하다. 이에 CNT의 농도와 분산을 주요 변수로 하여 UHPC의 레올로지 특성, 역학적 성능, 전기 및 열적 특성을 조사하였다. 물-바인더 비가 매우 낮은 UHPC에 고농도의 CNT를 효율적으로 분산하는 방법으로 건식 혼합이 제안되었다. 미세한 크기의 실리카 흄, CNT, 모래를 일정 시간 혼합하여 모래에 실리카 흄과 CNT로 얇게 코팅하였다. 기존의 초음파 분산 처리에 비해 더 낮은 침투 임계점을 달성할 수 있고, 초음파 분산 및 상용 분산액에 비해 강도 저하를 최소화할 수 있어, 경제적이고 에너지 효율이 높은 방식으로 평가되었다. 그러나 임계 농도 이상에서 CNT의 물과 감수제 흡수로 인해 작업성 확보가 어려우므로 시멘트 중량 대비 0.5%에서는 물과 감수제의 비율 조정이 불가피한 것으로 판단되었다. 침투 임계점 이상의 CNT를 상용액을 이용하여 균일하게 분산시킨 경우 시멘트 중량 대비 0.8%까지 첨가하였을 때, 공극률, 자기 수축, 압축 강도, 전기 및 열적 특성이 모두 기존 UHPC에 비해 향상되었다. 그러나 침투 임계점 농도에 해당하는 0.5% 이상에서는 작업성 저하와 과도한 CNT의 일부 응집으로 인해 향상 정도가 미미하였다. 시멘트 중량 대비 1.4%의 고농도 CNT의 첨가는 UHPC의 전기저항을 현저하게 낮추어 20V 내외의 저전압에서 발열 양생이 가능했으며 기존 성능의 저하는 발견되지 않았고 오히려 압축강도, 휨 변형 경화 등의 기계적 성능이 향상되었다. 또한, 휨 응력 하에서 뛰어난 균열 감지 능력을 발현하여 보수/보강과 스마트 특성의 통합이 실현될 수 있음을 보였다. 경화된 시멘트 매트릭스 내부의 CNT 분산 품질 및 농도의 비파괴 검사법 개발을 위해 공초점 라만 현미경 (confocal Raman micro-spectroscopy, CRM)을 활용하였다. 수백 나노미터의 매우 높은 공간적 분해능으로 수백 마이크로미터의 영역을 스캔하여 각 지점의 라만 신호를 획득하였다. 기저 분석을 통해 다상의 수화한 시멘트 페이스트 내부에 존재하는 미반응 클링커와 수화물을 구별할 수 있었으며 높은 감도로 CNT의 존재를 탐지할 수 있었다. 이를 상대 엔트로피 개념을 적용하여 CNT의 공간적 분포를 정량적으로 평가하는 지표를 개발하여 다양한 분산 방법의 평가법을 제시하였다. 개개의 탄소나노튜브로 존재하는 잘 분산된 상용액을 여러 농도로 첨가하여 시편을 제조하고 동일 방법으로 라만 맵핑을 수행하였다. 또한 주요 클링커 및 수화물의 라만 산란 민감도 (Raman sensitivity)를 레이저 강도를 높여가며 실험적으로 산정하였다. 이를 통해 한 지점에서 획득한 라만 신호에서 각 상의 라만 산란 민감도를 반영하여 CNT의 부피 농도를 나타내는 라만 신호 정규화를 시도하였다. 넓은 영역에서 정규화 된 CNT 라만 신호의 평균값과 CNT 첨가 농도는 로그 관계를 가졌으며 높은 상관계수를 보였다. 이를 이용하면 경화된 시멘트 내부의 CNT 농도 정량이 가능하다. 결론적으로 목적에 따라 적절한 양의 CNT를 UHPC에 혼입하면 노후화된 건물과 기반 시설의 일부를 보수/보강함과 동시에 현장 발열 양생, 균열 감지 등의 스마트 구조체로 활용이 가능하다. 또한 CRM을 이용한 라만 정량 분석을 통해 CNT의 첨가 농도와 분산 품질의 비파괴 정량 검사가 가능하여, 향후 현장에서의 품질 관리 및 검사법으로 활용이 가능하다.

This study covers two important design parameters, which are a uniformed dispersion of carbon nanotubes (CNTs) and their content, in developing multi-functional ultra-high performance concrete (UHPC) as an integrated system of repair/retrofitting and smart performance. Therefore, this study aims to seek viable dispersion methods incorporating highly concentrated CNT in UHPC and closely examine the rheological properties, mechanical strength, shrinkage, porosity, and electrothermal properties with various dispersion methods and CNT contents. Moreover, this thesis contributes to development of non-destructive evaluation of dispersion quality and CNT content in a relatively large area of hardened cementitious nanocomposites. To overcome the complexity and multi-scale characteristic of nanocomposites, confocal Raman micro-spectroscopy with high spatial resolution was adopted. Dry mixing of CNT powder, silica fume, and silica sand before wet mixing was compared with macro-dispersed ultrasonicated suspension with superplasticizer (SP) and commercially available nano-dispersed suspension. In terms of the efficiency of improving the electrical conductance and the minimization of adverse effect on mechanical strength, dry mixing showed the best performance, up to 0.5wt% of CNT content. Unexpectedly, the incorporation of CNTs in the form of ultrasonicated suspension had an insignificant increase in electrical conductivity due to damages on CNT by sonication energy. Two methods significantly affected the rheological properties of UHPC. When 0.5wt% of CNT was added, fresh UHPC concrete had no more self-compacting properties, and therefore the vibration process was needed to fabricate specimens. On the other hand, samples manufactured with the commercial CNT suspension with shorter CNTs had better workability at a concentration as high as 6wt%, but the compressive strength significantly decreased, and the percolation threshold was found to be much higher than two other methods. It could be attributed to a much shorter length and poor interfacial bonding between CNTs and matrix due to the surfactant. The results indicated that dry mixing, which was more cost-effective and energy-efficient, can be used when CNT content near the percolation threshold (~0.3wt%) is required for specific performance. The critical incorporation concentration for tailoring multi-functional UHPC was investigated with thorough examination on the pore structure, shrinkage, compressive strength, and multifunctional properties. The well-dispersed commercial suspension was used to minimize the reduction in flowability. At all concentrations, incorporating CNTs affects positively; in other words, no adverse effect was found at high CNT concentration. However, reduction in flowability could influence the porosity, leading to marginal increase or reduction in positive effect of CNT addition. The results confirmed that the multifunctionality of UHPC could be maximized by incorporating CNTs while mitigating autogenous shrinkage and utilizing superior mechanical performance and durability of UHPC. The CICs for various material properties were determined to 0.5 wt% considering percolation threshold and limited effect of CNTs on shrinkage and mechanical properties, a substantial reduction in volumetric heat capacity, and an increase in thermal diffusivity. When 6wt% of highly concentrated CNT suspension was used instead of mixing water, the electrical conductance was remarkably improved, and the electrical curing of UHPC instead of steam curing (heat treatment) was possible at a very low voltage of 19–23 V. There was no significant degradation due to electrical curing; the flexural strength was rather slightly increased with direct electrical curing. Mapping of phases in hydrated cement sample was conducted using confocal Raman micro-spectroscopy (CRM) to overcome the limitation of conventional techniques such as electron microscopy that can be applied for the examination of hardened cement. It was found that CRM can be used to identify not only most of phases in cement-based material including clinkers, hydration products, and mineral additives, but also the presence of CNTs in the hardened matrix. When a well-dispersed stable CNT suspension was utilized, CNT was detected in almost the entire area despite a very low CNT content, which indicated that CNTs were uniformly distributed in the matrix. Raman imaging the submicron CNT aggregates below the resolution of optical microscope were spread in the matrix in the specimen fabricated with CNT suspension with ultrasonication applied. The distribution and dispersion efficiency of CNTs was quantitatively evaluated with the metrics proposed in this study. Hence, non-destructive investigation of nanocomposite without any sample preparation can be possible to compare the efficiency of various dispersion methods using high resolution CRM. Quantitative Raman analysis was attempted to construct the calibration curve for the CNT concentration evaluation. To fully utilize the change in Raman intensity of phases in each measurement spot, the strategy for the normalization of CNT signal was to use the Raman sensitivity of material. The experimental Raman sensitivity was measured as the linear coefficient between the laser intensity and Raman intensity. Even though cement-based materials are multi-phase and complex material, it was possible to quantitatively compare the Raman intensity of CNT between spectrums through the normalization. The average of normalized CNT signal, which represented the relative volume fraction in mapping area, non-linear logarithmic relation with volumetric concentration of CNT. In conclusion, the overall understanding of engineering properties with varying CNT concentration and dispersion methods will expand the application of UHPC/CNT composite by helping engineers to choose appropriate CNT dosage and efficient dispersion methods. Furthermore, the quantitative analysis framework will enable the comparison of dispersion quality and CNT quantity in hardened cement composite, which is not limited to cement/CNT system, but has lots of potential to investigate even more complicated phenomena of cement-based materials.