Modeling of Mechanical Properties of Concrete Mixed with Expansive Additive

Hyeonggil Choi,Takafumi Noguchi 한국콘크리트학회 2015 International Journal of Concrete Structures and M Vol.9 No.4

This study modeled the compressive strength and elastic modulus of hardened cement that had been treated with an expansive additive to reduce shrinkage, in order to determine the mechanical properties of the material. In hardened cement paste with an expansive additive, hydrates are generated as a result of the hydration between the cement and expansive additive. These hydrates then fill up the pores in the hardened cement. Consequently, a dense, compact structure is formed through the contact between the particles of the expansive additive and the cement, which leads to the manifestation of the strength and elastic modulus. Hence, in this study, the compressive strength and elastic modulus were modeled based on the concept of the mutual contact area of the particles, taking into consideration the extent of the cohesion between particles and the structure formation by the particles. The compressive strength of the material was modeled by considering the relationship between the porosity and the distributional probability of the weakest points, i.e., points that could lead to fracture, in the continuum. The approach used for modeling the elastic modulus considered the pore structure between the particles, which are responsible for transmitting the tensile force, along with the state of compaction of the hydration products, as described by the coefficient of the effective radius. The results of an experimental verification of the model showed that the values predicted by the model correlated closely with the experimental values.

다양한 열화 환경 하에서의 폭로실험(일본사례 소개)

최형길(Choi, Hyeonggil),하마 유키오(Hama, Yukio) 대한건축학회 2016 建築 Vol.60 No.11

일본 시멘트협회 연구소

최형길(Hyeonggil Choi) 한국콘크리트학회 2014 콘크리트학회지 Vol.26 No.1

Evaluation on the Mechanical Performance of Low-Quality Recycled Aggregate Through Interface Enhancement Between Cement Matrix and Coarse Aggregate by Surface Modification Technology

Heesup Choi,Hyeonggil Choi,Myungkwan Lim,Masumi Inoue,Ryoma Kitagaki,Takafumi Noguchi 한국콘크리트학회 2016 International Journal of Concrete Structures and M Vol.10 No.1

In this study, a quantitative review was performed on the mechanical performance, permeation resistance of concrete, and durability of surface-modified coarse aggregates (SMCA) produced using low-quality recycled coarse aggregates, the surface of which was modified using a fine inorganic powder. The shear bond strength was first measured experimentally and the interface between the SMCA and the cement matrix was observed with field-emission scanning electron microscopy. The results showed that a reinforcement of the interfacial transition zone (ITZ), a weak part of the concrete, by coating the surface of the original coarse aggregate with surface-modification material, can help suppress the occurrence of microcracks and improve the mechanical performance of the aggregate. Also, the use of low-quality recycled coarse aggregates, the surfaces of which were modified using inorganic materials, resulted in improved strength, permeability, and durability of concrete. These results are thought to be due to the enhanced adhesion between the recycled coarse aggregates and the cement matrix, which resulted from the improved ITZ in the interface between a coarse aggregate and the cement matrix.

양생조건에 따른 섬유보강 시멘트계 복합재료(FRCC)의 균열 자기치유 특성

최희섭 ( Heesup Choi ),최형길 ( Hyeonggil Choi ) 한국건축시공학회 2016 한국건축시공학회지 Vol.16 No.4

양생조건의 차이가 섬유보강 시멘트계 복합재료(FRCC)의 자기치유에 미치는 영향을 검토하기 위해, 건습 반복시험 및 옥외 폭로시험을 실시하여 FRCC의 자기치유 효과에 대해 검토했다. 투수계수 및 마이크로스코프에 의한 균열조사에 의해 FRCC의 자기치유 효과에 대해 평가한 결과, 수중에의 침지시간이 길고, 건조온도가 낮은 경우 균열의 자기치유 효과는 큰 것을 확인할 수 있었다. 또한, 자기치유에 있어서의 수밀성능은 습윤 시간에 크게 영향을 받는것을 확인했다. 한편, 실환경 조건에 있어서 강우에 의한 수분 공급에 의해 자기치유는 진전하는 것을 확인했으며, 혼화재료를 적절히 병용하여 사용하는 것으로 균열의 자기치유 성능을 향상할 수 있다고 판단된다. In this study, the self-healing effect of a fiber-reinforced cement composite (FRCC) was examined using a drying-wetting test and an outdoor exposure test. The influence of various curing conditions on the self-healing effect of the FRCC was also investigated. The effect of self-healing was evaluated using a permeability coefficient and by investigating the cracks using a optical microscope. The results confirmed that the FRCC was capable of self-healing under a long wetting time and a low drying temperature. In addition, watertight performance by self-healing was shown to have a significant influence on wetting time. Meanwhile, this self-healing effect was enhanced by hydration as a result of rainfall when the FRCC was put under actual environmental conditions. Moreover, it was determined that cracking self-healing can be improved by using the appropriate admixture materials.

Strength Reduction of Concrete Subjected to High Temperatures: Effects of Various Aggregates

Wonchang Kim(Wonchang Kim),Keesin Jeong(Keesin Jeong),Hyeonggil Choi(Hyeonggil Choi),Taegyu Lee(Taegyu Lee) 한국화재소방학회 2023 International Journal of Fire Science and Engineer Vol.37 No.1

Previous studies on the strength degradation of concrete subjected to high temperatures were analyzed. To analyze the effect of the coarse-aggregate type on strength degradation, data from previous studies were collected, and the coarse aggregate used, physical properties of the aggregate, and heating conditions were analyzed. The concrete types were classified into normal, heavyweight, and lightweight concrete. Their high-temperature characteristics were analyzed and evaluated according to the mixed coarse aggregate. Finally, the correlations derived from the analysis results were compared with the CEB Code. The analysis results were different for different concrete and coarse-aggregate types, and different tendencies from the CEB Code were observed.

양생조건에 따른 섬유보강 시멘트계 복합재료(FRCC)의 균열 자기치유 특성

최희섭,최형길,Choi, Heesup,Choi, Hyeonggil 한국건축시공학회 2016 한국건축시공학회지 Vol.10 No.4

양생조건의 차이가 섬유보강 시멘트계 복합재료(FRCC)의 자기치유에 미치는 영향을 검토하기 위해, 건습 반복시험 및 옥외 폭로시험을 실시하여 FRCC의 자기치유 효과에 대해 검토했다. 투수계수 및 마이크로스코프에 의한 균열조사에 의해 FRCC의 자기치유 효과에 대해 평가한 결과, 수중에의 침지시간이 길고, 건조온도가 낮은 경우 균열의 자기치유 효과는 큰 것을 확인할 수 있었다. 또한, 자기치유에 있어서의 수밀성능은 습윤 시간에 크게 영향을 받는 것을 확인했다. 한편, 실환경 조건에 있어서 강우에 의한 수분 공급에 의해 자기치유는 진전하는 것을 확인했으며, 혼화재료를 적절히 병용하여 사용하는 것으로 균열의 자기치유 성능을 향상할 수 있다고 판단된다. In this study, the self-healing effect of a fiber-reinforced cement composite (FRCC) was examined using a drying-wetting test and an outdoor exposure test. The influence of various curing conditions on the self-healing effect of the FRCC was also investigated. The effect of self-healing was evaluated using a permeability coefficient and by investigating the cracks using a optical microscope. The results confirmed that the FRCC was capable of self-healing under a long wetting time and a low drying temperature. In addition, watertight performance by self-healing was shown to have a significant influence on wetting time. Meanwhile, this self-healing effect was enhanced by hydration as a result of rainfall when the FRCC was put under actual environmental conditions. Moreover, it was determined that cracking self-healing can be improved by using the appropriate admixture materials.

Modeling early age hydration reaction and predicting compressive strength of cement paste mixed with expansive additives

Van, Nguyen Duc,Choi, Hyeonggil,Hama, Yukio Elsevier 2019 Construction and Building Materials Vol.223 No.-

<P><B>Abstract</B></P> <P>Although expansive additives have been widely used to efficiently reduce shrinkage cracking in concrete, little research has been undertaken to model the hydration reaction of cement and expansive additive blends. This paper presents a hydration model that describes the hydration reaction that occurs when expansive additives are mixed at an early stage and an equation to predict the compressive strength of expansive concrete. Using the proposed model, this study predicted the hydration degree, rate of heat evolution, and gel/space ratio of the hardening cement–expansive additive paste. The results indicated good agreement of the calculated data with the measured data. From the relationship between the gel/space ratio and compressive strength, an equation was deduced for prediction of the development of compressive strength of expansive concrete. Furthermore, this work also found that the model parameters can be represented as a function of the mineral composition of the cement. The results demonstrate that the proposed model is effective and useful in predicting the hydration and mechanical properties of expansive concrete.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Proposed a hydration model of cement-expansive additive paste. </LI> <LI> Relationship between mineral composition and model parameters is clearly explained. </LI> <LI> Predicted gel/space ratio using proposed model. </LI> <LI> A conversion coefficient is deduced for predicting compressive strength. </LI> <LI> Predicted compressive strength development of expansive concrete. </LI> </UL> </P>

콘크리트의 강도 예측을 위한 초음파 속도법의 검토

김원창(Wonchang Kim),정기신(Keesin Jeong),최형길(Hyeonggil Choi),이태규(Taegyu Lee) 한국화재소방학회 2022 한국화재소방학회 학술대회 논문집 Vol.2022 No.추계

건축물 유지⋅보수 및 화재 후 복구 단계에서 콘크리트 건축물의 강도 예측은 건축물의 안전등급 평가 및 고온 이력을 받은 열화 부재의 보수⋅보강 범위 설정에 따른 안전성 및 경제성 측면에 있어서 매우 중요하다. 그러나 기존 측정하고자 하는 건축 부재의 코어 추출을 통한 파괴 시험은 콘크리트 부재의 손상을 가하는 방법으로서 구조적 안전성 문제가 발생 될 수 있다. 따라서, 비파괴 공법을 통한 평가 방법을 지향하고 있으며, 기존 연구자들은 초음파 속도법을 통해 콘크리트의 역학적 특성 예측 연구를 수행하였다. 그러나, UPV(ultrasonic pulse velocity)는 콘크리트 내부 수화물 생성에 따른 밀도 변화와 고온 이력에 따른 공극 및 균열 등의 열화에 영향을 받기 때문에 상온(20 ℃)에서의 강도 예측식과 고온 이력을 받은 콘크리트의 강도 예측식을 구분하여 사용되어야 한다고 판단된다. 또한, W/B에 따라 상이한 재료 혼합에 영향을 받을 수 있기 때문에 W/B 범위 및 혼합된 굵은 골재에 대한 분석도 필요하다고 판단된다. 본 연구에서는, 콘크리트의 재령별 강도 발현 및 UPV 변화와 고온 노출 후 강도 및 UPV 저하를 평가하였다. 다양한 재료적 영향성을 검토하기 위하여 보통골재를 혼합한 NC(Normal concrete) 및 경량골재를 혼합한 LC(Lightweight concrete)로 분류하였으며, 다양한 강도 범위에서의 강도 및 UPV를 분석하기 위하여 W/B를 0.41, 0.33 및 0.28로 설정하였다. 본 연구의 범위에서는, 콘크리트의 강도 및 UPV 상관관계 분석을 통한 강도 예측식을 제안하였다. 연구결과, 동일 W/B에서 재령에 따라 혼합된 굵은 골재의 다공성의 영향으로 LC 대비 NC의 높은 강도 발현 및 UPV 증가를 보였다. 그러나, 고온 이력을 받은 경우 골재의 열 팽창률 특성 차이에 의해 NC 대비 LC의 높은 잔존 강도 및 높은 잔존 UPV를 보였다.또한, NC 및 LC의 잔존 강도와 UPV의 상관관계를 나타냈으며, 모든 W/B 범위의 데이터를 분석하여 하나의 강도 예측식으로 제안하는 것 대비, 각각의 W/B 범위에서의 강도 예측식은 상대적으로 약 56% 높은 상관계수(R2)를 보였다.

Analysis of Ultrasonic Pulse Velocity Correlations for Estimating Residual Compressive Strength of Concrete Subjected to High-Temperature Hysteresis: Influence of Various Aggregates

Wonchang Kim,Keesin Jeong,Heeyong Choi,Hyeonggil Choi,Taegyu Lee 한국화재소방학회 2022 International Journal of Fire Science and Engineer Vol.36 No.1

In this study, the compressive strength and ultrasonic pulse velocity (UPV) behaviors of concrete at high temperatures by aggregate type were comparatively analyzed in accordance with the CEN and CEB Codes. The aggregates adopted were divided into limestone, granite, river gravel, and crushed stone, and the target temperatures were set to 100 °C, 200 °C, 300 °C, 400 °C, 500 °C, and 600 °C. The analysis items were specified as compressive strength (Fc), UPV, ratio of compressive strength (Fcratio), and UPV ratio (UPVratio); in addition, the correlations between Fcratio and UPV were analyzed and compared to the previously proposed estimation equations for concrete at room temperature. Accordingly, the Fcratio of limestone and river gravel aggregates exceeded the UPVratio at temperatures lower than 300-400 °C; however, UPVratio tended to be higher at subsequent temperatures. For granite aggregates, UPVratio was higher than Fcratio over the entire temperature range, while crushed stone aggregates exhibited higher Fcratio than UPVratio. Both Fcratio and UPVratio of granite aggregates exceeded those of limestone aggregates, while the UPVratio and Fcratio of river gravel aggregates were higher and lower than those of crushed stone aggregates, respectively. Consequently, a strength estimation equation was proposed by analyzing the correlations between Fcratio and UPV, and was inferred to be better than the existing strength estimation equations.