A Study on the Prediction of Compressive Strength of Concrete with Ground Granulated Blast Furnace Slag using Rate Constant and Apparent Activation Energy

Abstract

Recently, utilization of slag as an industrial byproduct is increasing as part of low-carbon green growth with respect to concrete industry. The Blast Furnace Slag(BFS) is the most promising building material and is being used the most. According to this, many researches on BFS are actively being reported. Various epidemiological characteristics on concrete with BFS being reported tell us that concrete with BFS has diverse merits such as improvement of long-term strength, chloride ion permeation protection, the inhibition of Alkali-Silica Reaction(ASR), watertightness improvement, reduction in heat of hydration and chemical resistance improvement while it has some shortcomings such as drastic quality variation of concrete, speediness of carbonation velocity, drying shrinkage according to retarding, reduced intial intensity, and the change of curing temperature. Meanwhile, compressive strength of concrete is a crucial factor to be considered to decide the time to remove the form, the period to prevent frost damage, control the curing temperature and the time to post tensioning of prestressed concrete when constructing buildings. These determinations of time are the critical respects for improving productivity of quality management and post treatment. Maturity method considering the relationship between temperature and age is being applied for the prediction of compressive strength of concrete when determining the time for construction. For the maturity method, the method of simply considering the relationship between temperature and age and the method of considering chemical reaction rate are suggested. The concrete with BFS shows more decrease of strength under same age than ordinary concrete, normal concrete, and prediction of compressive strength by existing models using maturity shows a problem of overpredicting compressive strength of BFS under the same temperature. Therefore, derivation of rate constant (kT) that quantitatively represents hydration reaction rate and apparent activation energy (Ea) that is minimum-energy required for chemical reaction is necessary for solving this problem and produce more accurate prediction. Also, predictive model selection that can reflect decreasing of strength at early-age of concrete with BFS is also required for accurate prediction of compressive strength. This study derives rate constant (kT) and apparent activation energy (Ea) considering BFS replacement ratio and curing temperature by ASTM C 1074 to predict compressive strength of concrete with BFS and conducts the prediction of compressive strength by Carino model. The results are as follows: 1) By measuring setting time of fresh BFS mortar with the penetration resistance test, it is shown that initial time and final time increase as BFS replacement ratio increases and curing temperature decreases. 2) The compressive strength of BFS mortar is measured at 2, 4, 8, 16, 32 and 64 times of the final setting of BFS mortar. It is understood that the amount of BFS replacement ratio and curing temperature makes a direct impact on compressive strength. 3) The rate constant (kT) and apparent activation energy (Ea) obtained by ASTM C 1074 show the following result. Rate constant (kT) decreases as BFS replacement ratio increases and curing temperature decreases whereas apparent activation energy(Ea) rises as BFS replacement ratio increases and curing temperature decreases. Furthermore, rate constant (kT) and apparent activation energy (Ea) proposed equations based on variables BFS replacement ratio and curing temperature is suggested. 4) This study comparatively reviews the actual measurement and predicted compressive strength by Carino model using rate constant (kT) and apparent activation energy (Ea) suggested by this study. This study predicts compressive strength of concrete with BFS and it is considered that prediction of compressive strength with high reliability is possible in the range of this experiment. The prediction of compressive strength of concrete with BFS will be possible by additional experiment and further interpretation with more varieties.| 최근 저탄소 녹색성장의 일환으로 콘크리트 산업의 경우도 산업부산물인 슬래그의 활용도가 높아지고 있다. 이중 건축 재료에서는 고로슬래그 미분말(BFS)의 사용이 양적으로 가장 많이 사용되고 있는 상황이다. 이에 따라 고로슬래그 미분말 혼입 콘크리트에 대한 연구가 보고되고 있는 실정이다. 고로슬래그 미분말 혼입 콘크리트에 대한 역학적 특징들이 많이 보고되고 있는데, 고로슬래그 미분말 혼입 콘크리트는 장기강도 향상, 염화물 이온 침투억제, 알칼리 실리카 반응(ASR)의 억제, 수밀성 향상, 수화열 저감, 화학저항성도 향상의 장점을 가지고 있으며, 반면 응결지연, 초기강도 저하, 양생온도 변화에 따른 급격한 콘크리트 품질 변화, 탄산화 속도 빠름, 건조수축 등의 단점을 나타내고 있다. 한편 콘크리트의 압축강도는 건설공사에 있어서 거푸집 탈영시기 결정, 초기동해 방기기간 결정, 양생온도 관리과 프리스트레스 콘크리트의 포스트 텐셔닝 시기 등을 결정하는데 있어서 중요한 요인으로 적용되며 이러한 시기 결정은 품질관리 및 후속공정의 원활한 진행을 위하여 중요한 사항이다. 이러한 건설공사 공정 시기 결정 시 압축강도 예측은 온도와 재령과의 관계를 복합적으로 고려하는 적산온도 방법이 적용되고 있는데 적산온도 방법은 재령과 온도의 관계를 단순히 고려하는 방법과 시멘트의 화학반응속도를 고려하는 방법 등이 제시되어 있다. 고로슬래그미분말 혼입 콘크리트는 보통콘크리트 보다 동일 재령에서 압축강도 저하가 나타나고 적산온도를 이용하는 기존의 모델을 통한 예측강도는 동일 적산온도에서 고로슬래그미분말의 압축강도다 높게 예측하는 문제가 발생하고 있다. 따라서 이와 같은 문제점을 해결하고 정확한 예측을 위해 수화반응속도를 정량적으로 나타내는 반응률상수 (kT)와 화학반응 시 필요한 최소에너지인 겉보기 활성화에너지 (Ea)의 도출이 필요하다. 또한 고로슬래그 미분말 혼입 콘크리트의 초기강도 저하현상을 반영할 수 있는 예측모델을 선정하여 정확한 압축강도 예측이 필요하다. 본 연구에서는 고로슬래그미분말 혼입 콘크리트의 압축강도 예측을 위해 고로슬래그미분말 혼입률과 양생온도를 고려하는 반응률상수 (kT)와 겉보기 활성화에너지 (Ea)를 ASTM C 1074 법으로 실험하여 도출하였고, 이를 통해 Carino 모델을 이용하여 압축강도 예측을 실시하였다.

1) 관입저항 시험을 통해 굳지 않은 BFS 모르타르의 응결시간을 측정하였고 BFS 혼입률이 증가하고 양생온도가 감소할수록 초결 및 종결시간은 증가하는 것으로 나타났다.

2) BFS 혼입 모르타르의 종결시간에 대한 일정 재령에서 압축강도 시험을 실시하였다. 이로부터 압축강도는 BFS 혼입률 및 양생온도의 크기가 직접적으로 영향을 미치는 것으로 판단할 수 있다.

3) ASTM C 1074 방법을 통하여 반응률상수(kT)와 겉보기 활성화에너지(Ea)를 도출하였다. 그 결과 반응률상수 (kT)는 BFS혼입률이 증가하고 양생온도가 감소할수록 감소되며, 겉보기 활성화에너지 (Ea)는 BFS 혼입률이 증가하고 양생온도가 감소할수록 증가하는 것으로 나타났다. 또한 BFS 혼입률과 양생온도를 변수로 하는 반응률상수 (kT) 및 겉보기 활성화에너지 (Ea) 도출식을 제안하였다.

4) 본 연구에서 제안한 반응률상수 (kT) 및 겉보기 활성화에너지 (Ea)를 이용하여 Carino 모델을 통해 압축강도를 예측하고 실험을 통한 실측 강도와 비교 검토를 실시하였다.

본 연구를 통해 고로슬래그미분말 혼입 콘크리트의 압축강도 예측을 실시하였고 본 연구의 실험범위 내에서 신뢰성 높은 압축강도 예측이 가능하다고 판단된다. 향후 다양한 수준을 통한 추가 실험과 해석을 통하여 고로슬래그미분말 혼입 콘크리트의 압축강도 예측이 가능하다고 사료 된다.; Recently, utilization of slag as an industrial byproduct is increasing as part of low-carbon green growth with respect to concrete industry. The Blast Furnace Slag(BFS) is the most promising building material and is being used the most. According to this, many researches on BFS are actively being reported. Various epidemiological characteristics on concrete with BFS being reported tell us that concrete with BFS has diverse merits such as improvement of long-term strength, chloride ion permeation protection, the inhibition of Alkali-Silica Reaction(ASR), watertightness improvement, reduction in heat of hydration and chemical resistance improvement while it has some shortcomings such as drastic quality variation of concrete, speediness of carbonation velocity, drying shrinkage according to retarding, reduced intial intensity, and the change of curing temperature. Meanwhile, compressive strength of concrete is a crucial factor to be considered to decide the time to remove the form, the period to prevent frost damage, control the curing temperature and the time to post tensioning of prestressed concrete when constructing buildings. These determinations of time are the critical respects for improving productivity of quality management and post treatment. Maturity method considering the relationship between temperature and age is being applied for the prediction of compressive strength of concrete when determining the time for construction. For the maturity method, the method of simply considering the relationship between temperature and age and the method of considering chemical reaction rate are suggested. The concrete with BFS shows more decrease of strength under same age than ordinary concrete, normal concrete, and prediction of compressive strength by existing models using maturity shows a problem of overpredicting compressive strength of BFS under the same temperature. Therefore, derivation of rate constant (kT) that quantitatively represents hydration reaction rate and apparent activation energy (Ea) that is minimum-energy required for chemical reaction is necessary for solving this problem and produce more accurate prediction. Also, predictive model selection that can reflect decreasing of strength at early-age of concrete with BFS is also required for accurate prediction of compressive strength. This study derives rate constant (kT) and apparent activation energy (Ea) considering BFS replacement ratio and curing temperature by ASTM C 1074 to predict compressive strength of concrete with BFS and conducts the prediction of compressive strength by Carino model. The results are as follows: 1) By measuring setting time of fresh BFS mortar with the penetration resistance test, it is shown that initial time and final time increase as BFS replacement ratio increases and curing temperature decreases. 2) The compressive strength of BFS mortar is measured at 2, 4, 8, 16, 32 and 64 times of the final setting of BFS mortar. It is understood that the amount of BFS replacement ratio and curing temperature makes a direct impact on compressive strength. 3) The rate constant (kT) and apparent activation energy (Ea) obtained by ASTM C 1074 show the following result. Rate constant (kT) decreases as BFS replacement ratio increases and curing temperature decreases whereas apparent activation energy(Ea) rises as BFS replacement ratio increases and curing temperature decreases. Furthermore, rate constant (kT) and apparent activation energy (Ea) proposed equations based on variables BFS replacement ratio and curing temperature is suggested. 4) This study comparatively reviews the actual measurement and predicted compressive strength by Carino model using rate constant (kT) and apparent activation energy (Ea) suggested by this study. This study predicts compressive strength of concrete with BFS and it is considered that prediction of compressive strength with high reliability is possible in the range of this experiment. The prediction of compressive strength of concrete with BFS will be possible by additional experiment and further interpretation with more varieties.