플라이애시 혼입 콘크리트의 압축강도 추정을 위한 결합효율 예측모델

Abstract

콘크리트의 압축강도는 역학적 특성값 및 내구성과의 높은 상관성으로 인해 콘크리트의 품질 평가에 가장 일반적으로 사용되는 기준값이다. 콘크리트 압축강도를 즉각적으로 예측하여 효율적으로 콘크리트의 품질을 관리하기 위해서는 배합요인에 의해 압축강도를 예측할 수 있는 모델이 필요하다. 다중선형회귀분석과 같은 통계적 분석방법에 의한 예측모델은 도출이 쉽고 강도 영향요인과 압축강도의 관계에 대한 정량적 이해가 용이하므로 압축강도 예측 외에도 내구성 평가, 배합설계 등의 실용적 측면의 활용성이 높다. 우리나라에서 사용되는 레디믹스트 콘크리트의 약 50% 이상에서 플라이애시(Fly-ash, 이하 FA)를 사용하고 있는데, 이는 FA 혼입 콘크리트의 우수한 역학적 특성 및 내구성, 산업부산물의 재활용에 의한 환경적·경제적 이점 때문이다. 하지만, 국내 콘크리트 산업에서 FA는 포졸란 반응에 의한 강도발현 특성에 대한 이해 부족 및 초기 재령에서의 강도 부족으로 인한 품질 문제, 수급자의 부정적 인식 등으로 인해 단순치환에 의한 최소한의 활용에 그치고 있어, FA의 장점을 극대화하지 못하고 있다. 콘크리트에서 FA의 활용 증진 및 최적 사용, 정확한 압축강도 예측을 통한 효율적인 품질관리를 위해서는 FA의 강도발현 특성을 통해 FA의 결합효율을 파악하는 것이 중요하다. FA 결합효율을 결정하기 위해, Smith는 포졸란 반응으로 인한 FA와 보통포틀랜드시멘트(Ordinary Portland Cement, 이하 OPC)의 강도발현 차이를 고려하여, FA의 질량(F)을 등가의 OPC 질량(kF)으로 변환시킬 수 있는 k값으로 FA의 결합효율을 나타냈다. 따라서 FA 혼입 배합에서 물결합재비(W/(C+F))를 FA 결합효율을 반영한 조정물시멘트비(W/(C+kF))로 변환하여 압축강도 예측 및 배합설계에 활용할 수 있다. 결합효율의 결정에는 일반적으로 압축강도를 사용하므로, FA 콘크리트의 강도에 영향을 미치는 요인들을 반영한 결합효율 예측모델을 제시한다면 압축강도 예측의 신뢰성을 향상시킬 수 있다. 기존 FA 결합효율 예측모델은 콘크리트 배합요인 중 FA 치환율의 영향만 반영하여, 치환율이 증가함에 따라 FA 결합효율이 감소한다고 설명한다. 그러나 FA의 결합효율을 정확하게 계산하여 강도 예측과 배합설계에 쉽고 즉각적으로 반영하기 위해서는, FA 치환율 외에도 FA의 포졸란 반응에 의한 강도발현에 영향을 미치는 배합요인을 추가한 FA 결합효율 예측모델을 제시할 필요가 있다. 이에 본 연구는 FA 콘크리트의 압축강도에 대한 배합요인의 상관관계분석 및 다중선형회귀분석 등의 통계적 분석방법을 활용하여, FA 치환율을 포함한 배합요인만으로 FA의 결합효율을 결정할 수 있는 예측모델을 제안하는데 목적이 있다. 또한, FA 결합효율 예측모델의 신뢰성을 실험을 통해 검증하고, 예측모델을 반영한 압축강도 추정의 정확성을 평가하여 FA 결합효율 예측모델의 적용성을 검토하고자 한다. 본 연구는 총 6장으로 구성되며, 각 장의 내용은 다음과 같다.

1장에서는 연구의 배경, 목적, 방법 및 범위에 대해 기술하였다. 2장은 콘크리트의 압축강도 예측방법, 압축강도 영향요인, 결합효율에 대한 문헌고찰을 통해 콘크리트의 압축강도 예측에 있어 결합효율의 적용에 대해 살펴보았다. 3장은 레디믹스트 콘크리트의 배합 및 압축강도 데이터를 수집하여, 이에 근거한 FA 결합효율 예측모델을 제안하였다. 콘크리트의 품질 수준을 확인한 후, 시멘트물비와 압축강도의 관계에 대한 Bolomey의 강도식과 OPC 배합에 대한 FA 배합의 압축강도비를 나타내는 상대강도의 개념, 결합효율의 개념을 활용하여 수집 데이터에 대한 다중선형회귀분석을 통해 콘크리트 배합요인으로 재령 28일 FA 결합효율을 파악할 수 있는 예측모델을 제안하였다. 4장에서는 FA 결합효율 예측모델을 검증하기 위해 모르타르 실험을 실시하여 예측모델의 재현성을 확인하였다. 또한 추정결합효율(kpm)을 반영한 (C+kpmF)/W에 의한 재령 28일 압축강도 추정의 정확성을 평가하여 FA 결합효율 예측모델의 신뢰성을 검토하였다. 5장에서는 새로운 FA 혼입 레디믹스트 콘크리트 데이터 및 연구문헌의 FA 콘크리트 실험 데이터를 수집하여, 4장의 결합효율 예측모델에 의한 (C+kpmF)/W를 통해 FA 결합효율 예측모델의 압축강도 예측을 위한 적용성을 확인하였다. 6장 결론에서는 제안된 FA 결합효율 예측모델의 특징 및 추정결합효율, 이를 반영한 압축강도 예측 등의 연구 결과를 다음과 같이 요약 제시하였다.

(1) 기존 FA 결합효율 예측모델은 배합요인 중 FA 치환율의 영향만 반영하여, FA 치환율 증가에 따라 결합효율이 감소한다. 그러나 본 연구에서 제시한 FA 결합효율 예측모델은 레디믹스트 콘크리트 실적 데이터를 활용함으로써 결합재물비의 영향과 FA 치환율 15%에서 결합효율에 대한 FA 치환의 영향이 역전되는 현상을 설명할 수 있었다.

(2) FA 결합효율 예측모델에 따르면, FA 콘크리트는 약 15%의 FA 치환율에서 강도발현이 최대인 것으로 분석되었다. 하지만, FA 혼입 레디믹스트 콘크리트 데이터의 절반 이상이 FA를 10% 이하로 치환하고 있는 것으로 나타나, FA를 효과적으로 활용하지 못하고 있는 것으로 나타났다.

(3) FA 모르타르 매트릭스에서의 결합효율 예측모델은 FA의 결합효율을 다음과 같이 제시하고 있다; 동일 FA 치환율에서 결합재물비가 낮을수록 FA 결합효율도 크다. FA 치환율 15% 이하에서는 결합재물비가 높을수록 FA 치환율 증가에 의한 결합효율의 증진이 크지만, FA 치환율 15% 초과에서는 결합재물비에 따른 결합효율의 차이는 크지 않으며 FA 치환율이 증가함에 따라 결합효율은 점차 감소한다.

(4) FA 결합효율 예측모델은 일반적인 FA 치환 범위에서 FA를 다량 치환한 경우까지 결합재물비 및 FA 치환율의 배합요인에 의해 FA의 결합효율을 계산할 수 있으므로, 강도발현 및 경제적 측면에서 FA의 사용을 증진시키고 최적화하는데 유용하게 활용될 수 있다.

(5) 3장의 레디미스트 콘크리트 데이터로부터 제안된 추정결합효율(kpc)는 결합재물비 1.67∼2.86, FA 치환율 5∼25% 범위에서 -0.19∼1.98이지만, 4장의 모르타르 실험에 근거하여 제시된 추정결합효율(kpm)은 결합재물비 1.67∼2.86, FA 치환율 0∼70% 범위에서 -0.71∼1.24로 나타났다. kpc와 kpm의 차이는 FA의 활성도지수 차이에 기인한 것으로 볼 수 있다.

(6) 레디믹스트 콘크리트 실적 데이터 및 문헌 데이터를 추가로 수집하여, 4장의 결합효율 예측모델을 적용해 재령 28일 압축강도를 예측한 결과, 결합효율을 미반영한 (C+F)/W보다 추정결합효율(kpm)을 반영한 (C+kpmF)/W에 의해 FA 콘크리트의 재령 28일 압축강도를 더 정확하게 설명할 수 있었다. 따라서 FA의 결합효율을 FA 콘크리트 매트릭스의 결합재물비 및 FA 치환율과 같은 배합만으로 즉시 결정할 수 있어 FA 콘크리트의 품질관리에 있어 효용성이 높다. |The compressive strength is the most universally used measure for concrete quality because of high correlations between compressive strength and other mechanical properties, and durability. For efficient quality control by estimating the concrete strength immediately, a model which can predicts the compressive strength from mix proportions of concrete through a multiple linear regression analysis is necessary. The statistical analysis method is easy to deduct a prediction model and understand the quantitative effects on factors influencing the compressive strength of concrete. Therefore, it has practical application such as mix design and durability evaluation as well as strength prediction of concrete. About 50% or more of ready-mixed concrete uses fly-ash(FA) because of environmental and economic benefit on recycling of industrial by-products and advantages such as excellent mechanical characteristics and durability of concrete containing FA. However, the effects of FA usage in concrete may not be maximized as it only uses FA minimally by simple replacement due to lack of understanding of strength development by Pozzolanic reaction, quality problems from a strength shortage in an initial age, and negative recognition of demanders. In order to use FA optimally, promote utilization of FA, and predict compressive strength of FA concrete exactly, it is important to determine cementing efficiency of FA based on the strength development of FA concrete. For determination of cementing efficiency, Smith suggested the cementing efficiency of FA with k value which could convert mass(F) of FA into an equivalent mass(kF) of ordinary Portland cement(OPC) considering the difference in strength development of FA and OPC due to Pozzolanic reaction. Therefore, it is possible to utilize the cementing efficiency of FA for predicting compressive strength and designing mixture by converting water-binder ratio(W/(C+F)) into modified water-cement ratio(W/(C+kF)). The compressive strength is generally used to determine the cementing efficiency. Therefore, if a model for determining the FA's cementing efficiency reflecting factors which have influences on compressive strength of FA concrete is suggested, it will be possible to improve reliability of concrete strength prediction. Existing models to determine the cementing efficiency of FA only reflect the influence of FA replacement ratio among concrete mixing factors, and they suggest that the cementing efficiency decrease as the FA replacement ratio increases. However, in order to calculate the cementing efficiency of FA exactly and to reflect it on strength prediction and concrete mix design easily and immediately, it is necessary to suggest a prediction model for cementing efficiency of FA adding mixing factors which have influences on strength development by Pozzolanic reaction of FA. This study aims to suggest a prediction model which can determine the cementing efficiency of FA only with the mixing factors including FA replacement ratio by using statistical analysis methods such as correlation analysis and multiple regression analysis between mixing factors and compressive strength of FA concrete. It is also the purpose of this study to verify the reliability of the suggested prediction model of FA cementing efficiency through experiment and to review the its applicability by evaluating the accuracy of prediction of compressive strength reflecting predicted cementing efficiency. This study is composed of total 6 chapters and contents of each chapter are as follows. The first chapter described the background, purpose, methods and scope of the study. The second chapter investigated application of the prediction model of cementing efficiency to predicting compressive strength of concrete through literature reviews about the prediction method on compressive strength of concrete, factors influencing compressive strength, and cementing efficiency. The third chapter suggested the prediction model for cementing efficiency of FA by using data on mixture and compressive strength of ready-mixed concrete. After confirming the quality level of concrete, it suggested a prediction model which could figure out the 28-day cementing efficiency of FA from mix proportions of FA concrete through multiple linear regression analysis of collected data. The cementing efficiency prediction model was based on the Bolomey’s equation on the relation between cement-water ratio and compressive strength, concept of relative strength which expressed the ratio of compressive strength of FA mixture to OPC mixture, and the concept of cementing efficiency. The fourth chapter confirmed the reproducibility of the cementing efficiency prediction model by conducting a mortar experiment, and also verified the reliability of the suggested model by the accuracy of strength prediction using (C+kpmF)/W. The 5th chapter collected data of new ready-mixed concrete containing FA and experimental data of FA concrete from research paper, and confirmed applicability for predicting compressive strength through (C+kpmF)/W using the FA cementing efficiency prediction model suggested in the 4th chapter. The 6th chapter, summarized the characteristics of suggested model for predicting cementing efficiency of FA and predicted cementing efficiency, and prediction of compressive strength. Conclusions of this study are as follows.

(1) Existing models for predicting cementing efficiency consider only FA replacement ratio among concrete mixing factors and suggest that the cementing efficiency of FA decreases as the FA replacement ratio increases. However, the FA cementing efficiency prediction model suggested in this study can explain not only the effect of binder-water ratio but also the phenomenon that the effect of FA replacement on cementing efficiency is reversed at about 15% of FA replacement due to using ready-mixed concrete data.

(2) It was analyzed that FA concrete showed the maximum strength development in about 15% of FA replacement ratio. However, as more than half of collected data substituted FA less than 10%, FA did not be used effectively for ready-mixed concrete.

(3) The cementing efficiency(kpm) estimated by the prediction model for cementing efficiency in FA mortar matrix are as follows; as binder-water ratio is lower in the same FA replacement ratio, the cementing efficiency is also higher. As binder-water ratio is higher in FA replacement ratio of 15% or less, the increase rate of the cementing efficiency is higher according to increase of FA replacement ratio. However, in FA replacement of 15% or more, there is little difference in cementing efficiency according to binder-water ratio and as FA replacement ratio increases, cementing efficiency gradually decreases.

(4) As the cementing efficiency prediction model suggested in the 4th chapter can calculate cementing efficiency of FA based on FA replacement ratio and binder-water ratio from general FA replacement to high volume FA replacement, it can be utilized to promote and optimize the use of FA in an aspect of strength development and economical benefit.

(5) Predicted cementing efficiency(kpc) suggested from ready-mixed concrete data in the 3rd chapter is -0.19~1.98 in the range of 1.67~2.86 of binder-water ratio and 5~25% of FA replacement ratio, but kpm suggested in the mortar experiment in the 4th chapter was -0.71∼1.24 in the range of 1.67~2.86 of binder-water ratio and 0~70% of FA replacement ratio. The disagreement in kpc and kpm is because of difference of strength activity index of FA such as 0.91~1.02 and 0.91, respectively.

(6) As a result of prediction of 28-day compressive strength of FA concrete using (C+kpmF)/W with kpm, based on data of ready-mixed concrete and research paper in the 5th chapter, it was possible to predict strength more accurately than using (C+F)/W without Kpm. That is, as it is possible to predict compressive strength of concrete through mixing factors of concrete matrix such as binder-water ratio and FA replacement ratio when using the cementing efficiency prediction model suggested in this study, it has more utility in quality control of FA concrete.; The compressive strength is the most universally used measure for concrete quality because of high correlations between compressive strength and other mechanical properties, and durability. For efficient quality control by estimating the concrete strength immediately, a model which can predicts the compressive strength from mix proportions of concrete through a multiple linear regression analysis is necessary. The statistical analysis method is easy to deduct a prediction model and understand the quantitative effects on factors influencing the compressive strength of concrete. Therefore, it has practical application such as mix design and durability evaluation as well as strength prediction of concrete. About 50% or more of ready-mixed concrete uses fly-ash(FA) because of environmental and economic benefit on recycling of industrial by-products and advantages such as excellent mechanical characteristics and durability of concrete containing FA. However, the effects of FA usage in concrete may not be maximized as it only uses FA minimally by simple replacement due to lack of understanding of strength development by Pozzolanic reaction, quality problems from a strength shortage in an initial age, and negative recognition of demanders. In order to use FA optimally, promote utilization of FA, and predict compressive strength of FA concrete exactly, it is important to determine cementing efficiency of FA based on the strength development of FA concrete. For determination of cementing efficiency, Smith suggested the cementing efficiency of FA with k value which could convert mass(F) of FA into an equivalent mass(kF) of ordinary Portland cement(OPC) considering the difference in strength development of FA and OPC due to Pozzolanic reaction. Therefore, it is possible to utilize the cementing efficiency of FA for predicting compressive strength and designing mixture by converting water-binder ratio(W/(C+F)) into modified water-cement ratio(W/(C+kF)). The compressive strength is generally used to determine the cementing efficiency. Therefore, if a model for determining the FA's cementing efficiency reflecting factors which have influences on compressive strength of FA concrete is suggested, it will be possible to improve reliability of concrete strength prediction. Existing models to determine the cementing efficiency of FA only reflect the influence of FA replacement ratio among concrete mixing factors, and they suggest that the cementing efficiency decrease as the FA replacement ratio increases. However, in order to calculate the cementing efficiency of FA exactly and to reflect it on strength prediction and concrete mix design easily and immediately, it is necessary to suggest a prediction model for cementing efficiency of FA adding mixing factors which have influences on strength development by Pozzolanic reaction of FA. This study aims to suggest a prediction model which can determine the cementing efficiency of FA only with the mixing factors including FA replacement ratio by using statistical analysis methods such as correlation analysis and multiple regression analysis between mixing factors and compressive strength of FA concrete. It is also the purpose of this study to verify the reliability of the suggested prediction model of FA cementing efficiency through experiment and to review the its applicability by evaluating the accuracy of prediction of compressive strength reflecting predicted cementing efficiency. This study is composed of total 6 chapters and contents of each chapter are as follows. The first chapter described the background, purpose, methods and scope of the study. The second chapter investigated application of the prediction model of cementing efficiency to predicting compressive strength of concrete through literature reviews about the prediction method on compressive strength of concrete, factors influencing compressive strength, and cementing efficiency. The third chapter suggested the prediction model for cementing efficiency of FA by using data on mixture and compressive strength of ready-mixed concrete. After confirming the quality level of concrete, it suggested a prediction model which could figure out the 28-day cementing efficiency of FA from mix proportions of FA concrete through multiple linear regression analysis of collected data. The cementing efficiency prediction model was based on the Bolomey’s equation on the relation between cement-water ratio and compressive strength, concept of relative strength which expressed the ratio of compressive strength of FA mixture to OPC mixture, and the concept of cementing efficiency. The fourth chapter confirmed the reproducibility of the cementing efficiency prediction model by conducting a mortar experiment, and also verified the reliability of the suggested model by the accuracy of strength prediction using (C+kpmF)/W. The 5th chapter collected data of new ready-mixed concrete containing FA and experimental data of FA concrete from research paper, and confirmed applicability for predicting compressive strength through (C+kpmF)/W using the FA cementing efficiency prediction model suggested in the 4th chapter. The 6th chapter, summarized the characteristics of suggested model for predicting cementing efficiency of FA and predicted cementing efficiency, and prediction of compressive strength. Conclusions of this study are as follows.

(1) Existing models for predicting cementing efficiency consider only FA replacement ratio among concrete mixing factors and suggest that the cementing efficiency of FA decreases as the FA replacement ratio increases. However, the FA cementing efficiency prediction model suggested in this study can explain not only the effect of binder-water ratio but also the phenomenon that the effect of FA replacement on cementing efficiency is reversed at about 15% of FA replacement due to using ready-mixed concrete data.

(2) It was analyzed that FA concrete showed the maximum strength development in about 15% of FA replacement ratio. However, as more than half of collected data substituted FA less than 10%, FA did not be used effectively for ready-mixed concrete.

(3) The cementing efficiency(kpm) estimated by the prediction model for cementing efficiency in FA mortar matrix are as follows