초음파속도법 및 반발경도법을 이용한 순환굵은골재 치환율과 플라이애쉬 혼입에 따른 콘크리트의 구조적 물성에 관한 연구

Abstract

본 연구에서는 재령 28일 설계기준강도를 목표로 21MPa, 27MPa, 35MPa에 대한 실험체를 제작하였으며, 각각의 배합에 순환 굵은골재를 0%, 30%, 50%, 100%를 치환⦁혼합하여 총 12배합을 설정하였다. 그리고 재령 1일, 3일, 7일, 28일, 160일에 대한 압축강도시험, 초음파속도법, 반발경도법을 실시하여 다음과 같은 결과를 도출하였다.

1) 공기량과 슬럼프값은 순환골재 품질기준에서 정하는 5.0±1.5% 와 120±15mm 사이인 3.5∼6% 와 105∼135mm 범위에서 측정되어 현장 적용에 문제가 없다고 판단된다. 다만, 순환골재 치환율에 따른 공기량과 슬럼프값의 상관관계는 뚜렷하지 않은 것으로 나타나서 순환골재 콘크리트 사용할 때, 사용 전 시험배합을 통하여 슬럼프 및 공기량에 대한 특성을 미리 파악해 놓아야 할 것으로 판단된다.

2) 재령 28일 기건양생 및 수중양생공시체에 대한 압축강도시험에서 순환골재가 혼입된 콘크리트는 천연골재 콘크리트보다 높은 강도 또는 거의 동등한 성능을 가지고 있었지만, 치환율에 따른 압축강도 상관관계는 적은 것으로 나타났다.

3) 기건양생공시체의 경우 21MPa는 평균 15% 증가, 27MPa는 평균 9% 증가, 35MPa는 평균 -1% 감소하였으며, 수중양생공시체의 경우 21MPa는 평균 7%, 27MPa는 평균 9%, 35MPa는 평균 1%가 증가하였다. 이러한 결과는 순환골재 사용 시 압축강도가 낮아진다는 기존에 알려진 사실과 반대되는 것으로서, 주된 원인으로 폐기물처리업체에서 고도 처리된 고품질 순환골재를 생산 시 발생하는 순환골재 미분말과 포졸란반응이라고 판단된다.

4) 35MPa의 경우 21MPa와 27MPa와는 다르게 천연골재와 거의 동등 또는 이하의 성능을 나타내었기에 강도에 영향을 줄 수 있는 순환골재 미분말과 플라이애쉬의 임계량이 존재할 것이라고 예상되며 이에 따른 추가적인 연구가 필요하다고 판단된다.

5) 재령 160일 기준 기건양생⦁수중양생⦁코어공시체에 대한 압축강도시험에 있어서 각각의 설계기준강도에 따른 순환 굵은골재의 치환율이 증가함에 따라 압축강도도 같이 증가하는 것을 알 수 있었으며 압축강도는 수중양생, 코어, 기건양생공시체 순으로 높게 측정되었다. 치환율이 높아질수록 압축강도가 증가함은 기존의 시멘트와 치환율에 따라 증가한 순환골재 미분말이 플라이애쉬와 반응하여 시멘트 수화 시, 수산화칼슘을 생성시키는 포졸란반응을 더욱더 활발히 일으켜 이미 수화되어 구성된 시멘트의 미세 조직에 생성물을 채워 넣어 덜 치밀한 부분까지 다시 채워주는 역할을 하기 때문이라고 판단된다. 6) 재령 28일 기준 변형률 게이지를 통하여 측정된 탄성계수는 순환 굵은골재의 치환율이 높아질수록 탄성계수가 높게 측정됨을 알 수 있었다. 다만, 현행 국내의 탄성계수 기준식이 해외의 기준식을 근거로 제정되어 있고, 국내 콘크리트 재료의 특성이 반영되어 있지 않아 국내 레미콘을 대상으로 한 포괄적인 실험을 통해 탄성계수 식이 제안되어야 할 것으로 판단된다.

7) 각각의 설계기준강도별 순환 굵은골재 치환율에 따라 측정된 포아송비는 일반적인 보통콘크리트 0.7의 응력에서 측정되는 0.15∼0.20의 범위 내에서 측정 되어 천연골재 콘크리트와 동등한 성능을 확보하고 있다고 판단된다.

8) 순환골재 치환율에 따른 초음파속도와의 상관관계는 적지만 재령일에 따른 강도 증가와 함께 파속도도 같이 증가함을 알 수 있었다. 재령일 160일 기준으로 측정된 초음파속도의 평균값은 21MPa는 4,205m/sec, 27MPa는 4,230m/sec, 35MPa는 4,314m/sec 로 압축강도가 증가함에 따라 파속도도 같이 증가함을 알 수 있었다. 이는, 초음파속도에 영향을 미치는 요인 중에서 콘크리트 내부의 밀도의 증가와 함께 함수량의 감소라고 판단된다.

9) 재령 28일과 재령 160일의 반발경도 R값을 비교해보면 순환 굵은골재의 치환율이 높아질수록 반발경도 R 값이 크게 나옴을 알 수 있었다. 이는 순환골재 콘크리트의 압축강도 추정 시 초음파속도법보다 유리할 것으로 판단된다.

10) 앞으로 연구 방향: 천연골재 콘크리트에 대한 고도 처리된 순환 굵은골재의 치환율의 따른 강도저하현상은 발견할 수 없었으며 순환골재에 대한 신빙성 있는 데이터 제공과 활용 증대 및 인식개선을 위하여 순환골재의 장기강도 저하현상과 구조체 적용을 통한 역학실험이 연속적으로 진행되어야 할 것이다. 또한, 순환골재 미분말과 플라이애쉬에 대한 상관관계와 임계량에 대한 추가적인 연구가 필요하다고 판단된다. | In this very research, specimens for 21MPa, 27MPa, and 35MPa were made to achieve the 28-day standard design compressive strength, where each mix was substituted and mixed with 0%, 30%, 50%, and 100% of recycled coarse aggregates to come up with the total of 12 different mixtures. In addition, a compression strength test, an ultrasonic pulse velocity test, and the rebound hardness test are carried out for 1-day, 3-day, 7-day, 28-day, and 160-day to induce following outcomes.

1) It is believed that there is no problematic issue in regards to the practical application of the values of the amount of air and slump since they were measured within the range defined by the quality standard of recycled aggregate 5.0±1.5% and120±15mm at 3.5∼6% and 105∼135mm. There are, however, uncertainties in regard to the correlation of changes in slump values and the values of amount of air for each recycled aggregate replacement ratio and this may require an apprehension of properties about slump and amount of air by creating a sample mixture prior to the actual use of recycled aggregate concrete.

2) As for the compressive strength test in regards to the 28-day curing test specimen and water curing test specimen, concrete mixed with recycled aggregate was identified to present higher or equal solidity relative to the natural aggregate concrete but the correlation with the compressive strength for each replacement ratio was low.

3) In case of a curing test specimen, 21MPa increased at an average of 15%, 27MPa with an average increase of 9%, but 35MPa decreased at an average of -1%. For water curing test specimen, 21MPa, 27MPa, and 35MPa all increased by averages of 7%, 9%, and 1%, respectively. Such a result contradicts the precedent research results where the property of compressive strength is found to decrease with the use of a recycled aggregate. The main factor that is causing this phenomenon is thought to be the grain powder of recycled aggregate along with the pozzolan phenomenon, which occur during the production process of a high quality recycled aggregate that has gone though a high-class treatment at the waste handling company.

4) In case of 35MPa, unlike 21MPa and 27MPa, it displayed an equal or a lower performance compared to the natural aggregate, thus it is expected that there exist critical mass of fly-ash and recycled aggregate grain and for this reason addition researches will be required in this scope.

5) As for the compressive strength test of curing test specimen, water curing test specimen, coring test specimen based on the 160-day standard, it was identified that the compressive strength increased along with an increase in the replacement ratio of recycled coarse aggregate for each specified compressive strength and the results of compressive strength are as follows in descending order of, water curing test specimen, coring test specimen and curing test specimen. The reason for an increase in the compressive strength along with a higher replacement ratio is because of the fact that the pozzolan reaction, which produces calcium hydroxide, is vitalized due from the reaction between the original cement and the increased amount of recycled aggregate grain powder in accordance with the replacement ratio during the process of cement hydration. This is turn is believed to fill in the refined structure of hydrated cement with products to further act as a role of refilling a rather less detailed sections.

6) The modulus of elasticity measured through a deformation rate gauge of 28-day standard was found to present higher value with an increase in the replace ratio of recycled coarse aggregate. However, the typical modulus of elasticity equation used in South Korea is derived based on the standard equation in foreign countries and it does not take into account the properties and characteristics of concrete materials in South Korea. For this reason, a further research subject to domestic ready-mixed concrete must be carried out for a comprehensive experiment to suggest an appropriate modulus of elasticity equation.

7) The measurement of Poisson’s ratio for each corresponding recycled coarse aggregate replacement ratio based on each category of specified compressive strength was carried out within the range of 0.15~0.20, which is typically applied for the stress of 0.7 with typical normal concrete, and the result showed that it bares equal level of performance when compared to the natural aggregate concrete.

8) The correlation with the ultrasonic pulse velocity according to the recycled aggregate replacement ratio was found to be very low but it has been identified that it can be increases along with an increase in the compressive strength at each set day. For the average value of ultrasonic pulse velocity measured according to the 160-day standard, 21MPa indicated 4.205m/sec, 27MPa with 4,230m/sec, and 35MPa with 4,314m/sec, which showed an increase in the ultrasonic pulse velocity along with an increase in the compressive strength. In other words, among the factors that affect the ultrasonic pulse velocity, an increase in the density of the inside of concrete causes a reduction in the total water content.

9) In a comparison of 28-day and 160-day rebound hardness value of R, an increase in the replacement ratio of recycled coarse aggregate increased the R value of rebound hardness. Generally speaking, this can be more useful in estimating the compressive strength of recycled aggregate concrete than the ultrasonic pulse velocity testing method.

10) Further Researches: From this research, a reduction of the compressive strength according to the replacement ratio of high-class treated recycled coarse aggregate in respect to natural aggregate concrete was not discovered. For this reason, a dynamical experiment with an application of structure and a long-term reduction in compressive strength of recycled aggregate must be continuously carried out in order to improve the awareness and increase the applications in addition to suggest credible data regarding recycled aggregates. In the end, it is judged that there is a necessity for an additional research in relation to the critical mass as well as the correlation between recycled aggregate grain and fly-ash.; In this very research, specimens for 21MPa, 27MPa, and 35MPa were made to achieve the 28-day standard design compressive strength, where each mix was substituted and mixed with 0%, 30%, 50%, and 100% of recycled coarse aggregates to come up with the total of 12 different mixtures. In addition, a compression strength test, an ultrasonic pulse velocity test, and the rebound hardness test are carried out for 1-day, 3-day, 7-day, 28-day, and 160-day to induce following outcomes.

1) It is believed that there is no problematic issue in regards to the practical application of the values of the amount of air and slump since they were measured within the range defined by the quality standard of recycled aggregate 5.0±1.5% and120±15mm at 3.5∼6% and 105∼135mm. There are, however, uncertainties in regard to the correlation of changes in slump values and the values of amount of air for each recycled aggregate replacement ratio and this may require an apprehension of properties about slump and amount of air by creating a sample mixture prior to the actual use of recycled aggregate concrete.

2) As for the compressive strength test in regards to the 28-day curing test specimen and water curing test specimen, concrete mixed with recycled aggregate was identified to present higher or equal solidity relative to the natural aggregate concrete but the correlation with the compressive strength for each replacement ratio was low.

3) In case of a curing test specimen, 21MPa increased at an average of 15%, 27MPa with an average increase of 9%, but 35MPa decreased at an average of -1%. For water curing test specimen, 21MPa, 27MPa, and 35MPa all increased by averages of 7%, 9%, and 1%, respectively. Such a result contradicts the precedent research results where the property of compressive strength is found to decrease with the use of a recycled aggregate. The main factor that is causing this phenomenon is thought to be the grain powder of recycled aggregate along with the pozzolan phenomenon, which occur during the production process of a high quality recycled aggregate that has gone though a high-class treatment at the waste handling company.

4) In case of 35MPa, unlike 21MPa and 27MPa, it displayed an equal or a lower performance compared to the natural aggregate, thus it is expected that there exist critical mass of fly-ash and recycled aggregate grain and for this reason addition researches will be required in this scope.

5) As for the compressive strength test of curing test specimen, water curing test specimen, coring test specimen based on the 160-day standard, it was identified that the compressive strength increased along with an increase in the replacement ratio of recycled coarse aggregate for each specified compressive strength and the results of compressive strength are as follows in descending order of, water curing test specimen, coring test specimen and curing test specimen. The reason for an increase in the compressive strength along with a higher replacement ratio is because of the fact that the pozzolan reaction, which produces calcium hydroxide, is vitalized due from the reaction between the original cement and the increased amount of recycled aggregate grain powder in accordance with the replacement ratio during the process of cement hydration. This is turn is believed to fill in the refined structure of hydrated cement with products to further act as a role of refilling a rather less detailed sections.

6) The modulus of elasticity measured through a deformation rate gauge of 28-day standard was found to present higher value with an increase in the replace ratio of recycled coarse aggregate. However, the typical modulus of elasticity equation used in South Korea is derived based on the standard equation in foreign countries and it does not take into account the properties and characteristics of concrete materials in South Korea. For this reason, a further research subject to domestic ready-mixed concrete must be carried out for a comprehensive experiment to suggest an appropriate modulus of elasticity equation.

7) The measurement of Poisson’s ratio for each corresponding recycled coarse aggregate replacement ratio based on each category of specified compressive strength was carried out within the range of 0.15~0.20, which is typically applied for the stress of 0.7 with typical normal concrete, and the result showed that it bares equal level of performance when compared to the natural aggregate concrete.

8) The correlation with the ultrasonic pulse velocity according to the recycled aggregate replacement ratio was found to be very low but it has been identified that it can be increases along with an increase in the compressive strength at each set day. For the average value of ultrasonic pulse velocity measured according to the 160-day standard, 21MPa indicated 4.205m/sec, 27MPa with 4,230m/sec, and 35MPa with 4,314m/sec, which showed an increase in the ultrasonic pulse velocity along with an increase in the compressive strength. In other words, among the factors that affect the ultrasonic pulse velocity, an increase in the density of the inside of concrete causes a reduction in the total water content.

9) In a comparison of 28-day and 160-day rebound hardness value of R, an increase in the replacement ratio of recycled coarse aggregate increased the R value of rebound hardness. Generally speaking, this can be more useful in estimating the compressive strength of recycled aggregate concrete than the ultrasonic pulse velocity testing method.

10) Further Researches: From this research, a reduction of the compressive strength according to the replacement ratio of high-class treated recycled coarse aggregate in respect to natural aggregate concrete was not discovered. For this reason, a dynamical experiment with an application of structure and a long-term reduction in compressive strength of recycled aggregate must be continuously carried out in order to improve the awareness and increase the applications in addition to suggest credible data regarding recycled aggregates. In the end, it is judged that there is a necessity for an additional research in relation to the critical mass as well as the correlation between recycled aggregate grain and fly-ash.