국내 건설폐기물은 매년 늘어나는 추세이며 2021년 총 폐기물 발생량의 42.5%를 차지한다. 이러한 건설폐기물 발생량을 줄이기 위해 순환골재를 사용할 수 있다. 순환골재는 건설폐기물을 재활용하여 만드는 친환경 골재이며 콘크리트 생산 시 발생하는 탄소배출량을 줄여 탄소중립을 실천할 수 있는 재료이다. 그러나 순환골재는 생산 과정에서 탈거되지 않은 부착모르타르와 분쇄 과정에서 생기는 미세 균열로 인해 일반골재에 비해 성능이 저하된다. 또한 순환골재의 생산 방법, 생산 지역, 모 콘크리트의 특성 등에 복합적으로 영향을 받아 품질에 큰 편차를 보인다. 이를 극복하기 위해 국내외에서 순환골재 콘크리트의 성능에 대한 많은 연구가 이루어지고 있으며 강관기둥에 충전하여 성능을 개선하는 방법이 제시되었다. 하지만 국내에서는 순환골재 콘크리트충전 강관기둥에 대한 연구가 진행되지 않았으며 해외에서만 일부 진행되어 순환골재 콘크리트충전 강관기둥의 연구가 필요한 실정이다. 콘크리트충전 강관기둥의 큰 이점은 구속효과로 인해 콘크리트의 강도가 상승하며 강관의 국부좌굴 영향을 감소시킨다는 점이다. 그러나 순환골재 콘크리트는 일반골재 콘크리트에 비해 낮은 성능을 보이고 편차가 크기 때문에 순환골재 콘크리트충전 강관기둥에 대한 구속효과에 대한 규명이 필요하다. 본 연구에서는 순환골재 콘크리트충전 강관기둥의 구속효과를 평가하는 것을 목적으로 한다. 순환골재의 특성과 구속효과 매커니즘 등 이론적 고찰을 통해 순환골재 콘크리트충전 강관기둥의 구속효과를 분석하고 실험을 통해 구속효과를 평가한다.
본 연구에서는 순환골재 콘크리트충전 강관기둥의 구속효과를 평가하기 위하여 순환골재 치환율과 강관 직경을 변수로 9개의 실험체를 제작하였고 순환골재 콘크리트의 영향을 평가하기 위해 콘크리트 단면에만 하중을 재하하여 압축실험을 진행하였다. 순환골재 치환율에 따른 구속효과와 강관 직경에 따른 구속효과를 분석하였으며 과거 제시된 구속된 콘크리트의 압축강도 모델들과 비교하여 구속효과를 평가하였다. 그 결과 다음과 같은 결론을 도출하였다.
(1) 순환골재 치환율 0, 25, 100% 콘크리트를 타설하여 순환골재 콘크리트충전 강관기둥의 콘크리트 단면에만 하중을 재하한 압축실험을 진행한 결과 순환골재 25% 치환 시 6.4 ~ 17.2%, 순환골재 100% 치환 시 15.9 ~ 22.8%의 내력이 감소하였다. 순환골재 100% 사용 시 성능 저하와 품질의 편차가 더 크게 기여하여 콘크리트 공시체 압축시험보다 실험체의 압축 내력이 더 크게 감소한 것으로 판단된다.
(2) 순환골재 치환율에 따른 순환골재 콘크리트충전 강관기둥의 구속효과를 평가하기 위해 콘크리트 단면의 내력과 실험 압축 내력의 비를 Strength Index로 나타내어 비교한 결과 순환골재 25% 치환 시 일반골재 콘크리트 대비 Strength Index가 0 ~ 12.9% 증가하였으며 순환골재 100% 치환 시 Strength Index가 1.1 ~ 9.1% 감소하였다. 따라서 순환골재 25% 치환 시 구속효과가 감소하지 않으며 100% 치환 시 순환골재의 성능 저하로 인해 구속효과가 감소하였다.
(3) 강관의 직경에 따른 순환골재 콘크리트충전 강관기둥의 구속효과를 평가하기 위해 세 가지 크기의 강관 직경에 따른 Strength Index를 비교한 결과 순환골재 치환율이 25, 100%에서 강관 직경이 증가함에 따라 구속효과의 감소율이 각각 4.2%에서 –0.3%, 6.7%에서 5.7%로 감소하였다. 이에 강관 직경의 크기는 순환골재 콘크리트충전 강관기둥의 구속효과에 영향을 미치지 않음을 확인하였다.
(4) 순환골재 콘크리트충전 강관기둥의 Strength Index와 구속된 콘크리트의 압축강도 예측 모델을 비교하였다. Hatzigeorgiou, Tang, Yang의 예측 모델에서 순환골재 100% 치환 콘크리트의 내력을 5 ~ 24% 크게 평가하고 있다. 따라서 순환골재 100% 치환 콘크리트 사용 시 순환골재의 성능 감소와 편차를 고려하여 감소된 구속효과를 적용해야 할 것으로 판단된다.

Domestic construction waste is increasing every year, accounting for 42.5% of the total waste generation in 2021. Recycled aggregate can be used to reduce the amount of construction waste. Recycled aggregate is an eco-friendly aggregate made by recycling construction waste, and it is a material that can practice carbon neutrality by reducing carbon emissions generated during concrete production. However, the performance of recycled aggregate is lower than that of normal aggregate due to the attached mortar that is not removed during the production process and the micro cracks generated during the crushing process. In addition, the production method of recycled aggregate, the production area, and the characteristics of the parent concrete are complexly affected, resulting in a large deviation in quality. In order to overcome these disadvantages, many studies have been conducted on the performance of recycled aggregate concrete at home and abroad, and methods for improving performance by filling recycled aggregate concrete into steel tube columns has been proposed. However, research on recycled aggregate concrete-filled steel tube columns has not been conducted in Korea, and only in foreign countries, research on recycled aggregate concrete-filled steel tube columns is needed. The great advantage of concrete-filled steel tube columns is that they increase the strength of concrete due to the confinement effect and reduce the local buckling effect of steel tubes. However, since recycled aggregate concrete shows lower performance than normal aggregate concrete and the variation is large, it is necessary to investigate the confinement effect on steel tube columns filled with recycled aggregate concrete. The purpose of this study is to evaluate the confinement effect of a steel tube column filled with recycled aggregate concrete. Through theoretical considerations such as the characteristics of recycled aggregate and the mechanism of confinement effect, the confinement effect of the steel tube column filled with recycled aggregate concrete is analyzed, and the confinement effect is evaluated through experiments.
In this study, in order to evaluate the confinement effect of a steel tube column filled with recycled aggregate concrete, nine specimens were produced using the recycled aggregate replacement rate and the diameter of the steel tube as variables. Compression experiments were conducted by loading only the surface of concrete. The confinement effect according to the replacement rate of recycled aggregate and the diameter of the steel tube were analyzed, and the confinement effect was evaluated by comparing with the compressive strength models of confined concrete presented in the past. As a result, the following conclusions were drawn.
(1) Concrete was placed according to the replacement rate of recycled aggregate of 0, 25, and 100%, and a compression test was conducted in which the load was applied only to the surface of concrete. The yield strength decreased by 6.4 to 17.2% when replacing 25% of recycled aggregate and by 15.9 to 22.8% when replacing 100% of recycled aggregate. When using 100% recycled aggregate, it is judged that the compressive strength of the test specimen decreased more significantly than the concrete specimen compression test due to the greater contribution of performance degradation and quality variance.
(2) In order to evaluate the confinement effect of recycled aggregate concrete-filled steel tube columns according to the replacement rate of recycled aggregate, the ratio of the strength of the concrete cross section and the experimental compressive strength was expressed as a Strength Index and compared. As a result, When 25% of recycled aggregate was replaced, the strength index increased by 0 to 12.9% compared to general aggregate concrete, and when 100% of recycled aggregate was replaced, the strength index decreased by 1.1 to 9.1%. Therefore, the confinement effect did not decrease when 25% of the recycled aggregate was replaced, and the confinement effect decreased due to the performance degradation of the recycled aggregate when 100% was replaced.
(3) In order to evaluate the confinement effect of steel tube columns filled with recycled aggregate concrete according to the diameter of the steel tube, the Strength Index according to the diameter of three sizes of steel tubes was compared. As a result, At 25 and 100% recycled aggregate replacement rates, as the diameter of the steel tube increased, the reduction rate of the confinement effect decreased from 4.2% to -0.3% and from 6.7% to 5.7%, respectively. Accordingly, it was confirmed that the size of the steel tube diameter did not affect the confinement effect of the steel tube column filled with recycled aggregate concrete.
(4) The strength index of the recycled aggregate concrete-filled steel tube column and the compressive strength prediction model of the confined concrete were compared. In the prediction model of Hatzigeorgiou, Tang, and Yang, the compressive strength of concrete replaced with 100% recycled aggregate is estimated to be 5 to 24% higher. Therefore, when 100% recycled aggregate replacement concrete is used, it is judged that the reduced confinement effect should be applied in consideration of the performance decrease and deviation of recycled aggregate.