초음파의 비선형 특성을 이용한 콘크리트 동결융해 손상 평가
Evaluation of Freeze-Thaw Damage on Concrete Using Nonlinear Ultrasound
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초록
구조체 열화, 손상 등에 의해 발생하는 누수는 동결융해에 의한 체적 변화를 유발하는 주요 원인 중 하나이며, 콘크리트 내부의 미세균열, 표면 스케일링 등을 유발한다. 이러한 손상은 염화물 등 외기 유해물질 침투 및 확산을 가속화시킨다. 시설물 성능평가 세부지침(2020)에서 피복 콘크리트 품질과 동해환경 지표가 새롭게 제시되었으며, 피복 콘크리트 품질은 반발경도시험으로, 동해환경은 동결융해 싸이클 수로평가한다. 본 논문에서는 빠른 동특성 기반 초음파 비선형성을 통해 동결융해에 의한 초기 미세손상을 평가하고자 하였다. 서로 다른 물-시멘트비(40%, 60%)와 공기량(1.5%, 3.0%)을 가지는 콘크리트 시험체를 제작하고, 동결융해 싸이클 수를 증가시키며 압축강도, 반발경도, 상대동탄성계수, 초음파 비선형성을 측정하였으며, SEM을 활용하여 미세균열 발생 및 진전을 분석하였다. 그 결과, 상대동탄성계수 및 반발경도로는 확인이 어려웠던 초기 미세손상을 공진주파수 비선형성 측정을 통해 탐지할 수 있었으며, 콘크리트의 동결융해 저항성능을 예측할 수 있다는 가능성을 확인하였다.
Abstract
Leakage due to deterioration and damage is one of the major causes of volume change by freezing and thawing, and it leads micro-crackingand surface scaling in concrete structures. The deterioration of damaged concrete accelerates with the chloride attack. Thus, in the detailed guidelinesfor facility performance evaluation (2020), the quality of cover concrete and the freeze-thaw (FT) repetition cycle were newly suggested for concretedurability assessment. The quality of cover concrete should be evaluated by the rebound hammer test and the FT repetition cycle should be also consideredin the deterioration environmental assessment. This study suggested the application of fast dynamic based nonlinear ultrasound method to monitorinitial micro-scale damage under freezing and thawing environment. Concrete specimens were fabricated with different water-cement ratios (40%,60%) and air contents (1.5% and 3.0%). The compressive strength, rebound number, relative dynamic modulus, and nonlinear ultrasound weremeasured with different FT cycles. The scanning electron microscopy was also performed to investigate the micro-scale FT damage. As a result, boththe rebound number and the relative dynamic modulus had difficulty to detect early damage but the proposed method showed a potential to detectinitial micro-scale damage and predict the FT resistance performance of concrete.
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