Journal of the Korea Concrete Institute (콘크리트학회논문집)

Korea Concrete Institute (한국콘크리트학회)
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Technique for the Measurement of Crack Widths at Notched / Unnotched Regions and Local Strains

콘크리트의 노치 및 비노치 구역에서의 균열폭 및 국부 변형률 정밀 측정기법

  • Choi, Sok-Hwan (Dept. of Civil and Environmental Engineering, Kookmin University) ;
  • Lim, Bub-Mook (Dept. of Civil and Environmental Engineering, Kookmin University) ;
  • Oh, Chang-Kook (Dept. of Civil and Environmental Engineering, Kookmin University) ;
  • Joh, Chang-Bin (SOC Research Institute, Korea Institute of Construction Technology)
  • 최석환 (국민대학교 건설시스템공학부) ;
  • 임법묵 (국민대학교 건설시스템공학부) ;
  • 오창국 (국민대학교 건설시스템공학부) ;
  • 조창빈 (한국건설기술연구원 SOC성능연구소)
  • Received : 2012.02.01
  • Accepted : 2012.03.15
  • Published : 2012.04.30
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Abstract

Crack widths play an important role in the serviceability limit state. When crack widths are controlled sufficiently, the reinforcement corrosion can be reduced using only existing concrete cover thickness due to low permeability in the region of finely distributed hair-cracks. Thus, the knowledge about the tensile crack opening is essential in designing more durable concrete structures. Therefore, numerous researches related to the topic have been performed. Nevertheless accurate measurement of a crack width is not a simple task due to several reasons such as unknown potential crack formation location and crack opening damaging strain gages. In order to overcome these difficulties and measure precise crack widths, a displacement measurement system was developed using digital image correlation. Accuracy calibration tests gave an average measurement error of 0.069 pixels and a standard deviation of 0.050 pixels. Direct tensile test was performed using ultra high performance concrete specimens. Crack widths at both notched and unnotched locations were measured and compared with clip-in gages at various loading steps to obtain crack opening profile. Tensile deformation characteristics of concrete were well visualized using displacement vectors and full-field displacement contour maps. The proposed technique made it possible to measure crack widths at arbitrary locations, which is difficult with conventional gages such as clip-in gages or displacement transducers.

균열폭은 콘크리트 구조물의 사용성을 평가하는 측면에서는 매우 중요하다. 균열폭을 일정값 이하로 유지할 수 있다면 낮은 투수성을 유지할 수 있으므로 콘크리트의 피복만으로도 염소이온에 의한 부식을 방지할 수 있다. 따라서 내구적인 구조물을 설계하기 위해서는 인장 균열에 대한 충분한 정보가 필요하다. 그러나 균열폭을 정확하게 계측하는 데는 몇 가지 어려움이 있다. 먼저, 균열의 생성 위치를 미리 알기 어렵다. 또한 변형률 게이지 등 탄성영역에서 사용되는 게이지는 사용할 수 없다. 이러한 문제를 극복하기 위해서 화상상관기법 및 고해상도 CCD를 이용한 균열 및 변위계측 시스템을 개발하였다. 이를 통해서 임의의 위치에 생성되는 인장균열폭을 측정하는 방법을 제시하였다. 변위계측 정밀도 검증을 실시한 결과 평균오차는 0.069 픽셀, 표준편차는 0.050 픽셀이었다. UHPC를 이용하여 직접인장 실험을 수행하였다. 노치 구역과 비노치 구역에서 각각 균열을 측정하는 방법을 제시하고, 하중단계에 따라서 클립인 게이지의 결과와 비교하여 설명하였다. 시편의 전면에서 변위벡터를 구성하고, 등변위도 및 변형률도를 작성하였다. 다양한 실험에 적용할 수 있는 범용의 기법이기 때문에 임의의 균열폭 혹은 전면변위 측정 분야에서 많이 활용될 수 있을 것이다.

Keywords

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