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콘크리트 장기거동 예측 모델은 많은 오차를 포함하고 있다. 구조물 장기거동 예측의 정확성을 높이기 위해 실험실 수준에서 콘크리트 재료의 장기거동 실험을 수행하여 예측 모델을 보정하여 사용하고 있다. 그러나 장기거동 실험은 시간 및 비용이 많이 소모되고 실험 여건에도 많은 제약이 따르기 때문에, 장기거동에 영향을 미치는 여러 가지 영향 인자들에 대해 모두 실험을 수행하기는 어렵다. 따라서 이 연구에서는 최소한의 실험으로 장기거동 특성을 예측하기 위한 최적의 장기거동 실험변수를 결정하는 방법을 제안하고자 하였다. 이를 위해 기존 장기거동 예측 모델을 분석하여 설계기준 압축강도 40 MPa 급의 콘크리트 장기거동 특성을 파악하였다. 파악된 장기거동 특성으로부터 크리프 및 건조수축 최적의 실험변수를 제안하였다. 크리프 실험변수에 대해서는 가압 시 재령 세 가지 변수(1~14일, 14~84일, 84~365일), 상대습도 두 가지 변수(40~60 %, 60~80 %), 부피-표면적비 세 가지 변수(37.5 mm, 75~200 mm, 200 mm 이상)로, 건조수축 실험변수에 대해서는 노출 시 재령 두 가지 변수(14일 기준 전?후 두 가지 재령), 상대습도 두 가지 변수(40~60 %, 60~80 %), 부피-표면적비 세 가지 변수(크리프 실험체와 동일한 크기)에 대해 실험하는 것이 재료모델을 구축하는데 타당한 것으로 나타났다. more...

In this study, quantitative evaluation on the enhancement in the ductility of 60-m long span prestressed concrete (PSC) girder to which the high-strength prestressed (PS) strands were applied will be made through the finite-element analysis. The extent of enhancement in the ductility was quantitatively evaluated by computing the deflection ductility index in accordance with the types of the PS strands. As the results of comparison with the deflection ductility index of PSC girder in which PS strands with a tensile strength of 1,860 MPa were used, the ductility was improved by about 1.38 folds when the PS strands with a tensile strength of 2,160 MPa are applied while there was approximately 1.71 folds enhancement in ductility when the PS strands with a tensile strength of 2,400 MPa were applied. In addition, the presence of large quantity of tendons can induce brittle failure of the girder due to the concrete fracturing in the compression region without the yielding of the PS strands. Therefore, it... more...

The primary objective of this study is to verify if the rheological properties of the lubricating layer obtained from the newly developed tribometer can give a good prediction of the real flow rates of pumped concrete. An equation for the calculation of the flow rates was derived considering the thickness and the rheological properties of the layer. Full-scale pumping tests were performed with pipelines of 350 and 548 m (383 and 599 yd) with seven different mixtures. From the test results and the comparison between the measured flow rates and the flow rates calculated from the equation, it was verified that the flow rates of pumped concrete can be accurately predicted before pumping according to the rheological properties of the lubricating layer measured with the tribometer. It was also found that the rheological properties of the lubricating layer remained constant during pumping, whereas the rheological properties of the concrete were found to vary. more...

The character of the lubricating layer formed near the wall of a pipe during the flow of pumped concrete is a dominant factor facilitating concrete pumping. To predict flow rates of concrete transported by pumping, it is necessary to measure the rheological properties of the lubricating layer before pumping. In this paper, a new version of a test instrument called a tribometer is developed by considering the level of shear rate that induces the lubricating layer during real pumping. A test method is also suggested based on numerical simulation of the shear flow inside the tribometer and, finally, equations capable of transforming the values measured in the new tribometer into the rheological properties of the lubricating layer are derived. Work in this paper will be extended in a subsequent paper in which full-scale pumping tests will be performed, and predictions for concrete pumping based on the measurements of the new tribometer will be verified from comparisons with the test results. more...