鋼管混凝土潛變行為之實驗研究

Abstract

隨著鋼管混凝土結構的應用越趨廣泛，各國對於鋼管混凝土相關研究也愈來愈多，尤其在對於鋼管混凝土結構耐震行為方面。然而對於鋼管混凝土之潛變行為研究則較為有限，在台灣對於鋼管混凝土結構之潛變相關研究則是更為稀少。 在現今長期對混凝土之研究與改良下，高強度、高性能之混凝土的使用日益頻繁，而在鋼管混凝土結構中，亦有將高強度自充填混凝土搭配進行使用的前例。而在鋼管混凝土結構中，混凝土未與空氣接觸，因此主要的收縮變形為自體收縮，為了減少因低水膠比及高漿體量而產生較多的自體收縮等問題，常會在混凝土配比中添加膨脹劑或其他摻料來達到目的，然而對於鋼管混凝土在添加膨脹劑後受軸壓之長期行為與混凝土本身在添加膨脹劑對於潛變與收縮之影響為何亦須要透過更多之實驗與研究來發現。 本研究以台北南山廣場之鋼管混凝土結構柱內灌漿配比進行潛變與收縮實驗，該配比為高強度且高性能自充填混凝土，配比中亦含有約3%之膨脹劑，藉由長期之軸壓實驗觀察鋼管混凝土與自充填混凝土之潛變與收縮行為，同時，因為混凝土產生潛變與收縮使得鋼管混凝土斷面應力重新分配，對於鋼管混凝土中應力轉移的現象進行分析，最後將潛變與收縮實驗的結果與本土化預測模型與其原始模型進行交叉比較與討論。 從實驗結果顯示，鋼管混凝土在有鋼管圍束的情況下使得內部混凝土在整體複合斷面強度提升下，對於混凝土潛變量的影響相較於無鋼管試體是較低的。鋼管混凝土在內部混凝土發生潛變與收縮的情況下使得鋼管所受的應力增加，在實驗齡期100天即增加了初始應力的30.8%。由於預測模型中皆未考慮膨脹劑的添加，因此預測模型相較於實驗結果的收縮量皆高出較多，顯示出膨脹劑的添加確實對於收縮量有產生減少的作用。而在預測模型對於實驗結果的比較中，潛變柔度函數的預測以Model B4-TW較為準確，相較於Model B4做了粒料強度之修正；且相較於資料庫量較少之Model B4-TW（SCC），Model B4-TW的預測值則更有可信度。

With the increasing application of concrete-filled steel tube (CFT) structures, there are more and more researches on performance of CFT structures, particularly in seismic behavior. However, the research on creep behavior of CFT is limited, and it is even rarer in Taiwan. As more and more improvement in concrete mix design, the uses of high strength and high performance concrete are increasing nowadays, and there are some precedent cases for the uses of high strength self-consolidating concrete in the CFT structures. In order to reduce the autogenous shrinkage caused by low water-to-cementitious ratio and high volume of paste in CFT structures, the concrete mix design is often added the expansion agent or other additives. However, the long-term behavior of CFT structures which are added expansion agent need to be discovered through more relative experiments and research. In this research, the creep and shrinkage experiments were carried out with the mix design of CFT columns in the Taipei Nan Shan Plaza, which is high strength and high performance self-consolidating concrete with 3% of expansion agent. The variables of creep experiments are the presence or absence of the lateral restraint and different loading ages; and the variables of shrinkage experiments are different curing ages and different curing condition. Then, owing to the occurrence of creep and shrinkage of internal concrete of CFT specimens, the phenomenon of stress transfer in CFT specimens could be analyzed. Finally, the experiment results were compared and discussed with the localized prediction models, Model B4-TW and Model B4-TW (SCC), and with its original model, Model B4. The experimental results show that the creep strain of internal concrete of CFT specimens which are under the lateral restraint of steel tubes is lower than that of the specimens without tubes. When the occurrence of creep in CFT specimen, the internal stress state in CFT specimen would change and made the stress of steel become larger. In this research, the results show that the stress in steel had grown 30.8% compared to initial stress in 100 days due to the creep in CFT specimen. Since the addition of the expansion agent is not considered in the prediction models, the shrinkage strain of prediction models is much higher than that of the experimental results, indicating that the addition of the expansion agent does reduce the amount of shrinkage strain. Last, in the comparison of the prediction models to the experimental results, the creep strain predicted by Model B4-TW is more accurate than the other two models, which means Model B4-TW is more credible in this research.