高強度鋼纖維鋼筋混凝土柱的軸壓行為與圍束效應之研究

Abstract

鋼筋混凝土由於其優異的耐久性以及成本低、易維護等優點，是目前世界上應用範圍最普遍的建築材料。隨著近代都市發展快速，在有限的土地面積下高層建築已成趨勢。日本政府在1988年到1992年間推行五年期國家型計畫New RC Project來滿足高層建築鋼筋混凝土構造高強度、輕量化之需求，此計畫大幅提升材料強度，鋼筋降伏強度從4000 kgf/cm2提升至7000 kgf/cm2，混凝土抗壓強度也從280 kgf/cm2提升至700 kgf/cm2，在1993年計劃結束以後，使用New RC的建物數量迅速增加，如此效益優良的材料目前台灣也在著手推行以及研究。 然而，如此高強度材料通常是極為脆性的，使用New RC的建物若不幸發生破壞，幾乎是沒有任何的緩衝時間，對居住安全帶來非常大的威脅，尤其是對位於地震帶的台灣而言在推廣前更是需要深思的議題。目前許多文獻指出，添加鋼纖維的混凝土比一般混凝土具有更佳的韌性，用於高強度混凝土效果更是顯著。一般而言，RC柱要透過增加圍束箍筋來達到耐震規範下的韌性標準，倘若能藉由鋼纖維取代圍束箍筋來達到同等的材料韌性，除了能降低鋼筋綁紮困難度，也能預防早期保護層剝落的現象，達到更優良的材料行為。 本研究主要探討鋼纖維鋼筋混凝土柱的軸壓行為與圍束效應，以不同之箍筋間距加入不同比例之鋼纖維來了解箍筋與鋼纖維所提供之圍束力，進而了解兩者間的材料取代效益。本研究同時針對鋼纖維混凝土與鋼纖維鋼筋混凝土之韌性指數以及應力應變曲線進行回歸整理，以利於工程界分析與應用。

From 1988 to 1992, the Japanese government carried on a five-year national project, New RC Project, to substantially increase the strength of the construction materials of the reinforced concrete for high rise buildings. The concrete strength rises from 40 MPa to 120 MPa, and the yielding strength of steel bar rises from 420 MPa to 685 MPa. Up to 2007, there are more than 500 New RC buildings in Japan. Due to brittleness of high strength concrete, much more confinement is needed to improve the ductility and satisfy seismic requirements for high strength reinforced concrete column, particularly under high axial loading demands. Furthermore, early cover spalling trigs substantial compressive strength loss and thus sudden failure occurs. Many studies show that addition of steel fibers can not only effectively prevent the early cover spalling, but also increase the toughness and ductility of high-strength concrete. The compressive behavior and confinement effect of high strength steel fiber reinforced concrete columns are investigated in this study. Regression relationships of toughness ratios and fiber/transverse reinforcement parameters are conducted based on the past experimental results. Nine high strength steel fiber reinforced concrete columns (200×200×900mm) were designed with the same toughness ratio to verify the validity and reliability of the proposed formulas. The results indicate that additions of steel fiber not only avoid the premature concrete cover spalling, but also increase the ultimate axial capacity. It is also found that the predicted toughness ratios obtained from the proposed equation have a good agreement with those got from the tests.