Cyclic Behavior of High-Strength Hook-Ended Steel Fiber Reinforced Concrete Exterior Beam-Column Joints

近年來高強度材料發展已漸趨成熟，如日本的New RC Project利用高強度材料可達到節省材料用量及減輕構件重量等多項益處。但伴隨而來的缺點是高強度混凝土十分脆性，其破壞模式與傳統混凝土並不相同，發生破壞時強度會瞬間驟降，顯然使用高強度混凝土有安全上的疑慮。 台灣地震頻繁，結構物必須具有足夠的韌性及耐震能力，傳統的方式就是增加橫向鋼筋以達到韌性需求。尤其梁柱接頭部分本就是梁柱主筋交會區，綁紮過密的橫向鋼筋易導致施工困難的問題。根據多項研究顯示，鋼纖維可明顯提升混凝土材料之韌性及抗拉、抗剪強度，且能在混凝土開裂時作為裂縫橋接並防止保護層過早剝落，故能取代部分橫向鋼筋的角色，改善綁紮過密造成之施工問題。本研究重點在探討高強度鋼纖維混凝土使用於梁柱交會區的可行性，並期望提出爾後設計鋼纖維梁柱接頭之配筋建議。 本研究設計四支實尺寸之外部梁柱接頭，分別在高軸力0.45A_g 〖f''〗_c及低軸力0.1A_g 〖f''〗_c下進行實驗。對照組為其中兩支綁紮緊密橫向鋼筋的試體，而另外兩支試體則為接頭處澆置體積取代率1.5%之鋼纖維混凝土取代全部箍筋繫筋。實驗結果顯示，在低軸力下，澆置鋼纖維混凝土的試體無論是破壞模式、位移韌性、能量消散、裂縫寬度控制…都與綁紮緊密橫向鋼筋的試體有相同甚至更佳的表現；而在高軸力下，澆置鋼纖維混凝土的試體因圍束不足而提早發生破壞，其表現不如綁紮緊密圍束箍筋之試體。但所有試體的實驗結果皆滿足規範之檢核，因此使用鋼纖維取代接頭處之橫向鋼筋是可行的，但建議可綁紮少量箍筋來更提升梁柱接頭處之圍束能力。In recent years, high strength materials are used more and more generally. For example, they were used to save consumption of materials and reduce weight of members in New RC Project. However, high strength concrete is too brittle. It will lose strength rapidly when failure occurs, which leads to the obstacle of application due to the concern of safety. In Taiwan, which is in the seismic region, structures need to be designed with sufficient toughness and ability of seismic restraint. Using high transverse reinforcement ratio arrangement is the general way. However, for beam-column joint, which is the connection of longitudinal bars of beam and column, intensive transverse reinforcement arrangement will lead to construction difficulty. According to many experiment results, adding steel fiber in concrete can obviously enhance toughness, tensile strength and shear strength of concrete and prevent cover spalling in the early stage so that it could replace the role of transverse reinforcements to solve the congestion problem. The objective of the study is to investigate the feasibility of casting high strength steel fiber concrete in beam-column joints to replace parts of transverse reinforcements. This study aims to propose suggestions of designing reinforcement detailing. Four full-scale exterior beam-column joints, including two specimens with intensive stirrups and two specimens cast with steel fiber concrete without any stirrup, are tested. The specimens are under low and high axial load ratio at 0.1 and 0.45, respectively. Test results indicate that specimen cast with steel fiber concrete performs as good behavior as specimen with intensive transverse reinforcements on failure mode, displacement ductility, energy dissipation and crack width control under low axial load. However, under high axial load, the specimen cast with steel fiber concrete fails earlier than specimen with stirrups due to lack of confinement. Test results of all specimens attain requirements, it means using steel fiber concrete to substitute stirrups is feasible. It is still proposed to use small amount of stirrups to enhance ability of confinement in beam-column joints