高強度鋼纖維混凝土之直線及擴頭鋼筋握裹行為與伸展長度研究

Abstract

添加鋼纖維於混凝土之中可以提升混凝土抵抗開裂之性能，鋼纖維的橋接效應可以在混凝土開裂時轉移一些拉應力，因此鋼纖維能提供混凝土更高的斷裂能以及更好的圍束性能。在混凝土發生裂縫時，外力會部分轉移至鋼纖維上，使載重於混凝土和鋼纖維之間重分配，能有效防止混凝土開裂，隨著載重增加，則會出現更多的裂縫，直到不再有新的裂縫後，鋼纖維就會被拉拔破壞。而鋼筋與混凝土之間的握裹強度若是在沒有橫向箍筋配置時，通常在劈裂破壞發生時，其握裹強度主要是依混凝土的抗拉強度主控，而添加鋼纖維能顯著提高混凝土之抗拉強度，由以上可說明於混凝土中添加鋼纖維後，因鋼纖維之橋接效應，能夠增強鋼筋之握裹強度。本研究藉由12組直線鋼筋以及12組擴頭鋼筋進行梁端鋼筋握裹偏心拉拔實驗，期能釐清鋼筋與高強度鋼纖維混凝土間的握裹行為以及鋼纖維在握裹行為中扮演的角色。 直線鋼筋實驗中，通過實驗發現即便破壞模式為拉拔破壞其握裹應力仍然會上升，且鋼纖維於直線鋼筋握裹性能之效益提升顯著，最終本研究提出鋼筋適用於高強度鋼纖維混凝土的直線拉力伸展長度之設計公式，其中鋼筋有效計算強度以及混凝土強度可分別適用至690MPa以及80MPa。 而擴頭鋼筋實驗中，觀察到鋼纖維能提升混凝土拉破強度，並且能縮短擴頭鋼筋於鋼纖維混凝土中之錨定長度，證明鋼纖維對鋼筋於混凝土中錨定能力是有正面效益，最後本研究提出有關鋼纖維混凝土拉破強度公式之上限值建議以及擴頭鋼筋於鋼纖維混凝土中錨定長度之建議。

Adding steel fibers to concrete can enhance its crack resistance. The bridging effect of steel fibers can transfer some tensile stress when the concrete cracks, thus providing higher fracture energy and better confinement performance. When cracks appear in the concrete, the external force is partially transferred to the steel fibers, allowing the load to be redistributed between the concrete and the steel fibers, effectively preventing the concrete from cracking. As the load increases, more cracks will appear until no new cracks form, and the steel fibers are eventually pulled out and damaged. The bond strength between the rebar and concrete, in the absence of transverse stirrups, is primarily governed by the tensile strength of the concrete during splitting failure. Adding steel fibers significantly enhances the tensile strength of the concrete, indicating that the bridging effect of steel fibers in concrete can improve the bond strength of the rebar. This study involves eccentric pullout tests of 12 sets of straight bars and 12 sets of headed bars at beam ends, aiming to clarify the bond behavior between rebar and high-strength steel fiber concrete and the role of steel fibers in bond behavior. In the straight bar tests, it was found that even if the failure mode was pullout failure, the bond stress still increased, and the benefits of steel fibers on the bond performance of straight bars were significant. The study ultimately proposed a design formula for the development length of bars suitable for high-strength steel fiber concrete, with applicable rebar effective strength and concrete strength up to 690 MPa and 90 MPa, respectively. In the headed bar tests, it was observed that steel fibers greatly increased the breakout strength of the concrete and significantly shortened the anchorage length of headed bars in steel fiber concrete, proving that steel fibers positively impact the anchorage capacity of bars in concrete.