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標題: | 添加鋼纖維對於高強度混凝土瓶狀壓拉桿行為之影響 The Effect of Steel Fiber on Bottle-Shaped Strut-and-Tie Behavior of High Strength Concrete |
作者: | Chung-Wen Hung 洪崇文 |
指導教授: | 廖文正(Wen-Cheng Liao) |
關鍵字: | 鋼纖維,高強度混凝土,壓拉桿行為,New RC, Steel fiber,High strength concrete,Strut-and-tie behavior,New RC, |
出版年 : | 2020 |
學位: | 碩士 |
摘要: | 高強度鋼筋混凝土材料已被推廣多年,國內規範亦逐漸更新並採用之,然而高強度混凝土的性質十分脆性,不同的破壞模式需要在設計時被考量,為了維持足夠耐震能力,於梁柱接頭等結構不連續區域需配置大量剪力箍筋而導致施工與澆置上的困難,而添加鋼纖維於混凝土中為可減緩脆性破壞之方法之一。 研究結果顯示,添加鋼纖維於混凝土中能提升其韌性與抗剪強度,使未配置橫向箍筋之鋼纖維混凝土梁柱接頭在勁度、強度或能量消散三部分皆通過現有規範檢核,鋼纖維提供之橋接效應亦能有效抑制混凝土裂縫生成,並解決箍筋過密的問題。然而,雖然已知使用鋼纖維混凝土於結構不連續區之顯著效果,但其力量傳遞行為仍有待釐清。 有鑑於此,本研究主要探討高強度鋼纖維混凝土於結構不連續區之力學行為,製作平版試體進行試驗以模擬瓶狀壓拉桿模式,並採鋼纖維體積取代率、橫向鋼筋比與橫向鋼筋位置做為設計參數,以了解添加鋼纖維之效益。實驗結果顯示,未配置鋼筋的純混凝土試體S0-N為脆性之劈裂破壞,於添加鋼纖維後即使未配置鋼筋,S075-N與S150-N試體之破壞模式仍可轉變為韌性之壓桿擠碎破壞,顯示添加鋼纖維可以提供額外拉桿力,並使試體擁有更佳的變形能力,造成破壞模式的轉變。 試體開裂強度與極限強度亦會隨著鋼纖維的添加而提升。相較於配置鋼筋試體,開裂強度與極限強度最高可分別提升40%與30%,無鋼筋系列試體則可提升80%開裂強度、40%極限強度。此外,低鋼筋比且鋼筋集中配置於接近試體承壓處之CL系列試體極限強度表現最佳,顯示除了鋼筋比,鋼筋配置位置亦為一設計重要因子。從試驗結果亦可得知,ACI 318-19訂定之壓桿有效係數相對保守,而添加足量鋼纖維之純混凝土試體能擁有與配置最小鋼筋量之普通混凝土試體相當的強度表現,顯示鋼纖維取代橫向鋼筋的潛在能力。 除了強度的提升,添加鋼纖維對於試體的韌性與消能表現亦有提升效果,對於裂縫控制更有顯著影響,能使寬度大的主裂縫轉變為較多細小裂縫,進一步造成主拉應變降低,而非原模型建議之定值。為顯示添加鋼纖維對於裂縫抑制的效益,並將評估鋼纖維抗拉拔能力之重要指標「有效握裹強度」納入參數,以完整呈現添加鋼纖維之效果,本研究透過實驗結果回歸,提出鋼纖維軟化係數之建議計算式,將建議之鋼纖維軟化壓拉桿模型用於評估深梁、外部梁柱接頭與平版試體之強度皆有一定的準確性與合理性,顯示添加鋼纖維之效益能有效以該建議之軟化係數評估。 High strength reinforced concrete has been promoted and utilized in high-rise buildings for several years; however, concrete becomes more brittle if its compressive strength increases. For the purpose of mitigating brittle behavior in high shear demanding area, such as beam-column joints, lots of shear reinforcement are required in design codes, which will cause steel congestion and construction difficulties. Through adding steel fiber into concrete, which is so-called steel fiber reinforced concrete (SFRC), brittle response can be improved and cracks can also be restrained effectively because steel fiber provides bridging action across microcracks in the matrix. Moreover, SFRC is expected to enhance shear strength and replace certain amount of transverse reinforcement for better construction workability. In this study, a series of panel experiments were conducted to verify the force mechanism of SFRC in discontinuity region. Three parameters were used, including volume fraction of steel fiber, ratio of transverse reinforcement and location arrangement of transverse reinforcement. The test result shows that when adding steel fiber in concrete, specimens with brittle splitting failure could change its failure mode to concrete crushing. Both cracking strength and ultimate strength increase with the presence of steel fiber. Compared to non-fibrous specimens with transverse reinforcement, panels with steel fiber can increase cracking strength and ultimate strength up to 40% and 30%, respectively. Furthermore, specimens designed with concentrated arrangement and low ratio of reinforcement have the highest ultimate strength. It indicates that apart from reinforcement ratio, location of reinforcement is one of the important design factors. After analysis on experiment data, the strut coefficient in ACI 318-19 is considered to be conservative. Specimens possessing enough amount of steel fiber could have excellent structural performance and behave like those with minimum distributed reinforcement ratio. In addition to strength development, toughness and energy dissipation increase in fibrous panels. The existence of steel fiber also has significant impact on crack control. Consequently, crack pattern changes from single huge main crack to several small cracks. The average tensile strain decreases instead of a constant value recommended by the original softened strut-and-tie model. In order to present the effect of steel fiber on crack restraint and highlight the importance of using equivalent bond strength as a parameter to illustrate the benefit of adding steel fiber into concrete, a modified formula of steel fiber softening coefficient is proposed. By applying the proposed coefficient, the steel fiber softened strut-and-tie model can accurately predict the strength of deep beams, exterior joints and panel specimens in the database. It indicates that the advantage of steel fiber can be evaluated by the proposed softening coefficient. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/8313 |
DOI: | 10.6342/NTU202002509 |
全文授權: | 同意授權(全球公開) |
顯示於系所單位: | 土木工程學系 |
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