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標題: 高流動性預力混凝土梁之剪力行為
Shear Behavior of Prestressed High-Flowability Concrete Beams
作者: 陳松堂
Chen, Sung-Tang
Contributors: 林建宏
土木工程學系所
關鍵字: 剪力;高流動性混凝土;預力混凝土梁
Shear;High-Flowability Concrete;Prestressed Concrete
日期: 2013
Issue Date: 2013-11-07 11:20:38 (UTC+8)
Publisher: 土木工程學系所
摘要: 本研究主要在探討不同流動性預力混凝土梁之混凝土種類、混凝土強度(f''c)、有效預力(Pe)、剪力筋間距(s)及剪跨比(a/dp),對於預力混凝土梁之剪力行為及ACI腹剪強度混凝土修正係數之影響。本研究共同製作10支普通混凝土(NC)預力梁及35支高流動性混凝土(HFC)預力梁。高流動性混凝土預力梁中包含10支高工作度混凝土(HWC)預力梁、15支自充填混凝土(SCC)預力梁(其中10支Type I及5支Type II)及10支砂-輕質自充填混凝土(LSCC)預力梁。經研究結果如下:
1.高流動性混凝土與普通混凝土預力梁之開裂剪力強度及極限剪力強度皆隨著混凝土強度增加、有效預力提高或剪跨比減小而提升,且剪力筋於剪力開裂後始發揮作用。
2.本試驗破壞模式皆控制為腹剪破壞,由最大剪力之支承面起向上成24°~45°間破壞,且皆在45°以下,表示ACI規範採45°作為混凝土標稱剪力強度計算標準應屬安全側。
3.以實際有效深度所計算之腹剪強度,比以ACI規範有效深度為0.8倍梁高所計算之腹剪強度為合理準確。
4.在有效深度斷面及相同變數條件下,高工作度、自充填Type I及輕質自充填混凝土預力梁之開裂剪力強度平均值分別為ACI腹剪強度之1.23、1.34及1.19倍,顯示ACI腹剪強度低估上述預力混凝土梁之開裂剪力強度。因高工作度、自充填Type I混凝土採用較多粗骨材,且使用飛灰及爐石粉取代部分水泥用量,使混凝土較為緻密,有利於剪力開裂之抵抗。
5.在有效梁深斷面及配置剪力筋條件下,普通、高工作度、自充填Type I、自充填Type II及輕質自充填混凝土之極限剪力強度平均值分別為ACI標稱剪力強度之1.52、1.73、1.68、1.72及1.66倍,顯示高流動性混凝土極限剪力強度均高於ACI標稱剪力強度1.5倍以上,尚稱合理。
6.有無剪力筋砂-輕質自充填混凝土預力梁之開裂剪力強度平均值分別為ACI腹剪強度之1.19及1.13倍,均顯示ACI以λ=0.85修正腹剪強度有偏保守現象。
7.依ACI 318-11規範,預力混凝土梁腹剪強度預測公式混凝土強度修正係數可修正為λm,則
普通混凝土: λm = 1.00
高工作度混凝土: λm = 1.20
自充填Type I 混凝土: λm = 1.35
自充填Type II混凝土: λm = 1.00
輕質自充填混凝土: λm = 1.00
8.各類混凝土之水固比 提高,則修正係數λm有下降之趨勢。
9.預力試體梁之粗細骨材比增加則勁度提高,而水固比增加則勁度下降。
10.不含輕質骨材之預力試體梁,其粗骨材體積或粗細骨材重量比增加,則預力損失皆減少。
The purpose of this study is to investigate the shear behavior and the modification factor of web-shear strength provided by concrete of prestressed beams with high-flowability concrete. Test parameters included the types of concrete, strength of concrete(f''c), amount of the effective prestress (Pe), pitch of the shear reinforcement(s), and shear span to depth ratio(a/dp). Ten normal concrete (NC) beams and 35 high-flowability concrete (HFC) beams were tested. The high-flowability concrete beams included 10 high-workability concrete (HWC) beams, 10 Type I self- consolidating concrete (SCC) beams, 5 Type II SCC beams, and 10 sand-lightweight SCC (LSCC) beams. The test results show the following:
1.The cracking and ultimate shear strength increase as the concrete strength , amount of the effective prestress increase, and the shear span to depth ratio decreases.
2.The inclination of major shear cracks range from 24 degrees to 45 degrees. The ACI uses 45 degrees and results in conservative predictions.
3.Use of ture effective depth dp results in more accurate shear strength than that dp=0.8h, as specified in the Code.
4.The cracking shear strength of HWC , Type I SCC and LSCC beams are higher than those of NC beams, due to larger amount of coarse aggregates contained in the concrete and the partial amount of cement replaced by fly ash and slag powder.
5.The ultimate shear strength of high-flowability beams are higher than those of NC beams. The ultimate shear strength of high-flowability beams are greater than 1.5 times of those of the ACI predictions.
6.The cracking shear strengths of LSCC beams of those of the ACI predictions without shear reinforcement are about 1.13 times, and 1.19 times for beams with shear reinforcement. It is conservative that the ACI uses a modification factor λ=0.85 for sand-lightweight concrete in calculating the nominal shear strength.
7.The λ value in ACI web-shear strength Vcw equation could be 1.00 for NC, 1.20 for HWC, 1.35 for Type I SCC, and 1.00 for Type II SCC and LSCC.
8.The λm value decreases as the water-solid ratio increases.
9.The stiffness of beam decreases as the water-solid ratio increases.
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