自充填高性能鋼筋混凝土梁剪力強度之測試與探討

Abstract

本研究主要在測試自充填高性能混凝土梁受靜態載重作用下之剪力及韌性行為。其目的在探討自充填高性能鋼筋混凝土梁在不同剪跨比、不同縱向鋼筋量、不同混凝土強度及剪力鋼筋量下之開裂剪力強度Vcr、極限剪力強度Vn及韌性的關係。並藉由試驗數據評估過去學者所發展的剪力強度公式及現行ACI 318-99規範對於剪力強度的適用性。 根據以上的目的，本研究規劃以三組剪跨比、四組剪力筋由rv=0，rv=rmin，rv=2rmin，rv=3rmin及三組縱向鋼筋量等變數加以探討外，另製作14根普通混凝土梁及14根自充填高性能混凝土梁，共28根梁試體進行試驗比較。試體尺寸斷面為180mm ×300mm。 試驗結果可歸納如下： 1.在變數相同的情況下，自充填高性能混凝土梁的剪力開裂強度與極限剪力強度大部分皆比普通混凝土佳。 2.自充填混凝土的剪力破壞模式仍類似於普通混凝土，但其斜裂縫的破裂面較為平整，且從其粗骨材剪裂，原因是有卜作嵐反應強化了骨材與水泥漿體的界面所致。 3.利用ACI、Zsutty及本研究之預測式推算自充填高性能混凝土梁與普通混凝土梁的剪力開裂強度及極限剪力強度，顯示ACI規範有趨於保守的現象，即ACI規範的規定亦適用於自充填高性能混凝土梁。而本文及Zsutty的預測式較ACI的預測式準確，適用於破壞強度的預測。 4.在變數條件相同的情況下，以SCC的韌性容量較高，尤以a/d=2拉力鋼筋適中，輔以剪力筋作用下，其試驗終點的最高韌性比高達52.34；比普通混凝土高出約17倍。顯示SCC有較好的耐震能力。 5.剪力開裂強度方面，有剪力筋者由於剪力筋的圍束作用比無剪力筋者來得大，尤以自充填混凝土更為明顯。 6.自充填高性能混凝土梁比普通混凝土梁之剪力撓度延展性佳，最高可達3倍。 7.自充填混凝土的剪力開裂強度及極限剪力強度，皆隨著跨深比之減少及拉力鋼筋比之增加而提高，此一現象與普通混凝土梁試體有相同的趨勢。而經試驗結果查知，ACI對剪力開裂強度之預估卻忽略了a/d的影響。 8.經分析剪力有效程度Se結果得知，Se值皆大於1，顯示ACI之公式有保守的現象。另Se與a/d有密切關係，且a/d愈大其Se愈小，本研究在a/d=4、混凝土強度為3000psi時Se值約為1.3，然a/d再增加時是否如ACI等於1，有待進一步確認。

ABSTRACT The purpose of this study is to investigate the behavior about shear and ductility of Self-compacting Concrete(SCC) beams under static load. The object of this research is to discuss the relationships among the cracking shear strength(Vcr), the ultimate shear strength(Vn) and the ductility of SCC high-performance beams(HPCs) with different groups of shear span-to-depth, various amounts of longitudinal steel, concrete strength and shear steel. Suitability of adoption for the empirical formulae developed by previous researchers and the current ACI 318-99 code was examined by the test results. Based upon the above objects, this study prepared the following variables for discussion: three groups of shear span-to-depth, three amounts of longitudinal steel and four shear steels with rv=0，rv=rmin，rv=2rmin，rv=3rmin included. In addition, 28 subjects of beams, with 14 regular ones and 14 SCC HPCs, were experimented and compared. The cross-section of the subjects was 180mm x 300mm. This study wraps up eight conclusions as belowed: 1. With same variables, most SCC HPCs perform better in Vcr and Vn than normal beams(NBs). 2. The breaking model of SCC HPCs is similar to that of NBs. However, owing to the Pozzolanic Reaction which consolidates the interface between aggregate and cement paste, the breaking plane of inclined cracking of SCC HPCs is much even and the origin of break is from the aggregate. 3. Comparing the inferences for Vcr and Vn of SCC HPCs and NBs by the predictive formulas from ACI, Zsutty and this research, the ACI code seems more conservative, which means that the ACI code also applys to SCC HPCs. Furthermore, the predictive formulas from Zsutty and this research are more precise, and are adoptable in the forecast of breaking strength. 4. With same variables, the ductile capacity of SCCs is higher, especially those of a/d=2 and medium tension steel with shear steel added. Highest ductility ratio at the end of the test reaches 52.34, which is 17 times to that of NBs, showing that SCCs possess better resistance against earthquake. 5. Due to the confinement of shear steel, Vcr of those with shear steel is bigger than that of those without. 6. SCC HPCs own better flexural ductility than NBs, with biggest difference of three times. 7. Vcr and Vn of SCCs enhance with the decrease of span-to-depth and the increase of tension-steel ratio. The trend of NBs is the same. From the test results, the ACI forecast about Vcr omits the effect of a/d. 8. From the analysis of the shear effectiveness(Se), all Se values are bigger than 1, which demonstrates the conservation of ACI formula again. Besides, there is a strong inverse relation between Se and a/d. The Se value of this research is about 1.3 with a/d=4 and concrete strength of 3000psi. However, whether the Se value approaches to 1 or not, as the ACI predicted, is in need of further confirmation.