考虑箍筋效应的PP-ECC梁抗剪试验研究

Experimental Investigation on Shear Behavior of PP-ECC Beams by Considering Effect of Stirrup

为了研究钢筋加强工程水泥基复合材料(ECC)梁中的箍筋抗剪加强效应,针对发生剪切破坏的普通钢筋混凝土(RC)构件,考虑箍筋率的影响,分别进行RC与ECC梁的四点加载试验研究。首先开展ECC材料试验,采用直接拉伸的加载方式,对聚丙烯纤维工程水泥基复合材料(PP-ECC)的拉伸力学性能进行试验研究。在材料试验结果的基础上,通过考虑箍筋效应,分别设计5根不同箍筋率的钢筋增强PP-ECC梁和2根普通钢筋混凝土梁试件,对7根梁进行四点加载试验,并在加载过程中对5根PP-ECC梁的斜裂缝进行观测,分析箍筋率对ECC梁斜裂缝开展行为的影响。通过修正桁架模型,分析箍筋率对ECC梁抗剪承载力的影响。试验结果表明:PP-ECC具有拟应变硬化和微裂缝的多缝开裂特征,其屈服拉伸强度和拉伸极限强度分别不小于2,3MPa,极限拉应变大于2.5%;PP-ECC梁有较好的剪切延性,随着箍筋率的增长,PP-ECC梁的抗剪承载力也逐渐加大;在相同箍筋率下PP-ECC梁的抗剪承载力大于RC梁,而无箍筋PP-ECC梁的抗剪承载力2倍于无箍筋RC梁;由于箍筋限制了R/ECC梁斜裂缝的开展,加剧了斜裂缝的剪切滑移,从而削弱了斜裂缝间的纤维桥接作用,导致PP-ECC承担的剪力随着箍筋率的增大而减小;现有规范未考虑斜裂缝的剪切滑移对PP-ECC抗剪承载力的削弱作用,使得计算剪切承载力过大而导致偏危险。

To investigate the shear reinforcing effect of stirrups in the steel reinforced engineered cementitious composite （ECC, R/ECC） beams, in allusion to the steel reinforced concrete （RC） structural member with shear failure, four-point loading tests were conducted on both RC and ECC beams by considering the effect of stirrup ratios. The tensile mechanical properties of polypropylene fiber reinforced engineered cementitious composites （PP-ECC） were firstly investigated by conducting uniaxial tensile tests. Based on the results, a total of 7 beams, classifying into 2 categories, namely, 5 steel reinforced PP-ECC and 2 RC beams, were designed and constructed. All beams were tested by four-point loading tests. In addition, the shear cracking behavior of 5 PP-ECC beams were carefully observed and measured during the loading, and the effect of stirrup ratio on the shear cracking behavior was analyzed. Through the modified truss model, the shear capacities of 5 PP-ECC beams were investigated by considering the effect of stirrup ratio. The experimental results show that PP-ECC exhibiting pseudo strain-hardening behavior and multiple fine cracking has tensile yield and tensile strength which is no less than 2 MPa and 3 MPa, respectively, and its tensile strain capacity is greater than 2.5%. ECC beams are with good ductility, whose shear capacity gradually increases with the increase of stirrup ratio. Under the same stirrup ratio, the shear capacity of R/ECC beam is larger than that of RC, and the R/ECC beam without stirrups shows twice shear capacity of RC beam. In addition, the shear carried by PP-ECC decreases with the increase of the stirrup ratio. The reason is that the higher stirrup ratio restricts the development of shear cracks, and the sliding in the critical shear cracks increases, thus decreasing the bridging effect of fibers. However, the current design and specification does not take the effect of shear sliding into considerations, which may result in overestimation on shear capacity of R/ECC beams.