腹筋对轻骨料混凝土深受弯构件受剪性能及尺寸效应影响的试验研究

Experimental analysis on effect of web reinforcement on shear behavior and size effect of lightweight aggregate concrete deep beams

完成对称集中荷载作用下的8根轻骨料混凝土深受弯构件及2根普通混凝土深受弯构件受剪性能试验,分析了腹筋配筋率及截面高度对破坏过程及形态、荷载-跨中挠度曲线、特征荷载、斜裂缝发展趋势、纵筋及腹筋应变等的影响机制与规律,重点明确了腹筋对深受弯构件抗剪承载力及尺寸效应影响的作用机理。为合理量化腹筋在传力机制中的贡献,建立以腹筋作为竖向拉杆的改进拉-压杆模型(STM),结合对典型压杆有效系数模型的分析和评价,验证了改进STM的有效性。试验结果表明:试件主要发生剪压破坏,其破坏形态与截面高度无关,腹筋可有效约束混凝土剥落;随腹筋配筋率增大,名义开裂/极限强度呈增大趋势,而最大斜裂缝宽度随之降低;腹筋可在抵抗斜截面内部分拉应力的同时对压杆形成有效侧向约束,削弱尺寸效应影响。计算结果表明:与简化STM相比,改进STM能够合理反映腹筋配筋率对受剪承载力的影响,基于ACI 318-19压杆有效系数的改进STM的预测值与试验值吻合良好。

Eight deep beams with lightweight aggregate concrete and two deep beams with normal weight concrete were tested under symmetrical concentrated load to investigate their shear behavior.The effect laws of web reinforcement ratio and beam depth on failure process and modes,load-deflection curve,characteristic load,diagonal crack propagation and strain in reinforcement were examined.The action mechanism of web reinforcement on the shear capacity and size effect of the deep beam was emphasized.To quantify the contribution of web reinforcement in the shear mechanism,a modified strut-and-tie model(STM)was established,in which the web reinforcements were considered as vertical ties.Moreover,based on evaluation and analysis on typical strut efficiency factor models,the accuracy of modified STM was verified.The test results indicate that all ten beams failed in the shear-compression mode,which was independent on size.The existence of web reinforcement can limit the spalling of concrete.The normalized diagonal cracking and ultimate shear load will increase with the web reinforcement ratio and a significant reduction in maximum diagonal cracking width can be observed.The web reinforcement can resist the internal tensile stress of the inclined section and form effective lateral restraint on the concrete strut,thus mitigating the effect of size effect.Compared with the original STM,the improved STM can reflect the contribution of vertical web reinforcement to the shear strength better.Generally,the predicted shear capacity by the improved STM with the ACI 318-19 strut efficiency factors is in good agreement with the experimental values.