U形UHPC永久模板RC无腹筋组合梁抗剪性能试验

为了研究UHPC永久模板RC无腹筋组合梁的抗剪性能，以UHPC永久模板的厚度和界面条件为试验参数，分别开展了UHPC材料力学性能与UHPC永久模板RC无腹筋组合梁四点加载试验。由于组合梁的抗剪性能与UHPC的基本力学性能密切相关，因此首先对UHPC的抗拉与抗压性能进行了试验研究。UHPC的力学性能试验结果表明，UHPC在单轴单调荷载作用下具有一定程度的应变硬化特征，其拉伸极限强度为4.87 MPa，极限拉应变为0.6%。在材料试验结果的基础上，通过考虑UHPC永久模板厚度与界面方式这2种试验参数，分别设计了1根RC参照梁，1根UHPC参照梁，以及2种UHPC/RC界面类型（光滑与均布剪力键）、3种永久模板厚度（15，20，25 mm）、共计6根U形UHPC永久模板RC无腹筋组合梁。在对这8根梁分别进行四点加载破坏试验的基础上，分析了UHPC永久模板不同厚度与界面类型对组合梁抗剪承载力的影响。结果表明：组合梁的抗剪承载力及其变形能力较相同尺寸及配筋的RC无腹筋梁至少提高了103.7%和117.7%；且无论何种界面类型下，抗剪承载力随着UHPC永久模板厚度的增加而增加；界面为均布剪力键的UHPC永久模板较光滑界面能提供更高的抗剪承载力与变形能力。最终，基于修正桁架模型理论，分析了UHPC永久模板与RC无腹筋梁的抗剪承载力及其抗剪构成，提出了UHPC永久模板RC无腹筋组合梁的抗剪承载力计算公式，且公式计算值与试验值吻合较好。

Shear Behavior of Reinforced Concrete Composite Beams Without Stirrups Using U-shaped UHPC Permanent Formwork

In this study, UHPC material tests and four-point loading tests on composite beams were performed to investigate the shear performance of reinforced concrete (RC) composite beams without stirrups using U-shaped ultra high performance concrete (UHPC) permanent formwork. The permanent formwork thickness and interfacial properties were adopted as experimental parameters. Because of the strong correlation between the UHPC tensile properties and the shear performance of composite beams, UHPC material tests were conducted. A monotonic uniaxial load was applied to the UHPC specimens to determine their tensile and compressive properties. The material test results show that the UHPC exhibits strain-hardening behavior under tension. The UHPC has an ultimate tensile strength of 4.87 MPa, and its ultimate tensile strain corresponding to the ultimate tensile strength is 0.006. Based on the material test results, one RC beam without stirrups and one UHPC beam without stirrups were used for reference, and another six composite beams, including two types of interfacial properties and three different permanent formwork thicknesses (15, 20, and 25 mm), were designed and subjected to four-point loading tests. The effects of the thickness and interfacial properties on the shear-resisting capacities of the permanent formwork were investigated. When a U-shaped UHPC permanent formwork is used, the shear-resisting and deformation capacities of the composite beams increased by 103.7% and 117.7%, respectively. Regardless of the interfacial properties, the shear-resisting capacity of the composite beams increased as the thickness of the permanent formwork increased. The U-shaped UHPC permanent formwork with a shear key interface is more effective than that with a smooth interface in enhancing the shear-resisting and deformation capacities. In addition, model and predictive equations for the shear-resisting capacity of RC composite beams using U-shaped UHPC formwork were derived and agreed well with the experimental results.