考虑纤维桥接退化的钢筋UHPC梁受弯承载能力计算方法

Calculation Method of Flexural Bearing Capacity of UHPC Beams Based on Fiber Bridging Degradation Considerations

现有UHPC梁受弯承载力计算方法均基于承载力极限状态下的受力模型推导,并用试验验证所得,未考虑正常使用极限状态到承载力极限状态纤维桥接作用退化对极限承载力的影响。对于UHPC梁,若正常使用极限状态允许其带裂缝工作,裂缝在活载作用下会存在闭合和张开的问题;若裂缝宽度达到一定值,伴随裂缝的闭合与张开,跨过裂缝的纤维-基体会因界面黏结疲劳导致黏结性能退化,纤维黏结性能的退化进一步引起UHPC梁跨中截面一定高度上的纤维桥接作用失效,影响其受弯承载能力计算的准确性。为此,充分研究了应变硬化与软化UHPC材料的力学性能,分析了单调和循环加载下纤维与基体的黏结滑移关系,确定了正常使用极限状态下裂缝不扩展的宽度值,然后考虑应变硬化UHPC极限拉应变和钢筋屈服应变的关系,基于循环荷载下跨裂缝纤维-基体黏结性能退化规律,分别建立了受弯全过程纤维桥接合理量化的钢筋UHPC梁受弯承载力计算方法。通过现有文献中UHPC梁的试验结果验证发现,对于应变硬化和应变软化UHPC梁,所提出公式的计算结果与试验结果平均比值分别为0.9和1.0,这表明所提出的计算方法可对应变硬化与软化UHPC梁受弯承载力进行较为准确的预测。另外,研究发现,对于UHPC梁,若要充分利用UHPC中纤维的桥接贡献,其正常使用极限状态下裂缝的宽度不宜大于0.05 mm。

The current flexural bearing capacity calculation method for UHPC beams is proposed based on the theoretical model of the ultimate bearing capacity without considering the influence of the fiber bridging effect degradation from the serviceability limit state(SLS)to the ultimate limit state(ULS).However,there will be a problem associated with the opening and closing of cracks when UHPC beams are exposed to live loads if cracks are allowed to exist in the SLS.When the crack width reaches the critical value,the fibers crossing the crack will experience interfacial adhesion fatigue owing to interfacial adhesion accompanied by the closure and opening of cracks.As a result,the degradation of fiber-matrix bond performance caused by the fatigue will lead to the fiber bridging failure at a certain mid-span height,weakening the flexural bearing capacity of the beam structure.In this paper,the mechanical properties of UHPC with strain hardening and softening were investigated.The crack width,which is not expanding under normal ULS conditions was determined,and the calculation method of the flexural capacity of UHPC beams was established.This method distinguishes strain hardening and softening UHPC types and considers the relationship between the ultimate tensile strain of UHPC and the yield strain of reinforcement,which can achieve reasonable quantification of fiber bridging during the entire bending process of UHPC beams.Regarding the strain-hardened and softened UHPC beams,the average ratios of the calculated results(using the proposed formulas)to the experimental results in this paper were 0.90 and 1.0,respectively.It is shown that this calculation method can accurately predict the flexural bearing capacity of UHPC beams.In addition,the research found that the crack width under SLS conditions should not be greater than 0.05 mm for UHPC beams to fully utilize the bridging contribution of crossing fibers.