Theory of cyclic creep of concrete based on Paris law for fatigue growth of subcritical microcracks
Theory of cyclic creep of concrete based on Paris law for fatigue growth of subcritical microcracks
复制标题
DOI：
10.1016/j.jmps.2013.09.010
复制
发表时间：
2014-02
影响因子：
5.3
通讯作者：
Z. Bažant;M. Hubler
中科院分区：
文献类型：
--
作者：
Z. Bažant;M. Hubler
最近由帕劳的一场灾难引发的调查显示，全世界有69座大跨度节段预应力混凝土箱梁桥遭受了数十年的过度挠度，而肯定存在更多。虽然过大的挠度被证明主要是由于静态蠕变的设计建议或规范过时造成的，但一些工程师怀疑循环蠕变可能是一个重要的额外原因。许多研究人员探索了混凝土的循环蠕变实验，但一个合理的数学模型，将锚定在微观结构，并允许外推到100年的寿命是缺乏的。在这里，它是假设的循环蠕变的原因是在水化水泥中预先存在的微裂纹的疲劳增长。通过将断裂力学应用于被认为是拉伸的或以破碎带的形式被认为是压缩的微裂纹来计算所得到的宏观应变。这导致了一个数学模型的循环蠕变压缩，这是验证和校准的实验室试验数据从文献中。循环蠕变被证明是成比例的应力的时间平均值和应力幅材料强度的比率的4次方。最近的发现支持4的幂，即在原子尺度上，巴黎定律的指数应该为2，并且由于尺度桥接，指数必须增加。指数4意味着循环蠕变挠度对所施加的循环应力的相对幅值非常敏感。对六个节段预应力混凝土箱梁的循环徐变效应的计算表明，由于自重的主导作用，对于大跨度（> 150 m），对挠度的影响绝对可以忽略。对于小跨度（< 40 m），循环徐变挠度不可忽略，但无关紧要，因为静态徐变会导致此类桥梁向上挠度。然而，循环徐变被证明在中小跨度（< 80 m）的桥梁中会导致显著的残余拉应变，这会在主梁的顶面或底面产生有害的拉伸开裂。
Recent investigations prompted by a disaster in Palau revealed that worldwide there are 69 long-span segmental prestressed-concrete box-girder bridges that suffered excessive multi-decade deflections, while many more surely exist. Although the excessive deflections were shown to be caused mainly by obsolescence of design recommendations or codes for static creep, some engineers suspect that cyclic creep might have been a significant additional cause. Many investigators explored the cyclic creep of concrete experimentally, but a rational mathematical model that would be anchored in the microstructure and would allow extrapolation to a 100-year lifetime is lacking. Here it is assumed that the cause of cyclic creep is the fatigue growth of pre-existing microcracks in hydrated cement. The resulting macroscopic strain is calculated by applying fracture mechanics to the microcracks considered as either tensile or, in the form of a crushing band, as compressive. This leads to a mathematical model for cyclic creep in compression, which is verified and calibrated by laboratory test data from the literature. The cyclic creep is shown to be proportional to the time average of stress and to the 4th power of the ratio of the stress amplitude to material strength. The power of 4 is supported by the recent finding that, on the atomistic scale, the Paris law should have the exponent of 2 and that the exponent must increase due to scale bridging. Exponent 4 implies that cyclic creep deflections are enormously sensitive to the relative amplitude of the applied cyclic stress. Calculations of the effects of cyclic creep in six segmental prestressed concrete box girders indicate that, because of self-weight dominance, the effect on deflections absolutely negligible for large spans (> 150 m). For small spans (< 40 m) the cyclic creep deflections are not negligible but do not matter since the static creep causes in such bridges upward deflections. However, the cyclic creep is shown to cause in bridges with medium and small spans (< 80 m) a significant residual tensile strain which can produce deleterious tensile cracking at top or bottom face of the girder.