粘贴纤维增强复合材料加固混凝土梁的蠕变特性研究进展

Research Progress on Creep Response of FRP-strengthened Reinforced Concrete Beams

自20世纪末起,外粘纤维增强复合材料(Fiber-reinforced polymers, FRP)加固混凝土技术由于施工方便、FRP性能优异等优势被逐步运用于结构改造、修复补强、抗震加固等实际工程中。FRP本身具有轻质、耐腐蚀、高强度等优点,又由于近年来成本的进一步降低,得到更广泛的应用,目前已基本取代钢材成为最主要的结构加固材料。建筑结构服役年限较长,结构在完成加固后仍将处于长期承受荷载的状态,然而,目前对加固结构在长期荷载下的行为研究不充分,实际加固设计时也未考虑结构的长期效应。鉴于目前粘贴FRP加固技术已广泛应用,大量已加固完成的结构在以上问题尚不十分清楚的情况下工作服役了相当长的时间,有必要对加固结构的长期受荷性能进行深入探讨,同时对该问题的研究、后续可能需要的补救措施的提出又具有迫切性。粘贴FRP加固混凝土结构在长期荷载作用下变形增大、结构内部出现复杂的应力重分布现象,本质上是由材料的蠕变行为引起。对混凝土的蠕变(徐变)特性研究起步较早,对混凝土的收缩、单轴受压徐变行为已有相对全面的认识,国际上有比较公认的理论模型。研究表明,用于粘贴FRP的环氧树脂(Epoxy resin)结构胶具有明显的蠕变特性,提高环境温度、湿度可增大环氧树脂胶的蠕变变形,甚至引起蠕变断裂。试验研究与数值模拟证实,环氧胶层的蠕变行为将导致粘贴界面的应力出现松弛现象。试验结果表明,用于结构加固的碳纤维增强复合材料(CFRP)、玻璃纤维增强复合材料(GFRP)的抗蠕变能力较强,其蠕变变形可适当忽略。长期承受外荷载时,粘贴FRP加固混凝土梁的变形随时间的延长而增大,持荷前期增大速率较快,然后逐渐降低,后期基本不再变形,趋于稳定。该现象与混凝土、环氧树脂胶在相应应力水平下的蠕变规律相似。加固对梁瞬时变形的降低效果比较明显,但对长期附加变形的降低效应不明显,其主要原因是环氧树脂胶的剪切蠕变引起外部FRP应力松弛,导致产生附加变形。基于正截面受力分析,可对粘贴FRP加固梁的长期行为开展理论分析与计算,但对胶层的剪切蠕变特性缺乏有效考虑。对材料蠕变行为的充分认识是对结构蠕变行为进行合理分析的基础。本文首先从材料层次上综述各材料组分(混凝土、环氧树脂胶、FRP)蠕变特性的研究进展。实际工程中,结构长期承受的是正常使用荷载,在该荷载水平下,钢筋应力低于屈服强度,可视为线弹性材料,其蠕变行为可适当忽略,该观点在结构工程领域被广泛认同,因此,本文不再总结钢筋的蠕变行为研究。然后从结构层次上综述粘贴FRP加固混凝土梁的蠕变特性研究进展,分为试验研究、理论分析两方面。试验研究方面主要综述长期试验的开展方法和相关试验现象及结论,理论分析方面主要综述结构长期变形的计算方法。最后,本文总结目前研究存在的问题,为今后的进一步研究工作提供建议与参考。

Due to the excellent performance of fiber-reinforced polymers (FRP) and ease of application, the techniques of strengthening concrete structures by externally bonding FRP had been widely applied in actual projects (e.g., structural modification, repairing, reinforcement and seismic retrofit), since the end of the last century. FRP presents low density, well corrosion-resistant and high strength. Recently, FRP has been more generally used since the material compensate is further declined. It should be noted that the service time of building structures is long. After strengthening, the strengthened structures still suffer sustained loads for long-period of time. However, so far, the long-term performance of strengthened structures under sustained load has not been fully explored. The practical strengthening designs do not consider the creep effects. To the actual fact that the strengthening techniques of bonding FRP externally have been already widely used, it is needed to further investigate the creep response of FRP-strengthened concrete structures. Under sustained load, the long-term deformation of FRP-strengthened structure increases with time, along with complicated stress redistribution occurs in structure. These phenomena are basically caused by the creep behaviors of materials. Researches on the time-dependent properties of concrete had been carried out long before. Up to now, there is relatively adequate understan- ding for the shrinkage and uniaxial compressive creep of concrete, and some generally accepted models worldwide are recommended to theoretically describe such behavior. Researches show epoxy resin, which is used as adhesive to bond FRP on concrete exhibits evident creep performance. Higher environmental temperature and relative humidity increase the creep deformation of adhesive, which may even induce creep fracture. Experimental researches and numerical studies have demonstrated that the viscous flow of epoxy leads to obvious relaxation of interfacial stress. The CFRP and GFRP used for structural strengthening present ignorable creep performance, which has been validated by tests. For the increasing of deformation of FRP-strengthened concrete structure under sustained load, the increasing rate is high in the earlier stage, but gradually reduces in the later stage, which is basically in consistent with the creep law of concrete and epoxy resin. Strengthening has evident effect on reducing the instantaneous deformation, whereas, its effect on reducing the long-term deformation is comparatively not pronounced. The believed reason is that the shear creep of adhesive relieves the stress in FRP, which degrades the strengthening effects and causes additional deformation. Based on cross section analysis, the long-term performance of FRP-strengthened concrete beams can be theoretically investigated, however, the shear creep of adhesive cannot be properly taken into account. Reasonable discussion on the time-dependent behavior of FRP-strengthened structures fundamentally dependents on the adequate understan- ding of the creep response of materials. Thus, this paper first review the research progress on the creep behaviors of concrete, epoxy adhesive and FRP. It is noteworthy that in actual engineering projects, the sustained load for structure is service load. Under such level of load, the stress in steel is lower than the yield strength, which can be considered as linear elastic materials, hence the creep of steel can be reasonably neglected. Therefore, this paper no longer addresses the creep behavior of steel. Afterwards, the recent study progress on the time-dependent response of FRP-strengthened concrete beam is reviewed in the aspects of experimental study and theoretical analysis. The reviewed experimental studies cover the methods, phenomena and conclusions of long-term test. The reviewed theoretical analyses mainly include the methods for calculating the long-term deformation. Finally, the problems of existed studies are concluded, and the suggestions for further researches are presented.