Effect of corrosion degree on mechanical properties of reinforcements buried for 60 years

Abstract This paper studies the influence of the corrosion degree calculated by the mass loss and by the smaller diameters on the yield strength, ultimate strength and final elongation. Reinforcements buried and naturally corroded for 60 years were studied. The mechanical properties of the protruding reinforcing steel were compared to reference bars, which also remained buried for 60 years, but without going through the corrosion process. Micrographs, besides the chemical composition and the characterization of the aggressiveness of the soil were realized. The micrographs and the chemical composition show the presence of pits in the reinforcements and sulfur contents for steel above the prescriptions of the time when the foundations were initially implement, respectively. The results also show that the effects of pitting corrosion on the mechanical properties of the naturally corroded bars may not be adequately expressed by the mass loss. This type of corrosion (pits) produces geometric variations in the cross sections along the length of the test specimens, generating stress gradients between successive sections. This has a noticeable impact on the mechanical properties of the reinforcements. In general, the effects of corrosion are more pronounced on the ductility of the reinforcement. Regarding the aggressiveness of the soil, high corrosion rates were identified in the reinforcement, even with the soil being classified as essentially non-corrosive.

摘要 本文研究了基于质量损失（mass loss）法与小直径参数计算的腐蚀程度对钢筋屈服强度（yield strength）、抗拉极限强度（ultimate strength）及断后伸长率（final elongation）的影响。研究对象为埋置并自然腐蚀60年的钢筋。将外露钢筋的力学性能与同样埋置60年但未经历腐蚀过程的对比钢筋（reference bars）进行了对比。此外，还开展了显微图像（micrographs）采集、化学成分分析以及土壤腐蚀性（aggressiveness of the soil）表征工作。显微图像与化学成分分析结果分别显示，钢筋内部存在点蚀坑（pits），且其硫含量超出了该基础工程初始建设时期的规范限值。研究结果同时表明，仅通过质量损失法无法充分反映点腐蚀（pitting corrosion）对自然腐蚀钢筋力学性能的影响。此类点蚀会沿试样长度方向造成截面几何形貌变化，在相邻截面间产生应力梯度（stress gradients），进而对钢筋的力学性能产生显著影响。总体而言，腐蚀作用对钢筋延性（ductility）的影响更为显著。关于土壤腐蚀性，尽管该土壤被归类为基本无腐蚀性，但检测到钢筋仍存在较高的腐蚀速率。