【作者】 沙建芳；

【导师】 孙伟； 詹炳根；

【作者基本信息】 东南大学 ， 材料学， 2005， 硕士

【摘要】 碱硅酸反应(ASR)、氯盐腐蚀是混凝土结构耐久性失效的两个主要原因。目前,对于单一因素作用下混凝土劣化过程的研究已有诸多报道,但关于混凝土在碱硅酸反应-氯盐腐蚀协同作用下的损伤失效过程及机理研究仍是空白。本文以提高水泥碱含量、复合碱溶液浸泡、提高环境温度等复合破坏方式进行混凝土加速劣化试验。混凝土材料损伤的表征为膨胀率和相对动弹。参照冻融破坏标准,当相对动弹损失40%时判定材料失效。实验中通过不同龄期混凝土膨胀率和动弹性模量的测量,详细研究了混凝土在碱硅酸反应-氯盐腐蚀协同破坏作用下的损伤发展过程。通过不同暴露时期混凝土不同深度处自由氯离子和总氯离子含量测定,揭示了在发生碱硅酸反应情况下混凝土氯离子扩散、吸附的规律。此外,通过光学显微镜、SEM-EDAX、XRD等微观测试手段的运用并结合宏观测试结果,分析了混凝土在ASR-氯盐腐蚀协同作用下的破坏过程,探讨了NaCl加速碱硅酸反应的原因以及煤矸石延迟碱硅酸膨胀的机理。经系统研究影响混凝土ASR膨胀的各因素后得出结论:在一定范围内随水泥碱含量增加膨胀增大;高浓度氯盐溶液对混凝土ASR膨胀前期加速显著,后期低浓度氯盐溶液的加速作用更为突出;煤矸石的掺入能有效延迟和减缓混凝土ASR膨胀。碱硅酸反应导致的材料损伤发展有一定的特殊性,其损伤发展过程分为三个突出的阶段,即早期相对动弹迅速下降;临近失效状态相对动弹下降大大放缓甚至较长时间内保持不变;最后又以极其缓慢的速率发展至完全失效。在碱硅酸反应协同作用情况下混凝土氯离子扩散规律为:混凝土中Cl-的扩散速率并不与其损伤程度成正比。表观扩散系数随暴露时间增加成负指数减小,表面氯离子浓度随暴露时间增加线性增大。ASR存在情况下混凝土氯离子结合能力很小,究其原因主要是剩余C3A含量少、孔溶液pH值高、暴露环境温度高。经光学显微镜原位追踪混凝土裂缝扩展情况,推测ASR-氯盐腐蚀协同作用下混凝土损伤失效机理为:ASR凝胶在界面反应区或迁移过程中吸水膨胀,导致混凝土内部出现微裂纹,NaCl等其它侵蚀性介质的侵入进一步加剧了混凝土损伤。经广泛综合前人研究成果并结合本实验微观测试结果,提出Cl-与碱硅凝胶之间相互作用使孔溶液保持高碱度是氯盐加速ASR膨胀的主要原因,Friedel盐填充在ASR引发的微裂纹中也加剧了混凝土的破坏性膨胀。煤矸石抑制碱硅酸反应的机理主要是,内部结构致密阻碍了离子侵入,反应速率受扩散过程控制。更多还原

【Abstract】 Alkali-silica reaction (ASR) and chloride corrosion are two major hazards to concrete durability. The deterioration of concrete under single destructive action have been broadly investigated, but the degradation evolution and the corresponding mechanisms subjected to coupled actions of ASR and chloride corrosion have not been studied yet .In this experiment, three measures are taken to accelerate concrete damage, including enhancing the alkali level of cement, curing in compound alkali solutions and elevating exposure temperature. In addition, two parameters, the expansion ratio and relative dynamic elasticity modulus (RDEM), are used to characterize concrete damage. When RDEM of concrete suffered from the combined actions of ASR and chloride corrosion has been dropped to a critical point, the concrete is supposed to be completely destructed. The damage evolution and rules of both chloride diffusion and binding are in detail investigated by respectively measuring expansion and RDEM and measuring free and total chloride concentration in different depth during different exposure time. Besides, the failure process of concrete is analyzed, mechanisms of chloride salts accelerating ASR and gangue effectively preventing ASR are further explored by means of optical microscope and SEM-EDAX, XRD et al.After systematically researching various factors affecting ASR expansion, three important conclusions are drawn. Firstly, ASR expansion is increased with the growth of cement alkali content when alkali level varies in a certain range; high concentration chloride solutions can accelerate ASR significantly in a short time, while in the long run, the effects of low concentration solutions on accelerating reaction is more remarkable. Secondly, the addition of gangue can effectively prevent or delay ASR expansion. And finally, The damage evolution of concrete caused by ASR presents some peculiarities, for that in the beginning its RDEM decreases rather quickly, later decreases with a greatly reduced velocity or remains same in a long period, finally gradually decreases to complete failure with a very little speed.In the occurrence of ASR, the diffusion rate of chloride ion in concrete is not proportional to its damage degree as anticipated. Apparent diffusion coefficient decays negatively exponentially with time, while surface chloride concentration increases linearly. As far as this experiment is concerned, the low capacity of concrete in binding chloride ion is presumed to be in relation to low residual C3A contents, high pH value of pore solution and high exposure temperature. According to the cracking observed by optical microscope, a degradation mechanism of concrete subjected to the combined actions of ASR and chloride corrosion is suggested. That is, ASR gels swell when absorbing water in reaction zone or during migration, induce microcracks and damage the concrete. Then the damage is severely aggravated by some nocuous medium such as NaCl solution intruding through microcraks. In combination with others’research results and our microscopic testing, high alkalinity in pore solution resulting from the interaction between chloride ion and ASR gels is considered to be mainly responsible for the acceleration of ASR, the precipitation of Friedel’s salt in microcracks also increases the ASR expansion. The reason of the inclusion of gangue to prevent ASR is suggested as follows: the migration of different ions is hampered by inner dense structure and consequently, the reaction is controlled by diffusion.更多还原