【作者】 李国栋；

【导师】 王宗林；

【作者基本信息】 哈尔滨工业大学 ， 桥梁与隧道工程， 2014， 博士

【摘要】 混凝土早期由于水化作用和处在不饱和环境中,会产生收缩变形,随着龄期收缩变形累积增加,在约束下就会加大混凝土早期开裂的几率,特别是高性能混凝土的早期收缩开裂尤为显著。混凝土桥梁建造过程中,高性能混凝土早期开裂现象时常出现,收缩变形是混凝土早期开裂的主要源动力之一。如何降低甚至避免桥梁高性能混凝土的早期开裂,是目前的科研热点和难点。桥梁高性能混凝土的宏观收缩变形与其内部的细观结构密切相关。因此,混凝土宏观收缩行为的研究有赖于从细观层次出发,考虑混凝土细观结构对宏观表象的影响,这样才能够从深层次上反映高性能混凝土的收缩变形和应力特性,揭示桥梁高性能混凝土早期收缩开裂的机理从而提出控制措施。论文基于桥梁高性能混凝土早期收缩裂缝形成的研究现状,并通过现场调研充分了解桥梁工程中高性能混凝土结构早期收缩开裂的时间、形式和特点。在此基础上,通过室内试验从宏观层次上阐明了水胶比、粉煤灰含量、含气量、集浆比等配合比参数对桥梁高性能混凝土早期收缩变形的影响,并提出了能较全面反映配合比参数影响混凝土早期收缩变形的预测模型,同时探讨了配筋率和配筋方式以及养护方式和养护时间对混凝土早期收缩变形的影响,并得到了相应的结论。采用宏细观相结合的方法,基于工业CT扫描图像和图像重建技术建立了混凝土细观力学模型,混凝土在细观层次上视为骨料和砂浆组成的两相复合材料,并应用有限元显式算法进行数值分析,模拟了桥梁高性能混凝土早期收缩变形的发展全过程。考虑了骨料和砂浆的力学特性和收缩特性的差异,从细观层次上分析了骨料的分布和粒径大小以及砂浆的收缩变形大小对混凝土早期收缩变形的影响,并得到了混凝土室内测试结果的验证。从模拟结果得到了自由收缩状态下混凝土内应力的分布情况以及与骨料分布和粒径大小的关系。结合弹性力学、细观损伤力学和断裂力学在细观层次上来分析混凝土的收缩变形以及由于收缩变形导致的损伤和破坏过程。基于弥散式裂缝模型理论建立起较为完整的混凝土细观力学分析体系来模拟混凝土早期收缩开裂过程,揭示因收缩变形导致混凝土早期裂缝萌生和发展过程的内在机理。从混凝土收缩开裂过程的模拟结果看出,混凝土的内部开裂位置是在骨料和砂浆的界面处以及空隙处。这是由于骨料表面对砂浆的约束效应最显著,界面处出现应力集中现象导致界面处最先开裂。混凝土细观收缩裂缝随着龄期逐渐向薄弱区域延伸。骨料与砂浆的界面面积越大,对砂浆的约束效应更加显著导致砂浆的内应力更大,成为混凝土早期裂缝的起裂点。微裂缝随着龄期从起裂点位置逐渐延伸、扩展和汇集最后形成了混凝土的宏观裂缝。论文通过宏细观结合分析了混凝土早期收缩裂缝形成的机理,从理论到试验再到实际工程探寻了高性能混凝土早期裂缝有效的防治途径,并在实际工程中得到了应用,效果显著。提出了桥梁高性能混凝土早期收缩裂缝的防治措施,对于减少甚至避免混凝土桥梁的早期开裂、提高混凝土结构耐久性、延长混凝土结构使用寿命具有重要意义。更多还原

【Abstract】 The concrete will produce shrinkage deformation on account of the hydration and the unsaturated environment in the early age. And the shrinkage strain accumulates by the age of the concrete. If the shrinkage deformation is restrained, the risk of cracking will increase in the early age, especially for the high performance concrete. During the construction period of concrete bridge, the cracking of high performance concrete is often observed, and the shrinkage deformation is treated as the main cause. So how to avoid the early-age cracking of concrete is becoming the difficulty and hot point of research now.The macroscopic shrinkage deformation of the high performance concrete is closely related to its mesostructure. So the study of the macroscopic shrinkage deformation must base on the mesostructure of concrete, and the influence of the mesostructure on the macro appearance must be take into consideration. Only in this way we can find the underlying cause of the shrinkage deformation of concrete, and then put forward measures to control the cracking of concrete in the early age.Based on the research status and field research of the early-age cracking of high performance concrete, it is realized the time, form and characteristic of the early-age cracking of concrete in the bridge engineering. On this basis, it is illuminated the influence of water to cementitious materials ratio, fly ash content, air content and aggregate to cementitious materials ratio on the early-age shrinkage strain by laboratory test. And it is proposed a predicted model of early-age shrinkage of concrete, which is able to reflect the influence of mix proportion parameter. Meanwhile, it is discussed the influence of reinforcement ratio, reinforcement way, curing way, curing time on the early-age shrinkage deformation of concrete, and get the corresponding conclusions.By the combination of macro and mesoscopic method, the scan imagery of industrial CT and image reconstruction technique, it is established the mesoscopic mechanics model of concrete. At mesoscopic level, the concrete is treated as a composite material of aggregate and mortar. It is simulated the development process of the early-age shrinkage deformation of high performance concrete by the the explicit algorithm of FEM. And considered the differences of the mechanical and shrinkage characters between the aggregate and mortar, it is analyzed the influences of the distribution and size of aggregate and the shrinkage of mortar on the early-age shrinkage deformance of concrete at mesoscopic level. And the analysis result is verified by the laboratory test. It is acquired by simulation the stress distribution of the concrete and its relationship with the size and distribution of aggregate which under the freedom of shrinkage.Unite the elastic mechanics, microscopic damage mechanics and fracture mechanics, it is analyzed the process of shrinkage deformation, damage and destory of concrete at the mesoscopic level. Based on the dispersion crack model, it is established a mesoscopic mechanical system of concrete which can stimulate the early-age cracking process of concrete and reveal its inner mechanism.From the stimulated result, it can be found that the position of cracking is mostly at on the interface of aggregate and mortar and the interstice. This is due to the most significant constraint effect of mortar on the aggregate surface, and the stress concentration of interface resulted in the first cracking of the interface. And the initial crack on the interface will extend to the other weak areas with age. The bigger the interface area is, the stronger the inner stress caused by the constraint effect is. And the interface becomes crack initiation. With the extending, expanding and coming together of the micro crack, the macro crack occurs on the concrete. The meso cracks from the crack point location are extension, extension, and gradually converge with age, and finlly form the macroscopic crack of concrete.The formation mechanism of early-age shrinkage cracking for high performance concrete is analyzed both in meso and macro methods. The prophylaxis and treatment approach of the early-age cracking is put forward in this paper. And the method is verified in theoretical analysis, laboratory test and practical engineering. The prophylaxis and treatment approach will play an important role in avoiding early-age crack, enhancing durability and extending the service life of the concrete structure.更多还原