【作者】 贺智敏；

【导师】 谢友均；

【作者基本信息】 中南大学 ， 土木工程， 2012， 博士

【摘要】 论文针对我国铁路预应力混凝土预制构件存在的脆性大和表层易开裂等关键技术难题和当前高速铁路建设中对预应力混凝土预制构件的重大需求,在总结国内外相关最新研究成果的基础上,基于高速铁路工程结构中主要的蒸养混凝土预制构件特点,运用混凝土材料科学、物理化学原理等相关理论知识,就蒸养热效应对混凝土微观结构和性能的损伤及其作用机制进行了较为系统深入的研究,取得的主要创新成果如下：(1)系统研究了蒸汽养护条件下混凝土在预养、升温、恒温及降温阶段的变形规律及其主要影响因素、蒸养混凝土微观结构和宏观性能及其相互关系,将蒸养过程对混凝土微观结构和性能的不利影响归纳为三种热损伤效应：肿胀变形效应、表层伤损效应和热脆化效应。(2)混凝土在蒸养过程中发生的不可恢复的残余变形定义为肿胀变形。基于肿胀变形机理及其关键因素的分析,建立了定量描述蒸养期内各阶段蒸养混凝土的肿胀变形模型,试验验证了模型的有效性；研究表明了蒸养混凝土肿胀变形在蒸养期内各阶段呈现不同特性；明确了蒸养混凝土体系内液(气)相含量是决定其肿胀变形大小的关键因素。(3)通过微观与宏观试验剖析了蒸养过程对水泥混凝土水化物相和微观结构的不利影响,而且这种不利影响对暴露于蒸养室的混凝土表层区尤甚,使得蒸养混凝土的微观结构与宏观性能(如毛细吸水性、氯离子迁移性能以及碳化性能等)由表层至内部呈现更大的梯度分布特征,匀质性降低；相对于标养混凝土和蒸养混凝土内部,蒸养混凝土暴露表层部分(约10mm范围)的微结构较为疏松,而内部结构相对更为致密,即蒸养混凝土表层伤损效应。论文揭示了混凝土蒸养表层区热损伤效应的变化规律及其发生机制,并提出了采用毛细吸水性试验的表征方法和吸水性系数评价指标。(4)通过测量带模养护、单面开放蒸养条件下混凝土内外的温度场分布,计算各截面温差应力,结果表明,近表层区混凝土由于温差过大极易受到损伤,而蒸养混凝土内部各截面处的温差很小,温差应力致使结构受损的可能性大为降低。通过分析蒸养过程中发生的混凝土表里热质传输,表明混凝土暴露表层区由于水分的蒸发而形成贯通毛细孔,表层区处于低约束热膨胀状态和降温阶段的冷缩应力亦增大了损伤程度。蒸养引发的表层与内部热效应不均匀造成了蒸养混凝土表层结构的损伤,是导致蒸养混凝土由表及里微结构梯度变化增大的主要原因。(5)通过测试不同养护温度下混凝土冲击韧性、拉压比及折压比性能,明确了蒸养对混凝土热脆化效应的影响规律。研究表明,蒸养混凝土的热脆化效应是导致蒸养预制构件出现开裂、崩裂及掉块等质量缺陷的主要原因。通过水化物相形貌观察和C-S-H组成分析,认为蒸养混凝土的粗晶结构、新生物比表面降低导致粘结力下降和蒸养降温阶段的冷缩导致过渡区产生应力集中或出现微裂缝是产生热脆化效应的主要机理。(6)基于对蒸养混凝土热损伤效应影响因素及其作用机理分析,研发了显著改善蒸养混凝土性能的胶凝材料体系与技术措施。掺用粉煤灰、矿渣或粉煤灰、矿渣和适量硅灰复掺,显著降低了蒸养混凝土的总孔隙率和最可几孔径；采用保湿蒸养工艺以及蒸养结束后预制构件的后续水养护方法可有效缓解蒸养混凝土的表层伤损效应和热脆化效应的不利影响。更多还原

【Abstract】 In recent years, the pre-stressed precast concrete elements are widely used in high-speed railways being built on a large scale in China. However, according to the survey of previous cases it was found that many key issues, such as surface cracks and brittleness of pre-stressed precast concrete, were still not full understood. In the present thesis, firstly, a state-of-the-art review was carried out and some issues that needed to be further studied were proposed, on the other hand, the common characteristic of steam-cured concrete elements widely used in the high-speed line was analyzed. Secondly, based on the above mentioned discussion, the influences of steam curing on microstructure deterioration and property degradation of concrete and corresponding mechanisms are systematically studied. Finally, according to the experimental results combining with the theoretical analysis of concrete material and physical chemistry principles, following creative achievements have been made:(1) The main influencing factors and development of deformation of concrete in the process of preheating, treatment and cooling duration were systematically studied, and the relationship between microstructure and macroscopic properties was also analyzed. According to the results, three heat effects causing degradation of microstructure and properties in the process of steam curing could be induced as follows:expansion effect, surface damage effect and embrittlement effect.(2) The irreversible residual size increment caused by steam curing is usually named as expansive deformation. Based on the anaysis on the mechanism of expansive deformation and the key factors affecting such deformation, a model was proposed to predict the expansive deformation of concrete occurring in steam curing process, and the validity of the model was verified by the experimental results. It was found that during the three stages of steam curing, i.e control heating, heating at constant temperature and control cooling, expansive deformation characteristics of concrete differs from each other for each stage. The quantity of gas and liquid phase in mixture before steam curing is the key factor influnecing expansive deformation of concrete.(3) The adverse effects of steam curing on hydration products and microstructure of cement paste were studied by means of micro and macro test methods. The experimental results showed that the adverse effects on surface concrete in contact with steam could cause a gradient distribution of micro and macro properties of concrete from exterior to interior of elements, such as capillary sorptivity, chloride transport and carbonation of concrete. Due to the surface damage effect, an surface layer with the depth of about10mm, showing larger porosity than inner concrete or standard-curing concrete, was formed. The mechanism of surface damage effect was disclosed by means of capillary sorptivity test, and the water absorption coefficient in the capillary sorptivity test was proposed to evaluate the development of surface damage effect.(4) In order to study the formation of gradient distribution of micro and macro properties of concrete, two groups of concrete specimens were cured in the molds with and without covers respectively. The temperature distributions in concrete were measured and the temperature stresses in different sections were calculated respectively. The results showed that the surface layer of specimen was severely damaged due to large temperature difference, however, concrete looks sound in the inner zone of specimen with little temperature difference. On the other hand, according to the experimental results of heat-mass transfer, it was found that a large amount of interconnected pores were formed in the surface layer, resulting in larger thermal expansion and contraction stress than the inner zone of concrete specimen. The deformation difference between the exterior and interior of concrete specimens resulted in the gradient distribution of micro and macro properties of concrete.(5) In order to study the influence of steam curing on the development of embrittlement effect on concrete, the impact toughness, tensile compressive ratio and flexure compression ratio of concrete were studied, the results showed that the embrittlement effect of steam curing should be the main cause for crack and peeling. On the other hand, in order to discuss the mechanism of embrittlement effect, the micro-morphologies and phase structures of C-S-H was observed, the experimental results showed that there were two main aspects. Firstly, the formation of coarse cement hydration product with low specific surface area due to steam curing caused the decrease of cohesive force, secondly, the stress concentration in the interfacial transition zone leaded to micro-crack formation.(6) Based on the abovementioned research results, specific experiments were carried out to study how to improve the steam cured concrete performance. The results showed that the total porosity of concrete and the size of pore with highest proportion in concrete could be significantly decreased by using double-mixture with ground slag and fly ash or multi-mixture with ground slag, fly ash and silica fume. On the other hand, the subsequent water curing after steam curing finished and steam curing on concrete with cover could significantly alleviate the surface damage and embrittlement effects.更多还原