【作者】 苏安双；

【导师】 巴恒静；

【作者基本信息】 哈尔滨工业大学 ， 结构工程， 2008， 博士

【摘要】 实际工程条件下,高性能混凝土早期开裂现象屡见不鲜。早期裂缝的出现降低了混凝土结构使用性能,劣化混凝土工程耐久性,缩短了其使用寿命。为优化高性能混凝土结构设计,确保其在实际工程中服役性能及长期耐久性,需深入研究早期开裂现象。结合早期凝结硬化、收缩变形、收缩应力、拉伸徐变、弹性模量及抗裂阻力观测,研究早期开裂机理;建立科学合理的开裂判据,较为准确的进行开裂预测。本文应用超声波无损检测技术,观测了通过高性能混凝土浆体的超声波声学参数随时间发展变化规律,并以此为基础确定高性能混凝土凝结时间。结果表明,超声波主频出现“跳跃式”增长时刻即为浆体初凝时间,波速由快速增长转为缓慢增长时刻为终凝时间;超声波法以胶凝材料水化反应进程及浆体内部微观结构发展为标准监测凝结、硬化,反映了浆体凝结的本质,较贯入阻力法具有操作上、技术上优势。监测了高性能混凝土早期密封条件下内部相对湿度变化,结合成熟度理论、水化程度理论,预测自干燥引发相对湿度变化;改进早期湿度扩散系数计算模型,并基于湿度迁移理论,预测单面干燥条件下高性能混凝土早期内部相对湿度分布。结果表明,水灰比、矿物细掺料对自干燥引发相对湿度变化具有显著影响作用;自干燥引发相对湿度变化与水化程度之间呈简单的线性关系,可基于此关系建立自干燥引发相对湿度变化预测模型,预测结果与试验观测结果具有较高相关性;基于Fick第二定律,结合自干燥与湿度扩散相互影响作用,预测HPC内部相对湿度分布较为合理。应用自行研制非接触式混凝土收缩变形测试仪,观测高性能混凝土早期自生收缩、单面干燥条件下收缩应变分布,研究其影响因素;基于毛细管压力理论,建立早期自生收缩预测模型;基于单面干燥条件下自由收缩与相对湿度线性关系,建立单面干燥条件下收缩应变分布预测模型。结果表明,水灰比、矿物细掺料对自生收缩、单面干燥条件下收缩应变分布具有显著影响作用;水胶比为0.40高性能混凝土,自生收缩预测结果与试验观测结果吻合性较好;低水灰比混凝土中浆体化学减缩明显,凝结过程持续时间较长,化学减缩对自生收缩贡献更为显著,导致毛细水张力预测自生收缩低于实测值;通过收缩应变分布预测结果与试验结果对比,证明受干燥影响区域存在层间约束;考虑层间约束后,收缩应变分布预测结果与试验结果吻合性较好。在观测高性能混凝土早期弹性模量、劈裂抗拉强度随龄期发展基础上,分析受约束高性能混凝土早期开裂敏感性;应用改进环向约束试验,研究高性能混凝土环向约束条件下收缩应力、应力梯度、拉伸徐变及其影响因素;应用显微技术,观测受约束圆环试件表面、内部开裂情况。在上述工作基础上,综合分析受高性能混凝土早期开裂趋势。结果表明,受约束高性能混凝土具有早期开裂敏感性;养护条件、超塑化剂、水灰比及矿物细掺料对收缩应力具有显著影响作用;养护条件、水灰比及矿物细掺料对早期徐变性能具有明显影响作用;降低水灰比、干燥条件作用及硅灰的掺加,增加了受约束高性能混凝土早期开裂趋势;磨细矿渣的掺入,对受约束高性能混凝土早期开裂趋势影响不明显。考虑混凝土早期粘弹性能,预测受约束高性能混凝土收缩应力;基于强度理论及能量理论,建立了早期开裂判据,预测受约束高性能混凝土早期开裂龄期。结果表明,残余收缩应力被释放前,考虑早期粘弹性能的收缩应力预测值与试验观测值较为接近;基于强度理论、能量理论预测的开裂龄期较试验观测结果有所提前,且相差不大,表明预测方法较为有效;能量理论确定的开裂抗力考虑了原始裂纹对早期开裂的影响作用,更适用于受约束高性能混凝土早期开裂预测,预测结果偏安全、可靠。更多还原

【Abstract】 Early-age cracking of High Performance Concrete (HPC) usually occurs in field engineering. The occurrence of early-age cracks reduces the service performance of concrete structures. Early-age cracking also deteriorates the durability of concrete engineering and shortens its service life. In order to optimize structural design of HPC and ensure its service ability and long-term durability, early-age cracking needs to be further studied. The mechanism of early-age cracking should be studied combining the investigation of setting and hardening, shrinkage, shrinkage stress, tensile creep and cracking resistance. Scientific and reasonable cracking criterion should be established to predict early-age cracking relative precisely.Ultrasonic nondestructive detection method was utilized to investigate the development of acoustic parameters of ultrasonic wave passing through the HPC mortar. According to the investigation results, the setting time was determined. The results indicated that initial setting time corresponded to the time at which the steep increase of ultrasonic dominant frequency occurred. Final setting time corresponded to the time at which ultrasonic pulse velocity started to increase slowly after rapid increase stage. Through ultrasonic method, setting and hardening were investigated according to hydration process of cementitious materials and development of internal micro-structures. So the results of ultrasonic method characterized the setting principle. Compared to penetration resistance method, ultrasonic method exhibited operational and technical advantages.The internal Relative Humidity (RH) change of HPC in sealed condition was investigated. The self-desiccation induced RH change was predicted based on the maturity and hydration degree theory. The calculation model of early-age moisture diffusion coefficient was modified. Based on moisture diffusion theory, the internal RH distributio of HPC under unilateral dryng condition was predicted. The results indicated that w/c (water to cement raio) and mineral admixtures have significant influence on internal RH change induced by self-desiccation. A simple linear relationship exsited between self-desiccation induced RH change and hydration degree. Prediction model of self-desiccation induced RH change could be established based on the mentioned linear relationship. Relatively high correlation was obtained between the predicted results and investigated results. Based on Fick’s second law, internal RH distribution prediction considering interaction between self-desiccation and moisture diffusion was relatively resonable.Self-developed non-contact shrinkage measuring device was applied to investigate early-age autogenous shrinkage and shrinkage distribution under unilateral drying condition. The influencing factors of autogenous shrinkage and shrinkage distribution were studied. Early-age autogenous shrinkage prediction model was established according to capillary pressure theory. Based on the linear relationship between free shrinkage and RH, prediction model of shrinkage distribution under unilateral drying condition was established. The results indicated that w/c and mineral admixtures had significant influence on autogenous shrinkage and shrinkage distribution. For HPC with water to binder ratio of 0.40, Relative high correlation existed between predicted autogenous shrinkage and investigated data. For HPC with lower w/c, chemical shrinkage of paste was more dramatic and setting process lasted longer time. So contribution of chemical shrinkage to autogenous shrinkage was more significant. This resulted that predicted autogenous shrinkage was lower than investigated data. Through the comparison of predicted shrinkage distribution and experimental data, existence of interlayer restraint was verified. Considering interlayer restraint, the predicted shrinkage distribution corresponded well with the experimental data.Based on investigated development of early-age elastic modulus and splitting tensile strength of HPC with age, cracking sensitivity of restrained HPC was analyzed. Through the modified restrained ring test, shrinkage stress, stress gradient, tensile creep of restrained HPC were studied. The influencing factors of the mechanical properties mention above were also studied. Through the application of micro-technique, surfacial and internal cracking of restrained concrete ring was investigated. Based on the experimental work mentioned above, early-age cracking tendency of restrained HPC was analyzed comprehensively. The results indicated that restrained HPC exhibted early-age cracking sensitivity. Curing conditions, superplaticizer, w/c and mineral admixtures had significant influences on shrinkage stress. Curing conditions, w/c and mineral admixtures had significant influences on early-age creep performance. Reduction of w/c, effect of drying condituon and addition of silica fume resulted in higher early-age cracking tendency of restrained HPC. The influence of addition of grouded blast-furnace slag on early-age cracking tendency was not evident.Considering early-age visco-elastic properties, shrinkage stress in restrained HPC was predicted. Early-age cracking criteria were established based on strength and energy theory. Accroding to the predicted shrinkage stress and established cracking criteria, early-age cracking age of restrained HPC was predicted. The results indicated that before release of residual shrinkage stress, predicted shrinkage stress considering visco-elastic properties approached to experimental data. Predicted cracking according to strength and energy theory occurred earlier than investigated cracking. The difference between predicted cracking age and investigated one was not great. This showed that early-age cracking prediction method mentioned above was relatively effective. The influence of initial crack on early-age cracking was considered during the determination of cracking resistance based on energy theory. So predition method based on energy theory was more suitable for early-age cracking prediction of restrained HPC. Predicted results according to energy theory presented greater security and reliability.更多还原