【作者】 余红发；

【导师】 孙伟；

【作者基本信息】 东南大学 ， 结构工程， 2004， 博士

【摘要】 我国大西北有4大盐湖区共上千个盐湖,其中,新疆盐湖区有102个,青海盐湖区有33个,内蒙古盐湖区有370多个,西藏盐湖区有220多个。盐湖地区的环境气候条件恶劣,混凝土耐久性问题异常严重。本文在现场调查研究的基础上,针对盐湖地区的(30%～50%)RH干燥条件,综合运用高性能混凝土(HPC)的配制原理、纤维的限缩阻裂和膨胀剂的补偿收缩等技术,研究了盐湖地区HPC的制备技术,同时制备出强度等级C30的普通混凝土(OPC)、C25的引气混凝土(APC)、C70的不掺活性掺合料的高强混凝土(HSC)、C70的三掺(硅灰+粉煤灰+矿渣)的HPC及其钢纤维增强HPC(SFRHPC)和高强高弹模聚乙烯纤维(PF)增强HPC(PFRHPC),分别研究了不同混凝土在4种盐湖地区的单一、双重和多重因素作用下损伤失效过程的规律、特点和氯离子吸附/结合规律,OPC、APC和HSC的损伤失效机理以及HPC高耐久性的形成机理。基于混凝土结构不同的失效机理,提出了混凝土结构损伤寿命的损伤演化方程预测方法,修正并完善了钢筋混凝土结构使用寿命的氯离子扩散理论预测方法。通过大量的室内外实验,初步建立了两种预测方法的理论体系及其基本参数数据,重点分析了影响盐湖地区混凝土使用寿命的因素和规律,对比了不同混凝土在典型盐湖卤水中的使用寿命,最后将HPC应用于青海盐湖地区的重点工程中,并探讨了我国大西北盐湖地区钢筋混凝土结构的耐久性参数设计问题。结果表明,高强度的非引气HPC同时具有抗卤水冻蚀、抗卤水腐蚀和长寿命的特性。本文提出混凝土使用寿命的两套预测方法具有普适意义。各章的主要研究内容与结果如下:第一章综述了混凝土在盐湖地区的耐久性和使用寿命预测方法的研究意义和最新研究进展,指出了当前研究存在的问题,在此基础上确定了本文的主要研究方向。第二章简要介绍了我国盐湖地区的环境气候条件,针对典型盐湖卤水中含有的对混凝土产生物理化学腐蚀的侵蚀性离子浓度,重新划分了盐湖卤水的类型,现场考察了混凝土与钢筋混凝土结构在青海盐湖地区的腐蚀与破坏现状。调查发现,混凝土和钢筋混凝土结构的腐蚀破坏非常严重。第三章重点研究了盐湖地区HPC的物理力学性能,运用XRD、DTA-TG、SEM-EDAX、IR和MIP等测试方法详细研究了HPC的水化产物、微观结构和孔结构。结果表明,HPC的主要水化产物是C/S比为0.97的非常致密的CSH凝胶和AFt晶体,在水化后期由于火山灰反应会形成一定数量的六方片状AFm,其孔结构以凝胶孔为主。纤维增强HPC在水化365d以后的微观结构发生了根本性转变,形成了一种异常致密的CSH凝胶板块结构——“类陶瓷结构”,对于提高混凝土在盐湖地区的耐久性具有十分重要的作用。第四章设计一种大尺寸混凝土棱柱体试件(100mm×100mm×400mm)的加载实验装置。研究了不同混凝土在盐湖地区的腐蚀、冻融循环、干湿循环和弯曲荷载等单一、双重和多重因素作用下损伤失效过程的规律和特点,探讨了干燥条件对混凝土损伤失效过程的影响。结果表明,在盐湖地区的严酷条件下,OPC和APC的耐久性不好,HSC具有优良的抗卤水冻蚀性,但是其抗卤水腐蚀性比较差,高强非引气HPC具有优异的抗卤水冻蚀性和抗卤水腐蚀性,纤维增强HPC的效果更佳。第五章采用XRD、DTA-TG、IR和SEM-EDAX方法研究了混凝土的腐蚀产物和微观结构的变化,探讨了OPC、APC和HSC等在盐湖地区单一、双重和多重因素作用下的损伤失效机理,提出混凝土冻融破坏的第三种机制——盐结晶压机制。将混凝土的Mg2+-Ca2+-Cl--SO42-复合型腐蚀机理扩展到Mg2+-Cl--SO42--CO32--HCO3-复合型腐蚀机理。结果表明,在单一冻融因素作用下,非引气HPC的冻融裂纹源于AFm向AFt转化时的膨胀压。在(冻融+盐湖卤水腐蚀)双因素作用下,OPC的抗卤水冻蚀性很差,其冻融破坏起因于Na2SO4·10H2O的结晶压作用。盐湖卤水对混凝土的冻融损伤作用,既有降低冰点、缓解冻融抑制损伤的正效应,又有促进盐类结晶、产生盐结晶压引起损伤的负效应。APC在西藏盐湖的抗卤水冻蚀性很差,在内蒙古盐湖卤水中会发生冻融破坏。在正常温度的单一腐蚀因素和(干湿循环+腐蚀)等双因素作用下,OPC、APC和HSC的腐蚀破坏以形成多种腐东南大学博士学位论文蚀产物的化学腐蚀为主,NaCl-KCl物理结晶腐蚀为辅。并且发现2种新的腐蚀产物——水化硅铝酸钙镁(C1-xMx)0.94(S1-yAy)H(x=0.4,y=0.13)球形晶体族和硅灰石膏CaCO3·CaSiO3·CaSO4·15H2O。第六章,运用XRD、DTA-TG、IR、SEM-EDAX和MIP分析了HSC-HPC的腐蚀产物、微观结构和孔结构,研究了高强非引气HPC在盐湖地区单一、双重和多重因素作用下的耐久性形成机理,提出了HPC结构的腐蚀优化机理。结果表明,非引气HSC-HPC因其细小孤立的湿胀或自收缩裂纹、过渡孔-凝胶孔为主的孔结构、强化的界面过渡区和致密的CSH凝胶等结构特征,造就其很高的抗卤水冻蚀性。HPC及纤维增强HPC在盐湖地区的腐蚀条件下,将发生水化产物的轻微腐蚀效应、基体CSH凝胶的腐蚀转化效应、FA等未水化活性掺合料颗粒的腐蚀诱导水化效应和微裂纹愈合效应等4个方面的有利作用。第七章根据不同混凝土在4种典型盐湖的单一、双重和多重因素作用下的大量数据,研究了混凝土对氯离子的吸附/结合规律,提出了线性氯离子结合能力和非线性系数的新概念。结果表明,在较低的自由氯离子浓度范围内,混凝土对氯离子的结合规律以线性吸附为主;在较高的自由氯离子浓度范围内,混凝土对氯离子的结合表现出Langmuir非线性吸附规律。通过实验确定了不同混凝土的线性氯离子结合能力及其非线性系数数值,可供应用。从化学结合与物理吸附方面探讨了混凝土的氯离子吸附/结合机理,其化学结合机理主要体现在AFt-AFm和CH分别与NaCl、KCl、CaCl2或MgCl2反应形成Friedels盐和含有MgCl2的络合物,其物理吸附机理包括CSH凝胶表面的吸附作用和水泥浆体孔隙内表面的吸附作用。第八章针对以冻融或腐蚀为主要失效特征的混凝土结构,研究了不同混凝土在单一冻融因素和(冻融+盐湖卤水腐蚀)双因素作用下的损伤失效规律,总结并提出了具有普适意义的损伤速度和损伤加速度的混凝土损伤演化方程。结果表明,混凝土在与冻融或腐蚀有关的耐久性因素作用下,开始时其损伤以一定的初速度产生,之后以一定的加速度发展。损伤初速度与损伤加速度取决于结构所处的环境、气候和受力状态,并与混凝土的原材料、配合比和养护条件密切相关。初步建立了一套基于损伤演化方程的预测混凝土结构使用寿命的基本方法与理论框架,并将这种预测方法应用于青海盐湖钾肥工程、南京地铁和润扬大桥等重大工程的混凝土结构使用寿命的预测。为今后解决非氯盐环境条件下重大混凝土工程的寿命设计和耐久性评估问题,提供了很好的借鉴作用。第九章在深入探讨当前混凝土氯离子扩散理论存在8个问题的基础上,对Fick第二定律进行了有效的理论修正,推导出综合考虑混凝土的氯离子结合能力、扩散系数的时间依赖性、结构微缺陷和荷载影响的氯离子扩散新方程,针对有限大体与无限大体、齐次边界条件与非齐次边界条件、线性氯离子结合与非线性氯离子结合问题的I维、II维与III维氯离子扩散新方程的解析解,得到适应不同条件的氯离子扩散理论新模型。提出了模型参数的测定方法,确定了关键参数的取值规律和建立初步数据库,分析了不同理论条件和实验因素对混凝土使用寿命的影响规律,对盐湖地区HPC的使用寿命进行耐久性设计,探索了不同盐湖地区HPC结构的耐久性设计参数问题。结果表明,采用高强非引气HPC,完全能够解决盐湖地区混凝土结构的寿命问题,适当增大保护层厚度,在盐湖地区则有可能实现西部混凝土结构百年寿命的设想。第十章归纳了全文结论和创新点,提出了进一步研究的建议和设立ChinaDuraCrete项目的设想。更多还原

【Abstract】 Thousands of salt lakes are distributed in the four salt lake regions in Northwest of China, among which there are 102 belong to Xinjiang salt lake region, 33 to Qinghai salt lake region, 370 to Neimenggu salt lake region, and 220 to Xizang salt lake region. Corrosion resistance of concrete exposed to salt lakes is of special importance because of the extremely harsh weather in salt lake regions. Aiming at the desiccate environment of salt lake regions, in which the relative humidity ranges within 30%～50%, High Performance Concrete (HPC) accommodated to the circumstance were proportioned on bases of field investigations, incorporated with proportion principals of HPC, cracking prevention effects of fibers and shrinkage compensation functions of expansion agent. C30 ordinary Portland cement concrete (OPC), C25 air-entraining concrete (APC), C70 high strength concrete (HSC) without active additions, C70 HPC with the composites of silica fume, fly ash and slag additions, steel fiber reinforced HPC (SFRHPC), and polyethylene fiber (PF) with high Young’s modulus reinforced HPC (PFRHPC), were prepared. For concretes exposed to salt lakes, chloride absorption or binding isotherm, deterioration process and features of concretes subjected to the actions of single, double or multiple factors were investigated. Degradation mechanism of OPC, APC and HSC and the high durable mechanism of HPC were also explored. A service life predicting method based on damage development of concrete structure was proposed, and the service life predicting method based on chloride diffusion was modified and improved. Basic parameters in both models were determined based on a large number of field and laboratorial experiments. Influential factors and the principles on life expectancy were analyzed in specialty. Service life of concretes exposed to four kinds of salt lakes was compared in this paper, and the HPC has been applied to a key project in Qinghai salt lake region. Durability design of concrete structures constructed in salt lake regions was discussed. Results show that high strength HPC without air entraining is of excellent bittern-freezing-thawing durability and perfect bittern attack resistance, result in a long life expectancy. Both of the proposed predicting methods can be generally applied.Main contents of each chapter are mentioned as follows:In Chapter 1, durability of concretes exposed to salt lakes and service life predicting methods were summarized. Problems remaining to be investigated were also proposed, and outline of this contribution was sketched.In Chapter 2, environment and climate in salt lake regions were briefly introduced. Bitterns were reclassified according to concentrations of corrosive ions. Field investigations on reinforced concrete structures in Qinghai salt lake region discovered that corrosions of concrete and steel reinforced concrete were particularly serious and universal.In chapter 3, physical and mechanical properties of HPC exposed to salt lake regions were studied. The hydration products, microstructures and pore structure were investigated by means of XRD, DTA-TG, SEM-EDAX, IR, and MIP. Results show that hydration products of HPC are mainly compact CSH gels with a C/S ratio of 0.97, AFt crystals, and some hexagonal AFm plates, which are formed in the later hydration phase as results of pozzolanic reactions, and the pore structures are mainly gel pores. Essential transformation of microstructures in fiber reinforced HPC exceeding a age of 365 days were observed, namely a kind of highly compact plate-massive CSH gel, so called“ceramic structure similative”, formed, which brings favorable effect in concrete durability.In chapter 4, a loading system for 100mm×100mm×400mm prism samples was designed. Damage development and the degradation features of concrete subjected to single or combined action of salt attack, freezing-thawing cycles, wetting-drying cycles, and flexural stresses were investigated. Effect of desiccation on damage development was also discussed. Results show that when exposed to the severe environment of salt lakes, both OPC and APC display poor durability, though HSC shows excellent bittern-freezing-thawing durability, the corrosion resistance against bitterns is not so good, while non-air-entraining HPC shows outstanding performance in both bittern-freezing-thawing durability and corrosion resistance against bitterns, and the performance can be farther improved by adding fibers.In chapter 5, corrosion products and microstructures of concretes were investigated by using XRD, DTA-TG, IR, and SEM-EDAX. Based on damage development process, degradation mechanism of OPC, APC, and HSC subjected to the actions of single, double, or multiple factors were discussed, the third damage mechanism of concrete, viz. salt crystallization induced pressure, was presented. Corrosion mechanism of concrete exposed to bitterns containing Mg2+-Ca2+-Cl--SO42- was extended to bitterns containing Mg2+-Cl--SO42--CO32--HCO3-. Results show that freezing-thawing caused crackles in non-air-entraining HPC subjected to freezing-thawing cycling are resulted from pressure occurred as AFm transforms to AFt. When subjected to combined actions of freezing-thawing cycling and bittern attack, OPC shows poor bittern-freezing-thawing durability, due to pressure resulted from crystallization of Na2SO4·10H2O. Effects of bittern on concretes undergoing bittern-freezing-thawing cycling comprise both positive ones, including lowering ice point and abating freezing-thawing damage, and negative ones, such as accelerating salt crystallization and inducing concrete expansion cracks. APC exhibits poor bittern-freezing-thawing durability in bittern from Xizang salt lakes, and collapses while undergoing freeing-thawing cycles in Neimenggu bittern. When subjected to single action of bittern attack or combined actions of bittern attack and wetting-drying cycles, damage of OPC, APC, and HSC are mainly due to chemical corrosions, resulting in several attack products, and also due to crystallization of NaCl-KCl. Two attack products were firstly reported, viz. spherical crystals of (C1-xMx)0.94(S1-yAy)H (x=0.4,y=0.13) and CaCO3·CaSiO3·CaSO4·15H2O.In chapter 6, corrosion products, microstructures and pore structures of HSC-HPC were analysised by XRD, DTA-TG, IR, SEM-EDAX, and MIP methods. Long durability mechanism of high strength non-air-entrained HPC exposed to salt lakes under the actions of single, double or multiple factors, was investigated, and a corrosion induced strengthening mechanism of HPC was proposed. The experimental results show that the excellent bittern-freezing-thawing durability of high strength non-air-entrained HPC benefits from the isolated tiny crackles resulted from wetting swelling or autogenous shrinkage, the pore structures containing transitional pores and gel pores. So the interfacial zone was strengthened, and gel was compacted. When HPC and fiber reinforced HPC are exposed to salt lakes, 4 advantageous effects, viz. mild corrosion of hydration products, transformation of CSH gel subjected to bittern attack, corrosion induced hydration of the remaining active additives, and close of tiny crackles, will occur.In chapter 7, chloride absorption or binding isotherm of concretes was investigated, linear chloride binding capacity and a non-linear coefficient were firstly proposed, based on a large amount of experimental data of concretes under the actions of single, double, or multiple factors in four typical bitterns. Experimental results show that linear absorption of chloride is dominating while the free chloride concentration is in a low range. However, non-linear Langmuir isotherm shows itself when the free chloride concentration is in a high range. Linear and non-linear coefficients of chloride binding capacity were determined from a large number of experimental data. Chloride absorption or binding isotherm of concretes were discussed based on physical absorption or chemical binding respectively, among which the chemical binding includes mainly chemical reactions of AFt-AFm and CH reacting with NaCl, KCl, CaCl2, or MgCl2, resulting in Friedels salt and complex salt with MgCl2, and the physical absorption comprises of absorption of chloride ions by CSH gel surfaces and pore surfaces in cement pastes.In chapter 8, damage and degradation of concretes subjected to single factor of bittern attack, or to double factors of combined bittern attack and freezing-thawing cycles were investigated. A universal damage developing equation, which including initial damage velocity and damage acceleration was deduced. Results show that when concrete subjected to corrosion or freeing-thawing related durability factors, the damage initials at a certain initial damage velocity and then accelerates at damage acceleration. Both the initial damage velocity and damage acceleration are determined by the environment, climate and the loading state. Based on the damage developing equation, a service life predicting method was established. This equation had been applied to concrete structures of a fertilize plant located in Qinghai salt lake, subway of Nanjing city, and the Runyang Bridge across Yangtse River. It also provided a method for service life predicting of key concrete projects constructed in environments without chloride attack.In chapter 9, the 8 problems remained to be resolved on chloride diffusion of concretes was discussed, and theoretical modifications on Fick’s second law was proposed. A diffusion equation, which took account of effects of concrete chloride binding capacity, time dependence on diffusion coefficients, defects in microstructures and loading, was proposed. For both linear and non-linear chloride binding, analytic solutions for one, two, or three-dimensional diffusion in finite or infinite space with different boundary conditions either independent or dependent on time change, were deduced. A theoretical model of chloride diffusion was also presented. Parameters of the model could be determined by certain procedures, and the key parameters were evaluated, a basic database was established also. Effects of theoretical conditions and experimental factors on concrete service life were analyzed. Durability design of HPC exposed to salt lake regions was carried out, and determination of design parameters was discussed. Results show that concrete structures constructed by non air-entrained HPC, with sufficient strength and reasonable protective covers, could attain a service life of 100 years.A summarization of this contribution was presented in chapter 10. Propositions for further researches and schemes for ChinaDuraCrete were also listed.更多还原