【作者】 张云清；

【导师】 余红发；

【作者基本信息】 南京航空航天大学 ， 道路与铁道工程， 2011， 博士

【摘要】 水泥混凝土是土木工程最主要的结构材料，混凝土结构的耐久性和服役寿命是国际国内工程界关注的重大科技问题。国内外学术界对混凝土在除冰盐作用下耐久性问题的研究主要集中在抗盐冻剥蚀性方面，即混凝土单面受到除冰盐溶液冻融作用下的表面剥蚀性能，但是没有同时研究混凝土结构内部的冻融损伤、氯离子浓度分布、钢筋锈蚀及结构承载力等变化规律，这对混凝土结构的耐久性和寿命研究是不全面的。本文在我国严寒地区城市主要立交桥和机场高速公路混凝土结构的实地调研分析的基础上，系统进行了引气与非引气的、不掺与掺加矿物掺合料的普通混凝土、高强混凝土和高性能混凝土在3.5％NaCl溶液(除冰盐)作用下的快速冻融耐久性实验，以及在北方冬季严寒的室外自然冻融环境中的除冰盐暴露耐久性实验；研究了混凝土构件在室内快速盐冻条件及室外自然暴露盐冻条件下的相对动弹性模量变化、表面剥蚀及其损伤劣化过程与机理、氯离子扩散规律、抗弯承载力与变形等规律，建立了盐冻作用下钢筋混凝土构件的承载力计算模型和服役寿命预测模型，为混凝土工程结构的耐久性设计提供了理论参考。本文主要研究内容和结果如下：1、通过对沈阳市立交桥的外观检查与无损检测发现，部分立交桥即使经过多次的维修与加固，仍然发生了粗集料外露、钢筋裸露锈蚀、箍筋锈断等非常严重的盐冻耐久性破坏；施工控制不严，混凝土保护层厚度不均，偏差比较大。混凝土碳化、盐冻等环境因素作用下，混凝土的碱性和密实性降低，加速了混凝土损伤及钢筋锈蚀。对现场混凝土及钢筋锈蚀物样品的化学分析和微观结构分析表明，结构混凝土内部的氯离子浓度很高，其浓度分布规律因受到雨水的冲刷影响，在保护层厚度范围内并不符合Fick扩散定律，其寿命设计的氯离子扩散模型必须考虑表层效应，并进行参数修正；结构混凝土的总氯离子浓度和自由氯离子浓度之间成线性关系；计算得出立交桥混凝土的氯离子结合能力只有0.0687，氯盐对混凝土中钢筋的锈蚀破坏作用很大。2、混凝土构件及混凝土试件的快速盐冻试验表明，C30混凝土的抗盐冻性能很差，其盐冻破坏源于混凝土的表面剥蚀现象，混凝土构件由于钢筋的约束作用，将延缓其相对动弹性模量的下降速度。高性能混凝土的抗盐冻破坏能力显著提高；但是过高的粉煤灰掺量将显著地降低高性能混凝土的抗盐冻性能；掺加硅灰的引气混凝土试件具有较高的抗盐冻性能，但是其混凝土构件的抗盐冻性能因其内部自收缩微裂纹的宏观扩展，出现较严重的劣化现象，这种劣化并不因强度等级的提高而有明显的改善。因此，对于掺加硅灰的混凝土，试件的盐冻破坏性能并不能可靠地反映钢筋混凝土构件的抗盐冻性能。3、利用低真空扫描电子显微镜（SEM）观察了硬化混凝土中的气泡结构形貌，运用图像分析软件计算气泡结构的特征参数。研究结果表明，混凝土具有较高抗盐冻性的含气量应提高至5.0%以上；对于掺矿物掺合料的高性能混凝土，具有良好抗盐冻性所要求的气泡间距与强度等级有关，当强度等级低于C50时，平均气泡间距必须小于250μm，当强度等级提高到C60以上时，平均气泡间距可以增大到700μm。4、通过超声平测法研究混凝土构件的盐冻损伤层厚度，探讨了钢筋对超声波测试结果的修正方法，提出了综合描述混凝土结构盐冻破坏的损伤度新概念，当混凝土的损伤层越厚、损伤层混凝土的声速越低时，表示其盐冻损伤度越大。5、对盐冻损伤的混凝土构件进行力学性能研究发现，混凝土受弯构件在初始荷载作用下，截面的应力状态仍满足混凝土构件平截面假定，因此可以采用现有的钢筋混凝土结构理论，建立盐冻作用下混凝土构件的结构设计模型和计算公式。在历经不同快速冻融循环次数的盐冻作用以后，混凝土内部出现盐冻损伤、表面疏松剥蚀，弹性阶段开裂点不明显，构件的弯曲刚度也随之减小，承载力极限值降低，跨中挠度相应增加。6、基于材料力学的平截面假定，参照现行《混凝土结构设计规范》(GB50010—2010)中受弯构件承载力、刚度和挠度的计算方法，引进盐冻作用的耐久性特征参数，提出了采用相对动弹性模量、盐冻损伤层厚度、钢筋锈蚀率作为盐冻环境下混凝土结构的耐久性设计参数，建立了考虑盐冻作用的钢筋混凝土结构的设计公式，其计算值与实验值吻合较好，可应用于实际工程结构的耐久性设计。7、根据混凝土构件承载力的耐久性退化作用和可靠度理论，建立了盐冻作用下混凝土结构的3阶段服役寿命理论模型：诱导期、劣化期和失效期。基于氯离子扩散、相对动弹性模量变化和混凝土表面剥蚀程度，确立了第一阶段服役寿命的计算依据——即盐冻作用下，随服役时间的延长，混凝土结构表面开始发生剥蚀、内部开始发生冻融损伤以及钢筋表面的自由氯离子浓度开始达到锈蚀临界值的时间时，混凝土结构的承载力才开始衰减，此时对应的时间即达到t1阶段的结束；基于结构混凝土表面开始剥蚀、抗压强度等力学性能开始下降、钢筋开始锈蚀以后，导致混凝土结构承载力的退化，确立了第二阶段服役寿命的计算依据——当结构混凝土的表面剥蚀量大于1500g/m2，内部相对动弹性模量下降至80%、以及钢筋锈蚀率超过6%时，即当混凝土结构的主要材料（混凝土和钢筋）达到其材料耐久性破坏的限值时，此时对应的时间即认为混凝土构件的第二阶段服役寿命t2；当结构继续使用，混凝土结构的承载力降低到设计极限状态时，此时对应的时间即认为混凝土构件的第三阶段服役寿命t3。8、依据系统的耐久性试验研究结果，通过本文建立的3阶段服役寿命理论模型，获得了引气与非引气的、不掺与掺加矿物掺合料的普通混凝土、高强混凝土和高性能混凝土构件在盐冻作用下的承载力退化规律曲线，计算了其服役寿命。结果表明，C30普通混凝土结构的服役寿命计算结果与实际工程调研的结果相符。9、运用本文建立的盐冻作用下混凝土结构的3阶段服役寿命理论模型，对典型盐冻环境下混凝土结构桥梁的主要构件进行了耐久性设计，提供了算例，比较了按照常规设计和耐久性设计结果之间的差异。为我国盐冻作用环境下混凝土结构按照50a或者100a寿命进行耐久性设计，提供了一种可行的设计方法。更多还原

【Abstract】 Concrete is the most important structural material in Civil Engineering, Concrete’s durability andservice life are major scientific and technological issues concerned by both international and domesticengineering. Domestic and foreign research on the durability of the de-icing salt on concrete mainlyconcentrated in the anti-erosion performance under salt freezing circumstance, viz.one side’ spallingproperties under freeze-thaw cycles in the de-icing salt solution. However, the freeze-thaw damage ininternal structure, chloride ion concentration, steel corrosion and changes of structural capacity havenot been systematically estimated. Therefore, the present research on the durability and service life ofstructural concrete is incomplete. In this study, based on the field research in the main airportexpressways and overpasses in the cold region of China, systematically experiments were carried outincluding various rapid freezing and thawing durability tests in ordinary concrete with gas and no gas,with admixture and no admixture, high strength concrete and high performance concrete in3.5%NaClsolution (de-icing salt). The freeze-thaw durability exposure test was also simultaneously performed inthe natural outdoor winter-cold environment. Therefore, the laws of the changes of relative dynamicelastic modulus, the process and mechanism of erosion and damage, the mechanism of diffusion ofchloride ions, bending resistance and deformation laws are presented. Furthermore, the bearingcapacity model of reinforced concrete and service life prediction model under the action of salt freezingwere established, which will provide a theoretical reference for the durability design in engineeringconcrete structures. The main contents and results in this paper are as follows:1. By visual inspection and nondestructive testing in some overpasses in Shenyang, some seriousdamage related to salt freezing durability, such as coarse aggregate exposure, bare steel corrosion andstirrups rust and breakage can be easily found, even in some overpasses repaired and reinforced severaltimes. Lack of strict construction control results in uneven thickness of concrete cover, inducingrelatively larger deviation. Because of carbonation, salt freezing and other environmental factors, thealkaline and density of concrete decrease. This results in accelerated steel corrosion and concretedamage. The chemical and microstructure analysis on the on-site concrete and the products of steelcorrosion indicate that the chloride ion concentration inside the structural concrete which is affected bythe erosion of rain is high. Besides, in the protective layer, the chloride ion concentration does notcomply with the Fick diffusion law. Therefore, the surface effects should be taken into account in thechloride diffusion model for life design and relative parameter should be modified. A linear relationship was found between the total and free chlorine ion concentration in structural concrete; the calculatedchloride binding capacity in overpass concrete is only0.0687, indicating that chloride salts play a keyrole in the corrosion damage of steel reinforcement in concrete.2. The rapid salt freezing tests on concrete structures and specimen show that the anti-salt freezingperformance of C30is poor. This damage results from the erosion on the surface of concrete. Thedeclining speed of relative dynamic elastic modulus will be delayed as the result of the restriction effectof reinforcement steel bar. The anti-salt freezing capacity of high performance concrete increasesdramatically; However, too much fly ash addition will significantly reduce the anti-salt freezingperformance of high performance concrete; Air-entrained concrete specimens with silica fume showhigher anti-salt freezing performance, but the corresponding concrete structures show seriousdeterioration due to the macro expansion of internal shrinkage cracks. And this deterioration will not beimproved as the strength grade increases. Therefore, for concrete with silica fume, the anti-salt freezingperformance of specimen will not reliably reflect the anti-salt freezing capacity of reinforced concretestructures.3. Low vacuum scanning electron microscopy (SEM) was used to observe the structure andmorphology of bubbles in hardened concrete. The characteristic parameters of bubble structure werecalculated by the matching image analysis software. The results show that the air content in concretewith good salt freezing tolerance should be increased to more than5.0%. For high performanceconcrete with mineral admixture, the bubble spacing required for good anti-salt freezing capacity haverelation to strength grade, i.e, when strength level is lower than the C50strength grade, the averagespacing must be less than250μm, while when the strength level increases to more than C60, theaverage spacing can be increased to700μm.4. The salt freezing damaged thickness of concrete structures was studied by ultrasonic levelmeasurement and a correction method reflecting the influence of reinforcement on the ultrasonic testresults. A new concept for comprehensive description of salt freezing damage in concrete structure wasproposed. The thicker the damaged layer of concrete and the lower speed of sound propagating in thedamaged layer, the greater the degree of salt freezing damage.5. When the mechanical properties of salt freezing damaged concrete structures were studied, itwas found that the stress state of cross section under initial load still met the plane section assumption.Therefore, the existing reinforced concrete structure theory can be used to establish the structuraldesign models and formulas under the action of salt freezing. After various rapid salt freezing circleswere conducted, the salt freezing damage and loose peeling phenomenon occured inside the concrete and in the surface, respectively. And cracking points are vague in elastic stage, the correspondingbending stiffness and capacity limits also decrease, the midspan deflection increases correspondingly.6. Based on plane assumption in Material Mechanics and the calculation methods in Design ofConcrete Structures(GB50010-2010) on flexural strength and stiffness and deflection, the salt freezingparameters are also introduced. The relative dynamic elastic modulus, thickness of salt freezing damageand steel corrosion rate were proposed to be design parameters of durability in the salt freezingenvironment. The design formula of reinforced concrete structures was also established, consideringthe effect of salt and freezing. The calculated values from the formula are in agreement with theexperimental values, which can be applied to the durability design for the actual engineering structures.7. According to the degradation of bearing capacity and the reliability theory, a three-stage servicelife theoretical model (Induction period, deterioration and expiration period) was established on theconcrete structures under the action of salt and freezing. Based on chloride ion diffusion, the changes ofrelative dynamic elastic modulus and the concrete surface’s denudation degree, the basis of service lifecalculation in the first stage can be established. Viz. with the service time prolonging under the actionof salt freezing, when the concrete surface erosion begins and inside,the freeze-thaw damageoccurs,and the free chloride ion concentration on the surface of reinforcement start to arrive atthreshold value to rust, the bearing capacity of concrete structures begin to decay, then thecorresponding time is the end of t1period; Based on the facts that the degradation of bearing capacityof concrete induced by the decrease of mechanical properties such as compressive strength and the steelbeginning to rust, the basis of service life calculation in the second stage can be established. viz. Whenthe surface of structural concrete spall greater than1500g/m2, the internal relative dynamic elasticmodulus decrease to80%, and steel corrosion rate is over6%, the main material of concrete structures(concrete and steel) reaches the durability of the material damage limit value, the corresponding time isregarded as the service life time in the second stage (t2). The calculated actual bearing capacity ofconcrete structures declines to the limit design load, the corresponding time is regarded as the servicelife time in the third stage (t3).8. Based on the systematic experiments on durability and three-stage theoretical service life modelestablished in this paper, various bearing capacity deterioration curves and service life of mixtures withgas and no-gas, with the admixture and noimineral admixture ordinary concrete, high strength concreteand high-performance concrete under the action of the salt freezing were respectively established. Thecalculated results of C30concrete service life match the one form the practical engineering.9. By the use of the three-stage theoretical service life model established in this paper, thedurability of the main concrete bridge structures in typical salt freezing environment was designed to provide a numerical example. The results were also compared between the conventional design anddurability design. The results show that a feasible design method is present for the durability design ofconcrete structure in the salt freezing environment with the life expectancy of50or100years.更多还原