SectionC混凝土耐久性

1、Section C 混凝土耐久性现今，在各种严酷环境条件下和伴随许多混凝土技术发展新问题的出现， 越来 越多的由于缺乏耐久性而不是强度不足的工程破坏实例引起了人们对混凝土耐 久性的关注。 时久性设计代替强度设计的观点越来越广泛地被人们所接受。 混凝 土的耐久性被定义为抵抗外部和内部原因导致的恶化的能力。 外部原因包括混凝 土遭受的环境的影响和工作条件的影响， 例如气候、 化学作用和磨损。 内部原因 是盐的影响， 特别是氯化物和硫酸盐， 以及组成材料之间的相互作用， 如碱骨料 反应、体积变化、吸水性和渗透性。为了生产高耐久性的混凝土，人们应该注意 选择合适的材料组分。 为生产均匀致密的混凝土，

2、使各组分材料配比得当， 这一 点也是很重要的。冻融由气候带来的混凝土的劣化通常是由混凝土内部自由水的冻融交替、 温度变 异和干湿交替引起的抑制作用下混凝土的膨胀、 收缩等引起的。 在冻期， 由冻融 循环导致的混凝土破坏是由孔隙水的膨胀引起的， 在一个特定条件的限制下， 如 果冻融循环重复足够的次数， 会导致水压力的发展有可能破坏混凝土。 路肩利石 板路，水坝和水库是非常容易遭受冻胀作用的结构。混凝土时冻融的能力可以通过提高混凝土的抗渗透性来改善。这可以通过采 用尽可能低的水灰比来达到， 但是必须具有经浇筑和捣实成内均匀体所需的和易 性。混凝土的时久性可以通过采用引气剂进一步改善，通常3%-6%

3、混凝土体积的含气量， 可以满足大多数应用的要求。 引气混凝土通常被用在撒除冰盐的路而 上。化学侵蚀一般情况下， 混凝土具有较低的耐化啦侵蚀能力。 很多化学制剂会与混凝土 发生反应， 但最常见的侵蚀有溶析、 碳化作用和氯化物、 硫酸盐侵蚀（如图 3.1）。 化学物品主要与硬化水泥浆体的某种成分发生反应， 因此，混凝土的耐化学侵蚀 能力通常受水泥标号的影响。 混凝土的耐化学侵蚀能力随着抗渗性的增强而明显 磨损混凝土磨损的土要原因是由快速流动的水、 水里的磨损材料、 风沙袭击和磨 耗以及交通的影响。一定条件下的水压流量会导致流动的水和混凝土表而之问空 隙的形成。这些空隙通常被有超高能量的水蒸气充满，

4、由于与混凝土表而频繁接 触，就导致了坑和洞的形成，称为气蚀。即使质量很好的混凝土也无法抗拒这种 劣化，因此，最好的补救办法是通过制造光滑的液压流来消除气穴。如有必要， 关键部位可以用有更好的耐气蚀材料作为内衬。一般情况下，混凝土的耐侵蚀性和耐磨损性随着强度的增长而增长。使用坚 硬和坚韧韧的骨料有助于提高混凝土的耐磨损性。目前在普通硅酸盐水泥中，某些天然的骨料与碱发生化学反应。 当这种反应 发生的时候，这些骨料扩大或膨胀，导致混凝土的开裂利破碎。体积的改变对于体积的改变土要的影响因素是水和水泥的化学结合、 随之而来的混凝土十 燥以及温度的变异和干湿交替。当粉煤灰等矿物掺合料掺入混凝土时， 其活性

5、成 分与水泥水化析出的氧氧化钙发生火山灰反应，生成体积胀大的硅酸钙水化物。当混凝土体积的改变被内部或外部力抑制时， 这将会产生裂缝，越是抑制，裂 缝就越严重。混凝土中裂缝的存在降低了它对溶析、 钢筋锈蚀、硫酸盐和其他化 学物质侵蚀、碱骨料反应利冻融损伤的抵抗力，从而导致混凝土破坏。特别是当 伴有交替的膨胀和收缩的时候，严重的裂缝能导致混凝土彻底破碎。体积的改变可以通过采用合适的材料组分以及与结构尺寸有关的配合比来减小。适当的湿度也能减小体积变化。吸水性和渗透性渗透性指水透过混凝土的难易程度， 不能与混凝生的吸水性相混淆，一者没 有必然的联系。吸水性可被定义为混凝土汲取水分进入孔洞的能力。 对于

6、水工结 构，低渗透性是一个很重要的要求，在某些情况下，可以认为混凝土的不透水性 比强度更为重要。其他条件相同的情况下，混凝土的低渗透性将会使其更时久。 容易吸收水分的混凝土更容易劣化。混凝土本身就是多孔材料，这是因为在混合料中使用了比水化的需要量更多 的水来使其具有足够的和易性，以及在捣实过程中空气难以完全从混凝土中排 出。如果孔隙相互连通，混凝土就会变成透水的，但是正常养护一的混凝土足以 满足大多数结构的不透水性要求。低渗透性的混凝土可以通过选择合适的材料组 分利比例并辅以精心浇筑、捣实利养护来实现。通常，对于允分捣实的混凝土， 混凝土的渗透性随着水灰比的减小而降低。渗透性还受水泥的 细度利

7、化学成分 的影响，低渗透混凝土应优先使用低孔隙率的骨料。 在浇筑过程年， 组成材料的 离析会严重影响混凝土的抗渗性能。混凝土耐久性的问题不仅影响建筑材料的许多性能， 而且还影响国家环境的 可持续发展目标。混凝土耐久性在许多领域影响颇为广泛。将性能、结构、过程 利环境等作为一个整体 的系统论方法来评估混凝土的耐久性被证明是有效的。Durability of ConcreteAt present, more and more destructive case of engineering due to lack of durability not insufficient strength und

8、er all kinds of serious conditions and many new questions companying withthe development of concrete technology make people pay attention to the durability of concrete.The idea of which concrete should be designed according to durability instead of strength was accepted widely.The durability of conc

9、rete can be defmed as its resistance to deterioration resulting from external and internal causes. The external causes include the effects of environmental and service conditions to which concrete is subjected, such as weathering, chemical actions and wear. The internal causesare the effects of salt

10、s, paticularly chlorides and sulphates in the constituent materials, interaction between the constituent materials, such as alkali aggregate reaction, volume changes, absorption and permeabilityIn order to produce a durable concrete care should be taken to select suitable constituent materials. It i

11、s also importnt that the mix contains adequate quantities of materials in proportions suitable for producing a homogeneous and fully compacted concrete mass. Freeze-thawingDeterioration of concrete by weathering is usually brought about by the disruptive action of alternate freezing and thawing of f

12、ree water within the concrete and expansion and contraction of the concrete, under restraint, resulting from variaHons in temperatureand alternate wetringand dryingDamage to concrete from freezing and thawing arise from the expansion of pore water during freezing, in a condition of restraint, if rep

13、eated a sufficient number of times, this results in the development of hydraulic pressure capable of disrupting concrete Road kerbs and slabs, dams and reservoirs are very susceptible to frost action.The resistance of concrete to freezing and thawing can be improved by increasing its impermeability.

14、 This can be achieved by using a mix with the lowest possible water cement ratio compatible with sufficient workability for placing and compacting into a homogenous mass.Durability can be fmther improved by using air entrainment, an air content of 3 t0 6 percent of thevolume of concrete normally bei

15、ng adequate for most applications. The use of air-entrained concrete is partcularly useful for roads where salts are used for deicing Chemical attackIn general, concrete has a low resistance to chemical attack There are several chemical agents, which react with concrete, but the most common forms of

16、 attack are those associated with leaching,carbonation, chlorides and sulphates (Fig 3.1) Chemical agents essentially react with certain compounds of the hardened cement paste and the resistance of concrete to chemical attack, therefore can be affected by the type of cement used The resistance to ch

17、emical attack improves with increased impermeability.WearThe main causes of wear of concrete are cavitation effects of fast-moving water, abrasive materialin water, wind blasting and attrition and impact of traffic Certain conditions of hydraulic flow result in the formation of cavities between the

18、flowing water and the concrete surface These cavities are usually filled with water vapor charged with extraordinarily high energy and repeated contact with the concrete surface results in the formation of pits and holes, known as cavitation erosion. Since even a good-quality concrete will not be ab

19、le to resist this kind of deterioration, the best remedy is therefore the elimination of cavitation by producing smooth hydraulic flow. Where necessary, the critical areas may be lined with materials having greater resistance to cavitaHon erosion.In general, the resistance of concrete to erosion and

20、 abrasion increases with increase in strength. The use of a hard and tough aggregate tends to improve concrete resistance to wear. Certain natural aggregatesreact chemically with the alkalis presentin Portland cement When this happens these aggregates expand or swell in resulting in cracking and dis

21、integration of concrete.Volume changePrincipal factors responsible for volume changes are the chemical combination of water and cement and the subsequent drying of concrete, variations in temperature and alternate wetting and drying. In the case of fly ash or silica fume mixed in the concrete, activ

22、ation composition of mineral adnuxture and calcium hydroxide precipitation of cement hydrate have pozzolanic reaction and generate calcium silicate, which expands in volume .When change in volume is resisted by internal or external forces, this can produce cracking, the greater the imposed restraint

23、, the more severe the cracking The presenceof cracks in concrete reduces its resistance to the action of leaching, corrosion of reinforcement, attack by sulphates and other chenucals, alkali-aggregate reaction and freezing and thawing, all of which may lead to disruption of concrete Severe cracHng c

24、an lead to complete disintegration of the concrete surface particularly when this is accompanied by alternate expansion and contraction.Volume changes can be minimized by using suitable constituent materials and mix proporlions having due regard to the size of structure Adequate moist is also essent

25、ial to minimize the effects of any volume changes Permeability and AbsorptionPermeability refers to the ease with which can pass through the concrete This should not be confused with the absorption property of concrete and the two are not necessarily related Absorption may be defined as the ability

26、of concrete to draw waterinto its voids Low permeability is an important requirement for hydraulic structures and in some cases water tightness of concrete may be considered to be more significant than strength although, other conditions being equal, concrete of low permeability will also be strong

27、and durable A concrete which readily absorbs water is susceptible to deteriorationConcrete is inherently a porous material. This arises from the use of water in excess of that required for the purpose of hydration in order to make the mix sufficiently workable and the difficulty of completely removing all the air from concrete during compaction. If the voids are interconnected concrete becomes pervious although with normal care concte is sufficiently impermeable for most purposes Concrete oflow permeability can be ob