Durability of Concrete

Besides its ability to sustain loads, concrete is also required to be durable. The durability of concrete can be defined 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 causes are the effects of salts, particularly chlorides and sulphates, in the constituent materials, interaction between the constituent materials, such as alkali-aggregate reaction, volume changes, absorption and permeability.

In order to produce a durable concrete, care should be taken to select suitable constituent materials. It is also important that the mix contains adequate quantities of materials in proportions suitable for producing a homogeneous and fully compacted concrete mass.

除了承受荷载的能力之外，混凝土还应该经久耐用。混凝土的耐久性可以定义为混凝土抵抗由外部或内部原因而造成的损坏的能力。外部原因包括环境的作 用和混凝土使用条件的作用，例如风化、化学作用和磨损。内部原因是组成材料中的盐，特别是氯化物和硫酸盐引起的组成材料中的相互作用，例如碱－集料反应、体积变化、吸水性和渗透性。

为了生产耐久性好的混凝土，应当认真地选择合适的组成材料。混合料中含有比例适当的，适量的材料以生产均质和密实的混凝土也是重要的。

Weathering

Deterioration of concrete by weathering is usually brought about by the disruptive action of alternate freezing and thawing of free water within the concrete and expansion and contraction of the concrete, under restraint, resulting from variations in temperature and alternate wetting and drying.

Damage to concrete from freezing and thawing arises from the expansion of pore water during freezing; in a condition of restraint, if repeated 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. 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 homogeneous mass. Durability can be further improved by using air-entrainment, an air content of 3 to 6 per cent of the volume of concrete normally being adequate for most applications. The use of air-entrained concrete is particularly useful for roads where salts are used for deicing.

风化

混凝土的风化损坏通常是由混凝土内的自由水冻融交替的破坏作用和混凝土在约束条件下膨胀和收缩引起的，这些都是由温度的变化和干湿交替造成的。混凝土的冻融破坏是由孔隙水结冰时的膨胀列起的，在受约束的状态下，如果冻融反复足够多的次数，水压力作用的结果就能使混凝土破裂。路缘和路面板、坝及水库对冰冻作用非常敏感。混凝土抵抗冻融的性能可以通过提高其抗渗性而得到改善。这可以通过采用尽可能低的水灰比来达到，但是必须具有能浇筑和捣实成为均匀体所需的和易性。耐久性可以通过使用引气剂得到进一步的改善，占混凝土体积3%～6%的空气含量，可以满足大多数应用的要求。加气混凝土特别适用于用盐做出冰雪剂的道路。

Chemical Attack

In general, concrete has a low resistance to chemical attack. There are several chemical agents which react with concrete but the most common forms of attack are those associated with leaching, carbonation, chlorides and sulphates. 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 chemical attack improves with increased impermeability.

化学侵蚀

一般来说，混凝土抵抗化学侵蚀的性能较低。有数种可以同混凝土反应的化学物品，但是最常见的侵蚀有溶析、碳化作用和氯化物 、硫酸盐侵蚀。化学物品主要和硬化水泥浆体的某种成分反应，因此，混凝土的抗化学侵蚀性主要取决于所用水泥的品种。抗化学侵蚀性能随抗渗性的提高而得到改善。

Wear

The main causes of wear of concrete are the cavitation effects of fast-moving water, abrasive material in water, wind blasting and attrition and impact of traffic. Certain conditions of hydraulic flow result in the formation of cavities between the flowing water and the concrete surface. These cavities are usually filled with water vapour 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 able 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 cavitation erosion.

In general, the resistance of concrete to erosion and abrasion increases with increase in strength. The use of a hard and tough aggregate tends to improve concrete resistance to wear.

磨损

混凝土磨损的主要原因是快速运动的水的空蚀作用、水中的磨料、风带起的砂粒以及交通带来的磨耗和冲击。水流在某种条件下能造成流水和混凝土表面间空穴的形成，这些空穴往往被带有极高能量的水汽所填充，这些水汽和混凝土表面反复接触，结果便形成小坑和孔洞，称为空蚀。因为即使是优质的混凝土也不能抵抗这种损坏，因此，最好的补救方法是通过产生平滑的水流来消除空蚀。在必要的地方，关键的区域可以使用抗空蚀性能好的材料来加固表面。

一般来说，混凝土抵抗空蚀和磨损的饱力随其强度的增加而增加。采用坚硬的集料有助于提高混凝土的耐磨性。

Alkali-Aggregate Reactions

Certain natural aggregates react chemically with the alkalis present in Portland cement. When this happens these aggregates expand or swell resulting in cracking and disintegration of concrete.

碱－集料反应

某些天然集料会与硅酸盐水泥中的碱起化学反应。当出现这种现象时，集料便膨胀或鼓起，造成混凝土的开裂和破碎。

Volume Changes

Principal 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. When a change in volume is resisted by internal or external forces this can produce cracking, the greater the imposed restraint, the more severe the cracking. The presence of cracks in concrete reduces its resistance to the action of leaching, corrosion of reinforcement, attack by sulphates and other chemicals, alkali-aggregate reaction and freezing and thawing, all of which may lead to disruption of concrete. Severe cracking can lead to complete disintegration of the concrete surface particularly when this is accompanied by alternate expansion and contraction.

Volume changes can be minimised by using suitable constituent materials and mix proportions having due regard to the size of structure. Adequate moist curing is also essential to minimise the effects of any volume changes.

体积变化

造成体积变化的主要因素是水和水泥的化学结合，以及随后混凝土的干燥、温度变化和干湿交替。当体积变化受到内力或外力抵抗时，就可能产生裂缝，所受到的约束越大，产生的裂缝就越严重。混凝土中裂缝的出现，降低其抵抗溶析作用、钢筋锈蚀、硫酸盐及其他化学物品的侵蚀、碱－集料反应和冻融作用的能力，所有这些都可以导致混凝土的破裂。严重的裂缝，特别是当伴随有交替的膨胩和收缩时，可以导致混凝土表面的完全碎裂。

可以采用适当的组成材料和考虑其结构尺寸的混合料配比，将体积变化减至最小。养护时，适当的湿度也能减少体积的变化。

Permeability and Absorption

Permeability refers to the ease with which water 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 of concrete to draw water into its voids. Low permeability is an important requirement for hydraulic structures and in some cases watertightness of concrete may be considered to be more significant than strength although, other conditions being equal, concrete of low permeability will also be strong and durable. A concrete which readily absorbs water is susceptible to deterioration.

Concrete 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 the concrete during compaction. If the voids are interconnected concrete becomes pervious although with normal care concrete is sufficiently impermeable for most purposes. Concrete of low permeability can be obtained by suitable selection of its constituent materials and their proportions followed by careful placing, compaction and curing. In general for a fully compacted concrete, the permeability decreases with decreasing water-cement ratio. Permeability is affected by both the fineness and the chemical composition of cement. Aggregates of low porosity are preferable when concrete with a low permeability is required. Segregation of the constituent materials during placing can adversely affect the impermeability of concrete.

渗透性和吸水性

渗透性是指水穿透过混凝土的容易程度。它不应该与混凝土的吸水性相混淆，二者没有必然的联系。吸水性可以定义为混凝土把水吸入其孔隙中的能力。低渗透性是水工建筑物的一个重要的要求，而且在某些情况下，可以认为混凝土的水密性比其强度更为重要，虽然在其他条件相同的情况下，低渗透性的混凝土也是高强度和耐久的。容易吸水的混凝土易于损坏。

混凝土的本性是一种多孔材料。这是因为在混合料中使用了比水化的需要量要多的水来使其具有足够的和易性，以及在捣实期间难以从混凝土中排除所有的空气。如果孔隙相互连找，混凝土就会变成透水的，但是对于大多数结构来说，正常养护的混凝土足以满足其抗渗性要求。通过适当选择组成材料和配合比，并且进行认真的浇筑、捣实和养护，就可以得到渗水性低的混凝土。通常，对于充分捣实的混凝土，其渗透性随水灰比的降低而降低。水泥的细度和化学成分对渗透性都有影响。当要求混凝土具有低渗透性时，应该使用低孔隙率的集料。浇筑时，组成材料的离析能够对混凝土的抗渗性产生有害的影响。