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.

除了承受荷載的能力之外，混凝土還應(yīng)該經(jīng)久耐用。混凝土的耐久性可以定義為混凝土抵抗由外部或內(nèi)部原因而造成的損壞的能力。外部原因包括環(huán)境的作 用和混凝土使用條件的作用，例如風(fēng)化、化學(xué)作用和磨損。內(nèi)部原因是組成材料中的鹽，特別是氯化物和硫酸鹽引起的組成材料中的相互作用，例如堿－集料反應(yīng)、體積變化、吸水性和滲透性。

為了生產(chǎn)耐久性好的混凝土，應(yīng)當認真地選擇合適的組成材料。混合料中含有比例適當?shù)?，適量的材料以生產(chǎn)均質(zhì)和密實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.

風(fēng)化

混凝土的風(fēng)化損壞通常是由混凝土內(nèi)的自由水凍融交替的破壞作用和混凝土在約束條件下膨脹和收縮引起的，這些都是由溫度的變化和干濕交替造成的?；炷恋膬鋈谄茐氖怯?/span>孔隙水結(jié)冰時的膨脹列起的，在受約束的狀態(tài)下，如果凍融反復(fù)足夠多的次數(shù)，水壓力作用的結(jié)果就能使混凝土破裂。路緣和路面板、壩及水庫對冰凍作用非常敏感?；炷恋挚箖鋈诘男阅芸梢酝ㄟ^提高其抗滲性而得到改善。這可以通過采用盡可能低的水灰比來達到，但是必須具有能澆筑和搗實成為均勻體所需的和易性。耐久性可以通過使用引氣劑得到進一步的改善，占混凝土體積3%～6%的空氣含量，可以滿足大多數(shù)應(yīng)用的要求。加氣混凝土特別適用于用鹽做出冰雪劑的道路。

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.

化學(xué)侵蝕

一般來說，混凝土抵抗化學(xué)侵蝕的性能較低。有數(shù)種可以同混凝土反應(yīng)的化學(xué)物品，但是最常見的侵蝕有溶析、碳化作用和氯化物 、硫酸鹽侵蝕?；瘜W(xué)物品主要和硬化水泥漿體的某種成分反應(yīng)，因此，混凝土的抗化學(xué)侵蝕性主要取決于所用水泥的品種?？够瘜W(xué)侵蝕性能隨抗?jié)B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.

磨損

混凝土磨損的主要原因是快速運動的水的空蝕作用、水中的磨料、風(fēng)帶起的砂粒以及交通帶來的磨耗和沖擊。水流在某種條件下能造成流水和混凝土表面間空穴的形成，這些空穴往往被帶有極高能量的水汽所填充，這些水汽和混凝土表面反復(fù)接觸，結(jié)果便形成小坑和孔洞，稱為空蝕。因為即使是優(yōu)質(zhì)的混凝土也不能抵抗這種損壞，因此，最好的補救方法是通過產(chǎn)生平滑的水流來消除空蝕。在必要的地方，關(guān)鍵的區(qū)域可以使用抗空蝕性能好的材料來加固表面。

一般來說，混凝土抵抗空蝕和磨損的飽力隨其強度的增加而增加。采用堅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.

堿－集料反應(yīng)

某些天然集料會與硅酸鹽水泥中的堿起化學(xué)反應(yīng)。當出現(xiàn)這種現(xiàn)象時，集料便膨脹或鼓起，造成混凝土的開裂和破碎。

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.

體積變化

造成體積變化的主要因素是水和水泥的化學(xué)結(jié)合，以及隨后混凝土的干燥、溫度變化和干濕交替。當體積變化受到內(nèi)力或外力抵抗時，就可能產(chǎn)生裂縫，所受到的約束越大，產(chǎn)生的裂縫就越嚴重?；炷林辛芽p的出現(xiàn)，降低其抵抗溶析作用、鋼筋銹蝕、硫酸鹽及其他化學(xué)物品的侵蝕、堿－集料反應(yīng)和凍融作用的能力，所有這些都可以導(dǎo)致混凝土的破裂。嚴重的裂縫，特別是當伴隨有交替的膨胩和收縮時，可以導(dǎo)致混凝土表面的完全碎裂。

可以采用適當?shù)慕M成材料和考慮其結(jié)構(gòu)尺寸的混合料配比，將體積變化減至最小。養(yǎng)護時，適當?shù)臐穸纫材軠p少體積的變化。

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.

滲透性和吸水性

滲透性是指水穿透過混凝土的容易程度。它不應(yīng)該與混凝土的吸水性相混淆，二者沒有必然的聯(lián)系。吸水性可以定義為混凝土把水吸入其孔隙中的能力。低滲透性是水工建筑物的一個重要的要求，而且在某些情況下，可以認為混凝土的水密性比其強度更為重要，雖然在其他條件相同的情況下，低滲透性的混凝土也是高強度和耐久的。容易吸水的混凝土易于損壞。

混凝土的本性是一種多孔材料。這是因為在混合料中使用了比水化的需要量要多的水來使其具有足夠的和易性，以及在搗實期間難以從混凝土中排除所有的空氣。如果孔隙相互連找，混凝土就會變成透水的，但是對于大多數(shù)結(jié)構(gòu)來說，正常養(yǎng)護的混凝土足以滿足其抗?jié)B性要求。通過適當選擇組成材料和配合比，并且進行認真的澆筑、搗實和養(yǎng)護，就可以得到滲水性低的混凝土。通常，對于充分搗實的混凝土，其滲透性隨水灰比的降低而降低。水泥的細度和化學(xué)成分對滲透性都有影響。當要求混凝土具有低滲透性時，應(yīng)該使用低孔隙率的集料。澆筑時，組成材料的離析能夠?qū)炷恋目節(jié)B性產(chǎn)生有害的影響。