Section C混凝土耐久性

1、 Section C混凝土耐久性 現(xiàn)今，在各種嚴酷環(huán)境條件下和伴隨許多混凝土技術發(fā)展新問題的出現(xiàn)，越來越多的由于缺乏耐久性而不是強度不足的工程破壞實例引起了人們對混凝土耐久性的關注。時久性設計代替強度設計的觀點越來越廣泛地被人們所接受?；炷恋哪途眯员欢x為抵抗外部和內部原因導致的惡化的能力。外部原因包括混凝土遭受的環(huán)境的影響和工作條件的影響，例如氣候、化學作用和磨損。內部原因是鹽的影響，特別是氯化物和硫酸鹽，以及組成材料之間的相互作用，如堿骨料反應、體積變化、吸水性和滲透性。為了生產(chǎn)高耐久性的混凝土，人們應該注意選擇合適的材料組分。為生產(chǎn)均勻致密的混凝土，使各組分材料配比得當，這一點也是很重

2、要的。凍融 由氣候帶來的混凝土的劣化通常是由混凝土內部自由水的凍融交替、溫度變異和干濕交替引起的抑制作用下混凝土的膨脹、收縮等引起的。在凍期，由凍融循環(huán)導致的混凝土破壞是由孔隙水的膨脹引起的，在一個特定條件的限制下，如果凍融循環(huán)重復足夠的次數(shù)，會導致水壓力的發(fā)展有可能破壞混凝土。路肩利石板路，水壩和水庫是非常容易遭受凍脹作用的結構。 混凝土時凍融的能力可以通過提高混凝土的抗?jié)B透性來改善。這可以通過采用盡可能低的水灰比來達到，但是必須具有經(jīng)澆筑和搗實成內均勻體所需的和易性?；炷恋臅r久性可以通過采用引氣劑進一步改善，通常3%-6%混凝土體積的含氣量，可以滿足大多數(shù)應用的要求。引氣混凝土通常被用在

3、撒除冰鹽的路而上?；瘜W侵蝕 一般情況下，混凝土具有較低的耐化啦侵蝕能力。很多化學制劑會與混凝土發(fā)生反應，但最常見的侵蝕有溶析、碳化作用和氯化物、硫酸鹽侵蝕（如圖3.1）?；瘜W物品主要與硬化水泥漿體的某種成分發(fā)生反應，因此，混凝土的耐化學侵蝕能力通常受水泥標號的影響。混凝土的耐化學侵蝕能力隨著抗?jié)B性的增強而明顯改善。磨損 混凝土磨損的土要原因是由快速流動的水、水里的磨損材料、風沙襲擊和磨耗以及交通的影響。一定條件下的水壓流量會導致流動的水和混凝土表而之問空隙的形成。這些空隙通常被有超高能量的水蒸氣充滿，由于與混凝土表而頻繁接觸，就導致了坑和洞的形成，稱為氣蝕。即使質量很好的混凝土也無法抗拒這種劣

4、化，因此，最好的補救辦法是通過制造光滑的液壓流來消除氣穴。如有必要，關鍵部位可以用有更好的耐氣蝕材料作為內襯。 一般情況下，混凝土的耐侵蝕性和耐磨損性隨著強度的增長而增長。使用堅硬和堅韌韌的骨料有助于提高混凝土的耐磨損性。 目前在普通硅酸鹽水泥中，某些天然的骨料與堿發(fā)生化學反應。當這種反應發(fā)生的時候，這些骨料擴大或膨脹，導致混凝土的開裂利破碎。體積的改變 對于體積的改變土要的影響因素是水和水泥的化學結合、隨之而來的混凝土十燥以及溫度的變異和干濕交替。當粉煤灰等礦物摻合料摻入混凝土時，其活性成分與水泥水化析出的氧氧化鈣發(fā)生火山灰反應，生成體積脹大的硅酸鈣水化物。 當混凝土體積的改變被內部或外部力

5、抑制時，這將會產(chǎn)生裂縫，越是抑制，裂縫就越嚴重?；炷林辛芽p的存在降低了它對溶析、鋼筋銹蝕、硫酸鹽和其他化學物質侵蝕、堿骨料反應利凍融損傷的抵抗力，從而導致混凝土破壞。特別是當伴有交替的膨脹和收縮的時候，嚴重的裂縫能導致混凝土徹底破碎。 體積的改變可以通過采用合適的材料組分以及與結構尺寸有關的配合比來減小。適當?shù)臐穸纫材軠p小體積變化。吸水性和滲透性 滲透性指水透過混凝土的難易程度，不能與混凝生的吸水性相混淆，一者沒有必然的聯(lián)系。吸水性可被定義為混凝土汲取水分進入孔洞的能力。對于水工結構，低滲透性是一個很重要的要求，在某些情況下，可以認為混凝土的不透水性比強度更為重要。其他條件相同的情況下，混凝

6、土的低滲透性將會使其更時久。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，這是因為在混合料中使用了比水化的需要量更多的水來使其具有足夠的和易性，以及在搗實過程中空氣難以完全從混凝土中排出。如果孔隙相互連通，混凝土就會變成透水的，但是正常養(yǎng)護一的混凝土足以滿足大多數(shù)結構的不透水性要求。低滲透性的混凝土可以通過選擇合適的材料組分利比例并輔以精心澆筑、搗實利養(yǎng)護來實現(xiàn)。通常，對于允分搗實的混凝土，混凝土的滲透性隨著水灰比的減小而降低。滲透性還受水泥的 細度利化學成分的影響，低滲透混凝土應優(yōu)先使用低孔隙率的骨料。在澆筑過程年，組成材料的離析會嚴重影響混凝土的抗?jié)B性能。 混凝土耐久性的問題不

7、僅影響建筑材料的許多性能，而且還影響國家環(huán)境的可持續(xù)發(fā)展目標?；炷聊途眯栽谠S多領域影響頗為廣泛。將性能、結構、過程利環(huán)境等作為一個整體 的系統(tǒng)論方法來評估混凝土的耐久性被證明是有效的。 Durability of Concrete At present, more and more destructive case of engineering due to lack of durability not insufficient strength under all kinds of serious conditions and many new questions companying w

8、iththe 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 concrete can be defmed as its resistance to deterioration resulting from

9、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, paticularly chlorides and sulphates in the constituent materials,

10、 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 importnt that the mix contains adequate quantities of materi

11、als in proportions suitable for producing a homogeneous and fully compacted concrete mass.Freeze-thawing 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

12、concrete, under restraint, resulting from variaHons in temperatureand alternate wetringand drying Damage to concrete from freezing and thawing arise from the expansion of pore water during freezing, in a condition of restraint, if repeated a sufficient number of times, this results in the developmen

13、t 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 wat

14、er 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 being adequate for most applications. The use of air-entrained concr

15、ete is partcularly useful for roads where salts are used for deicing 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, chloride

16、s 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 chemical attack improves with increased impermeability.Wear The ma

17、in 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 flowing water and the concrete surface These cavities are usual

18、ly 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 able to resist this kind of deterioration, the best remedy is the

19、refore 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 abrasion increases with increase in strength. The use of a ha

20、rd and tough aggregate tends to improve concrete resistance to wear. Certain natural aggregates react chemically with the alkalis presentin Portland cement When this happens these aggregates expand or swell in resulting in cracking and disintegration of concrete.Volume change Principal factors respo

21、nsible 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, activation composition of mineral adnuxture and calcium hydroxide

22、 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, the more severe the cracking The presence of cracks in co

23、ncrete 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 can lead to complete disintegration of the concrete surface

24、 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 essential to minimize the effects of any volume changesPermeabi

25、lity and Absorption Permeability 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 of concrete to draw waterinto its voids Low permeability

26、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 and durable A concrete which readily absorbswater is susc

27、eptible 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 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 obtained by