《专业英语》课件Unit_7.pptx
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1、Unit7Durability of ConcreteTextAt 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 with the development of concrete technology make people pay attention to the durability
2、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 defined as its resistance to deterioration resulting from external and internal causes.The external causes include the effects of environmental
3、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 cha
4、nges,absorption and permeability1.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.Tex
5、tWeatheringDeterioration 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 dr
6、ying2Damage 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
7、 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 homogeneo
8、us 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.TextFreeze-thawing Deterio
9、ration 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 con
10、crete 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 development of hydraulic pressure capable of disrupting concrete.Road kerbs and slabs,dams and reservoirs are very susceptib
11、le 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 homogenous mass.Durability c
12、an be further improved by using air entrainment,an air content of 3 to 6 percent 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.TextChemical attack In general,concrete has a l
13、ow 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 resist
14、ance of concrete to chemical attack,therefore can be affected by the type of cement used3.The resistance to chemical attack improves with increased impermeability.WearThe main causes of wear of concrete are cavitation effects of fast-moving water,abrasive material in water,wind blasting and attritio
15、n 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 vapor charged with extraordinarily high energy and repeated contact with the concrete surface results in th
16、e 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 materi
17、als having greater resistance to cavitation erosion.TextIn 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.Certain natural aggregates react chemically with the alkalis pr
18、esent in Portland cement.When this happens these aggregates expand or swell in resulting in cracking and disintegration 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 tempe
19、rature and alternate wetting and drying.In the case of fly ash or silica fume mixed in the concrete,activation composition of mineral admixture and calcium hydroxide precipitation of cement hydrate have pozzolanic reaction and generate calcium silicate,which expands in volume.When change in volume i
20、s 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 r
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