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
21、eaction and freezing and thawing,all of which may lead to disruption of concrete4.TextSevere cracking can 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
22、 materials and mix proportions having due regard to the size of structure.Adequate moist is also essential 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
23、 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 water tightness of concrete may be considered to be more significant
24、 than strength although,other conditions being equal,concrete of low permeability will also be strong and durable5.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 purp
25、ose of hydration in order to make the mix sufficiently workable and the difficulty of completely removing all the air from concrete during compaction.TextIf the voids are interconnected concrete becomes pervious although with normal care concrete is sufficiently impermeable for most purposes.Concret
26、e 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 th
27、e 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.The problems of concrete durability not only affec
28、t many properties of building materials,but also influence those goals of successive developments of a national environment.The concrete durability affects widely in many fields.It is pointed out that system theory method is favorable for researching the concrete durability and its evaluation with p
29、erformance,structure,process and environment as a whole.New Words and Phrasesdurable a.耐久的经久的，永久性的；n耐用物品durability n耐久性经久性，耐用年限chloride n氯化物，漂白粉sulphate n硫酸盐alkali n碱碱性，强碱absorption n吸收吸取，吸水性permeability n渗透性，透气性，渗透deterioration n变坏，变质，损坏，损伤weathering n风化(作用、层)，自然老化，大气侵蚀disruptive a.分裂的，摧毁的，破坏的thaw
30、v.使融化，解冻，熔化entrain n携带，传输使(空气)以气泡状存于混凝土中，产生leaching n浸出，浸析作用浸滤，溶析carbonation n碳化作用碳酸盐化blasting n破裂吹风气流加速运动，喷砂attrition n磨损，磨耗损耗hydraulic a.水力的水压的；水力cavity n空穴孔穴洞穴cavitation n气蚀空蚀，空化作用impermeability n不渗透性防水性气密性New Words and Phraseshydraulic structure n水工建筑物pervious a.透水的透光的有孔的能通过的aggregate a.集料骨料;n总计
31、合计homogeneous a.均质的均匀的单相的compact a.紧密的密实的；压实捣实kerb n路缘道牙建筑物上的边饰air-entrained concrete 引气混凝土Notes1.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 chan
32、ges,absorption permeability.内在原因是盐的影响，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，吸收性和渗透性。2.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 rest
33、raint,resulting from variations in temperature and alternate wetting and drying.由气候带来的混凝土劣化，通常是由混凝土中的自由水的冻融循环和在一定条件下由于温度变异和干湿交替变化所引起的混凝土的膨胀和收缩所造成的。3.Chemical agents essentially react with certain compounds of the hardened cement paste and the resistance of concrete to chemical attack therefore can b
34、e affected by the type of cement used.化学物质主要是与水泥石的某种成分发生反应，因此，混凝土的抗化学侵蚀能力受所使用的水泥的类型影响。Notes4.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 o
35、f which may lead to disruption of concrete.混凝土中的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的侵蚀、碱-骨料反应和冻融循环损坏的抵抗能力，所有这些都可能导致混凝土的破坏。5.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,oth
36、er conditions being equal,concrete of low permeability will also be strong and durable.对水工结构，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，混凝土的低渗透性将会使其更耐久。Exercises（Translate the following Chinese into English,or English into Chinese）1.Low permeability is an important requirement for hydraulic
37、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.2.The resistance of concrete to freezing and thawing can be improved by increasing its imp
38、ermeability.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.3.Certain aggregates can react chemically with the alkalin
39、e content of cement to form alkaline silica gel.When this happens these aggregates expand or swell resulting in cracking and disintegration of concrete.4.In order to produce a durable concrete care should be taken to select suitable constituent materials.It is also important that the mix contains ad
40、equate quantities of materials in proportions suitable for producing a homogeneous and fully compacted concrete mass.5.When a hardened concrete cured in water,is allowed to dry it first loses water from its voids and capillary pores and only starts to shrink during further drying when water is drawn
41、 out of its cement gel.Exercises（Translate the following Chinese into English,or English into Chinese）6.总之，混凝土的耐久性决定于材料的品质、内部密实度、施工质量三个方面。7.由于混凝土内部吸附水分蒸发，而引起凝胶体失水收缩，毛细管水分蒸发，混凝土系统内的颗粒受毛细管压力作用而产生体积收缩。8.化学收缩是指混凝土拌合物在水化过程中，由于水泥水化生成物体积比体积小，而产生的收缩。9.混凝土在长期荷载作用下，沿作用力方向的变形，且随时间的延长而增长，即荷载不变但变形仍随时间增长。10.混凝土抵抗环境介质作用并长期保持其良好的使用性能和外观完整性，从而维持混凝土结构的安全、正常使用的能力,叫混凝土耐久性。
