土木工程专业英语lesson 4 Volume Changes of Concrete.ppt

LESSON 4,Volume Changes of Concrete,Concrete undergoes volume changes during hardening.If it loses moisture by evaporation.it shrinks,but if the concrete hardens in water,it expands.The causes of the volume changes in concrete can be attributed to changes in moisture content,chemical reaction of the cement with water，variation in temperature，and applied load.,混凝土在硬化过程中产生了体积的变化。如果由于蒸发失去水分，它会缩小，但如果混凝土在水中变硬，它会膨胀。混凝土体积变化的原因可以归咎于水含量的变化，水泥与水的化学反应和所承受的荷载。,Undergo vt.经历，遭受，忍受Evaporation n.蒸发(作用）Shrinkage n.收缩,Shrinkage,The change in the volume of drying concrete is not equal to the volume of water removed.The evaporation of free water causes little or no shrinkage.As concrete continues to dry,water evaporates and the volume of the restrained cement paste changes,causing concrete to shrink,probably due to the capillary tension that develops in the water remaining in concrete.Emptying of the capillaries causes a loss of water without shrinkage.But once the absorbed water is removed,shrinkage occurs,参考译文,干燥中的混凝土体积的变化不等于失去水分的体积。自由水分的蒸发作用导致很少或没有收缩。随着混凝土继续干燥，导致混凝土收缩的水蒸发和有限的水泥浆体积的变化可能应归于在混凝土中的剩余的水中发展的毛细管张力。排空毛细管造成水丧失不会收缩。但是一旦丧失所吸收的水分，收缩产生了。,capillary a.毛细作用的moisture content 含水量，含湿度cement paste 水泥浆the capillary tension 毛细管张力，微张力,Shrinkage,1Many factors influence the shrinkage of concrete caused by the variations in moisture conditions 1Cement and water content.The more cement or water content in the concrete mix.the greater the shrinkage 2Composition and fineness of cement.High-early-strength and lowheat cements show more shrinkage than normal Portland cement.The finer the cement,the greater is the expansion under moist conditions,参考译文,1 许多因素影响混凝土中由水分条件变化引起的收缩。1、水泥和含水量。混凝土混合物中的水泥和含水量越多，收缩越大。2、水泥成分和细度。快硬水泥和低热水泥比普通硅酸盐水泥表现更多的收缩。水泥越细，在潮湿条件下的膨胀越大。,Shrinkage,3Type，amount，and gradation of aggregate.The smaller the size of aggregate particles,the greater is the shrinkage.The greater the aggregate content,the smaller is the shrinkage4Ambient conditions,moisture,and temperature.Concrete specimens subjected to moist conditions undergo an expansion of 200 to 30010-4,but if they are left to dry in air,they shrink.High temperature speeds the evaporation of water and,consequently,increases shrinkage,参考译文,3、骨料的品种,数量和级配。骨料粒子越小，收缩越大。骨料含量越大，收缩越小。4、环境条件、水分和温度。潮湿条件下混凝土试样会膨胀 200 到 300 10-4，但如果在干燥空气中就会缩小。高温加速了水蒸发，从而增加了收缩。,gradation of aggregate 骨料级配Ambient a周围的；n.周围环境Specimen n样品，样本，试样,Shrinkage,5Admixtures Admixtures that increase the water requirement of concrete increase.the shrinkage value6Size and shape of specimen.As shrinkage takes place in a reinforced concrete member,tension stresses develop in the concrete,and equal compressive stresses develop in the steel.These stresses are added to those developed by the loading action.Therefore,cracks may develop in concrete when a high percentage of steel is used.Proper distribution of reinforcement,by producing better distribution of tensile stresses in concrete，can reduce differential internal stresses,参考译文,5、添加剂。添加剂增加，需水量增加，收缩等值。6、试件的大小和形状。像钢筋混凝土构件产生收缩那样，张拉应力 在混凝土中产生，而等量的压应力作用在钢筋上。这些压力将与荷载作用产生的压力叠加。因此，当混凝土中使用高百分比钢筋时裂缝可能发育。通过在混凝土中产生更好的拉伸应力的分布，适当分布的钢筋，可以减少不同的内应力。,Shrinkage,The values of final shrinkage for ordinary concrete vary between 200 and 70010-6For normalweight concrete,a value of 30010-6 may be used.The British Code CPll0 gives a value of 50010-6，which represents an unrestrained shrinkage of 1.5mm in 3m length in thin,plain concrete sections.If the member is restrained,a tensile stress of about 10Nmm2(1400psi)arises.If concrete is kept moist for a certain period after setting,shrinkage is reduced;therefore,it is important to cure the concrete for a period of no fewer than 7 days,参考译文,普通混凝土的最终收缩值在200 到700 10-6之间变化。对常规混凝土，可使用300 10-6。英国混凝土规范 CP110 给出值 500 10-6，其表达的是3米长无钢混凝土有1.5 mm的自然收缩。如果构件受限，张拉应力上升约 10N/mm2(1400psi)。如果浇筑后混凝土保持湿润一段时间，收缩减少；因此，养护混凝土不少于 7 天是很重要的。,The British Code CP110 英国混凝土规范CP110,Shrinkage,Exposure of concrete to wind increases the shrinkage rate on the upwind side.Shrinkage causes an increase in the deflection of structural members,which in turn increases with time.Symmetrical reinforcement in the concrete section may prevent curvature and deflection due to shrinkage Generally,concrete shrinks at a high rate during the initial period of hardening,but at later stages the rate diminishes gradually.It can be said that 15 to 30 of the shrinkage value occurs in 2 weeks，40%to 80%occurs in 1 month.and 70to 85 occurs in l year,参考译文,暴露于风中的混凝土迎风一侧的收缩率增加。收缩导致的结构构件翘曲随时间增加而增加。混凝土构件的对称加固可以防止由收缩引起的弯曲和挠度。一般来说，混凝土在硬化初期将以高速率收缩，但后期逐渐减小。可以说收缩值的 15%至 30%发生在 2 星期内，40%至 80%发生在 1 个月内，70%至 85%发生在 l年内。,Expansion due to Rise in Temperature,Concrete expands with increasing temperature and contracts with decreasing temperature.The coefficient of thermal expansion of concrete varies between 4 and 710-6 per degree Fahrenheit.An average value of 5510-6 per degree Fahrenheit(1210-6 per degree Celsius)can be used for ordinary concrete,参考译文,混凝土随温度上升膨胀并温度下降而收缩。混凝土的热膨胀系数在4 至 7 10-6 每华氏度之间变化。普通混凝土可采用平均值 5.5 10-6 每华氏度（12 10-6 每摄氏度）。,The coefficient of thermal expansion of concrete 混凝土热膨胀系数,Expansion due to Rise in Temperature,The B.S.Code suggests a value of 10-5 per degree Celsius.This value represents a change of length of 10mm in a 30m member subjected to a change in temperature of 33 If the member is restrained and unreinforced.a stress of about 7Nmm2(1000 psi)may develop,参考译文,B.S.法规建议 10-5 每摄氏度。这一数值代表长度 30 米构件承受 33 温度变化有 10 毫米的变化。如果该构件受限且无钢筋，可能有 7N/mm2(1000 psi）的应力。,The BS Code 英国标准规范,Expansion due to Rise in Temperature,In long reinforced concrete structures,expansion joints must be provided at lengths of 100 to 200 ft(30 to 60m).The width of the expansion joint is about 1 in(25mm)Concrete is not a good conductor of heat,whereas steel is a good one.The ability of concrete to carry load is not much affected by temperature,参考译文,长的钢筋混凝土结构，必须在 100 到 200 英尺（30 到 60 米）的长度上设置伸缩缝。伸缩缝的宽度是约 1 英寸(25mm)。混凝土是不良导体热，而钢材是。混凝土的负载能力不太受温度影响。,the expansion joint 伸缩缝,Creep,Concrete is an elastoplastic material,and beginning with small stresses,plastic strains develop in addition to elastic ones.Under sustained load,plastic deformation continues to develop over a period that may last for years.Such deformation increases at a high rate during the first 4 months after application of the load.This slow plastic deformation under constant stress is called creep,参考译文,混凝土是一种弹塑性材料，从小的应力开始，除弹性应变外塑性应变亦发展。在持续荷载下,塑性变形持续发展可能会持续多年。这种变形在负载后的头 4 个月期间内以高速率增加。这种在恒定压力下缓慢的塑性变形称为蠕变。,Creep n.徐变,蠕变,Creep,Fig41 shows a concrete cylinder that is loaded.The instantaneous deformation is 1,which is equal to the stress divided by the modulus of elasticity.If the same stress is kept for a period of time,an additional strain 2，due to creep effect,can be recorded.If load is then released,the elastic strain,1,will be recovered,in addition to some creep strain.The final permanent plastic strain，3，will be left,as shown in Fig41In this case 3=(1一)2，where is the ratio of the recovered creep strain to the total creep strain,参考译文,Fig.4.1 显示加载的混凝土圆柱。瞬时变形是 1，其等于应力除以弹性模量。如果一段时间内保持相同的压力，可以记录到源自蠕变效应的额外应力2。如果负载被释放，除塑性应变外，弹性应变1将被复原。最终的永久塑性应变3会被留下，如Fig.4.1 所示。这里3=(1-)2，为复原的蠕变与总蠕变之比。,Creep,The ratio ranges between 0.1 and 0.2The magnitude of creep recovery varies with the previous creep and depends appreciably upon the period of the sustained load.Creep recovery rate will be less if the loading period is increased,probably due to the hardening of concrete while in a deformed condition,参考译文,比率在 0.1 和 0.2之间。蠕变的规模随以前的蠕变变化并明显取决于持续负载的期间。如果加载期间增加，蠕变恢复率将会减少，这可能缘于混凝土在变形条件下时硬化。,Creep,Creep,The ultimate magnitude of creep varies between 0.210-6 and 210-6 per unit stress(1 bin.2)per unit length.A value of 110-6 can be used in practice.The ratio of creep strain to elastic strain may be as high as 4Creep takes place in the hardened cement matrix around the strong aggregate.It may be attributed to slippage along planes within the crystal lattice,internal stresses caused by changes in the crystal lattice,and gradual loss of water from the cement gel in the concrete,参考译文,蠕变的最终量级在0.2 10-6 和 2 10-6 每单位应力（1 b/in.2）单位长度之间。在实践中，可以使用 1 10-6 的值。蠕变应变，弹性应变的比例可能高达 4。蠕变在强骨料周围已硬化的水泥基质中发生。它可能归因于沿晶格内位面的滑动，由晶格改变引起的内部应力，混凝土中水泥凝胶水分的逐步丧失。,crystal lattice 晶体点阵，晶格cement gel 水泥凝胶体,Creep,The different factors that affect the creep of concrete can be summarized as follows 1The level of stress 2Creep increases with an increase of stress in specimens made from concrete of the same strength and with the same duration of load 2Duration of loading.Creep increases with the loading period.About 80 of the creep occurs within the first 4 months；90 Occurs after about 2 years,参考译文,影响混凝土蠕变的不同因素可以归纳如下。1、应力水平。2 对相同强度混凝土制成的试件及相同的负载期限，蠕变随应力的增加而增加。2、加载的持续的时间。蠕变随加载期间增加。大约 80%的蠕变出现在头 4个月；90%发生在随后大约 2 年内。,Creep,3Strength and age of concrete.Creep tends to be smaller if concrete is loaded at a late age.Also,creep of 2000 psi-(14Nmm2)strength concrete is about 1.4110-6 whereas that of 4000 psi-(28Nmm2)strength concrete is about 0.810-6 per unit stress and length of time4Ambient conditions.Creep is reduced with an increase in the humidity of the ambient air5Rate of loading.Creep increases with an increase in the rate of loading when followed by prolonged loading,参考译文,3、强度和混凝土的年龄。如果混凝土晚期加载蠕变趋于更小。此外，2000 psi(14N/mm2)强度混凝土约为 1.41 10-6，而4000 psi(28N/mm2)强度混凝土约为0.8 10-6每单位应力和时间长度。4、环境条件。增加周围空气的湿度蠕变会减少。5、负荷率。蠕变随长时间加载率增加而增加。,Creep,6.Percentage and distribution of steel reinforcement in a reinforced concrete member.Creep tends to be smaller for higher proportion or better distribution of steel 7.Size of the concrete mass.Creep decreases with an increase in the size of the tested specimen 8.Type,fineness,and content of cement.The amount of cement greatly affects the final creep of concrete,as cement creeps about 15 times as much as concrete,参考译文,6、钢筋混凝土构件中钢筋的分布与百分比。钢筋比例越高或分布越好则蠕变趋于越小。7、混凝土质量的大小。蠕变随试件尺寸的增加而减小。8、品种，细度和水泥含量。水泥含量极大地影响混凝土最终的蠕变，因为水泥蠕变约为混凝土的 15倍。,Creep,9Watercement ratio.Creep increases with an increase in the water-cement ratio.10Type and grading of aggregate.Wellgraded aggregate will produce dense concrete and consequently a reduction in creep 11Type of curing.High-temperature steam curing of concrete as well as the proper use of a plasticizer will reduce the amount of creep.,参考译文,9、水灰比。蠕变随水灰比增加而增加。10、骨料类型与级配。良好级配的骨料将产生密实混凝土并因此减少蠕变。11、养护形式。混凝土高温蒸汽养护以及增塑剂的正确使用会减少蠕变量。,Creep,Creep develops not only in compression,but also in tension,bending,and torsionThe ratio of the rate of creep in tension to that in compression will be greater than l in the first 2 weeks,but this ratio decreases over longer periods.,参考译文,蠕变不仅在压缩情况下产生；也发生在张拉、弯曲、扭转情况下。张拉与压缩的蠕变率之比在前 2个星期将超过 1，不过这一比率会长期下降。,Creep,Creep in concrete under compression has been tested by many investigators.Troxell,Davis，and Raphael measured creep strains periodically for up to 20 years and estimated that of the total creep after 20 years，18 to 35occurred in 2 weeks，30to 70 occurred in 3 months,and 64 to 83%occurred in 1 year.,参考译文,许多研究者进行了压缩下混凝土蠕变的测试。Troxell、戴维斯和拉斐尔定期测量蠕变20 年并估计 20 年后的总蠕变，18%至 35%发生在 2 星期内，30%至 70%发生在 3 个月内。64%至 83%发生在 1 年内。,Creep,For normal concrete loaded after 28 days,Cr=0.13t，where Crcreep strain per unit stress per unit length Creep augments the deflection of reinforced concrete beams appreciably with time.In the design of reinforced concrete members，longterm deflection may be critical and has to be considered in proper design.Extensive deformation may influence the stability of the structure,参考译文,普通混凝土用加载后 28 天，Cr=0.13t，这里 Cr=蠕变每单位长度单位应力。蠕变随时间明显地增大了钢筋混凝土梁的变形。在钢筋混凝土构件的设计中，长期挠曲可能是危险的，必须在正确的设计中加以考虑。大量的变形可能会影响结构的稳定性。,Creep,Sustained loads affect the strength as well as the deformation of concrete.A reduction of up to 30 of the strength of unreinforced concrete may be expected when concrete is subjected to a concentric sustained load for 1 year,参考译文,持续的荷载影响混凝土强度及变形。当混凝土承受同心持续荷载 1 年，可预期无筋混凝土强度减少达 30%。,Creep,The fatigue strength of concrete is much smaller than its static strength.Repeated loading and unloading cycles in compression lead to a gradual accumulation of plasticdeformations If concrete in compression is subjected to about 2 million cycles,its fatigue 1imit is about 50%to 60%of the static compression strength.In beams,the fatigue limit of concrete is about 55 of its static strength,参考译文,混凝土的疲劳强度比其静力强度小得多。重复受压加载和卸载循环导致塑性变形逐渐积累。如果受压混凝土受到约 2 百万次循环，其疲劳极限约为静态压缩强度的 50%至 60%。梁的混凝土疲劳极限约为其静力强度的 55%。,