L4 Volumn changes of Concrete.ppt

1、Lesson 4 Volume Changes of Concrete,Background Knowledge,所涉及到的课程：建筑材料、钢筋混凝土基本原理。 混凝土的特性：混凝土是重要的人工混合材料，基本力学性能包括混凝土的抗压、抗拉、抗剪强度与混凝土的变形，影响混凝土力学性能的主要因素有重复加载、偏心受压、龄期、收缩与徐变等。 本课文涉及到混凝土的收缩、膨胀 、混凝土的徐变等。,The content of the text,The text can be divided into 3 parts.,New Words and Expression of Paragraph 1,shri

2、nk vi. 收缩 shrinkage expand vt. 扩张, 使膨胀 expansion expansion joint 伸缩缝 moisture content 含水量,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 attribu

3、ted to changes in moisture content, chemical reaction of the cement with water, variation in temperature, and applied loads.,Paragraph 1 (Introduction of concrete volume changes),volume n. 体积，卷，册，音量，大量，量 applied loads 外加荷载,New Words and Expression of Paragraph 2,the capillary tension 毛细管张力, 微张力 ceme

4、nt paste 水泥浆,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, probabl

5、y due to the capillary tension that develops in the water remaining in concrete.,Part 2 Shrinkage,变干燥的混凝土体积变化不等于移除的水的体积。,Paragraph 2,restrained adj. 克制的，受限制的 restrained deformation，restrained support,Emptying of the capillaries causes a loss of water without shrinkage. But once the absorbed water is

6、 removed, shrinkage occurs.,Paragraph 2,capillary kpilri, kpi- n. 毛细血管；微血管 absorbed water 吸附水；吸收水,New Words and Expression of Paragraph 39,Paragraph 4 concrete mix 混凝土混合物 concrete mixer truck Paragraph 5 fineness of cement 水泥细度 Paragraph 6 gradation of aggregate 骨料级配 Paragraph 8 admixture n. 混合；添加物；

7、掺和剂 Paragraph 9 reinforcement n. 加固；加强 reinforcement bar 钢筋条 reinforcement cage 钢筋笼,Many factors influence the shrinkage of concrete caused by the variations in moisture conditions. 1.Cement and water content. The more cement or water content in the concrete mix, the greater the shrinkage. 2.Composi

8、tion and fineness of cement. High-early-strength and low-heat cements show more shrinkage than normal Portland cement. The finer the cement, the greater is the expansion under moist conditions.,Paragraph 3-9 (factors influence the shrinkage of concrete ),3.Type, amount, and gradation of aggregate. T

9、he smaller the size of aggregate particles, the greater is the shrinkage. The greater the aggregate content, the smaller is the shrinkage. 4.Ambient conditions, moisture, and temperature. Concrete specimens subjected to moist conditions undergo（经历） an expansion of 200 to 30010-6 , but if they are le

10、ft to dry in air, they shrink. High temperature speeds （verb） the evaporation of water and, consequently, increases shrinkage.,Paragraph 3-9 (factors influence the shrinkage of concrete ),5.Admixtures. Admixtures that increase the water requirement of concrete increase the shrinkage value. 6.Size an

11、d 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.,Paragraph 3-9 (factors influence the shrinkage of concrete

12、),这些应力与由荷载作用产生的那些应力相加。,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.,Paragraph 3-9 (factors influence the shrinkage

13、of concrete ),differential adj. 微分的；差别的；特异的,The values of final shrinkage for ordinary concrete vary between 200 and 70010-6. For normal-weight concrete, a value of 30010-6may be used. The British Code CPll0 gives a value of 50010-6, Which represents an unrestrained shrinkage of 1.5mm in 3m length i

14、n thin, plain concrete sections.,Paragraph 10 (The values of final shrinkage for ordinary concrete ),section n. 章节, 地区, 截面,If the member is restrained, a tensile stress of about 10N/mm2 (1400psi) arises. If concrete is kept moist for a certain period after setting, shrinkage is reduced; therefore, i

15、t is important to cure the concrete for a period of no fewer than 7 days.,Paragraph 10,set vt. 树立, 点燃, 使凝结 vi. 凝固, 定型,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.

16、 Symmetrical reinforcement in the concrete section may prevent curvature and deflection due to shrinkage.,Paragraph 11 (Shrinkage can increase deflection of structural member),upwind adj. 逆风的 deflection n. 偏向；偏差；挠曲 curvature k:vt n. 弯曲，曲率,Generally, concrete shrinks at a high rate during the initial

17、 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 70% to 85% occurs in 1 year.,Paragraph 12 (shrinkage rate during different period of hardening),the coefficient of therm

18、al expansion 热膨胀系数 unreinforced adj. 无钢筋的 unreinforced concrete 无筋混凝土,New Words and Expression of Paragraph 1314,Concrete expands with increasing temperature and contracts with decreasing temperature. The coefficient of thermal expansion of concrete varies between 4 and 710-6per degree Fahrenheit. A

19、n average value of 5.5 10-6per degree Fahrenheit (12 10-6per degree Celsius) can be used for ordinary concrete.,Part 2 Expansion due to Rise in Temperature,Paragraph 13 (The coefficient of thermal expansion of concrete ),contract vi. 感染, 订约, 收缩 Fahrenheit frhait n. 华氏度，华氏温度计 Celsius n. 摄氏度,The B. S.

20、 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 7N/mm2 (1000 psi) may develop.,Paragraph 13 (The coefficient of thermal expa

21、nsion of concrete ),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

22、is not much affected by temperature.,Paragraph 14 (expansion joints),在混凝土大跨结构中，100英尺到200英尺（30米到60米的长度）必须设置伸缩缝。,creep n. 徐变 elastoplastic ilst,plsitik adj. 弹塑性的 sustained load 长期荷载, 永久荷载 the coefficient of thermal expansion 热膨胀系数 unreinforced adj. 无钢筋的 unreinforced concrete 无筋混凝土,New Words and Expres

23、sion of Paragraph 1516,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.,Part 3 Creep,Paragraph 15 (The conception of cre

24、ep),混凝土是一种弹塑性材料，随着很小的应力，除弹性应变外，塑性应变也相应发展。,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.,Paragraph 15 (The conception of creep),这种变形在荷载施加后的头4个月以很快的速度增长。,Fig. 4.1 shows a concrete

25、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.,Paragraph 16 (The procedure of creep),cylinder n. 圆柱状, 柱面 in

26、stantaneous ,instnteinjs adj. 即时的, 瞬间的,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 Fig. 4. 1. In this case 3 = (1- a) 2, where a is the ratio of the recovered creep strain to t

27、he total creep strain.,Paragraph 16 (The procedure of creep),此处a是恢复的徐变应变与总徐变应变的比率 。,Creep deformation,Elastic deformation,Shrinkage deformation,Elastic recovery,Creep recovery,Non recoverable deformation,Age in months,Deformation units,Fig.3.3 creep and elastic deformation,The ratio a ranges between

28、 0.1 and 0.2. The 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.,Paragraph 16 (The

29、procedure of creep),如果施加荷载的时间增加，徐变恢复速率会较小，可能的原因是在变形条件下混凝土的硬化 。,P34页图,lubrication,Vertical cracks,Frictional forces,Diagonal cracks,Compression platen,Fig.3.1(a),cube,cylinder,prism,The ultimate magnitude of creep varies between 0.2 X10-6 and 2 X10-6 per unit stress (lb/in. 2) per unit length. A valu

30、e of 1Xl0-6 can be used in practice. The ratio of creep strain to elastic strain may be as high as 4.,Paragraph 17 (The ultimate magnitude of creep ),Creep takes place in the hardened cement matrix around the strong aggregate. It may be attributed to slippage along planes within the crystal lattice,

31、 internal stresses caused by changes in the crystal lattice, and gradual loss of water from the cement gel in the concrete.,Paragraph 18 (the main reasons of creep ),cement matrix meitriks 水泥基，水泥浆 ？ crystal lattice 晶体点阵，晶格 cement gel 水泥凝胶体,这可以归因为晶格内的延面滑动，晶格变化引起的内应力，以及混凝土中水泥凝胶水分的逐渐丧失。,The different f

32、actors that affect the creep of concrete can be summarized as follows. 1. The level of stress. Creep increases with an increase of stress in specimens made from concrete of the same strength and with the same duration of load. 2. Duration of loading. Creep increases with the loading period. About 80

33、% of the creep occurs within the first 4 months; 90% occurs after about 2 years.,Paragraph 19-30 (factors affecting the creep of concrete ),3. Strength and age of concrete. Creep tends to be smaller if concrete is loaded at a late age. Also, creep of 2000 psi-(14N/mm2-) strength concrete is about 1.

34、41X10-6 whereas that of 4000 psi-(28N/mm2-) strength concrete is about 0.8X10-6 per unit stress and length of time. 4. Ambient conditions. Creep is reduced with an increase in the humidity of the ambient air. 5. Rate of loading. Creep increases with an increase in the rate of loading when followed b

35、y prolonged loading (长期荷载).,Paragraph 19-30 (factors affecting the creep of concrete ),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

36、 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.,Paragraph 19-30 (factors affecting the creep of concrete ),9. Water cement ratio. Cre

37、ep increases with an increase in the water-cement ratio. 10. Type and grading of aggregate. Well-graded aggregate will produce dense concrete and consequently a reduction in creep. 11. Type of curing. High temperature steam curing of concrete as well as the proper use of a plasticizer will reduce th

38、e amount of creep.,Paragraph 19-30 (factors affecting the creep of concrete ),curing 养护 curing condition 养护条件 plasticizer n. 塑化剂；可塑剂,Creep develops not only in compression, but also in tension, bending, and torsion. The ratio of the rate of creep in tension to that in compression will be greater tha

39、n 1 in the first 2 weeks, but this ratio decreases over longer periods.,Paragraph 31-32 (creep of concrete in tension, bending and torsion ),在受拉条件下徐变速度与受压条件下徐变速度的比率将。,Creep in concrete under compression has been tested by many investigators. Troxell, Davis, and Raphael measured creep strains periodi

40、cally for up to 20 years and estimated that of the total creep after 20 years, 18% to 35% occurred in 2 weeks, 30% to 70% occurred in 3 months, and 64% to 83% occurred in 1 year.,Paragraph 33 (Creep in concrete under compression),For normal concrete loaded after 28 days, , where Cr=creep strain per unit stress per unit length. Creep augments the deflection of reinforced concrete beams appreciably wi