毕业论文中英文翻译Thepropertiesofconcrete

1、the properties of concretethe strength properties of concrete are of great importance. concrete is strong in compression and relatively very weak in tension. the compressive strength of concrete has been taken to be the basic property of concrete and is usually obtained from cube, cylinder and prism

2、 tests. let us consider the test of a concrete cube. in compression the initial size of the cube decreases in the longitudinal direction and increases in the transverse direction. if the top and bottom surfaces, on which the uniaxial compressive forces are applied, are lubricated the cube is destroy

3、ed due to cracks developed in the direction of compressive forces (see fig. 1(a). this shows that the strength of concrete in compression in the longitudinal direction depends on the strength of concrete for tension in the transverse direction. if the end faces of the cube are not lubricated, fricti

4、onal resistance is developed between the compression platens and the faces of the cube that is pressed. this friction prevents the free deformation of the concrete in the lateral direction and the cube is destroyed by the formation of inclined cracks, as shown in fig. 1(b).the strength of the concre

5、te cube without lubrication is about twice as much as the strength of the same cube with lubrication. when we define the strength of concrete in compression, it is generally understood that it is the unlubricated strength. the strength of concrete also depends on the absolute size of the cube. when

6、the cube size is 10 cm x 10 cm x 10 cm, the strength of concrete is approximately 15% higher than the strength of the cube of 20 cm x 20 cm x 20 cm size. similarly, the strength of the 30 cm cube is found to be 10% less than that of the 20 cm cube. the new code specifies the compressive strength of

7、concrete as the strength of 15-cm cubes at 28 days, expressed in n/mm². based on this definition, concrete has been divided into nine grades which are designated as c10, c15, c20, c25, c30, c35, c40, c45 and c50, and it has also been stipulated that grades of concrete lower than c15 shall not b

8、e used in reinforced concrete.it is of interest to note that if we test concrete prisms, instead of cubes, the prism strength is lower than the strength of cubes of same transverse size. this is due to the fact that the influence of friction developed on the contact surfaces is reduced in the prisma

9、tic samples. with the increase in the height of the prism, the strength of concrete drops as shown in fig. 2.when we have a ratio of h/a 4, the prism strength of concrete becomes almost constant and approximately equals 0.7 to 0.8 times that of the corresponding cube.the strength of concrete al

10、so depends on the shape of the cross-section of the sample. for example, when there is the same length of specimen, the same areas of cross-sections and the same composition of concrete, the strength of a cylindrical sample is 10% less than that of the prismatic sample. the american practice is to s

11、pecify 15-cm diameter and 30 cm high cylinders as standard samples for compression tests, and the cylinder strength is approximately 0.80 times the 15-cm cube strength.the tensile strength of concrete assumes importance because of the cracking limit state requirements of the new codes of practice. i

12、t is not easy to conduct tensile rests on concrete specimens since the specimen breaks under very small loads and errors in testing may have significant influence on the test results. hence direct tension tests on concrete specimens have been avoided and indirect tension tests have been re

13、commended by various codes.the flexure test is more popular and is conducted on 70 cm long beams of 15 cm square crosssection. when the size of aggregates is less than 20 mm, 10 cm square and 50 cm long beams may be used. the rate of loading is 400kg/cm² and 180 kg/cm² per minute for 15 cm

14、 and 10 cm specimens respectively. the flexural strength is expressed as the modulus of rupture cr. the formula used is the familiar flexural formula,some countries have specified indirect tension tests, such as the cylinder and cube split tests as an alternative to flexure tests. the tensile streng

15、th obtained from these tests is generally lower than the value obtained as the modulus of rupture. it is to be noticed that increase in compressive strength does not proportionately increase the tensile strength.shrinkage of concrete has assumed greater importance in the new codes because of the def

16、lection limit state computations. as concrete loses moisture by evaporation, it shrinks. since moisture withdrawal is not uniform, differential shrinkage strains and stresses occur. these stresses can be quite large and this is one of the reasons for insisting on moist curing. in the case of unrestr

17、ained plain concrete under uniform shrinkage, no stresses are caused. in the case of reinforced concrete, even uniform shrinkage will cause stressescompression in steel and tension in concrete. the amount of shrinkage will depend on the exposure and the ingredients of concrete. exposure to wind grea

18、tly increases the shrinkage ratea humid atmosphere will reduce shrinkage.the total shrinkage of concrete depends on the constituents of the concrete, size of the member and environmental conditions. for a given environment, the total shrinkage of concrete is most influenced by the total amount of wa

19、ter present in the concrete at the time of mixing and to a lesser extent, by the cement content.engineers must realize that limit state design requires extensive data for the computation of serviceability requirements and the approximate values suggested in the code should be taken as guidelines for

20、 immediate use. for very important structures, shrinkage effects will have to be computed with greater care.creep of concrete is another phenomenon, which has assumed importance in the new codes of practice because of its influence on long-term deflections. the initial strain in concrete on first lo

21、ading is nearly elastic on the first loading cycle, but this strain increases with time, even under constant load, as shown in fig. 3.this increased deformation with time is called creep or plastic flow, and under ordinary conditions, it may exceed the elastic deformations.creep of concrete depends

22、on the constituents of concrete, size of the member, environmental conditions, stress levels, age at loading, and duration of loading. as long as the stress in concrete does not exceed one third of its characteristic compressive strength, creep may be assumed to be proportional to the stress.loading

23、 at an early age, using concrete with a high water-cement ratio, exposure of concrete to drying conditions, etc. influence the creep behaviors significantly. completely wet or dry concrete creeps very little, and in general, creep decreases with the age of concrete. creep is more rapid when the load

24、 is first applied and decreases somewhat exponentially with time. creep is one of the main sources of increase in deflection with time. in the case of reinforced concrete, the constant modulus of the steel causes strain readjustments with time. compression reinforcement reduces creep deflections eff

25、ectively by the transfer of stress from concrete to steel. in the case of reinforced concrete beams without compression reinforcement, the final deflections will be usually 2 to 3 times the initial deflection. it has been suggested that long-term deflections be calculated on the basis of creep coeff

26、icients. 混凝土的性能混凝土的强度特性是非常重要的。混凝土的抗压强度相对弱于抗拉强度。混凝土的基本强度值通常是由混凝土立方体抗压强度标准值表示。让我们来测试下混凝土立方体，在压缩试验中，混凝土立方体的尺寸横向增大，纵向减小。如果在其上下表面上，对其施加轴向压力，则使立方体产生裂纹并遭到破坏，其压缩导致的裂缝方向(见图1(a)。这说明，混凝土的纵向抗压强度取决于混凝土强度的横向拉力。如果立方体端面不是润滑摩擦，则摩擦阻力产生在压缩挡板和立方体之间的压力。这种摩擦防止混凝土在横向的自由变形和立方体形成斜裂缝被破坏，如图 1 (b) 中所示。无润滑条件下，混凝土立方体的强度是润滑条件下立方体

27、强度的两倍。所以我们在规定混凝土抗压强度时，一般理解他是在非润滑的强度下测定的。 混凝土强度还取决于立方体的绝对大小。当立方体的大小是10厘米×10厘米×10厘米，混凝土强度大约比20 厘米×20厘米×20厘米大小的立方体的强度高15%。同样，发现 30 厘米的立方体的强度比20厘米的立方体的强度少约10%。新规范里指定混凝土的抗压强度为15cm的立方体在第28天的强度，以n / mm²为单位。基于此定义，混凝土分为9个等级，我们说成c10, c15, c20, c25, c30, c35, c40, c45 和 c50，同时还规定在钢筋混凝土

28、中使用的混凝土强度等级不得低于c15。 值得注意的是，如果我们用混凝土的棱柱体代替立方体来测试，则在相同的横向尺寸下棱柱体的强度比立方体的强度弱，这是由于在实际中摩擦在棱柱样品中的接触表面比较少。则随着高度的增加，棱柱体强度下降如图2所示。当h/a 4时，棱柱体混凝土强度就几乎恒定且约等于0.7-0.8倍的立方体强度。混凝土强度也取决于样品的截面形状，例如，在相同长度，相同面积的横截面以及相同的材料组成的圆柱形混凝土试件，其强度小于棱柱体试件约10%左右。美国的做法是指定15cm直径和30cm高的圆柱体作为压缩实验的标准试件，其强度大约为0.8倍的15cm立方体混凝土试件强度。混凝土的抗拉强度的重要性在新的规定中承担了裂缝极限状态要求。这是基于混凝土试件进行荷载试验在破坏下很小的负载可能导致的不容易拉伸的结果的重大影响。因此各种规范里都建议避免进行直接拉伸试验和间接拉力测试。用70cm长15平方厘米的方形截面梁进行弯曲实验则更好。当骨料的大小小于20mm时可使用50cm长、10平方厘米的方形截面梁。当每分钟匀加载荷分别为400kg/cm2和180 kg/cm2时，试件则