毕业论文中英文翻译(Thepropertiesofconcrete).doc

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 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 destroyed 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, frictional 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 concrete 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 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 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 be 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 prismatic 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 also 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 specify 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. It 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 recommended 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 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 strength 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 deflection 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 unrestrained 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 greatly 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 water 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 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 loading 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 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 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 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 effectively 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 coefficients. 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，试件则变化为15cm和10cm。弯曲极限强度的模量用cr表示。常见的抗弯公式为下式，一些国家已经制定的间接抗拉测试，如圆柱体和立方体抗裂测试，可在其中选一个作为抗弯测试。这些测试所得的抗拉强度一般低于抗裂系数值。这让我们注意到抗压强度的增大并不会使抗拉强度按一定比例增大。混凝土的收缩假定在新的规范中具有较大的价值是因为需要计算极限状态挠度偏差。混凝土因为水分蒸发体积会缩小。而水分蒸发不一致，导致收缩变形不均匀而产生应力。这些应力可能很大，这也是为什么坚持潮湿固化的原因之一。素混凝土在自然条件下进行均匀收缩，则不会产生应力。而钢筋混凝土进行均匀收缩，则会产生应力应变压缩，钢筋作用的拉力不变。该结果表明混凝土的收缩取决于混凝土的配合比与环境。风化条件将大大增加收缩率潮湿的环境则减少收缩。混凝土的总收缩量取决于其混凝土的成分、配合比和环境条件。对于一个给定的环境，对混凝土的总收缩量造成最大影响的是水在总的混合成分中占据的比例，而在很小程度上取决于水泥含量。工程师必须认识到极限状态的设计需要大量的数据进行计算，其适用性要求和大致价值建议在编码中也应该作为其设计指导同时使用。对于非常重要的建筑结构，其收缩效应更要小心计算。混凝土的徐变假定在新的规范里也是非常重要的，这是因为徐变是混凝土在某一不变荷载的长期作用下, 其应变随时间而增长的现象。初始加荷时，混凝土的龄期越早，徐变越大，甚至是恒定负荷。其应力与徐变关系如图3所示。这个随时间增加而变形增大的过程被称为徐变或者黏性流动，一般情况下，其变形可能会超过弹性变形。混凝土的徐变取决于混凝土的成分，徐变时间，周围环境，应力条件，加荷龄期。只要混凝土的应力不超过其抗压强度的三分之一，可假定徐变的压力成正比。初始加荷时，混凝土的龄期越早，混凝土的水泥用量越多，水灰比越大，环境温度高，湿度小等条件下，徐变越明显。而相反的环境温度低，湿度大，徐变就越小。而一般来说，混凝土徐变的减小随着其龄期的增加。当初始加荷时，混凝土的龄期越早，徐变随时间以指数增大。徐变是挠度随时间变化增加的主要来源之一。在钢筋混凝土结构中，徐变可消除钢筋混凝土的应力集中，使应力较均匀的重新分布。徐变也可使受弯构件的挠度增大2到3倍，因此建议应该在徐变系数的基础上进行挠度的分析与计算。