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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/mm. 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
8、 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 sa
9、mples. 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 to0.8 times that of the corresponding cube.The strength of concrete also depends o
10、n 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
11、 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 eas
12、y 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 testresults. Hence direct tension tests on concretespecimens have been avoided and indirect tension tests have been recommended by various cod
13、es.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/cm and 180 kg/cm per minute for 15 cm and 10 cm specimens respectively. T
14、he 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 gene
15、rally 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
16、 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
