6concrete

6.1.1

The characteristic strength is defined as the strength of material below which not more than 5 percent of the test results are expected to fall.

6.1.2

The minimum grade of concrete for plain and reinforced concrete shall be as per Table 5.

6.1.3

Concrete of grades lower than those given in Table 5 may be used for plain concrete constructions, lean concrete, simple foundations, foundation for masonry walls and other simple or temporary reinforced concrete construction.

6.2.1 Increase of Strength with Age

There is normally a gain of strength beyond 28 days. The quantum of increase depends upon the grade and type of cement, curing and environmental conditions, etc. The design should be based on 28 days characteristic strength of concrete unless there is a evidence to

Table 2 Grades of Concrete

(Clause 6.1, 9.2.2, 15.1.1 and 36.1)

6.2.1.1

For concrete of grade M 30 and above, the rate of increase of compressive strength with age shall be based on actual investigations.

6.2.1.2

Where members are subjected to lower direct load during construction, they should be checked for stresses resulting from combination of direct load and bending during construction.

6.2.2 Tensile Strength of Concrete

The flexural and splitting tensile strengths shall be obtained as described in IS 516 and IS 5816 respectively. When the designer wishes to use an estimate of the tensile strength from the compressive strength, the following formula may be used:

Flexural strength, $f_{\mathrm{cr}}=0.7 \sqrt{f_{\mathrm{ck}}} \mathrm{N} / \mathrm{mm}^{2}$

where $f_{\mathrm{ck}}$ is the characteristic cube compressive strength of concrete in $\mathrm{N} / \mathrm{mm}^{2}$.

6.2.3 Elastic Deformation

The modulus of elasticity is primarily influenced by the elastic properties of the aggregate and to a lesser extent by the conditions of curing and age of the concrete, the mix proportions and the type of cement. The modulus of elasticity is normally related to the compressive strength of concrete.

6.2.3.1

The modulus of elasticity of concrete can be assumed as follows:

$E_{\mathrm{c}}=5000 \sqrt{f_{\mathrm{ck}}}$

where

$E_{\mathrm{c}}$ is the short term static modulus of elasticity in $\mathrm{N} / \mathrm{mm}^{2}$.

Actual measured values may differ by ± 20 percent from the values obtained from the above expression.

6.2.4 Shrinkage

The total shrinkage of concrete depends upon the constituents of concrete, size of the member and environmental conditions. For a given humidity and temperature, 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.

6.2.4.1

In the absence of test data, the approximate value of the total shrinkage strain for design may be taken as 0.000 3 (for more information, see IS 1343).

6.2.5 Creep of Concrete

Creep of concrete depends, in addition to the factors listed in 6.2.4, on the stress in the concrete, age at loading and the 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.

6.2.5.1

In the absence of experimental data and detailed information on the effect of the variables, the ultimate creep strain may be estimated from the following values of creep coefficient (that is, ultimate creep strain/ elastic strain at the age of loading); for long span structure, it is advisable to determine actual creep strain, likely to take place:

NOTE-The ultimate creep strain, estimated as described above does not include the elastic strain.

6.2.6 Thermal Expansion

The coefficient of thermal expansion depends on nature of cement, the aggregate, the cement content, the relative humidity and the size of sections. The value of coefficient of thermal expansion for concrete with different aggregates may be taken as below: