IS 456:2000 is the code for Plain and Reinforced Concrete. As per this code, the concrete is used for an RCC structure should have the grade of M20 or above.
The letter "M" here stands for mix and the number is the characteristic strength of standard cube of size 150 mm which when tested under compression load test after 28 days, not more than 5% of the cubes are expected to fail.
For example, if 100 standard cubes of M20 is tested after 28 days then at least 95 cubes should be able to resist more than 20 MPa before failure. If it is less than 95 then the test will be declared invalid and the concrete will be rejected.
But at a construction site, site engineers cannot wait for 28 days to test the compressive strength of the concrete used for construction. To know about the strength of the concrete at early stages they test the concrete cubes at 3 and 7 days. The 3-day strength of a concrete cube is nearly 40% of the 28-day strength and 7-day strength is nearly 65% of the 28-day strength.
This test is conducted on at least 3 cubes and the average of these 3 cubes is taken, provided that individual variation should not be more than $\pm $ 15 % average.
Characteristic Strength (${{f}_{ck}}$)
${{f}_{ck}}={{f}_{m}}-1.65\sigma $
Strength of concrete is found to decrease with increase in the size of the specimen. However, beyond 450 mm size, there is no decrease in the compressive strength of concrete.
Thus, compressive strength of concrete in structure is taken as $0.67{{f}_{ck}}$
Flexural Strength of Concrete (Modulus of Rupture)
Tensile strength of plain concrete is obtained by the splitting test.
Splitting tensile strength,
${{f}_{ct}}=\frac{2P}{\pi dL}=0.6{{f}_{cr}}$
Stress Strain Curve of Concrete
Maximum compressive stress occurs at a strain value of 0.002 i.e., 0.2%. The value of stress at 0.002 strain is called compressive strength of concrete.
Modulus of elasticity of concrete for all practical purpose is taken as secant modulus at a stress of around $0.33{{f}_{ck}}$.
Modulus of elasticity of concrete is primarily influenced by the elastic properties of aggregate and to a lesser exent by the condtions of curing, mix proportion and type of cememnt.
As per IS 456:2000 short term modulus of elasticity,
The letter "M" here stands for mix and the number is the characteristic strength of standard cube of size 150 mm which when tested under compression load test after 28 days, not more than 5% of the cubes are expected to fail.
For example, if 100 standard cubes of M20 is tested after 28 days then at least 95 cubes should be able to resist more than 20 MPa before failure. If it is less than 95 then the test will be declared invalid and the concrete will be rejected.
But at a construction site, site engineers cannot wait for 28 days to test the compressive strength of the concrete used for construction. To know about the strength of the concrete at early stages they test the concrete cubes at 3 and 7 days. The 3-day strength of a concrete cube is nearly 40% of the 28-day strength and 7-day strength is nearly 65% of the 28-day strength.
This test is conducted on at least 3 cubes and the average of these 3 cubes is taken, provided that individual variation should not be more than $\pm $ 15 % average.
Characteristic compressive strength compliance requirement (IS 456 Cl 16.1 and 16.3)
Specified Grade | Mean of the group of 4 Non-overlapping consecutive test results in $N/m{{m}^{2}}$ | Individual test results in $N/m{{m}^{2}}$ |
| M 15 | $\ge {{f}_{ck}}+0.825\times $ established standard deviation (rounded off to nearest $0.5 N/m{{m}^{2}}$) or ${{f}_{ck}}+3$ $N/m{{m}^{2}}$ , whichever is greater | ${{f}_{ck}}-3$ |
| M 20 or above | $\ge {{f}_{ck}}+0.825\times $ established standard deviation (rounded off to nearest $0.5 N/m{{m}^{2}}$) or ${{f}_{ck}}+4$ $N/m{{m}^{2}}$ , whichever is greater | ${{f}_{ck}}-4$ |
Characteristic Strength (${{f}_{ck}}$)
${{f}_{ck}}={{f}_{m}}-1.65\sigma $
Where, ${{f}_{m}}=$ Mean Strength
$ \sigma=$ Standard Deviation
$=\sqrt{\frac{\sum\limits_{{}}^{{}}{(f-{{f}_{m}})}}{m}}$ when test samples are $\ge$ 30
$=\sqrt{\frac{\sum\limits_{{}}^{{}}{(f-{{f}_{m}})}}{m-1}}$ when test samples are $<$ 30
$m=$ number of samples
Compressive Strength of Concrete in Structures
Strength of concrete is found to decrease with increase in the size of the specimen. However, beyond 450 mm size, there is no decrease in the compressive strength of concrete.
Thus, compressive strength of concrete in structure is taken as $0.67{{f}_{ck}}$
Flexural Strength of Concrete (Modulus of Rupture)
${{f}_{cr}}=0.7\sqrt{{{f}_{ck}}}$
Tensile Strength of Concrete
Tensile strength of plain concrete is obtained by the splitting test. Splitting tensile strength,
${{f}_{ct}}=\frac{2P}{\pi dL}=0.6{{f}_{cr}}$
Source:Research Gate
Stress Strain Curve of Concrete
Source: IS 456
Maximum compressive stress occurs at a strain value of 0.002 i.e., 0.2%. The value of stress at 0.002 strain is called compressive strength of concrete.
Modulus of elasticity of concrete for all practical purpose is taken as secant modulus at a stress of around $0.33{{f}_{ck}}$.
Modulus of elasticity of concrete is primarily influenced by the elastic properties of aggregate and to a lesser exent by the condtions of curing, mix proportion and type of cememnt.
As per IS 456:2000 short term modulus of elasticity,
${{E}_{C}}=5000\sqrt{{{f}_{ck}}}$
Long-term Modulus of elasticity depends on Creep,
${{E}_{ce}}=\frac{{{E}_{C}}}{1+\theta }$
$\theta=$ Creep Coefficient
| Age at loading | Creep Coefficient |
| 7 days | 2.2 |
| 28 days | 1.6 |
| 1 year | 1.1 |
Exposure Conditions
| Exposure | Minimum Grade | Minimum Cement Content ${KG}/{{{m}^{3}}}\;$ | Maximum freewater cement ratio | Nominal Cover, mm |
| Mild | M20 | 300 | 0.55 | 20 |
| Moderate | M25 | 300 | 0.50 | 30 |
| Severe | M30 | 320 | 0.45 | 45 |
| Very Severe | M35 | 340 | 0.45 | 50 |
| Extreme | M40 | 360 | 0.40 | 75 |
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