IS Code 456-2000 provides essential guidelines and standards for the design and construction of Plain and Reinforced concrete structures in India. This is a code of practice for the design of plain and reinforced concrete. It was first introduced in 1953. It was amended in 1957, 1984 and the latest amendment is in the year 2000. It gives specifications or specifies other codes requiring various materials used in making concrete. It classifies concrete into different grades based on the strength of the concrete cube at 28 days. It specifies the methods and care to be taken in the transportation, placing, compaction and use of concrete. It gives general design considerations, special design requirements and design procedure for various structural elements by limit state method approach. The design must meet both strength and serviceability requirements.

Here’s a summary of key points regarding concrete grades and properties.

• The characteristic strength of concrete is the strength below which not more than 5 percent of the test results are expected to fall.
• In IS Code 456–2000 defines various grades of concrete based on their characteristic strengths. Which is shown below in the Table 2

Minimum Grade of Concrete – IS Code 456-2000

• The Minimum grades of concrete for plain and reinforced concrete shall be as per below the table.
• Concrete grades lower than those given in the table below may be used for plain concrete construction, lean concrete, simple foundation, foundation for masonry walls, and other simple or temporary reinforced concrete construction.

Table 2 grade of concrete and their compressive Strength

Note In the designation of concrete mix, ‘M’ refers to the mix, and the number denotes the specified compressive strength of a 150 mm sized cube at 28 days in N/mm².

For concrete with compressive strength greater than ‘M55’, the design parameters given in the standards may not be directly applicable, and values may need to be derived from specifications and experimental results.

It is necessary to justify the greater strength of a particular structure due to age.

Table 5 Minimum Cement content, maximum water cement ration and minimum grade of concrete From Different exposure with normal weight aggregates of 20 mm nominal maximum size

Clauses 6.1.2, 8.2.4.1 and 9.1.2

Properties of Concrete – IS 456–2000 Guidelines

Increase strength with age

For concrete grade M30 and above, the rate of increase in compressive strength with age will be based on actual testing.

In cases where members are subjected to low direct loads during construction, they should be checked for stresses resulting from a combination of direct loads and bending during construction.

The tensile strength of concrete is determined using the methods described in IS 516 and IS 5816 for flexural and splitting tensile strength respectively. When estimating tensile strength from compressive strength, the following formula can be used:

Flexural strength (F_c) = k * √fck

Where fck is the specific cubic compressive strength of concrete in N/mm², and ‘k’ is a constant.

Elastic deformation:

The modulus of elasticity is mainly influenced by the elastic properties of the aggregate and to a lesser extent by the curing conditions and type of cement. It is generally related to the compressive strength of concrete.

The modulus of elasticity (EC) of concrete can be calculated as:

EC = 50000 / √fck

Shrinkage:

The total shrinkage of concrete depends on the components, member size and environmental conditions. The total amount of water present in the concrete at the time of mixing significantly affects shrinkage.

In the absence of test data, the approximate value of total shrinkage stress for the design can be taken as 0.0003.

Creep of Concrete:

The creep stress of concrete depends on the age of the concrete at the time of loading and the duration of loading. As long as the stress in the concrete does not exceed one third of its specific compressive strength, creep can be assumed to be proportional to the stress.

In the absence of experimental data and detailed information on the effects of variables, the ultimate creep stress can be estimated using the creep coefficient.

For long-span structures, it is advisable to determine the potential actual creep stress based on specific conditions and considerations.

Strength of concrete increases with age, and there is typically a continuous gain in strength beyond 28 days.

The rate of increase in strength depends on factors like curing, environmental conditions, and the concrete mix.

Designing is typically based on the characteristic strength of concrete at 28 days.

These guidelines are essential for ensuring the appropriate selection and use of concrete grades in various construction applications, considering factors such as strength, age, and intended purpose.

Note: The ultimate creep strain estimated as described above does not include the elastic strain.

Thermal Expansion

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

Workability of concrete; IS 456-2000

In the workability of concrete, the concrete mix proportions chosen must ensure that the concrete has adequate workability for the placing conditions and can be properly compacted with the available means. The suggested limits of workability of concrete measured as per IS 1199 are given below:

Note: For most placing conditions, internal vibrators or needle vibrators are suitable. The diameter of the needle shall be determined based on the density, spacing of reinforcement bars, and the thickness of the section. For tremie concrete, vibrators are not required to be used.

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