History of concrete:
In ancient Roman, people used a material that is remarkably close to Modern Cement Concrete to build many of their architectural marvels. The Romans also used animal products in their cement as early form of Admixtures (in additions to a mix to achieve certain goals)
What is concrete???
What is concrete: Cement Concrete consists of hard inorganic materials called aggregates such as gravel, sand, crushed sand, slag etc. cemented together with Portland cement and water. When the above materials are mixed together to form a workable mixture, it can be molded or cast into beams and slabs. A few hours after mixing, the materials undergo a chemical combination and the mixture solidifies and hardens. Concrete attains great strength with age. Due to this property is used widely in overall world.
Requirements of a Good Concrete:
Requirements of a Good Concrete are as follows,
- The aggregates must hard and durable.
- The aggregates must properly graded from fine to coarse.
- Cement need to be of sufficient quantity to produce the required water tightness and strength.
- The water used for mixing need to be free from organic materials and deleterious material
- The quantity of water need to be such as to produce the needed consistency.
- Mixing must be done thoroughly to produce the needed homogeneity.
- Concrete must fill in every element in the forms. This is done by ramming, puddling and vibrations
- Until the concrete is thoroughly hard it is necessary to keep the temperature above freezing point to
avoid retarded hardening.
Concrete is a mixture of cement, sand, aggregate mixed in a specific proportion with water. The concrete mix is generally referred to as 1: X: Y wherein 1 refers to the weight of cement, X refers to the weight of sand and Y refers to the weight of aggregate. The mixes are designated as Mz wherein Z refers to the strength of the concrete mix.
Hence M20 Grade concrete is one of which a 150mm cube of concrete cured for 28 days under standard testing conditions will yield at a compressive stress of 20N/mm².
The different grades of concrete along with their proportions:
Characteristic Strength means the strength of material below which not more than 5% of the test results are
expected to fail.
The different grades of concrete along with their proportions are mentioned as under,
|Mix Ratio||Characteristic Compressive Strength |
after 28 days curing in mpa (N/mm²)
|Ordinary concrete||M 10||1:3:6||10 mpa|
|M 15||1:2:4||15 mpa|
|M 20||1:1.5:3||20 mpa|
|Standard Concrete||M 25||1:1:2||25 mpa|
|M 30||Design mix||30 mpa|
|M 35||Design mix||35 mpa|
|M 40||Design mix||40 mpa|
|M 45||Design mix||45 mpa|
|M 50||Design mix||50 mpa|
|M 55||Design mix||55 mpa|
|M 60 7 above||Design mix||60 and above|
As can be easily seen the proportion of cement increases in each mix as we move down and cement is the main ingredient contributing to the strength of concrete. For mixes M 55 and above, design parameters given in I.S.456 are not applicable and values will have to be obtained from specialized literatures.
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