In M20 grade of concrete, M refers to mix(proportion) and 20 refers to characteristics strength of concrete at the age of 28 days.
Actually M20 is a standard mix, means it is mix proportion according to standard materials to be use and a standard working environment on site[up to M30 this standards are : 53 grade cement, maximum size of aggregate be 20 mm and temperature be 25 degree]
Now if we change material to be use or our site condition change being different as that prescribed in standard mixes, then we have to design the mix ration by concrete mix design.
Let us see what is concrete mix design
The process of selecting suitable ingredients of concrete and determining their relative amounts with the objective of producing a concrete of the required, strength, durability, and workability as economically as possible, is termed the concrete mix design. The proportioning of ingredient of concrete is governed by the required performance of concrete in 2 states, namely the plastic and the hardened states. If the plastic concrete is not workable, it cannot be properly placed and compacted. The property of workability, therefore, becomes of vital importance. The compressive strength of hardened concrete which is generally considered to be an index of its other properties depends upon many factors, e.g.quality and quantity of cement, water and aggregates; batching and mixing; placing, compaction and curing. The cost of concrete is made up of the cost of materials, plant and labor.
The variations in the cost of materials arise from the fact that the cement is several times costly than the aggregate, thus the aim is to produce as lean a mix as possible. From technical point of view the rich mixes may lead to high shrinkage and cracking in the structural concrete, and to evolution of high heat of hydration in mass concrete which may cause cracking. The actual cost of concrete is related to the cost of materials required for producing a minimum mean strength called characteristic strength that is specified by the designer of the structure. This depends on the quality control measures, but there is no doubt that the quality control adds to the cost of concrete. The extent of quality control is often an economic compromise, and depends on the size and type of job. The cost of labour depends on the workability of mix, e.g., a concrete mix of inadequate workability may result in a high cost of labour to obtain a degree of compaction with available equipment.
Requirements Of Concrete Mix Design:-
The requirements which form the basis of selection and proportioning of mix ingredients are:
1. The minimum compressive strength.
2. The adequate workability necessary for full compaction with the compacting equipment available.
3. Maximum water-cement ratio and/or maximum cement content to give adequate durability for the particular site conditions.
4. Maximum cement content to avoid shrinkage cracking due to temperature cycle in mass concrete. required from structural consideration.
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