02-10-2012, 05:21 PM
CONCRETE MIX DESIGN
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INTRODUCTION
Concrete is the second largest material consumed by human beings after food and water as per WHO. It is obtained by mixing cement, fine aggregate, coarse aggregate and water in required proportions. The mixture when placed informs and allowed to cure becomes hard like stone. The hardening is caused by chemical action between water and the cement due to which concrete grows stronger with age.
The strength, durability and other characteristics of concrete depend upon the properties of its ingredients, proportion of the mix, the method of compaction and other controls during placing, compaction and curing.
Concrete possess a high compressive strength and is usually more economical than steel and is non corrosive which can be made with locally available materials.
Hence concrete is used widely in all present-day constructions. The concrete is good in compression and bad in tension. Hence liable to be cracked when subjected to tensile load. In situations where tensile stresses are developed concrete is strengthened by steel bars forming a composite construction called Reinforced Cement Concrete (RCC).
CONCRETE MIX DESIGN
Concrete mix design may be defined as the art of selecting suitable ingredients of concrete and determining their relative proportions with the object of producing concrete of certain minimum strength and durability as economically as possible.
REQUIREMENTS OF CONCRETE MIX DESIGN
The requirements which form the basis of selection and proportioning of mix ingredients are :
The minimum compressive strength required from structural consideration
The adequate workability necessary for full compaction with the compacting equipment available.
Maximum water-cement ratio and/or maximum cement content to give adequate durability for the particular site conditions
Maximum cement content to avoid shrinkage cracking due to temperature cycle in mass concrete.
OBJECTIVES OF MIX DESIGN
The purpose of concrete mix design is to ensure the most optimum proportions of the constituent materials to fulfill the requirements of the structure being built.
To achieve the designed/desired workability in the plastic stage.
To achieve the desired minimum strength in the hardened stage.
To achieve the desired durability in the given environmental conditions.
To produce concrete as economically as possible.
NOMINAL MIXES
In the past the specifications for concrete prescribed the proportions of cement, fine and coarse aggregates. These mixes of fixed cement-aggregate ratio which ensures adequate strength are termed nominal mixes. These offer simplicity and under normal circumstances, have a margin of strength above that specified. However, due to the variability of mix ingredients the nominal concrete for a given workability varies widely in strength.
STANDARD MIXES
The nominal mixes of fixed cement-aggregate ratio (by volume) vary widely in strength and may result in under- or over-rich mixes. For this reason, the minimum compressive strength has been included in many specifications. These mixes are termed standard mixes.
IS 456-2000 has designated the concrete mixes into a number of grades as M10, M15, M20, M25, M30, M35 and M40. In this designation the letter M refers to the mix and the number to the specified 28 day cube strength of mix in N/mm2. The mixes of grades M10, M15, M20 and M25 correspond approximately to the mix proportions (1:3:6), (1:2:4), (1:1.5:3) and (1:1:2) respectively.