Concrete Mix Design

Concrete is a construction material composed of :

Cement (commonly Portland cement) as well as other cementitious materials such as fly ash and slag cement.

Aggregate (generally a coarse aggregate such as gravel, limestone, or granite, plus a fine aggregate such as sand).

Water and

Chemical admixtures.

The word concrete comes from the Latin word “concretus” (meaning compact or condensed), the past participle of “concresco”, from “com-” (together) and
“cresco”( to grow).Concrete solidifies and hardens after mixing with water and placement due to a chemical process known as hydration. The water reacts with the cement, which bonds the other components together, eventually creating a stone-like material. Concrete is used to make pavements, pipe, poles different structures, foundations, brick/block walls and footing for gates, fences and poles.

Ingredients of Concrete:

Cement:

Different types of cement.

Portland cement is the most common type of cement in general usage. It is a basic ingredient of concrete, mortar, and plaster.

Aggregates:

Fine and coarse aggregates make up the bulk of a concrete mixture. Sand, natural gravel and crushed stone are mainly used for this purpose.

Recycled aggregates (from construction, demolition and excavation waste) are increasingly used as partial replacements of natural aggregates, while a number of manufactured aggregates, including air-cooled blast furnace slag and bottom ash are also permitted.

Decorative stones such as quartzite, small river stones or crushed glass are sometimes added to the surface of concrete for a decorative “exposed aggregate” finish, popular among landscape designers.

Water

Combining water with a cementitious material forms a cement paste by the process of hydration. The cement paste glues the aggregate together, fills voids within it, and allows it to flow more easily.

Less water in the cement paste will yield a stronger, more durable concrete; more water will give an easier-flowing concrete with a higher slump.

Impure water used to make concrete can cause problems when setting or in causing premature
failure of the structure.

Hydration involves many different reactions, often occurring at the same time. As the reactions proceed, the products of the cement hydration process gradually bond together the individual sand and gravel particles, and other components of the concrete, to form a solid mass.

Chemical admixtures

Chemical admixtures are materials in the form of powder or fluids that are added to the concrete to give it certain characteristics not obtainable with plain concrete mixes.

Main uses of Accelerators:

•Winter conditions

•Emergency repair work

•Early removal of formwork

Disadvantages

•increased drying shrinkage

•reduced resistance to sulphate attack

•CaCl2 highrisk of corrosion of steel – not permitted in RC

Retarders:

Slow the hydration of concrete, and are used in large or difficult pours where partial setting before the pour is complete is undesirable. A typical retarder is sucrose (C12H22O11).

•Longer setting times, Slower strength gains

•Prevents the formation of cold joints

•Assist with long transit times

Main uses

•large concrete pours

•sliding formwork

•hot weather concreting

Overdosing can cause problems

•Excessive retardation

•Excessive bleeding

Air entraining agents:

add and distribute tiny air bubbles in the concrete, which will reduce damage during freeze-thaw cycles thereby increasing the concrete’s durability.

However, entrained air is a trade-off with strength, as each 1% of air may result in 5% decrease in compressive strength.

References

Concrete Technology by M.S. Shetty

Concrete Technology by A.M.Neville

Guidelines for Concrete mix Design IS10262- l 198

Plain and Reinforced Concrete Code of practice

IS 456 : 2000

Different parts of IS 4031 for Cement Test