PROPORTIONING AND MIXING CONCRETE

A concrete mix is designed to produce concrete that can be easily placed at the lowest cost.The concrete must be workable and cohesive when plastic,then set and harden to give strong and durable concrete.
The mix design must consider the environment that the concrete will be in;ie exposure to sea water,truck,cars,forklist,foot traffic or extreme of hot and cold.

PROPORTIONING
Concrete is a mixture of Cement,Water,Coarse and Fine Aggregates and Admixtures.The proportions of each material in the mixture affects the properties of the final hardened concrete.These proportions are best measured by weight.Measurement by volume is not as accurate,but is suitable for minor projects.

CEMENT CONTENT
As the cement content increases,so does strength and durability.Therefore to increase the strength,increase the cement content of a mix.

WATER CONTENT
Adding more water to a mix gives a weaker hardened concrete.Always use as little water as possible,only enough to make the mix workable.

WATER THE CEMENT RATIO
As the water to cement ratio increases,the strength and durability of hardened concrete decreases.To increase the strength and durability of concrete,decrease the water-cement ratio.Too much aggregate gives a sticky mix.Too much coarse aggregate gives a harsh or boney mix.

MIXING
Concrete must be mixed so the cement,water,aggregate and admixtures blend into an even mix.Concrete is normally mixed by machine.Machine mixing can be done on-site or be a pre-mixed concrete company.Pre-mixed concrete is batched(proportioned) at the plant to the job requirements.

TRUCK MIXING
The materials are normally added to the truck at batching plants and mixed for required time and speed at the plant.The trucks drum continues to rotate to agitate the concretes it is delivered to the site.

SITE MIXING
When site mixing begin by loading a measured amount of coarse aggregate into the mixer drum.Add the sand before the cement,both in measured amounts.

Mix materials together until there is no visible sand in the mix.Add enough water to get a workable mix.Be careful not to overload the mixer.Too much concrete in the mixer means each batch takes longer to properly mixed which causes costly delays in the long run or it will not mix at all.Always check how much the mixer holds so you know how much concrete can be produced at once.