Concrete Ingredients |

Concrete Mix Designs

For work requiring more than one cubic yard of material, concrete is usually ordered from a ready mix supplier for delivery to the job site. The supplier will need to know the minimum compressive strength, the maximum aggregate size, and any special requirements such as air entrainment for added freeze-thaw durability. The supplier will then select a mix design that is appropriate for your needs. If you are mix-ing small batches of concrete on site, you will need to understand the basic principles of concrete mix design yourself. The proportion of dry ingredients and the ratio of water to cement are the two most important factors.

Cement and aggregates provide strength, durability, and volume stability in concrete, but too much or too little of one in relation to the other reduces quality.

Lean or oversanded mixes with low cement content and high aggregate proportions are harsh and have poor workability.
Fat or undersanded mixes with high cement content and low aggregate proportions are sticky and expensive.

Within the range of normal concrete strengths, compressive strength is inversely related to water content. That is, the more water you use, the lower the concrete strength. But increasing water content increases fluidity and workability. Since water is required for workability, and since workability is required for high quality concrete, the low water requirements for strength and high water requirements for workability must be balanced. The ratio of water to cement is the weight of water divided by the weight of cement. Water cement ratio affects the consistency of a concrete mix. The consistency, in turn, affects how easily the concrete can be poured, moved around in the forms, compacted, and finished. Up to a point, a mix with more water is easier to work with than one that has less water and is therefore stiffer. Too much water, though, will cause the ingredients to separate during the pouring, placing, and handling and will destroy the integrity of the concrete. Too much water also lowers strength, increases the porosity and water permeability of the cured concrete, and makes it more prone to shrinkage cracking. The trick is to use enough water to make the fresh concrete workable, but not so much that it creates weak or porous structures.

Concrete Ingredients

The basic ingredients in concrete are cement, aggregates, and water. The type, quality, and proportioning of these ingredients affect the curing rate, compressive strength, and durability of the concrete. Chemical admixtures can be used to enhance one or more properties of the concrete or to improve its handling and placing characteristics.

Cement

Cement is not the same thing as concrete. Many people mistakenly refer to “cement” sidewalks or “cement” driveways and the like, but cement is only one of the ingredients in concrete. It is also an ingredient in masonry mortar, stucco, and other materials.

Cement + water = cement paste
Cement + water + sand =cement mortar
Cement + water + sand + lime = masonry mortar
Cement + water + sand + coarse aggregate = concrete

Aggregates

The aggregates most commonly used in concrete are sand, gravel, crushed stone, crushed slag, and pumice. Cement and water are mixed with aggregates to produce concrete. Concrete contains both fine and coarse aggregates. When cement is mixed only with fine aggregate, it is called cement mortar, which is used typically for patching and small repairs, or for coating a concrete surface to provide a smooth, even finish. Masonry mortar is different from a simple cement mortar because it contains other ingredients as well.

Water

As a rule of thumb, water used for mixing concrete should be drinkable. Any water that is drinkable is generally free of harmful impurities. In urban areas where municipal water supplies are available, contaminated water is usually not a problem.

Admixtures

Admixtures are substances other than cement, water, or aggregates which are added to concrete mixes for the purpose of altering properties of the fresh or hardened concrete. Admixtures are not generally required to produce high quality, low cost concrete, but they may sometimes be necessary or desirable to alter specific properties of the concrete for specific conditions or circumstances.

Chemical Admixtures

Set accelerators speed up the setting time and early strength development of concrete. This can be helpful in winter weather to reduce the length of time required for curing and protection and to compensate for the effects of low temperatures on strength development.