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Concrete

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Materials

Concrete is a hard and strong construction material consisting of a mixture of aggregate, cement, and water. It is used to build roads, bridges, factories, airports, waterways, and a wealth of other architectural structures.

The strength, durability, and consistency of the concrete are entirely reliant upon its mixture. Main ingredients include fine and coarse aggregate, cement, and water. A basic mixture is one part cement, two parts fine mineral aggregate, and four parts coarse mineral aggregate.[1] The amount of water added to the mix is of the utmost importance because it directly affects the strength and workability of the concrete. A low water to cement ratio results in stronger, but less fluid and workable, concrete, whereas a high water to cement ratio has the reverse effect.

Water chemically reacts with the cement through a process called “hydration,” which creates a paste to coat the aggregate. As the paste hardens, the aggregate is bound together.

There are also many types of materials that can be added to the mixture to alter the consistency, speed of curing, and strength of the concrete. These materials are called admixtures.

An accelerating admixture reduces the setting time, which is helpful for jobs that require the concrete to dry quickly (i.e. road repair).

A retarding admixture delays the setting time, which is often useful in warm climates or special finishing operations.

Fly Ash is another type of admixture. It can replace 15 to 30 percent of the cement in the mixture, which improves the concrete’s workability and reduces the amount of heat it generates.

An air-entraining admixture infuses the concrete with microscopic bubbles, which allows it to avoid being damaged by freezing, thawing, and deicing salts. Frozen water expands into the air bubbles instead of damaging the concrete. The air-entraining admixture also improves workability.

A water-reducing admixture reduces the amount of water needed for the concrete mixture.

A platicizing admixture helps to increase the concrete's workability so it can be easily placed. These mixtures usually have a reduced water content to improve the concrete’s strength and durability.

[edit] References

  1. Concrete. Answers.com. 2008-09-28.