CPCOZ CONSUMER INFORMATION
Concrete Basics
Commercial
Concrete Basics
In it's simplest form, concrete is a mixture of paste and aggregates (sand & rock). The paste, composed of cement and water, coats the surface of the fine (sand) and coarse aggregates (rocks) and binds them together into a rock-like mass known as concrete. Within this process lies the key to a remarkable trait of concrete: it's plastic and can be molded or formed into any shape when newly mixed, strong and durable when hardened. These qualities explain why one material, concrete, can build skyscrapers, bridges, sidewalks, and superhighways, houses and dams.
The key to achieving a strong, durable concrete rests on the careful proportioning
and mixing of the ingredients. A concrete mixture that does not have enough
paste to fill all the voids between the aggregates will be difficult to place
and will produce rough, honeycombed surfaced and porous concrete. A mixture
with and excess amount of cement paste will be easy to place and will produce
a smooth surface; however, the resulting concrete will be more likely to crack
an be uneconomical.
A properly proportioned concrete mixture will possess the desired workability for the hardened concrete. Typically, a mixture is by volume; about 10 to 15 percent cement, 60 to 70 percent aggregates and 15 to 20 percent water. Entrained air bubbles in many concrete mixtures may also take up another 5 to 8 percent.
Portland cement's chemistry comes to life in the presence of water. Cement
and water form a paste that surrounds and binds each particle of sand and
stone. Through a chemical reaction of cement and water called hydration, the
paste hardens and gains strength.
The character of concrete is determined by the equality of the paste. The
strength of the paste, in turn depends on the ratio of water to cement. The
water-cement ratio is the weight of the mixing water divided by the weight
of the cement. High-quality concrete is produced by lowering the water-cement
ratio as much as possible without sacrificing the workability of fresh concrete.
Generally, using less water produces higher quality concrete provided the
concrete is properly placed, consolidated and cured.
Besides Portland cement, concrete may contain other cementitious materials
including fly ash, a waste byproduct from coal burning electric power plants;
ground slag, a byproduct of iron and steel manufacturing; and silica fume,
a waste byproduct from the manufacturing of silicon or ferro-silicon metal.
Some of these cementitious materials are similar to the volcanic ashes the
Romans mixed with lime to obtain their cement binder. Some of these structures
still exist today! The concrete industry uses these
materials, which would normally have to be disposed in land-fill sites, to
the advantage of concrete. The materials participate in the hydration reaction
and significantly improve the strength, permeability and durability of concrete.