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Concrete Basics
In its simplest form, concrete is a mixture of paste and aggregates.
The paste, composed of portland cement and water, coats the surface
of fine and coarse aggregates and hardens to form the rock-like
mass recognizable as concrete. Within this process lies the key
to a remarkable trait of concrete: it is plastic and malleable when
newly mixed, strong and durable when hardened. These qualities explain
how this versatile material can build skyscrapers and ranch homes,
bridges and basements, sidewalks and superhighways.
Aggregates
account for 60 to 75 percent of the total volume of concrete. They
are inert granular materials such as sand, gravel or crushed stone
that are an essential ingredient in concrete. Aggregates strongly
influence concrete’s freshly mixed properties, mixture proportion
and economy.
Chemical
admixtures are the ingredients in concrete that are not portland
cement, water, or aggregate. Producers use admixtures to reduce
the cost of concrete construction; to modify the properties of concrete;
and to ensure the quality of concrete during mixing, transporting,
placing, and curing.
Supplementary
cementing materials, or mineral admixtures, also contribute
to the properties of concrete. This materials, such fly ash, ground
granulated blast-furnace slag, and silica fume, are often recycled
industrial byproducts. They make concretes more economical, reduce
permeability, increase strength, or influence other concrete properties.
One of the greatest advances in concrete technology was the development
of air-entrained
concrete in the late 1930s. Today, air entrainment is recommended
for nearly all concretes, principally to improve their resistance
to freezing when exposed to water and deicing chemicals.
Proper placing
and finishing of concrete is key to ensuring the desired end-result.
Mixing, transporting, and handling of the concrete needs to be carefully
coordinated with the placing and finishing operations.
Curing
has a strong influence on the properties of hardened concrete such
as durability, strength, water tightness, abrasion resistance, volume
stability, and resistance to freezing and thawing and deicer salts.
Adequate curing is vital to quality concrete.
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