 In its simplest form, concrete is a mixture of paste and aggregates. The paste,
composed of portland cement and water, coats the surface of the fine and coarse
aggregates. Through a chemical reaction called hydration, the paste hardens and
gains strength to form the rock-like mass known as concrete.
Within this process lies the key to a remarkable trait of concrete: it's
plastic and malleable 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.
Proportioning
The key to achieving a strong, durable concrete rests in 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 surfaces and porous
concrete. A mixture with an excess of cement paste will be easy to place and
will produce a smooth surface; however, the resulting concrete is likely to
shrink more and be uneconomical.
A properly designed concrete mixture will possess the desired workability for
the fresh concrete and the required durability and strength for the hardened
concrete. Typically, a mix is about 10 to 15 percent cement, 60 to 75 percent
aggregate and 15 to 20 percent water. Entrained air in many concrete mixes 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 coats each particle of stone and sand. Through a
chemical reaction called hydration, the cement paste hardens and gains
strength. The character of the concrete is determined by quality 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 a higher quality concrete
provided the concrete is properly placed, consolidated, and cured.
Other Ingredients
Although most drinking water is suitable for use in concrete, aggregates are
chosen carefully. Aggregates comprise 60 to 75 percent of the total volume of
concrete. The type and size of the aggregate mixture depends on the thickness
and purpose of the final concrete product.
Relatively thin building sections call for small coarse aggregate, while
aggregates up to six inches in diameter have been used in large dams.
A continuous gradation of particle sizes is desirable for efficient use of the
paste. In addition, aggregates should be clean and free from any matter that
might affect the quality of the concrete.
Hydration Begins
Soon after the aggregates, water, and the cement are combined, the mixture
starts to harden. All portland cements are hydraulic cements that set and
harden through a chemical reaction with water. During this reaction, called
hydration, a node forms on the surface of each cement particle. The node grows
and expands until it links up with nodes from other cement particles or adheres
to adjacent aggregates.
The building up process results in progressive stiffening, hardening, and
strength development. Once the concrete is thoroughly mixed and workable it
should be placed in forms before the mixture becomes too stiff.
During placement, the concrete is consolidated to compact it within the forms
and to eliminate potential flaws, such as honeycombs and air pockets. For
slabs, concrete is left to stand until the surface moisture film disappears.
After the film disappears from the surface, a wood or metal handfloat is used
to smooth off the concrete. Floating produces a relatively even, but slightly
rough, texture that has good slip resistance and is frequently used as a final
finish for exterior slabs. If a smooth, hard, dense surface is required,
floating is followed by steel troweling.
Curing begins after the exposed surfaces of the concrete have hardened
sufficiently to resist marring. Curing ensures the continued hydration of the
cement and the strength gain of the concrete. Concrete surfaces are cured by
sprinkling with water fog, or by using moisture-retaining fabrics such as
burlap or cotton mats. Other curing methods prevent evaporation of the water by
sealing the surface with plastic or special sprays (curing compounds).
Special techniques are used for curing concrete during extremely cold or hot
weather to protect the concrete. The longer the concrete is kept moist, the
stronger and more durable it will become. The rate of hardening depends upon
the composition and fineness of the cement, the mix proportions, and the
moisture and temperature conditions. Most of the hydration and strength gain
take place within the first month of concrete's life cycle, but hydration
continues at a slower rate for many years. Concrete continues to get stronger
as it gets older.
The Forms of Concrete
Concrete is produced in four basic forms, each with unique applications and
properties. Ready mixed concrete, by far the most common form, accounts for
nearly three-fourths of all concrete. It's batched at local plants for delivery
in the familiar trucks with revolving drums. Precast concrete products are cast
in a factory setting. These products benefit from tight quality control
achievable at a production plant. Precast products range from concrete bricks
and paving stones to bridge girders, structural components, and panels for
cladding.
Concrete masonry, another type of manufactured concrete, may be best known for
its conventional 8 x 8 x 16-inch block. Today's masonry units can be molded
into a wealth of shapes, configurations, colors, and textures to serve an
infinite spectrum of building applications and architectural needs.
Cement-based materials represent products that defy the label of
"concrete," yet share many of its qualities. Conventional materials
in this category include mortar, grout, and terrazzo. Soil-cement and
roller-compacted concrete-"cousins" of concrete-are used for
pavements and dams. Other products in this category include flowable fill and
cement-treated bases. A new generation of advanced products incorporates fibers
and special aggregate to create roofing tiles, shake shingles, lap siding, and
countertops. And an emerging market is the use of cement to treat and stabilize
waste.
(Content provided by Portland
Cement Association )
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