Relatively thin building sections call for small coarse

aggregate, though aggregates up to six inches (150
mm) 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.
More on concrete design and production.
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.

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