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 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. 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. Almost any natural water that is drinkable
and has no pronounced taste or odor may be used as mixing water
for concrete. However, some waters that are not fit for drinking
may be suitable for concrete. Excessive impurities in mixing water
not only may affect setting time and concrete strength, but also
may cause efflorescence, staining, corrosion of reinforcement,
volume instability, and reduced durability. Specifications usually
set limits on chlorides, sulfates, alkalis, and solids in mixing
water unless tests can be performed to determine the effect the
impurity has on various properties. 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.
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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