In its 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: itís 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 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 will be more likely to
crack and be uneconomical.
A properly proportioned concrete mixture will possess the
desired workability for the fresh concrete and the
required durability and strength for the hardened
concrete. Typically, a mixture is by volume about 10 to
15 percent cement, 60 to 75 percent aggregates and 15 to
20 percent water. Entrained air bubbles in many concrete
mixtures may also take up another 5 to 8 percent
The character of concrete is determined by the 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
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
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
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 hand float 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 toweling.
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
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.