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
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.
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.
More on concrete
design and production.
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
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
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.
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
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.
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
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.