Concrete is one of the most important con-
struction materials. It is comparatively economical,
easy to make, offers continuity and solidity, and will
bond with other materials. The keys to good-quality
concrete are the raw materials required to make
concrete and the mix design as specified in the project
specifications. In this chapter, well discuss the
characteristics of concrete, the ingredients of con-
crete, concrete mix designs, and mixing concrete.
Well conclude the chapter with a discussion of
precast and tilt-up concrete. At the end of the
discussion, we provide helpful references. You are
encouraged to study these references, as required, for
additional information on the topics discussed.
LEARNING OBJECTIVE: Upon completing
this section, you should be able to define the
characteristics of concrete.
Concrete is a synthetic construction material
made by mixing cement, fine aggregate (usually
sand), coarse aggregate (usually gravel or crushed
stone), and water in the proper proportions. The
product is not concrete unless all four of these
ingredients are present.
CONSTITUENTS OF CONCRETE
The fine and coarse aggregates in a concrete mix
are the inert, or inactive, ingredients. Cement and
water are the active ingredients. The inert ingredients
and the cement are first thoroughly mixed together.
As soon as the water is added, a chemical reaction
begins between the water and the cement. The
reaction, called hydration, causes the concrete to
harden. This is an important point. The hardening
process occurs through hydration of the cement by the
water, not by drying out of the mix. Instead of being
dried out, concrete must be kept as moist as possible
during the initial hydration process. Drying out
causes a drop in water content below that required for
satisfactory hydration of the cement. The fact that the
hardening process does not result from drying out is
clearly shown by the fact that concrete hardens just as
well underwater as it does in air.
CONCRETE AS BUILDING MATERIAL
Concrete may be cast into bricks, blocks, and
other relatively small building units, which are used
in concrete construction. Concrete has a great variety
of applications because it meets structural demands
and lends itself to architectural treatment. All
important building elements, foundations, columns,
walls, slabs, and roofs are made from concrete. Other
concrete applications are in roads, runways, bridges,
STRENGTH OF CONCRETE
The compressive strength of concrete (meaning
its ability to resist compression) is very high, but its
tensile strength (ability to resist stretching, bending,
or twisting) is relatively low. Consequently, concrete
which must resist a good deal of stretching, bending,
or twistingsuch as concrete in beams, girders,
walls, columns, and the likemust be reinforced with
steel. Concrete that must resist only compression
may not require reinforcement. As you will learn
later, the most important factor controlling the
strength of concrete is the water-cement ratio, or the
proportion of water to cement in the mix.
DURABILITY OF CONCRETE
The durability of concrete refers to the extent to
which the material is capable of resisting deteriora-
tion caused by exposure to service conditions.
Concrete is also strong and fireproof. Ordinary
structural concrete that is to be exposed to the
elements must be watertight and weather-resistant.
Concrete that is subject to wear, such as floor slabs
and pavements, must be capable of resisting abrasion.
The major factor that controls the durability of
concrete is its strength. The stronger the concrete, the
more durable it is. As we just mentioned, the chief
factor controlling the strength of concrete is the
water-cement ratio. However, the character, size, and
grading (distribution of particle sizes between the
largest permissible coarse and the smallest
permissible fine) of the aggregate also have important
effects on both strength and durability. However,