Advantages. Concrete and cement are among the most important construction materials. Concrete is fireproof,
watertight, comparatively economical, and easy to make. It offers surface continuity (absence of joints) and
solidity and bond with other materials.
Limitations. Certain limitations
of concrete cause cracking and other structural weaknesses that
appearance, serviceability, and useful life of concrete structures. Listed below are some principal limitations and
disadvantages of concrete:
Low tensile strength. Concrete members subject to tensile stress must be reinforced with steel (rebar) to
prevent excess cracking and failure.
Thermal movements. During setting and hardening, the heat of hydration raises the concrete temperature,
and then gradually cools. These temperature changes can cause severe thermal strains and early cracking.
In addition, hardened concrete expands and contracts with changes in temperature (at roughly the same
rate as steel); therefore, expansion and contraction joints must be provided in many types of concrete
structures to prevent failures.
Drying shrinkage and moisture movements. Concrete shrinks as it dries out and, even when hardened,
expands and contracts with wetting and drying. These movements require that control joints be provided
at intervals to avoid unsightly cracks. To prevent drying shrinkage in newly placed concrete, its surface is
kept moist continuously during the curing process. Moisture is applied as soon as the concrete is hard
enough to prevent damage to the concrete's surface.
deforms creeps gradually under load, and this deformation does
when the load is removed.
Permeability. Even the best quality concrete is not entirely impervious to moisture. It contains soluble
compounds that are leached out in varying amounts by water. Unless properly constructed, joints allow
water to enter the mass. Permeability is particularly important in reinforced concrete because the concrete
must prevent water from reaching the steel reinforcement.
Describing and Measuring Ingredients. The unit of measure for cement is the cubic foot (cf). Thus, a standard
sack of portland cement weighs 94 pounds and equals one loose cubic foot. Fine and coarse aggregate is
measured by loose volume, whereas water is measured by the gallon. Concrete is usually referred to by cubic
Plastic concrete in a relatively fluid state can be readily molded by hand like a clump of modeling clay. A plastic
mix keeps all grains of sand and pieces of gravel or stones encased and held in place (homogeneous). The degree
of plasicity influences the quality and character of the finished product Significant changes in the mix proportions
affect plasticity. Desirable properties of plastic concrete are listed below.
This property describes the relative ease or difficulty of placing and consolidating
concrete in the form.
Workability is largely determined by the proportions of fine and coarse
aggregate added to a given quantity of paste.
One characteristic of workability is consistency,
which is measured by the slump test (see Part C on page 1-20). A specific amount of slump is
necessary to obtain the workability required by the intended conditions and method of