Portland cement concrete is a simple material in appearance with a very complex
internal nature. In contrast to its internal complexity, concretes versatility,
durability, and economy have made it the worlds most used construction material.
This can be seen in the variety of structures it is used in; from highways, bridges,
buildings and dams, to floors, sidewalks, missile silos, and underground utility
structures. The use of concrete is unlimited and is not even earthbound, as indicated by
recent interest by NASA in concrete lunar structures.
Concrete is basically a mixture of two components: aggregates and cement paste. The
paste, comprised of portland cement and clean water, binds the aggregates (washed sand and
gravel or crushed stone) into a rocklike mass as the paste hardens. This occurs because of
the chemical reaction of the cement and water (hydration).
The quality of the concrete depends a great deal upon the quality of the paste. In
properly made concrete, each particle of aggregate is completely coated with paste, and
all of the spaces between aggregate particles are completely filled with paste. The
quality of hardened concrete is determined by the amount of water used in relation to the
amount of cement (water/cement ratio). The less water used, the better the quality of the
concrete. Smaller amounts of water result in stiffer mixtures, but with proper vibration,
these mixtures result in higher strength concrete.
The properties of concrete may be changed by adding admixtures (chemicals) to the
concrete, usually in liquid form, during batching. Admixtures are commonly used to (1)
adjust setting time or hardening, (2) reduce water demand, (3) increase workability, (4)
intentionally entrain air, and (5) adjust other concrete properties.
After completion of proper proportioning, batching, mixing, placing, consolidating,
finishing, and curing, hardened concrete becomes a strong, non rusting, noncombustible,
abrasion-resistant, and durable building material that requires little or no maintenance.
Concrete is also an excellent building material because it can be formed into a wide
variety of shapes, colors, and textures for use in an almost unlimited number of
When placed underground, concrete actually continues to gain strength for a long period