Concrete is a composite material. The use of concrete as a building material reaches back more than two thousand years. Evidence of sporadic use is found in the ancient eastern Mediterranean cultures, but the Roman Republic began making extensive use of concrete around 300 BC. The Colosseum (still standing) used concrete, the dome of the Roman Pantheon (still standing) is concrete, and the Roman aqueducts (many still standing) are concrete.
Concrete as a building material fell out of use after the decline of the Roman Empire in the 5th century. The modern revival of concrete is part of the Industrial Revolution. Various kinds of cement were developed in England starting in the 18th Century, and these led to the building of bigger and better freestanding structures.
Reinforced concrete was invented by Frenchman Joseph Monier, a gardener, who demonstrated his invention at the Paris Exposition of 1867. Reinforcing the concrete compensates for its major drawback, poor tensile strength. Reinforced concrete makes possible, for example, the tremendously large dams of the world (right).
Concrete as a finished product can be described as a kind of artificial stone made of many small stones in a binding agent. The major advantage of concrete over natural stone is that it is manufactured and poured as a viscous liquid into any desired shape, then it hardens. The recipe for concrete includes these several ingredients:
The aggregate is the main bulk of the concrete. This is irregular crushed stone, gravel, or sometimes waste and recycled materials.
Cement is the binding agent, usually the kind referred to as Portland cement. This is a grey powder consisting of lime and other minerals.
Water, when mixed in, turns the concrete temporarily into a slurry. The water gradually reacts with the cement and binds with its molecules, turning it into a hard solid. (Evaporation is not why concrete dries; this chemical reaction is.)
Reinforcement consists of metal bars ("rebar") or mesh embedded in the concrete. The resulting reinforced concrete has much greater tensile strength. Chemical polymers and carbon fiber are more recent reinforcements.
Decorations are important for some architectural or home landscaping purposes. A pigment will change the color of the concrete from its usual grey or white. Colored stones may be used as aggregate on the surface of, say, a walkway. The mold that the concrete is poured into may be sculpted into an artistic contour (right).
Huge volumes of concrete are produced to satisfy the construction industry of the world. Production of concrete is a local activity that takes place in and around most cities.
Concrete production can be described as wet or dry. Dry production mixes the cement powder, aggregate, and any additives before transport to the construction site. Wet production includes adding water and so must be done not long before use.
Depending on the final structure, concrete slurry is poured into trenches, wooden forms, or molds that give it shape. Concrete takes weeks or longer to harden. This is a chemical process that releases heat, so a very large volume of concrete, such as a dam, has to be cooled over all that curing duration. In a dry climate, the concrete should be kept wet so that it does not harden too quickly.
Concrete is a major structural element of buildings everywhere, including support columns and walls. Concrete is mixed in different varieties for different purposes. For example, the bottom support columns in a building might be designed for maximum strength, and the upper concrete portions might have minimum weight.
The highways of the United States and elsewhere are concrete, as are peripheral roads, sidewalks, and driveways.
Very large structures are commonly made of concrete, including walls, breakwaters, channels, and dams.
Use of concrete has a relatively low environmental impact. The energy required for manufacture and transport is low. On the other hand, cement, the major ingredient, generates considerable CO2 when made. Cement powder is caustic and does have to be handled with some care.
Concrete manufacture can accept a small amount of recycled material. Fly ash from burned coal can substitute for a portion of the cement. Blast furnace slag and other industrial wastes have also been tried. Finally, the aggregate may include construction material from previous demolitions.
Concrete lasts for generations, if not centuries, and leaves a residue equivalent to slightly alkaline rock. Old concrete can potentially be crushed and recycled for use as other filler material. Rebar can be recycled with other similar metal. Because concrete is insoluble and nonflammable, though, the volume of material does have to be disposed of somehow once the structure's useful life is past.