Concrete, a composite material whose key ingredient is a binding medium in which small pieces of rock or other materials are embedded. The small pieces are called aggregate; the binder is a cementing material. In portland-cement concrete, the binder is a mixture of portland cement and water. Asphalt and other cements are used to make some types of concrete, but the term "concrete" generally refers to portland-cement concrete.
Concrete is the most versatile and widely used building material. It is used in dams, canals, and aqueducts; in highways, pavements, and sidewalks; and in buildings, bridges, and other structures, both as a structural and as a decorative material.
Where concrete is not used as the primary structural material, it may be used for fireproofing, waterproofing, or soundproofing. Concrete also acts as a shield against damaging nuclear radiation.
Concrete is of such importance that almost every civil engineering structure uses it. On a worldwide basis the yearly production of concrete amounts to approximately one ton per capita.
Reinforced Concrete. The tensile strength of concrete is usually only about 10% as great as the compressive strength. In order to make practical use of concrete, steel reinforcement is placed within the concrete member to carry the tensile stresses.
Concrete and steel work well together in reinforced concrete because they have similar coefficients of thermal expansion, and a suitable bond can be made between the concrete and the steel rods. Concrete also protects the reinforcing steel from fire and corrosion. Reinforcing is placed in a structural member so that all tensile stresses are transferred from the concrete to the steel. In concrete beams between two supports, steel is placed in the lower portions of the beam where tension stresses due to bending are concentrated. Where beams or floors extend continuously over several supports, concrete may be subjected to tension in the upper side of the member. In this case it is necessary to use steel reinforcement in a position to withstand these tensile stresses. In columns, poles, and chimneys, it is necessary to reinforce all portions that may be subjected to tensile stress. (For weight saving and efficiency, reinforced concrete now is used sometimes as a substitute for cast iron or steel in the structures of heavy machines.)
Prestressed Concrete. The basic principle of prestressed concrete is that the reinforcing steel is under tension, while the concrete is under compression. When a prestressed concrete member is stretched by an applied tensile force, the stress is carried by the steel, which has very high tensile strength. In the concrete part of the member, the stretching relaxes the preexisting compression but does not ordinarily place the concrete under tension. Since concrete is about 10 times as strong in compression as it is in tension, prestressing thus makes use of concrete's characteristic strength. Also, because all of the concrete in prestressed members is in compression, there is negligible cracking due to shrinkage.
Prestressing is applied to steel or concrete for two purposes: first, to induce desirable strains and stresses in the structure, and second, to counterbalance undesirable strains and stresses. In prestressed concrete, the steel reinforcement is prestretched to avoid later excessive lengthening under service loads. The concrete is precompressed to prevent later cracking under tensile stress. This is an ideal combination of the compressive strength of concrete with the tensile strength of steel, and is responsible for the basic desirability of prestressed concrete.
Prestressing has made it possible to increase the span length of concrete members in bridges, roofs, floors, and other structural members. It also enables architects and engineers to design shallower, lighter, and more graceful concrete structures.
Concrete Shells. One of the more recent developments in concrete is the use of shells¡Xthin slabs formed into various curved surfaces. The various surfaces that are readily fabricated as concrete shells are cylindrical, hyperbolic, elliptical, and parabolic curves. More complex forms, some resembling seashells, have been used to produce unusual shapes for architectural beauty and grace. Reinforced concrete is quite suitable for shell structures, because the concrete in the shell needs to resist only compressive stresses. All tensile forces are carried by reinforcing steel in the supporting plane surrounding the shell.
The common egg illustrates the tremendous load-carrying capacity imparted by curved surfaces. The egg has exceptional strength in compression with a uniform load bearing around its surface. If an eggshell were to be formed in a thin sheet of the same thickness, however, it would break under a very light load.
William A. Cordon, Utah State University
Source: "Concrete." Encyclopedia Americana. Grolier Online http://ea.grolier.com/cgi-bin/article?assetid=0104230-00 (accessed August 10, 2007).