Why Concrete Is Reinforced With Steel

Most concrete structures are constructed with steel reinforcements. In almost all cases, it is not possible to see the steel because it is inside the concrete. Concrete structures built without steel reinforcement will not last long under the forces that will be acting on it over its lifetime.

Composition of concrete

Under normal circumstances, concrete usually consists of materials such as sand, crushed stone or gravel, commonly called aggregates, and cement to bind them together. Water is then added to make the mixture plastic enough for form work. For the concrete to be properly shaped accordingly, it should be in a plastic condition during mixing. The mixture is then placed in forms depending on the shape that is desired. These forms can be made of wood or steel. Inside the forms, steel reinforcement may be put depending on the strength required. The cementing material will later harden to give the concrete the appearance of a natural stone. The cementing material is a finely- ground powder manufactured by cement companies. Due to the chemical reaction with water, the cement later hardens. The mass begins to stiffen in about 45 minutes, and thereafter continues to harden indefinitely.

Steel reinforcement

Concrete beams reinforced with steel can be considered as a beam of two materials. To understand how the two materials will behave under stress or strain, it is necessary to transform the composite beam into an equivalent beam of one material. It is important to know that the strength of concrete is much greater in compression than in tension. It is strong in resisting compressive forces (those that tend to crush it) but weak in resisting tensile forces (those that tend to pull it apart). Hence concrete is the best material to use for members that are subjected to compressive forces such as posts or beams. When a slab or a beam is loaded, it bends or deflects, so it has one convex surface and one concave surface. For beams supported on both ends such as a floor slab, the top surface will be the concave side while the bottom surface will be the convex side. In any beam, the material on the convex side will be stretched therefore in tension while the material on the concave side will be shortened and therefore in compression. Compressive forces will be acting on the concave side of the concrete slab while tensile forces will be acting on the convex side. Hence a rectangular beam of concrete will fail from the tensile stresses on the convex side. The beam can be greatly strengthened by imbedding steel bars on the convex side. Since concrete grips the steel strongly, there will be no sliding of the steel bars with respect to the concrete during bending. In practice, the cross-sectional area of the steel bars is usually such that the tensile strength of the concrete on the convex side is overcome before yielding of the steel begins, and at larger loads the steel alone takes practically all the tension. Hence in establishing the bending stresses in reinforced-concrete beams, it is assumed that all the tension is taken by the steel and all the compression by the concrete.

To make concrete resist the forces that will act on it, steel is embedded in it for the purpose of resisting the tensile forces. Reinforced concrete is a combination of concrete and steel. The steel is arranged such that the two materials act together as a unit to carry the loads. To protect the steel from damage by fire, the reinforcement in beams and columns should not be placed nearer the exposed surface. It is advisable to leave a clearance of about 1.5 inches or approximately 4 cm. This specification fixes a lower limit for the depth of concrete below the reinforcing bars.