Concrete is good in handling compressive strength but weak in tensile strength. Structures undergo stresses; after adding steel, concrete can handle compressive and tensile stresses making concrete a stronger material. Concrete resists squeezing due to compressive strength and steel resists bending and stretching due to tensile strength. This enhanced ductility reduce long term deflections, or increase the flexible capacity. Steel bars are used, which are firmly anchored inside the concrete without the risk of slipping.
Steel in concrete is also helpful in preventing excessive cracking resulting from temperature variation and shrinkage cracks.
Structural steel is used as a construction material for making structural steel shapes. The shape is a profile, formed with a specific cross-section and following defined standards for chemical composition and mechanical properties. Examples of standardized structural steel shapes are the I-shape, Z-shape beam, angle, plate channel, and HSS-shape.
- Hot-rolled deformed bars
Those are the most commonly used type of reinforcement in concrete constructions. The deformations on the surfaces of the bars are aimed to bond with the concrete, minimizing slippage in the concrete. Deformed bars also possess more tensile stresses than mild steel plain bars.
- Mild steel plain bars
Those plain bars have a smooth surface as compared to deformed bars. Plain bars possess a lower tensile strength than deformed bars and are therefore used smaller construction projects.
- Cold worked steel reinforcement
When hot rolled steel undergoes cold working, the crystal structure of the metal is deformed (bent, twisted, compressed), resulting in the relatively uniform crystalline plains (from recrystallization anneal) moving over and past one another. This movement creates imperfections and faults, similar to faults in the earth’s crust during earthquakes. These discontinuities in the structure are dislocations.
These dislocations provide further resistance to deformation, which can be seen as an increase in hardness, as measured by the penetration of an indenter under load. The ultimate tensile strength and yield strength also increase due to the “locking effect” of those distorted and twisted grains (metallic crystals).
- Prestressing steel
Used in the concrete girders of bridges prestressing cables are made up of multiple wires hectically spun together into strands.
For different seismic zones, different types of steel bars are used. Apart from safety during earthquakes, steel also bears the tension forces generated while the concrete expands thermally during a fire or another temperature rise. Steel, together with concrete results in the stability of the structure.