STEEL RE-BAR PLANT

Steel reinforcement bar is also known as rebar, reinforcing bar, reinforcing steel and reinforcement steel. It is a versatile constructional material which is widely used in the construction industry for making of the reinforced concrete. Reinforcement concrete (RC) is a composite material made up of concrete and some form of reinforcement – most commonly steel rods, bars, wires or mesh of steel rods and steel wires. The steel reinforcement bars usually consists of such shape and size that they may easily be bent and placed in the concrete so as to form a monolithic structure.

The properties of thermal expansion for both steel and concrete are approximately the same. This along with excellent bendability property makes steel the best material as reinforcement in concrete structures. Another reason steel works effectively as reinforcement is that it bonds well with concrete. When passive reinforcement (steel bars) is employed, the structure is known as reinforced concrete structure. In pre-stressed concrete structure, the reinforcement (steel wire) is stressed prior to subjecting the structure to loading, which may be viewed as active reinforcement. Passive steel reinforcing bars, also known as rebars, should necessarily be strong in tension and, at the same time, be ductile enough to be shaped or bent.

Steel rebar is most commonly used as a tensioning devise to reinforce concrete to help hold the concrete in a compressed state. Concrete is a material that is very strong in compression, but virtually without strength in tension. To compensate for this imbalance in a concrete slab behavior, reinforcement bar is cast into it to carry the tensile loads. The surface of the reinforcement bar may be patterned to form a better bond with the concrete.

Reinforced concrete gets its strength from the two materials, steel and concrete, working together. To get them working together, it is critical that the steel be adequately bonded to the concrete. Achieving this bond is called developing the bar, and many aspects of reinforcement design are geared toward achieving development.

Steel rebars are the time proven match for reinforcing concrete structures. RC structures are designed on the principle that steel and concrete act together to withstand induced forces. The aim of the reinforced concrete designer is to combine the reinforcement with the concrete in such a manner that sufficient of the relatively expensive reinforcement is incorporated to resist tensile and shear forces, whilst utilizing the comparatively inexpensive concrete to resist the compressive forces.

To achieve this aim, the designer needs to determine, not only the amount of reinforcement to be used, but how it is to be distributed and where it is to be positioned. These decisions of the designer are critical to the successful performance of reinforced concrete and it is imperative that, during construction, reinforcement be positioned exactly as specified by the designer.

Originally concrete structures were made without reinforcement. The use of rebars has started in construction since at least the 18th century. Earlier cast iron was the materials for the rebars. This was because cast iron rebars were of high quality, and there was no corrosion on them for the life of the structure. Later the technique was refined by embedding the steel bars in the reinforced concrete structures. Plain mild steel rebars of strength 250 MPa were used widely till about 1960s. Square twisted steel bars (deformed bars) were introduced in 1960s. But these were phased out due to their inherent inadequacies.

Later the steel rebars of high yield strength were produced by raising carbon as well as manganese contents. After this high strength was incorporated to the steel rebars to a great extent by cold twisting. The cold twisted deformed (CTD) steel rebars were produced by cold working process, which was basically a mechanical process. It involved stretching and twisting of mild steel bars, beyond the yield plateau, and subsequently releasing the load. CTD round rebars having yield strength in the range of 415 MPa were introduced in the late 1960s. Since then, there has been an increasing demand for high strength deformed bars.

Quenched and self tempered (QST) steel bars were introduced during late 1970s. These steel bars are popularly known in India as thermo mechanical treated (TMT) steel rebars. Quenching and self tempering treatment of the steel rebars is a heat treatment process in which hot steel bars coming out of last rolling mill stand are rapidly quenched with water. Rapid quenching provides intensive cooling of surface resulting in the steel bars having hardened surface due to the martensitic structure with hot core. The steel rebars are then allowed to cool in ambient conditions. During the course of such cooling, the heat released from core tempers the hardened martensitic structure of the surface while core is turned into a ferrite pearlite structure. This quenching and self tempering process thus changes the structure of material to a composite structure of ductile ferrite pearlite composition in core and tough surface rim of tempered martensite providing an optimum combination of high strength, ductility, bendability and other desirable properties. The steel reinforcing bars can be produced with strength of 415 MPa, 500 MPa, and 550 MPa and even higher. Quenching and tempering treatment can also be given to steel rebars having composition strengthened with micro alloying.

1 gives typical cross section of steel rebars produced by quenching and self tempering.

1 Typical cross section of steel rebar

For engineering a sound and durable concrete structure, it is essential to use reinforcement of appropriate characteristics and quality. Characterization is a process to control and ensure the quality of a material. Principal objective of characterization of a material is to ensure that it possesses the requisite properties necessary for its intended engineering usage. Properties of steel rebars are influenced by the chemical composition of the steel from which it is manufactured. Characterization is generally performed by checking the chemical composition and certain specified physical properties. The particular chemical ingredients and physical properties, which are selected for characterization, again depend on the attributes of the material that are important for its specified application. Characterization of steel rebars is as important as that of concrete for a sound RC structure of desired strength.