There are different types of steel bars used in construction. The reinforcement bar is a steel bar that is used as tension in reinforced masonry structures and reinforced concrete to strength & aid the concrete under tension.
The term concrete is weak in tension & strong in compression. The reinforcement bar significantly increases the tensile strength of the structure. It is provided in combination with plain cement concrete to make it reinforced cement concrete.
What is reinforced concrete?
Reinforced concrete is a composite material that consists of concrete and reinforcement ( usually steel). Concrete is a mixture of water, cement, fine aggregate, coarse aggregate, and admixture in a suitable proportion.
Reinforced concrete is prepared by placing the cage of reinforcement bars in the formwork of desired shape and size and casting concrete inside it.
When the concrete hardens and develops strength, the formwork is removed. Thus, the prepared reinforced concrete is able to withstand the structural loads.
The main aim of reinforcing concrete is to improve its structural stability. Concrete possesses good compressive strength but its tensile strength is quite poor ( nearly 1/10th of its compressive strength ). Hence, providing reinforcement in the concrete enables it to resist tensile forces.
Why we use reinforcement in concrete?
The purpose of providing reinforcement material in concrete is as follows;
- Concrete resists only compressive strength, so the reinforcement bar is used for resists tensile strength.
- Concrete is brittle in nature and the reinforcement bar is ductile. More ductility means more elongation of reinforcement before failure. This results in insufficient warning time before failure.
- The coefficient of thermal expansion of the reinforcement bar should be nearly equal to that of concrete, for obtaining a good composite action.
- It should not react with other ingredients in RCC.
- The reinforcement bar should develop a perfect bond with the concrete to transfer stresses from one material to another.
Types of steel bars used in construction
|S.No.||Types of Reinforcement||Sub-type||Relevant Indian standard|
|1||Mild steel plain bar||Cold worked mild steel barsHot rolled mild steel bars||IS 432-1996 (part –I) Is 1139-1966 ( part-II)|
|2||High yield strength deformed bar ( Tor steel)||Cold worked deformed barsGrade Fe 415Grade 500Hot rolled deformed bars||IS 1786 – 1979 IS 1139 – 1996 (part – II)|
|3||Hard drawn steel wire fabric||Wire mesh||IS 1566 – 1967|
|4||Rolled steel members||Angles, T- sections joists, channels, etc.||IS 226 – 1975|
|5||Thermo-mechanically treated bars (TMT)Corrosion resistant steel (CRS) bars||Latest in use deformed bars|
Let us describe different types of steel bars used in construction one by one.
Mild steel reinforcement
Mild steel bars also are known as Fe 250 because the yield strength of this steel is 250 N/mm2. The stress-strain curve for mild steel is given in the figure. It shows a clear, definite yield point.
Mild steel bars are ductile, they are not preferred over high yield strength deformed bars because it has less strength and weak bond.
The modulus of elasticity of mild steel is taken as equal to 2 x 105 N/mm2. In columns, they are used as lateral ties and at places where nominal reinforcement is required. Mild steel plain bars are denoted by symbol .
High yield deformed bars
These are also known as HYSD bars. It has a higher percentage of carbon as compared to mild steel. Their strength is higher than that of mild steel, but the yield point is not clearly defined.
These bars are available as two types:
- Hot rolled high yield strength bars
- Cold worked high yield strength bars
The (b) form of steel is also known as CTD bars or Tor steel and are available in two grades. Deformed bars are represented by symbol #.
- Fe 415 or Tor 40
- Fe 500 or Tor 50
- Deformed bar
- Plain bar
A twisted deformed bar has nearly 50% higher yield stress than plain bars. There is corrugation on the surface of the twisted deformed bar. It increases the bond between concrete and steel.
These bars do not show a definite yield point. So, the yield point is taken as 0.2 % proof stress, which is determined from the stress-strain curve as follows:
Draw a line parallel to the initial stress – strain curve, near to a strain value of 0.2 %.
Where this line cuts the stress-strain curve, this point is taken as the yield stress or 0.2 percent proof stress.
TMT ( Thermomechanically treated ) steel bars
Among the consistent of RCC, steel is that the costliest, so the focus has been more on steel to create it better and better. This steel has more strength but has the same properties as HYSD. Mostly this types of steel bars used in the construction of the different structures.
These bars are manufactured by passing hot rolled steel bars through cold water. The outer surface of the bar becomes harder while the inner core is still softer, by doing this.
CRS ( Corrosion Resistant Steel) Bars
The latest development in steel bars in the production of CRS or corrosion-resistant steel bar. The amount of carbon content in the CRS bars is 0.18% as compared to 0.2 % of HYSD bars. The percentage of corrosion resisting elements (i. e. chromium) is high as 1.5%.
Steel wire fabric
Wire fabric is a fabric that is made by welding or weaving steel wire in the form of a mesh which is also known as steel wire mesh. This mesh is used as reinforcement in slabs, pavements, roads, and shells, etc.
For very heavy loaded elements such as foundations & columns rolled sections like rolled steel joists, angles or channels are embedded in concrete & used as reinforcement.
Characteristic strength of types of steel bars used in construction
The characteristic strength of steel is equal to the minimum yield stress, as per IS 456:2000. The table below gives the values of characteristic strength for various grades of steel and their minimum % elongation.
Characteristic strength and minimum percentage elongation for various types of steel
|Types of steel||Grade||Yield stress/0.2% proof stress or characteristic strength (N/mm2)||% Elongation ( minimum)|
|Mild steel||Fe 250||250||23%|
|High strength deformed steel (HYSD)||Fe 415 ( Tor 40) Fe 500 ( Tor 50) Fe 550 (Tor 55)||415 500 550||14.5% 12% 8%|
|TMT or CRS bars||Fe 500||500||12%|
|Steel wire fabric||–||480||7.5 over a gauge length of 8 diameter|
I hope this article remains helpful for you.
Happy Learning – Civil Concept
Civil Engineer – Pradip Thakur