In general, plain concrete is quite strong in compression, however, it is very weak in tension. Plain concrete takes about only 10% tensile load. Therefore, it is mandatory to enforce some other material so that it can take the tensile strength, as well as compression stress. There are many types of steel reinforcement available in the market. Therefore, in this post, I am going to share various types of steel reinforcement
In India, the following types of steel reinforcement are available, conforming to the relevant Indian Standards as mentioned in Table-1 below-
Mild Steel Reinforcement
Mild steel bars are also 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 Fig. 1.1. It shows a clear, definite yield point.
Although mild steel bars are very ductile, they are not preferred over high yield strength deformed bars because of their less strength and weak bond. The modulus of elasticity of mild steel is taken as equal to 2 x 10° N/mm2. However, they are used as lateral ties in columns and at places where nominal reinforcement is required. Mild steel plain bars are represented by the symbol ϕ.
High Yield Strength Deformed Bars
These are also known as HYSD bars. They have 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 as shown in Fig. 1.1. These bars are available as two types:
- Hot rolled high yield strength bars.
- Cold worked high yield strength bars.
The second type of steel is also called CTD (Cold Twisted Deformed) bars or Tor steel and are available in two grades. Deformed bars are represented by the symbol #.
- Fe-415 or Tor 40.
- Fe-500 or Tor 50.
A twisted deformed bar has about 50% higher yield stress than plain bars. A deformed bar has corrugation or ribs on the surface of the bar, as shown in Fig. 1.2, to increase the bond and prevent slipping of the bar in concrete. These bars do not show a definite yield point. So, the yield point is taken as 0.2 percent proof stress, which is determined from the stress-strain curve as follows:
- Draw a line parallel to the initial stress-strain curve, corresponding to a strain value of 0.002 (0.2 percent).
- The point where this line cuts the stress-strain curve is taken as the yield stress or 0.2 percent proof stress.
HYSD bars are preferred as reinforcement in R.C.C. over plain mild steel bars, due to following reasons.
- Higher Strength: HYSD bars have yield strength, higher than that of plain mild steel bars.
- Better Bond: The HYSD bars have better bond with concrete due to corrugations or ribs on the surface of the bars. As per IS 456: 2000, the bond strength of HYSD bars is 60 percent greater than the plain mild steel bars.
- Economy: The cost of HYSD bars is approximate same as mild steel but the use of HYSD bars leads to overall economy as the amount of steel required is less, due to its higher strength. Lot of steel (about 40 million tonnes) has been saved in India, since 1967, because of replacement of mild steel by the Tor steel, and the saving amounts to ~ 50,000 crore !!!
TMT (Thermomechanically Treated) Steel Bars
Among the constituents of R.C.C., steel is the costliest, so focus has been more on steel to make it better and better. TMT steel is new generation, high strength steel having superior properties as compared to common HYSD bars.
TMT bars are manufactured by passing hot rolled steel bars through cold water. By doing this, the outer surface of the bar becomes harder while the inner core is still softer.
In India, SAIL (Steel Authority of India Ltd.) and RINL (Rashtriya Ispat Nigam Ltd.) are producing TMT bars. The TMT bars have following advantages:
- High yield strength
- Better weldability
- Excellent ductility
- Superior corrosion resistance.
CRS (Corrosion Resistant Steel) Bars
The latest development in steel bars is the production of CRS or corrosion resistant steel bars. The carbon content in the CRS bars is 0.18% as compared to 0.2% of HYSD bars. The percentage of corrosion resisting elements such as chromium is as high as 1.5%. Thus, making the steel bars more corrosion resistant while keeping its other properties unchanged. In India, SAIL and TISCO (Tata Iron and Steel Company) are producing CRS bars.
Steel Wire Fabric Reinforcement
Wire fabric is a fabric made by welding or weaving steel wire in the form of a mesh which is also called as steel wire mesh. This mesh is used as reinforcement in slabs, shells, pavements and Toads etc.
Structural Steel Reinforcement
Sometimes for very heavily loaded elements such as foundations and columns rolled sections like rolled steel joists, channels or angles are embedded in concrete and used as reinforcement.
I hope this article will help you to understand the Advantages and Disadvantages of R.C.C. You may also want to see my other post from my Blog. If I have missed anything here, please let me know about that in the comment below this post.
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