Content of this Article : Reinforcing material, Suitability of Steel as Reinforcing Material, charectristic strength of steel and types of steel reinforcement
Concrete is weak in tension and it is to be reinforced properly with suitable material. The purpose of providing reinforcement in R.C.C. is :
- To take up all the tensile stresses developed in the structure.
- To increase the strength of concrete sections.
- To prevent the propagation of cracks developed due to temperature and shrinkage stresses.
- To make the sections thinner as compare to plain concrete section.
To fulfill above criteria the reinforcing material should satisfy the following requirements :
- The reinforcing material should develop a perfect bond with concrete to transfer stresses from one material to other.
- It should have high tensile strength.
- It should be cheap, easily available and durable.
- The coefficient of thermal expansion of the reinforcing material should be nearly same as that of concrete, for obtaining a good composite action.
- It should be workable i.e., easy to cut, bend and join.
- It should not react with other ingredients of R.C.C.
- It should be free from loose mill scales, loose rust and coats of paints, oil mud or any other substances which may destroy or reduce bond.
Suitability of Steel as Reinforcing Material
Many traditional materials such as bamboo and natural fibres have been tried as reinforcement in earlier times. But steel is found to be the most apropriate form of reinforcement.
It is the most suitable reinforcing material in R.C.C. because of following reasons :
- Steel is very strong in compression, tension, shear and torsion.
- Concrete develops very good bond with steel.
- Steel is ductile in behaviour. More ductility means more elongation of steel before failure.
- The steel bars can be cut, lifted and welded easily with commonly available tools and machines.
- Steel has longer life.
- Steel is easily available.
Steel reinforcement has various advantages as listed above, which make it a suitable reinforcing material. However, steel has a few disadvantages which are listed below.
- The biggest disadvantage of steel reinforcement is rusting. If concrete is porous or if cover to the reinforcement is not sufficient, steel gets rusted and loses strength.
- Steel loses its strength at high temperatures.
TYPES OF STEEL REINFORCEMENT
In India, following types of steel reinforcement are available, conforming to the relevant Indian standards as mentioned in the Table 1.9.
[table id=6 /]
Mild Steel Reinforcement
Mild steel bars are also known as Fe 250 because the yield strength of this steel is 250N/mm2. The stress-strain curve for mild steel is given inFig. 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 2×105 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 symbol f.
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 of mild steel, but the yield point is not clearly defined as shown in Fig. 1.1.
These bars are available as two types :
(i) Hot rolled high yield strength bars.
(ii) Cold worked high yield strength bars.
The (ii) type of steel is also called as CTD (Cold Twisted Deformed) bars or Tor steel and are available in two grades. Deformed bars are represented by symbol … or #.
(i) Fe 415 or Tor 40
(ii) 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 :
(i) Draw a line parallel to the initial stress-strain curve, corresponding to a strain value of 0.002 (0.2 percent).
(ii) 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 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 Rs. 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
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 roads etc.
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.
CHARACTERISTIC STRENGTH OF STEEL
The term characteristic strength means that value below which not more than 5% of the test results are expected to fall. As per IS 456:2000, the characteristic strength of steel is equal to the minimum yield stress or 0.2 percent proof stress. Table 1.10 give the values of characteristic strength for different grades of steel and their minimum percentage elongation.
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