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Making right choice of steel for reinforcement
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Reinforcement in a concrete structure accounts for one-third of the cost of the structure and hence choice of steel matters, writes A.R. Santhakumar
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— Photo: M. Karunakaran
Go for quality: One should know the varieties of steel available, their relative merits and quality requirements.
Concrete is reinforced to give it extra tensile strength; without reinforcement, many concrete buildings would not have been possible. Reinforcement in a concrete structure accounts for nearly one-third of the cost of the structure. The fabrication of steel adds almost 20 to 30 % of the basic cost of steel. It is important to choose correct steel and hence one should know the varieties of steel available, their relative merits and quality requirements.
The ribbed bars used as concrete reinforcements are made in grades of Fe 250, Fe 415, Fe 500 and Fe 550. Hot rolled and Cold Twisted bars are used for this purpose.
Cold Twisted Deformed Bars (CTD Bars)
The yield strength of CTD bars are 415, 500, 550 N/sq. mm which are well above the yield strength of 260 N/ sq.mm of plain round bars. The higher strength in these bars is obtained by cold twisting.
Thermo Mechanically Treated Bars (TMT Bars)
TMT bars are thermo-mechanically treated through a special technology for getting high yield strength. The process involves rapid quenching of the hot bars through a series of water jets. After they roll out of the mill stand the bar core cools down slowly. The strength of the bars is carefully controlled by optimising water pressure for their pearlitic core and a tough surface of tempered matensite. This provides optimum strength, ductility and toughness.
Corrosion resistant bars (SS bars and Galvanised bars)
Stainless steel bars offer a solution to corrosion in reinforced concrete structures. They help achieve a design life of more than 100 years. They are intended for special applications. In order to enhance the corrosion resistance bars are sometimes galvanised. They also offer better corrosion protection and are used for monumental structures such as Bahai temple.
Corrosion Resistant Steel Bars (CRS steel)
These types of bars offer better corrosion resistance than ordinary CTD bars. One should understand that they do not make the bar corrosion proof.
Using poor quality steel in construction is not only a risk through collapse of the structure, but can also cause deterioration due to rapid corrosion and fatigue stress. Therefore, it is imperative to choose steel conforming to standards. The presence of ISI mark on steel assures quality and should invariably be checked.
It should be capable of withstanding temperatures up to 500 degree C. Loss in strength under fire should be minimum.
Bond strength
The deformed bar should have higher bond strength than round bars by at least 40%.
Bending
Should satisfy bend and re-bend test without cracking as per IS 1786.
Welding
Should be capable of being butt welded or lap welded using ordinary coated electrodes.
Corrosion resistance
The bar should have better corrosion resistance due to lesser residual stress.
Earthquake resistance
Proper ductility leading to good earthquake resistance is necessary for the deformed bars
Advantages of using TMT bars
1. Good ductility properties
2. Optimum combination of high yield strength and bendability
3. Excellent fatigue properties.
4. Tendency for ageing and brittleness is less
5. It has better resistance to heat affected zones.
6. Good bond strength
7. Less prone to corrosion compared to CTD bars.
8. More uniform properties along the length of the bar.
9. Better for earthquake resistance construction
General precautions
Before use, clean the steel bars thoroughly to avoid dirt or foreign bodies adhering to steel. They will reduce the bond. Use only good quality G.I. wires to bind the steel bars and ensure that the wire is free from rust and that they are turned into core concrete and not left in the cover region of concrete. Check your steel for strength and percentage of elongation by testing a sample in a laboratory or verify the test certificate of the batch supplied to your site. Use only correct grade of steel. It is equally wrong to use a higher grade as it is to use a lower grade. It is also not correct to use a larger diameter bar instead of a smaller diameter bar because of non-availability. Use of higher grade in place of lower grade bar will make your structure over reinforced inviting the risk of a brittle collapse during an earthquake! Take correct measures to tested materials segregated and store the steel on a dry wooden platform to avoid contact with water and other corrosive matter.
It is the steel that make your home stand firm and hence its strength and ductility are of vital importance. Make sure you have them both in the steel with which you build your house.
The author is emeritus Professor, Department of Civil Engineering, IIT, Madras and former Dean, Anna University.
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