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Steel bars: Look for the D factor

Steel bars embedded in cement concrete help it bear heavy loads successfully and resist all forces acting on it.

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Jagvir Goyal

Steel bars embedded in cement concrete help it bear heavy loads successfully and resist all forces acting on it. These steel bars are known as reinforcement bars and the composite of steel and concrete is known as RCC. For RCC to remain strong forever, it is very important to choose steel of good quality and to embed it well in RCC, thus saving it from corrosion. Visibly, all steel bars look similar, but  in reality these differ significantly in strength and durability. We must, therefore, choose steel bars very carefully after examining certain important factors detailed below:

Choose TMT bars: Steel bars available in the market are either plain steel bars or HYSD bars or TMT bars. HYSD means High Yield Strength Deformed bars. While choosing steel for your house, choose TMT bars. TMT stands for Thermo Mechanically Treated bars. These are produced at a very high temperature and then cooled at periphery while their core remains hot. This hot and cold treatment shrinks the rods, makes them stronger at the periphery and keeps them ductile at the core, thus making them more earthquake resistant.

Appearance: Mild steel plain bars have a plain surface as the name suggests. Surface of HYSD bars is not plain but looks deformed with projecting lines running along the length of the bars. TMT bars have ribbed surfaces and you can note slanted and parallel ribs running along their entire length.

Designation: Depending upon their chemical composition, steel bars have different designations. These designations are listed in BIS code. Some of these are Fe 410, Fe 415, Fe 500, Fe 500 D, Fe 500 S, Fe 550, Fe 550 D, Fe 600. While the prefix Fe is symbol of Iron in chemistry, the figures of 410, 415, 500, 550, 600 and so on denote the yield strength of the steel. The suffixes like D and S indicate the ductility of the steel. These designations are clearly engraved on the steel bars at equal intervals along a length of the bar.

Chemical composition: Bars of above designations have different chemical composition. This chemical composition plays an important role in their performance, strength and durability. Content of carbon, sulphur and phosphorus in steel bars greatly affects their performance. Excess percentage of phosphorus makes steel bars brittle, while excess percentage of sulphur lowers their melting point. 

More percentage of carbon reduces their ductility which makes them less earthquake resistant. For each designation of steel, maximum permissible percentages of phosphorus, sulphur and carbon are defined in the BIS code and a ‘lot’ of steel bars is considered as ‘pass and fit for use’ only when its testing in the laboratory shows its chemical composition to be as per the BIS code. 

D-factor: While buying steel bars, one should look for the D factor in its designation. Therefore, among Fe 500 and Fe 500 D, the latter should be the choice. This is important because D denotes ductility which means that this steel is better to resist the effects of earthquakes. All builders and infrastructure contractors should also prefer 500 D or 550 D steel instead of 500 or 550 steel. For easy identification of designations, BIS should prescribe red, green and yellow colour paint on 4 inch end length of steel bars for 415, 500, 500 D designations and so on.

Brand choice: Choosing a good brand of steel is very important to ensure its quality. Reputed brands live up to their reputation and don’t compromise on quality of steel. Recently, during a steel testing campaign, only a few steel manufacturers could pass the tests while all others, having a good market share in supply of steel failed in tests of their samples.  

The choice of good quality steel will go waste if its correct cutting, bending, placement, spacing, lapping and binding in position is not done as per given design and drawings. Hiring the services of a structural engineer should be preferred to supervise the cutting and laying of steel in foundations, beams, columns, slabs etc.

— The writer is HoD and engineer-in-chief, Civil Engineering Department in a Punjab PSU


BIS Code number

MS round bars conform to BIS 432 Part I specifications. This IS mark is checked while buying MS bars. HYSD bars and TMT bars conform to IS 1786 specifications. For knowing permissible limits of carbon, sulphur and phosphorus for different designations of steel, these IS codes can be checked. Thus, the steel bars will carry the marks like ‘IS 1786 Fe500’ on them defining their confirmation to that particular designation of steel and IS 1786 provisions. 

 
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