Your guide to be steel smart

Jagvir Goyal

Cement concrete and steel are two such items that have helped the man achieve the unimaginable. While cement concrete can be moulded into any shape we want, steel helps in providing strength to cement concrete to resist all kinds of forces acting on it. The combination of cement concrete and steel has therefore resulted into reinforced cement concrete (RCC) that has revolutionised the construction scene. Each of the two items holds equal importance in making RCC strong and serves its desired purpose. In this article, we will discuss various aspects related to steel.

Reinforcement: The steel bars provided in RCC are known as reinforcement bars. The very purpose of their provision is to provide strength to concrete and resist tensile forces. Their quantity, diameter, spacing and way of placing are decided by the designer after considering all the forces and loads that are going to act on RCC components when it is laid in position. Services of a structural design engineer are essential to be hired for designing various RCC components of the house.

Types of steel bars: Two types of steel bars are used in RCC — plain steel bars, known as MS rounds, and deformed bars, commonly known as tor steel bars. MS rounds are rarely used these days. Use of deformed bars has become universal because of their better strength and bond with concrete.

IS codes: MS round bars, wherever used, should conform to IS 432 Part I specifications. This IS code further defines two types of plain steel. One is mild steel and the other is medium tensile steel. Mild steel bars further have two grades — Grade I and II. Grade II mild steel is not used in earthquake-prone areas. Deformed bars should conform to IS 1786 specifications. Full name of these bars is High Strength Deformed (HSD) bars.

Designation of steel bars: Carbon, sulphur and phosphorus in steel bars have a great bearing on their future service. Depending upon their chemical composition, steel bars are assigned different designations. Mild steel plain bars of Grade I are manufactured as per chemical composition of Fe 410 S designation steel. Mild steel plain bars of Grade II have chemical composition in accordance with Fe 410 designation. Medium tensile steel plain bars have chemical composition of designation Fe 540 W-HT. HSD steel bars have many designations and each designation defines maximum percentage of carbon, sulphur and phosphorus in steel bars. HSD steel bar designations are Fe 415, Fe 415 S, Fe 415 D, Fe 500, Fe 500 D, Fe 500 S, Fe 550, Fe550 D and Fe 600. While the figures 415, 500, 550 and 600 indicate yield strength of steel, D and S indicate steel having additional properties like better elongation.

Sulphur and phosphorus: Excess percentage of phosphorus in steel makes it brittle. Excess sulphur lowers its melting point. Excess carbon reduces ductility. Less carbon helps in better weldability of steel bars. For each steel designation like Fe 415 and so on, maximum permissible percentages of phosphorus, sulphur and carbon are given in BIS code.

Steel manufacturing: There are many steel manufacturers in India but those having BIS licence and plants equipped with advanced manufacturing technology and metallurgy control technique should be selected by the user while finalising the brand of steel. These plants use steel billets to manufacture reinforcement bars. Steel billets are manufactured from iron ore after multiple refining processes. These days, Thermo Mechanical Treatment (TMT) and hot rolling is done to produce steel bars from billets. TMT bars have a soft core but hard outer surface and take better stress before yielding because of their soft core.

Which steel to use: One should prefer HSDsteel to plain round steel as this steel is much stronger and helps in economising the cost. Both types of steel weigh equally. Weight of 12 mm diameter round or tor steel is same but a 10 mm diameter HSD steel bar has almost the same strength as 12 mm plain steel bar. Similarly, strength of an 8 mm diameter HSD steel bar is almost equal to that of 10 mm diameter plain bar.

Look for trademarks: Reputed manufacturers put their trademarks on steel bars. Ask the retailer to show them to you to confirm that you are buying genuine steel only. Reputed companies also use colour code to identify the grade of their bars but this colour coding is not standard and differs from company to company.

Test certificate: The buyer should ask the seller to show the test certificate of the lot being sold to him. These test certificates are supplied by the firms and show the lot number being sold to you, the diameter, strength and weight of steel. It is easy to know the standard weight of the steel bars of a particular diameter. One should compare the weight of steel shown in the test certificate with the standard weight to know whether the steel is under weight or over weight.

Go by weight: While finalising the steel lot, check the weight per metre length of the steel. Steel produced has variation in diameter. Steel categorised as of 8 mm diameter may in actual have 7 mm diametre and so on. This problem generally occurs in low diameters up to 16 mm diameter. Steel bars are sold by the stockists in numbers. Each bar has 12 metre length and rate is quoted per 12 meter length for each diameter of steel bars. Proper diameter avoids structural problems.

Check the condition of bars: One should always choose steel bars free of rust. Steel is supplied in straight as well coiled bars. Always prefer to buy straight steel bars as coiled steel bars will require extra labour for straightening up.

Design, detailing and quantity: Reinforcement design, detailing and estimating essentially require the services of a structural engineer. He can work out the loads and stresses on the beams, columns and slabs and design the quantity, shape and placement of steel bars. He can also work out right quantities required for each diameter of steel by avoiding wastage.

—The writer is HoD and engineer-in-chief, civil engineering department, in a Punjab PSU

(This column is published fortnightly)