Strength Vs. Ductility



Steel is a metal with great tensile strength and ductility. It can extend these qualities to concrete. In other words, it reinforces concrete. Steel in the form of bars has been doing this job for more than a century now. Naturally, the quality of steel has an important role in deciding the quality of concrete. Research has periodically produced various kinds of steel bars with qualities that make concrete more stable. It started with mild steel (MS); TOR steel and various grades of TMT followed.
Indian standards specify that the MS bars have a tensile strength of 250MPa. TOR steel is required to have 415MPa. This increase in power enhances the factor of safety in construction by a huge margin. Even today, many buildings are designed assuming a strength of 250MPa for Fe 415, higher-grade steel. Hence buildings built with Fe 415 grade steel provide a factor of safety that is much higher than what is required. But the higher strength of Fe 415 grade TOR steel rebars brings down ductility, or the ability to retain shape even under pressure, which is also a major requirement of steel reinforcement.

The TMT advantage

The advent of TMT bars helped overcome this handicap. In the TMT process, the hot steel rebars coming from the rolling mill are rapidly quenched for a short time in a water jet. Thus, the outer layer of the rebar is cooled, forming a martensite structure with high strength. The rebar is removed from the water jet quickly, leaving the core still hot and soft. Hence the rebars are endowed with a soft inner core with high ductility and a hard outer layer that provides it high strength.
The high ductility makes it easy to work with and gives the buildings the ability to withstand shocks such as earthquakes. Such rebars also have the advantage of being thinner, thereby reducing the quantity of steel required and saving space. Fe 415 TMT bars with much higher strength than mild steel bars provide an extra factor of safety to the design. And so, Fe 415 TMT bars became the most commonly used steel grade in the construction of houses in the State. Fe 415 may be the de facto standard in construction but specialized applications call for special products. Various other grades of steel – from Fe 500 to Fe 600 – are available today, to suit different needs.

Know your choices

High-rise buildings call for steel of higher strength, and hence higher grades of TMT steel – Fe 500 and above – are used in such buildings. This is because using Fe 415 steel will require a large number of rebars to provide the required strength. That would increase the size of the columns. This is a serious concern at a time when space is at a premium – else why should one go for high rises? Fe 500 is produced using the same TMT process that is used to produce Fe The beauty of the TMT process is that it can produce different grades of steel by making slight changes in the process. If the steel is quenched a little more, the outer martensite layer that provides the high strength to the steel re-bar becomes thicker at the expense of the soft inner core that endows it with ductility. This is exactly what happens in Fe 500-grade TMT steel. Many engineers recommend Fe500 grade steel today for ordinary buildings, too, citing its high strength.
If you design buildings considering the use of Fe 500 steel in construction, it could help reduce the volume of steel used and reduce the column size. And the savings on this count could more than compensate for the higher price of Fe 500 grade steel. The rosy part aside, Fe 500 grade steel could pose quite a few site-specific issues during construction, especially for small builders. Higher strength and lower ductility mean that Fe 500 bars do not bend easily. For example, a 12 mm rebar of Fe 500 breaks when bent into a perfect ‘U’ shape with zero radii, unlike similar rebar of Fe-415 grade. It might cause it to crack, too. Manual bending takes its toll on the masons. Hence hydraulic bending machines have to be used to bend the bar. Weldability too is an issue with Fe 500-grade steels.
The builders of most high rises that use Fe 500 grade rebars procure them in factory cut sizes, avoiding the need to work on them further. Their designers provide them with the steel detail that helps them procure items on the shelf. Small builders may not have such luxuries. In the first place, their design might not be for Fe 500 steel, negating the savings in the quantity of steel. Even if their design is for Fe 500 steel, they might not be able to take the due advantage due to a variety of reasons. They might not have access to a steel detail that gives the precise number of various types of structural steel elements needed for the building. Even if they have the steel detail, and can buy factory-cut steel bars based on it, transporting them to work sites in the interiors through roads that hardly allow a truck to pass is a difficult task. That is the main reason why factory-cut steel has not been picked up in Kerala. In such a situation they will have to resort to fashioning the required steel elements at the work site itself. And then, an absence of the machinery required to bend the Fe 500 rebars would lead to an increase in labor costs and a decline in quality. This is especially true if the rebar has to be shaped into tight curves. In, short, they would have to incur the extra cost without getting the perceived benefits. All this when a Fe 415 TMT rebar would have more than satisfied their requirements. A Fe 415 bar would be an optimal choice for normal buildings due to its right combination of strength and ductility. Adding more strength at the cost of ductility might not be the best solution for your dream home. In fact, IS:13920, the code of practice for ductile detailing for structures regarding seismic forces, recommends steel reinforcements of grade Fe 415 or less. Only select grades of Fe500 rebars have an elongation of more than 14.5 percent, against the normal 12 percent which can be used for the purpose. One should also be on the lookout for fraudsters who sell other grades of steel under the Fe 500 label.
The increasing demand for Fe 500 grade steel in the market and the inability to solve the problems associated with it provides ample room for such fraudsters. If the Fe 500 steel you bought bends easily and offers no issues with workability, you might have been taken for a ride. After all, India is a country where the government has not yet been able to mandate ISI certification for steel makers. So, before you set out to purchase steel for your dream home, assure yourself that you go for the right grade. If you have a premium for space, a design that has factored in the use of Fe 500 steel, and have access to factory-cut steel, it is all for you. For the rest, Fe 415 will be the choice. And when in doubt, ask your engineer.
IS:1786 (Fe 415)
Yield Stress,N/mm2: 415 (minimum)
Tensile Strength,N/mm2:10% over YS
Elongation ,%min:14.5(minimum)
IS:1786 (Fe 500)
Yield Stress,N/mm2: 500 (minimum)
Tensile Strength,N/mm2:10% over YS

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