By Faisal Mohammed B. Tech (CIVIL)
This article is written purely based on my research on the topic of steel and its applications in the construction field. Not being a Civil Structural Engineer, I understand and accept that my knowledge in this topic is limited to my research and I welcome any correction or improvisation from the audience to the details I share through this article.
As the most abundantly used metal, steel is considered the backbone in the construction of any structural building. And hence, the quality of the steel used while construction is the ultimate decisive factor for the sustainability of the structure, built with it.
Despite being the most cautiously chosen amenity, we usually take little pain in the selection of the brand, quality, or type of steel while constructing our building.
History of Steel Reinforcement
Historically, there is lots of evidence for the usage of iron in the construction of houses from the 15th century AD. But the iron metal was known to be used for several other purposes even centuries before Christ, and for the same reason, there is no clarity about the founder of the metal.
In 1849, a French gardener named Mr. Joseph Monier invented Reinforced Concrete by using iron mesh while making concrete pots.
In 1853, Mr. Francois Coignet used steel for the first time while constructing a four-storeyed building in Paris.
By the mid 19th century, the process of steel Reinforced Concrete Construction gained popularity, all around the globe.
There are 2 types of steels bars available in the market based on their manufacturing:
- Mild Steel Bars
Mild Steel bars are manufactured according to IS 432 Part 1, as flat and long bars. The advantages of such bars are that It is easier to bend and cut them when compared with the Deformed steel bars. They were widely used for building construction till the introduction of the TMT bars. Later, they lost their popularity due to their flat and grip-less surface which reduces the concrete bonding.
- Deformed Steel Bars
These types of steel bars are manufactured according to the IS1786 and are widely used for construction purposes in our area. Their main advantages are that it has vertical and horizontal rings (Lugs and ribs) around them. This feature enhances their grip that increases their capability for concrete bonding and even helps the structure to withstand earthquakes.
Deformed bars have more tensile strength when compared to the Mild Steel bars.
There are mainly 3 types of Deformed steel bars used in the construction industry. The latter two are not popular in our region and hence avoiding further descriptions.
- TMT steel bars
- High strength Deformed steel bars.
- Stainless steel bars.
TMT Steel bars (Thermo Mechanically Treated steel bars):
Though we have come across the claims from most companies that the steel bars they manufacture and supply are TMT Fe 500 or TME Fe 500 D steel bars, I wonder how many of us are familiar with or know exactly about these specifications.
TMT steel bars are manufactured using a unique method called Thermo Mechanical Treatment. They are accredited by the Bureau of Indian Standards for being used in the construction of buildings and other structures. Fe 500, Fe 500 D, Fe 550 D are among the best TMT bars available in India. Even though there are bars with lesser specifications like Fe 450 available in the market, it is always advisable to opt for Fe 500 or Fe 500 D or above.
TMT is the abbreviation for Thermo Mechanical Treated.
Steel gains optimum strength through a process called Cold Twisted Technology (CTT), where it is heated to a peak temperature and then chilled to a very low temperature later. The bars become flexible after this process so that they can even tolerate a 180-degree bend. The bars thus manufactured are called TMT steel bars.
Chemical Composition of TMT steel bars:
In Fe 500 and Fe 500 D, Fe denotes the chemical name of the metal – iron. 500 denotes the minimum yield stress in N/mm2 and the letter D denotes the higher values of ductility of such bars. The amount of phosphorus and sulphur will be minimum in TMT steel bars, the presence of which will result in degrading the quality of steel.
The property of elongation for Fe 500 D is 16% as compared to 12% in Fe 500.
The tensile strength of Fe 500 D is 565 Mpa whereas it is only 545 for Fe 500.
Therefore, it is highly recommended to use Fe 500 D only while constructing our dream home to get the advantage of the following reasons:
- Welding strength
- Earthquake resistance
- Temperature resistance
- Concrete bonding
- Rust resistance
The following are the tests conducted for the quality testing of the steel bars:
- Tension test
- Torsion Test
- Bend test
- Rebend test
- Hardness test
- Impact test
- Yield test
The requirement of steel for a concrete structure mainly depends on the kind of structure we build. Generally, for a slab structure, it is assumed that it should be a minimum of 78.50kg/m3. (1%(minimum needed) = (1/100) x 7850 (Density of steel) = 78.50 kg.)