Mild Steel Bar, Deformed Steel Bar - Rebar Types

In the construction industry, reinforcement bars, or rebars, play a crucial role in reinforced concrete structures. They provide tensile strength, ductility, and resistance to the structure. Today, mild steel bars and deformed steel bars are the most fundamental types of rebar that are widely used. Each type differs in strength, surface characteristics, manufacturing process, and applications.

Mild Steel Bar (Plain Bar)

Smooth and round surfaces are characteristics of mild steel bars. They are manufactured from low-carbon steel and known for their ductility and ease of bending. Without any lugs or deformations, they are simple to work with during the construction. They are simply used in structures that do not experience high tensile stress because they only have a yield strength of about 250 MPa.

Due to simplicity in the form of smooth surfaces, it works against mild steel bars. It provides poor bonding with concrete that led to slippage under load. So, to compensate, it requires greater development length or end hooks to achieve a secure grip within the concrete matrix.

In light-duty construction where there is not much load involved, mild steel bar is extensively used. It found applications in residential slabs, small foundations, footpaths, pavements, and temporary structures. They are also commonly employed as distribution bars in slabs and for tie purposes in columns.

However, as the industry is evolving, the demand for mild steel rebar is declining. Rebar types with higher strength and better bonding characteristics are higher in demand.

Deformed Steel Bar

Grade	Yield Strength (MPa)	Tensile Strength (MPa)	Elongation (%)	Typical Use	Features
Fe 415	415	≥ 485	≥ 14.5	Residential buildings, low-rise structures	Good ductility, suitable for seismic zones
Fe 500	500	≥ 545	≥ 12	Commercial buildings, bridges, industrial units	Higher strength than Fe 415, widely used in modern RCC
Fe 500D	500	≥ 565	≥ 16	Earthquake-prone zones, high-ductility needs	“D” stands for Ductile, better elongation properties
Fe 550	550	≥ 585	≥ 10	Large infrastructure like dams, flyovers	Higher load capacity, slightly lower ductility
Fe 550D	550	≥ 600	≥ 14	High-rise buildings, long-span bridges	Better combination of strength and ductility
Fe 600	600	≥ 660	≥ 10	Mega infrastructure, high load areas	Very high strength, requires careful design
TMT Bars	Varies (Fe 415–Fe 600)	As per grade	As per grade	General RCC use	Thermo-Mechanically Treated bars with enhanced properties
CRS Bars	As per grade	As per grade	As per grade	Coastal & marine structures	Corrosion Resistant Steel, designed for aggressive environments

Surface protrusions such as ribs, lugs, or indentations are characteristics of deformed steel bars. These deformations are designed to enhance the bond strength with concrete. It reduces the risk of slippage and is ideal for structural components that are subject to high loads and stresses. In modern construction, this type of rebar has become the standard choice in reinforced concrete construction, replacing mild steel bars.

There are two main types of deformed steel bars used in construction: TMT (Thermo Mechanically Treated) bars and HSD (High Strength Deformed) bars. Each type has its own manufacturing process, mechanical properties, and recommended applications.

a. TMT Bars (Thermo Mechanically Treated Bars)

Rapid quenching and tempering of bars in a controlled process produced TMT bars. The outer layer is cooled down quickly after rolling and forming a tough martensitic surface, while the core remains soft and ductile. TMT bars provide an excellent combination of high tensile strength, ductility, and thermal resistance.

TMT bars are corrosion-resistant due to the absence of surface stress or twisting, unlike older cold-worked bars. They are also earthquake-resistant, thanks to their ability to absorb energy through high elongation.

TMT bars are available in different grades, each denoting a specific yield strength:

b. HSD Bars (High Strength Deformed Bars)

HSD bars, also known as cold-twisted deformed (CTD) bars, are manufactured by hot-rolling followed by twisting the bars when cold to increase their yield strength. This mechanical treatment improves their strength but also introduces residual surface stresses that can lead to brittleness and corrosion over time.
HSD bars typically offer yield strengths in the range of 415 MPa to 500 MPa, similar to lower-grade TMT bars. However, the twisting process can make them less ductile and more prone to cracking, especially in seismic zones.

Due to these limitations, the use of HSD bars has diminished with the growing popularity of TMT bars, which provide higher strength, better ductility, and enhanced corrosion resistance without compromising safety. Nevertheless, HSD bars are still used in some regions or projects where cost is a significant constraint or TMT bars are not readily available.

Conclusion

In modern construction, the choice between different types of rebar can significantly affect the structural performance, safety, and longevity of a project. Mild steel bars, while easy to handle, are limited to light and temporary structures due to their low strength and poor bonding. On the other hand, deformed steel bars—especially TMT bars—have become the industry standard for reinforced concrete works due to their high tensile strength, superior ductility, and excellent bonding characteristics. HSD bars, once widely used, are now largely being replaced by TMT bars owing to advancements in material science and construction standards.

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