What Is Rebar?
Rebar is the shortest form of reinforcing bar usually made of steel and used to increase the tensile strength of concrete. We know concrete is sufficiently strong in compression but very weak in tension. By providing reinforcement to concrete, the overall performance of concrete structures under loaded conditions is improved. In this article, we will discuss different types of rebar used in steel reinforcement.
Why We Use Steel As Rebar In Concrete?
Following are the reasons why steel is used as a reinforcing bar.
i) During setting, cement concrete shrinks and grips the rebars perfectly. This effect develops a perfect bond between concrete and steel and enables concrete to transfer its stresses to steel after it has cracked.
ii) The coefficient of linear expansion of steel is almost the same as concrete. This helps to prevent setting up internal stresses within RCC members due to the increase or decrease of temperature.
iii) Tensile strength of steel is very high.
iv) Coating of cement concrete around steel bars protects the steel from corrosion and other atmospheric effects.
iii) Steel is the cheapest and easily available material.
Types Of Rebar:
1. Mild Steel Bar:
These bars have a plain and smooth surface without any ribs. The diameter of mild steel bars varies from 6 mm to 50 mm. These bars cannot develop a good bond with concrete hence hooks are essentially required at the ends.
Mild steel is generally classified into two grades: Grade I and Grade II. Mechanical properties of mild steel bars are given below:
Steel that exceeds the minimum ultimate tensile strength of 550 N/mm2 and 10000 N/mm2 are respectively known as medium tensile steel and high tensile steel.
Mild steel bars are used in small projects where the budget is low. With the introduction of deformed and twisted bars, the use of mild steel bars in construction is rapidly decreasing.
They are now only used in column spirals, dowels at expansion joints, contraction joints in roads, and runways.
2. Deformed Steel Bar:
Deformed steel bars have lugs, ribs, and other deformations on their surface which increase the bond strength between steel and concrete.
Deformed bars have high tensile strength, yield strength, and bond strength. Since deformed bars have greater bond strength than plain mild steel bars, hooks are not required at their ends.
Types Of Deformed Steel Bars:
The deformed steel bars are classified into two types of rebar.
- TMT bars,
- Cold twisted bars.
a) TMT bars:
TMT or thermo mechanical treatment is an advanced process in which red hot rebars are quenched through a series of water jets making a hardened outer layer surface surrounding a softer core.
Characteristics of TMT bars:
- High yield strength and ultimate tensile strength.
- Better ductility and elongation.
- Greater bond strength
- Better weldability
- High thermal resistance
- Corrosion resistance
- Earthquake resistance
TMT bar is the latest invention in the steel industry. These types of rebar are widely used as reinforcement for the construction of buildings, bridges, roads, and all other construction work.
b) Cold Twisted Bars:
Cold twisted bars (CTD) are high-strength deformed bars (HSD) whose quality is improved by various processes of cold working (stretching and twisting).
CTD bars have considerable large yield stress about 50% more than ordinary mild steel bars. The sizes of these rebars vary from 4mm to 50 mm. In India, these bars are known as TOR steel.
This type of rebar gives higher working stresses and saves the quantity of steel. One major drawback, there are surface stress and visible cracks due to cold twisting which leads to lower ductility, higher corrosion rate, and a reduction in elongation percent from 24% to 13%.
Characteristic Of HSD/CTD Bars:
i) Great Bond Strength: These bars have excellent bonding strength with concrete hence the end hooks are not required.
ii) High Tensile Strength: CTD bars possess high tensile strength.
iv) Low Carbon Content: Lower carbon content makes it ductile.
iii) Welding Capability: Welding is possible due to its lower carbon value.
iv) Better functional aspects & structural safety.
v) Less Steel Consumption: Construction with these bars consumes 30-40% less steel compared to other types of rebars.
vi) Economical: As it consumes less steel it is economical.
These bars are commonly used as reinforcement in concrete structures such as buildings, bridges, roads, precast concrete, water-retaining structures, etc.
Mechanical Properties Of HSD Bars:
3. Carbon Steel Rebar:
This is the most common type of rebar made up of carbon steel and is sometimes referred to as a black bar due to its carbon color.
One major drawback is that it corrodes easily, which makes it unsuitable in high humid areas or structures that are frequently exposed to water.
However, carbon steel rebars are one of the best choices for all other types of structures.
4. Epoxy Coated Rebar:
Epoxy coated rebar is nothing but black steel rebar having an epoxy coat over it. The epoxy coating is applied in the factory using an electrostatic spray.
This type of rebar is ideal for structures that are exposed to saltwater or where there is an increased risk of corrosion.
The epoxy coating increases the corrosion resistance from 70 to 1,700 times that of black bar.
However, it is very delicate and easy to being damaged which reduces the efficiency of the epoxy coating.
These green rebars are suitably used in marine structures, bridges, infrastructures, buildings, roadways, etc
5. Stainless Steel Rebars:
Stainless steel rebar is one of the best types of rebar used where high corrosion resistance is desirable or where repair is difficult and expensive.
However, these rebars are at least 8 times more costlier than epoxy-coated rebars. Some of the advantages of stainless steel rebars are
i) Highly durable and long long life,
ii) 1500 times more corrosion resistant than black bars.
iii) No maintenance is required.
iv) Do not get damaged during shipping or installation.
6. Galvanized Rebars:
In this rebar, a protective layer of Zinc (Zn) metal is added. Galvanized rebars have 40 times more corrosion resistance than carbon steel rebars.
However, they are costlier than black bars. These bars are suitable for structures that will be extremely exposed to moisture and humid conditions.
7. European Rebars:
These types of rebars are typically made of manganese so they are easily bendable. European rebars are not suitable in extreme weather conditions and natural disaster-prone areas such as earthquakes, tornadoes, hurricanes, etc.
8. Glass-Fiber-Reinforced-Polymer (GFRP) Rebars:
These are fiberglass rebars, sometimes also referred to as fiberglass-reinforced polymer rebar. They are typically made of carbon fiber. Some of the advantages of GFRP are
i) It will never corrode from exposure to salts or any other conditions.
ii) These bars possess higher tensile strength than steel.
iii) Better non-conductive electrical properties and thermal insulator properties than steel rebar
iv) Saves material handling and shipping costs due to its lower weight.
9. Welded Wire Fabric:
This rebar type is made from carbon or steel wires that are arranged at 90-degree angles and welded at all intersections. It will look like a square grid pattern or mesh.
WWF is ideal for well-compacted floor slabs, roads, sidewalks, small concrete pads, runway pavements, culverts, and small canal linings.