The word rebar is a short form of reinforcing bar. In general, its meaning is the use of steel rod with ridges for reinforced concrete. Basically, concrete is brittle material which is strong in compression but very weak in tension. Whenever tensile forces act on concrete it damages concrete by originating cracks in it. So, to avoid failure of concrete in tenson, steel bars are provided in concrete which are called reinforced bars or rebars.
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Why only steel is used for rebars?
Steel is only used as a reinforcing bar instead of any other material because of following characteristics:
- Tensile Strength of steel is very high.
- Coefficient of Thermal expansion of steel is nearly equal to that of concrete which prevents originating of cracks due to increase or decrease in temperature.
- When concrete sets it develops strong grip with concrete which provide strong bond of concrete with steel bars.
- It is the easily available material.
How Rebars are made?
Rebars are made in following way:
- Scrap Steel: In this process scrap steel is collected from different sources and then placed in ladle at steel mill where it is heated at temperature 1800-degree Fahrenheit till it get melted. This molten steel is then placed in casting.
- Molten Steel: Molten steel is poured into tundish which on drying gets the steel shape. The shape formed is known as billets which will use in later stages to ger rebars.
- Rough Mill Section: Rebars removed from casting are move into roughing mill where large billets hot rolled into small shape by reducing the billet at each strand on roughing portion of the mill.
- Intermediate and Finishing Sections: After passing through rough mill section, it moves through intermediate section of rolling mill. As it goes through strand, billets begin to get shape of bars. After intermediate section, the bars are passed through finished section where bars get its final shapes.
- Cut to Sze: After finished section, rebars are entered into cutting section where they are cut down and then placed into field for proper cooling.
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Types of steel bars pdf
Following type of steel reinforcement bars are used:
Mild Steel Bar:
These are plain and smooth steel bars without ribs on their surface. These bars are available in various sizes from 6mm to 50mm dia. There are not used in high rise building or in high loaded member because these plain bars do not make strong bars with concrete because of absence of ribs on surface. While these bars are used for special purposes like dowels at expansion joints, for contraction joints in roads or highways and for spirals in column. As, these are plain bars so they need hook at their end.
Grades of Mild Steel Bar:
Mild steel bars are available in following grades:
- Grade 1 which is designated as Fe-410s or Grade 60
- Grade-II which is designated as Fe-410-o or Grade 40
While its also available in grade for medium tensile steel bar like in Grade75 or Fe-540-w
Properties and specification of Mild steel bar:
Mild steel bars are characterized with following properties:
|Sr. No.||Type and nominal size of bar||Ultimate tensile stress N/mm2||Yield Stress
|1||Mild Steel Grade I
a. For bars up to and including 20mm
|b. For bars over 20mm and including 50mm||410||240||23
|2||Mild Steel Grade II
a. For bars upto and including 20mm
|b. For bars over 20mm and including 50mm||370||215||23|
|3||Medium tensile steel
a. For bars upto and included 16 mm
|b. for bars over 16mm, upto and included 32 mm||
|c. For bars over 32 mm upto and included 50mm||
Deformed Steel Bars:
These bars consist high strength as compared to smooth mild steel bars because of presence of lugs, ribs or deformation on their surface which prevents slippage of bars and helps in having strong bond with concrete. These bars make strong bond because when concrete gets dry it makes strong grip with ribs and interlocked bars which results in strong bond.
These bars are available in sizes from 6mm to 50mm diameter and use as reinforcement in heavy loaded structures, in ridges, buildings, roads, precast members and in water retaining structures. These bars have high tensile strength, yield strength and bond strength. Because of presence of ribs on their surface they do not need hooks at the end.
Types of deformed steel bars:
- TMT (Tharmo mechanical treatment) bars: These are hot treated bars that are high in strength and used as rebars. These are manufactured through advance process in which red hot rebars are quenched through water jet as a result hardened outer surface obtains that surrounds softer surface.
These bars consist of qualities that are superior to any other bars. These qualities include yield strength, tensile strength, bond strength, ductility, thermal resistance and economical.
- Cold Twisted Bars: These are high strength deformed bars that consists of qualities that are improved by various process of cold working. These bars are available in sizes from 4m to 50mm dia. These bars consist of 50% more tensile strength as compared to normal mild steel bar. These bars give high working stresses thus save quantity of steel that is needed.
Other Steel Bars:
- Carbon Steel Rebars: These bars are manufactured by using high carbon steel and called as black carbon because of black in color. These bars are environment sensitive that it gets corrosion easily whenever get contacts with water. Therefore, these bars are not used in humid regions.
- Stainless Steel Rebars: These bars are highly corrosive resistant bars that are used in region were there are more chances of corrosion. These bars are highly expensive bars and consists properties of high ductility, corrosive resistivity, and not damaged during shipping.
- Galvanized Steel Rebars: These bars consist of protective layer of zinc over its surface. These are also highly corrosive resistant and use in humid region and for structure that are more exposed in water.
- Glass Fiber Reinforced Polymer Rebars: These bars are made up of carbon fibers and consists of more tensile strength. These bars do not get corrosion whenever exposed to salt or any other condition.
- Prestressed Steel Rebars: These are steel bars thar are used as strand or tendons. In this type of rebars, strand that are made of multiple wires are employed into concrete to perform prestressing action. These wire strands are cold deformed and consists of high tensile strength in the range of 250000 to 270000 psi.
Different Sizes of rebars in structural members
Steel bars are available in different sizes and their diameter is different for different type of structural members which is shown below.
- In column, longitudinal reinforcement is provided which is called main reinforcement and transverse reinforcement is provided in the form of stirrups whose specifications are given below:
- Amount of reinforcement use in column 3-6% of gross section area.
- Minimum diameter of main reinforced bar is 12mm.
- Diameter of transverse reinforcement: It should be 1/4th diameter of main steel bar and should not be less than 7mm.
- Spacing: Spacing in rebars should not be more than 300mm.
- Diameter of Tie bar in Column: It should be more than 5mm and also 1/4th diameter of main bar.
- Cover Provided: To avoid corrosion, steel bars are provided cover of 40mm in column.
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- Amount of reinforcement in slab:
- When high strength deformed bars are used than the area of reinforced steel will be 0.12-4% of gross section area.
- When normal mild steel bar used than area of rebars will be 0.15-4% of gross section area.
- Diameter of rebars use in slab:
- For main bars, diameter used should be 8 to 16mm.
- For transverse rebars, diameter used should be 6 to 8mm.
- Diameter of rebars should not be more than 1/8th of slab thickness.
- Cover Provided for rebars: Cover provided for rebars in slab is
- Amount of Reinforcement in beam: Area of reinforced steel bars in slab is 0.15 to 4% of gross area.
- Diameter of rebars: It should be 8 to 16mm.
- Cover Provided: It should not be more than 15mm.
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- Amount of Rebars: It shares include 0.7 to 4.2% of total structural area.
- Cover Provided: In Footing concrete cover provided is 150mm.
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