Table of Content for Steel Reinforcement Facts-
- Why use steel reinforcement in concrete?
- Types of steel reinforcement.
- Grades of reinforcement.
- Positive & Negative reinforcement
- Cover to reinforcement
- Reinforcement requirement of the structural members
1. Why use steel reinforcement in concrete?
The property of concrete to take up compressive stresses makes it a popular construction material. However, the concrete is weak in tension. As per IS456:2000, the tensile strength of concrete is 0.7√fck where fck is the characteristic strength of the concrete. Thus, for designing purposes it can be said that concrete has negligible or no tensile strength.
The structural members are subjected to both compressive and tensile strength. The reinforcement is provided in the concrete structure to take up the tension as the concrete is weak in tension. The concrete embedded with steel bars is called Reinforced Cement Concrete (RCC). The reinforcement steel bar is also called rebar.
The steel reinforcement has equal strength in tension as well as compression. The strength of the steel bar higher than that of concrete. For the same value of the load, the steel requires a much lesser area than the concrete. Thus, it can also be used to increase the load-carrying capacity of the member under compression.
The coefficient of thermal expansion of concrete (13.5×10-6 m/oC per meter) and steel (11.5×10-6 m/oC per meter) is nearly equal. Thus, the bond between both of them remains intact in case of temperature variation. This property makes the steel suitable to be embedded in the concrete.
2. Types of Steel Reinforcement-
The rebars can be majorly categorized under following heads-
2.1 High Yield Strength Deformed Bars (HYSD)-
HYSD bars are commonly used for RCC works. They are characterized by the ribs present on its surface. The ribs are formed when the hot steel is rolled through the mill. These ribs help in better bonding with the concrete. The bond strength of these bars is 40-60% higher than the mild steel bars of the same size.
The HYSD Bars don’t have a well-defined yield point on the stress-strain curve. The strength of these bars is defined by the ‘characteristic strength’ that is equal to the stress at 0.2% elongation. The HYSD bars are available in Fe415, Fe500 & Fe550 grades.
IS1786-2015 is used for cold-worked HYSD bars.
2.2 Mild Steel Bars-
Mild Steel bars are plain bars. They have lesser yield and ultimate strength than the HYSD bars. The absence of ribs on the surface causes a weaker bond with the concrete.
The mild steel bars are seldom used for the RCC works. Their use is limited to small-scale low budget projects. They can also be used to provide nominal reinforcement and ties.
The yield stress of the mild steel bar is 250N/mm2. The mild steel bars are more ductile than HYSD bars but have low strength and forms a weak bond with the concrete. The mild steel bars are found to perform better under impact loads.
IS432-1996 (Part-I) is used for cold worked mild steel bars.
2.3 Prestressing Bars-
Prestressing Bars are made of few cold worked wires. It is also called tendons. A tendon is typically made up of 2, 3, or 7 wires. The tendons are easier to lay in any desired profile and have high ultimate tensile strength.
With the advancement in technology, new forms of rebars are being produced by the companies. Some of the commonly used rebars are TMT Bars, CRS Bars, and Coated Bars.
2.4 TMT Bars-
The term TMT Bar stands for Thermo-Mechanically Treated Bar. These bars are manufactured by passing the hot rolled bar through cold water. The passing of the hot bar through cold water hardens the surface of the bar but the core remains soft. The hard surface provides better durability while the soft core provides tensile strength.
2.5 CRS Bars-
The CRS Bars are Corrosion Resistant Steel Bars. The chemical composition of these bars is made in such a way to develop an inherent anti-corrosion nature. These bars are typically used in coastal areas, marine structures, acid tanks, etc.
2.6 Coated Bars-
Steel bars coated with polymers or epoxy are also available in the market. These bars are used in places where the steel is likely to corrode.
3. Grade of Steel Reinforcement-
The grade of rebar is represented by the term ‘Fe’ followed by a number. The term Fe represents the steel and the number followed represents the characteristic strength or yield strength of the bar. The Characteristic strength of the bar is used for HYSD bar and yield strength for mild steel.
The commonly used grades of rebar are Fe415, Fe500 and Fe550.
4. Positive & Negative Steel Reinforcement-
The members are subjected to various combinations of loads. These load combinations cause bending moments. The bending moment at a point can be either sagging or hogging. The sagging moment is called a positive moment and a hogging moment is called a negative moment.
The reinforcement provided to counteract sagging moment is called positive reinforcement. The reinforcement provided to counteract the hogging moment is called negative reinforcement.
Under general loading conditions, the positive reinforcement is provided near the bottom surface of the member, and the negative reinforcement is provided near the top surface.
5. Cover to Steel Reinforcement-
The steel has one major drawback i.e., corrosion. To protect the steel from corrosion, it should be covered completely with the concrete. The difference between the concrete surface and the steel is called ‘cover’.
The cover is of two types-
5.1 Effective Cover-
The distance between the rebar center and the concrete surface is called effective cover. It is used in designing the member as it defines the lever arm.
5.2 Nominal Cover-
The clear distance between the rebar outer and the concrete surface is called nominal cover. It is also called clear cover.
The cover to reinforcement is defined by the exposure condition of the concrete. The cover increases with the severity of the exposure condition. IS456:2000 defines the five exposure conditions namely mild, moderate, severe, very severe, and extreme and respective minimum nominal cover.
For the main rebar up to 12mm diameter, for mild exposure, the minimum nominal cover can be reduced by 5mm.
For general purposes, the nominal cover can be provided as follows-
However, the cover should be as mentioned above or the max. aggregate size+5mm, whichever is lesser.
6. Reinforcement Requirement of Structural Members-
The reinforcement required for each member is calculated on the basis of the load applied. The calculated reinforcement is checked against the minimum and maximum reinforcement specified for the member. If the calculated reinforcement is lower than the minimum or higher than the maximum value, the member is redesigned by changing its dimension.
IS456:2000 defines the minimum and maximum percentage of reinforcement. It also specifies the minimum diameter of the bar, the maximum and minimum distance between the bars, reinforcement splicing, curtailment details, etc.
The specification for main bars of beam, slab and column are as follows-
The minimum reinforcement for a beam is calculated by the formula written below. The maximum reinforcement is 4% of the cross-section area.
The minimum reinforcement for mild steel is 0.15% of the cross-section area and 0.12% for the HYSD bar. The diameter of the bar shall not exceed one-eighth of the depth of the slab.
The minimum & maximum reinforcement for column are 0.8% & 6% of gross cross-section area respectively. The minimum diameter of the bar specified for the column is 12mm and the minimum number of bars for square/rectangular and circular columns are 4 & 6 respectively.
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