What is Rolling Margin?
Rolling margin is the difference between the actual weight of round bars (Reinforcement) Vs. Theoretical weight of round bars.
This theoretical weight of round bars pick in IS Codes.
IS Codes 1786: 2008 ( High Strength Deformed Steel Bars and Wires for Concrete Reinforcement, Fourth Revision), this picks in Table-1 Nominal Cross-Section Area and mass, ( Dia Bars: 4mm, 5mm, 6mm, 8mm, 10mm, 12mm, 16mm, 20mm, 25mm, 28mm, 32mm, 36mm, 40mm).
This is because of the dia, which is used for casting of rebars. Over a period of time, the shape of dia changes, and as a result, the diameter of the steel bar also changes, which results in a difference in a unit weight of steel.
The size of the die is kept small, and as a result, the rolling margin is negative in the initial stage of production, there after it is more or less close to theoretical weight and then the size of die increases, which results in a rolling margin on the positive side.
As per IS code tolerance for reinforcement as below
As per above tolerance of reinforcement weight
1. Up to and including 10 mins: 4 mm to 10 mm dia of a bar, Individual weight of Up and Down 8%, The Batch weight of Up and Down 7%.
2. Over 10 up to and including 16 mins: 12 mm dia bar to 16 mm dia bar reinforcement, individual weight of Up and Down 6%, The Batch weight of Up and Down 5%.
3. Over 16 mins: 20 mm dia bar to 40mm dia bar reinforcement, individual weight of Up and Down 4%, The Batch weight of Up and Down 3%.
The Procedure of Rolling Margin
The above format is used in a rolling margin for reinforcement.
• In this method, one dia of bar minimum three sample requirement, Each sample length minimum 1 m. (1000 mm)
• All the above three sample take the induvial weight, Heter correct length because of cutting of reinforcement not perfect length (Example: 990 mm, 995 mm, 996 mm)
• So Convert in 1000mm length in weight Like 10 mm dia bar 990 length steel weight 0.598 kg, After converting 1000mm length steel weight 0.604 kg, all three weight fill in a block No. 8
• And block No. 9 actual weight of IS 1786, page- 5, table-1 consider
• Block No 10 is a difference between block no. 8 and block no. 9
• Here, Block no 11 is a negative all there sample average difference. Same as block no 12 positive block all three difference.
Why is Rolling Margin?
Rolling margin is a difference “-“and “+” depending on the tolerance of as per IS code 1786 : 2008. Here rolling margin show round bar is uber the weight or over the weight.