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Conventional way of rebar lapping / splicing as per BIS
As per BIS456-2000 some guidelines on lapping/splicing are as follows:
The guideline of lap length / development length of rebar required for lapping / splicing is as follows:
|Grade of RCC||M20||M25||M30||M35|
|Grade of Rebar||Lap length required in tension as per Cl. 26.2.1 of BIS456-2000:|
|Fe500||57 x d||50 x d||45 x d||40 x d|
|Fe600||68 x d||60 x d||55 x d||48 x d|
Note: d = nominal diameter of HYSD bar
Some disadvantages of this conventional method of rebar lapping / splicing:
The above table along with photo 1 clearly indicates how much lap length we need to provide for lapping, conversely how much rebar and binding wires we might save if we might use mechanical splicing for joining the rebar.
More lapping of rebar causes congestion, and if vibrator doesn’t percolate through the congestion and compact the surrounding concrete around the splice/lap location, it is likely to create honeycomb or voids in concrete, as shown in photo 2. That is one of the main reasons that although 6% rebar is permissible in columns as per Cl. 126.96.36.199 of BIS456-2000, not more than 4% rebar is advised as per NOTE of the clause.
As per Cl. 188.8.131.52 of BIS456-2000, at splice location longitudinal bars in a column are required to be inclined with the axis of column at a slope not exceeding 1 in 6, as shown in photo 3. Such profile is difficult to fabricate accurately without automated bending machine, which incur huge hire charges. Moreover such fabrication involves a lot of labour dependency and time engagement.
Now-a-days mechanical splicing system technology is developed where rebar couplers are designed to join reinforcement bars of dia ranging from 12mm to 40mm dia. The traditional method of connecting reinforcement bars with lap joints continuity in reinforced concrete need not be always appropriate. Advantages of simplicity and economy in lap splicing are limited to smaller diameter bars. Mechanical couplers offer the solution for splicing when large diameter bars are involved.
The processes and machines involved in rebar coupling process are as follows in photo 4:
The advantages of mechanical splicing system (shown in photo 5) over conventional rebar lapping system are as follows:
|Table for Cost Comparison between Rebar Coupler and conventional Lapping Process for a single lap|
|Bar Dia. d (mm)||Average Lap length for comparison = 50xd (m)||Weight of lapping (Kg)||Rebar Cost/Kg (Rs)||Rebar Cost (Rs)||Binding wire for lapping @ 5Kg/T (Kg)||Binding wire cost including tax (Rs)||Binding wire Cost (Rs)||Labour cost for bending @ Rs2/Kg||Total Lapping Cost (Rs)||Coupler Rate + rebar threading (Rs)||SAVINGS (Rs)|
Photo 5 : Mechanical splicing (rebar couplers) used in various projects