Most of the reinforced concrete structures won’t be provided with full length reinforced bars. The manufacture and transportation of long bars are difficult, which limit the use of full length reinforced bars.
The method used to join the reinforcement bars, so that the force is transferred effectively from one bar to the other is called as splicing. The integrity of the concrete structure is dependent on proper splicing of the reinforcement bars.
The forces are transferred from one bar to the other through bonds in concrete. Force is first transferred to the concrete through bond from one bar and then it is transferred to the other bar forming the splice through bond between it and concrete. Thus, concrete at the point of splicing is subjected to high shear and splitting stresses which may cause cracks in concrete. A properly designed splicing is the key element in transmitting the forces through the reinforcement bars by creating a proper load path.
Methods of Reinforcement Splicing
- Lap Splice
- Mechanical Splice
- Welded Splice
In India, the requirement of reinforcement bar splicing is covered in IS456 cl.25.2.5. The code also specifies that the splicing of flexural members should not be at sections where the bending moment is more than 50% of the moment of resistance, and not more than 50% of reinforcement bars should be spliced at any given section. The splicing of bars should be carried out for alternate bars if more than one bar has to be spliced.
The lap splice is the most common and economic splice employed in the construction. The welded splices and mechanical splices require more labour and skill compared to lap splicing.
The important points to be noted while providing lap splices in reinforcement bars:
- Laps in reinforcement should always be staggered. The center to center distances of laps should not be less than 1.3 times the required lap length of the bars. The bars to be lapped should be provided either vertically one above the other or horizontally one beside the other.
- The total lap length of bars including bends, hooks etc. in flexural tension should not be less than 30 times the diameter of the bar of the full development length Ld as calculated, whichever is greater.
- Lap length in direct tension should be 30 times diameter of bar (30) or 2 Ld whichever is greater. Tension splices should be enclosed in spirals made from 6mm bars with pitch not more than 100mm. Hooks are also to be provided at the end of the tension bars.
- Lap length is compression should be more than 24 or Ld in compression. When columns are subjected to bending, lap length may be also increased to the value in bending tension if the bar is found to be in tension.
- When two different diameters of bars have to be lapped, the lap length should be calculated based on the diameter of the smaller bar.
- Lap splicing of reinforcement bar more than 36mm in diameter should be avoided. In case such bars have to be lapped then they should be welded. When welding of cold bars is allowed, the special instructions applicable to these bars should be followed.
- Where the lapping of reinforcement bars have to be done in unusual circumstances such as splicing in areas of large moments or more than 50% of the bars have to be spliced, additional closely spaced spirals should be provided around the lapped bars and the length of lap should be increased.
- When bundled bars are to be spliced by lapping, one reinforcement bar at a time is to be spliced and the splicing should be staggered.
- If the general rules regarding laps cannot be obeyed in a construction, special welded splices or mechanical connections should be provided (Cl. 22.214.171.124 of IS 456).
- The use of lap splices will cause splice congestion issues that will demand for some other splicing method. Congestion of rebars will create critical stress points in rebars, difficulty for concrete to pass through, the splice length would be insufficient.
A mechanical splice or a joint make use of a coupler or a sleeve to splice two reinforcement bars. The mechanical splicing is a new type of splicing in Indian construction Industry.
The mechanical splice has many advantages compared to the conventional method of overlapping. Some of them are:
- A continuous reinforcement bar is obtained through this coupler splicing. The errors due to wrong lap length as in conventional method is avoided.
- The wastage of steel is reduced. Use of a mechanical joint helps to avoid lap length. This will save appreciable amount of steel.
- We can use couplers as dowel bars. This will save the formwork material.
- The mechanical joints will not create any steel congestion as the lapping of bars are eliminated.
- Couplers provides greater flexibility for the designers.
- The strength at the joint is easily analysed in the case of mechanical splice compared to conventional lap joint.
Mechanical couplers are the most commonly used mechanical splice or joint for reinforcement. The mechanical couplers can be of two types:
- Threaded Couplers
- Non-threaded couplers
Threaded Couplers: The threaded couplers are classified into two:
- Tapered Threaded Couplers: This type of tapered couplers are fitted to one end of the threaded rebar and the adjoining rebar is connected and tightened by means of a calibrated torque wrench. The procedure is performed at the site.
- Roll Threaded Couplers: In this type, the bars that are to be connected is pressed by a set of rollers. These pressed ends are connected by a couplers with matching and parallel threads.
Non-threaded Couplers: This type of couplers have variety of types which are used in areas where the threaded couplers cannot be employed. The different types are:
- Bolted Couplers
- Friction weld couplers
- Welded Couplers
- Swage Couplers
The main application of non-threaded couplers are in repair works than new construction works. These are higher in cost compared to threaded couplers and hence are not extensively employed. The couplers used for this process are bulky and installation process is slow.
Welded splicing is not commonly used as it may affect the strength of the reinforcement bars. Special conditions and rules are followed for welded splicing.
- The welded splicing is used for rebars with diameter greater than 36mm.
- If skilled labour is not there, the method is avoided
- Areas where there is rebar congestion and require greater moment strength, this method is adopted.
- If we make use of bars that have bad specifications for weldability, this method is not adopted.
Before undergoing welded reinforcement, it is necessary to have a proper chemical analysis of the steel rebars, field inspection, the quality of the steel and proper supervision.