Rebar Coupler / Mechanical Splice Manual
Professor Dr. Engr. Md. Jahangir Alam
Former Professor, Department of Civil Engineering, BUET
What is a Rebar Coupler / Mechanical Splice?
Steel rods (steel rebars or reinforcements) are used in the construction of buildings or other structures as reinforcement to enhance the strength of concrete. In columns and beams, rebars must be joined together or spliced together. Rebar coupler is one of the best methods to join rods. It is also called mechanical splice. A rebar coupler (also known as a mechanical splice) is a type of strong nut used to connect rods. The rods are threaded on both ends, and the rebar coupler joins them. No overlapping of rods is required in this case.
Methods of Splicing Rods
- There are three methods rebar splicing:
Lapping: Rods are joined by overlapping them at the joint. The length of the overlap is 50-70 times the diameter of the rod. The larger the rod, the greater the length of the overlap. - Rebar Coupler: A type of nut used to join rods. Threads are cut on both ends of the rods, and the rebar coupler is used to join them. No overlapping of rods is needed.
- Welding: Two rods are joined by overlapping and welding them. The overlap length required is less than that for lapping. However, welding can sometimes degrade the quality of the rod, so it is essential to ensure that the rod type is suitable for welding.
Among these three methods, rebar coupler is the most economical, easy, and earthquake-resistant method.
Methods of Splicing Rods
- There are three methods rebar splicing:
Lapping: Rods are joined by overlapping them at the joint. The length of the overlap is 50-70 times the diameter of the rod. The larger the rod, the greater the length of the overlap. - Rebar Coupler: A type of nut used to join rods. Threads are cut on both ends of the rods, and the rebar coupler is used to join them. No overlapping of rods is needed.
- Welding: Two rods are joined by overlapping and welding them. The overlap length required is less than that for lapping. However, welding can sometimes degrade the quality of the rod, so it is essential to ensure that the rod type is suitable for welding.
Among these three methods, rebar coupler is the most economical, easy, and earthquake-resistant method.
Advantages of Using Rebar Couplers:
- Cost-Effective: Using rebar couplers eliminates the need for the considerable length of overlap required for lapping. The larger the diameter of rebar, the greater the overlap length. Lapping must be done at specific locations in beams and columns. Lapping is prohibited within or near beam-column joints. If lapping must be done at specific locations, rods must be cut to precise lengths, resulting in many small, unusable pieces that are sold at a significantly lower price. Rebar couplers can join rods at any location without overlapping, reducing rod waste and cost.
- Earthquake Resistant: To build earthquake-resistant structures, the arrangement of rods and the anchorage of all rods must be done correctly. Lapping or welding joints must be avoided within or near beam-column joints, but often, workers place joints within or near these critical areas to reduce rod waste, increasing the risk of building collapse during an earthquake. Using rebar couplers allows more freedom in joining locations, making the building earthquake-resistant while reducing wastage and cost. In an earthquake, the concrete around the rebar may crack or fall off, but rods joined with rebar couplers can still carry the load. Thus, rebar couplers are essential for constructing earthquake-resistant buildings and structures.
- Independent of Concrete Strength: The strength of a joint made by lapping depends on the strength of concrete. If the concrete is weak for any reason, such as improper vibration, incorrect mix ratio, poor quality sand or aggregate, or excess water, the lapped joint become ineffective. However, a joint made with a rebar coupler remains effective even if the concrete quality is poor.
- Ease of Use: Joining rods with rebar couplers is very simple. The coupler supplier company provides a special machine on-site to make thread at the ends of the rods and supplies the rebar couplers. On-site workers can easily join the rods with wrenches and hand. Lapping requires special bending of rods, which can be difficult and time-consuming for large diameter rods. Rebar couplers eliminate these difficulties.
- Joining Thick Rods: Building codes in many countries encourage or even require the use of rebar couplers instead of lapping for joining large diameter rods (diameter 36mm or more). Therefore, rebar couplers are indispensable for joining thick rods.
- Avoiding Rod Congestion: Lapping creates a double layer of rods at the joint, potentially limiting concrete penetration. Rebar couplers prevent such congestion of rods / rebars / reinforcements.
- Construction Joints: For long structures, concrete is poured in stages. Keeping extra rods for lapping at construction joints is complex, unsafe, and time-consuming. Using rebar couplers eliminates the need for extra rods, simplifying and speeding up the process.
When is Using Rebar Couplers Economic and How much cost is saved?
Using rebar couplers is economic (save cost) when the rod diameter is more than 16 mm. For 8, 10, 12, and 16 mm diameter rods, using rebar couplers is not as cost-effective, but for practical purposes and to ensure earthquake-resistant construction, rebar couplers should be used even for 16 mm rods. Rebar couplers reduce the cost of rods and rod bending, prevent wastage, and result in approximately 5-10% cost savings.
When is Using Rebar Couplers Mandatory?
If the rod diameter is 36 mm or more, using rebar couplers is mandatory. Lapping is prohibited.
Guidelines for Using Rebar Couplers as per BNBC 2020 and ACI 328-19
According to the Bangladesh National Building Code 2020 (BNBC 2020) and the American Concrete Institute (ACI 318-19), the following guidelines apply for mechanical splices or rebar couplers:
- Staggering: Staggering means not joining all rods at the same position but joining them staggered way. Staggering is not mandatory but preferable. It is not advisable to join all rods at one place. However, it is not prohibited if there is a specific need. It is the best to stagger the rebar couplers by 2 feet forward or backward, meaning half should be joined at one location and the remaining half should be joined 2 feet above, below, or sideways.
- Type-1 Rebar Coupler Position: Type-1 rebar couplers or mechanical splices can only be used in the middle section of beams and columns. They must not be used at the ends of beams and columns. For OMRF and IMRF, Type-1 rebar can be used to splice up to 60 grade rebar at specified locations of column, beam and shear wall only.
- Type-2 Rebar Coupler Position: Type-2 rebar couplers or mechanical splices can be used in any part of the beams and columns. Type-2 rebar couplers can be used for any grade of steel rebar. They should not be used at the ends of beams and columns.
- Three Important Factors: The type of rebar coupler, seismic detailing type (OMRF, IMRF, SMRF), and steel rebar grade are the most critical factors determining the use and position of rebar couplers or mechanical splices. The following diagrams and figures clarify these aspects.
Rebar Coupler Quality Test:
To test the quality of rebar couplers, tensile strength tests are conducted. If the rod breaks within the rebar coupler during testing, it indicates poor quality of rebar coupler and it is not usable. Type-2 rebar couplers are preferable as they can be used in all types of structures. For Type-2 couplers, failure occurs in the rod outside the coupler, and the strength is equal to the rod’s strength. If the required strength is achieved and failure occurs outside the coupler at maximum load, the rebar coupler quality is satisfactory. For Type-1 couplers, failure occurs at a lower strength than the rod’s strength, and failure happens at the coupler’s end. Such rebar couplers are not suitable for constructing earthquake-resistant buildings and structures.
