In reinforced concrete structure longitudinal rebar connections are provided to ensure continuity of reinforcement. The reason that this requirement arises is the limitation in availability of a standard length of rebars. Due to transportation limits, the rebar length is normally restricted to 12 meters. In structural members, where reinforcement required length is longer than available, then two bars are joined for continuity. various types of rebar connections such as lapping or mechanical rebar couplers are used during construction of RCC structures. we will discuss practical aspects of rebar joints and compare rebar coupler vs lapping.
To transfer the axial forces from one rebar to the other, bars are spliced to each other. Conventionally, two longitudinal bars are either welded or lap splices are provided.
The most common way to join two longitudinal bars is a lap splice. It is easier to provide lap splices, but a lot of additional material is required to satisfy the minimum lap length requirements.
Providing welded splice is normally avoided as it involves huge electricity costs. However, the material is saved but qualified welders are required and also it is labor-intensive to provide welded splice.
Lap splicing or welding is not always desirable and practical. Sometimes, the use of lap splices is not possible due to already congested reinforcement in certain sections of the structure, or it may have been discouraged by the building code. To minimize the disadvantages of welded splicing and lap splicing, rebar couplers are used in modern construction.
Let’s see what are these rebar couplers.
Coupler Joint in Reinforcement
A coupler joint in reinforcement is provided using rebar couplers. Rebar couplers are mechanical coupling devices used as an alternative to providing rebar splicing. These are provided to safely transfer forces between two longitudinal bars without additional rebar or welding. Rebar couplers may be threaded, bolted, welded or grout filled.
Mechanical Rebar Couplers Uses
The requirement of using mechanical rebar couplers is arising due to the requirements for operational safety, seismic design requirements and to lower the costs of construction. The main feature of any rebar connection is its strength uniformity. Mostly the rebar couplers can be used for the following purposes.
- These rebar couplers may be used for precast as well as for in-situ construction.
- These can be used for renovation, modification, and retrofit jobs where already embedded rebar cannot be exposed to provide a complete lap splice.
- When diameter of the rebar is large and length of the lap splice could be avoided by providing rebar couplers. ACI 439.3R-2 also specifies when rebar couplers should be provided.
- To avoid congestion of heavily reinforced sections of concrete structure, a suitable type of rebar coupler should be provided to avoid further rebar congestion.
Although rebar couplers are being used in construction, however, not much guidance on ductile consideration is available in codes. Hence the application of rebar couplers under inelastic conditions is very limited.
Rebar Coupler Types
Many types of rebar couplers are commercially available consisting of various mechanisms like bar threading, bar swaging, swaged sleeve, grouted sleeve, or bolted. Some rebar-couplers are designed to interface with the concrete formwork to facilitate further construction activities. Once the concrete has hardened and formwork is removed, rebar-couplers are exposed to create continuity of reinforcement across construction joints.
We can classify rebar couplers according to the mechanism of the coupling system. Rebar couplers can also be classified according to the rebar function in a structural member, e.g. compression only, tension only, or tension-compression couplers.
Let us discuss the types and classifications of rebar-couplers.
We can classify the rebar-couplers according to the different rebar coupling systems as follows.
Threaded Rebar coupler
Threaded rebar couplers are the simplest form of a coupling system. It consists of a standard sleeve with internal threading (female threads). Rebars’ ends are threaded (male threads) to match the internal threads of the sleeve. Once the rebars are tightened with the threaded sleeve, forces are transferred between coupled bars by direct thread bearing. A threaded bearing may have parallel threads or tapered threads.
- Parallel threaded couplers: All Threads inside the coupling sleeve and rebar ends are of the same diameter. This is the simplest form of a rebar coupler and is easily installed without any special tool and skill.
- Tapered threaded couplers: A tapered threaded rebar coupler has continuously varying diameter for its threaded portion of the coupler sleeve, as well as the threaded portion of rebar ends. Using the tapered threaded couplers can be used for any length, shape, diameter, or combination of bar sizes. A tapered threaded coupler can be a positional coupler or transitional coupler.
- Positional couplers join two rebars of the same diameter.
- Transitional couplers are used for joining two different diameter rebars.
The benefit of parallel threaded couplers and tapered threaded couplers is their relatively easy and rapid installation as these couplers can be assembled using a hand wrench.
Swaged Rebar Couplers
Swaged rebar couplers are cold-forged mild steel sleeves placed over the ends of ribbed rebars. The sleeves are swaged on bars by using a portable hydraulic press. By pressing the steel sleeve forms in the shape of deformed rebar. When the structure is loaded, the formed dents on the sleeves interlock with the rebar ribs.
The benefit of swaged couplers is easy site installation; these are also convenient for repair. Using a wide sleeve swaged coupler, two bars can also be joined by lapping each other, and then the sleeve is swaged around both bars.
Mixed systems (Threaded and Swaged Couplers)
The mechanism of the threaded and swaged coupler can be used in combination to achieve more efficiency and time saving during construction. In a mixed system, the coupling sleeves have one threaded end and the other end is produced to be swaged around the rebar. These couplers may have parallel or tapered threads. Factory swaging facilitates installation, and assembly can be carried out manually. The use of these mixed-coupling systems eliminates the threading of rebar ends, hence site activities are reduced and time is saved. Another benefit of a mixed system is that rebar size transition is possible using this type of couplers.
Bar Lock Couplers
Bar lock or Shear Bolt lock coupling system consists of a steel sleeve with holes for hexagonal head shear bolts. These bolts are aligned along the longitudinal axis of the joining rebars. The shear bolts on the coupler are equally distributed on the joining rebars are tightened, using a wrench until their heads shear off. When tightened, the shear bolts exert force on rebars which create deformations in the form of a wedge. This creates an interlocking wedge action and the load is transferred between rebars.
A shear bolt lock mechanism can also be used for offset bolt lock couplers. In offset bolt lock couplers, two bars are lapped inside a sleeve and interlocked using shear bolts.
The benefit of these couplers is that bar threading or any other special bar-end treatment is not required. Also, no special installation equipment is required. Typically, a shear bolt lock mechanism is used for retrofitting concrete structures.
Grout Sleeve Rebar Couplers
In grouted sleeve coupling mechanism, the two joining bars are placed inside a sleeve and the sleeve is filled with high strength non-shrink grout. This mechanism has dual advantages the rebars are bonded with the grout as well as the joint is confined inside the sleeve.
In grouted sleeve couplers, internal ribs are also provided in sleeves so that the bond between grout and sleeve is more effective. The sleeves are also provided with rebar guides and stoppers to improve the assembly process of the joint.
Sometimes a mixed coupling system is also used to join two rebars. In mixed systems, one end of the coupler is provided with threads and wet grout at the other end. Such a coupling system is used for precast concrete structures. The threaded rebar is fixed at the factory or concreting yard, while the grouted end is inserted on site.
Other Rebar Coupler Types
Some other types of rebar couplers are also available in the market, like upset-headed couplers, rib-threaded couplers, metal-filled couplers, etc.
Upset Headed Rebar Couplers
In Upset-headed couplers, the ends of two reinforcing bars are forged into upset heads using a portable hydraulic machine. The upset-headed bars are then connected through a threaded steel collar assembly. The steel collar assembly is having threaded male and female coupling parts that join the two bars. In an upset-headed coupling system, compressional force is transferred through the heads whereas tensional force is transferred through the steel collar.
Rib-Threaded Rebar Couplers
Rib-threaded couplers rely on mechanical interlock between rebars and coupler sleeves. Female threads are provided in a sleeve that matches the rib configuration of the rebar. In these types of rebar couplers, grouting is done using high-flow mortar to provide a full grip of the sleeve over the rebars.
Weldable Rebar couplers
In weldable rebar couplers (also called a metal-filled coupling system) molten metal, mostly steel, is filled inside the coupling sleeve. The molten steel is filled by using an exothermic process in which the molten steel filler solidifies sound the rebar and internal grooves of the coupling sleeve.
Rebar Coupler vs Lapping
Conventionally, the easiest and most common way to provide reinforcement continuity is to provide lap splicing between two longitudinal bars. By providing the required length of lap splice the loads are transferred between spliced bars. For certain applications, due to limitations, alternatives to lap splicing have to be adopted, For example:
- For the largest diameter bars, the ACI code does not permit lap as for larger bar sizes, and in heavily reinforced members, lap splicing can lead to rebar congestion.
- When a structural member is expected to be loaded cyclically in their inelastic range, Lap Splicing may not be reliable.
Advantages of couplers
Reinforcement couplers have considerable advantages in comparison with conventional methods of lap splicing. Rebar couplers provide a better economy and can increase the speed of construction. Particularly these are very good to satisfy requirements for seismic detailing as the seismic provisions sometimes create the problem of reinforcement congestion. The construction industry is making ever-increasing use of rebar couplers due to the advantages as compared to lap splicing.
- Using rebar coupler vs lapping, reduction in reinforcement can be achieved. More material is saved when larger diameter bars are being used.
- When the structural members are heavily reinforced, the use of rebar couplers can lead to a significant reduction in reinforcement congestion.
- Rebar couplers can make precast and modular construction easier when rebars of two structural components have to be joined for reinforcement continuity. A considerable improvement in the speed of construction can be achieved using rebar couplers.
- The problem of reinforcement congestion becomes very challenging for ductile seismic detailing. Providing a rebar coupler will resolve the issue as further congestion at rebar joints will be avoided.
Rebar couplers are an attractive alternative that reduces the drawbacks of conventional reinforcement splicing.
Disadvantages of couplers
As we have seen that rebar couplers have many advantages over the conventional splicing method. Also, the use of reinforcement couplers is not specifically prohibited or restricted in most of the seismic codes.
- But we normally face this issue that reliable information on its inelastic performance is not available in the literature.
- Testing procedures for couplers are normally not well defined. The available testing procedures generally focus on the elastic slip, fatigue, and strength considerations, using idealized in-air uniaxial tests on rebar coupler arrangements. Due to the lesser guidance for testing protocols available in codes, the use of rebar couplers is not possible.
Couplers with a non-metallic material such as GFRP couplers are also being studied by researchers and may be available for commercial use shortly.
For our reader’s further reading we recommend the following book.
- Long-term performance of epoxy-bonded rebar-couplers
By Brungraber, G. (2009).
Retrieved from https://escholarship.org/uc/item/6tj707t8
- Stress-strain state of mechanical rebar couplings.
By Vasilij Klimenov, Artem Ovchinnikov, Artem Ustinov, and Artem Danilson,
AIP Conference Proceedings 1698, 070008 (2016)
- Ductility considerations for mechanical reinforcement couplers,
By D.V. Bompa, A.Y. Elghazouli,
Structures, Volume 12, 2017, Pages 115-119, ISSN 2352-0124,
- Quality control of rebar couplers in splicing of reinforcement bars,
By Radovanović, Željka. (2019).
Proceedings on Engineering Sciences. 1. 569-580. 10.24874/PES01.02.056.
- Mechanical performance for defective and repaired grouted sleeve connections under uniaxial and cyclic loadings,
By Guanyu Zheng, Zhiping Kuang, Jianzhuang Xiao, Zuanfeng Pan,
Construction and Building Materials, Volume 233, 2020, 117233, ISSN 0950-0618,
- Types of Mechanical Splices for Reinforcing Bars (ACI 439.3R-07)
ACI Committee 439 on Steel Reinforcement report 439.3R-07