For almost 100 years, construction practices in the building of concrete structures have focused on the use of steel reinforcement to transfer tension and shear forces. Lap splicing has become the traditional method of connecting the steel reinforcing bars, largely due to a misconception that lap splicing is “no-cost” splicing. Lap splicing requires the overlapping of two parallel bars. The overlap load transfer mechanism takes advantage of the bond between the steel and the concrete to transfer the load. The load in one bar is transferred to the concrete, and then from the concrete to the ongoing bar. However ACI R188.8.131.52 states that Lapped joints are not always an appropriate means of connecting reinforcing bars. The use of laps can be time consuming in terms of design and installation and can lead to greater congestion within the concrete because of the increased amount of rebar used. Lapped joints are also dependent upon the concrete for load transfer. For this reason any degradation in the integrity of the concrete could significantly affect the performance of the joint. Reinforcing bar couplers (Refer Figure below) available in the market have come across with a solution for this complexity as it provides a greater ease in design and construction of reinforced concrete and reduce the amount of reinforcement required. The strength of a mechanical splice is independent of the concrete in which it is located and will retain its strength despite loss of cover as a result of impact damage or seismic event.
Reinforcing Bar Coupler versus Lap Splicing
Lap splicing has traditionally been the accepted form of connecting steel reinforcing bars in concrete construction. The task of gaining acceptance of reinforcing bar coupler has been made more difficult by the misconception that lap splicing is a “low cost” method of splicing.
The time taken to prepare lap splices, the need for additional transverse reinforcement, the material used in forming the lap splice and the cost of rebar placement represent the hidden costs of lap splicing.
Need for an Alternative
Continuing research, more demanding designs in concrete construction, new materials, hybrid concrete and steel designs and other advances in the construction industry are calling for the use of alternatives to lap splicing.
Improved Structural Integrity
While Lap splicing systems depend on the bond between concrete and steel to transfer load, reinforcing bar coupler systems provide load path continuity independent of the condition or existence of surrounding concrete.
Improved Design Options
Lap splicing requires the use of small diameter reinforcing bars to offset problems with rebar congestion. This often means column dimensions must be unnecessarily increased. Reinforcing bar coupler use solves this problem.
Eliminates Time-Consuming Calculations
Compliance with current ACI & other codes requires tedious calculations, with rebar coupler, this problem does not exists.
Consistent Steel to Concrete Ratio
Rebar coupler’s achieve an ideal balance of steel and concrete by eliminating unnecessary rebar while providing greater structural integrity.
Rebar coupler’s eliminate concrete pour congestion, while minimizing potential job site problems.
Various Available Reinforcement Bar Coupling Systems:
A. Tapered Thread Bar Coupler
Tapered Thread couplers is designed to suit the majority of applications which call for the joining of reinforcing bars. Available to suit bar sizes 12mm to 40mm, the couplers are installed quickly and easily on site without the need for specially trained personnel or specialized, expensive machinery. The compact design of each coupler ensures suitability for use in confined situations where space is restricted or where the loss of cover must be minimised. The couplers are normally supplied fitted to the end of threaded bar, requiring only the engagement and tightening of the adjoining bar on site.
A. 1 Tapered Thread Standard Bar Coupler
The Standard Tapered Thread coupler is suitable for connecting two bars of the same diameter, where one bar can be rotated. It comprises an internally threaded sleeve with two right hand threads which are tapered towards the middle of the coupler. The bar ends are square cut and a tapered thread is cut onto the bar. The couplers are generally torqued onto the reinforcing bar in the bar threading shop, the internal threads protected by plastic end caps. The installation steps for tapered thread standard bar coupler is shown in Figure below.
A.2 Tapered Thread Positional Bar Coupler
Tapered Thread Positional coupler is designed to be used in applications in which neither bar can be rotated. Having a degree of adjustability, the Positional coupler can also be used as a closer between two fixed bars. The Positional coupler comprises three components, a male section, a female section and a locking nut. Plastic thread protectors are used to prevent damage to the threaded bar ends and the internal threads of the couplers are protected by plastic end caps. Installation Steps is shown in Figure below.
A.3 Tapered Thread Transition Bar Coupler
The Tapered Thread Transition coupler is used to join reinforcing bars of different diameters where one coupler can be rotated. With all the benefits of the Standard range, Transition couplers are designed to achieve failure loads greater than 115% of the characteristic strength of the smaller diameter grade 500 reinforcing bar. The Transition coupler comprises an internally threaded sleeve with two right hand threads both of which are tapered towards the middle of the couplter. Installation Steps is shown in Figure below.
A.4 Tapered Thread Weldable Bar Coupler
Tapered Thread Weldable couplers provide a convenient means of connecting reinforcing bars to structural steel plates or sections. Shorter than the standard coupler, it has a tapered thread at one end. The other end is welded directly to the steel. The couplers are manufactured from either Steel Grade 1045 to ASTM A576 or Steel Grade C45R to EN10083. The Tapered Thread Weldable coupler is suitable for welding to structural steels, Grade S275 or Grade S355. The first step involves the welding of the coupler to the steel surface.
A.5 Tapered Thread Headed Anchors
The Tapered Thread Headed Anchor provides an effective method of achieving rebar end anchorage within concrete. Anchorage of rebars within a concrete section is traditionally achieved by means of creating a long hooked end on the rebar. This can lead to problems when positioning the bar and can increase congestion. It can ultimately result in larger than necessary concrete sections at the location of the hooked ends. Typical applications include pile caps and beam to column connections.
B.1 Parallel Thread Standard Bar Coupler
Parallel Thread Standard Bar couplers offer a full strength connection together with enhanced fatigue resistance. They are suitable for projects of any size, including those requiring a high volume of couplers, such as road and rail bridges. Each end of the rebar to be joined is cut square and enlarged using a cold forge process. A thread is then formed on the enlarged bar end using a thread rolling machine. The thread is such that the cross sectional area of the bar ends are not reduced ensuring the strength of the connection matches or exceeds that of the parent bars. It is the application of the rolled thread that differentiates Parallel Thread Plus from other threaded rebar systems. Each thread-rolled bar end is proof-loaded to a force equal to the characteristic yield strength of the rebar. It is the combination of these processes that provide the connection with enhanced fatigue resistance.
Parallel Thread Standard Type A Bar Coupler
The Type A connection utilises an internally threaded coupler to join two cold-forged and thread-rolled bar ends together. Each bar end is threaded to half the length of the coupler. Type A connections are used where the continuation bar can be rotated. Installation steps of Bartch Type A Bar Coupler is shown in Figure below.
Parallel Thread Standard Type B Bar Coupler
The Type B connection utilises the same coupler as for the Type A, the difference being one bar end is threaded for the full coupler length. Type B connections are used where the continuation bar can be rotated for no more than one revolution. Installation steps of Parallel Thread Type A Bar Coupler is shown in Figure below.
Parallel Thread Standard Type C Bar Coupler
The Type C connection utilises the same coupler as for the Type A, together with locknuts and longer threads to the bar ends. Type C connections are used where the continuation bar cannot be rotated. Installation steps of Parallel Thread Type A Bar Coupler is shown in Figure below.
B.2 Parallel Thread Transition Couplers
Parallel Thread Transition couplers can be used to connect reinforcing bars of differing diameters, developing the full tensile strength of the smaller diameter bar together with the enhanced fatigue resistance. For transition connections, only the smaller diameter bar end is enlarged and thread-rolled. The larger diameter bar is simply skimmed and thread-rolled with the same thread form as applied to the smaller diameter bar, excluding the 32mm to 40mm connection. As explained for Parallel Thread Standard Bar coupler system, Parallel Thread Transition coupler also have three types A, B and C. Sample installation of Parallel Thread Transition Couplers Type A is shown below.
B.3 Parallel Thread Headed Anchor Couplers
The Parallel Thread Headed Anchor provides an effective and proven method of achieving rebar end anchorage within concrete. The Parallel Thread Headed Anchor is essentially an oversized coupler capable of carrying the full tension load of the bar when it bears against the concrete in which it is cast.
B.4 Parallel Thread Weldable Bar Couplers
Parallel Thread welded couplers provide a convenient means of connecting reinforcing bars to structural steel plates or sections. One end has the Parallel Thread thread form; the other end is prepared for welding to the steel.
C. Rebar Joining Couplers
Wide range of couplers provides a cost-effective method of joining reinforcing bars, particularly when the fixed bar is already in place and there is insufficient space for a hydraulic swaging press. Couplers are easy to install and achieve failure loads higher than 115% of the characteristic yield strength of grade 500 reinforcing bar. For applications involving replacement of corroded or damaged bars, the replacement bar is cut approximately 5mm shorter to allow clearance for insertion between the sound ends of the original bars. Rebar joining couplers are pushed fully over both ends of the replacement bar and temporarily held in position. The typical installation procedure of rebar joining couplers is shown below.
D. Grout Sleeve Couplers
The Grout Sleeve Coupler is a mechanical connector for the joining of reinforcing bars end to end. It was originally developed to cater for rebar tolerance/bar alignment issues and although it can be used in phased in-situ construction, it is particularly well suited to connecting precast concrete elements together. One end of the sleeve is internally threaded to accept a pre-threaded reinforcing bar; the other is open to receive a cementitious grout.
Case Study of Application of Reinforcing Bar Coupler in Burj Khalifa:
The Preshcon ‘IRON MAN’ BMS threaded rebar coupler system originated in Korea and has been developed to its current world leading standard by the Boo won Corporation in Seoul was used in construction of the tallest building of world Burj Khalifa. Since 1999 more than 50 million parallel threaded rebar couplers have been supplied by Preshcon for almost every form of civil and commercial structure imaginable, from Olympic stadiums to mass rapid transit systems, the prestigious Singaporean deep tunnel sewage system and high rise buildings including the Burj Khalifa. Refer Figure’s below for the application of parallel rebar couplers during the construction of Burj Khalifa.
Schematic Application of Reinforcing Bar Couplers in Various Structural Elements
Refer Figure below for various applications of mechanized splicing systems.
Corrosion Considerations of Reinforcement Bar Coupler:
Corrosion increases the size of the rebar causing the concrete cover to spall and crack. As lap splices depend on the “bond” between concrete and steel for strength, concrete degradation caused by corrosion results in lap splice failure. With Reinforcement Bar Couplers, structural integrity is maintained even with the loss of the concrete cover because mechanical couplers perform similar to a continuous piece of rebar.
As a result of changing concepts, ideas, and philosophies, a wide variety of rebar splicing methods has been developed, some better than others. Reinforcing Bar Coupler’s offer several economic benefits. Trends toward tougher buildings, increased geography impacted by seismic zone mapping, and tenant desires for safer structures may also impact the potential value of a building. Inclusion of reinforcing bar coupler today may predate future load path continuity requirements. The negligible short-term perceived economics of lap splicing are far outweighed by the many structural and economic benefits of rebar coupler, including continuity of reinforcing steel, and structural integrity. With many different types of reinforcing Coupler system available in the market, designers can easily incorporate them into future plans.