By Dr. N. Subramanian, Ph.D., FNAE
Mechanical splicing products (often called couplers) are widely used in the construction industry as an alternative to lap splices when joining two reinforcement bars, especially when there is no room for lap splices, when larger diameter of bars (larger than #11)are used, and in tension members. In addition, the use of mechanical couplers is useful where the reinforcement is congested or space does not allow for the full development length that a splice would require.
Coupler Advantages
Couplers have the following advantages: 1) improved structural integrity thus offering strength and toughness even during seismic events, 2) no reliance on concrete for load transfer, 3) elimination of lap-splice calculations, and 4) reduced material costs. This cost savings can be significant when expensive epoxy coated bars are used, since building codes require up to 50% longer splice laps for these bars than for standard rebars (Hurd, 1998). A few other hidden costs associated with providing lap splices, such as the time spent preparing lap splices and the necessity for additional transverse reinforcement, are also eliminated. (Jacinto, et al., 2023). Additionally, codes such as ACI 318:19, Building Code Requirements for Structural Concrete and Commentary on Building Code Requirements for Structural Concrete(clause 25.5.7.1) require that mechanical splices deliver higher performance than lap splices (typically 125% greater capacity). The use of mechanical couplers results in savings on rebar material and associated costs, including fabrication of lapping rebars. However, this cost advantage may be offset by the cost of couplers.
Types of Couplers
Several types of mechanical couplers are available-on the market
(Table 1). Couplers may be categorized as tension couplers or compression couplers. Unless specified otherwise, tension couplers should always be used. Couplers may also be categorized as in-line couplers, in which the centerline of each spliced bar coincides and offset couplers, where the centerlines have an eccentricity.
Mechanical splices, shown in Figure 1, generally use a threaded annular sleeve, slightly larger than the diameter of the bars, that is placed around the bars at the joint. The sleeves are normally cold pressed against the bars, forcing the ribs of the deformed bars to become embedded into the wall of the sleeve. A sleeve embedment length of only two bar diameters (2db) for each of the two bars may be sufficient to transfer the load of the bar in tension. The connection can also be formed by filling the annular space between the bars and the sleeve with molten metal. Direct threading of bars is avoided to prevent reduction in bar size, and consequently its strength. One solution to this is to increase the size of the ends over a small length by tapering the bars, as shown in Figure 1a.
A new type of coupler called a set screw coupler is not threaded, swaged, or metal-filled. Instead, they consist of a steel tube with a series of lock-shear bolts—generally six to eight—and two serrated strips that run the inside length of the coupler, as shown in Figure 1g. The installation of this coupler is carried out as follows: 1) after sliding one length of rebar halfway into the coupler, the bolts are tightened to fit snugly; 2) the second bar is then inserted into the coupler until it butts against the other bar end; and 3) the remaining bolts are tightened to a snug fit. A ratchet or an impact wrench is used to tighten the bolts until their heads shear off, implanting the bolt ends into the rebar and embedding the serrated strips into both the rebar and the interior coupler wall. This type of splicing system does not require any special bar-end preparation and can be used in projects where bars are in place or access is limited. More details about mechanical couplers may be found in ACI 439.3R-2007 Types of Mechanical Splices for Reinforcing Bars, BS 8597:2015 Steels for the reinforcement of concrete – reinforcement couplers – requirements and test methods, IS 16172:2023 Reinforcement Couplers for Mechanical Splices of Steel Bars in Concrete—Specification, SP 34:1987 Handbook on Concrete Reinforcement and Detailing, and Prakash Rao, 1991. The use of couplers in column reinforcement is shown in Figure 2.
Mechanical butt splices provide superior strength during load transfer. Other advantages include superior cyclic performance and greater structural integrity during seismic events. From the structural standpoint, the most important benefit of mechanical splices is that they ensure load path continuity of the structural reinforcement. Using mechanical butt splices allows the use of larger diameter rebar in a smaller column thereby minimizing congestion. Unlike lapsplices which often extend into the plastic hinge regions (thus violating code requirements), mechanical splices can be easily located outside these high stress regions. In addition, the use of mechanical splicing eliminates the tedious calculations needed to determine proper lap lengths and the possible errors associated with the calculations. Also, mechanical splices are fast to install.
However, mechanical splices also have a disadvantage compared to lap splices because of their complex process of installation. That is, rebars need to be prepared before installation by cutting the threads into the rebar so that these threads match the threads of the coupler. This process may need skilled labor, and two additional machines: the forging machine and the thread cutting machine (Damsara and Kulathunga, 2018). The installation must be carefully performed to ensure that the threads are well fitted. Also, the threaded ends of the rebars should be protected from corrosion before installation in order to obtain good fixity with the coupler.
Offset Splices
Based on experimental results of testing of offset mechanical splices, shown in Figure 3, Coogler et al, 2008,the following was found:
Offset splices are not recommended for use with bar sizes greater than #5(16 mm), unless tests indicate to satisfy the performance criteria.
Offset splices should not be used in applications subject to seismic load reversals.
Offset splices should be considered as a different category of mechanical splices (having a fatigue limit of 80 MPa).
Cost Comparisons
During 1998, Cagley and Apple made a cost comparison between lap splices and mechanical splices and found that the additional cost of using mechanical butt splices was about 0.2 percent of the total cost of the structure. However, since then the cost of mechanical splices have reduced and the cost advantage of using a coupler instead of lapping may increase with the increase in size of rebar (approximately 11% for #5 bars to 140% for #10 bars) (Damsara and Kulathunga, 2018 and Singh, et al., 2013).
Code Specifications
Clause 18.2.7.1 of ACI 318-19 Building Code Requirements for Structural Concrete and Commentary on Building Code Requirements for Structural Concrete (ACI 318R-19), defines two types of mechanical bar couplers, Type 1 and Type 2. According to clause 25.5.7.1 of this code, Type 1 coupler is a mechanical or welded splice that can develop 125% of the yield strength of the bars (i.e., 1.25Abfy, where Ab is the area of bar and fy is the yield strength of bar) in tension or compression. Type 2 couplers are those that both conform to the requirement of Type 1 couplers and can develop the specified tensile strength of the spliced bars (clause 18.2.7.1). Thus, the requirements for Type 2 mechanical splices are intended to avoid a splice failure when the reinforcement is subjected to expected stress levels in yielding regions. ACI 318-19 requires using Type 1 only in the locations in which the couplers shall not experience yielding. This is because (per commentary R18.2.7, Mechanical splices in special moment frames and special structural walls), in a structure undergoing inelastic deformations during an earthquake, the tensile stresses in reinforcement may approach the tensile strength of the reinforcement. Thus, this requirement ensures that premature failure, bar pullout, or coupler fracture will not impact the ductility of the structural system.
Clause 18.2.7.2 of the code stipulates that except for Type 2 mechanical splices on Grade 420 reinforcement, mechanical splices shall not be located within a distance equal to twice the member depth from the column or beam face for special moment frames or from critical sections, where yielding of the reinforcement is likely to occur as a result of lateral displacements beyond the linear range of behavior. Type 2 mechanical splices on Grade 420 reinforcement shall be permitted at any location, except that clause 18.9.2.1(c) stipulates that they shall be located not closer than h/2(h is the depth of beam) from the joint face of beams. This is because Type 1 mechanical splices on any grade of reinforcement and Type 2 mechanical splices on Grade 550 and Grade 690 reinforcement may not be capable of resisting the stress levels expected in yielding regions. The locations of these mechanical splices are restricted because tensile stresses in reinforcement in yielding regions can exceed the strength requirements of clause 18.2.7.1. These restrictions on all Type 1 mechanical splices and on Type 2 mechanical splices on Grade 550 and Grade 690 reinforcement apply to all reinforcement-resisting earthquake effects, including transverse reinforcement.
Although the ACI 318-19 requires mechanical splices should have at least 25% higher design strength than lap splices, clause 26.2.5.2 of IS 456:2000, Plain and Reinforced Concrete – Code of Practice, suggests 100% design strength be assumed for mechanical connections. Note that couplers should be provided with a cover similar to that specified for the reinforcement since there will be a reduction of cover at the splice due to the larger coupler size.
Staggering of Couplers
Staggering of mechanical splices in walls is generally recommended, but not required by most codes, except in seismic zones. There are several reasons for this:
Reduces stress concentration. When mechanical splices are concentrated in one area, it can create a weak point. Staggering the splices helps to distribute the stress evenly.
Improve concrete placement. Mechanical splices can make it difficult to place concrete properly, especially in congested areas. Staggering the splices gives the concrete more room to flow and helps to prevent voids.
Reduce the risk of cracking. Cracks are more likely to form at the location of mechanical splices. Staggering the splices helps to reduce the risk of cracking by spreading out the stress concentration.
In seismic zones, ACI 318 Code requires that mechanical splices in special moment frames and special structural walls be staggered at least 30 inches (750 mm) to help improve the performance of these walls during an earthquake.
According to clause 25.5.7.3 of ACI 318-19, mechanical or welded splices need not be staggered except as required by clause 25.5.7.4. Clause 25.5.7.4 stipulated that splices in tension tie members shall be made with a mechanical or welded splice provided that the splice develops in tension or compression at least 1.25fy of the bar. It also suggests that splices in adjacent bars shall be staggered at least 30 inches (750 mm). The commentary to clause 25.5.7.3 explains that although mechanical and welded splices need not be staggered, staggering is encouraged and may be necessary for constructability to provide enough space around the splice for installation or to meet the clear spacing requirements.
Mechanical splicing products are widely used in the construction industry to join two reinforcement bars as an alternative to lapping bars. Careful planning will ensure walls are properly reinforced and mechanical splices are properly staggered to reduce the risk of failure. ■