As defined in ACI 116 , a mechanical splice is “the complete assembly of an end bearing sleeve, a coupler, or a coupling sleeve, and possibly additional intervening material or other components to effect a connection of [two] reinforcing bars.” A mechanical splice is typically comprised of either a coupler or coupling sleeve as noted above. A coupler is a threaded device for joining reinforcing bars for the purpose of providing transfer of either axial compression or axial tension or both from one bar to the other. A coupling sleeve is a non-threaded device for joining reinforcing bars for the purpose of providing transfer of either axial compression or axial tension or both from one bar to the other. Both of these definitions are from ASTM A1034, which is introduced later herein.
Mechanical Splice/Joint is the common terminology for the complete assembly including the coupler or sleeve fitted to provide a splice of two reinforcement bars. Mechanical couplers are relatively, a new concept in India. Mechanical Splicing of Rebar is cost saving on steel consumption avoiding sacrificial steel wastages and has a Better load bearing due to the continuity of bars. There is no congestion of bars, thus avoiding honeycomb at the structural joints of both horizontal & vertical alignments. Moreover, it is easy to install and has a Faster splicing time when compared to Lapping of bars. The system allows full ductile elongation of bars. The primary purpose of couplings is to join two pieces of rotating equipment while permitting some degree of misalignment or end movement or both. By careful selection, installation and maintenance of couplings, substantial savings can be made in reduced maintenance costs and downtime.
Benefits of the system
- Cost Saving against lapping
- Continuity of Reinforcing bars
- No congestion of bars
- Construction cycle time reduced
- Steel wastage reduced
- Staggering of bars not required
- Dowels can be avoided enabling reuse of form works
- Allows full ductile elongation of bars
- No Torque wrenching required, Easy to install
- Bar Cross section area is not reduced
The choice of the mechanical coupler system for the project depends on various factors. Selection of the most suited coupler system should be made after considering all advantages and limitations of the coupler system, the project parameters, location of coupler joint and the preferences of the structural consultant for the project.
Broadly, Mechanical couplers can be classified in the following two main categories threaded and non-threaded couplers.
Threaded couplers are sub-categorized into taper and parallel threaded couplers. The cross-section and length of the coupler is determined by the grade of rebar for which it is designed. The general coupler systems are described below but one should note that certain modifications are possible in each system as per manufacturer and application requirements.
Taper Threaded Couplers:
A mechanical splicing system with tapered threaded couplers is one where the threading carried out on the rebar is at a slight incline. The slope of the threading and the coupler is kept the same to ensure engagement of all threads simultaneously in the coupler joint.
Tapered couplers are generally longer in length as compared to parallel couplers and number of threads is generally mentioned as a range. There is a tolerance allowance in tapered couplers for length of threading to be carried out on rebar. Taper threaded couplers are designed taking into account defects of the rebar such as undersizing, skewness and longness of the rebar.
Tapered threaded couplers are the simplest type of threaded couplers where the threads are cut out on the rebar at an angle. These type of couplers are suitable for columns in general and can be used in horizontal applications but it is necessary to ensure tightening of the joint. These couplers require that the rebar be turned for tightening and hence it is difficult to use such couplers in rafts or other applications where it is not possible to turn the rebar and tighten the same.
Roll Threaded Couplers
Roll Threaded couplers are a type of parallel threaded couplers where threads are formed by pressing the ends of the rebar using a set of rollers and are then connected by a coupler with matching parallel threads. The rebar end need to be cut perpendicular prior to threading as there is no allowance for additional threads inside the coupler in parallel threaded systems.
Roll Threaded couplers are suitable for both horizontal and vertical application. Due to parallel threads, in columns, more turns are required for tightening the coupler and hence installation takes more time as compared to tapered system. Roll Threaded couplers provide an option of position threading where one side of the joint can be threaded to the coupler length and the coupler can be turned to tighten the joint in location instead of rotating the rebar. However suitability of this application is to be determined based on the rebar available, as there will be a reduction in joint strength for this specific application. For vertical applications, full strength joint is achieved.
The machine for Roll Threaded splicing is more portable and such systems can be recommended for projects with coupler requirements in both raft/beams and columns. In such a system, it becomes necessary to ensure proper threading. Gauges are to be provided to ensure tolerance limits for threading on site. There can be situations where the splicing may have a play if the bars provided are undersized. In such case, it may become necessary to consider alternatives.
Cold Forged Couplers/Upset Parallel Threaded Couplers
Cold Forged Or Upset Parallel threaded couplers are formed by enlarging the rebar end by 10-30% under hydraulic pressure prior to cutting parallel threads on to the upsized rebar.
Cold Forged couplers are suitable for both horizontal and vertical application. Due to parallel threads, in columns, more turns are required for tightening the coupler and hence installation takes more time as compared to tapered system. Cold Forged couplers also provide an option of position threading. Due to forging of the rebar, there is no reduction in the joint strength even while using the position coupler option.
Precautions which are needed for cold forged couplers include ensuring that there is no play between the threads formed in this process and the coupler. Gauges are used to ensure tolerance of the threads in such cases. It is vital that the rebars used in this process have sufficient ductility or else there is a risk of micro-cracks being formed during the forging process. In such cases, the joint strength may be severely affected.
There are various subtypes of non-threaded couplers which are used mainly for installation of couplers in location where it is not possible to use threaded couplers or for specialized applications. This category includes but is not limited to crimple/swage couplers, welded couplers, Bolted couplers, couplers with injected sleeves, friction-weld couplers. They are also required to conform to the same standards as the threaded couplers unless specifically approved by the consultant.
The Non-threaded couplers are mainly used as repair aids and are not used extensively in new construction due to their higher costing, bulky couplers and slower installation procedure.
Using Mechanical Splice offers various advantages over the conventional method of overlapping of reinforcement bars.
Increases the tensile and compressive capacity of columns, long columns and long beams where the designed tensile loading is greater than normal situations. Tension couplers are so far the best solution for tension columns and tension piles.
Eliminates congestion of steel bars in heavily reinforced columns and core wall of the building. Thus reducing the formation of “honeycomb” in concrete structures.
Facilitates speedier concrete casting, as bigger space between reinforcements facilitates an easier flow of concrete.
Reduces cost and increases the speed of slip and jump formwork system in the construction of core walls and towers.
Avoids strains, corrosion, and breakage of starter bars left for future extension of building and bridges.
For the construction of structure like bridges in traffic congested area where the structure could be constructed in stages to allow the constant flow of traffic.
Provide connections for starter bars for underground slab and beams in the construction of diaphragm wall to prevent breakage of precast starter bars during the process of bending up to avoid tedious concrete hacking.
Where spacing between bars is insufficient to allow for lapping, e.g. in the construction of micro pile.
Reduces the size of the concrete section and ensures maximum usage of expensive floor space in office towers.
Enables connecting precast members to cast-in-site members, where a full tension splice is required.
Spliced rebar performs like continuous reinforcement due to mechanical joint, unlike lapping which has complete dependency on concrete. This eliminates errors due to providing wrong lap length, Reduction of concrete2 grade, compaction or segregation issues with concrete, concrete deterioration over time or due to sudden impact which causes reduction in lap joint strength.
Steel Wastage is reduced significantly. Using couplers saves lap length steel. While lapping can only be carried out in lap zones, it is possible to use couplers outside of lap zones. This also provides the added advantage of using standardized lengths. A 12m rebar can be divided into 3m or 4m as bars as required which also prevents the wastage of end pieces of steel which are not used in lapping.
Couplers can be used as a replacement to dowel bars which also saves formwork material.
Steel congestion is reduced due to elimination of laps. This also aides in proper flow of concrete in the critical zones and hence improves the quality of the overall structure.
Non-threaded coupler options can be used for extensions of steel in location as an alternative to welding.
Using couplers provides superior cyclic performance as compared to lap joint. It also allows greater flexibility for the designer.
It is possible to easily verify joint strength in case of Couplers as compared to lap splices where the testing is cumbersome and not regulated.
For the contractor, usage of couplers reduces labour cost for installation and handling of steel. The construction schedule is improved and there is saving on valuable crane time on the project.
In India, the structural consultant is free to define the code to which the couplers used for the project are required to conform to. While there are a number of different codes around the world for coupler manufacture and joint threading, some of the most common and relevant ones are listed below with a brief summary of their requirements.
The ACI 318 Building Code  governs the design of mechanical splices. Specifically, they are referenced in the following sections of the Code.
- 12.14.3 General requirements
- 12.15.6 Splices in tension tie members
- 12.17 Mechanical splices in columns
- 21.1.6 Mechanical splices in special moment frames and special structural walls
- 21.8.2(b) Mechanical splices in special moment frames constructed using precast concrete
- 18.104.22.168(d) Mechanical splices in special structural walls and coupling beams
- 22.214.171.124 Mechanical splices in structural diaphragms and trusses
- 126.96.36.199 Mechanical splices in members not designated as part of the seismic force-resisting system
A Type 1 mechanical splice, as defined in Section 188.8.131.52 of the ACI 318 Code, is a full mechanical splice that develops in tension or compression, as required, at least 1.25 fy of the bar, where fy is the specified yield strength of the reinforcing bar. A Type 2 mechanical splice, as defined in Section 184.108.40.206(b) of the ACI 318 Code, is a mechanical splice that meets the requirements of a Type 1 mechanical splice and also develops the specified tensile strength of the reinforcing bar.
Couplings find wide applications in industries such as oil industry, natural gas industry, water treatment companies and chemical industry. Additionally, the market has witnessed increase in adoption for coupling in gas pipe construction industry. However, increasing cost of labor and deteriorating exports are factors responsible for alteration of the couplings market globally. With industries worldwide focusing majorly on improvement of energy efficiency, the demand for couplings is likely to witness growth traction in near future. Additionally, strategic expansions pertaining to the coupling market in the MINT nations (Mexico, Indonesia, Nigeria and Turkey) are likely to provide higher growth opportunities. However, product technology of industrial couplings has matured and therefore customers seek value additions as a key purchase criterion for couplings.
Most mechanical splices join two reinforcing bars, in-air, prior to placing concrete around the bars. There are some special application splices which include the following: Weldable Couplers One end of the coupler can be welded to a structural steel plate or shape (i.e. column, beam, pile). Oftentimes, relatively small areas of a concrete member are exposed or “opened up” to permit installation of a repair or structural retrofit. These small areas do not always permit outof-plane maneuvering room for the installation of extra reinforcing bars. Moreover, existing bars cannot be rotated or displaced laterally. Special mechanical splices have been developed for these applications.
Joining reinforcing bars through formwork at a concrete construction joint is common. Several manufacturers make a flush-mounted coupler with a flange that can be attached to formwork. After the formwork is stripped, a temporary protection cap is removed, exposing the female end of the coupler. A threaded reinforcing bar, or an externally threaded coupler component, is then inserted into the internally threaded exposed coupler end, and the reinforcing bar is made continuous across the construction joint.
Mechanical splices are available in black (plain), zincplated, epoxy-coated, or hot-dipped galvanized. The epoxy coating on the coupler conforms to ASTM A775 [2007a] requirements. Galvanized, mechanical splices meet ASTM A767 [2009b] or ASTM A123 or ASTM A153 requirements. Mechanical couplers are also commercially available in stainless steel and other high strength corrosion resistant specialty steels. Manufacturers should be consulted for orders requiring specialty steel or coatings. As with any mechanical splice, test data should be utilized to determine suitability of available products.
Standard Testing method:
ASTM A1034, Standard Test Methods for Testing Mechanical Splices For Steel Reinforcing Bars, covers the methods for testing mechanical splices under the following conditions:
Monotonic Tensile Test – Used to measure the performance of a mechanical splice under an increasing tensile load. The load is increased until failure is reached.
- Monotonic Compression Test – Used to ascertain the performance of a mechanical splice under an increasing compressive load. The load is increased until failure is reached or a specified load is applied.
- Cyclic Load Test – Used to ascertain how a mechanical splice performs when subjected to alternating tension and compression loads for a given number of cycles. Each cycle may exceed the specified yield strain of the reinforcing bar and is intended to simulate an earthquake loading. High-Cyclic Fatigue Test Similar to a cyclic load test, but with the test load staying below the yield strength of the bar. The test is conducted until failure or a specified number of cycles are reached. The test is meant to simulate a mechanical splice in a bridge or other structure subjected to frequent elastic load cycles.
- Slip Test – Used to ascertain the plastic movement (slip) between the reinforcing bars within the mechanical splice, when loaded in tension.
- Low-Temperature Test – Used to test the suitability of a mechanical splice for use in a structure expected to operate at extremely low temperatures.
- Combination Tests – More than one of the tests described in ASTM A1034 is conducted concurrently
The coupling industry has witnessed innovations in product technology. Some of the major trends that this industry follow include developing couplings that require less space, usage of low weight materials with better inertia qualities for manufacturing couplings. Optimized disc springs and usage of wear free materials also play an important role while designing couplings. The growth of the global coupling market in developed economies is likely to witness a slower growth over the forecast period. The restricted capital expenditures across various end user industries such as the manufacturing and oil and gas industry, has considerably constrained the market growth. However, the demand in emerging nations such as India, China, Russia, Brazil and the Eastern European countries is anticipated to boost market growth. This is mainly due to the fact that, these regions have become the global centers of the manufacturing industry. The material handling sector in Middle East and the mining industry in Africa are also expected to be the fastest growing regions for couplings in near future.
Mechanical-coupling sales are ubiquitous enough to track the global economyand at present, the economy is being kind to couplings. Expansion in Mexico, Indonesia, Nigeria and Turkey (or MINT nations) are expected to show growth for couplings. China, Brazil and Russia should also show growth, albeit slower. Developed economies will likely offset the growth of the other regions but not enough to stagnate forward momentum.Materials innovation is also leading field. With overall coupling design remaining largely the same in the last 100 years, the technology is ripe for evolution.
A lot of R&D is going on in the field of rebar splicing systems, to further fine tune products, by leading manufacturers. Future breakthroughs are expected to provide even sleeker models of rebar couplers, which will also be stronger. With the gradual rise in awareness levels about the manifold benefits of rebar couplers their entry into the smaller tier-II and tier-III markets is expected to trigger a massive increase in demand. Any manufacturer trying for market leadership in a country like India therefore needs to first of all fine tune their distribution channel in order to get a firm foothold.
The Indian construction industry is progressing rapidly, with existing mega-cities expanding vertically due to the constraints on horizontal expansion and with upcoming new cities all over the country. Mechanical couplers, even though a relatively new system, has proved to be technically superior to conventional techniques, helping to increase the pace of execution and the quality of projects. It also saves steel which is now a valuable national resource and is also cost-effective in comparison to lap joints. Couplers have already been incorporated in the construction industry. It is necessary to ensure that proper checks are maintained on this product to ensure compliance with structural strength and ductility requirements.
With the country emerging as a hub for infrastructure development and the boom in the real estate sector in the last two decades, and with almost every modern technology entering this lucrative market, rebar couplers could not have been left behind. The recent upheavals in the economy have meant that contractors are under immense pressure to stick to project deadlines, reduce labour and material costs, apart from ensuring that stringent quality standards are adhered to. This is where rebar couplers have come as a boon. They ensure acceleration in the construction process. Additionally, they also ensure reduced wastage of construction material, making them the ideal choice of contractors.
Competition is fast picking up among manufacturers to garner their share of the Indian rebar splicing system market. Some of the leading players in fray include, Spliceman, Dextra and Unitech Couplers, to mention only some. With the rise in demand, several global industry leaders in the field are expected to enter the fray in the next couple of years.
Image and References:
- International Code Council ICC (2010), Acceptance Criteria for Mechanical Connector System for Steel Reinforcing Bars ICC-ES (AC 133-2010)
- International Code Council, Whittier, California, 7 pp. International Code Council ICC (2012), International Building Code, International Code Council, Country Club Hills, Illinois, 701 pp