Impact statementDuctility considerations for mechanical reinforcement couplers
Introduction
The construction industry is making ever-increasing use of mechanical couplers for reinforcement due to their respective merits compared to lap splices. The use of mechanical couplers can lead to significant reduction in reinforcement congestion in reinforced concrete (RC) structures, and they also offer the added benefit of facilitating site assembly in the case of precast concrete (PC). In comparison with typical reinforcement lapping, significant savings in materials can be made, normally more than offsetting the cost of mechanical couplers, and considerable enhancement in the speed of construction can be achieved.
The problem of reinforcement congestion becomes particularly challenging for ductile seismic detailing. Additionally, the significant lapping lengths and other codified detailing requirements for inelastic regions can often inadvertently alter the deformation capacity. Mechanical couplers can therefore offer an attractive alternative that alleviates the drawbacks of conventional reinforcement splicing. Nevertheless, although couplers are not specifically prohibited in most seismic codes, the lack of reliable information on their inelastic performance significantly inhibits their utilisation. Testing protocols for couplers (e.g. ISO 15835-1:2009) [1] typically focus on the elastic slip, fatigue and strength considerations, using idealised ‘in-air’ uniaxial tests on reinforcement-coupler arrangements.
The European seismic code, Eurocode 8 [2], stipulates that the use of mechanical couplers for reinforcement bar (rebar) splicing in dissipative zones needs to be verified by testing under conditions compatible with the selected ductility class (i.e. medium ductility: DCM, or high ductility: DCH). Depending on the ductility class of the structure, reinforcement Class B (minimum ultimate-to-yield ratio fu,b/fy,b of 1.08, and minimum elongation εuk,b of 5.0%) or Class C (minimum fu,b/fy,b of 1.15, and minimum εuk,b of 7.5%) may be used. For structures classified in DCM, reinforcement Class B and C are allowed, whilst for structures in DCH, the reinforcement grade is restricted to Class C. Accordingly, the ductility levels provided by reinforcement incorporating mechanical couplers should be a key parameter for evaluation. The actual performance in concrete can also be influenced by the shape of the couplers, with more compact forms mitigating possible stress concentration effects.
Using a collated database of > 350 tests extracted from the literature, this paper provides a comparative assessment of the performance of different types of mechanical coupling systems. The review includes uniaxial ‘in-air’ response and, where available, embedded ‘in-concrete’ behaviour. Key performance characteristics related to strength, ductility and size of mechanical splices in comparison with reference non-spliced bars are examined. Within the scope of the results considered, this comparative assessment offers some guidance for the selection and application of mechanical reinforcement couplers in inelastic regions.
Section snippets
Reinforcement coupling systems
Different types of mechanical reinforcement coupling systems can be broadly classified as either ‘threaded’, ‘swaged’, ‘bolt-lock’, ‘grouted sleeve’ or ‘headed’ couplers, as illustrated schematically in Fig. 1 and discussed below.
A threaded-rebar coupling system (TC) consists of standard sleeves with rolled-on grooves that have matching internal threads with the rebar, and forces are transferred by direct thread bearing. Parallel threaded couplers (PTC) are manufactured either by
Performance criteria
A detailed comparative assessment was carried out on existing sets of ‘in-air’ tests on couplers in order to determine their performance in terms of strength and ductility [3]. The results from tests on a total of 511 specimens, of which 244 were mechanical interlock-type (UHC, PTC, TTC, RTC, BLC, OBLC, SWC, OSWC, MFC) and the remaining 267 were grouted sleeve couplers (GSC), were compared in terms of static, as well as cyclic response where available. The typical monotonic and cyclic
Member behaviour
When embedded in concrete, the concrete confinement restricts sleeve dilation and may improve coupler performance. However, the complex concrete-reinforcement-coupler bond interactions can play a significant role in the local deformation response of a member. For example, for long and rigid couplers such as GSC and BLC, the inelastic deformations would tend to concentrate at the extremities of the coupler, hence modifying the plastic hinge mechanism and affecting the rotation capacity [5], [6].
Concluding summary
A comparative assessment, based on results extracted from over 350 in-air tests, was carried out to obtain an insight into the main characteristics of mechanical reinforcement splices with respect to strength, ductility and geometry. An evaluation of key ductility-related parameters for coupling systems, in combination with the reinforcement class specifications in the structural Eurocodes, points to the need for further detailed investigations since high ductility demands may not be met in
Nomenclature
- δy
member yield displacement
- δu
member ultimate displacement
- εu,sp
ultimate strain in splice
- εu,b
ultimate strain in non-spliced rebar
- εuk,b
ultimate characteristic strain in non-spliced rebar
- μδ,i = δy,i/δu,i
displacement ductility factor
- db
original rebar diameter
- dc
diameter of the coupler
- fu,sp
ultimate tensile strength of the splice
- fy,b
yield strength of the non-spliced rebar
- fu,b
ultimate strength of the non-spliced rebar
- Lc
length of the coupler
- Lcr
length of the coupler region
Subscripts
- b
bar
- c
coupler
- sp
splice
- co
Acknowledgments
This short article summarises a background comparative assessment carried out in preparation for a research project entitled ‘Seismic performance and application of mechanical splices in dissipative RC structures’ funded by a research award from the Institution of Structural Engineers.
References (12)
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(2009)Eurocode 8: Design of structures for earthquake resistance-part 1: general rules, seismic actions and rules for buildings
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Review on behaviour of mechanical reinforcement couplers in plastic hinge regions (internal report)
(2017) Eurocode 2: design of concrete structures: part 1–1: general rules and rules for buildings
(2004)Procedures to rehabilitate extremely damaged concrete members using innovative materials and devices PhD dissertation
(2013)- et al.
Seismic performance of precast columns with mechanically spliced column-footing connections
ACI Struct J
(2014)
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Seismic performance of prefabricated concrete columns with grouted sleeve connections, and a deformation-capacity estimation method
2022, Journal of Building EngineeringCitation Excerpt :Owing to the simple structure, clear force transfer mechanism and effective connection performance, GS connection has been widely applied in prefabricated structures. However, the incorporation of GSs in the plastic area will decrease the deformation capacity of the prefabricated columns [12,13]. In addition, some studies indicated that the displacement ductility of prefabricated members connected using GSs may be lower than that of traditional CIP members [14].