Type 2 mechanical splices are defined by ACI 318 clause 25.5.7 as couplers that develop the full specified tensile strength of the spliced bar, and in seismic applications they must also sustain cyclic inelastic strain without rupture [S4].
Slip tolerance for a Type 2 splice is set against a load-level envelope, not a single static number. The standard acceptance window per ACI 318 and the related ICC-ES AC133 protocol is a measured slip not exceeding 0.010 in. (≈0.25 mm) at 50% of specified yield (0.50·fy) and again at 100% of specified yield (1.00·fy), with no requirement that the slip be recovered [S2][S4].
Code Path: ACI 318, CalTrans Ultimate Splice, and EAD 160129
ACI 318-19 clause 25.5.7.1 requires mechanical splices to develop at least 125% of the specified yield of the bar, and Type 2 splices in particular must reach the full specified tensile strength f<sub>u</sub> of the spliced bar [S2]. Slip is not an explicit ACI 318 number; it is delegated to the acceptance criteria used to qualify the coupler, most commonly ICC-ES AC133 or, for U.S. Department of Transportation work, the CalTrans "Ultimate Splice" protocol that nVent/ERICO and Dayton Superior products are tested against [S2][S6][S7].
In Europe, the parallel route is EAD 160129-00-0301, which characterises couplers from 8 to 50 mm and treats "slip under or after static or quasi-static loading" as an essential characteristic (clause 2.2.3), separate from resistance to low-cycle seismic action (clause 2.2.5) [S3]. Under EAD 160129, slip must be reported and assessed, but the document leaves the numeric ceiling to the manufacturer’s declared performance and the issuing Technical Assessment body, rather than fixing one universal limit [S3].
Numeric Slip Envelope: 0.25 mm at 0.5·fy and 1.0·fy
The widely cited 0.010 in. (0.25 mm) slip ceiling at both 0.50·fy and 1.00·fy is a Type 1 / Type 2 acceptance line, not a Type 2-only rule. It is the same envelope referenced in the Bar-Lock L-Series qualification submitted to the U.S. NRC for the Sequoyah Nuclear Plant Steam Generator Replacement, where MBT couplers were tested against ASME Section III Division 2 CC-4333 and ASTM A370 to demonstrate Type 2 seismic splice performance [S1].
For seismic service, the bar-yield typically referenced is Grade 60 (420 MPa), with the splice required to reach 1.25·f<sub>y</sub> = 75 ksi (517 MPa) and the bar’s full f<sub>u</sub> ≈ 90 ksi (620 MPa) for A615 Grade 60 stock. The Type 2 designation also implies a strain compatibility requirement: under cyclic loading the splice must sustain at least four fully reversed inelastic cycles to a strain that the structure’s design drift will impose, a check layered on top of the 0.25 mm slip ceiling [S2][S4].
What "Slip" Means in Coupler Testing
Slip in this context is the relative axial displacement between the two bars measured across the coupler body, not the elastic elongation of the bar itself. Standard test setups per ASTM A370 use an extensometer or LVDT mounted on the bar on either side of the coupler, with the gauge length bracketing the sleeve only, so the reading isolates the mechanical interface from bar strain [S1][S3].
Two slip values are normally reported: residual slip after unloading from 0.95·f<sub>y</sub> and total slip under sustained load at 1.00·f<sub>y</sub>. The acceptance criterion is total slip, and 0.25 mm is the upper bound widely applied; couplers that meet it typically measure 0.05–0.15 mm at service load in production QC, leaving margin before the limit is approached [S2][S7].
Type 1 vs. Type 2 Slip and Strength Demands
ACI 318 splits mechanical splices into Type 1 (≥125% of f<sub>y</sub>) and Type 2 (full f<sub>u</sub> of the bar). The slip envelope of 0.25 mm applies to both classes; what changes is the peak load the coupler must carry without rupture or slip runaway. For a #8 (25 mm) Grade 60 bar, Type 1 means resisting ≥75 ksi (517 MPa) on the bar cross-section; Type 2 means resisting ≥90 ksi (620 MPa), a roughly 20% higher demand that pushes the design toward a full-tension threaded or shear-bolt system rather than a basic swage [S2][S4].
Dayton Superior’s D310C Taper-Lock High Strength coupler is a published example of a product qualified as Type 2, used where seismic or column-splice continuity demands the full tensile envelope. The same maker’s Bar Lock Transition coupler addresses diameter changes (e.g. #8 to #10) within the same Type 2 envelope, with slip tested to the same 0.25 mm ceiling across the transition joint [S7].
Coupler Family Comparison Against the Slip Limit
Four common coupler families behave differently against the 0.25 mm slip line. Cold-swaged sleeves reach 0.05–0.15 mm slip at 1.00·fy when properly pressed, but depend heavily on operator skill and rib engagement. Taper-thread couplers consistently land at 0.02–0.10 mm because the thread root carries the load mechanically rather than by friction. Shear-bolt (set-screw) couplers measure 0.08–0.20 mm, with the higher end of the band on bars whose ribs are worn or whose bolt torque sequence is interrupted. Grouted sleeve couplers used in precast typically show 0.10–0.25 mm and are the closest to the acceptance line, because the grout column itself contributes a small but measurable slip component under first load [S2][S4][S7].
Selection logic for a Type 2 splice: specify a coupler whose published ICC-ES AC133 or EAD 160129 report lists slip ≤0.10 mm at 1.00·fy with margin, and which carries a separate seismic/cyclic qualification clause. For columns in high seismic zones, prefer tapered-thread or high-strength swage over grouted sleeve, because the grouted geometry sits closest to the 0.25 mm ceiling and offers the least reserve under cyclic strain [S3][S4][S7].
Common Failure Modes and Field Slip Exceedances
Field failures on the 0.25 mm line almost always trace to one of three causes: bar-end preparation (thread undercut or swage under-pressing), bar diameter mismatch in transition couplers, and bolt torque sequence errors on shear-bolt systems. Epoxy-coated bars add a third-party variable: the coating thickness on the bar end must be removed before threading or swaging, otherwise the slip reading at 1.00·fy will routinely exceed 0.30 mm even on a coupler that passes on bare bar [S2][S7].
For rebar grades outside the U.S. default (e.g. Grade 550 / 690 MPa stock referenced in New Zealand and some EU markets), ACI 318 restrictions on Type 1 splices and on Type 2 splices at the higher grades apply, and the slip acceptance line remains 0.25 mm but the cyclic strain demand is higher in absolute terms, so coupler selection must be re-qualified against the higher f<sub>u</sub> rather than carried over from a Grade 60 data sheet [S2].
Specifying Slip on a Drawing or in a QC Plan
A spec-ready acceptance line reads: "Mechanical splices shall be Type 2 per ACI 318-19 clause 25.5.7. Slip shall not exceed 0.010 in. (0.25 mm) at 0.50·f<sub>y</sub> and at 1.00·f<sub>y</sub> per ICC-ES AC133 or equivalent. Cyclic qualification per AC133 seismic annex or EAD 160129 clause 2.2.5 shall be submitted. Epoxy coating shall be removed from bar ends prior to coupler installation." That line covers roughly 90% of what a structural engineer or QC inspector will check at the bench. [S2]
Trackable signals for the next review cycle: the ICC-ES AC133 revision status, the EAD 160129-00-0301 amendment log, and any DOT (CalTrans, TxDOT, NYSDOT) update to its Ultimate Splice qualified-products list, since these are the three live sources that publish slip-test data on production lots rather than on a single qualification coupon.
For engineers cross-checking instrumented vs. non-instrumented splices in adjacent systems, a primer on signal-level acceptance for process transmitters is unrelated but a useful contrast on what counts as a "passing reading" in a different spec family. Material-side context on the seismic rebar 1.25 TS/YS ratio sets the upstream chemistry the coupler is then asked to hold, and a cost-side read on coupler vs. lap splice is the usual reason a Type 2 line is added in the first place. A reference primer on rebar couplers ties the slip number back to the product category.
The underlying component specifications are covered under dry type transformer, and limit switch.