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Development Length and Splice Strength of High-Strength Reinforcing Bars in High-Strength Concrete
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Abstract
Thirty-nine tension lap-splice tests of high-strength reinforcement in high-strength concrete were performed and combined with test results in a large database to develop and evaluate design criteria for splice and development length. The experimental study included spliced No. 5, No. 8, No. 11, and No. 14 (16, 25, 36, and 43 mm) bars loaded to stresses at splice failure ranging from 70 to 150 ksi (483 to 1034 MPa) in concrete with compressive strengths between 6 and 16 ksi (41 to 110 MPa), with all but one specimen with strengths above 10 ksi (69 MPa). Other variables include the quantity of confining transverse reinforcement; minimum concrete cover [1 to 3.1 in. (25 to 79 mm)]; and splice lengths [17db to 54db], where db is the bar diameter. ASTM A1035 (Grade 120 (830)) bars were used to ensure splice failure prior to yielding.
The development/splice strength criteria developed in this study account for the effect of concrete cover, bar spacing, bar geometry, bar size, splice/development length, confining transverse reinforcement, and concrete compressive strength. The design expression incorporates a strength reduction factor based on a reliability-based approach and the Monte Carlo technique.
The contribution of concrete strength to bond for bars not confined by transverse reinforcement is best represented by the ¼ power of concrete compressive strength, and for bars confined by transverse reinforcement, the increase in bond strength due to transverse reinforcement is best represented by ½ power of concrete compressive strength. Increases in splice/development lengths above 60db are ineffective for bars not confined by transverse reinforcement.
The design criteria in ACI 318-19 for the development length of straight reinforcement are unconservative for bars not confined by transverse reinforcement, regardless of bar size and bar stress, and are unconservative for No. 18 (57 mm) bars confined by transverse reinforcement. ACI 318-19 is also unconservative for No. 6 (19 mm) and smaller bars confined by transverse reinforcement when (1) the smaller of the concrete side cover to the center of the bar or half the center-to-center bar spacing is less than the bottom (or top) cover to the center of the bar and (2) the clear distance of a splice to the nearest vertical leg of confining transverse reinforcement is greater than 4db. The descriptive equation in ACI 408R-03 progressively overpredicts bond strength as bar size increases, regardless of the presence of confining transverse reinforcement and for bars not confined by transverse reinforcement with an increase in splice length. The descriptive equation developed in this study is not biased with respect to bar size and not biased with respect to splice length for bars not confined by transverse reinforcement. It slightly underpredicts the bond strength with increased splice length for bars confined by transverse reinforcement. The descriptive equation and design criteria developed in this study are applicable for concrete compressive strengths up to 16 ksi (110 MPa) and bar stresses up to 125 ksi (862 MPa) for bars not confined by transverse reinforcement and 155 ksi (1069 MPa) for bars confined by transverse reinforcement.
Four modifications are proposed to reduce the unconservative nature of ACI 318-19 design criteria: (1) limit maximum splice/development length to 60db for bars without confining transverse reinforcement; (2) use a confinement factor of 1.15 and 1.0 for bars without and with confining transverse reinforcement, respectively; (3) use a bar size factor of 1.2 for bars larger than No. 11 (36 mm) not confined by transverse reinforcement and bars larger than No. 14 (43 mm) confined by transverse reinforcement; and (4) do not consider vertical confining reinforcement, and thus, take Ktr = 0, for No. 6 (19 mm) and smaller bars when (i) the smaller of the concrete side cover to the center of the bar and half the center-to-center bar spacing is less than the bottom (or top) cover to the center of the bar, and (ii) the clear distance of a splice to the nearest vertical leg of confining transverse reinforcement is greater than 4db.
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Date
2024-07
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Publisher
University of Kansas Center for Research, Inc.
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Keywords
ACI 318, ACI 408, Bond, Building code, Development, High-strength concrete, High-strength steel, Lap splice, Reinforced concrete, Reliability, Splitting, Structural engineering
Citation
Adhikari, R., Darwin, D., and O’Reilly, M. “Development Length and Splice Strength of High-Strength Reinforcing Bars in High-Strength Concrete,” SM Report No. 160, University of Kansas Center for Research, Inc., Lawrence, KS, July 2024, 401 pp.