Bond Strength and Development Length of Sand-Coated GFRP Bars Spaced and Bundled in Normal Strength and Ultra-High-Performance Fibre-Reinforced Concrete

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Kaufman, Lukas J.
bundled bars , spaced bars , bond , development length , sand-coated , GFRP bar
This thesis experimentally examines the bond strength and development length (Ld) of three sand- coated glass fibre-reinforced polymer (GFRP) bar sizes (#4, 5 and 8) embedded into ultra-high- performance fibre-reinforced concrete (UHPFRC) with 2% steel fibres. It also examines the Ld of sand-coated #5 GFRP bars bundled in two and three embedded into normal strength concrete (NSC). Both studies followed the RILEM RC5 standard notched beam bond test method. For the first study investigating GFRP bars with three bar diameters (db), concrete cover (C) varied from 20 mm to 55 mm, and embedment lengths (Le) were 4db, 9db and 14db, totalling 42 notched beams. 38 of 42 beams failed by splitting bond failure. ACI CODE-440.11-22 significantly overestimated Ld by factors of 2.0 to 3.5. The Michaud et al. (2021) design equation for Ld does not account for C or db. As such, at lower C/db ratios (i.e. at small concrete covers), it underestimated Ld while at higher C/db ratios (i.e. at larger cover), it overestimated Ld. A new equation for Ld has been developed, accounting for all critical parameters. For the second study investigating #5 GFRP bars spaced and bundled in groups of two and three, a total of 12 notched beams were tested. Clear concrete cover was consistent at 40 mm, while Le varied from 17 to 87db. It was found that bundling bars reduced the maximum attained longitudinal tensile stress at bond failure by 18-31% for two bars and 24-36% for three bars. At the full design tensile strength (ffu), the Ld of bundles of two and three bars were 1.4 and 1.5 times larger, respectively, than spaced bars. As the tensile stress ratio (ff/ffu) reduced from 1.0 to 0.34 (as in compression controlled failure), the Ld of bundles increased up to 1.9 and 2.5 times the spaced bars, respectively. An expression for this variable bundling factor is proposed. ACI CODE-440.11- 22 also underestimated Ld of spaced bars at ffu by 33%. A correction factor of 1.5 is recommended for spaced sand-coated bars with no deformations when the code equation is used.
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