Bundling Effect on Bond and Development Length of Sand-Coated GFRP Bars

One of the gaps in the new Building Code Requirements for Structural Concrete Reinforced with Glass Fiber-Reinforced Polymer (GFRP) Bars is design provisions for bundled GFRP bars. This study aims at establishing the bundling factor for the development length (Ld) of sand-coated GFRP bars embedded into normal strength concrete in bundles of two and three. A total of 12 notched beams were tested in flexure with spans ranging from 1 to 3 m, where the bars and surrounding concrete are under realistic tensile stress as opposed to pull-out tests. The embedment length (Le) of the bars varied from 17 to 87 times bar diameter (db), to establish a correlation with maximum tensile stress attained, thereby enabling calculating Ld. For each bundled arrangement, counterpart beams with spaced bars were also tested for comparison. It was found that bundling bars reduced the maximum attained longitudinal tensile stress at bond failure by 18%-30% for two bars and 20%-36% for three bars, within the studied Le range of 17-87db. At the full design tensile strength (ffu), Ld of bundles of two and three bars was 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), 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.