Effect of GFRP bar diameter and concrete cover on bond and development length in UHPFRC

Abstract

This study investigates the bond strength and development length of glass fiber-reinforced polymer (GFRP) sand-coated bars embedded into ultra-high-performance fiber-reinforced concrete (UHPFRC) with 2% steel fibers. A total of 42 notched beams following the RILEM RC5 standard bond test method were fabricated. The study focused on three GFRP bar sizes (#4, 5 and 8) with diameters (db) of 13.5 mm, 17.3 mm and 27.0 mm, while varying the concrete cover to the center of the bar (C) from 20 mm to 55 mm, which results in C/db ratio ranging from 1.0 to 3.7. Three embedment lengths (Le) for each case were used: 4db, 9db and 14db. The study established a correlation between development length (Ld) and (C/db). All beams, except for four, failed by splitting bond failure as would be expected. ACI440.11–22 code equations significantly overestimated bond strength at low (Le/db) ratios and underestimated it at high (Le/db) ratios as well as overestimated Ld by factors of 2.0 to 3.5. The Michaud et al. (2021) design equation for Ld which is intended for UHPFRC had limitations in that it 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 general equation for Ld has been developed using the combined database, accounting for all critical parameters.