Bond-Dependent Coefficient kb for New-Generation GFRP Bars

The new American Concrete Institute (ACI) design code for glass fiber-reinforced polymer (GFRP)-reinforced concrete (RC) members specifies a single value of 1.20 for the bond-dependent coefficient (k(b)) of all types of GFRP bars. This value was chosen based on test data from this project, as well as a compilation of data available in the literature. This paper reports on an experimental study that assessed the k(b) for the new generation of GFRP bars from five different manufacturers with different surface types: deformed/ribbed, helically deformed, helically grooved, double-helical wrap/sand-coated, and sand-coated. Two bar sizes (No. 5 and No. 8)-with 15.9 and 25.4 mm nominal diameters representing the typical range of GFRP-reinforcing bars used in practice as longitudinal reinforcement in concrete members subjected to bending-were selected from each of the manufacturers. Five RC beam replicates were used to increase experimental accuracy. Therefore, a total of 60 beams, including 50 beams reinforced with GFRP bars and 10 control beams reinforced with conventional steel bars for comparison purposes, were constructed and tested to failure according to a predetermined test method. Based on the analysis, the study confirms using the bond-dependent coefficient value of 1.20 as adopted in the new ACI design code for GFRP-RC members. This value is recommended for GFRP bars complying with (or exceeding) the material specification listed in the ASTM International standard specification for solid round GFRP bars for concrete reinforcement.