Study of bond-dependent coefficient for GFRP reinforcing bars

A consistent approach to reinforced concrete design requires that glass fiber reinforced polymer (GFRP) reinforcing bars be treated in a manner consistent with steel reinforcing bars. International design guides and standards have therefore taken the approach of normalizing the bond characteristics of GFRP bars to those of steel bars by adopting a so-called bond-dependent coefficient, referred to as kb in this work. The present study collects flexure-based bond tests reported in 19 previously reported studies encompassing different GFRP bars surface treatments and diameters embedded in a range of concrete strengths and flexural test geometries. The value of kb was found to be strongly correlated with the GFRP reinforcing bar stress at which kb is determined, while correlations with other factors including concrete strength, cover concrete and GFRP bar diameter were weak. Normalizing kb by the crack width at which was determined is proposed to allow a more consistent treatment of data from different sources. The calculation of the bond coefficient kb was found to be relatively insensitive to the surface treatment of the GFRP bar: whether deformed (ribbed) or sand-coated. The ACI 440-recommended default value, kb = 1.2, was shown to be appropriate for all bar types, although marginally conservative.