Geogrid reinforcement in hot-mix asphalt: Interlayer shear bond strength assessment

Abstract

With the increasing use of geogrid reinforcements to mitigate reflective cracking in hot-mix asphalt (HMA) overlays, interlayer (interface) bonding becomes an even more critical aspect of HMA placement/construction to mitigate delamination and debonding of the overlay. To comparatively evaluate the interlayer bond strength due to the effects of the geogrid reinforcements, the shear bond strength test was conducted in this laboratory study, using unreinforced control HMA samples as the reference datum. Cylindrical HMA samples (150 mm f) gyratory compacted in two 75-mm lift thicknesses, with the geogrid reinforcement in-between the two lifts, were used for testing at room temperature under a monotonically shear loading rate of 5 mm/min. Emulsified asphalt was used as the interlayer tack coat and six different geogrid materials, which are polyester-based (FA) and fiberglass-based (FG), were comparatively evaluated. As theoretically expected, the control (unreinforced) HMA samples exhibited superiority followed closely by samples reinforced with polyester-based geogrids. Although comparable to the values reported in the literature, HMA samples reinforced with fiberglass-based geogrids performed the poorest with the lowest interlayer bond strengths – that is the polyester-based outperformed the fiberglass-based geogrids. Overall, the interlayer bond strength exhibited a general decreasing trend with a decrease in the geogrid mesh size (open area), increase in the geogrid strand thickness, and material grade. Thus, in as much as reflective crack mitigation is structurally desired, due diligence must be cautiously exercised when selecting the geogrid type/grade for use in HMA reinforcement to ensure sufficient interlayer bonding and minimize any potential delamination/debonding problems in service.