By: Jim Penman
The Alabama Department of Transportation requested that GlasGrid 8501 be installed on the National Center for Asphalt Technology test track so that any construction difficulties or performance issues would be documented. In June 2000, GlasGrid 8501 was installed by Industrial Fabrics between a pair of 2-in. layers of stone matrix asphalt (SMA) on section W1 of the inaugural NCAT Pavement Test Track.
The entire track is supported by 20 in. of hot-mix asphalt (HMA) base to isolate distresses to the top 4 in. The first 100 ft of section W1 contained no reinforcement and acted as a control section for testing. The Marshall SMA test mix consisted of a 3/4-in. nominal maximum aggregate size, crushed granite and flyash mineral filler. A 6.2% SBR-modified PF76-22 liquid binder was specified. An emulsion tack coat of type CSS-1h was applied at a rate of 0.03 gal per sq yd before the placement of each lift of asphalt. The GlasGrid product was placed after the application of the tack coat on the binder course. Since section W1 was installed in 2000, a fleet of heavy, triple-axle trucks has applied over 20 million equivalent single axle loads (ESALs). Typically, it would take over 20 years for an interstate pavement to experience this level of load-induced damage. In 2006, the section was excavated and tested.
Following trafficking, longitudinal cracking was observed in the control test section. No longitudinal cracking was observed in the last 100 ft where GlasGrid was used to reinforce the asphalt. The cracking in the unreinforced area appeared along the centerline joint between the inside and outside lanes. A core sample removed prior to the main excavation revealed that the GlasGrid product was still intact and bonded to the sandwiching layers of the SMA mix.
Buzz Powell, test track manager of the NCAT organization, noted, “ GlasGrid did not create any problems during the installation of the product.” Introduced in 1989, the GlasGrid System consists of stiff environmentally friendly fiberglass material coated with an elostomeric polymer. The grid is rolled out over a thin leveling course placed before the main asphalt overlay. With its pressure-sensitive adhesive backing, installation of GlasGrid for reinforcement is easy and generally considered the most expedient installed interlayer system available.
