Over the past few decades, numerous research projects have been conducted that have studied how the physical properties of geogrids relate to performance. To date, no correlation between physical properties and performance has been documented. In an effort to claim equivalence according to construction specifications, many geosynthetic manufacturers and supplier sales representatives will routinely misrepresent this fact by publishing product data sheets that compare selected physical properties between products. The situation is further confused because many public agencies do not have performance-based specifications, so they default to specifying products based on physical properties.
The reliability of pavement design is compromised when product selection is based on physical properties, as these properties are not inputs to any validated flexible pavement design method. Proper pavement design looks at how all materials and loading conditions perform, including geosynthetics if used. Each geogrid product has different orientations, geometries, materials, rib structures and many more characteristics that determine how that product interacts with the aggregate. Differences in aggregate quality, asphalt types and thicknesses, and other variables must also be taken into account.
Geosynthetic performance must be empirically tested and evaluated in order to develop the appropriate parameters to use in a design. AASHTO has a documented process for evaluating and designing geosynthetic-stabilized aggregate base courses in flexible pavements. This standard of practice is described in AASHTO R50-09 and recommends that pavement designers verify the design parameters used. Design parameters should be based on predictive models developed through significant testing on each product considered.
Applied Research Associates (ARA) quotes from both its own review and reference to FHWA, 2013: “Products presented as equivalent shall have equivalent or better documented performance in pavement stabilization in laboratory tests, large-scale testing and full project experience for project conditions (base layer material and thickness, failure criterion, subsoil strength, etc.) Calibration and validation of the modified layer coefficient of the alternative geogrid have to be evaluated by a third party. The evaluation report, by the third party, must accompany the presentation and with the summary of qualifications of the third reviewer.”
