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What properties matter in a geosynthetic when it is being incorporated into a pavement design?

Numerous research projects over many years have tried to correlate the physical properties of geogrids with their tested performance in roads, but no correlation has yet been found. Many manufacturer and supplier sales representatives routinely misrepresent this fact in their efforts to claim equivalence under construction specifications, 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.

AASHTO has a documented process for evaluating and designing geosynthetic-stabilized aggregate base courses in flexible pavement. This standard of practice is outlined in AASHTO R50-09 and recommends that pavement designers verify the design parameters used.

Flexible pavement designs are based on expected design lives that can range from less than 100,000 ESALs to over 20 Million ESALs. Differences in aggregate quality, asphalt types and thicknesses, and other variables must be considered. Proper pavement design looks at how all materials and loading conditions work together, including geosynthetics if they are used.

Design parameters are not based on any of the physical properties reported on product data sheets which are often used to compare products. Physical geogrid properties are not inputs to any validated flexible pavement design method. Therefore basing product selection on physical properties compromises the reliability of the design.

Design parameters used for this application vary for each geogrid product, and also depend on the thickness of the pavement section, the materials used, and many other factors. 

The design parameters should be based on predictive models developed through significant testing on each product considered. Each product has different rib orientations, geometries, in-plane stiffnesses, rib structures, and many more features which determine how that product interacts with aggregate. Performance must be empirically tested and evaluated in order to develop the proper parameters to be used in a design.

"Products submitted as equivalent shall have documented equivalent or better performance in pavement stabilization in laboratory tests, full-scale tests, and completed project experience for the project conditions (base course material and thickness, failure criterion, subgrade strength, etc...) The alternate geogrid modified layer coefficient calibration and validation shall be evaluated by a third party. Assessment report, by the third party, shall accompany the submittal, and with the qualifications summary of the third party reviewer. (Quotes from ARA both from their own review and referencing FHWA, 2013)"