AASHTO R50-09, Standard of Practice
Design values need to be based on full-scale, highly controlled and monitored testing. Values and design methods should be reviewed by independent authorities with expertise in pavement design and geosynthetics.
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.
What testing and review should be performed to accurately develop design parameters and model performance of geogrids in roadbed applications?
Proper testing requires several steps, as outlined by pavement design experts. The process starts with laboratory testing to better understand how different geogrids interact with different materials. Once this is better understood, geogrids should be tested at full scale to validate the laboratory assumptions under actual wheel loads. Rigorous Quality Assurance and Quality Control procedures are needed to accurately measure and record performance. Changes in asphalt temperature, climatic conditions, pavement materials, loading conditions, layer thicknesses, and other conditions will affect performance, and therefore the measured benefit of the geogrid. The National Cooperative Highway Research Program (NCHRP) has published detailed procedures and requirements for this type of testing in NCHRP Report 512, Accelerated Pavement Testing: Data Guidelines.
This video presentation provides an overview of recent research conducted by the US Army Corps of Engineers at the US Army Engineering Research and Development Center. This testing is different from many previous studies on the use of geogrids in roadway applications. Most previous studies have been performed on unpaved sections over soft soils, or on paved section over soft soils. This recent testing was performed over stiff to very stiff subgrade soils. Additionally, trafficking for this study exceeded 800,000 ESALs. Finding from this study showed that Tensar TX geogrids could reduce asphalt by 25%, and aggregate base thicknesses by 25% while still improving pavement performance over stiff soils by approximately 62% and reducing deformation by up to 44%. Values used within Tensar’s 3rd party reviewed design software were also shown to be conservative.