Using Bender Elements to Investigate the Effect of a Multi-axial Geogrid on Aggregate Layer Stiffness

Geogrid inclusion in low-volume road applications has been shown to improve rutting performance. Although rutting performance and traditional instrumentation response data provide meaningful insights into pavement behavior, they do not provide a direct measurement of material behavior near the geogrid. Recently, the University of Illinois at Urbana-Champaign has developed field-deployable shear-wave transducers, that is, bender elements to measure the localized stiffness enhancement attributed to geogrid inclusion in unbound aggregate layers, which have been verified in laboratory and limited field applications. Bender element sensors were installed in a full-scale accelerated pavement test experiment at the U.S. Army Engineer Research and Development Center that included a recently developed multi-axial hexagonal geogrid. The sensors were installed directly on the geogrid and 10 cm above the geogrid in the unbound aggregate layer to measure the geogrid zone of influence. The test item was trafficked with a dual-wheel truck gear, and bender element measurements were made at select intervals throughout traffic application. To estimate the unbound aggregate moduli, the data were analyzed using two techniques: first arrival time and peak-to-peak arrival time. The bender element sensors were found to be sufficiently robust to survive installation, construction, and trafficking. Calculated modulus values 10 cm above the geogrid were found to be higher than those directly on the geogrid, which was an unexpected finding. However, a forensic investigation revealed moisture and fines migration into the bottom of the unbound aggregate layer, which could have dampened the shear-wave signal, resulting in lower calculated modulus values.