Bender element sensor technology to quantify local stiffness enhancement of geogrid-stabilized aggregate specimens

Geogrids improve pavement performance and extend lifespan by making the unbound aggregate base/subbase course stiffer. Extruded (i.e., punched and drawn) geogrids are widely used in pavement applications. This paper presents an experimental evaluation of multi-axial extruded geogrids with various aperture shapes for pavement applications. The study quantified modulus enhancement ratio and identified the extent of the geogrid-stiffened zone. Two types of aggregates (i.e., crushed limestone and partially crushed gravel), commonly used in Illinois for base or subbase course, were utilized. Repeated load triaxial tests were conducted on 6-in. diameter and 12-in. height cylindrical specimens instrumented with embedded shear wave transducers in the form of Bender Element (BE) sensors. The BE pairs were horizontally placed at three different heights above the geogrid. Resilient moduli of the aggregate samples were determined following the AASHTO T 307 standard test procedure. Concurrently, the localized small-strain moduli at different heights above the midheight of the specimen were determined from the shear wave velocities computed according to the first arrival times from the BE sensors recorded at the end of the application of each stress state. The results reveal that resilient modulus tests do not capture the geogrid effects, while BE sensors can quantify the local stiffness enhancement due to a geogrid, and the extent of the geogrid-stiffened zone. Results show that the multi-axial geogrids provided a 45% increase in small strain modulus for both aggregate types in the vicinity of the geogrid.