Characterization of As-Constructed Aggregate Base Layer Stiffness Using Embedded Shear Wave Transducers

Measurement of aggregate base layer modulus is one of the most important quality control and quality assurance concerns of constructed pavements. However, existing in situ testing methods have several limitations in measuring layer modulus due to the limited depth of influence of test equipment and unknown stress states in the layer required in the modulus estimation or back-calculation. This paper will introduce a new approach to evaluate in situ horizontal stiffness and stress state of constructed aggregate layers by using shear wave transducers at any layer depth of sensor installation. A pair of bender elements is used as embedded shear wave transducers, which generate and detect the shear waves transferred through the aggregate material. The bender elements are mounted in a steel frame with a span to be embedded into an aggregate layer. After construction of aggregate layer and installation of the bender element system, shear waves are recorded according to a static load applied by a single tire. As the wheel load is applied on the surface above the bender element system, the shear modulus increases. Using the power relation between the shear wave velocity and mean effective stress established in laboratory test, the mean effective stress of local area in aggregate layer is estimated. The bender element system proposed in this study can be used for monitoring of in situ stiffness and stress in aggregate layer.