Bender Element Field Sensor for the Measurement of Pavement Base and Subbase Stiffness Characteristics

Abstract

Layer modulus values are important input parameters in mechanistic pavement design and evaluation methods. Direct measurement of the stiffness characteristics of pavement base/subbase has been a challenging task. Nondestructive testing methods that are commonly used based on surface deflection measurements not only require a backcalculation process, but also have limitations on measuring local stiffness within the layer. This paper presents the result of a recent research effort at the University of Illinois aimed to develop a new sensor for the direct measurement of the in-situ moduli of constructed unbound pavement layers. The new sensor employs bender element (BE) shear wave transducers embedded in a granular base/subbase to evaluate the layer modulus from shear wave velocity measured at any depth and any orientation. To provide appropriate protection for the BE sensor and its cable connections, a stainless-steel cable guide, a sensor protection module, and a protection cover for the sensor were designed and optimized. A laboratory calibration box containing sand-sized crushed aggregates was used in the development stage of the BE sensor design. The BE sensor results were also studied for a typical dense-graded base course aggregate commonly used in Illinois. Finally, the BE sensor was installed in a field trial in newly constructed airport pavement test sections, and its layer modulus measurements were compared with results estimated from Dynamic Cone Penetrometer testing. The new BE field sensor has proven to be a viable direct measurement technique in transportation geotechnics applications to monitor stiffness characteristics of pavement granular base/subbase layers.

Airport Pavement Stiffness Monitoring and Assessment of Mechanical Stabilization using Bender Element Field Sensor

“…Based on the theory of elasticity, the elastic modulus (in a small-strain range) of an isotropic material is a function of shear modulus and Poisson’s ratio, and can be calculated as follows ( 12 , 19 , 22 ):…”

“…The BE field sensor, which was developed recently at the University of Illinois, was used to measure the shear wave velocity of constructed pavement unbound aggregate base. The BE field sensor estimates the in-situ stiffness of the pavement base and subbase layer through the shear wave velocity measurement ( 12 ). A pair of BE shear wave transducers attached at each end of the BE field sensor generates the shear wave and collects the shear wave velocity propagating through the constructed aggregate material.…”

“…A new bender element (BE) field sensor was recently developed to quantify the stiffness characteristics of the unbound aggregate layer of the pavement through the measurement of shear wave velocity ( 12 ). From a recent field application of the BE field sensor conducted at the University of Illinois, the capability of BE field sensors to estimate and monitor the in-service pavement modulus was demonstrated ( 13 , 14 ).…”

Full-scale evaluation of multi-axial geogrids in road applications

“…A literature review suggested that there are varying methodologies for the interpretation of bender element response data, particularly the designation of arrival time (Viggiani and Atkinson 1995;Arulnathan et al 1998;Fonseca et al 2008;Gu et al 2015;Kang et al 2021). Some have suggested that arrival time is the first indication of the received signal, while others have suggested an input peak to received peak methodology.…”

Degraded Ballast Stiffness Characterization Using Bender Element Field Sensor and PANDA® Penetrometer

“…A pair of bender element (BE) shear wave transducers can be made a field sensor, as recently developed for embedding in unbound aggregate layers of road and airfield pavements, to measure shear wave velocity of the granular medium and estimate layer stiffness characteristics. Multiple applications of BE field sensors both in laboratory and in full-scale field installations have been proposed and successfully demonstrated with validated sensor capabilities for measuring shear wave velocity accurately for a direct quantification of unbound aggregate layer stiffness (17)(18)(19)(20). However, such a BE field sensor has not yet been applied to the large and uniformsized railroad ballast.…”