A new non-destructive evaluation (NDE) methodology was investigated to more clearly identify and map deep subsurface defects in concrete bridges. To enhance the resolution of identified defects, an innovative measurement and data processing approach was developed using waveform-based ultrasonic tomography. The integrated linear-nonlinear ultrasonic tomography utilized the same sensor array for linear and nonlinear measurements to map subsurface defects, where received signals were decomposed into harmonics to generate subsequent inputs for linear-wave velocity-based tomography and acoustic nonlinearity coefficient-based tomography. In this light, deep defects were honed-in and accurately identified. Numerical simulations identified inclusions on a tomographic map produced via linear wave velocity using a frequency of 300 kHz and a configuration of 16 arrayed transmitter and receiver sensors. However, because linear ultrasonics limits resolution of the wavelength, especially in heterogeneous materials, nonlinear ultrasonics and sub-wavelength imaging, or second-order harmonics, was necessarily – and successfully – integrated. The dual-integrated tomography methodology was applied to coupon-scale concrete samples, a large-scale post-tested bridge girder that had been strengthened against damaging hurricane wave forces, and a bridge pier from the I-55 bridge overpass in Chicago. Although the first two series of tests successfully identified subsurface damage with excellent resolution, field testing brought to light a need to improve this integrated approach by making it more field-conducive, specifically in reducing the number of cables, the size of sensors, and by alleviating sensor-coupling challenges. The coupon-scale and large-scale results, however, confirm the viable promise of the integrated linear-nonlinear ultrasonics NDE technology.
The final report is available.