Several nondestructive testing methods can be used to estimate the extent of damage in a concrete structure. Pulse-velocity
and amplitude attenuation methods are very common in nondestructive ultrasonic evaluation. Velocity of propagation is not very sensitive to the degree of damage unless a great deal of micro-damages have evolved into localized macro-damages. The amplitude attenuation method is potentially more sensitive to damage than the pulse-velocity
method. However, this method depends strongly on the coupling conditions between the transducers and the
concrete and hence is unreliable. In a previous study, a new active modulation approach, Nonlinear Active Wave
Modulation Spectroscopy, was developed and found promising for early detection of damage in concrete. In this
procedure, a probe wave is passed through the system in a fashion similar to regular acoustic methods for inspection.
Simultaneously, a second, low-frequency modulating wave is applied to the system to effectively change the size and
stiffness of flaws microscopically and cyclically, thereby causing the frequency modulation to change cyclically as well.
The resulting amplified modulations can be correlated to the extent of damage and quantification of small damage
becomes possible. In this paper, we present the use of Hilbert-Huang transform to significantly enhance the damage
detection sensitivity of this modulation method by performing time-frequency decomposition of nonlinear non-stationary