Health monitoring of shotcrete on excavated rock surfaces is a crucialissue when considering the short-term and long-term stability of tunnels. The stateof the back-surface of shotcrete is investigated using impact-echo (IE) techniquesand ground penetration radar (GPR). A laboratory-scale experimental study wasperformed as a field application in this study. Three different bonding conditions(i.e., void, de-bonded, or fully bonded) were experimentally modeled. Signalsobtained from the IE tests were analyzed at the time-frequency domain. Using anintegrated active signal-processing technique coupled with a Short-Time FourierTransform (STFT) analysis, the bonding state of shotcrete can be effectivelyevaluated compared to GPR. The IE method coupled with the Short-Time FourierTransformation shows better performance than GPR for monitoring the quality ofshotcrete. Limited field tests were also performed to verify the evaluation chartfor the health monitoring of tunnel shotcrete linings.
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