Amplitude and phase measurements of continuous diffuse fields for structural health monitoring of concrete structures

Structural health monitoring systems for civil structures based on ultrasonic guided waves face the problem of very high attenuation of mechanical waves in concrete. This means that it is difficult to cover large areas with a reasonable number of transducers. Continuous acoustic transmission, as opposed to pulse transmission, enables narrow-band, coherent measurements, which can be used to detect meaningful signals for very low signal levels. This study investigates the use of amplitude and phase measurements of single-frequency continuous acoustic transmissions, between two piezoelectric transducers, as indicators of damage in a concrete slab. The sensitivity to damage of these measurements are compared to pulsed coda wave analysis. The measurements of the continuous signals are performed with a lock-in amplifier, which is known to be able to detect very low signal levels. The amplitudes of the transmitted signal is decreased in order to simulate increased distance between transmitting and receiving transducer. It is shown that amplitude and phase measurements of the continuous wave field provide indication of damage equal to that of coda wave analysis of transmitted pulses and that these can be detected at much lower signal amplitudes. This indicates the possibility to increase spacing in a network of reciprocal transducers in a structural health-monitoring system