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Evaluation of the Q-factor as a Metric for Distributed Damage in Concrete

Distributed damage in concrete is associated with microcracking due to delayed ettringite formation (DEF), alkali silicate reaction or freeze thaw (F-T) cycles. A possible metric for quantifying this type of damage by ultrasonic NDT is the Q factor, which is the ratio of the resonant frequency to the damping factor. The Q factor is readily obtained from the width of the resonant frequency peak in the frequency spectrum. Laboratory specimens of concrete were prepared and subjected to either DEF favorable conditions or freeze-thaw cycling for up to 220 days. The frequency response of the samples was measured using ASTM C597-95 “Standard Specification for Pulse Velocity Measurements in Concrete”. The F-T samples showed a significant decrease in Q with increasing cycles, however, the DEF specimens showed either no correlation with expansion, or a positive one indicating that concrete strength actually increased with time. Further analysis of the Q-factor showed that the damping factor does not correlate with damage in either case. This is apparently because the damping is dominated by the hydrated cement paste, which is not significantly affected by the DEF or F-T processes. Consequently the changes in the Q factor are the result of reductions in the resonant frequency. The conclusion is that microcracking is a significant mode of damage for F-T but not for DEF. This is consistent with theoretical models of the difference in the distribution of internal stresses between the two types of damage.

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