Evaluating Freeze-Thaw Damage in Concrete with Acoustic Emissions and Ultrasonics

Abstract

The purpose of the experiments described in this chapter was to apply acoustic emission technology in a passive mode to identify crack development in concrete specimens exposed to thermal cycling. Acousto-ultrasonics techniques, or using the acoustic emission sensors in an active mode to function as both pulse generators and receivers, were applied to periodically measure the speed of an ultrasonic wave sent across the specimen throughout thermal cycling. The observed reduction in wave velocity can be correlated to the degrading elastic properties of the concrete matrix due to damage development. These experiments were performed on concretes with a wide range of entrained air volumes which were conditioned to various degrees of saturation to identify the critical degree of saturation (S _crit) required for deterioration. As predicted, concrete specimens conditioned to high levels of saturation showed more damage than samples at or below the critical degree of saturation. This was made clear by studying the activity parameters of the recorded hit data from passive acoustic emission methods as well as by comparing the damage parameter determined from the acousto-ultrasonic method. Large amounts of acoustic emission activity were observed during periods of freezing resulting from cracking due to volume expansion as well as during thawing due to friction during crack closure. Small fluctuations in pulse velocity which clearly correlated to the temperature cycles were observed during testing of a control aluminum sample, showing a minor dependence of pulse velocity on temperature effects on the coupling agent alone. The combined approach using both active and passive acoustic emission proved to be highly informative, making it possible to quantify bulk material damage development while observing trends in acoustic emissions throughout multiple freezing and thawing cycles.