Combining X-Ray imaging and acoustic emission to measure damage progression in ultra-high-performance-concrete

The toughness and ductility of fiber reinforced concrete is made possible by a number of wellknown toughening mechanisms including, fiber-matrix debonding and pull-out, additional matrix cracking, as well as fiber bending and fracture. While it is sufficient for phenomenological modeling to know collective influence of these mechanisms, for material design and optimization we would like to measure the individual effects. In the work presented here, fracture of fiber reinforced ultra-high-performance concrete (UHPC) was examined using two complementary techniques: x-ray CT and acoustic emission (AE). 50-mm diameter specimens of two different fiber types were scanned both before and after load testing. From the CT images, fiber alignment was evaluated to establish optimum and pessimum specimen orientations. As expected, fiber orientation affected both the peak load and the toughness of the specimen, with the optimum toughness being between 20 and 30% higher than the pessimum. Cumulative AE energy was also affected commensurately. Post-test CT scans of the specimen were used to measure internal energy dissipation due to both matrix cracking and fiber pullout using calibration measurements for each. AE data, processed using an artificial neural network, was also used to classify energy dissipation. CT analysis showed that fiber pullout was the dominant energy dissipation mechanism, however, the sum of internal energy dissipation measured amounted to only 60% of the total energy dissipated by the specimens as measured by the net work of load. AE analysis showed a more balanced distribution of energy dissipation. AE data additionally showed how the dissipation mechanisms shift as damage accumulates.