Measurement of Energy Dissipation Mechanisms in Fracture of Fiber-Reinforced Ultrahigh-Strength Cement-Based Composites

In the study described here, reinforced and unreinforced specimens of reactive powder concrete were scanned using an X-ray computed tomography (CT) imaging system that allowed characterization and measurement of internal features. The X-ray CT imaging was done in conjunction with three-point bending tests of notched beam specimens. Unreinforced specimens were used to measure specific fracture energy in a way that accounts for the irregular shape of the fracture surface. For fiber-reinforced specimens, 3D digital image analysis techniques were used to measure fiber volume fraction, as well as the orientation of each individual fiber. In postfracture scans, the total amount of internal cracking was measured, as was the degree of fiber pullout relative to undamaged specimens. Measurements show that with a nominal steel fiber volume fraction between 3.5 and 4.0% there can be a greater than a 100-fold increase in the net work of load. Through quantitative analysis of the tomographic images, we could account for close to 90% of that increase. The analysis shows that roughly half of the internal energy dissipation comes from matrix cracking, including the crack branching and multiple crack systems facilitated by the fibers, while the remaining energy dissipation is a result of fiber pullout.