3D Measurements to Determine Micromechanical Energy Dissipation in Steel Fiber Reinforced Concrete
Publication Name: Proceedings of the 8th International Conference of Concrete and Concrete Structures
Adding sufficient quantities of steel fibers to concrete has long been known to transition a relatively brittle material to a relatively ductile one. That transition is made possible by a number of well known toughening mechanisms including, fiber-matrix debonding and pull-out, additional matrix cracking, as well as fiber bending and fracture. In the work here, we seek to measure these different energy dissipation mechanisms through the analysis of 3D microstructural images. Reinforced and unreinforced flexure specimens of ultra high performance concrete were scanned using an x-ray computed tomography (CT) imaging system that allowed quantitative measurement and characterization of internal features. The CT imaging was done in conjunction with three point bending tests of notched 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 individual fibers. In post-fracture 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 is a hundred-fold increase in energy dissipated. Through quantitative analysis of the tomographic images, we could account for roughly 90% of this 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 due to fiber pull-out.