Revisiting Critical Flaws in Cement-Based Composites

In this work, X-ray microtomographic images were analyzed to quantify the influence of void spaces in small mortar specimens, with a particular focus on the porosity of the interfacial transition zone (ITZ). Specimens were nominally 5-mm-diameter, 4-mm-long cylinders with 0.5-mm-diameter glass bead aggregates. Specimens were scanned via synchrotron-based X-ray microtomography while they were positioned in an in situ loading frame in a split cylinder configuration. Scans of undamaged specimens were evaluated for porosity both in the bulk paste and in the ITZ. Specifically, voids in the paste and porosity in the ITZ were superimposed onto a map of the principal tensile stress in the specimen in an attempt to identify critical flaws and to measure their role in split cylinder strength. Results indicate that a stress intensity factor-type approach can be used to identify critical flaws in cement paste specimens. Similarly, a critical ITZ region can be identified based on local principal stress and local ITZ porosity. In mortar specimens, this critical ITZ region could account for most of the splitting failures that were not accounted for by a critical flaw. However, some specimens exhibited neither a critical flaw nor a critical ITZ region, suggesting prepeak microcracking or some other nonlinear fracture phenomena. (C) 2016 American Society of Civil Engineers.