Measuring three-dimensional damage in concrete under compression

The fracture process of cement-based materials caused by compressive loading is a complex, three-dimensional phenomenon that occurs as a result of material heterogeneity and complicated mixed-mode cracking mechanisms. To investigate the relationship between surface and internal cracking, two high-resolution, nondestructive evaluation techniques are used to measure crack growth. Digital image correlation (DIC) is used to characterize the two-dimensional, surface fracture pattern, while three-dimensional, internal behavior is measured with x-ray microtomography (XMT). Rectangular mortar specimens (38.1 × 12.7 × 12.7 mm) containing both sand and graphite aggregates are examined. These techniques give complementary information about crack geometry and development: DIC is more effective at determining crack width and location of small cracks, while XMT depicts the shape of larger cracks more successfully and shows the influence of internal features on the fracture process. The effect of aggregate shape and strength and the subsequent influence of increased crack distribution on ductility are discussed.