Influence of entrapped air in cementitious materials on damage propagation

The increasing attention to numerical issues related to material modeling is still a strong incentive to develop sound mechanical models that can explain material behavior up to failure. A procedure to handle a robust geometric meso-scale reconstruction of concrete materials is here proposed, based on X-ray Computed Tomography (XCT). XCT as advanced detection technique has been adopted by several authors who aimed to investigate the behavior of concrete at the meso level, with satisfactory results. This study applies XCT on concrete made with calcareous aggregates: solid models have been generated with such technique and discretized in space to be numerically studied via the Finite Element (FE) method and the numerical results compared with uniaxial compression tests on the same scanned specimens. For the numerical analyses a specific non-associated elasto-plastic constitutive behavior, coupled with damage, has been developed for the cement matrix, whereas the coarse aggregates have been treated as elastic. The surface roughness of coarse aggregates has an obvious effect on the interfacial bond strength. Therefore a contact algorithm for the thin interface region known as Interfacial Transition Zone (ITZ) between aggregates and cement paste has been included in the numerical analyses. The presence of voids in cured concrete, in particular, is discussed in order to appreciate the influence of air segregation in ordinary concrete strength, when damage evolution and cracking mechanisms are evaluated included explicit ITZ modeling. While entrained air bubbles (dimension of voids below 0.1cm and spherical, typically) can be beneficial to concrete, especially for freezing/thawing cycles, and can be intentionally created by adding a liquid admixture specifically designed for this purpose, entrapped air voids (> 0.1cm) are usually irregular in shape and are created during improper mixing, consolidating and placement of concrete. Air pockets, or irregularly sized air voids can have negative effects on product appearance, strength and durability therefore they are usually removed with proper vibration techniques.