MICRO-SCALE AIR VOID STRUCTURES IN CONCRETE AND THEIR EFFECT ON FAILURE BEHAVIOR

Tomography images and image processing methods are extensively employed by researchers to investigate the micro-dimensional air voids that are formed in the internal structure of concrete.  Finite element method-based fracture analysis is required to investigate the effect of the mechanical behavior of concrete and micro-dimensional crack development; micro-dimensional voids cannot be experimentally observed because of their small scale.  Although concrete that is exposed to uniaxial compression remains in the elastic region, realistic brittle failure can be achieved using the damage plasticity model, which considers the effect of tension cracks that form around micro air voids, which in turn enhance cracking development and the compressive strength of concrete. Within the scope of this study, concrete cubes with a side dimension of 15 cm were prepared. Three groups of these cubes are composed; each group contains three specimens. The first group contains additive-free control specimens and the remaining two groups contain specimens that are mixed with two different ratios of air-entraining admixtures. After the concrete specimens were created,  core samples were prepared and scanned with micro computed tomography.  These 2D and high-resolution images are modeled using the image processing software Simpleware and exported to the FEM-based analysis software Abaqus.  The volume, void ratio and mass properties of fresh and hardened concrete, which are experimentally obtained, are compared with the physical properties of 3D-modeled specimens.  Based on the mass and volume analyses, these 3D models, which have micro-dimensional air voids that are assigned with the parametrized CDP material properties were simulated, and a uniaxial compression tests and fracture analysis were performed.  According to the analysis results, the relations between crack development and quantity and the distribution of entrained air were discussed.