Abstract

Size effect is a fundamental phenomenon in concrete. It is characterised by decreasing strength and increasing brittleness of concrete with increasing size. The thesis includes experimental and theoretical elements. The main goal of the thesis were investigations of a size effect at the aggregate level by taking fracture into account with the discrete element method (DEM) for various failure modes. Comprehensive experiments on a size effect were carried out for a tensile splitting test. In experiments, the micro-computed tomography (micro-CT) and digital image correlation (DIC) technique were used. DEM for concrete was calibrated with the aid of simple uniaxial tests of uniaxial compression and uniaxial tension. In numerical calculations with the real internal concrete structure, the distribution and evolution of inter-particle contacts, inter-particle forces, force chains, rotations, broken contacts and energies was analyzed (depending on the specimen size) for various failure mechanisms: quasi-brittle, brittle and very brittle (snap-back). Experimental results were directly compared with experiments. Good agreement was achieved. In addition, comprehensive size effect experiments on reinforced concrete beams under bending without and with stirrups were carried out that were scaled along the height or length.

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