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The paper describes experimental and numerical results of concrete fracture under quasi-static uniaxial compression. Experimental uniaxial compression tests were performed on concrete cubic specimens. Fracture in concrete was detected at the aggregate level by means of three non-destructive methods: three-dimensional X-ray microcomputed tomography, two-dimensional scanning electron microscope and manual two-dimensional digital microscope. The discrete element method was used to directly simulate experiments. Concrete was modelled as a random heterogeneous four-phase material composed of aggregate particles, cement matrix, interfacial transitional zones and macrovoids based on experimental images. Two- and three-dimensional analyses were carried out. In two-dimensional analyses, the real aggregate shape was created by means of clusters of spheres. In three-dimensional calculations, spheres were solely used. A satisfactory agreement between numerical and experimental results was achieved in two-dimensional analyses. The model was capable of accurately predicting complex crack paths and the corresponding stress–strain responses observed in experiments.

SUMMARYThe paper deals with numerical investigations of the behaviour of granular bodies during shearing. Shearing of a narrow layer of sand between two very rough boundaries under constant vertical pressure is numerically modelled with a "nite element method using a hypoplastic constitutive relation within a polar (Cosserat) continuum. The constitutive relation was obtained through an extension of a non-polar one by polar quantities, viz. rotations, curvatures, couple stresses using the mean grain diameter as a characteristic length. This relation can reproduce the essential features of granular bodies during shear localization. The material constants can be easily determined from element test results and can be estimated from granulometric properties. The attention is laid on the in#uence of the initial void ratio, pressure level, mean grain diameter and grain roughness on the thickness of shear zones. The results of shearing are also compared to solutions without the polar extensions. The FE-calculations demonstrate that polar e!ects manifested by the appearance of grain rotations and couple stresses are signi"cant in the shear zone, and its thickness is sensitive to the initial void ratio, mean grain diameter and layer height. The e!ect of the pressure level is rather low within the considered range.

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