SIZE EFFECTS ON TENSILE FRACTURE PROPERTIES - A UNIFIED EXPLANATION-BASED ON DISORDER AND FRACTALITY OF CONCRETE MICROSTRUCTURE

Tests were carried our using three independent jacks orthogonally disposed, making it possible to apply a purely tensile force, so that the secondary flexural stresses, If kept under control, constitute a degree of error comparable with the values allowed for normal testing apparatus. The method enables a stress versus strain curve to be plotted with the descending (softening) branch up to the point where the cross-section of the tensile specimen breaks away. The principal purpose is to avoid any spurious effect that might provide a fallacious explanation of the recurring size effects on apparent tensile strength and fictitious fracture energy. Once the secondary effects have been excluded, only the disorder and fractality of the concrete microstructure remain to explain such fundamental trends. In the case of tensile strength, the dimensional decrement represents self-similar weakening of the material ligament, hue to por es, voids, defects, cracks, aggregates, inclusions, etc. Analogously, in the case of fracture energy, the dimensional increment represents self-similar tortuosity of the fracture surface, as well as self-similar overlapping and distribution of microcracks in the direction orthogonal to that of the forming macrocrack.