In this paper, sandstones from three Czech localities were subjected to mechanical fracture tests in order to obtain their properties. Carboniferous sandstone from the Staříč site was primarily different from the two other Cretaceous sandstones from Podhorní Újezd and Javorka localities in the type of grain contact, as well as in their mineralogical composition of the rock matrix and cement. These differences were primarily reflected in different rock porosities. An advanced assessment of the fracture response of the chevron notch specimens made of sandstones subjected to three-point bending test was carried out by means of the GTDiPS program suggested for processing the loading diagrams. Bending Young's modulus, mode I fracture toughness, and fracture energy were subsequently calculated for all tested sandstone samples. Obtained outcomes show that the sandstone from the Staříč mine exhibits several times higher values of investigated properties than the Podhorní Újezd and Javorka sandstones. This was a result of a higher degree of rock compaction, siliciferous rock cement and, therefore, relatively low total porosity. Internal rock texture and mineralogical composition of matrix or cement are thus one of the most important factors influencing the values of mechanical fracture parameters of sandstones.
Content-based composites are traditionally a commonly used material in civil engineering structures. The basic representative of this type of material is concrete, a quasi-brittle composite in which crack resistance can be achieved by the addition of fibres. The double-K fracture model can be used to calculate the fracture-mechanical parameter values of structural concrete with and without popypropylene fibres. This model combines the concept of cohesive forces acting on the crack length with a criterion based on the stress intensity factor, using a ‘softening function‘ to determine the cohesive part of fracture toughness. In this paper, authors determine the effect of the type of this softening function on the evaluation of fracture tests performed on sets of concrete specimens with and without polypropylene fibres. and Obsahuje seznam literatury