Gefügeabhängigkeit technischer Gesteinseigenschaften

Abstract

By virtue of their usually complex geological past, natural stones represent a heterogeneous and anisotropic material. Their use demands good knowledge of their technical properties. In this study, 20 low-porosity crystalline rocks of different mineralogical compositions and rock fabrics (granitoids, gabbros, peridotite, quartzite, marble) were investigated with respect to the fabric dependency of the technical stone properties. The samples could be classified into three macrostructural groups according to their macroscopic fabric anisotropy. The mica texture was determined in 14 of the stones. The maximum intensity (Imax) increased proportionately to increasing fabric anisotropy, enabling definition of Imax for macrofabric groups I/II/III. Additionally, the crack density of the microcracks, their preferred orientation and the grain size were taken into account. The technical properties capillary water absorption, compression strength, tensile strength, flexural strength, abrasive resistance, E-modulus and thermal expansion were determined in at least in three directions perpendicular to each other and supplemented by the scalar index characteristics density and porosity. Relationships to the fabric characteristics were obtained both for the means (arithmetic mean from the three main fabric directions x, y, z axes) and the anisotropy of the technical characteristic values. In principle, the macroscopic fabric anisotropy shows a positive correlation with that of the technical properties. The means of the rock strengths (compression strength, tensile strength, flexural strength, abrasive resistance) mainly depend on the mechanical properties of the mineralogical composition and microcrack density, while grain size also plays a part. The anisotropy is determined by the texture of the rock-forming minerals and the preferred orientation of open microcracks. The same applies to the statically determined E-modulus, where the elastic properties of the mineralogical composition have a strong influence. Alongside the crack density, these properties are also crucial for the means of the dynamic E-modulus. Its anisotropy is a product of the spatial arrangement of the open microcracks and the texture of the minerals. The capillary water absorption, characterized by the w-value, is not dependent on mineralogical composition, but is dominated by the volume percentage of the capillary pores and their distribution in the rock fabric. In contrast, with regard to thermal expansion, which to a large extent depends on the thermal expansion properties of the rock-forming minerals, the influence of microcracks through crack buffering is less significant. In this context, thermally induced crack formation, which can lead to appreciably elevated thermal coefficients of expansion in calcite marble, is of greater importance. Rules for the cross-correlations of the technical stone parameters to each other can be derived from fabric dependencies of the individual technical characteristics and the processes used for their experimental determination. A good correlation of the (direction-independent) means can be expected when a) the technical characteristics show the most similar type of dependency on the same fabric characteristics, b) the technical parameter show a good correlation with respect to the rock forming minerals and c) the processes during the experimental determination of properties are similar. From a direction-dependent viewpoint, to achieve a representative relationship, the spatial position of the process zones must additionally be identical when determining technical characteristics. The use of stones, particularly those with a high fabric anisotropy, requires a directionally dependent analysis of the technical characteristics values. The anisotropy of the mechanical properties can reach 80% and that of thermal expansion 70%. But also the capillary water absorption and the E-modules can sometimes exhibit markedly anisotropic behavior, a factor that must be accounted for in construction engineering applications and damage analysis.