After all, road stone in standard loads can withstand intense okatyvanie. This means that, for example, under the railway individual pieces of gravel will rub one on another. And, of course, the harder the stone, the less resistance. Here requires a granite gravel as the most stable: crushing strength of crushed granite more than 200 MPa – a tiny tear. Gravel from the same sedimentary rocks such as limestone for a railway is ill-suited as a very intense abrasions.
An important factor is the form of gravel. Usually as a result of fragmentation and its shape is close to a cube or a tetrahedron. This is the optimal form. Worst of rubble with flat grains (flakiness) forms, as they break easily. This gravel is not used in construction. Why? Because it is necessary that the individual pieces that make up gravel, had more or less the same size in all three dimensions. Only then will she be strong. In the worst grades of gravel contained between 10 and 25% of pieces of flat shape.
If a gravel introduce as coarse aggregate in the concrete mix, then in the manufacturing process, they may lie parallel to each other. In this case, the concrete will have unequal strength in different directions. Involuntarily, a question arises. How can form flat fragments of the crushing of rock on the gravel? All due to the anisotropy of rocks. If anisotropic, ie having different properties in different areas of minerals in the rock are almost parallel to each other, form a flat fragments.