UNI 9724 and UNI 9725 define the tests of the most important characteristics for the acceptance of rock material and its appropriate use depending on its properties and behavior when in use. These tests include:
1. Petrological examination
2. Dimensions and shape
3. Apparent volumetric mass (weight/volume)
4. Compressive strength
5. Resistance to deformation (bending)
6. Coefficient of thermal expansion
7. Elasticity
8. Impact test
9. Knoop micro-hardness
10. Absorption coefficient
11. Freeze – thaw characteristics
12. Resistance to abrasion
The results of the physical and mechanical tests carried out on samples of rock defined as “porphyry” by the Department of Geology and resources of the Polytechnic of Turin follow. In order to make them easier to understand the results are accompanied by a few notes summarizing the techniques and apparatus employed, comments on the results and comparisons with other rock types.
Petrology
The rock is composed of microcrystalline groundmass with phenocrysts of average size 0.5 – 1 mm (maximum 5 mm) contained in it.
In the phenocrysts the following principal constituents are found (in decreasing order):
• Feldspar, in rather muddy crystals with inclusions of opaque minerals and sheet silicates;
• Quartz, in normally clear crystals, larger than average;
• Chloritized biotite, with abundant opaque inclusions
The rock could even be called a rhyolite or, based on slightly old-fashioned terminology still in use, a quartzite porphyry.
Apparent volumetric mass: UNI 9724/2
The apparent volumetric mass gives an indication of the maximum compactness of the rock. It is the ratio expressed in kg/m^3, between the mass and the apparent volume (determined by geometric measurements), i.e. the volume delimited by the external area, of samples of regular shape.
The weight of the unit of rock is linked to the mineralogical composition and its fabric. In a compact rock this may coincide with the “real specific weight” (unit volume of the rock from which is subtracted the volume of empty space), but in porous rocks it is lower. The real specific weight, compared with the apparent gives the level of compactness of the rock, i.e. the lesser or greater presence of empty space.
Porphyry – apparent volumetric mass, average value: 2555 kg/m^3
Compressive strength: UNI 9724/4
This determines the unit load, under simple compression expressed in M/mm (1MPa = 10.2 kgf/cm^2), necessary to produce breaking of the sample – either a cube with 7.1 cm sides or a cylinder with diameter between 4 and 8 cm and a height: diameter ratio of 2.
The load is applied with constant rate of 0.4 MPa/sec in a direction perpendicular to the planes of preferred splitting of the rock.
Knowing the characteristics like resistance to compression and resistance to deformation (bending), not to mention the modulus of normal elasticity, is clearly important for the necessary statistical evaluations of elements (tiles, blocks etc.) which undergo the effects of mechanical wear and tear. This applies not only in the case of load-bearing elements which are after all rare, but in relation to the particular effects of flexing caused by pressure, for example, due to the wind on cladding tiles.
Porphyry - resistance to monoaxial compression: average value 221.5 MPa.
