The main characteristics of stone are:
the ratio, in kg/m3, between the mass and the apparent volume. This provides a rough indication of density which is particularly important when calculating the stress of products such as load-bearing structures or cladding panels, when designing stone walls for protection against the sea or rivers or to contain land mass. In addition, a high density means that dust, odours, smoke, germs and pollen will not penetrate the surface layer of the material. The unit of measurement is Kg/m3.
This is its ability to absorb water assessed by the increase in weight obtained. It provides an indication of the density and durability under normal environmental conditions and in the case of prolonged contact with rain water or moist soil. The introduction of water in stone is one of the main causes of decay. And is therefore inversely proportionate to its mechanical resistance and closely linked to freezing. Since the absorption coefficient of most rocks is very low, the result is generally expressed as a percentage increase over initial weight.
It varies from average coefficients of 0.5% for granite to 15% for tuff.
The compressive strength of a rock is the resistance to fatigue where rocks fracture when crushed. Among the many factors that influence their resistance to compression (structure, texture, state of alteration, duration of load application, etc.) it is the structure of the rock that is particularly important. The cohesion of the rock depends on this, and will generally be higher in fine- and very fine-grained rocks. Stratified or otherwise oriented materials on the other hand generally have a preferential direction along which they show less resistance.
The unit of measurement is MPa.
This is the compressive strength of a rock that has undergone a cycle of freezing and thawing. Water penetrating the pores and increasing in volume as it freezes and changes from liquid to solid, is what impacts on the strength of the material. Rocks that have low resistance to freezing are said to be susceptible to frost, especially to temperatures that range from below zero to above zero. Factors that adversely affect frost susceptibility are a scaly structure, porosity, the absorption coefficient and clay content.
The unit of measurement is the MPa resulting from the average of four tests.
A typical example of bending stress consists of the pressure exerted by wind on panels of wall claddings, such action being among the most dangerous since it puts stress on the cladding, and testing this is one of the key issues in designing external claddings, especially where low thickness claddings are involved. Statistically speaking, it can roughly be said that average values ??recorded for a material correspond to 5% of its compressive strength.
The unit of measurement is the MPa resulting from the average of five tests.
This is the ratio between the compression rate exerted on a rock mass and the rate at which the length diminishes. It allows specific stability and static safety checks to be carried out on elements subjected to mechanical stress.
The elasticity modulus is thus defined as the ratio between the variation of the longitudinal tension and the unitary deflection in a longitudinal direction produced by a variation in stress and is expressed in MPa.
This allows evaluation of the behaviour of a material located in areas subject to friction, footfall and traffic of all kinds. This is the abrasion which alters the original surface characteristics of a material over time. The result is expressed as a coefficient relative to the abrasion, ie the ratio between the height of the abraded layers of the control material (S. Fedelino granite) and the material being tested.
Rocks that are more resistant than the San Fedelino granite have an index greater than 1, those less resistant have an index less than 1.
This defines the resistance to impact from a direct blow from a blunt instrument. It allows evaluation of the behaviour of stone materials in all situations where sudden impact may occur, such as heavy objects falling.
The result is given by the minimum drop height in centimetres of a 1kg steel ball. which breaks the panel of material when it lands in the centre.
Expansion coefficients revealed in the rock are mostly considered negligible when they are allowed the opportunity to expand; this is not the case when expansion is prevented, as when tension is created inside the rock which may exceed the compressive strength or cause irreversible deformation of the material. This is important in cases where the material is placed in close contact with walls to which it may transmit temperature changes caused by variations in internal and external environmental conditions, at varying rates.
The values ??depend on the structure, texture and mineral composition of the material: those of compact and fine-grained rocks are higher, and those of porous and coarse-grained rocks are lower.
The possibility of engraving or cutting rocks is tested by penetration with a diamond tool.
With this test the following can be distinguished:
HARD STONES: Porphyry/Basalt/Granite/Gneiss/Quartzite
- These contain mainly minerals such as quartz, feldspar and mica.
- They are resistant to acids and cutting.
- They are particularly suitable for floors subject to heavy usage, cladding and kitchen worktops.
SOFT STONES: Marble/Travertine/Sandstone/Limestone
- These contain calcite, clay or mica.
- They are non-acid resistant.
- Suitable for internal and external wall cladding.
The color of stone materials can vary over time, especially in materials located externally and exposed to the direct action of weathering.
Gradual discoloration can sometimes take place, white materials generally tend to assume yellowish or grey tones, dark ones tend to lighten, green, red and yellow tones become duller.
Natural stones are inert to fire, do not emit toxic substances and ensure that stone-clad buildings are effectively protected.