Physical and mechanical properties

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Rocks worked and subsequently installed are subjected to numerous stresses from disturbing agents presents in the environment. These phenomena can be physical (dilation, sunstroke, freeze-thaw cycles, salt), mechanical (permanent loads, accidental loads, friction, wear, collision), chemical and physico-chemical (water, oxidation, corrosion caused by air pollution, acid rain) and biological (mosses, lichens). These actions, combined or not with each other, can cause permanent damage to the surface, or even more serious to the structure of stone material, compromising the integrity and function for which the object was created. Physical-mechanical properties of interest for the qualification of natural stones are reported.

Bulk density: is the ratio of the mass of the material and the apparent volume, i.e. volume bounded by outer surface samples having a standard geometric shape. It is expressed in kg/m3. The bulk density is a very important indicator to define the compactness of a stone element and it is also fundamental for calculation of loads of the products (especially in load-bearing structures). The bulk density calculation is determined on two cubic specimens with side of 7 centimeters and it represents the ratio between the weight of the sample dried at a temperature of 110° C (Figure 1) until constant weight, and volume. The final result is the average value of two measurements.

Imbibition coefficient: imbibition is that physical phenomenon whereby all materials, including more compact appearance, immersed in a liquid, assume it in variable amounts, reaching sometimes the state of saturation. Imbibition coefficient, for stone materials, is the maximum amount of deionized water absorbed at room temperature and pressure. This measurement provides important information on the degree of compactness and durability in normal environmental conditions and in the event of prolonged contact with rainwater or soil moisture (capillary rise) of a stone material. The introduction of water into the interstices of rocks is one of the most responsible mechanisms of degradation of materials, because in the water there are also dissolved substances (such as mineral salts) that will accelerate the process. Imbibition test is carried out on five specimens with a weight of 200 gr; the coefficient is obtained from the percentage increase of the weight of a sample after a prolonged immersion in water. The final result is the average value of five measurements. For some types of material imbibition coefficient is expressed in ‰, because it is very low. (Figure 2).

Compressive breaking load: compression strength of a stone material represents the unit load necessary to cause the breakdown of material specimens. The tensile test in simple compression is one of the most commonly used tests nationally and internationally, since it provides a precise indication of permanent maximum load supported by a stone product. The structure of the rock is one of the factors influencing the compressive strength; it depends on the type of cohesion of the rock itself, especially the most cohesive are rocks with fine grain and/or thin because the cohesion develops in contact between the outer surfaces of the crystals. Layered materials like many crystalline marbles (sandstones) generally have a preferred direction along which have greater fragility and where it is most desirable to rupture the specimen. The compressive strength of these materials has very variable and differentiated values: it appears to be highest in the direction normal to the direction, minimum in the direction parallel to it. This is very important since, often in ornamental objects used for building, the cut is made along the lines of greater fragility that often do not coincide with the directions that offer the best value of compressive strength. Simple compressive strength test consists in submit a specimen of stone material to a load that progressively increases of 2 MPa generated by a hydraulic press which has an auto-lock at the time of rupture of the sample. (Figure 3) The test is carried out on four samples of cubic shape with dimensions of 7.1 cm each side (or on cylindrical specimens with diameters between 4 and 8 cm and a height/diameter ratio equal to 2) dry state (dried to about 30° C until constant weight).The hydraulic press normally acts in the direction perpendicular to the planes of preferential divisibility of the rock.

Compressive breaking load with freeze-thaw cycles: rocks with a little resistant to frost, especially in freeze-thaw cycles, i.e. the alternations of periods with cold temperatures, below 0°C, and periods with hot temperatures, above 0°C, are defined frozen stones. The water in the pore of the rocks freezing during the winter causes the material fracture. (Figure 4) The test, carried out to verify the results of this phenomenon is very important to choose adequately the type of rock to be used for external cladding able to withstand temperatures and bad weather conditions. The test consists in determining the resistance to simple compression according to procedures similar to those described in the previous paragraph, on specimens subjected to cyclic variations of temperature from a minimum of -10 ° C to a maximum of + 35 ° C. During each cycle of "freezing", the specimen is immersed in water for about three hours at a temperature of 35 ° C, and then placed in a cold room at a temperature of -10 ° C. The rock passes the test and is not considered a frozen stone if, after such heat treatment, showing a reduction of the breaking load in compression of less than 25% compared to the comparable value measured on the untreated specimens. The value that determines the outcome is taken from the average of four samples and is expressed in MPa.

Flexural strength: flexural stresses result in shear at plate constraints that sometimes reach the breaking point. The wind is one of the most dangerous agents since it has fatiguing character, for that reason this is one of the key issues during the aying of stone coatings (especially those of reduced thickness) to the should be paid particular attention during the then buildings with high altitude or in buildings constructed in areas with a strong wind. Bending stresses are also present in the realization of stairs in buildings with cover function, floors and architectural elements such as shelves, balconies or lintels. The flexural breaking load test s carried out on five specimens having dimensions of 12x3x2, leaning against two knives with rounded edge, load on centre line by another knife also with rounded edge. (Figure 5) Usually the test is performed with the load that acts perpendicular to the planes of preferential rock Division; the result, expressed in MPa, is the average of the five values obtained.

Normal elastic modulus: is the ratio of the value of the compression, expressed in MPa, applied to the stone material during testing, and the progressive decrease of the length that the body suffers. This test checks the static stability and security of items subjected to mechanical stress. The module defines the degree of elasticity of a material and is calculated, in most cases, for those products used for coatings or for load-bearing structures. The test shall be performed on two contact sheets block with a square base with standard dimensions of 20x5x5 cm, or on cylindrical specimens with a diameter of at least 5 centimeters and a ratio between the height and the diameter of 3. A compressive acts on the specimens, usually along their longitudinal axis, then some load (at least 10 values) corresponding with longitudinal deformations suffered in order to determine the stress-strain curve of the material are measured. (Figure 6) Elastic modulus, espressed in MPa or in GPa, is the ratio between the variation of longitudinal tension and in longitudinal unitary direction deformation produced by the voltage variation. Even in this case, as in the tests described in the previous paragraph, the test is carried out with the load agent in the direction perpendicular to the planes of material preferential divisibility.

Impact resistance: this physical propery defines the toughness or fragility of the material, i.e. the ability to resist breakage after the surface of the material has been subjected to an impact with a blunt instrument. It is helpful to understand the degree of impact resistance especially for uses of stone material in industrial floors, exterior staircases, pedestals, and generally in all those circumstances which may arise the possibility to undergo sudden and instant hits, such as heavy objects falling from high altitudes. The test shall be carried out on four slabs of standard dimensions 20x20x3 centimeters, which are placed on a layer of sand about 10 inches thick. The result is expressed by the minimum height of fall, expressed in centimeters, of a steel ball weighing one kilogram, hitting the centre of the surface of the slab, the crack. (Figure 7)

Linear thermal expansion coefficient: in the rocks there are two types of thermal expansion: linear with length variations of the material and the volumetric expansion, with volume variations. The linear thermal expansion coefficient shows the possible length variations of stone materials, once laid, as a result of temperature increases. The values for the coefficients of thermal expansion of the stone material are believed to be normal when there is the possibility to dilate, dangerous are the impediments to the thermal expansion that can create internal stresses in the material itself that often cause bending and warping of the buildings. For external applications (floors or walls) must always be provided an expansion joint, which width is proportional to the size of the slabs. The test is performed on two test specimens, having a cylindrical shape with a diameter of 3 cm and a length of 20 centimeters, which are placed in a special tool, dilatometer in silica glass (Figure 8), and subjected to substantial temperature fluctuations between the 0 °C and 60 °C. The variations in length of the specimens are measured by millesimal comparison. The result, expressed in 10-6 / ° C, is the average of the values of the two specimen.

Wear caused by friction: this property allows to evaluate the behavior of a stone material placed in areas prone to rubbing, foot traffic or transit of people, vehicles or animals. The wear caused by friction, with the passage of time, causes of changes on the original surface of the material, an example is given by hollows present on the stone steps of secular buildings. The test is determined by means of a tribometer (Amsler) on two standard-sized specimens of 7, 1 x 7, 1 x 2 .5 cm pressed against a load of 0.03 MPa against a rotating track covered with a layer consisting primarily of abrasive grit carborundum with a particle size of less than 0.15 millimeters and greased with petroleum jelly. (Figure 9) Simultaneously, the same test is carried out on a material known (Saint Fedelino granite), which are the bedrock values that you want to examine. (Figure 10) The result is a comparison of the heights of the abraded layers in the reference material and in the tested material. Then the rocks that are more resistant to Saint Fedelino granite have a coefficient greater than 1, while those less resistant have a coefficient of less than 1.

Microhardness Knoop: Knoop test records the micro hardness of a stone material using a diamond penetrator. (Figure 11) As the test covers portions of material with submillimetric sizes, the value obtained on the hardness cannot be expressed at the end of a single proof or as the average of the results of more tests; this is because the material is homogeneous to a not so small scale and hardness of a rock is a different property for each piece of material. Therefore several tests are performed on the same specimen and results are then plotted on a cumulative frequency curve. Usually the test is carried out on 20 or 40 footprints, depending on the type of material of the specimen, with a distance of one millimeter and obtained using a diamond penetrator pressed on the surface with a particular predetermined and workload. The values obtained are a graph whose performance corresponds to that of the cumulative frequency diagram of micro hardness of the rock. (Figure 12)