Technical features of stone (compressive, bending, tensile, abrasion, impact and hardness)

A- Compressive strength

The compressive strength of the stone shows the limit of its stability against the applied compressive forces. The uniaxial compressive strength test of stone is the most common laboratory test for the study of stones, which, despite its simple appearance, is very difficult to perform accurately. The stones that are used for construction purposes rarely show a resistance of less than 40 MPa, and in many cases, especially in igneous rocks, from 140 to 200 MPa and even more than this value can be observed. While in the wake of the tallest buildings, the pressure will probably not be more than one megapascal. The compressive strength of rock is usually obtained by means of a compression jack by breaking cubic or cylindrical samples. In order for the results of different tests to be comparable, the test conditions must be exactly the same. The reason for this is the effect of various factors on the test results, including the extraction method, the amount of stone weathering, the sample preparation method, the degree of dryness of the stone, the temperature of the test environment, the direction of loading relative to the layering, the characteristics of the plates holding the samples, etc. can be named; Therefore, it is clear that the test should be standard and for all works, the standard method should be followed. ASTM C: 170 standard can be used to measure compressive strength.

In general, the factors affecting the uniaxial compressive strength of rocks can be divided into two major groups: internal and external factors. Internal factors such as mineralogical composition, density, porosity, size and shape of grains, porosity index and anisotropy are factors that depend on the inherent characteristics of the stone. External factors depend on the test method, environmental conditions and the tester, which include factors such as sample dimensions, sample geometric shape, height-to-diameter ratio, friction between the loading plates of the device and the sample, loading speed, and temperature.

The compressive strength of the stone depends on the state of the stone granulation. In clastic rocks, the amount and type of cement, and in igneous or metamorphic rocks, the way the grains are connected is very important. In layered rocks, the compressive strength perpendicular to the layering surface is usually higher than its value in the direction parallel to the layering. In the absence of experimental testing, it is better to cut and install the stone in such a way that the loading is perpendicular to the layer.

The compressive strength of building stone should be measured in both dry and wet conditions. These two are often very different, and in the wetter state, the compressive strength is reduced. The greatest reduction occurs when the rock cement is of a type that has the ability to loosen in contact with water. The compressive strength of most rocks decreases after several periods of freezing; Therefore, it is recommended to test this issue before choosing.

B- Bending strength

The bending strength of stone can be expressed as its ability to withstand bending stresses. The importance of this test has not been paid much attention and therefore it is performed less often. Many times it has been observed that the stone used for window sills is broken due to bending forces. The reason for this fracture is its low bending strength. Stones that are placed on two bases (such as bridge spans) or stones that are involved on one side and the stone is suspended (such as some stairs) or stones that are only involved in the middle and have two free wings (such as stones on walls) and fences) will break if the pressure exceeds the bending strength of the stone to its free part.

It is not uncommon for such forces to enter as a result of building settlement. It should be noted that there is no direct relationship between the bending strength and the compressive strength of the stone. Wide variations in flexural strength can be seen between different samples of the same type of stone. The resistance of stone usually decreases due to moisture. ASTM C: 99 standard can be used to measure the bending strength of bending stone.

C- Tensile strength

Tensile strength is the tensile stress required to break a sample of a certain shape and size. The tensile strength of rocks depends on the loading direction, for example, in gneisses, it is higher in the direction of schistosity than in the direction perpendicular to it. Also, the tensile strength depends on the moisture content of the stone. The amount of tensile strength of stones is lower than their compressive strength and usually varies between 10 and 40%.

E- Abrasion resistance

Abrasion resistance of stone depends on the state of granulation and hardness of the constituent minerals. Some stones suffer from irregular erosion due to the different hardness of its makers, and in this sense, they may be less interesting than stones with low but uniform hardness.

Abrasion resistance of stone is important when it is subjected to abrasion. Among others, we can mention stairs, pavements, sidewalks and floors of buildings or water and sewage channels. In addition, in dry areas where wind erosion is significant or along the coasts where the presence of sand and wind can cause abrasion, this issue should be considered. Testing the wear resistance of stones can be done in different ways, including frictional wear and sand throwing.

In the friction wear test, the amount of weight or volume of the worn layer is determined from a flat sample placed in the vicinity of a rotating plate with a certain speed and worn by standard abrasive materials. This test is performed for floor stones, stairs, sidewalks, etc. The test method is according to the national standard of Iran No. 619.

The sand throwing test is performed to check the wear resistance of rubble stones or coarse aggregates for road and railway use. This test is suitable for rocks that are subject to wind and storm erosion.

F- Difficulty

Hardness is another characteristic of stones that is usually determined by resistance to scratching, abrasion, cutting and perforation. This characteristic depends on the characteristics of the rock minerals, density, grain size, construction and texture, degree of weathering, degree and type of cementation. For rocks that are composed of several minerals, the degree of hardness changes in different places. Hardness in terms of scratches can be estimated by comparing with the Moss table, and in terms of erosion, frictional wear tests can also be used.

G- Impact resistance

The resistance that a stone shows against a dynamic impact (for example, a hammer blow or an object falling on it) is called impact resistance. A stone that shows little resistance is called brittle. Impact resistance depends on the hardness and structure of the stone. Usually, the finer-grained the stone, the harder it is. This property is very important in the stones that are used for the construction of ports, floors, sidewalks and railways. There are several methods to measure the impact resistance, the most common method of which is to place the sample on a sand bag and then throw a one-kilogram iron weight on the stone from a certain height. They increase the height of the bullet so much that the sample breaks. The impact resistance of the sample is obtained from the product of height (meters) and weight (kg).