What determines the water absorption of materials? Physical properties of building materials. State parameters, methods for their determination. The influence of porosity on the properties of materials
Water resistance is the ability of a material to maintain strength when saturated with water. For some materials (for example, cement concrete), the strength increases when saturated with water, while for others (for example, gypsum materials) it sharply decreases.
An indicator of water resistance is the softening coefficient K size, which is defined as the ratio of the ultimate strength (in compression) of a material in a water-saturated state R cx to the ultimate strength of a dry material R compress: K size= R‘ compress / R compress
Softening coefficient values for various materials are in the range from 0 (unfired clay materials) to 1 (glass, metals, bitumen, porcelain). Materials with a softening coefficient of at least 0.8 are considered waterproof. Waterproof, for example, are quartzite, granite, marble, etc.
Porosity
Porosity P is the degree of filling of the material volume with pores. Determined by the formula P = (1 – γ/ρ) 100%; where γ is the average density of the material, kg/m3; ρ – true density of the material, kg/m3.
For bulk materials, intergranular porosity (voidness) is calculated. It is determined by the same formula, only for the calculation, instead of the true density, the average density, or bulk average density, is taken.
The volume of the material can simultaneously contain pores and voids. Pores (from the Greek poros - exit, hole) are small cells in the material filled with air or water, voids are larger cells and cavities between pieces of loosely poured material, filled with air.
The porosity of a material significantly affects its properties such as average density, strength, water absorption, humidity, water permeability, frost resistance, thermal conductivity, etc.
Approximate porosity values, %, for some building materials are given below:
metals and glass 0
quartzite Up to 1
marble 0.8-3.0
granite 1-3
brick 25-35
volcanic tuff 20-60
wood 50-75
Porosity is a physical property used in indirectly assessing the water absorption of rocks, their durability, etc. Porosity is calculated by known values average and true density.
Water absorption
Water absorption is the ability of a material to absorb and retain water in its pores. As a rule, it does not characterize the true porosity of the material, since some of the pores are inaccessible to water, and air partially remains in the water-filled pores. Water absorption is determined by mass B wt or volume B vol as a percentage.
Water absorption B wt is equal to the ratio of the mass of water absorbed by the sample upon saturation to the mass of the dry sample: B wt = [(m 1 – m) / m] 100%, where m is the mass of the dry sample, kg; m 1 – mass of the sample saturated with water, kg.
Water absorption V vol is equal to the ratio of the mass of water absorbed by the sample upon saturation to the volume of the sample: V vol = [(m 1 – m) / V] 100%
To convert Bmass to Babout, use the formula Bvol = Vmass γ, which is derived from the equation Bvol / Bmass = (m 1 – m) / V: (m 1 – m) / m = m / V = γ .
Humidification and saturation of building materials with water, as a rule, negatively affects their basic properties - it increases the average density, thermal and electrical conductivity, and reduces strength. Water absorption depends on the number and nature of the pores. Approximate water absorption values, %, for various materials are given below:
quartzite 0.17
granite 0.09-0.65
marble 0.05-0.3
ceramic tiles for floors 1-4
brick 8-20
Water absorption is an important physical property of stone, which is used to roughly assess its durability. So, for example, if the specified parameter for a rock does not exceed 0.5%, it is not tested for frost resistance, assuming that the rock has sufficient durability (in the standards for blocks and side stones). For rocks used in the production of wall materials, water absorption should not exceed: for volcanic tuffs - 50, for other rocks - 30%.
Determination of water absorption of rock is carried out on five samples - cubes with an edge size of 40-50 mm or cylinders with a diameter and height of 40-50 mm. Each sample is cleaned with a brush from loose particles and dust and dried to a constant weight. After the samples have completely cooled in air, they are weighed on a table or dial scale, placed in a vessel with water at room temperature in one row (the water level in the vessel should be 20-100 mm above the top edge of the samples) and kept for 48 hours. Next, the samples taken out of the vessel, wiped dry with a soft cloth and weighed individually. In this case, the mass of water leaked from the pores of the sample onto the scale is included in the mass of the water-saturated sample.
The water absorption of rock is calculated as the arithmetic mean of the results of determining the water absorption of five samples. The values of this indicator for the most common types of facing stone in the CIS are given in the appendix.
Humidity
Humidity is the amount of moisture contained in a material, related to the mass of the material in a dry state. Humidity W is calculated using the formula W = [(m 1 – m) / m] 100% (here m is the mass of the dry sample, kg; m 1
– mass of the wet sample, kg).
Humidity is taken into account during transportation, storage and acceptance of materials by weight. It affects thermal conductivity, resistance to decay and some other properties of materials.
Water permeability
Water permeability is the property of a material to allow water to pass under pressure. This is one of the main performance characteristics of roofing and waterproofing materials, tarpaulins, leather. The amount of water permeability is determined by the amount of water (ml) that the material passes per unit time (hour) through an area (1 cm2) at constant pressure.
The opposite property - water resistance - is characterized by particularly dense materials (for example, steel, glass, bitumen) and dense materials with closed pores (for example, concrete of a specially selected composition).
True Densityρ (g/cm3) - mass per unit volume of material in an absolutely dense state, that is, without pores and voids
Average densityρ 0 (kg/m3) - mass per unit volume of material in its natural state, that is, together with pores and voids
Bulk Densityρ n (kg/m 3) - the ratio of the mass of the material in a loose-free-flowing state to its volume. Porosity- degree of filling of the volume of material with pores. There are general, open and closed porosity.
Open porosity By - the number of open pores in the volume of the material (determined by water absorption)
Closed porosity P z - the number of closed pores in the volume of material
HYDROPHYSICAL PROPERTIES - these are the properties of building materials in relation to the action of water
Hygroscopicity - the property of a porous material to absorb water vapor from the air.
Humidity characterizes the relative water content in the material as a percentage.
Water absorption - the ability of a material to absorb and retain water in direct contact with it. The amount of water absorption depends on the structure of the material and, above all, on the open (capillary) porosity.
Distinguish Water absorption by mass In m (%) - the ratio of the mass of water absorbed by the material mв to the mass of the material in an absolutely dry state m
Volumetric water absorption V o (%) - the ratio of the volume of water absorbed by the material m in /ρ in to its volume in the water-saturated state V 2:
Humidity deformations- This is shrinkage and swelling. Shrinkage (shrinkage) is a reduction in the volume and size of a material as it dries. It is caused by a decrease in the thickness of the layers of water surrounding the particles of the material and the action of capillary forces tending to bring them closer together. Swelling(swelling) - an increase in the volume and size of a material when it is moistened. It occurs due to the wedging effect of water and a decrease in capillary forces.
Water permeability- the ability of a material to pass water through its thickness. It is characterized by the value of the filtration coefficient Kf (m 2 /h), which is determined by the amount of water passing through 1 m 2 of area for 1 hour at constant pressure.
Waterproof- the ability of a material not to allow water to pass through, and it is related to the filtration coefficient by an inverse relationship.
Water resistance characterized by softening coefficient K p
Frost resistance - the ability of a material to withstand repeated and alternating freezing and thawing in a water-saturated state.
THERMAL PHYSICAL PROPERTIES characterize the relationship of a material to the action of heat.
Thermal conductivity- the ability of a material to transfer heat from a body with a higher temperature to a less warm one.
Thermal resistanceR, (m 2 o C)/W, construction thickness δ is equal to
Heat capacity is determined by the amount of heat that must be imparted to 1 kg of a given material in order to increase its temperature by 1 o C. Fire resistance- the ability of the material to withstand prolonged exposure to high temperatures under load.
Fire resistance - the ability of the material to withstand short-term exposure to open fire
MECHANICAL PROPERTIES characterize the ability of a material to resist internal stresses and deformations under the influence of force, thermal, shrinkage or other influences.
Mechanical properties are divided into:
- deformative (elasticity, plasticity and others) and
- strength (tensile strength in compression, tension, bending, shearing; impact strength or impact resistance; abrasion resistance).
Plastic - the property of a material, when loaded, to change its size and shape within significant limits without the formation of cracks and breaks, and to maintain this shape after removing the load.
Fragility - the property of a material to collapse under load without noticeable plastic deformation.
STRENGTH PROPERTIES- these are the properties of a material to resist, without collapsing, internal stresses and deformations that arise under the influence of load or other factors.
The strength of a material is assessed by the tensile strength (temporary resistance) determined for a given type of deformation. For fragile materials (natural stone materials, concrete, mortars, brick) the main strength characteristic is the compressive strength.
Compressive strengthRszh(MPa) is equal to the maximum compressive stress that caused the destruction of the material
Psize- destructive force, N; F - cross-sectional area before testing, m 2
The compressive strength is determined by loading standard samples to failure on special presses (testing machines).
The same formula is used to determine tensile strength for those materials that resist tensile stresses and deformations (wood, metals, etc.).
For many materials (concrete, brick, wood, etc.) determine
Bending tensile strengthR izg (MPa) according to the formulas:
Hardness - the property of a material to resist the penetration of another, harder, material into it.
The hardness of stone materials and glass is assessed using the Mohs hardness scale, consisting of 10 minerals, arranged according to the degree of increasing their hardness (1 - talc, ... 10 - diamond).
Impact strength (dynamic strength) - the property of a material to resist impact loads, characterized by the amount of work spent on destroying a standard sample using special devices called impact drivers, per unit volume (J/cm 3)
Abrasion resistance - the property of a material to resist abrasive influences, characterized by abrasion - weight loss when a sample is abraded on abrasion circles, related to its area (g/cm2)
The simultaneous effects of abrasion and impact characterize wear resistance material
Properties characterizing the characteristics of the physical state of materials.
The physical state of building materials is quite fully characterized by average and true density and porosity.
Average density ρс is the mass per unit volume of the material in its natural state, i.e. with pores. It can be dry material, in a state of natural or other humidity specified in GOST. Average density (in kg/m3, kg/dm3, g/cm3) is calculated using the formula:
where m is the mass of the material, kg, g; Ve - volume of material, m3, dm3, cm3.
When the temperature and humidity of the environment surrounding the material changes, its humidity changes, and therefore the average density. Therefore, the average density indicator is determined after preliminary drying of the material to constant weight or calculated using the formula:
ω is the amount of water in the material (fraction of its mass);
rmv and rms are the average density of wet and dry material.
The average density of bulk materials - crushed stone, gravel, sand, cement, etc. - is called bulk density. The volume includes pores directly in the material and voids between grains.
The average density of most materials is usually less than their true density. Individual materials, such as steel, glass, bitumen, as well as liquid ones, have almost the same true and average density.
The average density of a material is an important characteristic when calculating the strength of a structure taking into account its own weight, to determine the method and cost of transporting material, to calculate warehouses and handling equipment. The value of the average density indirectly judges some other properties of the material. For example, for stone materials there is an approximate relationship between average density and thermal conductivity, and for wood and some stone materials (limestone) - between strength and density.
True density ρu is the mass per unit volume of an absolutely dense material, i.e., without pores and voids. It is calculated in kg/m3, kg/dm3, g/cm3 using the formula:
where m is the mass of the material, kg, g; Va is the volume of material in a dense state, m3, dm3, cm3.
The true density of each material is a constant physical characteristic that cannot be changed without changing it chemical composition or molecular structure.
Relative density d is the ratio of the average density of the material to the density of the standard substance. Water at a temperature of 4°C and having a density of 1000 kg/m3 is taken as the standard substance. Relative density (dimensionless value) is determined by the formula:
Most building materials have pores, so their true density is always greater than average. Only for dense materials (steel, glass, bitumen, etc.) is the true and average density almost equal, since the volume of internal pores is negligible.
Porosity P is the degree of filling of the material volume with pores. Calculated in % using the formula:
where ρс, ρu are the average and true densities of the material.
For building materials P ranges from 0 to 90%.
Porosity is divided into general, open and closed:
Po-open porosity;
M1, m2 - masses of materials in dry and water-saturated states, respectively
Pz-closed porosity:
For bulk materials, voidness (intergranular porosity) is determined. True, average density and porosity of materials are interrelated quantities. Strength, thermal conductivity, frost resistance and other properties of materials depend on them.
The experimental (direct) method for determining porosity is based on replacing the pore space in the material with liquefied helium and was described earlier.
Pores are cells that are not filled with structural material. They can range in size from millionths of a millimeter to several millimeters.
Larger pores, for example between grains of bulk materials, or cavities present in some products (hollow brick, reinforced concrete panels) are called voids. The pores are usually filled with air or water; in voids, especially in wide-cavity ones, water cannot be retained and flows out.
The most important properties of the material depend on the amount of porosity and its nature (size and shape of pores, uniform distribution of pores throughout the volume of the material, their structure - communicating pores or closed ones): density, strength, durability, thermal conductivity, water absorption, water resistance, etc. For example, open pores increase the permeability and water absorption of the material and impair its frost resistance. However, in sound-absorbing materials, open pores are desirable because they absorb sound energy. Increasing closed porosity at the expense of open porosity increases the durability of the material and reduces its thermal conductivity.
Information about the porosity of a material makes it possible to determine appropriate areas of its application.
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