Properties of building materials True density. Physical properties of building materials. Status parameters, methods for their definition. The effect of porosity on the properties of materials
True densityρ (g / cm 3) - mass in a unit of material volume in absolutely dense state, that is, without pores and emptiness
Average densityρ 0 (kg / m 3) - mass per unit volume of material in natural condition, that is, with the pores and voids
Bulk densityρ N. (kg / m 3) - the ratio of mass of the material in the loose-bulk state to its volume. Porosity- The degree of filling the volume of the material by pores. Distinguish between the 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 PZ - Number of closed pores in the volume of material
Hydrophysical properties - These properties building materials in relation to water
Hygroscopicity - property porous material Absorb water vapor from air.
Humidity characterizes the relative water content in the material in percent.
Water absorption - The ability of the material to absorb and hold the water with direct contact with it. The magnitude of water absorption depends on the structure of the material and, primarily from the open (capillary) porosity.
Distinguish Water absorption by weight In M (%) - the ratio of the mass by the absorbed water material MV to mass of the material in a completely dry state M
Volume water absorption In (%) - the ratio of the volume by the absorbed material of water M B / ρ in its volume in a water-saturated state V 2:
Humidity deformations- This is a shrinkage and swelling. Shrinkage, (deck) - decrease in the volume and size of the material when it is dried. 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 seeking them to bring together. Swelling(swelling) - an increase in the volume and size of the material during its moisture. It occurs due to the propagating action of water and reduce the capillary forces.
Passenger- The ability of the material to skip water through its thickness. It is characterized by the magnitude of the filtration coefficient to F (m 2 / h), which is determined by the amount of water passed through 1 m 2 of the area for 1 hour at a constant pressure.
Waterproof - The ability of the material does not pass water, and it is associated with the filtration coefficient inversely dependence.
Water resistance characterized by the softening coefficient k r
Frost resistance - the ability of the material to withstand multiple and alternate freezing and thawing in a saturated water.
Thermophysical properties characterize the ratio of the material to the action of heat.
Thermal conductivity- the ability of the material to transmit heat from the body to a larger temperature to less heat.
Thermal resistanceR., (m 2. about C) / W, the design thickness Δ is equal
Heat capacitydetermined by the amount of heat that must be reported to 1 kg of this material in order to increase its temperature 1 o C. Fireproof- the ability of the material to withstand a long effect high temperatures under load.
Fire resistance - material ability to withstand the short-term exposure of open fire
MECHANICAL PROPERTIES characterize the body's ability to resist internal stresses and deformations under the influence of power, thermal, shrinking or other effects.
Mechanical properties are divided into:
- deformative (elasticity, plasticity and others) and
- strength (strength limits for compression, tension, bend, rocking; impact strength or impact resistance; abrasion resistance).
Plastic - The property of material during loading in large limits to change the size and form without forming cracks and breaks and maintain this form after removing the load.
Fragility - The property of the material under the action of the load is collapsed without noticeable plastic deformation.
Thunderstanding properties - These are the properties of the material to resist, not destroying, internal stresses and deformations arising from the influence of the load or other factors.
The material strength is evaluated by the strength (time resistance) determined at this form of deformation. For fragile materials (natural stone Materials, concrete, construction solutions, Brick) The main strength characteristic is the tensile strength.
Compression strengthR.szh.(MPa) is equal to the maximum compressive voltage caused by the destruction of the material
P. raid - destructive power, H; F. - Size area to test, m 2
The tensile strength is determined by loading to the destruction of standard samples on special presses (testing machines).
By the same formula define ultimate tensile strengthfor Tex materials, which resist with tensile stresses and deformities (wood, metals, etc.).
For many materials, (concrete, brick, wood, etc.) determine
Tensile strength when bendingR. izg (MPa) by formulas:
Hardness - The property of the material to resist the penetration of another, more solid, material into it.
The hardness of the stone materials, the glass is estimated using a MOOS hardness scale consisting of 10 minerals located in the degree of increase in their hardness (1 -tall, ... 10 - diamond).
Shock viscosity (dynamic strength) - the property of the material to resist shock loads, is characterized by the amount of work spent on the destruction of a standard sample on special devices, called coppers, referred to a unit of volume (J / cm 3)
Abrasion resistance - the property of the material is resistive to abrupt effects, is characterized by abrasibility - loss of mass during abrasion of the sample on abrasion circles, referred to its area (g / cm 2)
The simultaneous impact of abrasion and impact characterizes wear resistancematerial
Properties characterizing the features of the physical condition of materials.
The physical condition of the building materials is fully characterized by the average and true density and porosity.
The average density ρc is the mass of the volume of material volume in a natural state, i.e. with pores. It can be dry material, in a state of natural or other humidity, indicated in GOST. Medium density (in kg / m3, kg / dm3, g / cm3) are calculated by the formula:
where m is the mass of material, kg, r; Ve is the volume of material, m3, dm3, cm3.
When changing the temperature and humidity of the medium surrounding the material, its humidity changes, and consequently, the average density. Therefore, the average density indicator is determined after the pretty drying of the material to constant mass or calculate according to the formula:
ω-amount of water in the material (share of its mass);
rMW and RMS-average density of wet and dry material.
The average density of bulk materials - rubble, gravel, sand, cement, etc. - called bulk density. The volume includes pores directly in the material and emptiness between the grains.
The average density of most materials is usually less than their true density. Separate materials, such as steel, glass, bitumen, as well as liquid, have almost the same true and average density.
The average density of the material is an important characteristic when calculating the strength of the structure, taking into account its own mass, to determine the method and value of the transportation of the material, to calculate warehouses and lifting and transport equipment. The magnitude of the average density is indirectly judged on some other material properties. 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.
The true density ρu is the mass of the volume of absolutely dense material, i.e., without pores and emptiness. It is calculated in kg / m3, kg / dm3, g / cm3 by the formula:
where m is the mass of material, kg, r; 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.
The relative density D is the ratio of the average density of the material to the density of the standard substance. Water was accepted for the standard substance at a temperature of 4 ° C, having a density of 1000 kg / m3. Relative density (dimensionless value) is determined by the formula:
Most building materials have pores, so they have true density Always more average. Only in dense materials (steel, glass, bitumen, etc.) True and average densities are practically equal, since the volume of the internal pores they have insignificantly small.
Porosity P is the degree of filling material by pores. It is calculated in% by the formula:
where ρc, ρu is the average and true density of the material.
For building materials P fluctuates from 0 to 90%.
Possess the porosity shared, open and closed:
Open porosity;
M1, M2 masses of materials in a dry and water-saturated state, respectively
PZ-closed porosity:
For bulk materials, emptiness is determined (intercolored porosity). True, average density and porosity of materials are interrelated values. They depend strength, thermal conductivity, frost resistance and other properties of materials.
Experimental (straight) Method for determining porosity is based on the substitution of pore space in the material with liquefied helium and is described earlier.
Pores are cells, not filled structural material. In terms of magnitude, they can be from millions of millimeters to several millimeters.
Larger pores, for example, between grains of bulk materials, or cavity, available, in some products ( hollow brick, panels from reinforced concrete), called voids. Pores are usually filled with air or water; In voids, especially in broadband, water can not be lingering and follows.
From the magnitude of the porosity and its character (size and form of pores, the uniform distribution of pores in terms of material, their structures - the reporting pores or closed) depend on the most important properties of the material: density, strength, durability, thermal conductivity, water absorption, waterproof, etc. For example, open Pores increase the permeability and water absorption of the material and worsen its frost resistance. However, in sound-absorbing materials, open pores are desirable, as they absorb sound energy. An increase in closed porosity due to the open increases the durability of the material and reduces its thermal conductivity.
Information about the material porosity makes it possible to determine the appropriate areas of its application.
Water resistance - material ability to maintain strength when saturated with water. In some materials (for example, at cement concrete), water saturation strength increases, in others (for example, gypsum Materials) - sharply decreases.
The water resistance rate is the softening coefficient to size, which is defined as the ratio of the strength of the strength (in compression) of the material in a saturated water of the state R Cx to the strength of the dry material R SZh: To size \u003d R 'SZh / R SZh
The values \u200b\u200bof the softening coefficient for various materials are in the range of 0 (unreleased clay materials) to 1 (glass, metals, bitumen, china). Materials with a coefficient of softening of at least 0.8 are apparent. Waterproof, for example, are quartzite, granite, marble, etc.
Porosity
Porosity P is the degree of filling material by pores. Determined by the formula n \u003d (1 - γ / ρ) · 100%; where γ is the average density of the material, kg / m 3; ρ is the true density of the material, kg / m 3.
For bulk materials, intercontrolle porosity (emptiness) is calculated. It is determined in the same formula, only for calculating instead of true density take middle density, or bulk average density.
In the volume of material, pores and emptiness can be simultaneously. Pores (from the Greek Piple - Outlet, Hole) are small cells in the material filled with air or water, the voids are larger cells and cavities between slices of loose-filled material filled with air.
The material porosity significantly affects its properties such as average density, strength, water absorption, humidity, water permeability, frost resistance, thermal conductivity, etc.
Exemplary porosity values,%, for some building materials are shown below:
metals and Glass 0
quartzite to 1.
marble 0.8-3.0
granite 1-3.
brick 25-35.
tuf Volcanic 20-60
wood 50-75
Porosity is a physical property used in an indirect estimation of water absorption of rocks, their durability, etc. Porosity is calculated by known values \u200b\u200bof medium and true density.
Water absorption
Water absorption - the ability of the material to absorb and hold water in the pores. It, as a rule, does not characterize the true porosity of the material, as part of the pores turns out to be inaccessible for water, and the air remains partially in the water filled with water. Water absorption is determined by weight in Mas or volume in OB in percent.
Water absorption B max is equal to the ratio of the mass of water absorbed by the sample during saturation, to the mass of dry sample: B Mas \u003d [(M 1 - M) / M] 100%, where M is a mass of dry sample, kg; M 1 - Mass of a saturated water of the sample, kg.
Water absorption in equals the ratio of the mass of water absorbed by the sample during saturation, to the volume of the sample: in OB \u003d [(M 1 - M) / V] 100%
For the transition B Mas k in about the form of a torment in Ob \u003d V wt Γ, which is derived from the equation in circular to MAC \u003d (M 1 - M) / V: (M 1 - m) / m \u003d m / v \u003d γ .
Moisturizing and saturation of building materials with water, as a rule, negatively affects their basic properties - increases the average density, heat and electrical conductivity, reduces strength. Water absorption depends on the number and nature of pores. Exemplary values \u200b\u200bof water absorption,%, for various materials are shown below:
quartzite 0.17
granite 0.09-0.65
marble 0.05-0.3
ceramic tile for floors 1-4
brick 8-20.
Water absorption is an important physical property of the stone, which enjoy the estimated estimate of its durability. For example, if the specified parameter in the rock does not exceed 0.5%, it is not experienced for frost resistance, believing that the breed has quite sufficient durability (in standards on blocks and onboard stones). At rocks used in the production of wall materials, water absorption should not exceed: for volcanic tuffs - 50, for other breeds-30%.
Determination of water absorption rock breed They produce on five samples - cubes with a rib size 40-50 mm or cylinders with a diameter and a height of 40-50 mm. Each sample is purified by a brush from loose particles, dust and dried to constant mass. After completely cooling the samples in the air, they are weighed on desktop or dial scales, laid into a vessel with water temperature in one row (the water level in the vessel should be 20-100 mm above the upper face of the samples) and maintained for 48 hours. Next samples Remove from the vessel, - wipe dry drying with a soft cloth and weighed the piece. In this case, the mass of water flowing from the pores of the sample to the scale of the scales is included in the mass of the sample saturated water.
The water absorption of rock formation is calculated as the arithmetic average of the determination of the water absorption of five samples. The values \u200b\u200bof this indicator for the most common species facing stone The CIS is given in the appendix.
Humidity
Humidity is the amount of moisture contained in the material, referred to the mass of the material in a dry state. The moisture w is calculated by the formula w \u003d [(M 1 - M) / M] 100% (here m - the mass of dry sample, kg; m 1
- Mass of a wet sample, kg).
Humidity takes into account when transporting, storing and accepting materials by weight. It affects thermal conductivity, resistance to rotting and some other properties of materials.
Passenger
Passenger - the properties of the material to pass water under pressure. This is one of the main performance characteristics of roofing and waterproofing materials, tarpaulin, skin, the magnitude of the water permeability is determined by the amount of water (ml), which passes the material per unit of time (H) through the area (1 cm 2) at constant pressure.
Reverse property - waterproofability - characterized by particularly dense materials (for example, steel, glass, bitumen) and dense materials with closed pores (for example, concrete of a specially selected composition).