Строительные материа ...

 

Unit 1. CHIEF PROPERTIES OF BUILDING MATERIALS

The properties of building materials dictate their fields of appli­cation. Strong, durable and efficient buildings and installations may be erected only when the quality, i.e., the properties of mate­rials they are built of have been correctly evaluated.

The properties of building materials are generally classified as physical, chemical and mechanical. Physical properties of materials include their characteristics relating to weight and density, their permeability to liquids, gases, heat and radioactive rays as well as their resistance to aggressive environmental conditions. This latter characteristic governs the service life of materials, and in the long run, that of construction. Chemical properties of materials are essen­tially evaluated by their resistance to acids, alkalis and salt solu­tions which may initiate exchange reactions in the materials and cause their failure. The ability of materials to resist compression, tension, impact, penetration by a foreign body and other actions involving force, are generally known as mechanical properties. The above characteristics are complemented by workability which is the amenability of materials to working. All these properties are dealt with in the present course in application to specific materials.

1. Physical Properties of Materials

The specific (unit) weight of a homogeneous body is the weight, expressed in newtons, of 1 cubic metre (m³) of the body.

The specific (unit) weight is a variable which depends on the value of if at the point of measurement and thus cannot serve as a charac­teristic of a substance.

The density (kg/m³) is the mass of a unit volume of a substance, i.e., the ratio of a mass at test to its volume:

p=m/V

where m=mass of the substance, kg V=volume of the substance, m⁸ The densities of some of the construction materials are listed in Table 1.

 

Table 1

Densities of Some Construction Materials

 

Bulk density is the mass of a unit volume of a material (compo­nent) in natural state (voids and pores included)

g_o = m₁/V₁

where m₁=mass of a material, kg

V₁-volume of a material, m³_.

 

The bulk density of a material may vary as a function of porosity and void content. Loose materials (sand, crushed stone, cement and others) are characterized by their bulk mass. The volume of these materials is considered to include not only the pores and voids inside the grains of the material, but also the voids between them. The bulk density of a material predetermines to a great extent its physico-mechanical properties, such as strength and thermal conductivity. These characteristics are used to determine the thickness of (he enclosing walls of heated buildings, the dimensions of building structures, to calculate the amount and the capacity of the required transportation means and handling and lifting equipment.

Table 2

Bulk Density of Some Construction Materials

 

 

Bulk densities of construction materials vary over a wide range.

Bulk density also depends on the porosity and the moisture content of a material, increasing with the latter. The bulk density value is, to a certain extent, a measure of economic effectiveness.

Pores are small cells in a material filled with air or water. Pores inay be open or closed, fine and coarse. Fine closed pores filled with dlr impart construction materials good heat insulation properties. The porosity of a material may also be indicative of its other major properties, such as bulk density, strength, water absorption, thermal conductivity, durability, etc. Structures, which are required to be Very strong or impervious to water, are built of denser materials, whereas walls of buildings are generally made of substantially porous materials which possess good heat insulation properties.

Void content. Voids are empty spaces between grains of a loose material (sand, crushed stone, etc.) or cavities in some of the compo­nents, such as hollow brick and reinforced concrete panels. The void content of sand and crushed stone ranges from 35 to 45%, and that of hollow brick, from 15 to 50%.

The properties of materials with respect to the action of water are characterized by their capacity to absorb water on moistening and to give it off on drying. The water permeability of a material is ar­bitrarily classed with this kind of properties.

Material may be saturated with water either in vapour or in liquid form. Accordingly, distinction is made between hygroscopicity and water absorption.

Hygroscopicity is the property of a material to absorb water va­pour from air. It is governed by air temperature and relative humi­dity, by the type of pores, their number and size, and, finally, by the nature of the substance involved. Surfaces of some materials (called water-retaining, or hydrophilic) avidly attract water, while surfaces o( others (known as water-repellents) repulse water. Hydrophilic substances readily dissolve in water, whereas water-repellents strongly resist the action of water media.

With all other conditions being equal, the hygroscopicity of mate­rial depends on its surface area, including that of pores and capillary channels. Materials of equal porosity, but with smaller pores and capillary channels prove to be more hygroscopic than materials with larger pores.

Water absorption is the ability of material to absorb and retain water. It is described by the amount of water absorbed by an initi­ally dry material fully immersed in water, and is expressed in per cent of the mass (water absorption by mass) or of the volume (water ab­sorption by volume) of the dry material.

The value of water absorption by mass (%) is determined by means of the formula  and that by volume (kg/m³).

 Water absorption is always less than the true (absolute) porosity since some of the pores are closed, i.e., isolated from the surrounding medium and not accessible to water. Water absorption by volume is always less than 100%, whereas water absorption by mass of very po­rous materials may exceed 100%.

Water absorption of building materials is governed chiefly by the volume of the pores, their shape and size. It is also influenced by the nature of materials and their water-retaining properties.

Saturation with water greatly affects the properties of materials: the bulk density and heat conductivity of some go up, whereas oth­ers, e.g., wood, clay, tend to swell, so that their volume increases and their strength decreases because the links between the particles are broken by the penetrating molecules of water.

The ratio of the compressive strength of material saturated with water (R_s) to that in dry state (R_d) is called the coefficient of sof­tening

K_S=R_S/R_d

and describes the water resistance of materials. For readily soaking materials, it equals 0 (clay), whereas others (metal, glass) fully re­tain their strength under the action of water, and their coefficient of softening is 1. Materials with a coefficient of softening of 0.8 and higher are referred to as water-resisting materials. Materials with a coefficient of softening less than 0.8 should not be permanently ex­posed to the action of moisture.

Water release. Materials placed in the air retain moisture as long UN it is in equilibrium with the relative humidity of the air. Should the latter fall balow the equilibrium value, the materials will release moisture to the surrounding medium, or dry. This property is reverse to water absorption. The rate of drying depends, first, on the diffe­rence between the moisture of the material and the relative humidity of the air-the greater the difference, the more intensive is the drying of the material; second, the rate of drying is affected by the proper­ties of the material itself and the nature of its porosity. Water-repel­lents and materials with large pores release their moisture more rea­dily than fine-porous and hydrophilic materials.

Under natural conditions, the water release for construction mate­rials is described by the intensity of water loss at a relative air humi­dity w of 60% and temperature.

There is always some water vapour (moisture) in the atmosphere, mid therefore, a moist material dries in the air not completely, but to a certain degree, called the equilibrium humidity, the material then being called air-dry. Under room conditions, when the relative humidity of the air seldom exceeds 60%, wood has a moisture content nf 8-10%, and the exterior walls of buildings, 4-6%. The moisture content of hydrophilic materials varies with the relative humidity of the air.

Variations in humidity are accompanied in many a material by changes in volume: the materials swell, when moisture content in­creases, and shrink, when it decreases. Repeated moistening and drying cause alternating stresses in the material of building construc­tions and may result, in the course of time, in the loss of load-bearing capacity (failure). In this connection the concept of weathering re­finance of materials has been introduced.

Weathering resistance of material is its ability to endure repeated moistening and drying over prolonged periods of time without either Suffering considerable deformation or losing mechanical strength. Materials behave differently when exposed to varying humidity. Concrete, for example, tends to fail because the cement stone shrinks In the process of drying, while the aggregate suffers practically no Volumetric changes. This causes tensile stresses in the cement stone which contracts and tears itself off the aggregate. Wood undergoes alternating stresses when the humidity fluctuates.

Weathering resistance of materials may be enhanced by means of hydrophobic additives which make the materials water-repellent.

Water permeability, or the capacity of material to let through Water under pressure, is described by the amount of water penetrating III I h at a constant pressure through 1 m" of the material being test­ed, Dense materials (steel, glass, bitumen, most plastics) are imper­vious to water (waterproof).

Frost resistance is the ability of a water-saturated material to en­dure repeated freezing and thawing without visible signs of failure or considerable decrease of mechanical strength.

Systematic observations have shown that materials subjected to repeated saturation with water and freezing tend to disintegrate gra­dually. This is due to the fact that water contained inside their pores increases in volume by up to 9% in the process of freezing. The ma­ximum expansion of water in the process of freezing is observed at a temperature of -4°C, while further fall of temperature has no effect upon the volume of ice. The walls of water-filled pores, subjected to freezing, experience considerable stresses and may even fail.

Frost resistance of materials is determined by freezing water- saturated specimens at a temperature between -15 and -17°C and subsequently thawing them out. The reason for freezing the speci­mens at such a low temperature is that water congeals in capillary channels only at temperatures below -10°C.

The frost resistance of material depends on its density and the degree of its saturation with water. Dense materials are frost resis­tant. Of the porous materials, frost resistant are only those in which most of the pores are closed or filled with water to less than 90% of their volume. Material is considered frost resistant when its strength decreases by not more than 15 to 25%, and the loss in weight as a result of spalling does not exceed 5% after a prescribed number of freezing-and-thawing cycles.

A frost resistant material is one whose coefficient K is not less than 0.75.

By the number of freezing-and-thawing cycles they are capable of withstanding, materials are subdivided into grades of frost resistance labeled Mp3 10, 15, 25, 35, 50, 100, 150, 200 and over.

Under laboratory conditions, specimens are frozen in refrigerating chambers. One or two freezing cycles in the chamber are equivalent to 3 or 5 years of atmospheric exposure. There is also a faster testing method in which specimens are soaked in a saturated solution of so­dium sulphate and then dried at a temperature of 100 to 110°C. Crys­tals of decahydrate sodium sulphate, formed inside the pores of the stone, press against the walls of the pores even stronger than the freez­ing water. This kind of test is particularly severe. One cycle of testing in a solution of sodium sulphate is equivalent to 5-10 or even 20 cyc­les of direct freezing tests.

Heat conductivity of a material is its ability to conduct heat. All materials conduct heat, though to a different degree. The heat con­ductivity of material is quantitatively evaluated by a coefficient which is .equal to the quantity of heat flowing in 1 hour through a specimen of 1 m2 area and 1 m thick when the temperature difference between its opposite and parallel flat surfaces is 1°C.

The heat conductivity of material is governed by a number of fa­ctors: nature of the material, its structure, porosity, character oi pores, humidity and mean temperature at which the heat exchange takes place. Materials with closed pores have a lower heat conducti­vity than those with communicating pores. Fine-porous materials have a lower heat conductivity than those with large pores. This is due to the fact that the air inside the large and communicating pores is more free to move, which enhances heat transfer. Heat conductivity of a homogeneous material depends on its bulk density: when bulk density decreases, heat conductivity drops, and vice versa. No general relationship between the bulk density of a material and its coefficient of heat conductivity has been established; however, this relationship is in evidence for some materials with a moisture content of 1 to 7% by volume.

Table 3

Coefficients of Heat Conductivity of Some Construction Materials

 

Heat conductivity is greatly affected by humidity. Moist materials have a higher heat conductivity than dry ones. Heat conductivity of water is 25 times higher than that of air. Table 3 allows a com­parison of bulk densities and coefficients of heat conductivity of some building materials.

The coefficient of heat conductivity is the basis for subdividing various construction materials into heat-insulating, structural-and-heat-insulating and structural categories.

 

The values of heat conductivity for various categories of materials have been set as follows (W/m×°C):

For heat insulating materials:

Class A............................    up to 0.082      

Class B............................    0.082 to 0.116

Class B............................    0.116 to 0.174

Class T ...........................    0.174 to 0.210

For structural-and-heat-insulating and for structural materials above 0.210

The heat conductivity of a material may also be characterized by the thermal resistance, which is the reciprocal of heat con­ductivity.

Heat conductivity is of major importance for materials used to build walls of heated buildings and to insulate refrigerators and various thermal units (boilers, heat supply mains, etc.). The coeffi­cient of heat conductivity directly governs the cost of heating, which is a major factor in evaluating the economic effectiveness of walls in dwelling houses, etc.

The heat capacity of a material is its ability to absorb or give off heat on respectively heating or cooling, this ability being described in quantitative terms by the specific heat which is the quantity of heat required to heat 1 kg of a material by 1°C.

Heat capacity is of vital importance when heat accumulation is to be taken into account (e.g., in the calculation of thermal stability of walls of heated buildings), so as to prevent sharp temperature varia­tions caused by outside temperature fluctuations; in the calculation of the heating of a material, e.g., for winter concrete laying; in the calculation of furnaces, etc.

Fire-resistance is the ability of a material to resist the action of high temperature without losing its load-bearing capacity (i.e., without substantial deformation or loss of strength). This property is of particular value in case of fire, and, since fire-fighting involves the use of water, the fire-resistance of a material is tested by the combined actions of high temperature and water.

By their fire-resistance, building materials may be subdivided into non-combustible, fire-resistive and combustible. Non-combustible mate­rials neither smoulder nor char under the action of high temperature. Natural and artificial non-organic mineral materials and metals belong to this category. However, some of these materials neither crack nor lose shape (e.g., clay brick) when exposed to high temperatures, whe­reas some other (as, for example, steel) suffer considerable defor­mation. This is why steel structures cannot be referred to non-combusble materials. Fire-resistive materials are ones which char, smoulder, mid ignite with difficulty when exposed to the action of flame or high temperature but continue to burn or smoulder only in the presence of flame. Wood impregnated with fire-proofing compounds may be referred to this class of materials. Combustible materials burn and imoulder under the action of fire or high temperature and go on burn­ing after the starting flame is removed. All organic materials, not treated with fire-proofing compounds, fall into this category.

Refractoriness is the ability of a material to withstand prolonged net Ion of high temperature without melting or losing shape. In this category there are three varieties of material-refractory, high-meltlng and low-melting. Materials capable of resisting a prolonged Notion of temperatures from 1580°C and higher are known as refra­ctory. High-melting materials withstand temperatures from 1350 to 1580°C and low-melting materials-temperature below 1350°C (clay brick).

Spelling resistance of a material is characterized by its ability to endure a certain number of cycles of sharp temperature variations endure failing. Spalling resistance depends on the degree of homoge­neity of the material and the coefficients of linear expansion of its constituents. The lower the latter values, the higher is the spalling resistance of the material. Glass and granite may be mentioned as examples of materials with poor spalling resistance.

 

Permeability to nuclear decay radiation. Among the great variety of physical properties required of materials intended for the atomic industry, a major one is the ability to arrest gamma-rays and neutron fluxes which are dangerous to living organisms. For this reason, some parts of installations at atomic industry, plants and research institutes are required to serve as biological shields. A radioactive flux impinging upon a shielding construction made of specific material absorbed to a degree depending on the thickness of the shield, the lint lire of the radiation and the properties of the shield material.

 

Protection against neutron radiation is provided by materials

Chemical resistance is the ability of material to withstand the action of acids, alkalis, salt solutions and gases.

Sanitary facilities, sewer pipes and hydraulic engineering instal­lations are most frequently attacked by corrosive liquids and gases,| and, in some cases, by sea water which contains a large quantity of dissolved salts.

Natural stone materials, such as limestone, marble and dolomite, are eroded even by weak acids; the resistance of wood to the action of acids and alkalis is low; bitumen disintegrates rapidly when pla­ced in contact with concentrated alkali liquors. Most resistive acids and alkalis are ceramic materials and manufactured item and plastic articles.

Durability is the property of material to resist the combined action of atmospheric and other factors. These include variations in temperature and humidity, attack by gases contained in the air or salts dissolved in water or the combined action of water, frost and insolation. The loss of mechanical strength may be due then to loss of structural continuity (cracks), to exchange reactions with sub­stances in the surrounding medium and also to changes in the state of the material (change of crystal lattice, recrystallization, transation from amorphous to crystalline state).

The rate of gradual deterioration of materials under service conditions is their major characteristic, as the durability and resistance of materials to chemical attack are directly linked to expenses in running buildings and installations. The task of increasing the durability and chemical resistance of building materials becomes a pressing one from the standpoint of both engineering and economics!

 

2. Mechanical Properties of Materials

Mechanical properties are characterized by the ability of a mate­rial to resist all external actions involving the application of force. The various mechanical properties are generally divided into the following categories: compressive strength, bending strength, im­pact strength, torsional strength, etc., hardness, plasticity, elasti-1 city, abrasion resistance.

Strength is the ability of material to resist failure under the action of stresses caused by a load. The study of this property of materials; is the concern of a special science, the strength of materials. General concepts relevant to the study of the main properties of building materials are discussed below.

Materials incorporated in a construction may be subjected to different types of loads, the most common of them being compression, tension, bending and impacts. Stone materials (granite, concrete).

have adequate compressive strength, but a much lower resistance (from 1/5 to 1/50-th) to tensile, bending and impact stresses, and this is why they are chiefly used as members in compression. Metals and timber have high compression, bending and tensile strengths which make them particularly suitable for constructions subjected to compression, tension and bending.

The strength of building materials is described by their ultimate strength, which is the stress corresponding to the load destructing a specimen of the material. Ultimate strength (Pa) in compression and tension R (called compressive and tensile strength) is computed with the aid of the formula

R=P/F

where P=breaking load, N

F=area of the initial cross specimen, m²

The compressive strength #_com of various materials ranges from 0.5 to 1000 MPa and over.

Compressive strength of material is found experimentally by test­ing specimens in mechanical or hydraulic presses. Speci­mens shaped as cubes with sides measuring from 2 to 30 cm are spe­cially prepared for the purpose. Specimens from homogeneous mate­rials are prepared in smaller sizes, and those from less homogeneous materials, in larger sizes.

Sometimes specimens for com­pression tests are shaped as cylinders or prisms. Specimens of metals te­sted for tensile strength are round bars or strips; specimens for testing binding materials are shaped as fi­gure eight.

Specimens for ultimate strength tests are prepared in strict com­pliance with specifications of the corresponding. State Standards. The size and shape of the specimens must be strictly standard since they affect the test results. For example, prisms and cylinders have lower resistance to compression than cubes of the same cross-sectional area; on the other hand, low prisms (with heights inferior to their sides) have greater compressive strength than cubes. This is due to the fact that when a specimen is compressed, the press plates are for ced tight against its bases and the resultant friction forces preven; the expansion of the adjoining faces, while the central lateral part: of the specimen suffer transversal expansion which is counteracted only by the adhesive forces between the particles of the material. Therefore, the further away is a cross section from the press plates, the easier it fails and so does the entire specimen. For the same rea­son during testing of brittle ma­terials (stone, concrete, brick and others) they acquire a typical form of two top-to-top truncated prisms (Fig. 1).

The strength of material is affec­ted not only by the shape and the size of the specimen, but also by the character of its surface and the rate of load application. Therefore, comparable results can be obtained only by following the standard tes­ting methods specified for a given material.

 

Fig. 1. Cubic specimen after comp­ression testing in a hydraulic press

 

The strength of material depends also on its structure, bulk density (porosity), moisture content, direction of load application. Bending tests are performed on specimens shaped as small bars, supported at their ends and loaded with one or two concentrated weights which are gradually increased until the bar breaks.

In building materials stresses are not allowed to exceed a certain part of their ultimate strength, this providing a margin of safety (or safety factor). When considering the value of a margin of safety, ac­count is taken of the inhomogeneity of the material: the less homo­geneous is the material, the greater is the required margin of safety.

When deciding on the correct value of the safety factor, one must take into consideration the service conditions and the nature of load­ing. An aggressive medium and loads of alternating signs, causing fatigue of materials, call for a greater safety factor. The values of safety factors, which are so vital for the preservation and the service life of constructions, are specified by design standards and depend on the type and the quality of the material, service conditions, ser­vice life rating of the building and, finally, special engineering and economic calculations.

In the last few years the building practice has been enriched by novel techniques which allow non-destructive testing of specimens or elements of constructions for strength. These techniques can be used to test items and constructions either in the process of their manufacture at plants or directly at construction sites after they are mounted. Of these, the acoustic (ultrasonic) methods are the best known ones; the most widely used are the impulse and the resonance va­riants. Such methods are based on the principle that physical proper­ties of material can be evaluated indirectly by the propagation ve­locity "of ultrasonic waves, or the propagation time of shock waves required to penetrate the material, and also by the frequency of na­tural oscillations of the material and their attenuation.

Hardness is the ability of material to resist penetration by a harder body. It sometimes fails to correlate with the strength of the mate­rial and may be determined by a number of methods. Hardness of stone materials is found with the aid of Mohs' scale of hardness  which is a list of ten minerals arranged in the order of increas­ing hardness. The hardness of material lies between the hardnesses of two adjacent materials, one of which scratches and the other is scratched by the tested material.

Hardness of metals and plastics is found by indentation of a steel ball.

Hardness of materials is interrelated with their abrasion resis­tance and is a major factor affecting their workability and their use for floors and road surfaces.

The abrasion resistance of a material is characterized by the loss of its initial mass referred to 1 sq. metre (m²) of the surface area being abraded. Tested for abrasion resistance are materials intended for floors, roadway wearing courses, stair treads and the like. Some materials are also tested for wear.

Wear is the failure of material under the combined action of ab­rasion and impacts. Wear resistance is evaluated by the loss in mass expressed in per cent. Subjected to wear are road surfaces and railway ballast.

Impact strength is of prime importance in materials for floors and road surfaces. The ultimate strength of material subjected to im­pact is described by the quantity of work required to cause the fa­ilure of a specimen per its unit volume, J/m⁸. Materials are tested for impact strength in an impact testing machine.

Elastic and plastic properties. When loaded, materials tend to deform, i.e., change their shapes and dimensions. The extent and the character of these deformations may differ greatly. A deformation is called elastic when the specimen restores its size and shape after the load is removed; a deformation is plastic when the specimen retains partly or fully the change in shape after the load is removed.

Elasticity is the ability of material to restore its initial form and dimensions after the load is removed. The elastic limit is the mag­nitude of stress under which the residual strain for the first time reaches a certain very small value (specified for each material indivi­dually).

Plasticity is the ability of material to change its shape under load without cracking and to retain this shape after the load is removed. Materials may be divided into plastic and brittle ones. Plastic ma­terials may be exemplified by steel, copper, clay grout, hot bitu­men, etc. Brittle materials fail suddenly without any considerable deformation preceding the failure. Rock materials are generally brittle. Brittle materials have adequate resistance to compression only and a poor resistance to tension, bending and impacts.