Principal Properties of Building Materials

Sunday, June 13, 2010 , Posted by HB at 8:21 PM

Building.Materials.3rd

1. INTRODUCTION

Building materials have an important role to play in this modern age of technology. Althoughtheir most important use is in construction activities, no field of engineering is conceivablewithout their use. Also, the building materials industry is an important contributor in ournational economy as its output governs both the rate and the quality of construction work.There are certain general factors which affect the choice of materials for a particular scheme.Perhaps the most important of these is the climatic background. Obviously, different materialsand forms of construction have developed in different parts of the world as a result of climaticdifferences. Another factor is the economic aspect of the choice of materials. The rapid advanceof constructional methods, the increasing introduction of mechanical tools and plants, andchanges in the organisation of the building industry may appreciably influence the choice ofmaterials.Due to the great diversity in the usage of buildings and installations and the variousprocesses of production, a great variety of requirements are placed upon building materialscalling for a very wide range of their properties: strength at low and high temperatures,resistance to ordinary water and sea water, acids and alkalis etc. Also, materials for interiordecoration of residential and public buildings, gardens and parks, etc. should be, by their verypurpose, pleasant to the eye, durable and strong. Specific properties of building materialsserve as a basis for subdividing them into separate groups. For example, mineral bindingmaterials are subdivided into air and hydraulic-setting varieties. The principal properties ofbuilding materials predetermine their applications. Only a comprehensive knowledge of theproperties of materials allows a rational choice of materials for specific service conditions.

The importance of standardisation cannot be over emphasised. It requires the quality of
materials and manufactured items to be not below a specific standard level. However, the
importance of standardisation is not limited to this factor alone, since each revised standard places higher requirements upon the products than the preceding one, with the effect that the industry concerned has to keep up with the standards and improved production techniques.Thus, the industry of building materials gains both in quantity and quality, so that new, more efficient products are manufactured and the output of conventional materials is increased. To develop products of greater economic efficiency, it is important to compare the performance of similar kinds of materials under specific service conditions. Expenditures for running an installation can be minimised by improving the quality of building materials and products. Building industry economists are thus required to have a good working knowledge, first, of the building materials, second, of their optimum applications on the basis of their principal properties, and, third, of their manufacturing techniques, in order that the buildings and installations may have optimum engineering, economic performance and efficiency. Having acquired adequate knowledge, an economist specialising in construction becomes an active participant in the development of the building industry and the manufacture of building materials.

2. PHYSICAL PROPERTIES

Denity(p) is the mass of a unit volume of homogeneous material denoted by

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For most materials, bulk density is less than density but for liquids and materials like glassand dense stone materials, these parameters are practically the same. Properties like strengthand heat conductivity are greatly affected by their bulk density. Bulk densities of some of thebuilding materials are as follows:

Halaman 3-1It indicates the degree to which the volume of a material is filled with solid matter. Foralmost all building materials po is less than 1.0 because there are no absolutely dense bodies innature.

Halaman 3-2Specific weight (y) can be used in civil engineering to determine the weight of a structuredesigned to carry certain loads while remaining intact and remaining within limits regardingdeformation. It is also used in fluid dynamics as a property of the fluid (e.g., the specific weightof water on Earth is 9.80 kN/m3 at 4°C). The terms specific gravity, and less often specific weight, are also used for relative density.

Spesific Gravity (Gs) of solid particles of a material is the ratio of weight/mass of a givenvolume of solids to the weight/mass of an equal volume of water at 4°C.

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True of Absolute Spesific Gravity (Ga) If both the permeable and impermeable voids areexcluded to determine the true volume of solids, the specific gravity is called true or absolutespecific gravity.

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The absolute specific gravity is not much of practical use.

Appearance of Mass Spesific Gravity (Gm) If both the permeable and impermeable voids areincluded to determine the true volume of solids, the specific gravity is called apparent specificgravity. It is the ratio of mass density of fine grained material to the mass density of water.

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Porosity (n) is the degree to which volume of the material of the material is interspersed withpores. It is expressed as a ratio of the volume of pores to that of the specimen.

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Porosity is indicative of other major properties of material, such as bulk density, heatconductivity, durability, etc. Dense materials, which have low porosity, are used for constructionsrequiring high mechanical strength on other hand, walls of buildings are commonly built ofmaterials, featuring considerable porosity.

Following inter relationship exists between void ratio and the porosity.

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Void Ratio (e) is defined as the ratio of volume of voids (Vv) to the volume of solids (Vs).

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If an aggregate is poured into a container of any sort it will be observed that not all of the space within the container is filled. To the vacant spaces between the particles of aggregate the name voids is applied. Necessarily, the percentage of voids like the specific weight is affected by the compactness of the aggregate and the amount of moisture which it contains. Generally void determinations are made on material measured loose.

There are two classes of methods commonly employed for measuring voids, the direct and the indirect. The most-used direct method consists in determining the volume of liquid, generally water, which is required to fill the voids in a given quantity of material. Since in poring water into fine aggregate it is impossible to expel all the air between the particles, the measured voids are smaller than the actual. It therefore becomes evident that the above direct method should not be used with fine aggregate unless the test is conducted in a vacuum. By the indirect method, the solid volume of a known quantity of aggregate is obtained by pouring the material into a calibrated tank partially filled with water; the difference between the apparent volume of material and the volume of water displaced equals the voids. If very accurate results are desired void measurements should be corrected for the porosity of the aggregate and moisture it contains.

Hygroscopicity is the property of a material to absorb water vapour from air. It is influenced by air-temperature and relative humidity; pores—their types, number and size, and by the nature of substance involved.

Water Absorption denotes the ability of the material to absorb and retain water. It is expressed as percentage in weight or of the volume of dry material :

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where :

M1 =mass of saturated material (g)

M =mass of dry material (g)

V =volume of material including the pores (mm3)

Water absorption by volume is always less than 100 per cent, whereas that by weight of porous material may exceed 100 per cent.

The properties of building materials are greatly influenced when saturated. The ratio of compressive strength of material saturated with water to that in dry state is known as coefficient of softening and describes the water resistance of materials. For materials like clay which soak readily it is zero, whereas for materials like glass and metals it is one. Materials with coefficient of softening less than 0.8 should not be recommended in the situations permanently exposed to the action of moisture.

Weathering Resistance is the ability of a material to endure alternate wet and dry conditions for a long period without considerable deformation and loss of mechanical strength.

Water Permeability is the capacity of a material to allow water to penetrate under pressure.Materials like glass, steel and bitumen are impervious.

Frost Resistance denotes the ability of a water-saturated material to endure repeated freezing and thawing with considerable decrease of mechanical strength. Under such conditions the water contained by the pores increases in volume even up to 9 per cent on freezing. Thus the walls of the pores experience considerable stresses and may even fail.

Heat Conductivity is the ability of a material to conduct heat. It is influenced by nature of material, its structure, porosity, character of pores and mean temperature at which heat exchange takes place. Materials with large size pores have high heat conductivity because the air inside the pores enhances heat transfer. Moist materials have a higher heat conductivity than drier ones. This property is of major concern for materials used in the walls of heated buildings since it will affect dwelling houses.

Thermal Capacity is the property of a material to absorb heat described by its specific heat.Thermal capacity is of concern in the calculation of thermal stability of walls of heated buildings and heating of a material, e.g. for concrete laying in winter.

Fire Ressistance the ability of a material to resist the action of high temperature without any appreciable deformation and substantial loss of strength. Fire resistive materials are those which char, smoulder, and ignite with difficulty when subjected to fire or high temperatures for long period but continue to burn or smoulder only in the presence of flame, e.g. wood impregnated with fire proofing chemicals. Non-combustible materials neither smoulder nor char under the action of temperature. Some of the materials neither crack nor lose shape such as clay bricks, whereas some others like steel suffer considerable deformation under the action of high temperature.

Refractoriness denotes the ability of a material to withstand prolonged action of high temperature without melting or losing shape. Materials resisting prolonged temperatures of 1580°C or more are known as refractory.

High-melting materials can withstand temperature from 1350–1580°C, whereas low-melting materials withstand temperature below 1350°C.

Chemical Resistance is the ability of a material to withstand the action of acids, alkalis, seawater and gases. Natural stone materials, e.g. limestone, marble and dolomite are eroded even by weak acids, wood has low resistance to acids and alkalis, bitumen disintegrates under the action of alkali liquors.

Durability is the ability of a material to resist the combined effects of atmospheric and other factors.

3. MECHANICAL PROPERTIES

The important mechanical properties considered for building materials are: strength,compressive, tensile, bending, impact, hardness, plasticity, elasticity and abrasion resistance.

Strength is the ability of the material to resist failure under the action of stresses caused by loads, the most common being compression, tension, bending and impact. The importance of studying the various strengths will be highlighted from the fact that materials such as stones and concrete have high compressive strength but a low (1/5 to 1/50) tensile, bending and impact strengths.

Compressive Strength is found from tests on standard cylinders, prisms and cubes—smaller for homogeneous materials and larger for less homogeneous ones. Prisms and cylinders have lower resistance than cubes of the same cross-sectional area, on the other hand prisms with heights smaller than their sides have greater strength than cubes. This is due to the fact that when a specimen is compressed the plattens of the compression testing machine within which the specimen is placed, press tight the bases of the specimen and the resultant friction forces prevent the expansion of the adjoining faces, while the central lateral parts of the specimen undergoes transversal expansion. The only force to counteract this expansion is the adhesive force between the particles of the material. That is why a section away from the press plates fails early.

The test specimens of metals for tensile strength are round bars or strips and that of binding materials are of the shape of figure eight.

Bending Strength tests are performed on small bars (beams) supported at their ends and subjected to one or two concentrated loads which are gradually increased until failure takes place.

Hardness is the ability of a material to resist penetration by a harder body. Mohs scale is used to find the hardness of materials. It is a list of ten minerals arranged in the order of increasing hardness (Section 3.2). Hardness of metals and plastics is found by indentation of a steel ball.

Elasticity is the ability of a material to restore its initial form and dimensions after the load is removed. Within the limits of elasticity of solid bodies, the deformation is proportional to the stress. Ratio of unit stress to unit deformation is termed as modulus of elasticity. A large value of it represents a material with very small deformation.

Plasticity is the ability of a material to change its shape under load without cracking and to retain this shape after the load is removed. Some of the examples of plastic materials are steel,copper and hot bitumen.

4. CHARACTERISTIC BEHAVIOUR UNDER STRESS

The common characteristics of building materials under stress are ductility, brittleness, stiffness,flexibility, toughness, malleability and hardness.

The ductile materials can be drawn out without necking down, the examples being copper and wrought iron. Brittle materials have little or no plasticity. They fail suddenly without warning. Cast iron, stone, brick and concrete are comparatively brittle materials having aconsiderable amount of plasticity. Stiff materials have a high modulus of elasticity permitting small deformation for a given load. Flexible materials on the other hand have low modulus of elasticity and bend considerably without breakdown. Tough materials withstand heavy shocks.Toughness depends upon strength and flexibility. Malleable materials can be hammered into sheets without rupture. It depends upon ductility and softness of material. Copper is the mostmalleable material. Hard materials resist scratching and denting, for example cast iron and chrome steel. Materials resistant to abrasion such as manganese are also known as hard materials.

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