What are the properties of building materials

 Building materials have an important role to play in this modern age of technology. Although their most important use is in construction activities, no field of engineering is conceivable without their use. Also, the building materials industry is an important contributor in our national 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 materials and forms of construction have developed in different parts of the world as a result of climatic differences. Another factor is the economic aspect of the choice of materials. The rapid advance of constructional methods, the increasing introduction of mechanical tools and plants, and changes in the organisation of the building industry may appreciably influence the choice of materials.Due to the great diversity in the usage of buildings and installations and the various processes of production, a great variety of requirements are placed upon building materials calling 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 interior decoration of residential and public buildings, gardens and parks, etc. should be, by their very purpose, pleasant to the eye,durable and strong. Specific properties of building materials serve as a basis for subdividing them into separate groups. For example, mineral binding materials are subdivided into air and hydraulic-setting varieties. The principal properties of building materials predetermine their applications. Only a comprehensive knowledge of the properties of materials allows a rational choice of materials for specific service conditions.

Building Materials

 PHYSICAL PROPERTIES

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

p = M V g / cm3

where                               M = mass (g)

                                         V = volume (cm3)

Density of some building materials is as follows:

Material                           Density (g/cm3)

Brick                                2.5–2.8

Granite                             2.6–2.9

Portland cement               2.9–3.1

Wood                               1.5–1.6

Steel                                 7.8–7.9

Bulk Density (p_b) is the mass of a unit volume of material in its natural state (with pores and voids)

calculated as : p = M V kg / m3

where                               M = mass of specimen (kg), V = volume of specimen in its natural state (m3)

Note: Bulk density may be expressed in g/cm³ but this presents some inconveniences, and this is

why it ix generally expressed in kg/m³. For example, the bulk density of reinforced cement concrete is preferably expressed ax 2500 kg/m³ rather than 2.5 g/cm³.

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

Material

Bulk density (kg/m3)

Brick

1600–1800

 

Granite

2500–2700

Sand

1450–1650

Pine wood

500–600

Steel

7850

Density Index (p_o) is the ratio, p = bulk density

density

= p_b p  

It indicates the degree to which the volume of a material is filled with solid matter. For almost all building materials p_o is less than 1.0 because there are no absolutely dense bodies in nature.

Specific Weight (y) also known as the unit weight) is the weight per unit volume of material,

y = p . g

where, y = specific weight (kN / m3), p = density of the material (kg / m) g, = gravity

(m / s2)

Specific weight can be used in civil engineering to determine the weight of a structure designed to carry certain loads while remaining intact and remaining within limits regarding deformation. It is also used in fluid dynamics as a property of the fluid (e.g., the specific weight             of 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.

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

True or absolute specific gravity (Ga) If both the permeable and impermeable voids are excluded to determine the true volume of solids, the specific gravity is called true or absolute specific gravity.

 Apparent or mass specific gravity (Gm) If both the permeable and impermeable voids are included to determine the true volume of solids, the specific gravity is called apparent specific gravity. It is the ratio of mass density of fine grained material to the mass density of water.

Porosity (n) is the degree to which volume of the material of the material is interspersed with pores. It is

expressed as a ratio of the volume of pores to that of the specimen. n = Vv / V

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

Void Ratio (e) is defined as the ratio of volume of voids (V_v) to the volume of solids (V_s). e = Vv/V_s

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 Resistance is the ability of a material to resist the action of high temperature without any appreciable deformation and substantial loss of strength. Fire resistivity materials are those which char, smolder, and ignite with difficulty when subjected to fire or high temperatures for long period but continue to burn or smolder only in the presence of flame, e.g. wood impregnated with fire proofing chemicals. Non-combustible materials neither smolder 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, alkalies, sea water and gases. Natural stone materials, e.g. limestone, marble and dolomite are eroded even by weak acids, wood has low resistance to acids and alkalies, 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.

 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.