Following terms in connection with the physical properties of engineering materials are defined and explained:
(1) Bulk density: The term bulk density is used to mean the mass of a unit volume material in its natural state i. e. including pores and voids. It is obtained by finding out the ratio of mass of specimen to the volume of specimen in its natural state. The technical properties of the material such as strength, heat, conductivity, etc. are greatly influenced by its bulk density and hence the performance of a material will depend upon its bulk density.
(2) Chemical resistance: The ability of material to withstand the action of acids, alkalis, gases and salt solutions is known as its chemical resistance. This property is carefully examined while selecting material for sewer pipes hydraulic engineering installations, sanitary facilities, etc.
(3) Coefficient of softening: The ratio of compressive strength of material saturated with water to that in dry state is known as the coefficient of softening. The materials such as glass and metal are not affected by the presence of water and their coefficient of softening is unity. On the other hand, the materials like clay easily lose their strength when soaked in water and hence, their coefficient of softening is zero.
(4) Density: The term density of a material is declined as the mass of a unit Volume of homogeneous material. It is obtained by working out the ratio of mass of material to the volume of material in homogenous state. The physical properties of a material are greatly influenced by its density.
(5) Density index: The ratio of bulk density of a material to its density is known as its density index and it thus denotes the degree to which its volume is filled up with solid matter. As there are practically no dense substances in nature, the density index of most of the building materials is less than unity.
(6) Durability: The property of a material to resist the combined action of 'atmospheric and other factors is known as its durcibility. The running or maintenance cost of a building will naturally depend upon the durability of the materials of which it is composed
(7) Fire resistance: The term fire resistance is used to mean the ability of a material to resist the action of high temperature without losing its load-bearing capacity i .e, without substantial loss of strength or deformation in shape. This property of a material is of great importance in case of a fire and as the operation of fire-fighting is usually accompanied by water, this property of a material is tested by the combined actions of high temperature and water. The material should be sufficiently fireproof to afford safety and stability in case of a fire.
(8) Frost resistance: The ability of a water-saturated material to resist repeated freezing and thawing without considerable decrease of mechanical strength or visible signs of failure is known as the frost resistance, the frost resistance of a material depends upon the density of material and its degree of saturation with water. In general, the dense materials are frost resistant. The porous materials whose pores are closed or filled with water to less than 90% of their volume are frost resistant
(9) Hygroscopicity: The property of a material to absorb water vapour from air is 'known as the hygroscopicity and it is governed by the nature of substance involved, number of pores, air temperature, relative humidity, etc. The water-retaining or hydrophilic substances readily dissolve in water.
(10) Porosity: The term porosity is used to indicate the degree by which the volume of a material is occupied by pores. It is expressed as a ratio of volume of pores to that of the specimen. The porosity of a material is indicative of its various properties such as strength, bulk density, water absorption, thermal conductivity, durability, etc, and hence it is to be carefully studied and analyses.
(11) Refractoriness: The ability of a material to withstand prolonged action of high temperature without melting or loosing shape is known as its refractoriness.
(12) Spalling resistance: The ability of a material to endure a certain number of cycles of sharp temperature variations without failing is known as its spalling resistance and it mainly depends on the coefficients of linear expansion of its constituents
(13) Specific heat: The term specific heat is defined as the quantity of heat, expressed in kilocalories, required to heat 1 N of material by 1Â°C. The specific account. The specific heats of steel, stone and wood are as follows:
(14) Thermal capacity: The property of a material to absorb heat is known as its thermal capacity and it is worked out by the following equation:
T = (H / M x (T2-T1))
T = Thermal capacity in J/NãC
H = Quantity of heat required to increase the temperature of material from T1 to T2 in J
M = Mass of material in N
TJ = Temperature of material before heating in Â°C
T2 = Temperature of material after heating in Â°C
(5) Thermal conductivity: The thermal conductivity of a material is the amount of heat in kilocalories that will flow through unit area of the material with unit thickness in unit time when difference of temperature on its faces is also unity. The unit of thermal conductivity is J per m hr. C and it is usually denoted by K. The thermal conductivity of a material depends on its density, porosity, moisture content and temperature. The term thermal resistivity of a material is used to mean the reciprocal its thermal conductivity. The thermal resistance of a material is equal to thermal resistivity multiplied by its thickness.
(16) Water absorption: The ability of a material to absorb and retain water is known as its water absorption. The dry material is fully immersed in water and then the water absorption is worked out either as percentage of weight or percentage of volume of dry material. It mainly depends on the volume, size and shape of pores, present in the material.(17) Water permeability: The capacity of a material to allow water to pass through h it under pressure is known as its water permeability and it is described as the quantity of water that will pass through the material in one hour at constant pressure, the cross sectional area of the specimen being I cm2. The dense materials like glass, steel, etc. are water-proof or impervious to the water.
(18) Weathering resistance: The term weathering resistance is used to express the ability of a material to resist alternating wet and dry conditions without seriously affecting its shape and mechanical strength, It thus, indicates the behaviour of materials when exposed to changing conditions of humidity.
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