QUALITIES OF CONSTRUCTION MATERIALS
Properties of construction materials, which are of use
in engineering works, are classified as follows:
1. Physical properties
2. Mechanical properties
3. Electrical properties
4. Magnetic properties
5. Chemical properties
Properties of construction materials that are relevant
to building construction are dealt with in the following sections.
Physical Properties
The important physical properties of building materials
are as follows:
1. Bulk
Density
Bulk density is the mass
per unit volume of material in its natural state. This is obtained by finding
the ratio of the mass of the material to the volume of the material. Most of
the technical properties such as strength, conductivity, etc., depending on the
bulk density of the material. It is represented by the unit kN/m3. Bulk
densities of some of the naturally available materials are
Granite: 25 to 27 kn/m3,
Limestone: 18 to 24 kn/m3,
Pinewood: 5 to 6 kn/m3.
Mass per unit volume of homogeneous material is termed density. The density of steel is 78.5 kN/m3.
Density index is the ratio of bulk density to its density. Evidently, the
density index of most of the building materials is less than one.
2. Chemical
Resistance
It is the ability of a material to withstand the action
of chemicals like acids, alkalis, salt solutions and gaseous substances.
3. Fire
Resistance
Fire resistance is the capacity of a material to sustain
the action of high temperature without losing strength and change in the
original shape of the structure. Wood, one of the building materials, is
highly affected by the fire. Such materials should be treated so as to resist high
temperatures and fire.
4. Frost
Resistance
When voids of building material are filled with water
and subjected to sub-zero temperature, it is affected by a process known as
freezing. Thus, the ability of water-saturated material to resist freezing and
thawing is known as frost resistance. It depends on the density of the material
and its degree of water saturation. Generally, dense materials are frost
resistant.
5. Spalling
Resistance
Materials that could sustain repeated temperature
changes is said to be of high spalling resistance. This mainly depends on the
coefficient of linear expansion of the material.
6. Weathering
Resistance
It is the capacity of a material to resist alternate wet
and dry conditions without getting affected. Weathering causes a change in
shape and a decrease in mechanical strength.
7. Porosity
The porosity of a material is the ratio of the volume of voids
or pores to the total volume of the material. Denser the material, the lesser will
be its porosity. Bulk density, water absorption, thermal conductivity,
strength, durability, etc., depending on the porosity of the material.
8. Water
Absorption
The ability of a material to absorb and retain water is
called water absorption. It depends on the porosity of the material, the size
and shape of the pores.
9. Water
Permeability
It is the capacity of a material to allow water to pass through it under pressure. For example, wood may have high water permeability compared to steel.
10.
Hygroscopicity
It is the property of a material to absorb water vapour
from the air. It is governed by the porosity of the material, air temperature
and the nature of the substance involved.
11.
Coefficient of Softening
It is the ratio of compressive strength of a fully
saturated material to that of the same material when dry. Materials that have
a coefficient of softening more than 0.8 are said to be water-resistant
material.
12.
Durability
The property of a material to resist the action of
atmospheric and other factors continuously is called the durability of the
material. For example, the durability of a building depends on the resistance
offered by various materials used to construct it under different conditions.
Mechanical Properties
Various mechanical properties of materials are discussed
below:
1. Abrasion
Abrasion is the property of the material which resists
the force caused by a moving load.
2. Elasticity
Elasticity is the ability
of a material, which under stress, recovers its original shape after the
removal of the external load. If the material regains its original shape then
it is said to be perfectly elastic. Steel, copper, aluminium, etc., may be grouped
under perfectly elastic materials within certain limits of deformation. Thus,
for each material, there is a critical value of the load, generally known as the
elastic limit, which makes for the partial breakdown of elasticity. Loading
the material beyond this point leads to permanent deformation.
3. Plasticity
Plasticity is the property of a material by which a
strained material retains the deformed position even after the removal of the
external load which caused the deformation. Under large loads of forces, most
materials become plastic.
4. Ductility
The ability of a material by which it can be drawn into
a wire by external forces is known as ductility. Thus, a ductile material can
withstand large deformation before failure. During the process of extension, a
ductile material may show a certain degree of elasticity together with a
considerable extent of plasticity. Some of the ductile materials include
copper, aluminium, gold, etc.
5. Brittleness
Brittleness is the property
of a material by which it is not capable of undergoing a significant amount of
deformation due to the application of an external load but breaks or ruptures
suddenly. This is the most undesirable property of construction material.
Some brittle materials include glass, porcelain, etc.
6. Malleability
Malleability is the property of a material by which it
can be uniformly lengthened or widened by hammering or rolling without rupture.
A malleable material possesses a high degree of plasticity. This property has wide used in forging, hot rolling, drop stamping, etc. Some malleable materials
are wrought iron, copper, mild steel, etc.
7. Strength
Strength is the property of
a material determined by the maximum stress that the material can withstand
prior to failure. Strength is defined based on the nature of loading and the
nature of stress. There is no unique value that can define strength in all
cases.
For example, an adequately
designed structural member is not expected to fail under normal operating
conditions. This is ensured when the material of the member is strong enough to
withstand the force exerted on it.
8. Hardness
The hardness of a material is the ability of the material to
resist penetration by a hard material or object. The hardest material is
diamond, and the one with the least hardness is talcum.
9. Toughness
Toughness is the property of a material that enables
the material to absorb energy without fracture. This is a very useful
property of a material, which is applicable in cyclic or instantaneous loading.
10.
Stiffness
Stiffness is the property that enables a material to
withstand high stress without large deformation. The stiffness of a material
depends on its elastic property. For example, in a structural element, large
deformations are undesirable, and the material should be stiff enough to
withstand the load.
11.
Fatigue
When a material is subjected to repetitive fluctuating
stress, it will fail at stress well below that required to cause a
fracture under steady loads. This property is known as fatigue.
12.
Stability
Stability is the overall property of a member to
maintain the overall equilibrium preventing complete collapse. For example, a
component member made out of a particular material is just long enough to
prevent buckling when subjected to a force acting along its axis.
Electrical Properties
Physical properties and
mechanical properties are important for solid materials, which are used as
building construction materials. However, knowledge of electrical, magnetic and
chemical properties is also needed to deal with construction activities of
buildings in different environments. The electrical properties of materials are discussed
briefly below.
1. Resistivity
It is the property of the material to resist the flow of
electricity through it. It is a useful property of a conductor.
2. Conductivity
Electrical conductivity is the property of a material
due to which the electric current flows easily through the material. It is the
reciprocal of electrical resistivity.
3. Dielectric
Strength
Dielectric strength is the insulating capacity of a
material against high voltage. Hence, a material having high dielectric
strength can withstand sufficient high voltage before it will break down and
conduct.
4. Superconductivity
It is the phenomenon of an abrupt drop of resistivity of
some metals at a temperature called superconductivity transition temperature.
Magnetic Properties
Magnetic materials are those in which a state of
magnetization can be induced. Such mate-rials create a magnetic field in the
surrounding space. Some of the important magnetic properties are as follows:
(i) Permeability
(ii) Coercive force
(iii) Magnetic hysteresis
Permeability is the ratio of the flux density in a
material to the magnetizing force-producing that flux density.
Coercive force is the magnetizing force that is
necessary to neutralize the magnetism completely in an electromagnetic field.
Hysteresis is the quality of a magnetic substance due
to which energy is dissipated in it on reversal of its magnetism.
Chemical Properties
Engineering materials when they come in contact with
other substances are likely to react and suffer from chemical deterioration.
Some of the chemical properties of importance are corrosion resistance,
chemical composition and acidity or alkalinity.
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