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 which are relevant to building construction are dealt with in the following sections.

1.3.1 Physical Properties

The important physical properties of building materials are as follows:

1. Bulk Density

Bulk density is the mass per unit volume of a 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., depend on the bulk density of the material. It is represented by the unit kN/m³. Bulk densities of some of 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 a homogeneous material is termed as density. 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 loosing strength and change in the original shape of the structure. Wood, one of the building materials, is highly affected by fire. Such materials should be treated so as to resist high temperature and fire.

4. Frost Resistance

When voids of a 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 decrease in mechanical strength.

7. Porosity

Porosity of a material is the ratio of volume of voids or pores to the total volume of the material. Denser the material, lesser will be its porosity. Bulk density, water absorption, thermal conductivity, strength, durability, etc., depend 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.

1.3.2 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 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 a 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 a wide use 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 which 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

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 least hardness is talcum.

9. Toughness

Toughness is the property of a material which 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. 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 a stress well below than 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.

1.3.3 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 under different environments. 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 that 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 abrupt drop of resistivity of some metals at a temperature called superconductivity transition temperature.

1.3.4 Magnetic Properties

Magnetic materials are those in which a state of magnetization can be induced. Such materials create a magnetic field in the surrounding space. Some of the important magnetic properties are as follows:

1. Permeability
2. Coercive force
3. 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 that quality of a magnetic substance due to which energy is dissipated in it on reversal of its magnetism.