MATERIALS

by Ikmalzatul Abdullah

THERMAL MOVEMENT Thermal properties – connection with heat transfer

and heat loss. Thermal movement is caused by the expansion or

contraction of materials. Materials generally expand when heated and

contract when cooled. Problems arise related to the thermal movement:

• Large walls and glass in windows may crack due to this process if they are restrained.

The actual amount of thermal movement depends on the type of materials used.

MOISTURE CONTENT & WATER ABSORPTION

Many building materials contain water or moisture to a greater or lesser extent.

The moisture may be the remainder after the material was prepared and used or it can occur within the substance naturally.

Inappropriate water content causes many problems. (e.g: timber)

Cont’d Some materials do not absorb or give

out any moisture (e.g: bituminous materials, metals and most plastics)

These materials frequently used for water proofing, as vapor barriers to stop the penetration of water vapor and for damp-proof courses respectively.

Cont’d The actual level of moisture within a given

material depends also on the temperature and relative humidity of the surrounding air.

In damper environments moisture levels in materials are higher than in drier environments.

The moisture content can also be greatly modified depending on what the material is in contact with.

POROSITY As the void content of a material increases, so

does the ability to absorb water by capillary action.

The ability to absorb moisture depends on the size of the voids and how accessible the voids are to the water at the surface.

Generally, there is no direct correlation between porosity and other properties such as durability.– Porosity = volume of pores / bulk volume x

100– Porosity = (solid density – bulk density) /

solid density x 100

ELASTICITY Elasticity is the forces cause the material in a

building to squash, stretch and bend. The forces frequently caused by gravity. Squashing forces = compressive Stretching forces = tensile The columns / walls / foundations holding a

building up are in compression while the wires holding up a suspended walkway are in tension.

Beams have bending forces acting on them and are partly in compression and partly in tension.

Cont’d Some materials behave well in tension while

others perform better in compression. Most materials have elastic behavior up to a

certain level of applied force. Glass, steel, iron, timber are examples of

materials which exhibits elastic behavior to a greater or lesser extent.

Fresh putty and sealants do not have any elastic properties.

Elastic behavior is important because it is predictable.

STRAIN The strain measures the change in length of a

material compared with the original length. A measure of the deformation of a material. A material in tension has tensile strain and in

compression it has compressive strain.

FORCE Measured in Newton (N). Can be applied in any direction. Gravity causes vertical forces to act. The force of gravity acts on all objects.

– Force = mass x gravity

STRESS A measure of the force causing deformation. Stress in the ‘concentration’ of the applied

force. Stress is very important concept. Suppose a given load bearing column is at the

maximum allowed stress. For a given stress the load is directly

proportional to its cross-sectional area.– Stress = force / cross sectional area

YOUNG MODULUS

If a force is applied to a given sample material, the strain can be measure as the stress is increased.– Young modulus = stress / strain

Also known as the elastic modulus. A measure of the stiffness of the material or it

resistance to squashing or stretching.– Unit = N/mm2

STRENGTH OF MATERIALS

Yield stress is the stress at which the material starts to change from elastic to plastic behavior.– Yield stress = force at yield point / original

area

Breaking stress = the stress at the fracture, that is at the point the material breaks. – Breaking stress = force at fracture / original

area

Cont’d Ultimate stress = the greatest value of stress

the material can withstand before failure.– Ultimate stress = maximum force / original

area Yield stress, breaking stress and ultimate

stress are all measured N/mm2 or N/m2. These are very important quantities for

building engineers. These are all measures of the tensile strength

of materials.– Compressive strength = force applied at

failure / area of specimen

OTHER RELEVANT MECHANICAL PROPERTIES

Materials which have substantial plastic phase when tensile forces are applied are said to be ductile.

Ductility = also the ability of a material to be drawn into wires.

For engineering purposes, ductility is an important consideration.

The property of ductility allows loaded materials to redistribute localized stresses which may build up at the position of notches.

Cont’d Localized stresses may build up as

there is no yielding and continue to do so until failure suddenly occurs.

Hardness is the resistance of a material to becoming indented.

Hardness is an important factor to consider for the construction of floor and walls.

Cont’d For metals, hardness is determined by

measuring the resistance offered by the material to the penetration of either a hardened steel ball or a diamond into its surface under standard loading conditions.

Tough materials can easily absorb energy impact.– For example, ceramic materials tend to be

very strong in compression but they are not tough.

– They are easy to break by sudden blows.

ELECTRICAL INSULATION Materials which are good electrical

conductors can pass large electric current. A very good electrical conductor is copper.

– Copper wires have very low electrical resistance and are frequently used power cables, telephone wires, etc.

Aluminum too, has long been in use for electrical conductors.

Cont’d At the other extreme, plastics are very poor

conductors – to be electrical insulators and have very high resistances.– Plastics are used to electrically insulate

conducting wires and electrical fittings inside buildings.

The ability of a given type of material to oppose the flow of an electric current is called the electrical resistivity.

Cont’d Refer to the table given.

– From the table, metals have very low resisitvity.

– Most non-metals have very high resistivities.– Metals are very good conductors while non-

metals are very good insulators of electricity. Electrical resisitivity = (area of cross section x

resistance) / length

CHEMICAL PROPERTIES Besides the physical properties, chemical

properties are also one of the important properties to be discussed.

In a chemical reaction, the atoms or molecules of two or more substances combine irreversibly to form a new material.

Metals suffer from various chemical reactions which cause corrosion.

These reactions result in degradation of the material.

OXIDATION A common chemical reaction between oxygen

and many metals. This reaction often causes damage to the

building material. Examples of the oxidation process:

• Unprotected iron and steel will react with oxygen in air only in the presence of water:

–Iron + oxygen = iron oxide (rust)• The water is essential in this reaction to

take place.• The reaction is speeded up when salts are

present.

Cont’d The rust formed flakes away from the

remaining metal and not only spoils the appearance but more importantly reduces the strength of the structure.

Where moisture is present, it is important to protect the steel from rusting.

Unlike rust, aluminum oxide sticks to the surface of the metal and protects the material from further oxidation.

Cont’d– Nevertheless, aluminum is usually

anodized.

– In this process, the natural oxide film is thickened. This increases the resistance to degradation.

– The process of anodizing also improves the appearance of the surface.

ELECTROLYTIC CORROSION If two different metals are joined together, for

example in plumbing, electrolytic corrosion may occur if precautions are not taken.

In this electrical and chemical process, one of the two metals is gradually ‘eaten away’ by corrosion.

For any two metals joined together, it is easy to find out which of them will corrode.

When two different metals are connected together, the one having the more negative electrode potential has an increased willingness to supply electrons.

CORROSION PROCESS The corrosion process can be accelerated

further:• As the salt or acid concentration in the

solution is increased• If the area of the anode is small

compared with the cathode• If there is an increase in temperature of

the solution