Thursday, 15 January 2009

material for construction

Materials for construction

Required properties of construction materials:
1. strength
The strength of a material refers to its load-carrying capacity in tension, compression, shear and torsion.

The strength may vary appreciably with
-the rate and frequency of loading
-In non-homogenous materials, with the direction of loading.
-Moisture content (timber)
-temperature (rubber)

the own weight of the material usually ‘use up’ a considerable proportion of its strength.

Permissible stress= characteristic strength/factor of safety

2. ductility
ductility generally decreases with strength in tension. Materials that are linearly elastic up to failure are highly unsuitable for structural purposes. They give no warning of approaching failure: moreover, they often shatter under impact.

3. durability
All materials deteriorate with time, but at different rates. Direct or indirect causes of deterioration include the following:
(a) Corrosion of materials- corrosion usually occurs in moist conditions and in the presence of atmospheric pollution or flue gases.
(b) Biological agencies- insects attack organic materials, mainly timber.
(c) Water- some materials, such as limestone, slowly dissolve in water; timber becomes weaker when it is wet; water also promote fungal growth; water tapped in pores or interstices causes cracking and bursting when it expands during freezing.
(d) Sulphate attack- ground waters and industrial wastes often contain soluble sulphates which attack cement products(such as concrete) and metals.

4. fire resistance
In fires, materials may
-melt, loose strength
-expand, and crack

-timber, plastics
-bricks, stones, concrete and metal

Steel may expand and cause collapse.
Asbestos cement sheets may spall, shrink and allow the passage of flame through cracks.

Fire resistance is a property of a structural element, e.g. walls, columns, floors, beams, and not of individual materials. It is expressed as the period of time (in hours) during which a structural element survives the standard fire test while continuing to perform its normal structural function.
Vary according to the use of the structure, its height, floor area and cubic capacity.

Selection of structural materials:

Factors to be considered:
1. structural form
2. foundation
3. site condition
4. construction method
5. availability of the materials and the relative cost
6. time available for the construction