EXP 2 Week 2 Materials

Reinforced Concrete

A composite material in which concrete's relatively low tensile strength and ductility are counteracted by the inclusion of a high tensile strength reinforcement, usually steel reinforcement bars (rebars) embedded in the concrete before it sets. The concrete is usually pre-stressed to improve the behaviour of the structure under working loads.

The rebars have to be well bonded to the concrete and act well under thermal pressures (ie. don't expand and contract to much), and resist corrosion. Also, the thermal expansion of concrete is similar to that of steel, eliminating large amounts of internal stress.

Chemicals within the cement paste used in the concrete react with the steel to create a passivating film on the surface of the steel making it more resistant to corrosion.

How it Corrodes:

• Mechanical Failure - cracking allowing corrosive elements into the structure, poor design.
• Carbonation - carbon dioxide attacking rebars too close to the concrete surface.
• Chlorides - salts from salt water (sea) or from chlorinated water in concrete mix (now disused)

Glass

Glass used in architecture is soda-lime glass, made up of 75% silicon dioxide, sodium oxide, lime and several minor additives. It is melted in gas fired units, homogenized and refined (bubbles removed), then formed. For windows, the float glass process is used - molten glass is floated on a molten metal surface. The top surfaces are then subjected to nitrogen under pressure to obtain a polished finish. It is finally annealed to remove stresses.

Glass is quite corrosion resistant. Only a few chemicals actually attack glass - hydrofluoric acid, concentrated phosphoric acid, hot concentrated alkali solutions and super heated water. However, in architecture, these acids do not usually come into contact with glass.

The most common way glass corrodes is by water. Alkali ions in the glass are extracted by hydrogen in water, leaving a silica-rich layer on the surface of the glass. More and more pH will touch the glass, rapidly increasing the corrosion process.


Timber

The making of timber is a simple process. Trees are felled, transported to a saw mill and cut into pieces of timber.

Timber is widely used in construction as it is one of the few natural building materials. It is renewable, low in production cost, a good insulator, readily available and easy to work.

However, wood destroys easily, mainly from fungal effects and being eaten by insects, mainly termites. Termites do not have a huge effect on timber structures at the moment, but without people to keep termites away from buildings, termites could eat an entire timber structure within a year. Timber is also susceptible to fire. Usually timber is the first material to corrode in a building, however it does stand well against things such as earthquakes.