Composites

Question 1

Timber

Answer 1

Timber is a composite of cellulose fibres or tracheids and natural resin. Timber can come in two forms; softwood and hardwood. These terms can be rather misleading however, and so a more poignant way of defining them might be pored (hardwood) and non-pored (softwood). While timber does offer excellent specific strength and reasonable performance in bending, susceptibility to weather and attacks from pests make it so that few ancient timber structures remain.

Question 2

Mortar

Answer 2

Mortar is the material used between bricks in building. Prior to the development of Portland cement, mortar consisted of slaked lime, sand and water. This creates a slow hardening paste that hardens, with the lime reacting with the carbon dioxide to produce calcium carbonate, which replaces the sand/slake lime paste with a hard solid.

Question 3

Concrete

Answer 3

Concrete is a composite made of cement, sand and aggregate; the aggregate provides the strength, the sand fills in the gaps and the cement acts as a binder. Concrete offers far greater strength than cement alone and is cheaper, so it is used extensively.

Concrete behaves in a similar way to stone, as it is strong in compression, weak in tension and has low toughness. It is also fireproof and does not corrode.

Question 4

Reinforced concrete

Answer 4

Because of concrete’s inherent weakness in tension, there may be problems to do with sagging. To solve this issue, concrete may be reinforced with steel rods or mesh that take the tensile loads and make the concrete more resistant to failure.

Question 5

Asphalt

Answer 5

Asphalt is a composite with hard aggregate and bitumen as the matrix. It is advantageous for roads as it is tough and crack resistant due to the bitumen, yet hard wearing due to the exposed aggregate.

Question 6

Laminates

Answer 6

Laminates are materials that consist of varying materials sandwiched together.
E.g. Plywood is a laminate of timer with grain arranged at 90* to each successive layer
E.g. Laminated Veneer Lumber (LVL) is similar to plywood in appearance but the veneers are not arranged at 90* to the adjacent one, hence each veneer runs along the beam.
E.g. Laminated glass is used when shatter resistant glass is needed. Two layers of glass are passed through rollers that compress a polymer sheet lying between them.