Building Materials and Their Properties
Building Materials and Their Properties tests whether you can select a material for a job and justify it with properties and simple numbers. The exam expects strength, durability, thermal behaviour, workability and specification.
Core rules
- Strength resists load; durability resists weathering and decay; density affects weight and sound insulation.
- Thermal conductivity (λ) measures heat flow through a material; lower λ means better insulation.
- Thermal resistance R = d/λ, where d is thickness in metres. U-value = 1/Rtotal, so lower U is better.
- Concrete mixes are often given by volume, e.g. 1:2:4 cement:sand:aggregate; water-cement ratio controls strength and durability.
- Mortar should be weaker than the units it joins so cracks occur in the mortar, not the blockwork.
Worked example
A 100 mm insulation board has λ = 0.035 W/mK. Convert 100 mm to 0.100 m. R = 0.100/0.035 = 2.86 m²K/W. If the rest of the wall has R = 0.50 m²K/W, total R = 3.36 m²K/W, so U = 1/3.36 = 0.30 W/m²K. The reasoning is: thickness must be in metres, then add resistances before taking the reciprocal.
Traps
- Mixing millimetres and metres in R = d/λ.
- Confusing R and U; a high R is good, a high U is poor.
- Assuming a denser material is always stronger or always better insulated.
- Ignoring wetting, which can raise λ and reduce insulation performance.
Specify material, property and reason in one sentence.