ZIMSEC O Level · Building Technology and Design · Form 1, Form 2
Bricks and blocks
Burnt clay bricks (common in Zimbabwe), concrete blocks and cement-stabilised soil blocks. Good bricks are well burnt, hard, uniform in size and give a ringing sound when struck together.
Cement, lime and sand
- Ordinary Portland cement hardens by reacting with water (hydration). Store it dry, off the floor, and use it before it goes lumpy.
- Lime improves workability of mortar.
- Sand: clean, sharp sand for concrete and mortar; dirty sand weakens the mix.
Mortar
Cement, sand and water used to bond bricks. A common mix is 1 part cement to 4–6 parts sand.
Concrete
Cement, sand (fine aggregate), crushed stone (coarse aggregate) and water. A common mix is 1 : 2 : 4 (cement : sand : stone). Too much water weakens concrete.
- Curing: keep concrete damp for about 7 days so it gains strength.
- Reinforced concrete uses steel bars to resist tension (lintels, slabs, beams).
Other materials
Timber (roof trusses, door frames), roofing sheets (corrugated iron, IBR), roof tiles, thatch, glass, steel and plastics (pipes and DPC membranes).
Exam tip: Learn the mix ratios with the order of materials – examiners expect “cement : sand : stone”.
Check yourself
- Give the common mix ratio for concrete.
- Why is concrete cured?
- State two qualities of a good brick.
Show answers
- 1 : 2 : 4 (cement : sand : stone).
- Keeping it moist allows proper hydration so it reaches full strength without cracking.
- Any two: well burnt, hard, uniform size and shape, ringing sound, free from cracks.
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