Construction Materials and Technology
What the exam tests
This domain tests concrete, steel, masonry, timber and bituminous materials, including specification, testing, batching, placing, curing and durability. Expect questions on concrete mix adjustment, cube and slump tests, reinforcement cover, mortar and timber moisture, and common construction sequences.
Core rules
- Concrete strength depends mainly on water/cement ratio, cement type and content, compaction and curing. Lower w/c generally increases strength and reduces permeability, but too low a w/c harms workability.
- Workability is measured by slump or flow; strength is normally checked on standard cubes or cylinders at 7 and 28 days.
- Curing must keep concrete moist and at a suitable temperature; early drying reduces strength and durability.
- Reinforcement needs adequate cover to protect steel and provide fire resistance. Cover is measured to the outermost bar.
- Steel reinforcement is specified by grade, diameter, spacing and lap length; laps and anchorage transfer force.
- Masonry mortar should be weaker than the units in many situations so minor movement cracks the mortar, not the masonry.
- Timber moisture content affects shrinkage, strength and movement; specify moisture content for the service condition.
Worked example
A 1 m³ concrete mix contains cement 350 kg, water 175 kg, sand 700 kg and coarse aggregate 1150 kg. The sand has 4% moisture and the coarse aggregate 1%. Find the added water and as-delivered aggregate masses. Free water in sand = 700 × 0.04 = 28 kg. Free water in coarse aggregate = 1150 × 0.01 = 11.5 kg. Added water = 175 − 28 − 11.5 = 135.5 kg. As-delivered sand = 700 + 28 = 728 kg. As-delivered coarse aggregate = 1150 + 11.5 = 1161.5 kg. The w/c ratio remains 175/350 = 0.50; if water is added without reducing the free water from aggregates, the w/c rises and strength falls.
Traps
Adding water on site to improve slump without adjusting the mix; ignoring aggregate moisture; specifying materials by loose volume instead of mass; confusing nominal mix proportions with designed mixes; forgetting curing; and measuring cover to the centre of a bar instead of the outer surface.