Reinforcement, Formwork and Concrete Finishing
This domain tests the details that make reinforced concrete work: the right bar, the right cover, the right lap, formwork that holds its shape, and a finish that is cured rather than just trowelled.
Reinforcement
- Mild steel bars (R) have a characteristic strength around 250 N/mm²; high-yield deformed bars (Y) around 460–500 N/mm².
- Common sizes: 6, 8, 10, 12, 16, 20, 25 mm diameter.
- Nominal cover: about 25 mm for slabs, 30 mm for beams and columns, 40–50 mm for foundations cast against earth.
- Tension lap length about 40 × bar diameter for deformed bars (45 × for mild steel); stagger laps and lap no more than half the bars at any one section.
- Use chairs, spacers and 1.6 mm annealed binding wire; keep bars off the ground and free of mud and oil.
Formwork
Timber or 18 mm plywood shutters, strong, rigid, watertight, accurate and clean, coated with release agent. Props and braces must be adequate and founded on a solid base. Strike sides after about 24 hours; slab soffits after about 7 days (props left longer); beam soffits 14–21 days depending on span and loading - always follow the specification.
Finishing and curing
Screed to level, float with a wooden float, then steel-trowel for a hard surface, or brush/broom for a non-slip finish. Edge and joint as detailed, then cure moist for at least 7 days.
Worked example
A slab 4.0 m × 3.0 m with 12 mm bars at 200 mm centres both ways and 25 mm cover. Bars running the 4.0 m way: (3000 − 2 × 25) ÷ 200 = 14.75, so 15 spaces and 16 bars. Each bar is 4.0 − 0.05 = 3.95 m, total 63.2 m. Add 5% for laps and waste = 66.4 m. At 0.888 kg/m for 12 mm bar, mass = 66.4 × 0.888 ≈ 59 kg. Reasoning: count spaces, not bars, then add one; deduct cover from both ends.
Traps that cost marks
Letting bars sit on the ground so cover ends up zero; putting all laps in one section; formwork not oiled or not cleaned out; striking soffits early to save time; measuring cover to the wrong face; trowelling while bleed water is still present; skipping curing.