Concrete and RCC Works
What this domain tests
You must know concrete grades and nominal mixes, water–cement ratio, workability and cube tests, curing, formwork stripping and reinforcement cover.
Core rules and concepts
- Grades (IS 456): M10 (1:3:6), M15 (1:2:4), M20 (1:1.5:3) nominal mixes; M25 and above by design mix. M20 is the minimum for reinforced concrete in mild exposure.
- Water–cement ratio: about 0.5 for RCC; minimum cement content 300 kg/m³ in moderate exposure; never add water on site to improve workability - it drops strength.
- Workability: slump test; 25–75 mm typical for RCC.
- Cubes: 150 mm cubes, three tested at 28 days; compaction by vibrator or tamping.
- Curing: 7 days for OPC, 10 days for blended cement, 14 days in hot, dry or marine conditions; keep surfaces continuously moist.
- Cover (mild exposure): slab 20 mm, beam 25 mm, column 40 mm, footing 50 mm; not less than the bar diameter.
- Stripping: vertical sides 16–24 hours; slab soffit props 3 days; beam soffit 7 days; 14 days for longer spans.
Worked example
A slab 4 m × 3 m × 0.12 m in M20 nominal mix 1:1.5:3. Wet volume = 4 × 3 × 0.12 = 1.44 m³. Dry volume = 1.44 × 1.54 = 2.218 m³. Sum of mix parts = 1 + 1.5 + 3 = 5.5. Cement = 2.218 ÷ 5.5 = 0.403 m³ × 1440 kg/m³ = 581 kg ≈ 11.6 bags, so 12 bags.
Traps that cost marks
Testing cubes at 7 days and calling it the grade; using dry volume factor 1.0 instead of 1.52–1.54; forgetting that one bag is 50 kg; stripping beam soffits in 3 days; leaving cover out of the bar bending schedule.