Concrete and Plastering
Concrete and Plastering tests batching, mixing, placing, compaction and curing of concrete, plus plaster thickness, mix proportions, surface preparation, bond and finishing. The exam links mix design and workmanship to strength, shrinkage and appearance.
Core rules
- Concrete grades such as M10, M15, M20 and M25 denote characteristic cube strength in N/mm2; use the specified grade and slump.
- Measure cement by weight, aggregates by consistent volume or weight, and control water-cement ratio; extra water reduces strength and increases shrinkage.
- Compact with a vibrator or punning, avoiding segregation; cure moist for at least 7 days for ordinary Portland cement concrete, longer for blended cements.
- Plaster is usually 12 mm thick in one coat or about 20 mm in two coats; common mixes are 1:4 to 1:6 cement:sand.
- Prepare the background, wet it, use chicken mesh at dissimilar junctions, and cure plaster to reduce shrinkage cracks.
Worked example
Plaster 100 m2 at 12 mm thick has wet volume 100 × 0.012 = 1.2 m3. For 1:6 mortar, dry volume = 1.2 × 1.33 = 1.596 m3. Cement volume = 1.596 ÷ 7 = 0.228 m3; cement mass = 0.228 × 1,440 = 328 kg, about 6.6 bags. Sand = 1.596 × 6 ÷ 7 = 1.368 m3. Add a separate allowance for uneven backgrounds and wastage.
Traps that cost marks
Adding water on site to make concrete easier to place, plastering over dust without a bond coat, and skipping curing are frequent errors. A 25 mm single-coat plaster is not a substitute for two properly bonded coats.
Key points
- M20 means characteristic cube strength of 20 N/mm2.