Mixing and Placing
Mixing and placing tests the practical chain from batching to curing: how you measure, how long you mix, how far you can drop concrete, how you compact it, and what you do about Ghanaian heat and rain.
Batching and mixing
Batch by volume with gauge boxes, not shovels, and correct for bulking of damp sand. Dry-mix until the colour is uniform, then add water gradually. Machine mix for at least 2–3 minutes after the last water; hand mixing on a watertight platform means turning the pile at least three times. Batch only what you can place within about 30 minutes of mixing (up to an hour with a retarder).
Transport and placing
Wheelbarrow, chute or skip, without segregation. Free-fall height should not exceed about 1.5 m; deeper than that, use a chute or tremie pipe. Place in horizontal layers of 300–500 mm and compact each layer before the next. Avoid cold joints: if a stoppage is unavoidable, key and roughen the surface, clean it, wet it and start with a rich mortar layer.
Compaction
Poker vibrator: insert vertically and quickly, withdraw slowly, 5–10 seconds per point, points spaced not more than about 1.5 times the radius of action (roughly 450 mm), and do not touch the formwork or the reinforcement. Over-vibration causes segregation. Hand compaction is acceptable only for blinding and very thin sections.
Hot weather and rain
Shade the aggregates, use cool water, avoid placing at midday, use a retarder, wet the formwork and substrate, and cover the finished surface with polythene. Stop placing in heavy rain and protect fresh concrete from washout.
Worked example
A strip footing 12.0 m long × 0.60 m wide × 0.225 m deep in 1:2:4 concrete. Wet volume = 12.0 × 0.60 × 0.225 = 1.62 m³; add 5% for waste and uneven excavation = 1.70 m³. Cement = 1.70 × 6.3 = 10.7, so order 11–12 bags. Sand = 1.70 × 0.44 = 0.75 m³; coarse aggregate = 1.70 × 0.88 = 1.50 m³. Water at w/c 0.50 ≈ 0.50 × (1.70 × 317) = 269 litres, adjusted on site for damp aggregates.
Traps that cost marks
Adding water to a stiff mix instead of remixing; mixing more than can be placed before it stiffens; vibrating the reinforcement or formwork; over-vibrating and causing segregation; placing on a dry, dusty, uncleaned base; forgetting to cure once the surface is finished.