Skills · skill
Mixing and Placing Concrete
Building · Beginner
- Difficulty
- Beginner
- Safety
- Wet cement is caustic and burns skin over hours without you noticing; gloves, and wash off splashes. Dust: mask when handling dry cement.
- Done When
- It sets hard overnight, rings rather than thuds after a week, and shows no surface dusting or fine cracks.
- Category
- Building
- Summary
- Making the world's most-used material — cement, sand, gravel and water — and putting it where it will set into stone.
Steps
Gauge by volume: 1 cement, 2 sand, 3 gravel for general work.
Mix the dry ingredients until one colour, then add water a little at a time to a stiff, workable mix that holds a shape.
Dampen the formwork or hole; place the concrete within an hour of mixing.
Tamp or rod it to drive the air out, especially at edges and around bar.
Strike off level, then float once the surface sheen has gone.
Cover and keep damp for a week; do not load for a month.
Tools
Cement, sharp sand, 20 mm gravel, clean water; a mixer or a board and shovel; buckets for gauging; a trowel, float, and something to tamp with; polythene to cover it.
The Science
Cement does not dry, it hydrates: calcium silicates react with water to grow interlocking crystals, and keep doing so for weeks. Too much water leaves pores when it evaporates — every extra bucket costs strength; too little and it will not compact. Concrete is strong in compression and weak in tension, which is why it is reinforced with steel bar, and why it cracks if it dries out fast before it has cured.
How It Fails
Weak, dusty concrete from too much water or from drying out uncovered in sun. Honeycombing — voids at the surface — from poor compaction. Cracks from a slab with no joints, or curing too fast.
Referenced by
element
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Open on the Skills shelf — the interactive card, the filters, and everything alongside it.