General construction: how buildings go together
Concrete basics: mix, pour, cure and joints
Concrete is cement, water, sand and stone that hardens by a chemical reaction, not by drying. How it is mixed, placed, cured and jointed decides whether it lasts. A plain-language guide for buyers.
Concrete is cement, water, sand and stone that hardens by a chemical reaction, not by drying. How it is mixed, placed, cured and jointed decides whether it lasts. A plain-language guide for buyers.
What is in the mix
Cement and water form a paste that binds sand and stone (aggregate). The ratio of water to cement is the biggest single driver of strength and durability: more water makes it easier to pour but weaker and more porous. The mix is specified by the engineer, usually by strength class, and a supplier batches to that.
Pouring and placing
Concrete should be placed on prepared, compacted ground or formwork that is clean, braced and the right shape. It is compacted or vibrated to remove air pockets, then levelled. Do not let anyone add water on site to make it flow, because it quietly lowers the strength you paid for.
Curing is not drying
Concrete gains strength only while it stays moist and within a suitable temperature range. Curing means keeping the surface damp or covered in the first days, which prevents surface cracking and weak, dusty skin. Hot, windy or cold weather needs specific precautions, and the engineer or supplier should state them.
Joints and cracks
Concrete shrinks as it cures, and it will crack somewhere. Control joints are cut or formed so the cracks happen in straight lines you chose. Expansion joints separate slabs from walls and columns so movement does not push them into each other. A hairline crack is common; a wide, growing or stepped crack is a reason to call the engineer.
Testing and records
Ask what testing is planned, commonly slump checks and cylinder or cube samples broken at set ages, and who keeps the results. Keep the delivery tickets, which record the mix and time of batching.
Quick answers
- How long until concrete is at full strength? Strength builds over weeks, and the design strength is normally defined at a set age such as 28 days. Loading earlier should follow the engineer's instruction.
- Why does concrete crack? Mainly because it shrinks as it cures. Joints and reinforcement are there to control where and how wide cracks form.
- Can I pour in the rain or heat? Only with precautions set by the engineer or supplier. Weather affects strength and finish.
Keep reading
- Foundation types explained: slab, crawlspace, basement and piles
- Waterproofing and damp control basics
- Wind and seismic resilience basics
Always check the project drawings and specification, and confirm requirements with the engineer of record and the local authority.
Questions people ask
How long until concrete is at full strength?
Strength builds over weeks, and the design strength is normally defined at a set age such as 28 days. Loading earlier should follow the engineer's instruction.
Why does concrete crack?
Mainly because it shrinks as it cures. Joints and reinforcement are there to control where and how wide cracks form.
Can I pour in the rain or heat?
Only with precautions set by the engineer or supplier. Weather affects strength and finish.
Sources and further reading
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