How do I mix concrete by hand?

The ratios for common jobs, the slump test, and why adding extra water is the mistake that halves the strength.

Difficulty: intermediate
Time: 1 hr 30 min
Read: 4 min
Safety: warning

Short answer

For general use, 1 part cement to 5 parts ballast, or 1:2:3 cement, sand and aggregate. Mix dry until uniform, add water slowly, and stop as soon as it holds together. Extra water is easy and it dramatically weakens the finished concrete.

Hand-mixed concrete is entirely adequate for domestic work. The only variable that really matters is how much water goes in.

Safety

Wet concrete causes severe chemical burns through prolonged skin contact — cement burns develop slowly and people frequently do not notice until damage is done. Waterproof gloves, long sleeves, boots and eye protection, and never kneel in it. Dry cement dust contains crystalline silica; wear a mask. Bags of cement and ballast are heavy, so lift carefully.

What you'll need

Cement, and ballast or separate sand and aggregate
A mixing board, barrow or tub, and a shovel
Waterproof gloves, boots and eye protection

Step by step

Choose the mix for the job.1:5 cement to all-in ballast for general work such as fence posts and paths. 1:2:3 cement, sharp sand and 20mm aggregate for slabs and foundations. Postcrete for posts where speed matters.
Measure by volume with the same bucket.Consistency matters more than precision. Use one bucket for everything so the ratios stay the same between batches.
Mix the dry materials thoroughly.Turn the pile over at least three times until the colour is completely uniform. Streaks mean uneven strength through the pour.
Form a crater and add water slowly.A well in the centre, a little water, then fold the dry mix in from the outside. Repeat. This gives you far more control than pouring water over the whole heap.
Do the slump test.The mix should hold its shape when heaped and slump only slightly. If it flows or puddles it has too much water. If it crumbles, it needs a little more.
Resist adding water to make it easier.Workability is tempting and water is the single biggest factor in final strength. A wet mix can be less than half the strength of a correct one.
Mix only what you can place.Concrete starts setting within about two hours. Mix in barrow-loads you can place and finish before it stiffens.
Place and compact it.Fill in layers and tamp or rod each one to remove air pockets. Voids left in the pour are weak points and show on the surface.
Level and finish appropriately.A straight edge tamped across the form for a level surface. A wood float for a grippy finish, a steel float for smooth — a smooth outdoor slab is slippery in the wet.
Cure it properly.Keep it damp and covered for several days. Concrete gains strength by hydration, not by drying — letting it dry out fast in the sun weakens it substantially.
Protect it from frost.Concrete that freezes before it has set is permanently damaged. Cover with insulation in cold weather or wait for milder conditions.

Tips

Water is the enemy of strength. Mix it as dry as you can still work with.
Use the same bucket for every ingredient so the ratio stays consistent.
Keep it damp for several days. Concrete cures by hydration, not by drying out.

Common mistakes

Adding extra water to make it easier to work — Water content is the dominant factor in concrete strength. A sloppy mix can end up under half the strength of a correctly proportioned one, and there is no way to correct it afterwards.
Letting it dry out quickly in the sun — Concrete needs moisture to cure. Drying out fast stops the chemical reaction early, leaving a weak, dusty surface that never reaches full strength.

If it doesn't work

Mix too wet and running

Cause: Too much water — Fix: Add more dry material in the correct ratio. Never just add cement.

Surface dusty and weak after curing

Cause: Dried too fast, or over-watered — Fix: Cannot be rectified properly. Keep future pours damp and covered for several days.

Honeycombing and voids in the face

Cause: Not compacted — Fix: Tamp or rod each layer as you fill. Patch existing voids with a repair mortar.

Cracks appearing within days

Cause: Shrinkage from drying too fast, or no movement joints — Fix: Cure damp, and include movement joints in larger slabs.

Concrete not setting

Cause: Frost, or very old cement — Fix: Cement has a shelf life and absorbs moisture in the bag. Use fresh cement and protect from frost.

Questions people ask

What is the standard concrete mix ratio?

For general domestic work, 1 part cement to 5 parts all-in ballast. For slabs and foundations, 1:2:3 cement, sharp sand and 20mm aggregate is a common specification.

How much water should I add?

As little as you can while still being able to place and compact it. There is no fixed figure because the ballast carries moisture — judge by the slump test instead.

Is Postcrete worth using for fence posts?

For speed, yes — it sets in minutes and needs no mixing. It costs more per post, so for a long run of fencing a mixed concrete is more economical.