Mixing Concrete by Hand for Farm Builds: Ratios, Bags and Curing in the Short Rains

October is building season on many Kenyan farms. The long rains harvest is in, there is a little cash in hand, and the short rains are a reminder of every place where water pools, mud builds up and floors crack. So the jobs go on the list: a concrete base for the new water tank, a floor for the zero-grazing unit, a feed trough, a slab under the chaff cutter, footings for a goat house. Most of them are small enough that nobody orders ready-mix. A fundi turns up with a shovel, a few bags of cement, a heap of sand and a heap of ballast, and the concrete is mixed by hand on the spot.

Hand-mixed concrete can be excellent. It can also be weak, dusty and cracked within a year, using exactly the same cement. The difference is almost never the brand of cement. It is how the materials are measured, how much water goes in, how well the mix is turned and how the slab is treated for the first week of its life. Those are the four things this guide is about.

Below are the common mix ratios, a worked bag estimate, a step-by-step hand-mixing method, when to hire a mixer, and how to cure concrete in the short rains. Ratios here are the nominal mixes widely used on small Kenyan sites; for anything structural, such as a tank stand carrying many tonnes or a suspended slab, have the design checked by a qualified engineer.

Choose the Mix for the Job, Not Out of Habit

Concrete is cement, sand (fine aggregate), ballast (coarse aggregate) and water. The cement and water react to form a paste that glues the aggregates together. On small sites the mix is described as a ratio by volume of cement to sand to ballast. Three nominal mixes cover most farm work.

The 1 : 2 : 4 mix is the workhorse, and it is what most fundis mean when they say "normal concrete". The richer 1 : 1.5 : 3 mix costs more cement per cubic metre but gives a denser, stronger concrete, which is worth it where the slab carries a full water tank or a posho mill that vibrates all day. A lean 1 : 3 : 6 mix is fine as a clean, level layer under a slab, but it should never be the slab itself.

Measure by Bucket or Gauge Box, Never by Shovel

The most common fault on small sites is measuring by shovelfuls. Shovels of damp sand and dry ballast hold different volumes, and every labourer loads differently, so by evening the "1 : 2 : 4" has drifted into something else.

Use a standard container for every material. A 20-litre bucket works well for small batches; a simple timber gauge box, a bottomless box sized to hold an exact volume, is faster for larger pours. Fill each measure level, not heaped. Keep one bucket for cement only, so it stays dry and you are not adding wet sand clinging to its walls.

Damp sand bulks up, so a bucket of damp sand holds less actual sand than a bucket of dry sand. Keep sand covered and draw it from the same heap, in the same condition, for the whole pour.

How Many Bags Do You Need?

Too little cement stops the pour halfway, leaving a weak joint between today's concrete and tomorrow's. Too much ties up cash in bags that harden in a damp store. A quick estimate avoids both.

Start with the wet volume: length times width times thickness, in metres. A slab 4 metres by 3 metres and 0.1 metres (100 mm) thick is 1.2 cubic metres. Dry materials shrink in volume when water is added and the fine particles fill the gaps between the coarse ones, so builders multiply the wet volume by about 1.5 to 1.55 to get the dry volume of materials. Here that is roughly 1.85 cubic metres of dry material.

In a 1 : 2 : 4 mix there are 7 parts in total, and cement is one of them. So cement is about 1.85 divided by 7, roughly 0.26 cubic metres. At about 1,440 kg per cubic metre, that is around 380 kg, or about 7.6 bags of 50 kg. Round up and buy 8, plus one spare bag for waste and for the inevitable extra corner. The same sum gives about 0.53 cubic metres of sand and 1.06 cubic metres of ballast. Use the steel measuring tape to measure the actual area rather than pacing it out; our guide to measuring and layout tools covers squaring a slab before you pour.

The Hand-Mixing Method, Step by Step

Good hand mixing follows a simple order: dry materials first, mixed until uniform, and only then water, added a little at a time. Every shortcut in this order shows up later as patches of weak concrete.

1. Prepare a mixing platform. Mix on a clean, hard, non-absorbent surface: an old slab, a steel sheet or a large timber board. Never mix on bare soil. Soil mixes into the concrete and weakens it, and dry ground sucks water out of the mix unevenly.

2. Spread the ballast, then the sand. Spread the measured ballast in a flat layer, then spread the sand on top of it.

3. Add the cement and dry-mix. Spread the cement over the sand and turn the whole heap over with a shovel from one side to the other, then back again. Keep turning until the colour is completely even, with no streaks of grey cement or brown sand. Three full turns is a common rule; more if the colour is still patchy.

4. Form a crater and add water gradually. Shape the heap into a ring with a hollow in the middle and pour in part of the water. Draw dry material from the sides into the water, turning carefully so water does not escape. Add the rest of the water in small amounts while turning.

5. Turn until uniform, then stop adding water. The mix is right when it is plastic and cohesive: it holds its shape when squeezed in a gloved hand without water running out, and a shovel cut through the heap leaves a clean face that does not slump flat. If it is too stiff to place, add a little water. If it is runny, you have gone too far; the fix is a little more of the dry materials in the correct ratio, never more water.

6. Place it quickly. Mix only what you can place in roughly the next half hour to an hour, and sooner in hot sun. Concrete that has started to stiffen should not be "revived" with more water.

Placing and Compacting: Getting the Air Out

Freshly placed concrete is full of trapped air. Left in, those pockets become honeycombing, the rough, stony voids you see on the sides of poorly built bases. On small jobs compaction is done by hand: rodding the concrete with a steel bar or a length of timber, tamping the surface with a straight board, and tapping the sides of the formwork with a hammer until the surface shows a thin film of paste and bubbles stop rising.

Place concrete close to its final position rather than dragging it across the slab, which separates stones from paste; a wheelbarrow tipped straight into the formwork beats a chain of shovels. On deeper bases, pour and compact in layers.

For animal house floors, think about drainage before you pour. A slight fall towards a drain channel stops urine and wash water pooling, which matters for hoof health and for hygiene. Our zero-grazing unit guide covers floor layout for dairy cows. Finish floors that animals walk on with a wooden float or a broom rather than a steel trowel, because a glass-smooth floor becomes slippery when wet. Brick and plastering trowels are on the price list at the time of writing for edges and patches.

Curing in the Short Rains: Damp Is Good, Rain on Fresh Concrete Is Not

Concrete does not dry hard. It hardens because cement reacts chemically with water, a reaction that continues for weeks while moisture is present. If the surface dries out in the first days, the reaction stops near the surface, leaving a weak, dusty skin that wears away under hooves and wheelbarrows. Curing simply means keeping the concrete damp so the reaction can continue.

The common guidance for ordinary cement is to keep concrete continuously moist for at least seven days. Cover it with polythene sheet, wet sacks or a layer of damp sand, and wet it down morning and evening if it is in sun. Shade the slab if you can on hot afternoons.

The short rains help and hurt. Humid, overcast weather makes curing easier, but heavy rain falling on concrete that has not yet set washes cement paste off the surface and pits it. Watch the sky before a pour, start early in the day, and have polythene sheeting and something to weigh it down on site before the first bucket is mixed. Once the concrete has set firm, usually after some hours, rain is no longer a threat to the surface and becomes free curing water.

Keep traffic off for the first days, and keep heavy loads off for longer. A tank base should be well cured before a full tank is placed on it; our guide to tank stands and gravity flow explains how heavy a full tank really is and why foundations are where stands fail.

Hand Mixing or a Mixer?

Hand mixing suits small, spread-out jobs: a trough base, a post footing, a floor patch. The limit is labour: as the team tires, the turning gets shorter and the water goes up.

A hired drum mixer makes sense for one large continuous pour, such as a full animal house floor or a big tank base, where a joint halfway through would be a weakness. It mixes more consistently with less effort, but still needs the same discipline about measuring and water. A mixer that is fed by shovel and topped up with water by eye makes bad concrete faster.

A practical rule many farm builders use: if the job can be done comfortably in a few batches by a small team in a morning, mix by hand on a good platform. If it needs a continuous pour bigger than the team can mix and place without rushing, hire a mixer or split the job at sensible joints planned in advance.

Pre-Pour Checklist

• Mix chosen for the job, and an engineer consulted for anything structural or heavily loaded.
• Area measured and squared; bags, sand and ballast calculated, plus a spare bag.
• Cement stored dry, off the floor, and used in date order; lumpy cement left out.
• Clean sand and ballast, free of soil, roots and rubbish.
• Clean mixing platform, standard buckets or gauge box, shovels, a tamping board and floats on site.
• Formwork firm, level and oiled or lined; ground below compacted and dampened.
• Clean water to hand; polythene sheeting and weights ready for rain and curing.
• Gloves, boots and eye protection for the team, since wet cement is alkaline and burns skin with long contact. Our workshop safety and PPE guide covers the basics.

For the tools that make a pour go smoothly, from shovels and trowels to mason hammers, concrete chisels and tapes, see the workshop and industrial tools range or the full price list, and talk to any Fuga branch or WhatsApp 0716 961 018 before you start.

Frequently Asked Questions