Concreting a 150 mm ground-bearing slab

Slab thickness is not a matter of taste. It depends on the load, how firm the ground is and, for any habitable building, on the structural engineer's design. The price list covers three common thicknesses. They differ mainly in what the slab can carry and how much concrete goes into it.

100 mm slab

This suits light structures and areas without heavy point loads: the floor of a garden shed, a log store, a carport for bikes or a patio that people only walk on. It is also often used as a blinding or base layer under other build-ups. One square metre usually needs about 0.1 m³ of concrete.

150 mm slab

This is the thickness you see most often. It goes under single garages, workshops, larger garden rooms, small timber-frame buildings and floors that carry heavier items. With good reinforcing mesh and a well-compacted sub-base it handles normal loads without trouble. It uses roughly 0.15 m³ of concrete per m².

200 mm slab

This is used for houses built on a raft slab, under masonry walls, on weaker ground and under heavy machinery. It often has two layers of reinforcement and thickened edges under loadbearing walls. It needs roughly 0.2 m³ per m². For a house, the thickness, concrete class and reinforcement are always set by a structural engineer.

If the slab belongs to a building that falls under building regulations, do not assume that a garage or large garden room is automatically exempt. Rules vary by country and region. Check with your local building authority what your particular project needs before you order any concrete.

A crew that turns up to an unprepared site either waits around or pours onto a poor base. Neither is good. Before pour day, the following should be done.

Sub-base and compaction

Topsoil, roots and soft ground are dug out down to firm, undisturbed soil. A well-graded crushed stone sub-base (in the UK often Type 1) is then brought in, usually around 150 to 300 mm thick depending on the ground. It is compacted with a plate compactor in layers, not the full depth in one go. On clay or made-up ground it is sensible to lay a geotextile under the stone so it does not get pushed into the soil.

Why does everyone keep going on about compaction? Concrete is strong only when it sits on something that does not move. If the stone settles over time, the slab loses support and cracks. These cracks often appear months later, when the floor finish or the walls are already on top.

Formwork and levels

The slab edges are formed with timber boards, battens or system formwork. Insulated slabs sometimes use permanent formwork made from XPS. The formwork must be firmly staked, because wet concrete pushes hard against it. The top edge is set to the exact level with a dumpy level or laser. On a 150 mm slab there are usually a few extra level pegs marked in the middle of the area too.

Reinforcement

Most slabs use welded steel mesh. The key point is that it must not lie on the stone. It sits on plastic or concrete spacers (chairs) so the concrete can surround it on all sides. Cover is typically about 30 to 50 mm, according to the design. Sheets overlap by at least one full mesh square, or as shown on the reinforcement drawing for a house. On a 200 mm slab with two layers, steel chairs hold the top mesh above the bottom one.

Service entries, earthing and membranes

Anything that passes through the slab has to be in place beforehand: drainage, the water supply, ducts for electricity and broadband, and perhaps a duct for a heat pump. Where a foundation earth electrode is planned, usually galvanised steel strip that forms part of the building's earthing, it is installed by a qualified electrician. This is not a job for anyone else.

For habitable buildings you need to know in advance where the damp-proof membrane and insulation go. Sometimes they sit below the slab, sometimes on top. If your area has a raised radon risk, the design will also include a radon barrier. A polythene sheet is normally laid under the slab to stop cement paste draining into the stone.

Access for the truck and pump

A ready-mix truck is heavy and needs firm access. If it cannot reverse right up to the formwork, you will need a concrete pump. Agree beforehand where the pump will stand, check it is clear of overhead power lines and trees, and let the neighbours know if the street will be blocked for a while.

When the area is properly prepared, everything happens quickly and in a clear order. A 150 mm slab of a few dozen square metres usually fits into one morning.

1. Final check: the crew checks formwork levels, mesh on spacers and service entries. On housing projects a site supervisor or building inspector often looks again.
2. Damping down: in warm weather the sub-base and formwork are lightly wetted so they do not draw water out of the fresh concrete.
3. Placing: concrete is placed evenly, working out from one corner, not dumped in one heap and raked a long way.
4. Compacting: a poker vibrator or vibrating screed drives out trapped air, so concrete fills around the mesh and along the formwork.
5. Screeding to level: a straightedge or vibrating beam is pulled along the formwork and level pegs until the surface is flat.
6. Finishing: depending on what comes next, the surface is left textured, floated by hand, or power-floated once it has stiffened.
7. Curing: as soon as the surface allows, the slab is covered with polythene or kept wet.

Power floating waits until the concrete is firm enough that a footprint sinks in only a few millimetres. In summer that can be a few hours after the pour, in spring and autumn much later. This is why the crew sometimes stays until evening.

Concrete class

Smaller slabs under garden buildings usually use C16/20 or C20/25. Garages and houses typically use C20/25 to C25/30. If the slab is outside and exposed to frost or de-icing salt, a garage driveway apron for example, the concrete also needs the right exposure class under EN 206. The design sets the class. If you have no design, the ready-mix supplier will advise when you tell them what the slab is for.

Consistence

Slabs are usually ordered at consistence class S3 or S4, which spreads and compacts well. Nobody should add water on site. Wetter concrete is easier to work, but it is weaker and shrinks more. If you need a more fluid mix, for pumping say, order it with an admixture straight from the plant.

Reinforcement: one mesh, two meshes or fibres

One layer of mesh in the lower third to half of the thickness is enough for most 100 and 150 mm slabs. Two meshes, top and bottom, go into thicker and more heavily loaded slabs, typically a 200 mm house raft. Fibre-reinforced concrete with steel or polypropylene fibres limits small shrinkage cracks and replaces mesh in some industrial floors. For a house foundation, only the structural engineer can approve it.

Surface finish to suit what goes on top

If the slab will be built on and later screeded, a flat, slightly rough surface is fine. If it will be the actual floor of a garage or workshop, power floating is worth it, perhaps with a dry-shake hardener to improve wear resistance. Outdoor patios are often given a light brushed finish so they are not slippery when wet.

The price list counts this job per square metre, yet two slabs of the same area can be very different to build. The job gets easier with:

• Good access, so the truck can reverse right to the formwork
• A simple rectangular shape without projections
• Complete, checked preparation: sub-base, formwork, mesh and service entries
• Mild weather, with no frost, heatwave or rain

It gets harder with:

• The need for a pump, for a back garden or a sloping site for example
• An irregular shape, thickened edges and many service entries that need hand compaction around them
• A small volume, where the truck runs part-loaded and the supplier adds a small-load charge
• A demanding finish, especially power floating, which means the crew has to wait
• Working in winter or a heatwave, when you need admixtures, covers and longer curing

Greater thickness means more concrete and somewhat more compaction work. On a 200 mm slab with two meshes, checking and placing also take longer. Quotes often list the sub-base, formwork and reinforcement as separate items, so when you compare them, check exactly what each one includes.

Preparation, meaning digging, sub-base, compaction, formwork and mesh, usually takes several days for a garage or small house slab. The pour itself normally takes half a day to a full day, including finishing.

Concrete gains strength gradually. After one to two days you can walk on it carefully. Formwork usually comes off after two to three days. Bricklaying typically starts after about a week, later in cold weather. The concrete normally reaches its design strength after 28 days, so hold off on full loads, such as parking a car in the garage, until then.

Curing is often neglected, yet it decides the quality of the surface. Concrete needs water to harden. If sun and wind dry it too fast, the surface crazes and later turns dusty. Keep the slab moist for at least the first seven days: covered with polythene, under a geotextile that is kept wet, or treated with a curing compound applied as the manufacturer directs. In summer, do not spray cold water onto it in full sun; water it in the morning and evening instead.

Below roughly +5 °C, concrete is poured only with precautions: an accelerating admixture, insulated covers, and never onto frozen ground. In heat above roughly +30 °C it is better to start early in the morning.

Most mistakes are invisible on the day. The slab looks fine after the pour, and the problem appears a year later.

• Poorly compacted sub-base: the slab settles unevenly, cracks run through its full depth and tiles or floor finishes on top break.
• Mesh lying on the stone: the reinforcement sits under the concrete rather than in it. It does nothing and slowly corrodes, so the slab behaves as if it were unreinforced.
• Water added on site: lower strength, more shrinkage, more cracking and a dusty surface.
• Too little vibration: honeycombing along the edges and under the mesh, where damp later creeps in.
• Forgotten service entry: later core drilling or breaking out, which damages the reinforcement and the membrane.
• Uneven surface: with thin-joint blocks the first course has to be packed level, and floors need a thicker screed.
• No curing: a network of fine surface cracks that appears within the first few days.
• No joints in large outdoor areas: patios and yards crack wherever they like. Contraction joints are therefore usually saw-cut within one to two days of the pour.

Leave a house foundation slab to professionals, because it is a structural element designed by calculation. A small 100 to 150 mm slab for a garden shed, log store or bin area is something you can manage yourself, with a helper and ready-mix concrete. Mixing a larger quantity by hand is exhausting and the result tends to be patchy.

1. Set out the area with string lines and pegs, and check the diagonals with a tape measure.
2. Dig out the topsoil to roughly the depth of the slab plus the sub-base.
3. Bring in crushed stone and compact it in layers with a hired plate compactor.
4. Build timber formwork, stake it firmly and set it to the exact level with a spirit level or laser.
5. Lay polythene sheet and place the mesh on spacers.
6. Work out the volume: area times thickness, plus a margin of about 5 to 10 %.
7. Order concrete of a suitable class and arrange access for the truck.
8. Spread the concrete, tamp it or use a poker vibrator, and screed it off along the formwork.
9. Float the surface smooth or give it a light brushed finish.
10. Cover the slab with polythene and keep it moist for at least a week.

Wear gloves, wellingtons and eye protection when handling wet concrete. Cement paste is alkaline and can cause chemical burns after prolonged skin contact.

Look over the slab a day or two after the pour, and again before the bricklayers or floor layers start. Keep all paperwork, as you will need it for building control sign-off and for any complaint.

• Dimensions and diagonals match the drawings
• The top surface matches the datum level
• Flatness checked with a 2 m straightedge, with no significant hollows
• All service entries are in place, not filled with concrete or displaced
• The earth electrode comes out where the electrician needs it
• Edges are free of honeycombing and broken arrises once the formwork is off
• No cracks on the surface wide enough to catch a fingernail
• Delivery tickets from the supplier showing the concrete class and volume have been handed over
• Photos of the mesh and service entries taken before the pour are available
• Curing was agreed and carried out