Concrete basics for building: Concrete floors for domestic applications

We expect concrete floors to last for many years, despite heavy loads and hard wear.

Pay special attention to foundation material, the thickness of the slab and the strength of the mix, and provide joints to allow for contraction. And cure the slab well. This, as well as proper batching, mixing, transporting, placing and curing will ensure that high quality is built in at the outset and that future trouble and costly maintenance will be avoided.

1. Foundations

The performance of a concrete floor on the ground depends largely on the underlying material providing uniform support.

Provide a carefully compacted 100 – 150 mm thick layer of suitable fill material immediately below the concrete. Avoid clayey materials.

Finish the surface of this foundation layer smoothly, true to level.

1.1 Dampproofing

Where dampproofing is necessary, cover the whole surface of the foundation with polyethylene (plastic) sheeting, at least 0,25 mm thick. Where strips join, overlap them by about 200 mm. Turn the plastic sheeting up around the edge of the floor slab by at least the thickness of the slab. Where possible build the excess plastic into adjacent brickwork.

The plastic prevents the concrete from absorbing moisture from the ground below, which can penetrate normal concretes and may:

• Lift or damage the floor covering.
• Damage goods stored directly on the floor.

Immediately before placing the slab, carefully and thoroughly dampen any foundation or abutting part of the building not covered with plastic sheeting to prevent absorption of moisture from the concrete.

The floor slab

Concrete floor slabs for domestic and other light duties should be at least 100 mm thick, cast with medium strength concrete.

Lay the floor in panels not more than 3 m wide and long. Square panels are preferable, but where rectangular panels are required, the ratio of length to width should not exceed 1,25.

See Concrete basics for building: Concrete for making, placing and curing concrete.

Don’t add more water to the mix than that necessary to compact the concrete fully with the available equipment, and pay special attention to compacting the concrete close to the panel edges. A thin layer of mortar will appear on the surface when the concrete has been fully compacted.

Start damp curing the surface and any exposed edges as soon as possible, and keep the slab covered for at least seven days.

Except in special circumstances, don’t add a screed or topping. Well-proportioned concrete can be trowelled to almost any desired degree of smoothness and, if the delayed trowelling technique is used, the resulting surface will be hard and strong enough for most applications in houses and outbuildings.

Finishing: delayed trowelling

Delayed trowelling will result in a hardwearing, smooth finish.

After wood-floating the concrete surface in the normal manner as rapidly as possible after compaction, leave the slab undisturbed until bleeding has ceased, the bleedwater has evaporated (or has been removed) and the concrete is starting to stiffen ( a footprint should barely show). This typically takes from 2 to 4 hours – plan ahead!

Then (and only then!), use a steel trowel to give the surface the desired degree of smoothness – a number of trowellings may be needed. Heavy pressure is required on the trowel – if possible, use power-operated equipment.

Remember: correct timing of the trowelling is of prime importance, and it is essential that no bleedwater is trowelled back into the surface!

2.2 Non-slip finish

If areas on the slab or stair-treads need a non-slip surface, finishing should be with a wood float rather than a steel trowel.

3. Screeds and toppings

Most concrete floors don’t need a sand-cement screed or concrete topping if the top surface is well finished without over-trowelling. A common cause of cracking is poor bond between topping and base course, so it is generally better to provide a full-depth concrete slab.

However, when a screed is necessary for floors to be covered with carpet, plastic tiles or linoleum, or a concrete topping is required for a wearing surface in store rooms or garages, use the following guidelines.

3.1 Preparation

Successful screeds and concrete toppings depend on proper preparation of the underlying floor surface.

The most commonly used method is for the screed or topping to be laid as a separate operation and bonded to the hardened concrete.

• Chip the surface lightly with sharp tools (i.e. scrabble or hack) to remove the whole of the surface skin and expose clean hard concrete.
• Remove all debris and use an industrial vacuum cleaner to remove all dust (the cost of hiring this equipment is well worth it for the higher strength and greater reliability of the bond that can be obtained).
• The day before laying the screed or topping, test the concrete for absorptiveness by pouring a cupful of water onto the surface. When the water is absorbed within a few minutes, the surface suction is good, and the surface should be wetted and kept wet for four hours (remove all free water before grouting).

If the test water is not visibly absorbed by the concrete during the first few minutes, suction is low, so apply the grout to the dry concrete.

• Make up grout by adding about half a litre of water per kg of cement (or 1½ litres of loose cement to one litre of water), and mix well. Stir continuously and use within half-an-hour.
• Scrub the grout into the surface with a brush (try a stiff carpet brush). Scrub the grout into all surface irregularities, then brush it off to form a thin coating – there must be no pools of grout in depressions.
• Lay the screed or topping 25 to 30 mm thick in 3 m square panels on the visibly wet grout, i.e. within 10 to 20 minutes of applying the grout.

3.2 Mix proportions

• Sand-cement screeds: 130 ? of concreting sand per 50 kg bag of cement and enough water to give a plastic, workable consistence.
• Concrete toppings: as given for medium strength concrete, for applications where abrasion is not a consideration.

3.3 Finishing

To ensure a hard and durable surface, use the delayed trowelling technique described in section 2.1.

Source: Cement & Concrete Institute. Photographs: The Family Handy Man and Natal Park Homes.