Concrete slab floors come in many forms and can be used to provide great thermal comfort and lifestyle advantages. Slabs can be on-ground, suspended, or a mix of both. They can be insulated, both underneath and on the edges. Conventional concrete has high embodied energy. It has been the most common material used in slabs but several new materials are available with dramatically reduced ecological impact.
Some types of concrete slabs may be more suitable to a particular site and climate zone than others.
Slab-on-ground is the most common and has two variants: conventional slabs with deep excavated beams and waffle pod slabs, which sit near ground level and have a grid of expanded polystyrene foam pods as void formers creating a maze of beams in between. Conventional slabs can be insulated beneath the broad floor panels; waffle pods are by definition insulated beneath. Both may benefit from slab edge insulation.
Suspended slabs are formed and poured in situ, with either removable or ‘lost’ non-loadbearing formwork, or permanent formwork which forms part of the reinforcement.
Precast slabs are manufactured off site and craned into place, either in finished form or with an additional thin pour of concrete over the top. They can be made from conventional or post-tensioned reinforced concrete, or from autoclaved aerated concrete (AAC) (see Autoclaved aerated concrete).
Benefits of concrete slabs
‘Thermal mass’ describes the potential of a material to store and re-release thermal energy. It is sometimes referred to as ‘building conditioning’, which is much more effective than air conditioning. Materials with high thermal mass, such as concrete slabs or heavyweight walls, can help regulate indoor comfort by acting like a temperature flywheel: by radiating or absorbing heat, they create a heating or cooling effect on the human body (see Thermal mass).
Thermal mass is useful in most climates, and works particularly well in cool climates and climates with a high day–night temperature range. To be effective, thermal mass must be used in conjunction with good passive design and should also consider the inclusion of high mass walls, as they can provide the benefits of ‘building conditioning’ instead of, or as well as, concrete slab floors (see Design for climate; Passive solar heating; Passive cooling).
In winter, slabs should be designed so they can absorb heat from the sun (or other low energy sources). This heat is stored by the thermal mass and re-radiated for many hours afterwards.
In summer, slabs must be protected from direct sunlight and exposed to cooling night breezes and night sky radiation so that heat collected during the day can dissipate.
A slab-on-ground can be ground coupled (uninsulated) or insulated. An uninsulated slab in a good passively designed house has a surface temperature approximately the same as the stable ground temperature at about 3m depth. Depending upon your location, this may or may not be desirable. Ground coupling in mild climate zones such as Pretoria, Johannesburg or coastal Durban allows the floor slab of a well insulated house to achieve the stable temperature of the earth: cooler in summer, warmer in winter. In winter, added solar gain boosts the surface temperature of the slab to a very comfortable level.
In climates with colder winters, such as Freestate or the colder winters of Northern Cape , the deep ground temperature is too low to allow passive solar heating to be effective enough. In these locations, slabs should be insulated underneath, which reduces the amount of heat required to achieve comfortable temperatures.
Long life — Concrete’s high embodied energy can be offset by its permanence. If reinforcement is correctly designed and placed, and if the concrete is placed and compacted well so there are no voids or porous areas, concrete slabs can have an almost unlimited life span.
To ensure longevity of the slab, control cracking with:
- proper preparation of foundations
- appropriate water content: excess water causes cracking and weakens the slab
- appropriate placing and compaction
- appropriate curing, employing a curing membrane in the first 3–7 days (continuous wetting is a common practice but also consumes large amounts of water)
- appropriate construction scheduling allowing 28 days, or the duration specified by your structural engineer, for the concrete to reach design strength before placing significant loads.
Termite resistance — For minimum termite risk construction, concrete slabs should be designed and constructed in accordance with Australian Standards to have minimal shrinkage cracking. Joints, penetrations and the edge of the slab should be treated.
- Slab edge treatment can be achieved simply by exposing a minimum 100mm of slab edge above the ground or pavers, forming an inspection zone at ground level.
- Where a brick cavity extends below ground, physical barriers must be installed using sheet materials including stainless steel, a termiticide-impregnated polyethylene vapour barrier (tPVC) and/or damp course, a fine stainless steel mesh, or finely graded stone.
- Pipe penetrations through concrete slabs require a physical barrier. Options include sheet materials such as tPVC, stainless steel mesh or graded stone.
- Although physical barriers are environmentally preferable, chemical deterrents are also available, which must be reapplied at regular intervals to maintain efficacy. Benign natural deterrents can be applied by permanent reticulation pipework similar to a drip irrigation system.