Concrete is strong, durable, fire-resistant and good at storing heat.

It's also a relatively economical and well understood building material. 

Key features of concrete construction

Concrete is made from Portland cement mixed with sand and gravel (aggregates) and water. Cement is made by heating limestone and clay, then grinding it with gypsum. The powder, when mixed with water and cured over several weeks, turns into a hard solid.

Cost

Concrete is relatively cheap to use. However, transport can be costly.

Concrete masonry walls are usually used for exterior and load-bearing walls only.

Concrete slabs on the ground are cheaper than timber floors, and are now standard practice in many homes.  A damp-proofing barrier is necessary.

Strength

In general, concrete is strong in compression but needs reinforcing steel to make it strong in tension.

Ease of construction

Concrete is familiar to the majority of building professionals.

Durability and weathertightness

Concrete can be very durable. There are many buildings overseas made from concrete that survive from Roman times.

Though concrete will allow moisture to penetrate, it can be made water resistant with the right additives and sealers.

Earthquake-resistance

Concrete homes can be made earthquake resistant if they are correctly designed, and have sufficient reinforcing steel in the right places. 

Fire resistance

Concrete is fire resistant.

Pest resistance

Concrete is vermin proof.

Is it compatible with passive design?

Concrete is good at absorbing and retaining heat during the day, and radiating it out at night when temperatures fall. On summer days, it can absorb heat from the surrounding air and keeps your home cooler (see Using thermal mass for heating and cooling).

However, concrete is not a good insulator. To keep this heat from escaping into the colder surrounding ground, a concrete slab floor should be insulated both underneath and around the edges of the slab. Exterior concrete walls, used as  thermal mass, should be insulated on the exterior only.

With concrete construction, doors and windows can be placed where needed to make maximum use of the sun’s warmth and breezes for cooling.

Concrete is a good sound insulator. However, without carpet, sounds inside your home may echo.

Is it right for my site?

Topography and site impact

Concrete floor slabs are generally used on level sites. However, a suspended concrete floor (see concrete slabs below) can be used to minimise the excavation required on a sloping site.

Climate

Concrete homes are used in all climate conditions.

Types of concrete construction

Concrete slabs

Concrete slab construction is now the most common way to lay a ground floor on flat ground.

Thermal mass

In winter, a slab floor can absorb solar heat and then radiate it out during the evening to keep your home warm. It should be insulated underneath and around the edges to stop heat escaping into the ground.

Concrete takes at least four months to dry out – longer in winter – and it will not perform well as a thermal mass until it is dry.

If you’re using concrete as a source of warmth, don’t cover it with carpet where the sun shines on it - this stops the slab from warming up. However, not having a floor covering can also make the concrete floor less forgiving if you trip or fall.  Consider partially covering the slab - for example, with exposed concrete or tiles around the perimeter and carpet or mats in the centre.

Reinforcing

All concrete floor slabs on ‘good ground’ are required to have reinforcing steel mesh and all perimeter foundations are required to be tied to the concrete slab with reinforcing steel; you can’t use unreinforced slabs anywhere in New Zealand.

The purpose of the mesh is to stop or limit the spread of a crack if it opens up. Without mesh, a crack in an unreinforced slab is likely to widen and spread across the whole slab. Tying the perimeter foundation to the slab reinforcement limits movement and damage from earthquake shaking.  Large slabs are designed to have some movement joints.

Heating systems

Slabs can have water pipes or electrical cables embedded in them for heating and cooling. If anything goes wrong with these embedded systems they are very hard to fix.

Insulation

In most climate conditions a concrete slab will need to be insulated around the edge, and underneath.

Waffle pod concrete flooring incorporates polystyrene pods to reduce amount of concrete required.  However, this insulation is not continuous under the slab, and does not extend up the edges – this leaves more gaps for heat loss and makes it less suitable to act as a thermal mass.

Suspended floors

Commercial buildings usually have reinforced concrete slab floors on all levels. These slabs can be pre-cast to speed on-site construction. This can also be done in domestic construction but will need to be insulated on the underside.

Concrete blocks

Walls can be made from hollow concrete blocks which are laid like bricks. These blocks are also called concrete masonry units (CMUs).

Reinforcing

Block walls are reinforced with steel during construction. After the reinforcing is put in place, concrete is poured into the hollows void.

Insulation

Standard concrete blocks don't have enough insulation to meet the Building Code's minimum requirements. Insulation can be added to the outside or inside of the wall.

Design and installation

To avoid cutting blocks to size it is preferable to design buildings to fit standard block sizes.

Pallet-loads of blocks are heavy, and are likely to need trucks with cranes, or tractors  to move them. This may not be possible in some locations.

Block laying should be done by a skilled block layer.

Curing concrete

All fresh concrete should be cured for the first week to slow down the drying process so cracks do not appear. This can be achieved using a fine water spray, laying a polythene sheet or wet hessian over the top, or using a chemical curing compound. If water is used, sand can be piled up around the edges of the slab to retain a wet surface layer.