Build: Concrete slabs

When building a house, you will require concrete slab work of some kind, either on the ground or in the form of suspended floors.

Ground slab options

Caste slab

  • Once foundation work is complete, soil is put back in area where the slab will sit and compacted to just below the top of the brick plinths (two rows of bricks will contain the concrete mix)
  • High quality building plastic is laid down over the compacted soil to prevent ground moisture (note, if you have a lot of water on site, plastic is not enough and drains will be required)
  • Steel mesh is then laid over this for strength
  • Polystyrene can be laid as an extra insulation material
  • Some councils will insist on pest control spray around platform
  • Any pipe work has to also be completed before the concrete casting can begin

Suspended/floating concrete slabs

If you have a lot of water on your site, a pre-ordered, suspended slab is highly advised. This means your house does not rest on compacted soil, but the engineered concrete arrives on your site in precast panels and is hoisted in with a crane to sit on top of your foundation plinths (with an air gap underneath).

Pre-engineered, suspended slab.

Raft slabs

Raft slab are sometimes advised for unstable soil conditions. They are cast as a continuous foundation, like an upside-down waffle and are more costly.

First floor options

Cast slab

If you are building your own slab with a contractor and structural engineer, you will need to supply a carpenter will all the raw material, props and support work to construct the form work as per design. Otherwise concrete specialists can provide this service.

The following has to be completed inside this formwork:

1.Steel framework (The structural engineer will provide a steel company with a bending schedule for the rebar steel, which, once delivered, has to be constructed as per design and tied up with wire.

2. All electrical and plumbing works in the upstairs level.

3. Any concrete detailing if you want an exposed concrete ceiling below. Also drip lines.

Precast slab

Companies can install precast slabs (also called ‘Rib and block’) or ready-made slabs as detailed above. All conduiting will then have to be completed above or below this, before a top layer of concrete is applied. It can save time and money, but is not as flexible. Also not as attractive as an exposed ceiling.

Mezzanine

A suspended floor can by created using steel or timber beams and a timber floor. Cheaper option, but very little noise insulation and less weight bearing capacity.

Final stage

  • The appointed structural engineer and NHBRC have to inspect all slab work before the final concrete arrives on site, including precast slabs that still require a layer of fresh concrete on top.
  • Concrete trucks will have to caste slabs in one go. Best to order them first thing in the morning as they can run very late for this reason.
  • Use plastic and water to keep concrete from setting too quickly. The longer it takes, the stronger it gets. It also prevents cracks from forming.
  • Concrete drivers are obliged to dump any leftover concrete before they return to their depot. Work out quantities carefully or prepare any extra area to use left-over concrete.

Polished concrete floors

Although any cured concrete can be polished, there is now pre-mixed concrete available that includes marble chips and extra fibre to reduce cracking supplied by Metier and LaFarge.

If you want a good polish finish, it’s essential to vibrate and float (‘bull’ or ‘power’) the concrete shortly after casting. This means getting in casting specialists that smooth out the wet concrete. This is mainly to avoid bubbles, ridges or patterns (the usual method of spreading involves patting concrete down with rakes or wooden planks, which leaves a rough service and makes polishing much harder).

Note that all floating methods have to be done straight after the pour and can take hours. Order your concrete delivery for first thing in the morning.

Bull floating is done manually with wooden floats. Power floating (using machines) can take the longest and has to be done continuously till set. To be avoided if you have neighbours near by as it can go on till the early hours of the morning and is very noisy.

Wetting and covering concrete is even more crucial for a good finish. Have plastic sheets and water hoses on site.

Polishing contractors will ‘cut’ (rough polish) the set slab a few weeks later, followed by the polishing just before the house is complete (to avoid damage during construction). Depending on your budget, the polishing can be between level 3 -7, seven being a super smooth, mirror polished look. For a tighter budget, level 4/5 is fine.

This is a very messy process and must be done before joinery or painting starts. Also ensure any residue is cleaned up while wet. Any set concrete sludge becomes very hard to clean after.

For more about the superstructure, read here.

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