Case Study 1 – Earthworks and Slab

EARTHWORKS AND SLAB STAGE

In this stage of our build, the home has reached the slab down stage.

Earthworks, Drains (stormwater & sewer connection), and Repeg & Set out have been completed.

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This stage of proceedings is very important. For the first time you can start to get an idea of the positioning of the home on your property, and it’s dimensions.

If the slab looks smaller than you expected don’t worry, it is a common feeling and you will start to get a better feeling for the size of the home once the brickwork starts to take shape.

There are a couple of things to look for when the concrete slab goes down:

  1. Set out dimensions. Your supervisor should check the setbacks to the boundaries. Sounds intuitive but so many leave it to the concretor these days. It is important to check, because if there is a problem, better to know it now rather than when the brick paver picks it up when he is trying to pave down the side of the home. Any problems are easier to fix now.
  2. Plumbing pre-lay (see the white PVC sticking up out of the slab in the pictures above) should be in place and accurate for your wet areas.
  3. The wet areas should be sloping towards the wastes. This is important because if they don’t slope correctly you are going to get pooling water in your bathrooms after the tiling is finished. Your shower areas should also be dropped to allow for screeding an additional slope in your showers. The high water volume in these areas mean it is important so as to contain the water within the recesses (see front of picture 2 above).
  4. There should be galvanised steel rods in place for any piers (see in the rear of the photos). If you don’t have these then the roof isn’t going to meet the Australian Standards for tie downs.
  5. The earthworks should be complete, and an engineer have signed off on the compaction of the site. You should be able to ask your builder for a copy of the compaction certificate.
  6. Finally the finish on the slab should be even and relatively unblemished. Good concretors can give you a firm flat surface that is not too chalky in finish.

You may also hear some people talking about the curing of your slab. Curing is the process where the cement in the slab gradually breaks down into its’ chemical components and strengthens the final product.

Curing is a relatively slow process (in building terms). It is commonly thought that the slab will reach what is effectively its’ final strength at around 4 weeks, however it will reach 90% of its’ finished strength after just 7 days.

Transition across the slab (i.e. to remove footing sand) can take place after about 4 days, however I wouldn’t load a slab (put bricks on it) until after that initial week. Any earlier runs the risk of damaging the slabs surface.

In winter the slab can be left to its own devices to cure, since it wont dry out too quickly (drying out too quickly reduces the overall final strength of the slab since the water content is too low for all the cement to react). In summer, an inhibitor should be added to the concrete mix which slows the curing process and reduces the rate the slab dries out.