Owner Building a Strawbale House
in the Bega Valley,
NSW,
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A Summary of Polished Concrete Finishing

We have looked at how to attain a polished concrete floor in the house – with the theory that it is: a good passive solar absorber; easy to clean; hard wearing; presentable; and good for asthmatics.

We already assume that we are having an “integral raft slab on grade”, so polishing the surface sounded like a cheap and interesting floor finish.

It doesn't quite work like that, there are lots of gotcha's and hidden costs.

Methods for a laying a polished concrete floor:

  1. You can pour your slab, and polish it directly

    • You could have added colour to the entire concrete slab

    • You could have added colour only to the top layer of the slab

    • You can add interesting aggregate to the entire concrete slab

    • You can add interesting aggregate only to the top layer of the slab

  2. You can pour your slab and later add a layer (25 to 40mm) of extra concrete

    • This can have colour through it

    • This can have interesting aggregate through it

If you polish your concrete slab it really needs to be crack free (most concrete slabs do crack, and this is generally cosmetic not structural). To get your slab crack free you need to have about twice as much reo (extra thick reo, instead of F72 you might need F82 or stronger), and you need to have very stiff concrete (slump test less than 40mm) – this needs to be specified to the readymix supplier and your concreter needs to work harder (i.e.: more hours, therefore more cost) to lay the stiffer mixture.

The local concrete ready-mix place gave us a figure of about $10 per cubic metre more for 32MPA (harder finish) concrete than 20MPA concrete. Normally you'd use 32MPA if you were using the slab as part of your termite barrier (as we are).

If you add colour to your entire concrete slab it adds a lot of cost (some 10% of the concrete mix is colouring); plus the colour detracts from the strength of the concrete, so you need to get a stronger, more expensive base concrete mix (see above). The cost from our local place was an about an extra $80 to $90 per cubic metre for concrete with a colour added. If you wanted to specify a particular colour aggregate you are limited by what is available in your area and costs vary. In other words start adding more dollars per cubic metre.

If you add colour to the top layer (such as the “dry shake” method, of sprinkling the colour on top of the concrete and trowelling it in) you run the risk of polishing through it to the uncoloured concrete below (but you save on cost with the colouring agent).

If you add interesting aggregate to the entire slab you need to be careful of the strength of the slab, plus how well the aggregatemixes with the rest of the concrete (so that it doesn't sink, etc.) – and cost becomes a factor.

If you add interesting aggregate to the top layer you save on material cost, and you can sprinkle it on and trowel it in reasonably easily.

If you pour your slab and add an extra layer later (a) you should add this layer straight after pouring the slab, while the concrete is still in a “plastic” stage – so that the two layers bond together (b) you are up for extra cost! The layer needs to be 25 to 40mm thick (depending upon the size of the interesting aggregate you want), and this will cost roughly ¼ of the slab cost, plus the colour and interesting aggregate.

Methods for polishing the floor

Pay someone to do it

We got a quote to have the floor professionally polished. It was about $65/m2 (or around $13,000 for our 200m2). This doesn't include the costs for extra reo, colour, aggregate, laying costs etc.

Buy the equipment and sell it afterwards

We got a quote to buy a polisher new for around $8,000 ($13,000 with a nice vacuum attachment to reduce the mess). It would do an excellent job on the polishing but would take a long time on the initial grinding back. Of course, the $8,000 doesn't include the consumable grinding/polishing disks, so add another $3,000.

Hire the equipment and do it ourselves

Kennards Concrete – Big Boys Concrete Toys! 24X7 hire location in Sydney. They are used to their equipment being freighted around. They supplied a lot of useful info. If we were closer we'd go and have a look at the concrete in their yard that has apparently been polished and “looks pretty shmick”.

They suggested hiring a grinder first as it would get through the rough cutting faster and is cheaper to hire. Then hire the polishing unit to get the final finish.

Our costs

On the house, carport, and dual-bed pavilion (about 200 m2) we would need the following:


ITEM

days

Extra dollars

1

Lay slab with heavier reo and a stiffer mix (to get the least number of cracks possible)

0

2000

2

add a 25mm layer on top of coloured concrete with large chunks of interesting aggregate

2

2000

3

grind the concrete back some 4mm with a dual head grinder (hire)

7

2000

4

polish the result to 1500 grit with a triple head planetary polisher (hire)

7

3000


TOTAL

16

9000

This amounts to about $45/m2.

The costs shown above in items 3 & 4 assume we hire the equipment and do the polishing ourselves.

Issues

  1. There are some logistics problems with laying the extra 25mm coloured layer. How would we get the right amount delivered to cover just the area that had just been put down? There will be multiple concrete trucks for the house slab and to get the coloured on at the plastic stage means multiple small deliveries of coloured concrete or a lot of physical work hand mixing the coloured concrete on-site and keeping up with the main truck deliveries. It would be easier to have the whole slab coloured and that adds up to about an extra $1,000 on the costs worked out above.

  2. There is no way for us to know exactly what our finished product will look like. We're unlikely to be able to get the aggregate & oxide proportions into a test piece we can polish and seal to see what the final look will be. Of course, we have a fair idea having seen plenty of pictures and other floors.

  3. The physical job of doing the polishing ourselves is likely not to be enjoyable – lots of vibration, noise and mess.

The alternative of just putting down tiles is looking better every day...