Easy access: grinding the edge of the slab

Polishing the Slab – part 1

Andrew Kerrby author page

2 comments Construction

Monday July 27 / Tuesday July 28

How would you like your concrete served?

Concrete is available in a variety of finishes. You can choose to select different colour and size aggregate (bits of stone) in the concrete mix if you are going to grind a concrete slab – it all depends on the effect you would like to see. It was a late decision to grind the slab so I didn’t specify any particular aggregate. I thought the cost of a polished slab would be prohibitive, but when you consider the cost of the alternatives (floor coverings such as carpet, tiles or overlay flooring) and the benefits that a polished slab offers (exposed thermal mass, easy to clean) it made sense to.

 

Dean fixing the discs to the grinder

Dean fixing the discs to the grinder

Preparation

It’s best to grind a slab around 3-4 weeks after it has been poured so it’s not too soft and not too hard. It had been 5 weeks since we poured the slab of 60k House. Stuart the concrete finisher had done a really good job with the slab – the concrete was dense and compact, with no undulations that would make it difficult for the grinder.

 

Slow going: working around the slab

Slow going: working around the slab

Add power

A concrete grinder needs power, a lot of power. We still didn’t have power connected to the site so I borrowed some extension leads (around 100m) and ran them from the neighbour’s shed. We knew there might be ‘voltage drop’ (a loss of power over such a big distance) but the power-hungry grinder worked! For 2 minutes, before the warning light came on indicating it wasn’t getting enough reliable power. I called a plant hire company and set off up the road to hire a generator.

 

The hire generator (power supply) for the concrete grinder

The hire generator (power supply) for the concrete grinder

Some water

It’s best to either grind a concrete slab when it is wet or it is dry. We didn’t have water at the building site (Flowerpot doesn’t have town water) so I used an old home-brew keg to collect water from the dam at the bottom of the block.

 

Collecting water from the dam for the concrete grinder

Collecting water from the dam for the concrete grinder

Grind slowly

Grinding a slab is slow going. Dean hooked up the earphones, put the earmuffs over the top and set to work. It took around 10 hours over 2 days to grind 50 square metres. This is only half of the job finished, done now before the walls go up and the roof goes on, making it easier to get into the corners. Once the building is lined and nearly complete it will take another couple of days to add densifier to the slab (to make it even harder), polish it some more before applying a sealant to finish it off.

 

Scrape it off: removing the paste made from the water and concrete dust

Scrape it off: removing the paste made from the water and concrete dust

 

Getting there: revealing the concrete finish beneath the sludge

Getting there: revealing the concrete finish beneath the sludge

Serve

Because concrete is a natural product you never know exactly how it’s going to turn out. The finish achieved here is described as the ‘salt and pepper’ look and is apparently all the rage on the mainland. I’m not interested in trends or fads – I just think it looks good.  Most importantly the slab  will be durable and work well as thermal mass.

 

Looks more impressive when wet

Looks more impressive when wet

 

The (nearly) finished product: the salt and pepper look

The (nearly) finished product: the salt and pepper look

 

Costs: grinding – $1,500 ($3,000 total)

 

Disclaimer: Any advice contained within this blog is of a general nature only and cannot be relied upon. Details provided are in good faith and relate specifically to this project. Any author will not be held responsible for advice or information presented.

2 Comments

    1. Andrew Kerr

      If you mean using geofoam instead of gravel and FCR for the fill then no, not really – it was only a couple of truck loads of fill. The thickness of the concrete slab would still need to be the same (110mm to allow for grinding). There is sub-floor insulation to improve thermal performance.

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