Thinking of doing concrete benches? Read this first!
I often joke with people that when doing the rammed earth, we would be mixing large amounts just adding ingredients by hand and eye – no real measurements – then when we came to the concrete benches we were literally using drug scales and measuring 20 different products at 0.1g accuracy!! Talk about chalk and cheese.
Side note:
Just in case you were wondering – yes the scales used to be used for drugs. Something else Jesus has redeemed 😂. I was amazed that there was an actual legitimate use for them – and each time we use them I am thankful for how He saved me from so much. Not *just* hell, but drugs too…I better stop before I get carried away. (If you struggle with drugs or addiction click here)
Concrete benches are far more complex than I had imagined – the method, ingredients, setup, sealing and even the weather all adding to the complexity. So I thought I would write a short go to article to help those who have asked for recipes / methods or are keen on doing it themselves…
As some of you may have read, (or seen) we have installed concrete bench tops in the kitchen, bathroom and laundry.
They look amazing, though are not perfect and some have a few issues (like surface cracks).
First I would suggest you decide if you are going to do traditional concrete (which is thick and heavy) or if you are going to go with composite or glass fibre reinforced (GFRC).
Traditional Concrete
Typically 6 – 10cm thick
Sand, Cement, Aggregate, Steel Reinforcing bars,
Mixed in cement mixer, Wet forms, trowels, vibrator
For traditional, you need to look at how strong your cabinets are as there is a *lot* of extra weight. A lot of standard cabinetry is not capable of withstanding the extra weight.
It is generally cast in place as it is very difficult (and weak) to move so the form work needs to be able to handle the wet mix and keep it from seeping on to the cabinets.
If you have large spans or hang overs (for say a breakfast bar) you may not get the strength out of traditional concrete without lots of reinforcing bars or added uprights.
Composite Concrete
Composite concretes can be either cast in place or made in forms and then moved into place and installed.
They can be much thinner (typically 4cm) and can be quite strong and span despite the light weight. It doesn’t use stone aggregate, but uses special glass fibres for strength.
The cost is higher and generally you can’t use standard equipment (ie a standard cement mixer won’t work) – we would say that you will likely spend a minimum of $2000 – $3000 on products, materials & tools to be able to do a decent job. Specialised one-use forms, though not essential can save huge amounts of time but do add to the cost.
Our tools & supply business (BonzaBuy!) can help you save some $ on the tools & some products, though they are not all listed.
SCC / GFRC
There are a few main methods like SCC (Self consolidating) and GFRC. SCC is poured into the moulds, GFRC can be done by spraying, pouring in thin layers or packing with a dry mixes.
Each one has a different process, different set up and requires different tools and products so it is really worthwhile doing your research first and picking which suits you or our project best.
Once you have decided on the type of concrete, you need to choose the manufacturer of the products that you will use. Some manufacturers specialise in wet or sprayed concrete – others in dry ‘pack-in’ mixes.
INGREDIENTS
Only once you have decided on the type of concrete and the process can you start to look at the ingredients.
For consistency, standard grey cement is never used in these types of mixes. Standard sand is never used – its make up and different sizes would be a huge problem in these mixes with unexpected results.
Most specialist companies have some type of ‘all in one’ product that could contain 10 – 20 different additives in the correct proportions for your mix. You then add the correct cement, particular sand, fibres, colouring and maybe a few extra needed elements.
We opted for GFRC that would be sprayed in – which worked very well on pieces that were turned over to be installed.
We used an all in one product, then about 10 other additives that gave different benefits or needed characteristics. Some speed up the drying, others make it take longer, some improve the surface fish, some make it stronger, some reduce the moisture content (to reduce cracking), some fill in the gaps, add strength etc etc etc!
FINE TUNING
We started with small test samples and then went up in size and complexity with tests from planks to shelves to a desk to bathroom bench. Each time we adjusted the recipe to see the results and change accordingly.
This recipe still changes depending on the project, the weather, the temperature of the water to be added, the air temperature, the method it will be installed with and the finish needed.
When using our adjusted mix, for our main benches (which had to be done in place), we got surface cracks that we are yet to try to buff out (assuming we don’t just leave them there!).
Apart from this, we are very happy with the results and the benches are incredibly strong, despite their thin and light make up. The finish is not as good as the benches that we sprayed and then turned upside down.
CONCLUSION
We are still learning and have laid over 30m2 in around 20 different items / projects.
As we gain more experience, we will update with tips and ideas and have days where people can come and learn and practise.
We have already seen that composite mixes are incredible – though that they need experience and knowledge to be done correctly.