Cast-In-Place of Cast Insitu concrete is literally just that. Cast in the place that its poured. So in essence you build a mould to take your concrete, most commonly a mould sat on top of your cabinets or block walls or timber frame, and cast the concrete into it.
But as with everything the devil is in the details and in this article I will try to demystify a Cast-In-Place project!
So to start it’s probably worth talking a bit about concrete in general, how it cures and how it relates to a Cast Insitu project.
What is Concrete?
According to Dictionary.com Concrete is defined as;
“an artificial, stonelike material used for various structural purposes, made by mixing cement and various aggregates, as sand, pebbles, gravel, or shale, with water and allowing the mixture to harden.”
This is so brief it barely touches on the detail. “Harden” in this sense actually means “chemical cure”. In a standard concrete mix, when the cement mixes with the water, it kicks off a chemical reaction that lasts 50 years! That’s correct. 50 YEARS!
Admittedly most of the work is done in the first 30 days by which time most of the “hardening” is done and then it slows to a snail pace.
It’s worth having a look at the graph below. Its a bit nerdy but tells the story!
With a Cast-In-Place project the longer you can keep your concrete covered over to keep the moisture in the stronger it will become. But and its a big BUT, if it gets too strong then any finessing becomes more difficult. I’ll discuss that in a bit!
So back to Cast-In-Place and a few more Q & A’s.
We’ve discussed general concrete and how it hardens. We know broadly what cast-in-place or cast insitu means. But how does this information feed into a countertop project?
Countertop or Worktop Cast-In-Place Concrete
The concrete mix for a bridge or road is different to a worktop. Concrete for a bridge is full of large aggregates and is vast in terms of volume and effectively has its own reservoir of water which it can use to help the cement react. For a worktop the mix has much smaller aggregates and in our case we’ve swapped them out for glass fibres so the concrete can be made much thinner without the need for extra reinforcement. Being thinner though means much less water by volume which means that it has to be managed more effectively to get your concrete good and strong.
Now the relevance of the strength and curing is that once your concrete is poured you will leave it 4 or 5 hours or so to reach a strength that you can smooth it off with a trowel. During this time the concrete will be air-drying so timing is key.
Remember the boring graph above? Look at the air cured concrete with the dotted lines. It’s weaker than moist-cured concrete.
So once the concrete has been troweled lovely and smooth and its hard enough … you need to tent plastic sheets over the top of it to keep as much moisture in as possible.
The following day you’ll uncover your concrete and give it a sand with an orbital sander and wet-dry sanding discs to get that final polish whilst wetting it down with more water, thereby reintroducing more water into the surface.
By the way, we recommend these discs - https://amzn.to/4d38lvK - which fit on to a standard orbital sander. No dust extraction is required because you’re going to be wetting the concrete down which will suppress any dust.
The concrete mix design is all important in the early stages and that is why our mix design has a curing agent which hold the moisture in the mix to prevent too much early drying BUT this doesn’t kick in for about 18 hours so again those early stages are key.
Finally after sanding/polishing a water-based sealer can be applied which does two things. It seals the concrete but also reintroduces water which will further assist curing.
I know. I’ve gone really technical here but those of you who like a bit of detail will hopefully appreciate it?
Take a look at this video here which captures the start-to-finish of a Cast-In-Place project for an outdoor kitchen.
So this article may have prompted a few more questions such as how do I build my mould and what do I build my mould out of? Well here are some resources for that;
Another question - what materials and how much do I need? Use this calculator here;
Now as ever if you have any more questions you can always get in touch with us directly by Chat, Email or Phone and don’t be shy. After all there’s no such thing as a stupid question.
Bye for now!