Concrete countertop - why not add waterproofing additive

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  #1  
Old 05-15-18, 01:06 AM
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Concrete countertop - why not add waterproofing additive

Hi folks,

I am embarking on a concrete counter top project.

The internet says that the main disadvantage of concrete counter tops is that they stain easily and need to be sealed and re-sealed regularly. I suppose this is because of the porosity of concrete.

I'm wondering why no one adds a waterproofing agent to the mix? This is readily available here in France along with your plasticizers etc. and commonly used in swimming pools etc. So why couldn't you add some of this to your concrete counter top mix to improve resistance to stains?

Since I haven't come across this idea anywhere, I'm assuming there is a glaringly simple reason for it?

Another couple of specific questions:
- How much plastisizer should I add? Normally products come with a dosage range - in my case 0.2 to 1% of the cement mass. I am assuming I should shoot for the upper end of the range?

- Do you wait for the full cure time to begin sanding / polishing? What is the optimum time to start this and what should I look out for, as I guess it all depends on mix and temperature etc.
 
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Old 05-15-18, 05:17 AM
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Concrete requires water to cure. So, adding a waterproofing agent to the mix doesn't work so well. You need thorough wetting of the concrete for it to cure and it really needs to remain wet or at least damp for almost a month for it to come up to close to full strength.

Unfortunately concrete is not what I would call the best for counters. It is the current fad but it does have it's drawbacks. One of which is the need to keep it sealed.

You can begin sanding and polishing almost immediately after you remove the forms. Be mindful that concrete cures very slowly. Even though it appears solid the general rule of thumb is four weeks (28 days) for it to be effectively fully cured though it continues to strengthen after that.

If you start finishing early the cement part of the mix is much easier to grind and sand as it is still relatively soft. The problem is that the aggregate (rocks and sand) in the mix are quite hard so it's easy to have high spots where rocks are as you grind/sand down the cement in between. Waiting longer allows the concrete to harden more so the aggregate and cement are a more similar hardness so it's a bit easier to get a consistent and straight/smooth surface though the grinding is more difficult.
 
  #3  
Old 05-15-18, 01:49 PM
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This would seem to make sense - but they do sell additives to make concrete less porous - that is used in swimming pools and even marine construction - so it is possible, while permitting curing also - so why isn't this used in counter tops I wonder?
 
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Old 05-15-18, 11:23 PM
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The technical term seems to be "permeability-reducing admixtures" or PRAs.

See more here: http://www.concreteconstruction.net/...for-concrete_o

So why has no one tried these for counter tops?
 
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