I’ve done several cast concrete projects over the years, so I understand and am aware of most pitfalls and techniques. However, for a current countertop project, I will be grinding the surface for the first time. I’ve now done lots of samples (for color selection) and am having trouble vibrating the form sufficienty to have a nearly void free surface. I’m casting the tops inverted.
I’ve found that it is easy to get the air voids to rise just off the surface of the inverted form, but they become exposed with grinding. I’m vibrating with a pnumatic hammer pounding the form, an old vibrating sander mushed into the wet mud, and even tried using a Fein tool with an attachment I made.
How have others solved this? I sure appreciate any guidance.
Jeff
Replies
re - and even tried using a Fein tool with an attachment I made.
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I don't have any suggestions....
Just wanted to ask if your attachment copies Fein's accessory that I see listed for this purpose.
Also, the FH concrete countertop book seems to show a Makita made specifically for this purpose. I don't know where my copy is at the moment to check...
Hey Jeff, Although I haven't done my first countertop yet, I was able to take a workshop from Cheng design studios in Berkely. It was discussed that getting out all the voids is very difficult, if not impossible, so we were shown that after the grinding process, you simply make a slurry mix, and using I think it was a plastic blade, scraping it over the top of the counter. Really easy, and in some cases, they talked about using a different color even which gave a great look. Don't know if it helps....getting ready to do my first complete counter in a few weeks. Good luck.
Thanks for the interest and responses. My attachment to the Fein tool is simply a bent length of 1/8 X 1" flat steel. I made it long enough to keep the tool out of the mud but not so large that it's mass would hog down the motor. It works great to vibrate in small areas but lacks the poop to do much for the greater mass.
I know that removing all voids is unrealistic, but am hoping for something better than what I've been getting. I've been filling the voids with the slurry, but I simply want to try for better. I don't like the look of contrasting colors in the voids, although some might. In my last sample, I had some large voids and filled those with a sand/slurry mix. This is a vast improvement in the look of things as far as I'm concerned.
If others have any more suggestions, I'd certainly like to hear about them. If anyone has any questions that I might answer, please ask.
Jeff
I wouldn't say that I've 'solved' it... but two things I've done have helped a lot:
1. I use a water reducer/super plasticizer in my mix
2. a sawzall sans blade to vibrate the forms works pretty well
Other than that, I think you've just got to fill the voids.
When I cast plastic, I use a vacuum pump to remove the bubbles. I should think it'd work with concrete, although I've never tried it.
re - I use a vacuum pump to remove the bubbles.
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Jamie -
I'm trying to picture this setup -
Would you mind giving a bit more detail ?
Thanks !
As for others mentioning filling the void with slurry mix, I like that idea and see many possibilities.
Thank-you for mentioning this.
Just did my first countertop. Same method. My vibrating was not as good as yours, had some air pockets especially in one section. My vibrator person, my wife, was not really into the job. The slurry mentioned earlier, I used portland cement, dye and water. Worked well.
Hi, this is Viliam's wife.
We did concrete counter for our kitchen 6 years ago.
We mixed in polymer grout stuff into the cement mix.
We vibrated only minimally and had no probs.
We made our slurry from cement, silica sand and instead of water used that liquid glue stuff for cement (you know the stuff you paint on before you pour new concrete to stick to old concrete).
We also made the counter in place, so no moving. The counter was u-shaped.
I did baby the counter for 3 weeks. Every day , twice a day, I put fresh wet compresses on it to slow curing as much as poss. We only got 1 crack and that was near the sink area where the concrete was just 3' wide.
Good Luck!
I love concrete and always like to hear what people do with it.
The anarchist Gardener/Renovator