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RichC 09-19-2013 08:10 AM

definitive guide to concrete countertop?
 
Hi Guys, my dome is in, covered with fibreblanket and about 75~100mm of vermicrete. My next step is to cast a concrete countertop in situ, after which I'll render the dome. The intention is that the countertop slopes gently away from the dome and the plastered finish on the dome sits on the countertop so water is naturally carried away. My intention is to complete the countertop as follows(inspired by Amacs concrete countertop).

1. Cast the countertop in situ using marine ply or something like that with an overhang, about 50mm probably.
2. Theres a product I havent seen and I believe it some kind of fibre, like hair, that can be used to reinforce the counter. Is this a better product than steel to use as the counter might be a bit thin for steel? Whats this product called?
3. Make a white mix of mortar, white cement, and can I buy white gravel or should I just use a white sand/cement mix? have never seen white gravel.
4. Should my mix be fairly dry as I see a lot of instructional vides that seem to use a 'very' dry nearly putty like mix? I would have preferred a wetter mix to ease getting air bubbles out?
5. Pour the mix into the shutter until nearly full then wait for it to start to go off, a few hours maybe. Make another mix with broken glass in it and fill to top of shutter. Is this the correct technique or should I have glass right through the mix? would too much glass weaken it structurally?
6. When cured, grind surface back with progressively finer pads on a 9" angle grinder. Do I really need a variable speed grinder?

I'd like to get a good shiny finish but I dont expect it to end up like granite kitchen countertops.
Any comments on my technique appreciated?

Thanks

Richard

rsandler 09-19-2013 08:38 AM

Re: definitive guide to concrete countertop?
 
A couple of thoughts:

1. Melamine coated particle board is the standard for concrete countertops--it is very smooth and gives a nice surface to start polishing
2. Often stainless steel mesh is used in place of rebar in countertops. In addition to metal reinforcement, it is a good idea to add nylon reinforcing fibers.
3. You can do concrete counters with or without gravel. If you include gravel, you will likely see some exposed aggregate after polishing. Just depends on what you want.
4. I recall from my research that a sand-cement mix can be more putty-like. I haven't tried it.
5. Much easier to just fill the form to the top, sprinkle glass on top and float it in. The counters in my build were done this way.
6. You can even use a cheap 5" orbital sander, plus run a hose over the surface.

FYI, if you really want your surface nice and smooth, you'll get better results casting "face down".

RichC 09-19-2013 09:09 AM

Re: definitive guide to concrete countertop?
 
Thanks RSandler,
I intend doing my counter the whole way around the oven so it makes sense(to me anyway:)) to cast in situ as the alternative would be to cast in sections and then get them to try and fit around the dome.
Is the mesh really necessary when using the nylon fibres? Will it actually add strength? If I remember correctly when reinforcing a slab you'd ususally have a min of 50mm cover from the steel to the edge of the shutter. In this case thats impossible becuase the slab is no more than 50mm thick.
Will the countertop be stronger with or without gravel? I would have assumed 'with' but I'm not sure?
I like the idea of using an orbital sander!

Thanks

Richard

deejayoh 09-19-2013 09:22 AM

Re: definitive guide to concrete countertop?
 
on #2, there are fibers that you can add to the mix to prevent cracks - but you still need to use rebar, grid, or fiber mesh. I used High Zirconia Alkali-Resistant Fiberglass Net in mine instead of metal (in addition to the fibers) and have had good results. Go to directcolors dot com and you will find both the mesh and the fibers

#4, a wetter mix is not as strong. You should have to push the mix into the form. Use something to vibrate the bubbles out as much as you can - and then fill with slurry. It works surprisingly well. Bug holes are really not an issue to fill.

#5, Glass goes on top - but be careful - there is an issue where the glass can react with the concrete and cause the concrete to degrade. If you want glass in the mix, I would personally source glass that has been treated to prevent this.

#6, most pads are not rated to run at the 7-10k rpm that a grinder runs at. You can get a router speed controller at HF or amazon for <$20 (ok, this is US) that will allow you to reduce the speed. Edit: and a 9" grinder is probably too big. A smaller grinder works fine, and the pads are much cheaper. I'm not sure about an orbital sander - but I used a $15 chinese made angle grinder with the speed controller and it worked great.

My experience was opposite rsandler's - I did both cast in place and form cast, and found that the cast in place section had better surface because of the troweling than the form cast section. In the end they both turned out fine, but I definitely had more bug holes to fill in the cast section.

UtahBeehiver 09-19-2013 03:36 PM

Re: definitive guide to concrete countertop?
 
There is a link on Lburou's Forno Bravo Treasure Archives in the Newbie Section that has hyperlinks to several builds with various concrete counter projects

RichC 09-19-2013 05:22 PM

Re: definitive guide to concrete countertop?
 
Quote:

Originally Posted by UtahBeehiver (Post 162145)
There is a link on Lburou's Forno Bravo Treasure Archives in the Newbie Section that has hyperlinks to several builds with various concrete counter projects

Thanks Russel, I had seen that before but coudl never find it again. Added to favourites!

ATK406 09-19-2013 07:45 PM

Re: definitive guide to concrete countertop?
 
Use of a Superplasticizer and/or Pozzoloans will help to make your mixture design easier to place - but they are a little expensive (much less than the premixed countertop mix though). Due to the relatively thin cross section of the countertop and the fact that it may be used to span an unsupported opening, every effort should be made to make your concrete as strong as possible. I.E. Low water to cement ratio, well graded aggregate (fine, medium and course), reinforcement (pencil thin rebar or mesh) and the use of admixtures if you want to go the extra mile. Here is another good link for both GFRC and Cast concrete countertops - Concrete countertop training : Concrete Countertops, Institute, Concrete Connections, Raleigh, NC : The Concrete Countertop Institute.

Regards,
AT

ATK406 09-19-2013 08:14 PM

Re: definitive guide to concrete countertop?
 
Sorry, let me try that link again, the blog posts are more informative;
cast in place | Concrete Countertops Blog

RichC 09-21-2013 06:26 PM

Re: definitive guide to concrete countertop?
 
Thanks Guys, ATK they're great links, thx again

CoyoteVB 09-26-2013 09:28 PM

Re: definitive guide to concrete countertop?
 
I built my entire outdoor kitchen with concrete countertops. Here is my input. Mix concrete with polyester or stainless fibers. Pour the concrete into your forms. Vibrate to remove air bubbles an to get the slurry give you that shiny look on the edges of your forms. Pull forms, cover with plastic an slow cure with water. At least a week, the longer the better to gain strength and minimize cracks. If you have pits, fill with a mixture of fine sand and cement. If you want a granite look polish with diamond pads. Start with 60g grit and go down to 800 grit. seal with a poly urethane sealer.


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