#61  
Old 06-28-2011, 05:56 AM
Tscarborough's Avatar
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Default Re: Polished concrete tables.

The wetter the mix, the more prone to shrinkage cracking. I would not fix the corner, although you could simply by mixing up a little bit somewhat dry, applying a bonding agent, and then filling it. It will not match perfectly, but no one but you will ever notice it. I pulled the edge forms within 2 hours of finishing the pour and then filled the bugholes while it was still workable and I had the same material to do so.
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  #62  
Old 06-28-2011, 07:38 AM
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Default Re: Polished concrete tables.

Mike

That counter looks great! I wouldn't worry too mich about the chip - very small and on the bottom. Perhaps slurry and polish the sides and call it good...because it really looks good!

gene
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  #63  
Old 06-28-2011, 07:38 AM
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Default Re: Polished concrete tables.

Well, IME, the only times I have had issues with losing bits was when I got impatient with pulling the forms. From what I have seen, that type of issue declines in direct proportion to time in the form. Obviously...more cure time means more strength.
I disagree with the 24 hour advice, but what do I know.

Inside corners almost always crack. Yours has the added bonus of wrapping around the whole front, right? I suspect that multiplies the inside corner cracking issue by a lot. That said, I do not have any direct experience with that situation.
Of course "wet" and "dry" are relative, but to my eye the look of what's going on in the edges of your slab say a wetter pour than not.

It looks good. What are you going to address the height difference between the hearth and the counter?
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  #64  
Old 06-28-2011, 07:52 AM
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Default Re: Polished concrete tables.

Splatgirl, it depends upon the design of the formwork. Always use screws, and for cantilevered pours, always make it such that the form for the bottom stays while the form for the face is removable. Then you can leave the support and still dress the face.
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  #65  
Old 06-28-2011, 08:40 AM
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Default Re: Polished concrete tables.

I am familiar with the ins and outs of formwork. That is by no means the only or even the biggest factor in extracting a perfect slab with all bits intact, even assuming construction with a good exit plan in mind and careful removal. Sometimes all it takes to break off a corner or an edge of a very green slab is the force of the actual face board coming away from the material.

"Dressing the face" really has no bearing on a 24 hour old slab. As you know, it's not wet enough to dress and not cured enough to have any strength. If it's a DIY, pulling the form early like that makes absolutely no sense to me. Leaving it on a good few days-- and reaping the added bonus of more moisture in the slab over a longer period-- is a cheap and easy insurance policy against this and other problems IMO.

OTOH, when a guy wants to get paid and get outta there, well...
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  #66  
Old 06-28-2011, 08:41 AM
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Default Re: Polished concrete tables.

I am talking about dressing it within a couple of hours, not 24. You never pull the formwork away, you slide it, and always into the corners.
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  #67  
Old 06-28-2011, 10:07 AM
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Default Re: Polished concrete tables.

Mike I think your work looks great. with more practice and experience you could have worked things like those bug holes out while pouring but some slurrying will get those patched right up.

as for the corner cracking and pulling forms at given time points etc. one thing I noticed looking back through the process leading up to the pour is that the forms were made from dimensional lumber. When I am working on counter top projects I always use melamine not lumber, this is something so natural to me that I don't even think to discuss it. Un -coated dimensional lumber will wick moisture from a fresh pour, as well as any surface imperfections and pores will fill with concrete and want to pull away pieces like the corner did.

At 24 hours Mikes counter top should have been at 50% hardness so roughly 2500psi which is more than sufficient to strip the forms on a simple counter like this. Also with all the additives and counter flow the mix should have appeared to be wet, I wouldn't be too concerned about cracks, especially with a 4" pour.
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  #68  
Old 06-28-2011, 10:48 AM
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Default Re: Polished concrete tables.

Yes, melamine is my M.O. for anything like this, too. And in situations where the use of melamine isn't feasible, a layer of packing tape on the interior face of the form board.

As you know, Nic, 2500 psi is descriptive of compressive strength only. Not a very applicable measure of what a cast decorative piece is going to be subjected to. In this case, it looks to be more about tensile strength which even fully cured concrete has very little of.

Anyway, my point remains that leaving a piece in the form longer is the best preventative, and that if 24 hours were "more than sufficient", he probably wouldn't have lost that bit. But who knows. I am just speaking based on what I have seen in practice. Even under the most perfect of circumstances, concrete always has a little bit of a mind of it's own. Absolute predictable perfection is not it's strong suit, but it's also why it's so fun and so interesting, right?
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  #69  
Old 06-28-2011, 09:25 PM
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Default Re: Polished concrete tables.

Thanks for all the support everyone,

I think that corner broke off because my friend was vibrating the hell out of that corner (before I was able to tell him to do the whole thing not just that corner). That was my fault, I was busy mixing and moving the cement. The aggregate sunk really hard and that corner that broken piece was mostly aggregate.

- I don't know what I was thinking building the forms with lumber. My test piece was with melamine. I think I was on auto pilot from the top slab of the hearth. The side counter will be done with melamine.

-I pulled the forms after 24hs to polish the counter while it was still a little soft. I don't have a wet polisher so I was doing it by hand with the 5" diamond pads. My arms are rockin right now but I think things are working out.
-I will try to epoxy that piece on and put a slurry around it and see how it goes.
-overall, I think it is going to come out fantastic. Thanks for all the help.

Mike
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  #70  
Old 06-28-2011, 09:30 PM
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Default Re: Polished concrete tables.

Splatgirl-

The front area will get a row of brick under the cooking floor that will extend out to rest on top of them. This will connect to the outer arch/vent area. I needed to do this counter first to move on to finish the arch/vent transition. This has been a long road of education, but that first pizza is going to be dam good.

Mike
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