Polished concrete tables.

[Tscarborough]

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.

[Mike D]

Re: Polished concrete tables.

I don't think I removed the form too soon, I just didn't have any help pulling it off, as well as that corner was a little weird. Nic said (as other things I read) pull forms after 24 hrs for a pour in place counter.

-The mix was fairly dry, the top you see is polished down with 200 grit. The more I polish it down the less the corner bothers me.

-That's what I taught about the slurry, I will try to epoxy it and see what happens.

- Why do you think it will crack?

Mike

[splatgirl]

Re: Polished concrete tables.

From the looks of it, you had quite a wet mix. FWIW, the slab is probably going to develop cracks which might brighten your outlook about the corner chip.

[splatgirl]

Re: Polished concrete tables.

Demerits for pulling the forms too soon
That is going to be tough to fix perfectly and permanently.
Slurry won't "glue" it. Concrete has little to no adhesive properties.

In the past, I've built up and fixed spots like that with a thick pasty slurry, but because it's not part of the original matrix and doesn't have any aggregate, it's always a weak spot. In a corner or an edge, it will break off with very little provocation. My oven landing slab has basically that same defect. It was fixed but didn't stay that way. Patina is the nature of the material, IMO, so I don't sweat it.
Try epoxying it and see what you get. It's probably going to show, but it will at least have a chance of staying put. I've never tried that route. I guess I'd let it cure for a week or so first.

[Mike D]

Re: Polished concrete tables.

Ok-

So I poured the counter over the weekend. The color came out good, but when I was removing the form I broke one of the corners. It was a combination of that one corner got vibrated too much (I didn't use the in-concrete vibrator, I read that it would make all the aggregate sink to the bottom. I guess that would be fine if I was going to flip it over.).

-What should I do about the corner? Should I epoxy the chipped piece back on and then apply the slurry or would the slurry do the job of gluing the piece on?

-The top doesn't have a lot of holes, it was the sides that didn't come out super smooth.

Mike

[splatgirl]

Re: Polished concrete tables.

I wouldn't worry about that even a little. It will never get hot enough to do anything.

[Mike D]

Re: Polished concrete tables.

Do you guys think I need to leave a expansion gap (a piece of cardboard thickness) between the insulation and the counter? I have a double insulation layer (fiberboard on top of vermicucrete). Is the counter around my oven prone to cracking from the heat moving into the insulation layers.

Here are some pictures of the form

Mike

[splatgirl]

Re: Polished concrete tables.

I use a palm sander.
Once you slurry, keep the slabs wet by covering them with plastic wrap for several hours or a day. My experience has been that even when wet down beforehand, the slab sucks up all the water and the slurry doesn't cure properly unless the whole thing is kept wet with repeated spraying and covering.

[Mike D]

Re: Polished concrete tables.

Here check this cheng video out Concrete Exchange - Pouring the Concrete

He calls it a stick vibrator. I will use a palm sander and a random orbital sander on the sides, but the stick vibrator is to get out the bubbles in the middle so there are less holes to fill when I sand/polish it down.

Mike

[ggoose]

Re: Polished concrete tables.

I have a DVD where Rhodes uses a palm sander or similar; I have seen a video of Cheng (it has been a while...I'm pretty sure it was Cheng) using a pencil vibrator attached to the end of a drill...seemed like overkill, especially in light of how well the palm/random orbit sander works.

Check out this link at 3:14...very brief; in the DVD version, it shows them vibrating until the cream just begins to vibrate over the lip of the form, bringing the air bubbles with it.

gene

[Tscarborough]

Re: Polished concrete tables.

I have met the Old Master Cheng and the new, Mr Rhodes, and while I have not observed their process in full, my guess is that they are using a purpose built vibrator, and not the one that is available from your local rental company.

[Mike D]

Re: Polished concrete tables.

Tscarborough- why do you say not to use a vibrator. When I watch all the videos (cheng, rhodes) they use a vibrator (in concrete, a vibrating stick you move around in the mix). My counters will be 2 1/2 inches thick.

Thanks for the slurry mix Nic.

Mike

[Tscarborough]

Re: Polished concrete tables.

I am not using a bonding agent as I am acid staining it, but other than that I agree 100%

[Nic The Landscaper]

Re: Polished concrete tables.

there is a huge range of things you can do with slurry to get it to do what you want. But a very basic, proven slurry mix is as follows.

Straight Portland cement (grey), sifted with a flour sifter (this is important lumps in the Portland make for a very poor finish). about 2 cups
Acrylic Modifier (I like a product called Tammsweld) about 1 cup or until mix is paste consistency
Add water until the mix is the consistency of ketchup. (Some say yogurt but i think that is a little too thick still, ketchup flows better)

Add color to the mix until the color of the mix matches the color of your pour when wet.

Use an electric or cordless drill with a paint mixer or small taping mud mixer on high speed in a 1 or 2 quart mixing container to mix material. Mix until all lumps are out. Water will act as an activator in this mix and so it will start curing as soon as the water is added, you will have about 20 minutes until slurry is no good.

Optional Items you can add to mix.

Metakaolin: about 20% by volume of portland cement.
MicroSpheres: Microscopic ceramic beads that will help mix flow (Over rated IMO)

DO Not add sand or any aggregate, or use a grout type material. The sand will make it impossible to get a good fill on the holes.

It will generally take 2 to 4 passes with slurry coats to completely fill in all the voids.

Spritz your slab with clean water out of a spray bottle and then scrape slurry mix across the entire surface with a putty knife. the idea is to just push it into the holes and not build up the mix across the entire surface.

[Tscarborough]

Re: Polished concrete tables.

I am going to slurry mine this weekend, I will be using straight masonry cement with color added.

The vibrator they have is probably an in-concrete vibrator and it is not what you should use. It will cause too much segregation of the aggregate in that shallow of a pour.