I just started to demold a few of my smaller pieces and I noticed some white blotching on my tops. Doesn't seem to come off with water. What caused this and will it come out when I begin polishing? It's only where it had contact with the melamine and not on the back. I know I have some bug holes as well. Guess I could have spent some more time vibrating. I attached a picture to illustrate. Sorry for all the questions!

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To follow up, I think  I see what went wrong. When I look at the melamine I see that same white haze (I used black melamine) and also the little dimples you see in the concrete I have raised bumps on the melamine. The only thing I can think of is that I was too aggressive with the stick vibrator and maybe put little nicks in the melamine which then absorbed water. So the question is how do I fix it. I was planning to go with a light grind but that's probably out at this point. I was hoping to grind deeper and expose aggregate. Hopefully that will take care of the white blotchiness. Then I assume the slurry will fill the little dips. Any suggestions are appreciated. I hate to recast at this point. I have quite a large area and don't know if I have it in me to start over.

What kind of recipe did you use?
Is this GFRC?
How did the grinding go?
I used Cheng pro mix with sakrete 5000. Haven't done all the pieces but the good news is I'm able to get the white blotches and dimples out with a lot of grinding. Wasn't originally planning to do a deep grind but now I like it. Only issue is it's alot of work with the 50 grit pad to do the deep grind. When I focus on removing those dimples I expose alot of aggregate in those areas which means I need to do it everywhere to look even. Any way to speed the process up? I thought about using a cup wheel on my grinder but I'm afraid that it'll make everything wavy and uneven.

Any way to speed the process up? I thought about using a cup wheel on my grinder but I'm afraid that it'll make everything wavy and uneven.


One way to speed up the process is to remove from the concrete from mold sooner. I would demold after 2-3 days and start wet polishing while the concrete is still somewhat soft. But you'll need to keep that polishing pad flat on the surface as possible, with the concrete still soft it's easy to create divots.   

Well it's too late to that at this point. The concrete is 6 days old. Would a cup wheel be too risky. I'd rather take the time if I need to than ruin it.

The cup wheel did the trick. Exposed a nice amount of aggregate and removed all the dimples and inconsistencies with ease.

You only fear the cup until you use it. I was the same way. Experience breeds confidence.

I love that thing now.

I'm guessing the cup exposes a lot faster cause of the smaller surface area? Am I correct to assume that?

So i will just start with this,if you are making a mix that doesn't suck you should demold in about 18 hours and sooner if the temperature is up. You do not want to pop your concrete out the mold early to diamond polish because it is softer. 5000 psi is the point of processing with diamonds. If you have aggregate/glass in your piece and you start grinding it the glass or agg will be much harder than your concrete.Think about it.

 
Urban West said:

Any way to speed the process up? I thought about using a cup wheel on my grinder but I'm afraid that it'll make everything wavy and uneven.


One way to speed up the process is to remove from the concrete from mold sooner. I would demold after 2-3 days and start wet polishing while the concrete is still somewhat soft. But you'll need to keep that polishing pad flat on the surface as possible, with the concrete still soft it's easy to create divots.   

Josh, can you be more specific regarding temperature? I had a bench that got up to 136 degrees, at which point I pulled the blanket off and misted it with water. I noticed small cracking, not sure if that was the cause.

If you have any knowledge of what is happening chemically at that temperature, please share.
Thank you.
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Ya I would try to keep 130 or lower.if it gets to hot it can crack,but the heat is good for hi early strength.

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