Pesky air bubbles

rhonda2006March 11, 2006

Hi all, it's me again. Ok so what's the trick to getting rid of the air bubbles in the concrete?? This time I added more water to my mix and it seems to have helped some. I have a hand held vibrator that I used as well as tapping on the surface. I really didn't see that many bubbles coming to the top. My molds are plastic. One is the size of my palm up to the side of a stepping stone. I have been using canola oil as a mold release. You have been so helpful, any suggestions would be appreciated.

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Air bubbles are a curse. You can vibrate the mix in the bowl before pouring it into the mould, as well. And the wetter the mix the fewer bubbles, as it allows the bubbles to travel up more easily. And rock the mould around as you vibrate it, to allow the bubbles to free themselves from the oil, try to guide them into the centre of the mix where they won't show. The wetter the mix the longer it takes to set hard, so you'll have to leave it in the mould longer. Are you using a super-plasticiser?

    Bookmark   March 11, 2006 at 4:27PM
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Forgot to say - you can try adding a bit more cement to the mix to speed up the set, as well.

    Bookmark   March 11, 2006 at 4:32PM
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Thanks running dog. This is a totally new thing for me. I poured my first molds last week and my second batch yesterday.
I know very little, although I have learned alot through this site. I don't know enough about it though to know what to use or what not. I'm not using super-plasticiser. (yet) If it would help I might.

    Bookmark   March 11, 2006 at 8:58PM
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Take a look at the info on Smooth-On's website. Excellent iformation on avoiding "bugholes". See the link.

Here is a link that might be useful: Smooth-On...Bugholes in Castings

    Bookmark   March 11, 2006 at 10:57PM
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Thanks Tango, you are such a great source of information. :)

    Bookmark   March 12, 2006 at 10:29AM
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buckyforce(10 So. Fla.)

Hi Rhonda,

I usually "thump" the bucket or container, in which I have mixed the concrete, on the ground a bunch of times and then let it sit for a minute or two, before I pour or cast. Seems to get the majority of the bubbles out that were created by the mixing. Don't be shy either - thump that bucket HARD! and watch the bubbles rise.

Hope this helps!


    Bookmark   March 12, 2006 at 4:15PM
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If you're going to thump the outside of the mould, it's worth buying a rubber hammer so you don't damage the mould in the process.

    Bookmark   March 13, 2006 at 7:07AM
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I went to a flea market yesterday, picked up a rubber mallet to use and this guy talked me out of it. He makes stepping stones, urns etc. He has a pretty big operation with a commercial mixer and the whole works. He kept telling me I need a vibrating table and talked me out of the mallet. I just want this to be a hobby, make it fun and hopefully make a little money.

    Bookmark   March 13, 2006 at 9:20AM
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daisy_ny6(z6 NY)

I was working on something just last night. Lately, I've been really bugged by those holes even though I vibrated my concrete.

For an experiment, I vibrated the mixing bowl (I just have a hand-held orbital sander with no paper covered with a plastic bag to keep it clean). While it's being vibrated, the mixture seems to liquify but when then seems much denser to handle. I scooped some out of the smooth plastic bowl and found a lot of bug holes on the concrete that had been in contact with the smooth surface.

This last time after vibrating in the mold, I re-pressed the concrete with my hands (I need a tiny trowel). This morning when I peeked by peeling back a bit of the mold, the part I could see was good and smooth.

This was with a vermiculite-sand-cement mix which was not in the least bit pourable, perhaps a tad bit drier than Playdough.

I think the vibrating distributes the water-admixture and the various size particles more evenly. When the mix was drier, marble-like spheres formed on top, but I added more water and it smoothed out on top, but the bugholes formed below.

I have a lot more experimenting to do. I'll have to try longer vibrating times vs more and less liquid. Vibrating slurry bring the air bubbles up instantly.

What do the rest of you all do for vibrating? I just salvaged a thin metal deskdrawer to make a sort of table - I think if I set the sander and the bowl of cement on it, it might vibrate sufficiently to be useful as well as hands-free.

I'll also check out the Smooth-On link. I got their body-casting video. Unfortunately, my nephew was watching it with me and it freaked him out so much, it took a year of persuasion and bribes before he'd let me do his face. I've still never done a whole-head casting.

    Bookmark   March 13, 2006 at 12:18PM
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I have tried a wetter mix and my back massager and have gotten better results.

I may of had 1 piece completely free of holes. Takes time and practice. I have been looking for a Paper Jogger table to help with my stepping stones mold but haven't found one yet for the price I want to pay.

    Bookmark   March 13, 2006 at 12:54PM
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Sounds like there is no easy solution to this. Something I came across this morning while trying to research it more is a website that shows you how to make your own vibrating table using the above pictured Homedics vibrator for about $40, which may be appealing to some. The website is:

    Bookmark   March 13, 2006 at 1:12PM
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Good ideas there for making the vibrating table.

Daisy - I wonder if vibrating a mix that's as dry as you describe would make any difference at all? The vibration helps to move the bubbles up through the liquid mix, if the mix is very dry the bubbles would have nowhere to go.

    Bookmark   March 13, 2006 at 1:49PM
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daisy_ny6(z6 NY)

The interesting thing in vibrating a relatively dry mix is that it becomes liquid and levels out - it's kind of neat to watch.

I noticed that it also seems to bring the excess liquid to the top. I am using superplasticizer with vermiculite mix; I don't like what it does to pure sand-cement - it gets gloppy too easily.

    Bookmark   March 13, 2006 at 4:57PM
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I think the type and amount of mold release may have some effect also. I know when I have used motor oil/denatured alcohol I get more holes and it sticks more to the mold (the crete) than when I use Smooth On's Mold Release for my plastic molds. Also I heard oil is not good for rubber and latex molds. My 2 cents.

    Bookmark   March 13, 2006 at 8:46PM
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I've heard that also paws. The gal that I bought my last batch of plastic molds from recommended the Smooth On. I have been using canola oil. I think it's a bit too heavy. Some of my molds are pretty detailed. I talked to the tech people at Quikrete and they told me I need to tamp it with a rod. I think it's back to experimenting. I'm determined to find something that works. Ever feel like you're back in chemistry class??? ha!

    Bookmark   March 13, 2006 at 11:41PM
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rhonda - tamping with a rod is just a way of achieving the same effect as vibrating - it's a way of moving the concrete around and liquifying it so that it releases the air-bubbles and allows them to rise to the surface. You'll never get good detail on the surface by poking the mix with a stick lol.

There are two types of casting - wet mix using vibration to get rid of air-bubbles, and dry tamped mix. When using the dry-tamped method you don't have to worry about air entrapment. The method that most people on here seem to use with the hypertufa is a mixture of both - the mixture is slightly wetter than would normally be used in a dry-tamp mix, but dryer than would be used in a wet-vibration mix.

I think I'm making things sound much more complicated than they are. When using the hypertufa method advised in most of the tutorials on here, you don't have to worry about air-bubbles, or about vibrating the mould. The mix is being pushed down manually into the moulds, air voids are being forced out by the pressure as you press on the moulds. If there are voids on the surface, it just adds to the nice aged look.

A mould is vibrated if you're using a wet sloppy mix (achieved by adding super-plasticiser) to achieve a closed, highly detailed surface, and/or if you can't reach areas of the mould to force a dry mix in.

Vibrating a semi-dry hypertufa mix, rather than pressing it into the mould, doesn't seem to me like it would achieve anything. You're just giving yourself extra work for no return.

I spent many years casting small sculptures, using vibrated wet-mix. I used to use, mainly, a 3:1 mix of crushed granite and white cement. The mix going into the mould would be as liquid as pourable porridge. Any excess water that rose to the surface over the next few hours, I'd just carefully sponge off. The castings would be ready for release within 24 hours, less if the weather was warm.

I've attached a link to some photos at the bottom. Although a couple of the photos show terracotta slipcast versions, (I made separate plaster moulds for the slipcasting) all of them were produced on a small commercial scale using the wet-cast method. They were acid-dipped after a few days, to expose the mica in the granite.

Now...dry-tamping. Commercially, that's done with a very fine sand/cement mix. That's mixed very well in a cement-mixer, then (as a friend who uses this method described it) you 'frighten it with water', and continue mixing. He meant that you make it only barely damp. You put handfuls of the mix into the mould, ram it very hard using a stick, or electric ramming tool. More of the mix is added and rammed, you fill the mould right up ramming each layer as you go. The surface finish looks like sandstone, and needs no acid-cleaning.

Whew, wonder if I've just confused matters further for you? :o(

The bottom line is that it's an either/or situation - you EITHER have a wet mix and vibrate the mould OR you have a dryer mix and press the mix into the mould manually.

Here is a link that might be useful: Wet mix, vibrated

    Bookmark   March 14, 2006 at 3:49AM
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Thanks so much RD. I'm using a sand mix just to experiment with. Friday I tried a mixed white portland/white sand. My only variation would be using perlite or vermiculite to lighten the weight a little. Not sure which one of those to use yet. After reading as much as I could on the tamp issue, I really didn't think it was for me.
Several years ago, I did crafts with my family. I was such a perfectionist that if it wasn't perfect I would give up and try something else. So, I guess in a way that's what I'm trying to achieve. I have some money invested in this so that's what keeps me going. I'm afraid that if they have a bunch of holes in them, they won't be very appealing.
When I took this on, I didn't know there was so much to it.
I'm still having fun. I appreciate the people on here who have helped me so much.
PS. Your link didn't work.

    Bookmark   March 14, 2006 at 9:50AM
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club_53(z5 MA)

Not being an expert at anything, I have to put this out: how are you getting airbubbles if you are working with hypertufa?

I thought, strictly speaking, hypertufa was a mixture of portland, peat and perlite and/or vermiculite and supposed to be somewhat dry so that you couldn't pour into a mold if you wanted to. So, if you can't pour it, how are you going to vibrate it? Isn't hypertufa supposed to be packed into molds, and that it is the packing (compression) that makes the walls of the item strong. What I learned from reading, etc. is that if "hypertufa" was wet and sloppy enough to pour, it was not a good thing -- the more water the weaker the mix. It is supposed to cure out and look like old, weathered limestone -- an imitation of tufa rock...hence the name.

It seems (to me anyway :) ) that some of these beautiful things made by may contributors are really not, strictly speaking, hypertufa, but very talented workings with concrete mixtures.

Am I off the beam?

    Bookmark   March 14, 2006 at 10:47AM
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I'm not making hypertufa. In the future I want to experiment with a mix of portland,vermiculite or perlite and part sand (no peat) to lighten up my plaques and pieces to hang.

    Bookmark   March 14, 2006 at 11:00AM
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daisy_ny6(z6 NY)


Sounds like you got fun times ahead experimenting.

I mentioned perlite mainly because you seemed to want white and I think it's less dense and therefore lighter weight than vermiculite. However, I do like the way vermiculite affects the working texture of the mix and the gold fleck effect on the finished pieces. Perlite is a little more difficult to polish and a lot harder to carve. I use both with sand or peat (sand for solidity, peat for porosity and hopefully quicker mossy growth) in varying proportions (usually 1 part sand to 2 parts vermiculite-perlite mix).

You're just not going to know ahead of time what's going to suit you best.

Vibrating the drier mix does seem to have a densifying effect (even while it liquifies and looses its shape) and I think it eliminates any dry spots. There is certainly an effect - I'm just uncertain whether it's for better or worse. I think I'll try it both only in the mixing bowl & thumb-tamped into the mold and then for a longer time in the mold, just to see.

    Bookmark   March 14, 2006 at 3:00PM
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daybees(B.C. canada)

Daisy how does your metal desk vibrating table work?
That's an really neat idea.
Thanks for the pics running-dog, beautiful work!!!

    Bookmark   March 14, 2006 at 10:02PM
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Thanks Debbie :)

Today I'm going to try making that small vibrating table (instructions linked above). I have an idea for how to make it slightly differently - the cardboard stuck on to the bottom of the foam wouldn't hold it in position with a heavy concrete mould on top. If it works I'll post photographs, if it doesn't it's back to the drawing board lol

    Bookmark   March 15, 2006 at 2:50AM
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Be very careful with the vibration of the mix. If you vibrate it too long, the fines(sand,cement etc.)have a tendency to seperate and float to the top. Also, if you are wanting to rod the mix to achieve the same result, make sure you use a smooth round rod with a rounded tip and tap the mold or whatever gently after you do so. It's the same thing with adding water- be careful about adding too much. Experiment with it until you get the right feel for the mix. Also, if you want a ready made border for your garden, check with your local concrete plant and see if they have any old "test" cylinders they may want to give away. They should be 12 inches long and 6 inches in diameter. It's not for everyone, but it's worth alook.

    Bookmark   March 15, 2006 at 7:40AM
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Ok, so with all this discussion on "pesky air bubbles" I expect progress reports on those participating. lol
I was going to experiment today, I had too many other things to do, I will let you know. The saga continues . . . . .

    Bookmark   March 15, 2006 at 6:06PM
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Mike Larkin


    Bookmark   August 28, 2008 at 1:59PM
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daisy_ny6(z6 NY)

Re the vibrating table - it doesn't work at all well - noisy and indifferent results at best, increased bug holes at worst.

I have had better luck with "painting" the mold with a thick slurry of cement, marble dust, pigment & latex admixture (with a bit of superplasticizer) before adding the cement mix.

I've found that the slicker the mold surface, the greater the likelihood of getting hemispherical bug holes. I have not been able to eliminate them from coke-bottle type plastic. I'm thinking of sanding it lightly with 800 grit sandpaper might work. One of the problems seems to be that the slurry won't stick to the super slick surface.

Of course, the flip side is that the set cement sticks a bit too well when demolding time comes.

    Bookmark   August 30, 2008 at 3:11PM
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