Using Decorative Cement Mold Hints

Those nasty air bubbles !!

Posted in Decorative Concrete Technical Help by CementMoldGuy on February 5, 2009
Tags: , , ,

SO we pour our concrete into the mold and let it dry for the recommend 24 hours. The next day we come back on open up the mold only to find a pin and potholed disaster….WHAT HAPPENED !! ugh..sigh…disappointment.

So what can I do different next time to not have these?????

Fat Fish Concrete Sculpture


Fat Fish Concrete Sculpture

The MIX                                                                                                        Firstly is the mix itself correct? It needs to be much wetter then the mix you used to pour you front step or back patio. The mix needs to just fall off the shovel or mixer blades. Should be very loose.

Release Agent                                                                                                        A release agent creates a space between your mold and the concrete for the air to move. With the hints below the air will move to the top of the mold and out the top.                                                                                       

So you have poured the concrete into the mold now what ???Well how about  … Vibrating,  Prodding,  Bumping or Tilting. 

  • Prodding the concrete mix in the mould with a rod or reinforcing rod many times gently, will help break water and air pockets and aid in the settling of the concrete mix.
  • Use a Rubber Mallet and gently hit the side of the mould during and after pouring will greatly reduce the number of surface air holes and will help in filling undercuts.
  • Depending on the shape of the mold it is sometimes necessary to tilt the mold as you fill it to ensure no undercuts. Continue filling your mold until your mold is slightly overfilled. This will help the water flow off your mix.
  • Use a Vibrating Table by placing the empty mould on a vibrating table. Pour in some mix and switch on vibrator. Whilst vibrating continue to fill mould. Switch off and remove mould to cure. Over vibrating can cause extra wear on your mould and the mix to separate so keep to around 1 minute.

I hope these hints will help you get the beautiful sculptures from your molds!!


11 Responses to 'Those nasty air bubbles !!'

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  1. kristian said,

    who sell’s this release agent .and what is it made off

    • Hi Kristian
      A very common release agent is a simple mix of castor oil and denatured alcohol (rubbing alcohol). You mix at a ratio of 20% castor oil and 80% alcohol. You can purchase these at Walmart, or other ratil pharmacys.

  2. elizabeth said,

    where do you get vibrating tables and how much

    • HI Elizabeth
      Depends where you are located. In the US, Houston Vibrators if you want a new one. Otherwise look on ebay or Craigslist for used ones in your area.

  3. Terr y said,

    I actually made a vibrating table, looked for a small vibrating motor and had someone weld it on to a metal small table created just for this use. total cost. around 250 us motor cost 200 but it is new and I know what I have some time you buy used stuff and well you don’t know what you are getting this is a small table just next to another table so i can slide the molds off to let sit.

  4. […] Those nasty air bubbles !! February 2009 5 comments 4 […]

  5. ash said,

    i got a back massager from walmart for $30, i use a rubber bungie cord strapped across the table to hold it tight, i turn on the massager and the whole table vibrates fine. $30 back massager and $2 bungie cord vs. $400 for a vibrating table….. table is a metal desk i bought for $10 surplus. Works great, easy to level etc.

  6. terry said,

    I am located in Costa Rica. so I do with what I can anyway the vibrating table works great for any size peaces.

  7. Denease said,

    I recieved a tip from a pro and it works great. We vibrated our concrete and still had bubbles and a pro told me to wet the sand and let it sit for 40 minutes before adding cement and aggregate. The air in the sand is pushed out by the water as it is absorbed by the sand and by letting it sit, you get the air out before you finish your mix. Once you let it sit for 40 minutes, add cement and aggregate and whatever extra water you may need to get the consistency needed for your pour. We have had great luck with this. We still vibrate the pours as well and we are seeing very little to no holes from air bubbles.

    • Good point. Yes wet sand is much better to use.

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