Prevent Air Bubbles and Bug Holes in Concrete When Casting Cement Stone Molds

After over nineteen years in the concrete stone mold business, some of our do-it-yourself stone, paver and tile making customers and homeowners using our moulds still have problems with air bubbles or bug holes in the cast concrete products they make. Almost one-hundred percent of the time, it’s because they don’t follow the simple, free instructions provided with each purchase. So here are the instructions and techniques again in the form of a Home Improvement DIY article.

Is a mold release really necessary when casting concrete? – We always suggest using a mold release of some kind, even when using rubber molds. Mold release will help prolong the life of concrete, cement, or plaster molds. Using a mold release will also make de-molding the stone, tile, pavers, or other items you are duplicating, an easier and faster chore.

Make a home-made liquid mold release for use with Plaster of Paris, molding plaster and other plasters. When working with plaster, a simple mixture of a tablespoonful of liquid dish detergent to a cup of water makes a good release agent. Keep in mind that different plasters offer different shrinkage properties. The less a plaster shrinks, the more difficult it may be to de-mold the plaster item from a plastic or other semi-rigid mold. Be sure to read the instructions and properties on the package of all plasters before choosing one for your project. We also recommend using a soapy release when working with rubber molds and plaster. It makes a cleaner release and smoother finished piece, as well as helping to keep the plaster from sticking to the rubber molds— thus saving cleaning time and effort.

How to Make a Mold Release for use with Concrete and Cement – There are a number of commercially prepared concrete mold releases on the market. We actual offer our customers two types— a water-based and an oil-based mold release. We market it as a convenience to our customers. A light vegetable oil works just about as good as a commercial blend. The commercial releases may contain a suspended wax or other agents to make it easier to use, or offer other benefits.

Regular vegetable oil is fine, with a “light” vegetable oil being even better. But don’t worry about that. The problem with air bubbles in concrete castings is seldom the type of release used— it is usually the quantity left behind before pouring. It’s usually excess release, or it’s too much vibrating used to dislodge any air attached to the surface of the mold— or should I say between the mold and the freshly poured concrete. Have you heard the old saying about “too much of a good thing”? That’s normally the problem. With too much vibration, you can actually introduce air bubbles into the mix. And over-vibrating— especially with only half of the concrete batch poured into the mold first, can certainly cause excess air.

How much oil should be left to coat the mold? – For best results, be sure to wipe most of the oil out of your molds prior to filling them with concrete. You need to be especially aware of the low crevices of the mold where the oil can puddle. You should barely be able to see that there is a coating on the mold. Then fill your mold about half to three-quarters with your concrete mix. On a flat, level surface, lift each side of the mold about a half-inch or so and let it fall back flat to the table or other surface you are using. If it is a larger mold, say over 12″ in diameter lift it higher so that the center gets the benefit of the vibration as well. Do this about four or five times on all four sides. Then fill the mold to the finished depth that you want. This time, lift all four corners one at a time four or five times, letting them fall back down. Now shake the entire mold a couple of times in each direction to be sure that the concrete is level. Cover with plastic sheeting and wait for it to harden. This technique allows you to dislodge any air bubbles from what will be the surface of your stone.

Why is there air in my concrete mix? – Yes, having a certain amount of air in your mix is actually desirable… especially if the stone is for an exterior application. This is called air entrainment. It allows a space for any moisture within the concrete to expand and contract in a freezing environment. This is called freeze-thaw. When you use our mix additives, there is actually an air-entrainment agent in the additive to put air into the concrete! Normally, air entrainment in concrete and cement should only be about 5% though. That air really has little to do with “bubbles” or what are called “bug holes” in the industry to appear on the surface of your stone casting.

Think of how happy your arms are going to be now that they don’t have to vibrate that heavy concrete as much!