Prevent air bubbles and insect holes in concrete when casting cement stone molds
After over nineteen years in the concrete stone mold business, some of our customers and homeowners who use our molds to make DIY stones, pavers and tiles still have problems with air bubbles or holes in the molds. cast concrete products they manufacture. Nearly 100 percent of the time, it’s because they don’t follow the free, easy instructions provided with every purchase. So here are the instructions and techniques again in the form of a DIY home improvement article.
Is a release agent really necessary when pouring concrete? – We always suggest using some kind of release agent, even when using rubber moulds. The release agent will help extend the life of concrete, cement or plaster molds. Using a release agent will also make unmolding the stone, tile, pavers, or other items you’re duplicating easier and faster.
Make a homemade liquid release agent to use with plaster of paris, plaster for casting, and other plasters. When working with plaster, a simple mixture of one tablespoon of liquid dish detergent in one cup of water is a good release agent. Please note that different plasters offer different shrinkage properties. The less a cast shrinks, the more difficult it can be to unmold the cast item from a plastic or other semi-rigid mold. Be sure to read the package directions and properties of all plasters before choosing one for your project. We also recommend using a soapy release agent when working with rubber and plaster molds. Produces a cleaner release and a smoother finished part, plus helps prevent plaster from sticking to rubber molds, saving cleanup time and effort.
How to make a release agent for use with concrete and cement – There are several commercially prepared concrete release agents on the market. We currently offer our customers two types: a water-based and an oil-based release agent. We market it as a convenience to our customers. A light vegetable oil works almost as well as a commercial blend. Commercial communications may contain a suspended wax or other agents for ease of use or other benefits.
Regular vegetable oil is fine, and a “light” vegetable oil is even better. But don’t worry about that. The problem with air bubbles in concrete pours is rarely the type of release agent used; it is usually the amount remaining before dumping. Usually it’s too much release, or too much vibration being used to dislodge air stuck 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 is usually the problem. With too much vibration, you can introduce air bubbles into the mix. And excess vibration, especially when only pouring half the batch of concrete into the mold first, can certainly cause excess air.
How much oil should be left to cover the pan? – For best results, be sure to wipe most of the oil out of your molds before filling them with concrete. You should be especially aware of low crevices in the mold where oil can collect. You should just be able to see that there is a layer on the mold. Then fill your mold half to three-quarters full with your concrete mix. On a flat, level surface, lift each side of the pan about a half inch and drop it onto the table or other surface you’re using. If it is a larger mold, say over 12″ in diameter, raise it higher so that the center also benefits from the vibration. Do this about four or five times on all four sides. Then fill the mold to the final depth. This time, lift the four corners up one at a time four or five times, letting them drop down. Now shake the entire mold a couple of times in each direction to make sure the concrete is level. Cover with a sheet plastic and wait for it to harden.This technique allows you to dislodge 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 really desirable…especially if the stone is for an exterior application. This is called air entrainment. It allows space for any moisture within the concrete to expand and contract in a frozen environment. This is called freeze-thaw. When you use our admixtures, there is actually an air-entraining agent in the admixture to put air into the concrete! However, normally the air entrainment in concrete and cement should only be 5%. That air really has little to do with the “bubbles” or what are called “bug holes” in the industry that appear on the surface of your stone casting.
Think how happy your arms will be now that they don’t have to vibrate that heavy concrete as much!