Minimizing Air Bubbles

Question:
Is there an admixture I can use to get rid of air bubbles in my countertops? They leave behind unattractive pinholes in the surface.

Answer:
No. All concrete will have air trapped in the mix due to the mixing process. And there’s no admixture you can add to make this air disappear, dissolve or not get entrapped.

Fine aggregates and sand tend to trap air bubbles, and a stiff cement paste won’t allow the air to rise and escape. The only way to cast concrete so there are no air bubbles (large or small) on the surface is to design the mix so that the fresh concrete is very fluid or can be made very fluid by vibration. Only a very fluid concrete will allow the air bubbles to push their way through the concrete and rise to the top and dissipate.

Often the cast surface will be pinhole free but any significant grinding will reveal small pinholes just below the surface. Large air bubbles have enough buoyancy to push aside the aggregate and escape, while the smallest bubbles get left behind because they are too small and not buoyant enough to push their way through the concrete. This is why I don’t try to vibrate out all the air bubbles. Instead, I simply grind off the cement skin and use a slurry, or grout, to fill in the inevitable pinholes. I call this the “grind and grout” method.

Many people do use vibration to eliminate most air bubbles, and the result is a smooth cast cement look. I call this the “pop a perfect slab” method. The two methods are so different that I have planned a seminar at the 2007 Concrete Countertop Industry Conference comparing them, called “Should I ‘Grind and Grout’ or ‘Pop a Perfect Slab’?”