The use of glass as a special aggregate for decorative concrete is on the increase due to more color possibilities. One might think this is a questionable practice in light of the potential for alkali-silica reaction (ASR) between the glass and the cement, but according to Lane Howell, a partner in American Specialty Glass, Salt Lake City, “We know there is the possibility for such a reaction but we have yet to hear of a single case of ASR involving glass aggregates.”
Glass aggregate for decorative finishes can be exposed in several ways. You can use a typical exposed aggregate procedure (surface retarders), sandblast the surface, or diamond polish the surface. Glass aggregates can be integrally mixed into the concrete but more often they are broadcast onto the surface of fresh concrete. This technique is increasingly being used to add interest to diamond polished floors and concrete countertops.
The right kind of aggregate
A logical assumption might be that glass aggregate would come from recycled glass bottles, but that isn't the case. Recycled glass has too much contamination to be suitable for use in concrete. And, recycled glass tends to come in long, flat shapes that aren't considered good aggregate for concrete. The best concrete aggregates are cubical in shape, which is the shape in which glass aggregate is supplied. Howell says for the aggregate his company supplies, they collect refuse glass from manufacturers of glass products such as windows, French goblets, mirrors from Connecticut, bottle manufacturers, and Mexican glass products. They don't use “dirty” glass products, such as glass with labels glued to them. Recycled glass can be sorted by color or it can have color added. In either case it's melted and crushed instead of being quenched with water (which fractures the glass too much). Then it's sorted by size and the aggregate edges are dulled so they aren't sharp. “Aggregates can range in size from fine aggregate powder to 10-inch diameters,” adds Howell.
Glass aggregate can be purchased in 20 different colors, the most expensive ones being red and orange. Expect to pay approximately $85 per 50-pound bag and $150 for reds and oranges, with discounts for volume purchases.
Dave Blasdel, owner of Blazes Concrete Impressions, Kalispell, Mont., says that he uses colored glass aggregates for concrete countertop and floor construction. He places them integrally for countertop construction and seeds them for floor work. For seeded work he takes the added step of soaking them overnight in polymer to increase the bond to concrete. Before the placement he removes the glass from the polymer and lets it dry to the point where it still feels a little sticky. He broadcasts the aggregate from planks to achieve the most even broadcast. If he can't do that he places the aggregate every 10 lineal feet during the concrete placement. He typically uses a pea gravel mix when he does glass aggregate work and some of these stones are revealed in the exposing process, adding a nice look to the finish.
After the aggregates are in place, the finishing steps are the same as with any exposed aggregate surface. He floats the aggregate into the surface with a wood bull float, then with a magnesium bullfloat, and finally closes the surface with a steel trowel finish.
When work is diamond polished, Blasdel recommends starting with 200-grit diamond pads instead of the 100-grit pads more commonly used for plain concrete. Glass is more brittle than stone, so the first pass barely removes the top surface, minimizing any tearing and damage to the surface. Then he doubles the grit size with each successive grind, going up to a 1500- or 3000-grit polish.
Mike Miller, owner of the concretist, Benicia, Calif., also installs glass aggregate seeded concrete floors and uses diamond tools for exposing. But he refers to his process as “grinding” instead of diamond “polishing,” preferring a rougher finish that is sealed with a solvent acrylic sealer afterwards.
Glen Roman, senior technical representative for Brickform, Rancho Cucamonga, Calif., sandblasted a stencil logo graphic for an automobile dealer. The concrete contractor for the project seeded black glass aggregate and finished the slab so that no aggregate showed on the surface. After a month Roman adhered stencil patterns to the concrete and using slag as the blast medium (which hits harder than silica sand), blasted a fairly deep reveal, showing a nice contrast between smooth finished concrete and exposed aggregate.
Glass aggregate increasingly is being mixed with epoxy binders for terrazzo floors too.