How to Seed Glass Aggregate: Tips from a Pro

Harry Gressette of Grescon Construction LLC in Charleston, S.C., puts enough glass in his concrete each year to fill two tractor trailers. So he knows the ins and outs of exposed glass aggregate. Here are a few tips for creating exposed glass aggregate floors that he shared with Concrete Decor:

1. Make sure the substrate is clean — in other words, shotblasted.

2. To get good coverage when seeding, use at least 3 pounds of glass per square foot of floor.

3. Seed the edges as thoroughly as the rest of the floor. There’s nothing worse than an otherwise good-looking exposed glass aggregate floor with little or no glass on the edges.

4. Wash the glass well, and keep it moist while seeding. This will reduce pop-outs.

5. Make sure the glass is not thicker than the depth of the pour.

6. When seeding, always start at the back of the room and work toward the door so you don’t step in what you’ve just broadcast.

7. Wear spiked shoes and always lift your feet straight up and down. Never drag the spikes through the concrete, and don’t even graze the surface. You can’t float out tears that go all the way down to the subfloor. Four days later, when you start to polish, you’ll find voids that look like pine straw and holes that look like golf tees. Ask me how I know. (I would have had to fire myself for that one!)

8. For large rooms, have someone floating or running a blue steel smoother over the seeded areas to embed the glass as you go.

9. Glass sinks in wet concrete faster than it does in stiff concrete. When pouring, keep a wet edge, and when seeding, move fast so you can get the aggregate in before the slump changes. That way, if there’s any settling of the aggregate, it’s all at the same rate.

10. Seed heavily around protruding pipes, electrical boxes, etc.

11. Screed with a gauge rake before seeding, and float with a blue steel smoother or large magic trowel. Don’t use a bull float — it’s too heavy and will move the glass around, causing bald spots.

12. Wait at least four days before polishing, so you get good aggregate lock.

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