Q: We were placing a large concrete patio that was to have an exposed-aggregate finish. After placing and bullfloating the slab, we spread a layer of decorative aggregate across the surface, pressed it down so it was completely embedded in the concrete, then hand-floated the entire surface, slightly covering the aggregate with mortar. After waiting for the concrete to harden, we weren’t able to wash and brush away the paste to get a finish that would satisfy the owner. He insisted we come back and sandblast the surface before he would accept the work. How can we avoid this problem in the future?
A: In the future, consider using a surface retarding product that is sprayed onto the surface after floating. These products come in several strengths so you can accomplish the needed profile depth. These products also are colored so that you know they are on the surface in a uniform concentration. Some of them also include polymers that form a crust on the surface to limit moisture loss from the concrete and protect the slab in the event of rain. You can roll or spray them onto a surface and remove the retarded cement paste with a power washer several hours later.
In projects like these, it’s a good idea to make preconstruction test panels. That way, you can work out the timing and techniques needed to expose the aggregate particles without dislodging them. You also can use the mockups to get aesthetic approval from the architect and owner.