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Cheng prefers a 6-inch slump, and uses his fingers to push his dense oatmeal-like NeoMix into the mold's nooks and crannies.
To help ensure consistency, Gobillot says, "We pour the whole kitchen at once, in multiple mixes, but each batch we weigh out to the 100th of a pound, especially with dyes."
Frequently, special aggregates (such as colored glass) are incorporated in a mix, and are later exposed by grinding.
Casting and finishing techniques
Each precast contractor has his own preference as to tools, but many use standard concrete, woodworking or granite tools. Gobillot says his preference is for gardening tools, which "seem to work better than construction tools."
From Ralston's perspective, "It's almost a necessity for precasters to have a whole repertoire of tools that let you get into little nooks and crannies."
Latex or rubber gloves are standard gear, as are respirators to protect from the ill effects of portland cement and sand.
Whether a concrete countertop needs to be vibrated - or how much so - depends on whether you plan to grout the surface with a slurry mix for creative effect, Girard says.
Vibrating the poured concrete will help eliminate bug holes and voids.
Finally, you want to screed off the excess concrete so it is level with the top of the mold.
How soon you unmold depends on the concrete mix you use.
"We de-form between two to four days after pouring," Gobillot says. "We begin sealing after three weeks. Our surfaces come out like a mirror finish so we go straight to sealing them."
Cheng recommends placing a moist blanket over the piece after it sets up, and leaving it there for one to three days while it hydrates and cures. He typically demolds four days after pouring; then lie waits another two to three days before grinding. |
