Q.: What are the differences in appearance and cost for smooth-rubbed, grout-cleaned, and cork-float finishes for formed concrete surfaces?
A.: Cleveland concrete contractor Ron Simonetti says a smooth-rubbed finish (done with an abrasive stone) should look the same as a grout-cleaned (sacked) finish. However, stone rubbing has to be done soon after the forms are removed--within a day or two at the latest, while enough moisture is still in the concrete to create a paste that will fill the bug holes. If you must wait longer than 2 days, you'll need a power grinder to remove fins, and you won't be able to work up much, if any, paste with the stone.
Another contractor says that when his crews did stone rubbing, the finishers carried a bucket of low-water-cement-ratio paste that they used to fill big bug holes. Then they rubbed over the filled holes so the color was the same as the rest of the wall. However, instructions for producing a smooth-rubbed finish in ACI 301-99, "Standard Specifications for Structural Concrete," prohibit using cement grout other than cement paste drawn from the concrete itself.
It's much harder to fill all the bug holes by rubbing than it is to fill them by the grout-cleaning method. Thus Simonetti prefers a grout-cleaned finish. He says his company has never been asked to do a cork-float finish in its 55 years of business.
ACI 301-99 describes the procedure for applying a cork-float finish as follows: "Compress grout into voids by grinding the surface with a slow-speed grinder. Produce the final finish with a cork float, using a swirling motion." This would seem to indicate that a swirl pattern would appear on a cork-float finish.
The 1997 R.S. Means estimating guide gives the following estimated costs for the finishes:
- 75¢ per square foot for a grout-cleaned finish (one man can do about 450 sq ft/day).
- $1.02/sq ft for a float finish 1/16 inch thick (one man can do about 300 sq ft/day). [We assume this is equivalent to a cork-float finish.]
- $1.78/sq ft for a wet-rub finish with a carborundum stone (one man can do only about 175 sq ft/day).
Simonetti says he'd estimate that the grout-cleaned finish would cost about $1.25 to $1.50/sq ft. That's about double the 1997 R.S. Means value, but we think this cost will vary significantly depending on whether the work is done from the ground or a ladder or if it requires scaffolding, as for a multistory wall.