We're working with a specification that calls for final troweling until the trowel makes a ringing sound as it moves over the surface. This is to be followed by scarifying the surface with a soft-bristled broom. I don't think the broom will scarify a hard-troweled surface, but is there anything in writing to prove this?
You're correct. We talked with two experienced concrete finishers, Carl Peterson, former coordinator of the Operative Plasterers' and Cement Masons' Job Corps Training Program, and Jerry Woods, executive director of the Illinois Ready Mixed Concrete Association. They agree that once the trowel makes a ringing sound, the surface is too hard for a broom finish. The written documents we've seen don't clearly indicate the correct timing for a broom finish. For instance, one common specification for a trowel-and-fine-broom finish says to apply a trowel finish as specified, then immediately follow by slightly scarifying the surface with a fine broom. What's the meaning of "a trowel finish as specified"? The specification is ambiguous on this point because it says in a previous section that for a trowel finish you begin final troweling when the surface produces a ringing sound. The section on brooming in the Portland Cement Association's Cement Mason's Guide (6th ed., 1995) can also be misinterpreted. It says to slightly roughen a hard, steel-troweled surface by drawing a damp, soft-bristled broom across the surface after troweling. The word "hard" in this description makes it sound as if several trowelings are done before the broom is used. Peterson and Woods say that brooming must be done immediately after the second troweling. If you wait, says Peterson, the concrete may get too hard for the soft bristles to leave brush marks. And Woods reminds finishers to keep the trowel flat during the second troweling. Tilting the trowel leaves ripple marks that won't be taken out by brooming.