I am glad to see someone finally decided to challenge the age-old notion that a hard-troweled floor requires the same degree of curing that a light machine or handtroweled floor requires. We worked with CTLGroup, Skokie, Ill., several years ago evaluating the 72-hour moisture retention differences of ASTM C156 using the specified wood-screed finish compared to a hand-applied, steel-troweled finish. Our results indicated that a hand-applied, steel-troweled finish retained approximately 20% more moisture over the 72-hour time period than the wood-screed finish. Obviously, as your test indicated, a machine, hard-troweled surface will retain even more moisture.
This has been proven to me over the years in tilt-up construction where it has been common place for contractors to successfully use bond-breakers (that do not meet ASTM C309) as curing compounds on their casting/floor slabs without any evidence of inadequate curing. Maybe some day a less restrictive curing specification will be developed for hard-troweled floors that replaces ASTM C309.
As for your concern about the adhesion of floor coverings on hard-troweled floors, burnished floors are common in tilt-up construction and often have floor coverings. Assuming proper surface preparation is performed, good adhesion can be obtained.
Another concern that needs to addressed is the use of pozzolanic materials in the concrete mix, which tend to slow down the set time and strength development contributing to more moisture loss. Thanks for performing your test and supplying the information to the industry.
Michael A. Linn
Nox-Crete Products Group Inc.
Thanks for the added information and your thoughts. Regarding adhesives applied to these finishes, the increase in pH at the surface may contribute to adhesive failures.
CONCRETE CONSTRUCTION would like to obtain cylinders from a similarly finished floor outside a building enclosure, preferably in an arid climate so that permeability studies can be performed. This would help to identify differences (if any) from the indoor slab in our study.
— CONCRETE CONSTRUCTION Editorial Staff
Very interesting article. We are having a lot of problems in Ontario with failed vinyl floors. Do you think that this finish would qualify as equivalent to a surface membrane allowing sheet vinyl to be applied earlier than current conditions?
Testing with the MVER kits would provide information on the moisture emissions at various times after placing the concrete to determine if the emissions are below the 3-pound threshold of most adhesives.
Brampton, Ontario, Canada
The dense hard-troweled surface region retains moisture in the floors but does not have nearly the resistance to moisture required for an ASTM E1745 vapor retarder. Therefore, a hard-troweled surface does not provide enough moisture protection over a wet slab for adhered resilient floor coverings. The dense surface also likely reduces the measured moisture vapor emission rate (ASTM F1869) giving a false low result, which does not indicate the true moisture condition deeper in the slab.
—Howard Kanare, senior principal scientist, CTLGroup, Skokie, Ill., responding on behalf of CONCRETE CONSTRUCTION