Question: In the February 2013 issue of CC, we ran an article on garage floor slabs. We received a couple of questions about the recommendation to use air-entrained concrete for a garage floor, where the owner is typically going to want a troweled surface. Can’t hard-trowelling air-entrained concrete lead to delaminations? We asked our friend Jerry Holland, Structural Services Inc., what he thinks.
Answer: The trowel versus air-entrained concrete issue is a tough balancing act. These difficulties and compromises should be discussed with the owner, who should be shown similarly finished garage slabs and give approval of the finish ahead of the slab construction (and remember that all important discussions should be followed up in writing). Another discussion to have with the owner is that the garage slab must be kept as clean and dry as feasible in cold weather. The options are:
Light troweling with air-entrainment
If the owner decides to go with a lightly trowelled air-entrained garage slab, even in the northernmost or mountainous locations, I suggest considering a moderate air content (4.5 percent +/-1.5 percent) rather than the more typical 6.0 percent that is used for an exterior broomed slab in severe freeze-thaw areas. The reasons for this are twofold. First, the potential for delaminations increases exponentially as the air content increases. And second, even lightly troweled slabs decrease the moisture and chloride ingress into the top of the slab, thereby making it less likely that the slab will be fully saturated to a significant depth when freeze-thaw cycles occur.
When troweling air-entrained concrete, I suggest the contractor consider the following points:
- Delay each finishing step as long as possible.
- Through the finishing steps, keep the surface as “open” as feasible, and finally close the surface as late as possible.
- When lightly troweling the surface at the end of finishing, “leave some fuzz on the surface,” as the finishers say.
Troweling without air entrainment
If the owner decides not to air-entrain a garage slab in a severe freeze-thaw environment, I suggest troweling the surface as densely as feasible to minimize moisture and chloride ingress, and use a silane sealer.
Another discussion the contractor should have with the owner has to do with deicer usage on the driveway leading up to the garage (which can be directly tracked into the garage). For new concrete going through its first season of snow and ice, use sand only and no deicer. Sand can improve traction and slip-resistance but will not act as a deicer. However, if a deicer appears to be necessary for safety, it should be used sparingly and everyone should understand that the propensity for concrete surface disruptions is increased (both outside and inside the garage). For the second winter, in addition to sand, use a sodium chloride deicer, such as rock salt. Other deicers tend to attack the concrete surface.