Q: How does hard-troweled air-entrainment impact strong concrete?
A: It’s pretty well known in the concrete industry that hard-troweled surfaces and air-entrained concrete don’t mix. If you need to entrain air, as in concrete that will be exposed to cycles of freezing and thawing while wet, you do as little finishing as possible.
If you want strong concrete, one with a hard, smooth, dense surface, you don’t entrain air in it. ACI 302.1R says, “An air-entraining agent should not be
specified or used for concrete to be given a smooth, dense, hard-troweled finish because blistering or delamination may occur. These troublesome finishing problems can develop any time the total air content is in excess of 3%.” But why is that?
The solid ingredients in concrete – cement, slag cement, fly ash, fine and coarse aggregates – are all denser than the water. Once the concrete is placed, the solids settle and the water begins to appear at the surface. We call this “bleeding.” Concrete finishers must wait until the bleeding stops before they start working. The reason for this is that hard troweling densifies the surface so that water becomes trapped below it.
Under normal circumstances, it’s pretty easy to tell when the concrete is bleeding – the wet sheen on the surface is plain to see. But when the surface dries faster than the bleed water appears, it’s harder to tell. On a hot, dry, windy day it’s obvious that the surface will dry quickly. That’s why it needs to be protected from drying.
But air entrainment slows the bleeding. It’s hard to tell whether the bleeding has stopped when it’s slow to begin with: is the surface dry because the bleeding has stopped or because it’s too slow to keep up with the drying? It looks pretty much the same to the finisher.
You can see from the microscope photo that the dense, hard-troweled surface looks darker than the rest of the concrete. You can also see about a half inch below the surface where bleed water was trapped. It leaves a void that may eventually cause delamination.