“You groan deep down inside and instantly know one of 2 things…”
Its every concrete finisher’s nightmare.
It is two weeks after the floor is poured and you get a call from the customer. He says, “Something sounds hollow under the surface of my floor, and one spot is flaking a little bit. If I kick a stone across it or tap with a hammer, some places it sounds fine, and other places it sounds hollow or like it could crack or something.”
You groan deep down inside and instantly know one of 2 things. Either you know the concrete was air-entrained and you hard-troweled it, or you fully thought it wasn’t air-trained but somehow through miscommunication or accident it was anyway.
You pull a logging chain out of the trailer toolbox or the corner of the shop and drive out to the jobsite. In the 2 weeks since the floor was poured, the overhead doors have been installed and the interior nicely lined and trimmed. You drag the chain around on the floor and listen for good sounds and bad sounds. Sure enough, about 20 percent of the floor sounds hollow and crackly. The spots are scattered all over the floor. There are no isolated areas or rhyme to it.
What to do?
There are 2 questions that immediately surface. Whose fault is this? And what can we do to fix it.
It’s complicated. Theoretically it’s the air-entrainment’s fault. If the air wasn’t in the concrete, it wouldn’t have happened. Furthermore, it’s the batch plant’s fault, you asked for non air-entrained, but they sent air-entrained anyway. But the call-recording and ticket system was down that day so nobody can prove who said what.
The harsh part is, regardless of what happened, it can always be technically blamed on the finisher.
What causes the hollowness is when the surface is troweled and sealed off before the slab is done bleeding. The leftover bleed water collects under the finish, and after some time it evaporates or dissipates, leaving a void under the surface. Air-entrainment vastly reduces the speed of the bleeding process and typically the slab is firmed up and ready for machines before the bleed-water is out. This can also happen on slabs with no air-entrainment. For example, if the concrete is heated and has calcium in it, it may flash dry when poured and you need to get machines on right away. The bleed-water can be trapped, and you may experience delamination.
So, while the rule book may say you should never steel-finish air-entrained concrete, in purely technical terms, if the slab would have been done bleeding before putting a machine on, this wouldn’t have happened.
Both ways it’s your fault.
How to fix it? Typically, the conversation goes something like this. We could try to chip out the bad spots and patch it, but that would look terrible. We could perhaps grind down and polish the whole floor. But it will take a lot of grinding because the bad spots are already ¼ inch lower than the good spots. So, to get consistency it will take a terrible amount of grinding and be very expensive. We could fill in the bad spots with patch material, grind everything a little bit and apply a coating, but we run the risk of the coating not adhering properly. We could repour 2 inches over the whole floor, but all the doors are already set, and the interior trimming is done.
Or we could tear out the whole floor and start over. 9 times out of 10 this is the most economical option.
When air-entrainment is specified on an interior finished floor, the first question is whether it was a simple copy and paste without thought by the designer. Even if its code, such as a residential garage floor my rule of thumb is to cut it back to low or none. From my experience failure is more likely to happen from finishing air-entrained concrete than it is from excluding the air altogether. When finishing an interior concrete floor in an uninsulated, unheated environment that may be susceptible to moisture, I can cut the air back to 3 or 4 percent, make sure you let it bleed as long as possible, finish it smooth but do not polish. Usually this is a good compromise. Pouring on vapor barrier or insulation exacerbates the challenge as the bleed water can only go up. Pouring on dirt offers more forgiveness as the water can escape into the subgrade.
This finishing with air dilemma happens to every finisher at some point. All the above has happened to me throughout my short concrete career. Like so many things, the way the world is supposed to work and the way it works every day on the job does not always coincide. That concrete floors constantly get spec’d with air and simultaneously are expected to be polished, is one of those things.
When failure occurs, our first impulse is to point fingers at someone, but too often they can technically be pointed back at us.
So more importantly, keep a good relationship with your ready-mix supplier. When you experience delamination and communication seemed the primary culprit, that relationship just may ease the pain and save the day.
