Q.: In the past year or so I've had problems with scaling and delaminations while placing and finishing floors. Lately it's been occurring more often. I've been in business on my own for 6 years and have made a good name for our company. We work hard and get very frustrated when this happens. We show the customer the problem and explain the causes, reasons, and how this happens, but I'm still the one who has to fix it, and the company gets a bad rap for shabby work. I wonder if you have any suggestions that would help me prevent this from happening again.
Here's a situation we recently faced:
- On a fall morning, about 40 degrees, sunny, it warmed to about 50 degrees through the day and cooled back down in the afternoon.
- Concrete was placed on 1/4 to 3/8 size pearock.
- The concrete mix included 3500# natural rock, no fly ash with 2% calcium chloride, and air entrainment.
- We used a Morrison "super-screed" for the larger section of the pour and a Lindey "Vibra-Strike" for the smaller portion.
- After screeding we immediately bullfloated the surface with a magnesium bullfloat.
We wanted to use a wood bullfloat, but we figured the sun would flash set the floor, so we opted for the mag bullfloat. We waited as long as we possibly dared and started with a walk-behind trowel with combo blades. We left no imprints at all, but it did open the surface allowing it to bleed a little.
As the floor started to get hard we started finishing with a razor-back riding trowel with pans to open the surface more. As the floor got harder, we removed the pans and started troweling operations. At the same time, the top was getting hard because the sun was drying the slab fast. As I was finishing I noticed a few blisters, so I threw the pans back on to open the surface. After this I achieved a good polish and thought everything was fine. But the next morning I walked the slab and the surface began to ôpopö under my feet. There were large areas of delaminations and scaling.
Why is this happening and how can we patch the problem areas? Most of it will be under carpets.
A.: There are probably two factors causing the problems you have described. You mentioned that your mix design includes air entrainment. Some ready-mix producers include 1% or 2% air entrainment in concrete used for floor construction. This small amount helps to control excessive bleed and should not pose any special problems for finishing with power trowels. But air entrainment dosages designed to provide freeze/thaw resistance could cause the scaling problems you describe. If the slab you are casting is for outdoor use, you shouldn't use power trowels because they can cause the problems you describe. They also will deplete the air entrainment in the top surface of the concrete. Freeze/thaw scaling will result.
The conditions you describe may also be ideal for surface crusting problems. If the subgrade and ready mix are fairly cool and the sun causes the surface to get hot, the top layer of concrete dries out and initial set occurs more rapidly than the concrete below. Finishing becomes very difficult as a result. If you know this after the concrete is placed, fogging the area and keeping the surface open as long as possible will help. If you know conditions are favorable for surface crusting, the accelerator you added to the concrete will help the bottom thickness of concrete to set faster than the top making finishing steps easier. But you have an adequate number of workers to handle the situation.
Regarding repairing scaled areas, be sure that all unsound material is removed. Then, use polymer cements (there are many different brands in the market place) to make the repairsùand they should bond and perform very well over time. Any patch material is difficult to color-match with surrounding concrete.