Flaking Concrete Slab - Why? 1

We just had a concrete patio poured at our church. Right after we were allowed to walk on the patio we noticed some dime-sized (circumference and thickness) depressions where it appeared as though a layer of the top had flaked off. Those areas have now grown in size but not in depth. It truly is that the top layer is "flaking" off. What caused this and what should we do to have it rectified? Does our contractor need to cutout and repour? We are not getting many straight answers...>Church Patio.

 

[JAE]

It is probably delamination due to incorrect construction workmanship. When you place concrete and first strike it off, it is important to let it sit for a short time as excess bleed water in the concrete mix will migrate to the top surface. You will actually see the bleed water appear slowly and the correct course of action is to squeegy it off BEFORE you do you finish trowelling. All this happens while the concrete is still plastic.

If the contractor didn't wait for the bleed water, but went ahead and trowelled right away, he was essentially sealing off the top surface with a thin, tightly condensed layer (dime size thickness?) of cement. The bleed water rises, and then cannot work through the top layer, remaining in place, just below the surface. After hardening and the water eventually disappearing, you have a thin layer of concrete surface, with possible voids or weak layers directly below.

Thus, you get the flaking off of the hard surface.

The fix? You've got a poor surface from which to work....with the weaker concrete and all.. I would suggest having the contractor do either one of two things (others in this forum can also chime in!)

1. Remove the slab and start over.

2. Have the contractor grind down the top 3/4" of surface, leaving it roughened. Apply a bonding agent to it, and apply a patching material or other concrete overlay. Not knowing where you reside its difficult to come up with an exact solution. If this slab is of considerable area, and expense, you might want to bring in a local engineer to study and recommend a solution.

Option 2 above will not give you, really, what you paid for as the "patch" will eventually work loose if you have freeze/thaw action and/or you have significant wheel loads on it (patio tractor )

 

[phuduhudu]

I would agree with JAE though I think it should be possible to apply a screed which will stay on after removing the top surface. You should talk to a local engineer or direct to a local construction chemical supplier to find the right product and application method. The other option you could offer the contractor is to remove the loose surface and then tile it at his expense (or you might negotiate a share of the cost if that is what you want). Otherwise your fall back position is always to say give me what I paid for. Carl Bauer

http://www.bauerconsultbotswana.com

 

[jheidt2543]

JAE has the cause right but, I would suggest a little bit different fix. Use a terrazzo grinder to take off the layer of poor concrete, about 1/4" thick as you have described it. Wash the slab clean or use a shop vac to take up the ground slurry. Don't dispose of this down the sewer! Then, coat the slab with a liquid hardener (not a sealer), there are many types, but "Lapidolith" is the easiest to work with, follow the directions and use two coats. You can get on the slab almost right away. I've used this many times and it is quick, inexpensive and give very good results if the problem is only in the surface of the slab.

Good Luck!!