floor slab curling

huangyhg

floor slab curling
i am investigating the cause of some uneven concrete floor slabs.  the building had been used for an ice skating rink.  the existing floor slab consists of a 3 1/2" slab on grade, in good condition, around what was the ice rink.  the new owner of the building hired a contractor to "fill in" the area which was the ice rink with a 3 1/2" concrete slab.  the slab was poured in sections approx. 85'x43' with zip strips placed creating slabs averaging 20'x 20'.  the new slab has cracks at about every 10' and has apparently curled adjacent to existing slab in some areas as much as 3/4".  the new slab was installed with heating tube.  the new slab was placed on 3" of crush aggregate with fines which was on top of the existing 1" foam insulation.  the issue is the cracking and more importantly the apparent curling which has created a tripping hazard.  the contractor is claiming that the owner turned on the heating system to soon and caused the cracking and excessive curling.  the owner apparently turned on the heat 30 days after the concrete was poured. i think that some of the curling was not curling but poor workmanship. does anybody have any experience with this or know of any references pertaining to curing slabs with heating tubes?
first of all, control joints should have been placed closer together. for a 3.5" slab, 10 feet would have been better. so, the cracking is not a surprise. curling usually occurs when the top of the slab dries faster than the bottom. in slabs, bleeding and pour curing tend do produce higher drying shrinkage potential at the top of the slab relative to the bottom. thin slabs and long control joint spacing tends to increase the chance of curling. closer joints would have reduced this potential for curling and cracking.
temperature gradient difference can also cause curling, so the heat can add to the problem.  there are also numerous other things that can be done to avoid curling but it would be an article in length. hope this helps.
agree with idecharlotte.
also - there probably isn't much you can do to "fix" the slab at this point since it is only 3 1/2" thick.  you could consider grinding the slab down level, but this would reduce the thickness to 2 3/4" thickness at the 3/4" curl points and this probably isn't acceptable.
quite the mess.
if curling is the cause, then it should be noticeable... not poor workmanship.
having said that, a good concrete contractor should have been aware of the outcome...
dik
heat adds to the problem... interesting thought.  
i assume that you mean difference in temperature, which could resally come into play here if, having been an ice arena for many years, permafrost developed below the slab.  i may be all wet here, but was a concrete core done to test for this possibility?  
actually, it's well below freezing, sunny, with four inches of snow on the ground, so i'm not all wet, just cold.
mike mccann
mmc engineering
the contractor is claiming that the heat in the slab caused the top of the slab to dry faster than the bottom (more than would normally occur) and that the insulation under the slab trapped the water on the bottom side of the slab.  he is also stating that the slab should have been cured for 90 days before the heat was turned on.   my comment on the poor workmanship was based upon the fact that not all of the slabs are raised the 3/4".  some of the corners of the slabs have moved less than 3/8".  we are going to measure the flatness with a 10' straight edge to see how the curling compares to the 3/4" measurement.  i think that if the 3/4" is all curling, then we should see it with the straight edge.
thanks for all your replies.
1) what type of curing method was used for the slab; liquid curing compound, moisture retaining cover (blankets), continuous application of water, nothing at all?
2) who specified the curing method?  were there specfications, or was it left to the contractor?  if there were not specifications, was the contractor the "expert" and supposed to know how to avoid curling with proper techniques similar to the above?
it is obvious the top of the slab cured faster than the bottom, creating the "curling" effect.  the real question lies with the responsiblity of knowing how to prevent the curling.
i would guess that the heating tubes in the slab had little to do with the curling, but rather that the frozen ground under the slab, as mike suggested, had a lot to do with it.  the surface of the slab would have dried and set much quicker than the cooler and protected bottom.  so i think that the contractor is right in suggesting that heat was an issue, but not the heat of the tubes.  does anyone really know when the curling occurred?