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We placed some concrete for a porch recently. It was placed on gravel backfill and rests on the concrete block foundation at the outside of the porch. We installed a fair amount #4 steel in the slab and sealed the slab with a curing compound when we were finished with the surface work. A week or two later, we noted that at the outside corners of the block porch wall, the block had been lifted up about a 1/16th of an inch. There is no settlement. The block had been lifted! The slab appears to have curled there. My first reaction is that the slab simply cured faster on the top surface than the bottom surface which was gravel and probably damp. The curing compound is supposed to stop this, however, so I am not sure what the mechanism is. No one seems to have had a similar experience as far as the concrete finishers or block subs are concerned. In retrospect, we should have isolated the block from the slab, but the slab would still have curled. Any thoughts?
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My profession is the auto business, and I've done a lot of work with the state agencies responsible for road smoothness. Yes, concrete does curl. Road slabs will curl at the expansion joint, with each side rising by about 1/16" to 1/8". Temperature is the cause, so the road can be perfectly smoothe in the morning, and by afternoon, that small rise will cause a terrible ride, especially if the joint spacing, vehicle speed, and frequency of the vehicle all line up. I do know that with as little as 0.040" input, I can shake a vehicle to death with the correct frequency input.
I would consider a temperature variation, and how your slab is restrained, as things to look at as the cause of the rise.
*Joe, This is a common problem. You must isolate the slab from the block or it will pull and crack them every time. You can use almost anything that will keep the two surfaces from contacting each other. This is a good lesson for other people attempting to do the same kind of job.
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We placed some concrete for a porch recently. It was placed on gravel backfill and rests on the concrete block foundation at the outside of the porch. We installed a fair amount #4 steel in the slab and sealed the slab with a curing compound when we were finished with the surface work. A week or two later, we noted that at the outside corners of the block porch wall, the block had been lifted up about a 1/16th of an inch. There is no settlement. The block had been lifted! The slab appears to have curled there. My first reaction is that the slab simply cured faster on the top surface than the bottom surface which was gravel and probably damp. The curing compound is supposed to stop this, however, so I am not sure what the mechanism is. No one seems to have had a similar experience as far as the concrete finishers or block subs are concerned. In retrospect, we should have isolated the block from the slab, but the slab would still have curled. Any thoughts?