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The concrete is too thin.
The concrete is the wrong mix.
There's no reinforcing, or insufficient reinforcing in the slabs.
There are insufficient, incorrectly constructed, and/or inadequate control joints, expansion joints, and construction joints in the slabs.
The base material isn't free-draining gravel.
The base gravel is too thin.
The base gravel wasn't compacted correctly.
The existing sub-base wasn't prepared correctly.
There may be other reasons, but the baloney about the slab "settling" into place is just that...baloney.
[This message has been edited by treebeard (edited April 09, 2003).]