A.Bill Palmer, former editor of Concrete Construction magazine and president of Complete Construction Consultants in Lyons, Colo., responds: Cracks seldom degrade from freeze-thaw action, since water within them isn't confined and therefore can't cause damage when it freezes. In addition, most "hairline" cracks are surface cracks; they don't go all the way through the slab, so water can't get through and degrade the subbase. For cracks less than .04 inch wide (about the thickness of a standard paper clip wire), the aggregate interlock will prevent any differential movement across them, and repair is unnecessary. So if these truly are hairline cracks, the best approach is probably to do nothing: Whatever crack-repair technique you use will look worse than the crack.
Even a crack wide enough to allow water all the way through the slab shouldn't cause any damage if there's a well-compacted and well-drained subbase; without that, uneven settlement could occur around the crack. But keep in mind that cracks this size are active — they widen in cold weather and narrow in warm weather — so any sealant must have...
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