We supplied concrete for a slab that now has a number of cracks in it. The owner thinks drying shrinkage caused the cracks and believes our concrete is at fault, but the concrete was placed on a hot, dry, windy day, so we think the cracks were caused by plastic shrinkage. Can visual examination give clues as to the cause of cracking?
The photos show examples of typical drying-shrinkage and plastic-shrinkage cracking. Plastic-shrinkage cracks are generally randomly oriented and usually don't extend to the edge of the slab.
 Plastic shrinkage cracks usually don't extend to the edge of the slab.Portland Cement Association
Drying-shrinkage cracks are typically perpendicular to the long dimension of the slab, often occur at locations where concrete width or thickness changes (such as at a re-entrant corner) and usually extend to the edge of the slab.
 Drying shrinkage cracks usually extend to the edge of the slab.Ward Malisch