The following are the most common that we encounter. The joints were not cut deep enough to induce the crack, leaving the slab to crack randomly. If the slab thickness varies, the slab contraction will encounter restraint, causing the slab to crack. The shrinkage stresses in the slab exceeded the strength of the concrete before the joints were cut. The base under the slab must be smooth and evenly, densely compacted.
Uneven bases create restraint, causing the slab to crack.
If the dowels at construction joints are misaligned, or if rebar is used instead of smooth, greased dowels, the joints will be restrained and random cracks will occur, usually parallel to the joints. In tilt-up construction the crane's outrigger may rest on a panel corner and break it off. All concrete curls upward at the ends of pours. If this curl is severe enough, leaving the panel end cantilevered, the slab may crack when loads are imposed. Additives may cause excessive curl.