Question: I do a lot of troubleshooting on residential concrete flatwork -- mainly sidewalks or driveways. Is there a way to tell the difference between structural cracking caused by overload or soil movement, and nonstructural cracking caused by volume changes related to drying shrinkage or thermal contraction? I sometimes suspect that cracks I see were caused by driving a vehicle on the driveway too soon after it was placed, or driving on a sidewalk slab that wasn't intended for that heavy a load.

Answer: Structural cracking in driveways or sidewalks often occurs diagonally across corners or as curved or half-circle cracks at the edges of a slab (see drawing). If the crack is caused by soil settlement, the slab on one side of the crack may drop below the adjoining part of the slab.

Nonstructural cracks are most commonly caused by plastic shrinkage, thermal effects such as cooling, or drying shrinkage. The time at which the cracking occurs gives a clue to the cause.

  • Plastic-shrinkage cracks appear within hours after concrete placing, are usually parallel to each other, and often don't extend to the slab edges.
  • Cracks caused by cooling appear within several hours to a few days after concrete placing and generally extend from edge to edge of the slab. That's about the same time you might see cracks caused by early loading.
  • Drying-shrinkage cracks normally appear weeks to months after concrete placing and also generally extend from edge to edge of the slab. They are often indistinguishable from cracks caused by cooling.

For more information about differences between structural and nonstructural cracking, see the list of references.

References
  1. Harvey Haynes, Avoid Cracks in Concrete Slabs-on-Grade, (videotapes and summary notes), Moyer Publishing, 2 Moyer Place, Oakland, CA 94611.
  2. Non-Structural Cracks in Concrete, Technical Report No. 22, The Concrete Society, 1992, 48 pp.