In reviewing the structure, CES found the occurrence of hairline cracks in the slab. Based on our observations and analysis, the cracks in this foundation fall under the category of “shrinkage” or “curing” type cracks. Shrinkage cracks are very common in post-tension foundations and do not constitute a structural concern.

Shrinkage cracks start at the onset of cement hydration. For the concrete to be workable enough to be placed, virtually all concrete is produced with about twice as much water as required to hydrate the cement. Because water can only evaporate from the upper surfaces of slabs on grade, uncombined water creates moisture gradients between the top and bottom of the slab. Evaporation of moisture from the top surface of a slab causes the upper half of the slab to shrink more than the lower half, although some shrinkage occurs in all three dimensions. This is especially true during hot temperatures and/or windy conditions. The resultant cracking can often run across the entire width of the foundation and can be in excess of 1/32” in width. Shrinkage cracks can become slightly agitated by some post-construction movement but that is to be expected. One of the advantages of post-tension foundations slabs-on-ground is the control of shrinkage cracks. Typically, shrinkage cracks do not affect the integrity of the foundation.

Shrinkage cracks can be reduced by limiting the amount of total water content in the concrete mix (but not less than the required water-cement ratio) or by the addition of a sealer on top of the slab immediately after the slab is poured.

Where tile is to be placed where shrinkage cracks currently exist, a flexible plastic membrane or elasticized mastic material should be used over the shrinkage crack(s). When tile is placed over existing cracks, vibratory cracks can occur where the shrinkage cracks exist as the vibrations run through the foundation and are transmitted to the tile at the cracks. The membrane buffers the vibratory effects and controls future cracking.