Slab cracks in slab foundations can cause great anxiety to homeowners. However, many slab cracks do not pose a structural problem to the foundation. For example, many times, slab cracks appear soon after the concrete is installed and are usually caused by poor quality control by the concrete contractor. Most of the time, these types of cracks are hairline in width and are only superficial (do not penetrate deep into the slab) – however, there are exceptions. Generally speaking, these types of cracks are related to the excessive loss of moisture in the concrete and are called plastic shrinkage cracking. Plastic cracking is related to many factors during the slab installation process, including: wind, low humidity, excessive water content in the concrete mix, exposure to the sun, etc. These types of cracks will not usually have a negative impact on the structural ability of the slab.
The cracks that concern us the most are the cracks caused by foundation movement. Most of these types of cracks are hairline to 1/16” wide or so, however, I have seen slab cracks that are ¼” and wider. Many times, an extensive network of slab cracks is an indication of poor rebar placement and/or significant foundation movement.
Unless a slab crack is covered with floor tile (which will also crack) or the slab crack allows excessive moisture penetration from the subgrade soils, most slab cracks cannot be easily detected under carpet or wood flooring.
The usual remedy for a slab crack caused by foundation movement is twofold:
- Stabilize the slab so it cannot move. This is typically done by installing piers.
- Epoxy inject cracks that are 1/32” wide or wider. This type of work needs to be done by an experienced technician.
There are other reasons concrete slabs crack but that is another topic for another day.
Jim McNeme, P.E.