Slab foundations consist of perimeter footings with a concrete slab that is the floor of the home. Typically, these footings are a minimum of 12” deep and 12” wide with horizontal reinforcing steel (rebar) inside the footings for extra strength. The concrete slab portion of the foundation is usually 4” thick and generally reinforced with rebar or reinforcing steel. In areas of very expansive soil, some builders have used post-tensioned foundations, which consist of a slab & footing system utilizing steel cables tightened through the slab. In San Diego County, most slab-on-grade construction was started in the mid to late 1950’s tract developments. This continues to be the prevalent foundation system used in new construction, except where the building sites are on hilly terrain, etc.
It is important to note that not all cracks in a concrete slab are the result of some differential settlement or distress. A significant portion of floor slab cracks are harmless drying shrinkage cracks that do not get worse, and are structurally insignificant. Usually they exhibit no vertical displacement and are generally less than 1/8’ in width.
Structurally significant slab cracks occur when the foundation has experienced distress. This occurs when the footings have either settled or heaved which in turn puts pressure on the slab and causes a crack. A floor level survey is performed using an electronic digital manometer to determine the amount of the differential settlement has occurred by measuring the high and low points of the slab. The structural significance of the crack is best determined by this survey along with the findings from a complete examination of the superstructure of the building.
Foundation Movement Indicators:
- Unlevel floors, cracked or broken floor tiles, ripples in vinyl flooring
- Windows that are hard to open and close
- Doors that stick or bind in the door frame; doors that are out of square
- Cracks in interior walls and ceiling
- Diagonal cracks above doors and windows
- Significant cracks in exterior walls and surfaces
- Chimney cracked or pulling away from structure
- Horizontal or vertical cracks in foundation footings
- Patios, flatwork, or garage slab separating or migrating away from exterior walls
- Horizontal or vertical cracks visible in footings
Repair Methods for Slab Foundations:
- Epoxy injection by high pressure
- Epoxy injection with steel rebar reinforcements (aka slots, stitches)
- Sawcut and replace portions of existing slab
- Pin with rebar & epoxy (construction joints, additions, etc.)
- Deepened Footings
- Underpinning / Stabilization by installing pipe piles and or grade beams.
- Drainage repairs and improvements