A slab foundation consists of footings for the perimeter, with a concrete slab that is the floor of the house, with no access underneath. Footings depths for single story houses are a minimum of 12” wide by 12” deep, reinforced with two pieces of ½” rebar placed horizontally. The concrete slabs are typically 4” thick, and preferably reinforced with rebar or wire mesh. Depending on its age, there may be a moisture barrier between the slab and the soil, likely embedded in a sand sub-base under the slab.
The quality of the slabs and footings installed has a direct impact on the future performance of the foundation. Older slabs are often placed without adequate reinforcing steel, or with steel that is not placed in the middle of the slab, making them more likely to crack. Steel without cover by the concrete are likely to rust, causing cracks and not performing to hold the concrete together. Slabs placed on expansive soil without a sand sub base are more likely to heave when moisture expands the clay. Leaking plumbing under a slab can affect the soil, causing slabs to crack. Tree roots can extend under a slab, lifting and cracking the concrete. Poor quality concrete or excessive heat during pouring can cause shrinkage cracks as the concrete cures.
Slab foundations were used starting in the late 1940’s. In San Diego at this time, more areas were developed, and in order to find flat lots to build on the hillsides and canyons were graded. Grading a lot flat entails cutting into the hillside on the uphill side, and using that soil to push down the slope. Often, the soil was not compacted, which entails using machinery to pound the soil as it is placed in layers on the downhill side of the lot. If not properly compacted, soil will compress, allowing the structure of the house to move. Also, the soil may have been placed without benching the slope, basically cutting steps into the slope prior to placing the fill.
Crack repairs for Slabs
Repairing slabs can be accomplished in several ways. Where there are small cracks in the slab (3/16” wide) with no height differential on each side of the crack, the typical repair is to fill the crack by epoxy injection. This method of repair includes attaching ports to the crack on the top, sealing the crack with epoxy, and then using the ports to inject epoxy into the crack with a pneumatic epoxy-mixing tool. Then, the ports and excess epoxy are removed. If done correctly, the epoxy fills the crack, bonding it together stronger than the original concrete.
Where there is a larger crack, or the crack has differential, repairs in addition to injection can be implemented. Rebar stitches consist of reinforcing steel laid in slots cut into the concrete perpendicular to the crack, then filled with an epoxy grout. This extends the repair past the crack area, strengthening the slab.
When a slab is cracked in multiple directions, or where there is heaving or settling of the slab, a portion or the entire slab can be removed and replaced. This consists of saw cutting the concrete slabs, removing the slab and a portion of the sub area, and then pouring a new slab. The new slab should be placed on top of a minimum of 4” of sand or decomposed granite, with a moisture barrier of 10-mil plastic sheet. The new slab needs to have the reinforcing steel epoxy dowelled into the remaining slabs and footings.