My new house, built on a concrete slab, is about 60% done, framing and drywall are in. My concern is I have several hairline cracks throughout the slab. I purposely had the slab made thicker (6 inches) with heavier rebar and a fiber- type additive to hopefully minimize this from happening.
Is there anything I should do prior to having my flooring installed or something I can do to minimize what’s already happened? My contractor did tell me it was going to crack even though I made it thicker. – Larry B.
A crack in a slab of 1/8 inch or less is typically a normal shrinkage crack and not a cause for concern. If the crack is larger or growing larger (an “active” crack), or one side of the crack is higher than the other, then you may need to have the work reviewed by a structural engineer.
Crack Types & Solutions
There are three main types of cracks in concrete. Each has its own cause and strategies to prevent or minimize.
Plastic shrinkage cracks. These occur during the first few hours when the concrete is still in a “plastic” state. They are caused when the surface moisture evaporates too quickly, usually during hot or windy weather. Synthetic fiber additives can help reduce this type of cracking, but do little once the concrete has cured.
Drying shrinkage cracks. These occur as moisture leaves the concrete after the slab has hardened. The main cause is concrete that is too wet, referred to as a “high-slump” mix. The best solution is to use less water in the concrete mix. Concrete suppliers sometimes add water to make the concrete easier to work with, but this weakens the concrete.
Welded wire mesh can also help reduce shrinkage cracking, but only if it is placed in the middle or upper half of the slab, but at least 2 inches below the surface. Wire mesh also helps keep small cracks from growing. In too many cases, however, the wire mesh ends up on the bottom of the slab where it does nothing.
Shrinkage cracking can be managed by the use of control joints placed in the slab. Some contractors cut or form a grid of small grooves in the slab to keep the shrinkage cracks in an orderly grid, which looks better than random cracks, but functions the same way. If you are placing tile on the slab, it’s important the control joint line up with a control joint in the tile – easier said than done. So random cracking might be a better approach for tile.
Structural cracks. Concrete can support a lot of weight in compression, but is weak in tension. For example, a concrete wall can support tons of weight from above, but will crack easily if pushed sideways forcing it to bend. Similarly, a slab will crack if too much weight is placed in one spot, or if the soil settles unevenly, bending the slab.
A water leak in the piping under a slab can also lead to soil movement and hydrostatic pressure that results in cracking. Wet spots in and around the slab, and water leakage, are also symptoms of underground water leaks.
The best protection against structural cracking in residential structures is good compaction of the soil and gravel underneath the slab. In addition, rebar should be placed in the footings around the perimeter of the slab and at post bases within the slab.
Rebar is generally not needed in the field of the slab in residential projects and is difficult to place properly in a slab less than 6 inches thick. However, it can be useful in garages and other areas subject to structural loading. It may also be required in areas with problem soils or in seismic zones. To be effective, rebar should be placed near the bottom of the slab, about 1/3 of the way up. The rebar must be fully embedded in concrete, with sufficient coverage, to prevent corrosion. To keep the rebar in place during the pour, it is wired together are joints and supported by metal or plastic “chairs”.
Underslab Vapor Barrier
I strongly support the use of a vapor barrier below the slab to prevent moisture migration into the building. Concrete might look solid as rock, but it is highly permeable to moisture. This can wreck havoc with many types of flooring introduce excessive moisture into the home. Some contractors like to place a layer of sand over the vapor barrier, but most have adapted to placing the concrete directly on top of a heavy-duty vapor barrier. Read more on vapor barriers in concrete slabs.
If you are installing ceramic tile on the slab, you will definitely want to install a crack-isolation membrane on the slab before installing the tile. There are a number of excellent membranes on the market. If you are installing wood flooring, carpeting, or resilient flooring, no extra precautions should be necessary.
Symptoms of Foundation Problems
Once the slab is covered with flooring, it may be difficult to detect problems with cracking or uneven settlement. During the first few years, every new home experiences some degree of adjustment as the framing and other materials acclimate to the moisture and temperature conditions of the home, and react to daily and seasonal changes.
nail pops in the drywall and small joint separations at ceiling corners are normal and should be fixed by the contractor. Similarly, you may see some small gaps in hardwood flooring during the first heating season.
If you start seeing more extensive or systemic problems, however, you may need to call in the experts. Problems might include:
- Growing “active” cracks in concrete slabs or walls
- Sticking or gaps in doors and windows
- Cracks in tile floors
- Cracks in the drywall or stucco/brick at window corners
- Diagonal or “stair-step” cracks in drywall, stucco, or brick
- Floors that are sloped, humped, or out of alignment
- Displacement of cabinets, wood work, or other building elements
Any of these problems could indicate excessive or uneven settlement or other significant problems with the foundation or soil under the building. Get a structural engineer or other qualified professional to take a look. These problems can be difficult and expensive to address, but the sooner you start, the better. – Steve Bliss, BuildingAdvisor