The Inside Outside Guys: That sinking feeling concrete slabs, porches and patios – The Detroit News

Ken Calverley and Chuck Breidenstein, Special to The Detroit News Published 5:53 p.m. ET Jan. 14, 2021

If you are a homeowner, you know the feeling. You walk into your garage or on to your front porch and note the concrete is sinking! It wasnt that way yesterday, was it? What happened?

The Guys talk all the time about the importance of base and supporting soils under concrete and asphalt.

Keep in mind that the ground beneath a construction ultimately supports and distributes the load from the building or driveway. If that soil is not well compacted or it disappears or shifts in any way, it can impact whatever is resting on it.

When we begin to build a house or place slab-on-grade concrete for sidewalks, patios and driveways we first seek undisturbed soils. These would be soils that have not been dug or otherwise manipulated for many years. Undisturbed soils are not as likely to further settle or compact. We can create such soil conditions through measured compaction with mechanical devices.

Driveways need the proper material below them for stability.(Photo: Paul Brennan , Pixabay)

Soil composition is also important. Undisturbed clays can be a good base in many cases. But clays have two faults. They can absorb and retain lots of water which makes them dimensionally unstable. Clays that retain water in freezing conditions can lead to frost heave which will lift concrete. Expansion and contraction due to gain or loss of water in the clay can lead to shifting concrete as well.

Soils that are well-drained and well compacted are ideal. Well-drained soil mimics the sand on the beach your kids play in; no matter how much water they take from the lake to dump in that castle moat they are building, the water simply runs down through the sand.

But sand under concrete has issues as well. Water running adjacent to a concrete driveway may sweep away the sand beneath it and undermine the slab. The Guys have seen this many times in brand new drives and sidewalks.

A great alternative is to use a product like crushed concrete or limestone on top of an undisturbed base. It will compact and drain and it is less likely to ever wash out. When your slab is placed on this base, it is formed such that the slab is above the surrounding grade and it will shed any surface water. A cross slope of 1 inch in 60 inches is enough to shed surface water.

When surface water is shed and directed away from the slab and subsurface water cannot accumulate below the slab, the slab will not shift due to weathering conditions as long as the base remains stable. At that point the biggest danger to your slab is overloading. If you allow a too heavy truck or piece of equipment on the slab it could create damage.

As to porches and garage slabs, the same thinking applies but with additional considerations. When we excavate to build a basement, we over-dig; that is we dig a bigger hole than the construction will occupy. When we fill this back in or backfill we should again use readily compacted, well drained materials like sand or pea-gravel. Many builders simply use native soils like the clay they took out.

The use of native soils may sound ecologically friendly, but from a construction standpoint it can possibly wreak havoc. Clays can take years to settle and when they do whatever they support will settle as well; like your garage slab or patio or front porch.

We should pin these slabs and porches to the house foundations with steel rod or even provide a shelf or notch in the foundation for the concrete to rest upon. In theory, the house foundation should never move or settle so tying these constructions directly to the house foundation can virtually assure they will never move, either.

Keep in mind there is a solution for those unbroken, settled, slabs. Brett Butler from A-1 Concrete Leveling says they can save most such slabs and add years to their serviceable life.

Now you better understand the potential problem and are armed with good solutions!

For more Home Improvement advice listen to the Inside Outside Guys every Saturday and Sunday on News/Talk 760, WJR-AM from 10 a.m. to noon or contact us at insideoutsideguys.com with your questions.

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The Inside Outside Guys: That sinking feeling concrete slabs, porches and patios - The Detroit News