That sinking feeling – concrete slabs, porches and patios

If you are a homeowner you know the feeling. You walk into your garage or porch and find that the concrete is sinking! It wasn’t like that yesterday, was it? What happened?

The guys talk about the importance of sub and support floors under concrete and asphalt all the time.

Remember that the floor under a structure ultimately carries and distributes the load from the building or driveway. If this soil is not well compacted or disappears or shifts in some way, it can affect everything that rests on it.

When we start building a house or placing concrete slabs for sidewalks, patios and driveways, we first look for “undisturbed floors”. This would be soils that have not been excavated or otherwise manipulated for many years. Undisturbed soils are less likely to continue to settle or compact. We can “create” such soil conditions through measured compaction with mechanical devices.

Soil composition is also important. Undisturbed sound can be a good basis in many cases. But clays have two “flaws”. They can absorb and hold back a lot of water, making them dimensionally unstable. Clays that retain water in freezing conditions can cause “freeze swings” that lift the concrete. Expansion and contraction due to water gain or loss in the clay can also cause concrete to shift.

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 throw in the moat they are building, the water just runs through the sand.

But sand under concrete also has problems. Water flowing next to a concrete driveway can sweep away the sand underneath and undermine the slab. The guys have seen this on brand new drives and sidewalks many times before.

A good alternative is to use a product such as crushed concrete or limestone on an undisturbed surface. It thickens and drains and is less likely to ever wash out. When your slab is placed on this base it is shaped so that the slab is above the surrounding degree and surface water drains away. A 1 inch by 60 inch bank is enough to drain surface water.

If surface water is thrown off the slab and drained away and water cannot collect under the surface under the slab, the slab will not shift due to weather conditions as long as the soil remains stable. At this point, the greatest danger to your disk is overload. Leaving a truck or equipment that is too heavy on the plate can cause damage.

The same thinking applies to porches and garage panels, but with additional considerations. When we dig up to build a basement, we “dig”; That means we’re digging a bigger hole than the structure will occupy. When we fill this up again, or “refill” it, we should again use easily compact, well-permeable materials such as sand or pea gravel. Many builders simply use “native floors” like the clay they took out.

Using native soils may sound environmentally friendly, but it can potentially wreak havoc from a structural standpoint. Clays can take years to settle, and if they do what they support, they’ll settle too. like your garage slab or patio or porch.

We should “nail” these slabs and porches to the house foundations with a ½ inch steel rod, or even provide a “shelf” or notch in the foundation for the concrete to rest on. In theory, the house foundation should never move or settle down, so tying these structures directly to the house foundation virtually ensures that they never move either.

Remember, there is a solution for these unbroken, stepped panels. Brett Butler of A-1 Concrete Leveling says they “can save most of these slabs and add years to their life”.

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

For more DIY tips, listen to the Inside Outside Guys every Saturday and Sunday from 10 a.m. to 12 p.m. on News / Talk 760, WJR-AM or contact us at if you have any questions.

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