Concrete Settlement Common Around Nashville

Why is concrete settlement common around Nashville? When we are talking about slab settlement, we are talking about the concrete slabs that can be found in a number of areas in and around your home. These may include:

Interior Floor Slabs
Patios
Pool Decks
Driveways
Porches
Sidewalks
Steps
Garage Floors

Each of these areas may sink and settle for different reasons, but before we can talk about the causes of concrete settlement, it’s important to first understand the process of how concrete is poured. Once these basic steps for concrete placement are clear, the reasons for why the concrete settles will be very apparent. So let’s get to it.

1. Site preparation. The area where the concrete is poured is cleared of grass, debris, rocks, etc., and the site is usually dug down about 4”.
2. Forms are set. Forms, typically made of wood, are set to act as a temporary mold when the concrete is poured. This approach is used when pouring concrete for driveways, sidewalks, patios and so on. For the slab floor in your home, the footing and foundation walls serve as the form.
3. Pour the Concrete. Sometimes rebar is placed in a grid pattern to reinforce the strength of the concrete. However, this is very uncommon in older homes and almost never occurs when pouring concrete for sidewalks, pool decks, patios and driveways. Concrete is then poured into the forms on top of the soil and is leveled.
4. Finish the Concrete. The surface of the concrete is finished using tools such as trowels and brooms.
5. Concrete cures. The concrete rests and begins to cure (harden). This process takes several days or weeks.

What is Slab Settlement?

Simply put, slab or concrete settlement is the movement a concrete slab experiences when the soil below can no longer support the weight of it.

You may be asking yourself, “Hey, the soil used to support my (driveway, patio, garage, slab floor) just fine. So what changed?”

Most likely, the soil changed. And while there are many reasons the soil may change underneath your concrete slab, let’s focus on the three most common reasons:

1. Changes in the soil’s moisture content
2. Wash-out of soil under the slab
3. Poorly compacted fill soil

Changes in the Soil’s Moisture Content

You will notice in Step 3 of the Basic Steps for Concrete Placement, we bring to your attention that concrete is poured “into the forms on top of the soil.” So let’s talk about soil and its relationship with moisture.

The most common soil type in Middle Tennessee is clay soil.

When clay is wet, it holds onto the water and expands in size. When it is dry, it shrinks.

Look at this sample of moist clay soil. When this soil is allowed to dry out, even the naked eye can see how much the soil shrinks. Imagine how dramatic this is when several feet of clay soil exists below your concrete slab – whether it be your driveway, patio, or beneath the concrete floor in your home!

Concrete Settlement

Imagine how dramatic this is when several feet of clay soil exists below your concrete slab

Soil can dry out for several reasons, including drought conditions, tree roots drawing valuable moisture from the soil, and leaking HVAC systems under your floor slabs. In any case, as the soil beneath your concrete slab shrinks in size, it creates an empty space for the slab to settle into. The result? Cracked and unlevel concrete.

Wash-out of Soil Under the Concrete

As water moves beneath your concrete slab, it washes away the soil that’s supporting the weight of the concrete. This is caused by plumbing leaks, erosion, large rains, and a multitude of other reasons.

Poorly Compacted Fill Soil

During the construction of a home, driveway, patio or sidewalk, soil is commonly moved around or spread out to get to the desired grade level. Oftentimes, the concrete slab is poured right on top of these fill soils that have been moved around. If the fill is poorly compacted, the fill soil compresses and settles, and a void is created under the concrete. Again, with nothing to support it, the concrete cracks, breaks, and settles into the void.

Concrete Settlement

If the fill was poorly compacted, the fill soil compresses and settles, and a void is created under the concrete.