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Why is Your Concrete Slab Settling?

No matter how long you have had supported concrete slabs, it isn’t uncommon for homeowners to eventually experience unsupported slabs. This settlement can be shown by the unleveled, cracked, and/or sinking appearance of the concrete. 

uneven concrete

How does this happen? Settlement  can occur due to the various changes that the soil underneath the concrete can endure.

There are three main causes of settlement:


1. The Change in Moisture Content

The type of soil that is used underneath the concrete is crucial to recognize before establishing the impact moisture will create. There are two basic types of soil that are most commonly used: sand and clay.

  • Sand

Fortunately, sand isn’t extremely affected by the high or low moisture levels. When sand is wet or dry it doesn’t expand or shrink. The main issue with this soil type is that sand does wash out beneath the concrete. 

  • Clay

Contrary to sand, clay is highly impacted by the amount of moisture. Clay will expand and shrink according to the amount of moisture, leading to cracked and unleveled concrete.


2. Wash-Out Under the Slab

• When large amounts of water settle around the soil supporting the slab, it’s likely that the soil underneath will wash away. This occurs due to heavy rains or plumbing leaks and usually occurs with sandy soils.


3. Poorly Compacted Soil

One of the most vital steps to ensure that the soil will be able to support the slab is how well the soil is compacted before concrete is poured. When the soil is poorly compacted, voids are formed. Voids essentially are empty spaces that do not provide any support to the cement above. These voids then produce cracks and breaks throughout the concrete.

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