Strong soil drains and compacts well, with a uniform surface that settles evenly. Uneven settling from moisture in the soil or freeze-thaw action creates tension in a concrete slab and results in cracks that can lead to structural deterioration. Footings distribute the weight of the house and transfer it to the soil. The greatest pressure is on the soil immediately beneath the footing. This material needs to be well-compacted, with organic and clay soils replaced with granular material.
Differential settling of the soil is the main culprit in footing failure. Moisture in expansive, or clay, soils will cause it to settle at different rates. Concrete will shift and crack in response. Foundations constructed at different depths may also be subject to differential settlement. Footings sometimes fail because the foundation design isn’t adequate for the load or when house additions increase loads beyond the capacity of the foundation’s design. Building on old foundations is another source of differential settlement.
Flaws in the concrete mix and placing concrete in freezing temperatures also lead to footing failure. Too much water in the mix is the most common reason for weak concrete. Concrete loses strength when the water content is increased. Concrete can be poured in cold weather, but special measures need to be in place. Otherwise, the hydration process, by which concrete gains strength, is interrupted. Laying concrete on frozen ground results in differential settlement when the ground thaws.
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