Concrete is really heavy! It is so heavy that it can literally squeeze air and water from soils. When this happens, the soil sinks and the results are telegraphed to the slab.

(Photo: Hugh Cairns)

Concrete cracking and sinking is likely caused by water action or improper soil preparation. Since soil movement is rarely uniform, the slab is subjected to a differential or upheaval.

Concrete slabs, whether your basement floor, a patio, or just a sidewalk by your home - all are subject to the same natural forces. When they shift or crack, it is often an indicator of what is happening around or beneath them. If part of the concrete slab is properly supported by soils and other sections are not, the unsupported pieces may crack and dislodge from an adjacent section. In residential properties, slab settlement problems can result in potential accidents and loss of real estate value. Poor drainage, tripping hazards, and unsightly cracks may also result from concrete slab settlement.

Telltale signs:

• Sinking Concrete Steps or Sidewalks
• Concrete Driveways & Patios Sinking Or Tilting Down
• Cracking & Sinking Sections Of Slabs
• Washout Of Soils Under Concrete Slabs

The soils underneath an outdoor concrete slab can fail to support the weight of the slab for two primary reasons:

Soil shrinkage, compaction and settling

Different types of soil have different load-bearing capacities. When a soil's load-bearing capacity is exceeded, the soil will compact and settle.

It's also possible for the soil beneath a slab to settle and compact of its own accord. Soil that contains large amounts of clay and/or silt will shrink substantially when it dries. As the soil compacts underneath a concrete slab, the unsupported slab will sink as well, usually cracking in one or more spots.

Washout of soils

The soil that is underneath your driveway, patio, or concrete steps may not necessarily remain there to provide adequate support for the concrete slab.

As water moves underneath your concrete slabs, it can wash away the soil that's supporting the weight of the concrete. As this happens, it creates a void, or empty space, underneath. Over time, with nothing to support it, your concrete slab can begin to sink or cave in. Broken and/or misaligned concrete (sidewalks, patios, driveways, etc.) poses a safety hazard, while also detracting from appearance and real estate value.

Before tackling concrete slab, consider contacting a professional familiar to inspect your property and do some soil analysis. If you don't find a way to fix the cause of the eroding soil, it will likely soon reoccur.

For more information on concrete cracking click here.

Hugh Cairns welcomes your questions, comments and suggestions. Contact him through www.subject2homeinspections.com or interiorinfrared.com.