Settlement Cracks in Concrete Slab on Grade
2011-10-27
A concrete slab on grade relies on the underlying soil for its support. As such, the soil beneath the slab is considered to be a primary component of this structural system. Changes that occur in the soil after a slab is placed can have a significant detrimental effects on the slab, and may include consolidation of organic material or loose sand, variations in the water content of clayey soils, and erosion of the soil from beneath the slab. Many of these changes and their harmful effects occur over a significant period of time. The soil beneath a slab must be properly prepared prior to concrete placement in order to avoid or minimize such harmful changes and their effects on the structure. The possible effects of sinkhole activity are not considered in this discussion. Full support of the underlying soil allows a slab on grade to be constructed with little or no steel reinforcement. As concrete is strong in compression, a fully supported concrete slab easily transfers normal residential loads into the soil directly beneath the slab. However, the tensile strength of plain concrete is usually only 10 to 15% of its compressive strength. Therefore, without reinforcement, concrete is very susceptible to cracking due to bending, which induces tensile forces within the slab. Shear is another potential cause of cracks in concrete slabs. The shear strength of plain concrete is a function of its compressive strength and the area potentially affected by shear failure. Bending and shear develop within a floor slab or other structure that is partially supported or only supported at intervals. These forces are relieved when the slab cracks and settles, often under its own weight. A small amount of settlement is normally considered in new building design since all new structures founded on shallow foundations settle. Structural damage does not usually occur as the result of settlement alone. For instance, if an entire building with all its foundations 'sank' exactly one foot into the earth at the exact same rate, the building itself would not be structurally scathed by the settlement alone. It is differential settlement that usually causes structural problems. Differential settlement occurs when one portion of a building or structure settles more than another. This differential movement induces unexpected and potentially severe forces within a structure, causing structural damage. Settlement occurs for a number of reasons usually related to the initial preparation of the soil beneath the slab. Cohesionless soil (sand) that is not properly compacted prior to slab placement is susceptible to consolidation due to applied pressure and fluctuations in the water table. These fluctuations may occur during periods of heavy rain. In dry sand, internal friction and shear resistance increase with applied pressure. However, hydrostatic buoyancy reduces the pressures between the sand particles by about 50%. This translates into an approximately equal loss in soil bearing capacity, and effectively doubles the expected settlement. Organic materials left beneath a slab may decay or move with applied pressure over time. Clayey soils are susceptible to plastic deformation, and may swell or shrink with more or less water content. Erosion of support soil beneath or at the perimeter of a slab on grade is also a likely cause of settlement. This may occur as the result of missing or poorly directed gutter downspouts, groundwater runoff at steep grades, etc. In summary, concrete slabs on grade need complete support from the underlying soil as they are not able to sustain their own unsupported weight. As such, a slab may crack and settle if the support soil is adversely affected, lost or diminished. The following photograph shows significant settlement at the corner of a concrete driveway slab:
