Settlement Cracks in CMU Walls
2011-11-03
In general, cracks in a concrete masonry unit (CMU) wall occur as the result of excessive structural loading, inadequate or adversely affected support, shrinkage and thermal movement, or a combination of the above. Excessive structural loading may be the least common cause of cracks in residential CMU walls, and is not considered further here.
Shrinkage cracks occur due to a reduction in moisture content, thermal effects, and carbonation. The focus of this discussion is the adverse soil support conditions that may cause significant settlement of a CMU wall's foundations and related cracking in the CMU wall itself.
Shallow foundations, such as spread footings and monolithic footings, are often used to support CMU walls, and rely on the underlying soil for support. As such, the soil beneath the foundation may be considered a primary component of this structural system. Changes that occur in the soil after a foundation is placed may have significant detrimental effects on the foundation and whatever it supports.
Such changes include the consolidation of organic material or loose sand, variations in the water content of clayey soils, erosion of the soil from beneath the foundation, and the potentially harmful effects of root growth. Many of these changes and their harmful effects occur over a significant period of time. Proper construction requires that soil beneath a foundation be properly prepared prior to concrete placement in order to avoid or minimize such changes and their harmful effects on the supported structure. The possible effects of sinkhole activity are not considered in this discussion.
Full support of the underlying soil allows most residential shallow footings to be constructed with relatively little reinforcement steel. As the tensile strength of plain concrete is typically only 10 to 15% of its compressive strength, it is susceptible to cracking if bending develops within the footing. Bending forces cause tension that must be resisted by the presence of reinforcement steel.
Shear is also a cause of cracks in shallow foundations affected by a loss of soil support. Plain concrete's resistance to shear is a function of its compressive strength and the area potentially affected by the shear failure. The loss of proper soil support may cause damaging bending and shear forces within a lightly reinforced footing. These forces are relieved when the footing cracks and settles. This has the potential to adversely affect a CMU wall or other structure supported by the footing.
A small amount of settlement is normally taken into consideration in new building design since new structures with shallow foundations settle to some degree. 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 causes structural problems. Differential settlement occurs when one portion of a foundation or structure settles more than another. This differential movement induces unexpected and sometimes severe internal forces which cause structural damage.
Significant settlement occurs for a number of reasons which are usually related to the initial preparation of the soil beneath the footing. Cohesionless soil (sand) that is not properly compacted prior to footing placement is susceptible to consolidation due to applied pressure and significant 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 footing 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 the footing 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. Root growth, particularly when trees are located near the home, has the potential to damage a CMU wall foundation and cause cracks in a supported wall.
Foundations are usually buried and therefore hidden. As such, the only indication of significant settlement may be its effects on a supported wall or other structure. CMU is a brittle material, and significant movement or settlement of its support is likely to cause cracks in the wall. The width, orientation, and location of settlement cracks vary and depend largely upon the amount, location, and type of foundation settlement.
In summary, shallow footings require full support from their underlying soil, and are otherwise unable to support their design loads. A footing may crack and settle if the support soil is adversely affected, lost, or diminished. This settlement or movement has the potential to cause cracks in a supported CMU wall or other structure.
The following photograph shows significant settlement cracks at the corner of a CMU building:
