Editor's note: This story is adapted from the U.S. Department of Housing and Urban Development's Residential Rehabilitation Inspection Guide, 2000.Click here for other stories in this series.
Parts of this story: Introduction ~~ Seismic and wind resistance ~~Cracking and deterioration of masonry, general issues ~~Masonry foundations & piers~~Above ground masonry walls ~~Chimneys ~~Wood structural components ~~Iron and steel structural components ~~ Concrete structural components
Concrete is commonly used for grade and below-grade level floors and for footings. It also may be used for foundations, beams, floors above grade, porches or patios built on grade, exterior stairs and stoops, sills, and occasionally as a pre-cast or poured-in-place lintel or beam over masonry openings. Concrete structural components are reinforced. Welded steel wire mesh is used in floors at and below grade, patios built on grade, walks and drives, and short-span, light-load lintels. All other concrete structural components usually are reinforced with steel bars.
Inspect for the following:
- Cracking at corners or openings in concrete foundations below masonry exterior walls cue to drying shrinkage of concrete walls that are prevented from contracting by the mass of the masonry above. This cracking will occur early in the life of the building. Minor cracks can be filled with mortar and major cracks with concrete epoxy grout.
- Cracking of interior slabs on grade is usually due to shrinkage or minor settlement below the slab. If cracking is near and parallel to foundation walls, it may have been caused by the movement of the walls or footers. Cracking can also result from soil swelling beneath the slab, a condition that may be caused by water from clogged or broken basement or footer drains. Rarely is such cracking structurally harmful to the building.
- Cracking of exterior concrete elements, such as porches, patios, and stairs, is usually due to heaving from frost or nearby tree roots, freeze-thaw cycles, settlement, or a combination of these conditions. It is compounded by the use of deicing salts. Such cracking rarely presents a structural problem to the building, but is often a practical problem that can best be remedied by replacing the concrete and pro-viding the new work with more stable support. Cracks in existing concrete elements that are not seriously deteriorated may be cyclical and can be filled with a flexible sealant.
- Fire damage to concrete structural components should be thoroughly evaluated. Concrete heated in a building fire will lose some compressive strength, although when its temperature does not exceed 550 F (290 C) most of its strength eventually will be recovered. If the concrete surface is intact, it can usually be assumed to be in adequate condition. Superficial cracking can be ignored. Major cracks that could influence structural behavior are generally obvious and should be treated on a case-by-case basis. Cracks can be sealed by injecting concrete epoxy grout. Paints are available to restore the appearance of finely cracked or crazed concrete surfaces.
Two specialized tests may sometimes be useful for estimating the quality, uniformity, and compressive strength of insitu concrete.
1) The first is the Windsor Probe, a device that fires a hardened steel probe into concrete. See ASTM C803, Standard Test Method for Penetration Resistance of Hardened Concrete.
2) The second test is the Schmidt Hammer, which measures the rebound of a hardened steel hammer dropped on concrete. See ASTM C805, Standard Test Method for Rebound Number of Hardened Concrete.
For additional information about inspecting and repairing concrete, consult the following publications from the American Concrete Institute:
ACI 201.1R, Guide for Making a Condition Survey of Concrete in Service;
ACI 364.1R, Guide for Evaluation of Concrete Structures Prior to Rehabilitation; and ACI 546.1R, Concrete Repair Guide.
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