Housing Giants - September 24, 2008 - (Page 16)

Process Here, the bearing soil isn’t properly leveled and compacted. The result is bulges, which hamper efforts to properly post-tension the slab once the concrete is poured. inconsistent or poor bearing capacities, such as expansive and organic soils. In the U.S., post-tension slabs are primarily constructed in the Southwest, where there are expansive (clay) soils, and in the Southeast, where there are organic (loam) soils. Expansive soil is a clay soil that shrinks when the water content lessens and swells when the water content increases. As the soil shrinks and swells, it creates serious problems for the home that appear as cracks that can range from small cracks to cracks that run deep, reducing the slab’s strength and durability. The most extensive damage to homes occurs in areas that have a semiarid climate where periods of rainfall are followed by periods of no rainfall, such as in Arizona. In these areas, constructing a post-tension slab instead of a traditional slab is a good choice, because posttensioning strengthens the foundation, allowing it to resist the effects of shrinking and swelling soil. Organic soil is a type of soil that contains a high percentage of organic matter. Organic matter, such as grass, roots and leaves, will decay under the slab, causing the foundation to settle and crack. In general, organic soil is too compressible for a traditional slab-ongrade foundation; this type of soil can’t be relied on for long-term support. As a result, it’s best to construct a posttension slab, giving the slab the strength to resist stresses caused by the soil. Photo courtesy of BuildIQ Errors that lEad to cracks When errors are made on-site during the post-tensioning process, the ability of the reinforcement to prevent cracking is decreased. As a result, even though posttensioning is meant to help prevent a slab from cracking, the slab can still crack if For more information, the Post-Tensioning Institute offers the “Construction and Maintenance Manual for Post-Tensioned Slab-on-Ground Foundations.” You can purchase it on PTI’s Web site in the bookstore. 16 hoUsING GIaNts.9.24.08 www.hoUsINGGIaNts.com http://www.azgs.az.gov/HomeOwners-OCR/HG4_problemsoils.pdf http://www.concreteconstruction.net/industry-news.asp?sectionID=715&articleID=366219&artnum=1 http://www.post-tensioning.org/ http://www.housinggiants.com

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Housing Giants - September 24, 2008

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