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Re: Post-Tensioned Cable vs. Rebar Slab on Grade Foundation

From: info@builderswebsource.com
Category: Foundations and Basements
Remote Name: 63.198.181.43
Date: 11 Aug 2001
Time: 11:54 AM

Comments

You are planning to build a home in North-central Texas where heavy clay is present and your builder is recommending a post-tensioned slab-on-grade foundation over traditional rebar.

First, it's important to understand that expansive clay is a foundation's worst enemy. Millions of homeowners in Texas and other parts of the country routinely experience differential foundation settlement or heaving due to improperly engineered and constructed foundations. Therefore, I applaud you for asking these questions before you build your home. In fact, a good local source of information on this topic is available from Atkinson Engineering's FAQ section at:

 http://www.atkinsonusa.com/faq.htm.

Here you'll find Q&A on typical slab-related questions for the Central Texas region.

Next, before building a structure, you really should have a geotechnical study done. The company will drill soil cores to determine the state and make up of the existing soil. As part of this study, it will determine how deep the "bad clay" goes, what the moisture content is and where competent soil exists. Scarification of existing soil, import of non-expansive fill, and compaction of the building pad may be required to minimize foundation trouble.

Generally, the best foundation for heavy clay is a pier and grade beam foundation. The piers (typically 12-16" in diameter) are drilled into the ground, past the the clay, into more stable soil. The skin friction of the soil is used to determine the depth of the piers based on the weight of the structure and location of load-bearing walls. Generally, this type of construction results in a "raised" foundation with a crawlspace. Personally, I like this type of foundation as it also provides maintenance access to plumbing, HVAC and electrical service underneath the home. Additional information on pier and grade beam foundations is available for a nominal fee in our online book, Building a Dream at:

http://www.builderswebsource.com/book/foundations.htm

Now, if you decide to pour a slab foundation, be advised that most slabs have performed poorly in Texas where expansive soils are present. Their popularity, however, is consistent with lower installed cost versus other methods.

Traditional rebar is considered "passive reinforcement" meaning that it only provides reinforcement as the concrete is forced into tension. Since concrete is very weak in tension (but strong in compression), rebar helps to prevent the slab from separating or pulling apart. However, cracks or settlement are still possible, particularly on clay soils where moisture content varies throughout the year from moist to dry.

Post-tensioned slabs, however, put the slab in tension after the concrete has cured. This can help to minimize cracking and differential settlement in adverse conditions. An excellent 2-page reference on post-tensioned concrete is available from, of all places, the Post-Tensioning Institute (PTI). This document explains the benefits of post-tensioned concrete, including slabs on expansive soils. It's in PDF format, so you'll need the Adobe Acrobat reader (free from Adobe).

http://www.post-tensioning.org/pdfs/what-pt.pdf

Be advised that installation of post-tensioned concrete requires specialty training. Don't let an unskilled builder do this without proper experience or training. Some builders are PTI-certified, having undergone the proper installation training.

In summary, post-tensioned concrete slabs when properly installed can provide important advantages over traditional rebar-reinforced slabs on expansive soils. However, we recommend first that a geotechnical company perform a soils test and make recommendations as to the foundation type, including provisions for drainage and grading of the building pad.

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