In the 1950s a method of actively reinforcing concrete was introduced to the United States. This innovation, known as prestressing, allowed builders to take advantage of concrete's superior compressive strength properties while helping to overcome its relatively weak tensile strength. By prestressing the concrete, that is, using high-strength steel to squeeze the concrete and put it into compression before the concrete member has any other outside loads applied to it, the tensile properties of the concrete member could be greatly increased. The development of systems and methods that allowed prestressing steel to be located inside the concrete and tensioned after the concrete hardened led to the rapid expansion of the post-tensioned concrete industry.
1960s, '70s, and '80s saw many innovations in this industry as well as the development of some precursors to the modern post-tensioned reinforcing systems now in use. During this time, many office buildings, residential towers, parking structures, and containment vessels were constructed using post-tensioned concrete. At the same time, builders and designers were experimenting with post-tensioned reinforcing to construct residential slab-on-grade foundations. Officially accepted by the Federal Housing Administration in 1969, the use of this type of foundation exploded in popularity in the 1990s. Not only did manufacturers find ways to improve the long-term durability of the systems, but groups like the Post-Tensioning Institute codified and published rational design procedures that have been incorporated into model building codes allowing easy adoption by builders and designers.
Today, residential foundations account for more than 50% of the post-tensioned concrete market, and it is estimated that over 375,000 houses are constructed on post-tensioned foundations each year. In Arizona, builders have turned to post-tensioned foundations to support single and multifamily homes built on soils that shrink and swell as they get wet and dry out. Builders also like the added advantage of fewer cracks in their slabs since the post-tensioned reinforcing actively compresses the concrete, squeezing it together to keep it from tearing apart as it shrinks, bends, and moves on top of the soil that supports it.
These builders use post-tensioned reinforcing systems manufactured in modern facilities that are inspected and certified by the Post-Tensioning Institute. These facilities automate the task of taking bulk packages of steel, coating it with special post-tensioning grease, and encasing it in a seamless plastic sheathing. This finished product is then moved to a fabrication line, where it is cut to precise lengths, anchorages are attached, and it is bundled into the packages that are delivered to housing tracts.
—Contributed by Jim Rogers, President of Evaluation & Certification Services, LLC, Alexandria, Va.