The first U.S. use of post-tensioned concrete was on the Walnut Lane Bridge in Philadelphia in 1949 with precast post-tensioned girders. The first post-tensioning in buildings was in the late 1950s in lift-slab construction. Engineers with lift-slab companies were aware that post-tensioning could control deflection and reduce slab thickness. Once the lifting companies started post-tensioning their slabs, the deflection problems virtually disappeared. The lift-slab guys were clever and packaged the tendons into their bids for lift-slab construction, effectively shutting out independent post-tensioning companies. But the result was that this encouraged aggressive post-tensioning firms to develop and submit alternate bids for cast-in-place floors, competing directly with lift-slab. The post-tensioning companies formed an alliance with companies that were developing large-panel flying forms. It turned out that post-tensioned cast-in-place slabs formed with large panel flying form systems were highly competitive with lift slab buildings, and by the late 1960s became the preferred method for multistory slab buildings.

In the early days everyone was using button-headed tendon systems in cast-in-place buildings. The button headed system was replaced by tendons using seven-wire prestressing strand and wedge anchors. The strand system was much more economical than the button-headed tendon system and eliminated all of its major construction drawbacks. Virtually all post-tensioned tendons sold in America for building construction today are strand tendons.

When the earliest post-tensioned buildings were about 15 years old, tendon corrosion problems started to surface, and we realized that some tendon sheathings and coatings could not adequately resist corrosion. Improvements in sheathing and coatings, based on material specifications developed by PTI in the mid-1970s, have largely solved the corrosion problems.

—Kenneth B. Bondy

Read more highlights from 50 Years of Concrete Construction Progress.