University Park, Pa -- Post-tensioned construction can be a quick way to build beautiful bridges and with new, improved grout mixtures, it can be a safer, longer lasting option, too, says a Penn State engineer.
Dr. Andrea J. Schokker, the Henderson professor of civil and environmental engineering, is the lead author of a new study that has identified the best grouts for bonded post-tensioning applications.
With the use of these grouts in new construction, it may be possible to significantly increase the service life of a bridge, she says. For example, in a coastal environment where structural elements are exposed to seawater or spray or on inland bridges exposed to deicing salts, corrosion could cause problems before the end of the expected service life of the bridge. With the new grouts, the same structure may last up to twice as long depending on circumstances.
In post-tensioned construction, a portland cement grout acts as a "last line of defense" to prevent deicing salt or seawater chlorides from reaching a structure's reinforcing strands, composed of seven twisted steel wires, and causing corrosion.
At the construction site, the strands are inserted into plastic or galvanized steel ducts or tubular channels that run the length of each pre-cast concrete segment such as a beam or block. After insertion, the strands are stretched to put them under tension and anchored at the ends, forcing the concrete into compression. The ducts are then filled with portland cement grout. The grout not only protects the steel strands from corrosion but also distributes the bond with the strand along its full length.
Recently, problems with corrosion, due to voids in the grout, have been noted in some structures. Schokker notes there have also been three confirmed post-tensioned structure collapses in Europe which have caused Britain to re-evaluate their grouting procedures. She cautions that, while corrosion of post-tensioned bridges does not appear to be a major U.S. problem at this point because many post-tensioned structures are less than 30 years old, it is prudent to adopt high-performance grouts and grouting methods in new construction.
In experiments at the University of Texas at Austin where Schokker earned her doctorate, she tested more than 30 grout designs. Fresh property tests were conducted to determine which grouts, when freshly mixed, were fluid enough to be workable but still provide good corrosion protection for the reinforcing strand when hardened. Some grout mixtures, while workable when fresh, had a tendency to separate into individual pockets of water and grout. The pockets of separated water eventually evaporate, leaving voids when the grout hardens. The tendency of water to separate, called bleed, is especially significant in tall structures and was determined for each of the 30 grout mixtures.
With the grouts that passed the fresh property tests, Schokker conducted an accelerated corrosion test. Finally, the grouts that passed the corrosion test were subjected to a large-scale duct test that simulated field conditions. In the field simulation, a duct, containing a bundle of reinforcing strands, was arranged in a horizontal serpentine pattern and grout was pumped in to investigate flow and the formation of water pockets due to changes in duct elevation.
Two grouts emerged from the testing as recommended: for horizontal applications a 0.35 water-cement ratio grout with 30 percent cement weight replacement fly ash (Class C) and for vertical applications (such as tall bridge piers) a 0.33 water-cement ratio grout containing anti-bleed admixture.
Schokker is continuing her investigations of grout bleed characteristics in laboratory and large-scale testing at Penn State.
Her results to date have been detailed in a paper, "High Performance Grouts for Bonded Post-Tensioning," presented at the American Segmental Bridge Institute Board of Directors Meeting in Amelia Island Plantation, Fla. in October 1999. Her co-authors are her former University of Texas dissertation adviser, Dr. J. E. Breen and her co-adviser Dr. M.E. Kreger.
The results will also be released February in a research report, Development of High Performance Grouts for Bonded Post-Tensioned Structures, by the Center for Transportation Research Bureau of Engineering Research of the University of Texas as Austin. Her research report co-authors are B. D. Koester, research assistant, Dr. J. E. Breen and Dr. M. E. Kreger, all at the University of Texas. The Texas Department of Transportation supported the research in cooperation with the U.S. Department of Transportation Federal Highway Administration.
EDITORS: Dr. Schokker is at (814) 863-2786 or at ajs19@psu.edu by email.