Prestressed or post-tensioned concrete is widely used in bridge construction. It is an efficient and cost-effective method that allows for lighter, stronger structures that can cross wider spans. Stressing is a method of adding strength to concrete by including steel cables under stress which run through plastic ducts in the concrete. Cementitious grout is then injected into the duct and allowed to harden. The grout is intended to bond the cables (called tendons) and duct into a single unit structurally and, by displacing all the air and moisture in the duct, to prevent corrosion. This has been the accepted method of protecting the tendons for many years.
However, after discovering a number of corroded grouted tendons in coastal Florida bridges, the FDOT Structures Design Office began investigating causes. These tendons commonly failed from either poor grouting practices or defective grout materials. Often, these grouts had been improperly mixed or contained excess water, allowing solids and liquids to separate and form a jelly-like soft grout that retains water and does not harden. Rather than protect the steel, these defective grout applications actually hasten corrosion.
While the Structures Research Center investigated several alternatives, including unbonded tendons and individually coated strands, problems with ultimate strength, cracking, and corrosion disqualified them. Flexible fillers, on the other hand, seemed a promising alternative: wax or grease, rather than grout, is injected into the duct. This concept is not new. Nuclear facilities in the United States have been using flexible fillers for many years and, more recently, some bridges in Europe have utilized flexible filler in post-tensioned tendons.
In a series of research projects, University of Florida researchers worked with the FDOT Structures team to perform full-scale mockup investigations to work through the constructability and structural implications of flexible fillers. The process requires microcrystalline wax to be heated to between 212°F and 240°F and injected into the duct. As it cools, the wax congeals and forms a barrier around the steel, protecting it from moisture and contaminates.
For a project cost increase of less than 1%, the critical steel components are protected. Because the wax repels water and bonds more strongly to steel than to water, it provides protection even when the duct is not fully filled. Moreover, since the wax is a homogenous material, there is no possibility it will experience separation like grout.
Flexible filler was implemented in Florida with the FDOT Structures Design Bulletin in April 2014 and subsequently in the January 2016 Specifications. A number of upcoming construction projects may include flexible filler, including the replacement for the Pensacola Bay Bridge.
References
BDV31-977-15 Replaceable Unbonded Tendons for Post-Tensioned Bridges
Florida Department of Transportation Research Showcase 