Florida rewrites specifications to prevent corrosion in post-tensioned bridges. 

When corrosion occurs in the post-tensioning for bridges and other structures, the effects can be disastrous. Following random inspections of Florida bridges that revealed corrosion in a few of the structures' post-tensioning, the Florida Department of Transportation (FDOT) called for a full investigation in late 2000 to determine the degree of corrosion in all FDOT bridges reinforced with post-tensioning. In response to concerns over the loss of permanent reinforcing, FDOT wanted to ensure these bridges could handle the loads they were designed to carry. The investigation examined more than 70 major post-tensioned bridges built during the last 46 years in Florida, covering a total deck area of nearly 16 million square feet. 

Following substantial research from the state bridge evaluations, FDOT decided to rewrite the state's specifications for post-tensioning in bridges with a goal of producing a design, construction, and maintenance environment that consistently produces durable post-tensioned bridges. FDOT based the new specifications on findings from its research regarding the causes of corrosion. Corrosion in post-tensioning in Florida bridges can be traced to several contributing factors. One factor is Florida's humid, saltwater environment. In addition, bridges were not watertight in some cases. Grout that bonds the post-tensioning tendon to the surrounding concrete via corrugated ducts, which are usually made of galvanized steel, was also intended to fill the duct and prevent corrosion from the ingress of contaminants. But in many instances, the grouted tendons were later found to contain air voids. 

According to FDOT, the investigation revealed other compromising situations with the bridges, including shrinkage cracks at construction joints and cracks in concrete pour backs. In certain instances, flawed sealing of epoxy joints in precast segmental bridges compromised the corrosion protection of internal tendons. Insufficient grouting procedures produced voids in the tendons. Furthermore, discontinuous ducts at precast segment joints, along with imperfect epoxy joint seals, allowed direct access for water to tendons that were not fully grouted. High-density polyethylene ducts of some external tendons suffered splits, allowing moisture direct access to grout or strands. Shrinkage cracks and leaks in some applications compromised anchor protection by ordinary concrete pour-backs. In response to these findings, FDOT's new specifications outline standards for preventing corrosion in post-tensioning. 

In light of the new specifications, post-tensioning suppliers faced a big task in developing new, enhanced post-tensioning systems. But despite the challenges, FDOT, suppliers, and contractors alike desired one goal: stronger, more durable post-tensioned bridges that would serve their design purpose throughout the intended life of the bridge, while requiring only routine inspection and maintenance. 

New specifications 

FDOT's new specifications for post-tensioning in bridges are based upon a five-part strategy. The first strategy for producing more durable post-tensioned bridges states that all post-tensioning tendons must be fabricated using enhanced post-tensioning systems. To ensure post-tensioning systems are enhanced, FDOT must approve all post-tensioning systems for use in Florida bridges and will list approved systems on the state's Qualified Product List (QPL). FDOT's definition of the qualities that constitute an enhanced post-tensioning system include: a three-level system of corrosion protection; tendons placed within plastic ducts; positively sealed duct connections; pre-bagged and pre-approved grout for post-tensioning tendons; post-tensioning tendons capped with permanent, heavy-duty plastic caps incorporating an O-ring seal; elastomeric coating over pour-back areas; and pressure-testing of all post-tensioning tendon ducts. 

According to the second strategy, all post-tensioning tendons must be filled completely with grout during construction. This requirement also includes the condition that all anchorages must be accessible for stressing, grouting, and inspection throughout all processes of installation and protection. 

To meet requirements for the third strategy, all post-tensioning tendon anchors must have a minimum of four levels of corrosion protection. The fourth strategy requires all bridge decks of post-tensioned bridges to be watertight. Finally, post-tensioned bridges must be designed to provide increased redundancy with multiple tendon paths, using a greater number of smaller-sized tendons. 

A new system 

Post-tensioning supplier VSL submitted to FDOT its ECI System components and the testing data required, and in 2004 became the first post-tensioning supplier to gain FDOT approval and listing on the state's QPL. VSL's new post-tensioning system meets all Florida requirements for external unbonded and cast-in-place internal bonded post-tensioning systems. 

A view of the backside of the bulkhead of Pier 3 on the Eastbound I-4 Bridge shows the ECI 6-19 bearing plate with the trumpet installed. The 76-mm plastic duct is connected to the trumpet and sealed with heat shrink. These anchorages are ready for concrete. Photo credit: Kiewit Southern

The ECI system's anchorages offer galvanized protection. The anchorage's bearing plate is hot-dipped into galvanized steel to provide a galvanized coating. Further, all ducts are both plastic and UV-resistant to meet the new specifications. To ensure proper grouting, ECI System anchorages have a dual-inspection port for post-grouting inspection. Ports on the ECI System allow an inspector to inspect the anchorage with a borescope to determine visually if it is completely filled with grout. Since all duct and anchorage connections must be air- and water-tight, the ECI System uses a combination of mechanical couplers and heat-shrink sleeves. Because the new FDOT standards require improved grout material with zero-bleed characteristics, all admixtures used to grout the tendons are pre-mixed and prebagged. VSL's ECI System is currently available for 0.6-inch strand in configurations of four, seven, 12, and 19 strands. A 31-strand system is in development. 

Having met the new criteria, VSL's ECI System has been installed on a variety of post-tensioned bridge projects in Florida, including the Ernest Lyons Bridge, the Robert's Landing Bridge in Sopchoppy, the Golden Gate Parkway in Naples, the I-4 interchange in Tampa, and the Treasure Island Bascule Bridge in Treasure Island. 

The Ernest Lyon's Bridge in Stuart is one of the first post-tensioned bridges in Florida benefiting from the new ECI System. PCL Civil Constructors, headquartered in Denver, began constructing the new precast segmental box girder bridge in 2003. FDOT called for the bridge replacement to increase the bridge's load rating and to expand the bridge from two to four lanes. For this project, all of the precast concrete segments are poured prior to the bridge's erection. The ECI System was installed in the concrete at the project's casting yard during the year-long casting phase. Bridge erection began in 2005; the project is expected to be completed in March 2007. This project marked the first use of the ECI 6-19 external post-tensioning system and 1-3/8-inch, internal post-tensioned bars. 

The first use of the ECI 6-7 internal post-tensioning system is for the Golden Gate Parkway in Naples, Fla.—a new bridge over I-75 with ramps. MCM of Miami was hired to construct the bridge and work began in December 2005. The estimated completion date is 2007; post-tensioning work was expected to be complete by the end of 2006. The project consists of 10 pier caps poured monolithically with a concrete deck. VSL serves as both supplier and installer of the 10 integral cast-in-place pier caps. 

The ECI 6-19 internal post-tensioning system was selected for the I-4 interchange in Tampa in 2004, under construction by Gilbert Southern Corporation of Atlanta. The ECI System was used for six integral cast-in-place pier caps. This project involves widening the bridges and ramps from four to six lanes for increased vehicular traffic and marks the first time the ECI system has been used on internal tendons in the state of Florida. 

Yet another use of the ECI system is for the Robert's Landing project—a new precast, flat slab bridge using the SA6-4 internal post-tensioning system. The contractor is Fairchild Florida based in Tallahassee, Fla. Further, Johnson Brothers, Bartow, Fla., is using the ECI System for the Treasure Island Causeway in Treasure Island. This project is a precast, flat slab bridge with transverse post-tensioning tendons. Casting of segments for this bridge began in April 2005. VSL provided post-tensioning systems two years ago for the East/West Bridge of this causeway under the old specifications and is now supplying the new ECI System for the West Bridge portion, a bascule bridge. This project marked the first use of the ECI 6-12 internal post-tensioning system. 

Revised post-tensioning specifications are benefiting Florida's bridges with more durable construction. Consequently, other states experiencing similar post-tensioning corrosion will likely follow Florida's lead and revise their post-tensioning requirements. 

About the Author 

Clyde Ellis is Branch Manager for VSL in Sprinfield, VA. Previously, he worked for VSL's Multistrand Division in South Florida, specializing in post-tensioning systems and specialty equipment for large projects. He has been involved with the Ringling Causeway Bridge, Crosstown Expressway Bridge, Memorial Causeway, Ernest Lyons Bridge and Sunshine Skyway Bridge. He can be contacted at 703-451-4300 or via e-mail at cellis@structural.net.