New Florida Department of Transportation (FDOT) specifications for bridge post-tensioning challenged engineering and construction interests to develop
New Florida Department of Transportation (FDOT) specifications for bridge post-tensioning challenged engineering and construction interests to develop an improved strand and anchor system yielding stronger, more durable structures that require only routine inspection and maintenance. In 2004, VSL became the first post-tensioning supplier to gain approval according to revised FDOT guidelines and listing on the state’s Qualified Product List (QPL). Known as the ECI System, VSL’s post-tensioning technology meets all Florida requirements for external unbonded and cast-in-place internal bonded post-tensioning systems.
CORROSION PROBLEM EXPOSED
To varying degrees of depth, an investigation of Florida bridges initiated by FDOT in 2000 examined 70-plus major post-tensioned spans, comprising a total deck area of nearly 16 million square feet built over the last 46 years. The study was prompted by concerns over the loss of permanent reinforcement following random inspections of the state’s bridges that revealed corrosion in several structures’ post-tensioning. As post-tensioning corrosion can have disastrous effects, FDOT aimed to determine the degree of corrosion in all its post-tensioned bridges to ensure these structures could handle the loads they were designed to carry.
Substantial research derived from the Florida bridge evaluations led FDOT to rewrite the state’s specifications for bridge post-tensioning with the goal of ensuring a design, construction and maintenance environment that consistently produces durable post-tensioned spans. Accordingly, latest specifications reflect the study’s findings regarding causes of post-tensioned reinforcement corrosion in Florida bridges. Primary among factors contributing to corrosion cited in the research are (1) humid, saltwater environment; and (2) lack of watertight construction, i.e., grouted tendons were compromised by air voids. Insufficient procedures for grouting intended to bond post-tensioning tendons to the surrounding concrete via corrugated ducts, as well as fill the duct and prevent contaminant ingress, produced voids in the tendons.
Other deleterious conditions discovered during the investigation of Florida bridges included shrinkage cracks at construction joints and cracks in concrete pour backs jeopardizing anchor protection. Additionally, corrosion protection of internal tendons was sometimes compromised by flawed sealing of epoxy joints in precast segmental bridges. Discontinuous ducts at precast segment joints in combination with imperfect epoxy joint seals allowed water direct access to tendons that were not always fully grouted. High-density polyethylene ducts of some external tendons suffered splits, furthermore, exposing grout or strands to moisture. In response, FDOT addressed such issues in its revised specifications, which outline standards for preventing corrosion in post-tensioning.
A five-part strategy underlies FDOT’s specifications for post-tensioning in bridges. First, all post-tensioning tendons must be fabricated using enhanced post-tensioning technology, i.e., one of the systems approved by FDOT for use in Florida bridges and cited on the state’s QPL. Agency criteria for an enhanced post-tensioning system include: a three-level system of corrosion protection; tendons placed within plastic ducts; positively sealed duct connections; prebagged and preapproved grout for post-tensioning tendons; capping of post-tensioning tendons 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.
The second strategy dictates that all post-tensioning tendons must be completely filled with grout during construction. Also included in this requirement is the condition that all anchorages be accessible for stressing, grouting and inspection throughout installation and protection implementation phases. The third strategy involves ensuring that all post-tensioning tendon anchors have a minimum of four levels of corrosion protection. The fourth strategy requires all decks of post-tensioned bridges to be watertight. Finally, increased redundancy in post-tensioned bridges must be achieved by means of multiple tendon paths using a greater number of smaller-sized tendons.
The ECI System anchorages’ hot-dipped galvanized bearing plates resist corrosion. Further, all ducts are plastic and UV-resistant to meet the new specifications. Proper grouting is ensured as anchorages have a dual-inspection port for post-grouting inspection, allowing examination with a borescope to visually determine if the anchorage is completely filled with grout. Since all duct and anchorage connections must be air- and watertight, the system employs a combination of mechanical couplers and/or heat-shrink sleeves. Compliance with FDOT standards requiring improved grout material with zero-bleed characteristics is assured by using all premixed and prebagged mixtures to grout the tendons. VSL’s ECI System is currently available for .6-in. strand in configurations of four, seven, 12 and 19 strands. A 31-strand system is currently in development.
Having met the latest FDOT specifications, the ECI System has been installed on a variety of post-tensioned bridge projects in Florida. Notable among these are the Ernest Lyons Bridge in Stuart, Robert’s Landing Bridge in Sopchoppy, Golden Gate Parkway in Naples, I-4 interchange in Tampa, and the Treasure Island Bascule Bridge.
A precast segmental box girder crossing, the Ernest Lyons Bridge was one of the first post-tensioned spans in Florida to incorporate the ECI, marking the initial use of the system’s 6-19 external post-tensioning and 1?-in. internal post-tensioned bar configuration. Denver, Colo.-based PCL Civil Constructors began the bridge replacement project in 2003, fulfilling an FDOT mandate to increase the structure’s load rating and expand its width from two to four lanes. All precast segments were fabricated prior to bridge erection. The ECI System was installed in concrete components at the precast yard during the 12-month fabrication phase. Bridge erection was started in 2005 and continued through the end of 2006.
Construction of the Golden Gate Parkway, a new bridge over Interstate 75, occasioned the first use of the ECI 6-7 internal post-tensioning system. Begun in December 2005 under the direction of MCM of Miami, the project consists of 10 pier caps poured monolithically with a concrete deck. VSL served as both supplier and installer of the 10 integral cast-in-place pier caps.
For the Interstate 4 interchange in Tampa, ECI 6-19 internal post-tensioning was employed in 2004. Gilbert Southern Corp. of Atlanta was enlisted to widen the bridges and ramps from four to six lanes for increased vehicular traffic. The ECI System provided six integral cast-in-place pier caps and was used Û for the first time in Florida Û on internal tendons.
Robert’s Landing, a new precast flat slab bridge using SA6-4 internal post-tensioning, comprises yet another ECI applications. Tallahassee-based Fairchild Florida served as lead contractor.
The Treasure Island Causeway in Treasure Island is a precast, flat slab bridge with transverse post-tensioning tendons for which Johnson Brothers of Bartow selected the ECI System. Casting of segments for the structure began in April of 2005. In contrast to post-tensioning systems provided by VSL for the causeway’s East/West Bridge several years ago under the old specs, ECI 6-12 internal post-tensioning was used Û in its first application Û on the West Bridge portion, a bascule span.
Thus, in meeting latest FDOT specifications, VSL representatives note, its ECI System contributes to more durable post-tensioned bridges in Florida. With revised post-tensioning specifications on the horizon for other states, they anticipate widespread application of the ECI System for improved post-tensioned bridge construction nationwide. Û www.vsl.net