THE CONCRETE BRIDGE MAGAZINE

Spring 2019

ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.

Issue link: http://www.aspiremagazinebyengineers.com/i/1093138

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C O N C R E T E B R I D G E T E C H N O L O G Y 36 | ASPIRE Spring 2019 by Dr. Clay Naito, Lehigh University, and Dr. Christina Cercone, Manhattan College Implementation of an Electrically Isolated Tendon Post-Tensioning System in the United States Post-tensioned concrete bridges represent an important component of the current U.S. bridge inventory. These systems, whether cast-in-place or precast concrete construction, can be used to create long- span continuous bridge superstructures with long service lives. However, ensuring the quality of the post-tensioning duct sys tem and grou t material used to encapsulate the strands has been a concern. The post-tensioning ducts are conventionally located within the web of the concrete girders, and critical sections— such as splices and anchorages—are heavily reinforced to provide adequate transfer of forces into the members. These practical requirements of post-tensioning (PT) make visual and nondestructive inspection techniques difficult to implement during construction and throughout the life of the bridge. Quality control methods are well established for PT installation and grouting, but traditional inspection methods are not easy to implement during regular service-life inspections of internal tendons. To a d d r e s s t hi s i s s u e, el e c t ri c a l ly isolated tendon (EIT) systems have been developed. These systems, which can be easily integrated into new construction, provide full electrical isolation of the steel PT tendons from the surrounding concrete of the bridge structure. With isolation, the PT steel cannot form a corrosion cell with the exterior reinforcement, and corrosion of the tendon is nearly eliminated because the amount of oxygen within the duct is low. System Components The EIT system is composed of an enhanced tendon and anchorage system. The anchorage includes an isolation plate, a plastic trumpet, and proper encapsulation of the anchorage head. The tendon system uses a plastic duct, with plastic duct couplers and plastic half shells to protect the integrity of the corrosion protection system along the length of the tendon. The plastic half shells are used between the plastic duct and the transverse reinforcement to provide additional protection to the duct during member fabrication and tensioning. Several companies commercially produce the required components of the EIT system for the European market, making this an off-the-shelf technology. For the past 20 years, EIT systems have been used in various post-tensioned concrete bridges in Europe, including the Piacenza Viaduct and Marchiazza Viaduct in Italy and the P.S. du Milieu and the Wiesebrücke Basel in Switzerland. With the EIT system, the encapsulation s y s t e m i s a s s e s s e d f ro m t im e o f construction through the service life of the bridge by using commercially a va il a bl e in d u c t a n c e / c a p a ci t a n c e / resistance (LCR) meters to measure the AC resistance, capacitance, and loss factor between the isolated PT strands and the reinforcing steel located in the surrounding bridge s truc ture. These readings can be compared to limiting acceptance criteria values 1 to provide owners with an assessment of the quality of the encapsulation and any unexpected corrosion initiation. These measurements a r e o f t e n t a ke n a t ke y s t a g e s o f construction, such as during tensioning or tendon installation, before and after grout installation, during deck placement, and during service. The EIT system has the potential to provide long-term condition assessment of the tendon system over the life of the bridge. First U.S. Demonstration Project The many benefits of the EIT system and its successful use in Europe have prompted research and development efforts geared toward implementation in the United States, including a series of The main unit of the Coplay-Northampton Bridge consists of a 540-ft-long, three-span section over the Lehigh River. The superstructure consists of five prestressed concrete PA bulb tees, which were spliced and post-tensioned. All Photos: Dr. Clay Naito.

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