THE CONCRETE BRIDGE MAGAZINE

SPRING 2017

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/806277

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SAFETY AND SERVICEABILITY Post-Tensioned Concrete: An Inherently Durable System by Dr. Andrea Schokker, University of Minnesota Duluth I teach courses in prestressed concrete and bridge design, and I am freshly reminded each semester about the inherent durability we have in our post-tensioned (PT) concrete bridge elements. While the phrase "belt-and suspenders approach" may be overused, I think it truly defines a PT system. The steel strand is separated from aggressive agents, such as moisture and chloride, by the concrete, the PT duct, and grout (or other material or coating). Additionally, we have seen active development in each of these areas over the past 15 years, particularly in PT bridge systems. I’ve outlined the progression of each component below with a focus on a grouted PT system. Concrete and Steel Low-permeability concrete with high quality placement has become far more common, particularly in the quality controlled, tight-tolerance environment of a bridge casting operation. We know far more about concrete materials and admixtures than we did a decade ago and the knowledge base continues to expand each year. Quality concrete, combined with the active prestressing force from the steel PT tendon, provides the foundation for a highly durable structure. With today’s low-permeability concrete, cracks are the primary path for corrosive agents to reach the steel. The precompression of PT concrete keeps cracks from forming under service loads and forces any cracks that might form to close after an overload is removed. This combination of high-strength steel strand and concrete brings the best out in both materials for a solution that is customized (through PT tendon layout)to the expected loads. Duct and Anchor Protection The PT duct and anchorage protection system has been revolutionized over the past 15 years. As a PhD student with Dr. Jack Breen at the University of Texas at Austin in the late 1990s, I was part of a team studying a wide array of options for improving durability of PT systems, including duct types, grout types, anchor protection, strand coatings, galvanized strands, and prestress level.1–3 Most of the specimens contained the standard system at the time: a galvanized duct with duct-taped splices and attachments to the end trumpet, no end cap, and a nonshrink grout pour back in the anchor recess.

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