Precast Solutions - May/June 2008 - (Page 9)

GLENMORE LEGSBY PEDESTRIAN BRIDGE The Glenmore/Legsby Pedestrian Overpass in Calgary, Alberta, is a single-span, 53-meter bridge that stretches across eight lanes of traffic. It consists of two cantilevered, high-performance concrete abutments and a drop-in “T-section” UHPC (Ductal) girder with an arch. The girder is 33.6 meters long, 1.1 meters deep at mid-span with a 3.6-meter-wide deck and weighs approximately 100 tons. It is constructed with 13-millimeter steel fibers and post-tensioned with 42 15-millimeter strands. GFRP (Glass fiber-reinforced plastic) bars were also used as a redundant, passive reinforcing system. The girder required 40 cubic meters of material, resulting in the largest single monolithic pour of Ductal in the world to date. To ensure proper, efficient mixing, a high-shear mixer is recommended. Since the precast facility’s high-shear mixer could mix only 1.25 cubic meters at a time, the entire UHPC amount was prepared in four readymixed trucks over a 16-hour batch cycle. A specific filling order ensured the same average pot life across the prepared material. The UHPC was kept agitated in the trucks at a low revolution until ready for casting. The material’s workability and plastic behavior remained consistent throughout the entire agitation and casting period. This unprecedented, monolithic girder pour was achieved by filling up the stem of the T-section from one end. Random fiber orientation in the deck was ensured by using a concrete bucket the same width as the top flange of the girder. After filling the mold, the top surface was completely enclosed due to the longitudinal curvature and transversal drainage slopes. After the girder reached an initial strength of 35 megaPascals (MPa), form restraints were released to allow uniform shrinkage and prevent potential cracking during its initial curing process. A detailed study about the shrinkage behavior of this particular girder shape (during initial set) was performed prior to casting to ensure uniform setting of the UHPC within the controlled curing environment. After the required transfer strength of 100 MPa was reached, the girder was stripped and post-tensioned. Thermal treatment of 90 C for 48 hours ensured the required design strength and durability. Before the girder was placed into service, the owner (The City of Calgary) required completion of two simply supported tests. The test results indicated displacements smaller than predicted and the girder fully recovered upon load removal, indicating no inelastic behavior. The increased stiffness was attributed to an actual higher E-Modulus compared to theoretical. Finally, the girder was installed using two cranes at both abutment ends and lifted into position during the night to minimize traffic disruption. the full article at www.precast.org/sustainability. “It is only when the engineer, precast producer, material suppliers and designer sit down together to New precast concrete technology enables ultra-thin, lightweight construction as in the Glenmore Legsby Pedestrian Bridge in Calgary, Alberta. Photo by Tucker Photography COLLABORATE UP-FRONT FOR discuss sustainability goals in a project’s early phases that we find maximum potential for green construction and sustainability/LEED initiatives,” says Professor Tess Ahlborn, director of the Center for Structural Durability at Michigan Technological University. She adds that it is in taking advantage of the available brain trust that exists in the precast concrete industry’s professionals that “we can attain in practice what we espouse as the potential of the sustainability movement.” COMPETITIVE SUSTAINABLE DESIGN Without exception, project participants agree that fruitful initial collaboration between owner, designer, engineer, precaster and erector is the critical process that leads to unprecedented sustainable designs and green buildings. Whether the project is the construction of a DOT bridge, erection of a high-rise building or production of specialty precast products for landscaping, it is the initial teamwork and preliminary collaboration that garners and energizes the best ideas from design conception to finished product. Sue McCraven, NPCA senior technical consultant, is a civil engineer, technical writer and editor, and environmental scientist who has contributed numerous articles and studies to prominent scientific journals. MAY/JUNE 2008 | PRECAST SOLUTIONS 9 http://www.precast.org/sustainability

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Precast Solutions - May/June 2008 Contents Sustainability and Practicality Security Blanket A Frank Lloyd Wright Takes Shape Concrete Joins MENSA Precast Lightens Up

Precast Solutions - May/June 2008

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