THE CONCRETE BRIDGE MAGAZINE

WINTER 2016

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/622975

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bleeding, in-place concrete density, and mass concrete. “Concrete in drilled shafts in Hawaii historically has demonstrated undesirable behavior,” explains Hashimoto. The bleed water migrates to the top of the shaft during placement, resulting in lower-density concrete at the top of the shaft. That often interferes with the tie-in to the structure, creating a weak point and adding stress. The combination of materials available in Hawaii also increases the heat of hydration, especially with larger shafts. The increased heat can reduce long-term durability. KSF’s engineers experimented with options and devised a concrete mixture that would account for the typical characteristics of local concrete to reduce heat of hydration and bleed water. Lower heat minimized internal cracking, and the reduction of bleed water resulted in more uniformity of the concrete. The result was a drilled shaft superior to previous designs. The second innovation focused on meeting challenges presented by decks with greater water-cementitious materials ratios, which could lead to shrinkage and cracking. “Our goal was to create a tougher mixture that exhibited less shrinkage,” says Hashimoto. “We looked at mixture options and optimized them to reduce the factors causing problems. Then we looked at blends of fibers that could increase toughness. The result was a high-performance concrete that contains entrained air and a combination of macro and micro fibers that minimizes shrinkage and creep characteristics typically encountered with local deck concrete mixtures. The materials and proportions minimized bleeding and improved toughness of the concrete. Both innovations were applied to the North-South Road bridges on the Island of Oahu, which consist of twin, 165-ft-long, single-span, integral-abutment bridges. They feature 5.5-ft-deep, post-tensioned spliced girders and precast concrete deck panels with a high-performance concrete deck topping. Five-ft-diameter drilled shafts at each abutment support the structures. “This project provided a good opportunity to try both ideas,” says Hashimoto. “It was the first one we’d worked on for some time that was not a repair or replacement project, which gave us an opportunity to design strictly for the site. It was a chance to try these fresh ideas, and that gave the research a major push.” KSF used a different shrinkagereducing admixture for its work on the Kii Bridge, an 80-ft-long, singlespan, integral-abutment bridge. The superstructure consists of 2-ft-deep, precast, prestressed concrete planks with a 5-in.-thick topping and is supported by 3-ft-diameter concrete drilled shafts. The admixture’s impact was examined with laboratory tests, field measurements, strain-gage readings, and finite-element analysis. Staged construction, post-tensioning, and soil-structure interaction were modeled with a finite-element program. An additional research project studying soil-structure interaction also was performed. Contractor Input Critical Creative results are often achieved in conjunction with contractors, a process enhanced by design-build method and value engineering. “We always work closely with the contractor from conceptual phase to project completion. Good relationships and collaboration result in better designs and construction

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