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Mixtures also had to be selected without opportunity for development or qualification testing. Cemstone scrutinized and pulled from its vast collection of performance-based concrete mixtures, choosing those that met the varying project requirements. These included low heats of hydration, high early strength, high resistance to chloride ion penetration, freeze/thaw durability, low shrinkage, dimensional stability, and high abrasion wear. Cemstone also was first to develop mixtures for photocatalytic self-cleaning concrete in North America and the worldwide first use of this cement in self-consolidating concrete.
Providing the appropriate mixtures was only one component of meeting the rigorous project requirements. Efficient and consistent concrete production for this project, while minimizing interruption of other scheduled projects, was essential.
Fortuitously, a Cemstone batch plant sits on the Mississippi Rivier five miles from the I-35W bridge site. This allowed easy access to both banks where the project was being constructed and to the precast yard which had been set up on an unused portion of the highway. A second Cemstone plant, located in the Midway region between St. Paul and Minneapolis, is almost as close and served as a backup.
Both plants were outfitted with central mixers and each had the capability of stockpiling large quantities of aggregate, a necessity for nighttime and weekend placements outside of the normal material delivery schedules.Pump parameters
Cemstone partnered with Nordic Contracting Inc. for pumping operations. This included a pair of placing towers for the cast-in-place portions of the bridge deck, and pumps with boom lengths ranging from 17 to 61 meters for all aspects of bridge construction (foundation through precast elements).
In addition to having the right mixtures, equipment, and plants, Cemstone's senior management and quality control personnel were at the site continuously, so the could respond to delays in scheduled placements due to inspections, a necessity for avoiding the inherent risks of fast-track construction.
Multiple levels of inspection occasionally resulted in rescheduling of placement deadlines. Cemstone plant staff also frequently determined moisture content of incoming aggregates and checked air content and other properties.
Cemstone's dispatch center normally controls movement of trucks and materials, but during off hours, local dispatch operated from the batch plant. Consequently, the producer had a very responsive control system for delivering the materials.
Cemstone's engineering department was heavily involved in modeling life-cycle costs, including time-to-corrosion and cracking. It performed thermal modeling with assistance from CTLGroup of Skokie, Ill. Mass pours were performed in extremes temperatures, ranging from 100° F to -40° F. The concrete contained more pozzolan than portland cement, and no cracking or other issues arose because of temperatures.
Cemstone's preparedness allowed it to meet the concrete needs of the new I-35W bridge construction. The expansive bridge opened Sept. 18, 2008, well ahead of the original fast-track Dec. 24 deadline.
Kevin MacDonald is Cemstone Products' vice president, engineering services. He is a licensed professional engineer in Minnesota and Ontario, Canada, and is a fellow of ACI. For more on the producer, visit www.cemstone.com.