In 2006, the City of Omaha made state history by awarding Nebraska’s first design-build project: the Bob Kerrey Pedestrian Bridge, also known as the Skyline Bridge. Connecting Omaha with the City of Council Bluffs, Iowa, the 3,000-foot structure is one of the longest pedestrian bridges ever built.
It’s also visually stunning: A 200-foot-tall steel pylon stands on each side of the Missouri River, anchored to the concrete deck by 40 post-tensioned (PT) cables 1.25 inches to 2.3 inches in diameter. When they’re at the center of the bridge, people are about 60 feet above the river on a walkway that spans 15 feet.
As the state’s first design-build contract, all eyes were on the project team to deliver a durable as well as aesthetically pleasing structure. To fill the channels that house the cables, the team wanted a cementitious grout that would enhance the performance of the PT design in addition to protecting the steel cables from moisture, salts, air, and other elements that cause corrosion, deterioration, and failure.
Design/builder HNTB Corp. specified grout made with Type K cement, a blend of local portland cement and Komponent® shrinkage-compensating cement additive. Generally classified as shrinkage-compensating or self-stressing depending on intended use and designed expansion, Type K cement creates expansion that, when restrained, induces sufficient compressive stresses to offset the tensile stresses caused by drying shrinkage. Designed expansion (i.e., proper dosage) is verified by ASTM C806 (Standard Test Method for Restrained Expansion of Expansive Cement Mortar) and ensures long-term residual expansion is achieved to keep the grout in compression.
Komponent more efficiently and effectively utilizes mix water during hydration than portland cement, improving the quality of the cement paste and producing a dense, low-permeability grout material that provides long-term dimensional stability.
Komponent manufacturer CTS Cement Manufacturing Corp. of Garden Grove, Calif., worked with Ready Mixed Concrete Co./Lyman Richey Corp. and placement contractor APAC Central Inc. to develop a mix design that would be easy to pump and ensure residual compression. The mix was bulk-batched at the plant to eliminate the costly material and labor associated with prepackaged bags.
The mix had a specified strength of 4000 psi (27.6 MPa) using only Type K cement. No fly ash or other supplementary materials were used. To facilitate pumping, maximum aggregate size was 3/8 inch (9.5mm). The mix design dosage of Komponent was optimized to ensure residual compression.
Various mixes were tested per ASTM C806 at the CTS lab and evaluated for optimized performance. These mix designs and test results were presented to HNTB for review, and one then selected for the project.
Fourteen years after the bridge opened in 2008, the iconic structure represents what can be achieved when innovative design and proven high-performance materials are constructed with collaboration and best practices.
Type K shrinkage-compensating cement grout designs meet ASTM C1107—Standard Specification for Packaged, Dry, Hydraulic-Cement Grout (Non-Shrink) as well as CRD-C 621—U.S. Army Corps of Engineers Specification for Non-Shrink Grout.