The Bob Kerrey Pedestrian Bridge, also known as the “Skyline Bridge,” opened Sept. 28, 2008, and is one of the longest pedestrian bridges ever built. The bridge connects Omaha, Neb., and Council Bluffs, Iowa, crossing the Missouri River.
This impressive cable-stayed superstructure has a dramatic look that comes from single-tower pylons, standing above the water on both sides of the Missouri River. The total length of the bridge is 3,000 feet and the towers carry a unique curved 506-ft. (914.4m) main span and two 253-ft. (154.2m) back spans. The bridge is north of the I-480 girder bridge and connects the Port of Omaha’s Miller Landing in Omaha to One Renaissance Center in the former Dodge Park Playland in Council Bluffs.
The bridge’s deck features a constant, unobstructed width of 15 ft. (4.6m) over the entire length of the bridge, widening out to 24 ft. (7.3m) on the Omaha landing, allowing enough width for both cyclists and pedestrians to share the path.
The $22-million project was largely funded by a $17-million federal transportation grant secured by Sen. Bob Kerrey in 2000. Other funding came from private donations, including $1 million each from The Peter Kiewit Foundation and Iowa West Foundation, a $1.7 million federal planning grant, $1 million from the Papio-Missouri River Natural Resources District, and $1.5 million each from the states of Nebraska and Iowa.
HNTB – Kansas City, Mo., was chosen to design and build this iconic superstructure. Their $22-million design-build bid incorporated innovative and cost-effective approaches for construction and was the first time a design-build process was used in Nebraska. Groundbreaking was initiated on Oct. 26, 2006.
The longitudinal post-tensioning (PT) design used for the Skyline Drive bridge is a critical aspect of the structure. Protection of the PT tendons with a high-performance cementitious grout is essential to its long-term durability and the life span of the structure.
Type K shrinkage-compensating cement was used for this critical grouting detail. Its engineered performance creates a dense, low-permeability grout mix designed to prevent negative volume change. Combined with the alkaline environment a cementitious grout creates, optimum corrosion protection is achieved.
This expansive cement effectively bonds to the PT tendons building expansive strains during the wet cure period. These strains are simply relieved when the wet cure is removed. Designed expansion (i.e., proper dosage) is verified by ASTM C806 (Standard Test Method for Restrained Expansion of Expansive Cement Mortar) and ensures that long-term residual expansion is achieved. Residual expansion keeps the shrinkage-compensating grout in compression for the life of the placement.
CTS Cement worked with Ready Mixed Concrete Co. and APAC to develop a shrinkage-compensating grout mix design that would ensure residual compression and be easy to pump in the field. Using the expansive cement additive (marketed as Komponent®) to create the grout mix allowed the dosage to be tailored to the specific requirements of the project and bulk batched at the plant. This eliminated the use of pre-packaged bagged goods and the costly material and labor associated with them.
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.
Komponent, used to create Type K shrinkage-compensating cement grout, efficiently consumes more mix water than portland cement during hydration. More efficient consumption of mix water improves the quality of the cement paste resulting in a dense, low-permeability grout material that provides long-term dimensional stability.
By preventing negative volume change and resulting drying shrinkage cracking, lowering permeability, and increasing density, PT tendons are effectively protected from moisture, salts, air and other elements that promote corrosion, deterioration and failure.
The use of a Type K shrinkage-compensating cement is a chemically post-tensioned approach to non-shrink grout materials that complement engineered longitudinal PT systems. Together, the concrete and the grout are kept in compression.
This harmonized performance achieves exceptional durability and bolsters the grout’s function in a bonded PT system of facilitating the transfer of prestressing force to the structural member.
The Type K cement grout mix was used to fill the longitudinal open channels that house the post-tensioned cables. 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 ensure efficient pumping, the maximum aggregate size was 3/8 in. (9.5mm). The mix design dosage of Komponent was optimized to ensure residual compression.
Over 20 years post-construction, this iconic structure represents what can be achieved when innovative design and proven high-performance materials are constructed with best practices and collaboration. CTS is proud to be a part of this distinguished project, with a shrinkage-compensating grout solution that will protect the integrity of the PT design for generations to come.
SHRINKAGE-COMPENSATING AND SELF-STRESSING GROUTING SOLUTIONS
Type K expansive cement is generally classified as shrinkage-compensating or self-stressing. Shrinkage-compensating concrete, mortar and grout designs create expansion that, when restrained, induce sufficient compressive stresses to offset the tensile stresses caused by drying shrinkage. Self-stressing designs create expansion that, when restrained, induce substantial compressive stresses after drying shrinkage has occurred. If unrestrained, self-stressing designs produce a greater volume increase than unrestrained shrinkage-compensating designs.
Both shrinkage-compensating and self-stressing designs have been used to create grout and mortar mixes for various applications, including posttension grouting applications, oil well caps, pipe abandonment, borehole plugging and nuclear waste containment. These designs often must be pumpable with extended working time, impermeable with low porosity, exhibit strong bonding characteristics, provide sulfate resistance, and be durable for decades and beyond.
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.
Assistance is available to develop project specific mixes with unique project requirements. Contact a member of CTS’ Komponent® or Technical Service teams for support. All mixes should be tested in a qualified lab using methods designed for shrinkage-compensating cement.
For information on how Komponent® can be used to provide a high-performance shrinkage-compensating concreting or grouting solution on your next project, contact a member of the CTS Cement Engineering Team at (800) 929-3030 or info@CTScement.com.
Thank you to the Iowa DOT for providing information and photos for this piece. Retrieved from: https://iowadot.gov/autotrails...
Additional information for this project was obtained from: The Transportation Research Board, National Cooperative Highway Research Program, NCHRP Report 584