Instrumentation and Long-Term Monitoring of the North Halawa Valley Viaduct

Aerial View of North Halawa Valley Viaduct during Construction

The newest member of Hawaii's highway system is the H-3 Freeway, connecting the Leeward and Windward sides of Oahu. The freeway spans approximately sixteen miles from Haiku Valley, through Koolau Mountain, to the North Halawa Valley. The freeway consists of two systems of highways and viaducts leading to two parallel Trans-Koolau Tunnels. The project is the largest ever undertaken by the State of Hawaii Department of Transportation (HDOT), the cost totaling nearly one billion dollars.

Viaduct Cross-section during Construction

One of the largest sections of the H-3 Freeway is the North Halawa Valley Viaduct. This Viaduct is a segmental cast-in-place post-tensioned concrete box-girder bridge. The bridge was built by the cantilever construction method. With the construction of the North Halawa Valley Viaduct, the State has an opportunity to evaluate the long term performance of an important bridge type and to advance the state of the art in the design of prestressed concrete bridges. The focus of this study is to instrument the North Halawa Valley Viaduct in order to monitor the creep and shrinkage strains in the structure. This information will greatly improve the constitutive models currently used in analysis and design programs for this type of structure, leading to improved prediction of performance in future structures. In addition, the study will produce vital knowledge for a better assessment of the remaining structural capacity and long-term performance of the viaduct.

Seven sections of the inbound Viaduct were selected for instrumentation to provide an adequate representation of the Viaduct behavior. The necessary measurements to achieve the project goals are concrete and reinforcing steel strain, tendon forces, concrete and ambient temperatures, span deflections, support rotations, span shortening, and concrete creep and shrinkage strains. The instrumentation system used to make the above measurements includes vibrating wire strain gages, electrical resistance strain gages, demec strain gages, load cells, thermocouples, a base-line system, tiltmeters, extensometers, and an automated monitoring system. During the construction of the bridge, all instrumentation was installed by personnel from the University of Hawaii (UH) and Construction Technology Laboratories (CTL). CTL, a Structural/Architectural Engineering, Consulting, and Material Technology company located in Skokie, Illinois, also performed concrete testing to determine the compressive strength, static modulus of elasticity, and coefficient of thermal expansion.

Instruments in Stem of Box-Girder

Automatic Datalogger

Standard creep and shrinkage tests are being performed under laboratory conditions in accordance with the American Society for Testing and Materials (ASTM) and in atmospheric conditions similar to those of the structure. The laboratory testing is being conducted by CTL in Skokie, Illinois, while the on-site tests are located inside and outside of the box girder bridge in Halawa Valley.

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