The Hoover Dam and Lake Mead are located in Black Canyon on the Colorado River in the USA on the borders of Arizona and Nevada. Construction of the 220m high dam began in 1931 and more than 4 million m³ of concrete were placed by the time it was completed in 1936. Highway 93 is built into the top of the dam to cross the canyon, but the sharp bends and narrow widths of the highway, combined with the number of tourists and necessary security, led to an astronomical increase in traffic, so that long delays and bottlenecks had become the norm.
After a long debate due to the sensitive environment, the final decision was for a composite bridge structure with a reinforced concrete deck, over steel box girders, supported on concrete piers from twin concrete rib arches. For the cast-in-place reinforced concrete foundations and ribs of the arch, a High Performance Concrete mix design was specified that had to achieve a compressive strength of 69 MPas (10’000 psi), together with strict limits on shrinkage and longterm creep.
This had to be developed and then adapted to provide consistent performance in the extreme temperature conditions, which can vary from below -20°C in winter, to higher than +40°C in summer.
The precast concrete pier segments had to have an excellent visual finish and also required very high early strengths of up to 30 MPas (4’500 psi), which had to be achieved within 12 – 16 hours, even during the cold winter months, so that the formwork could be stripped and prepared for pouring the next segment.
Finally, after tensioning the cables also had to be sealed and protected in their ducts with a non-shrink, high flow grouting system to prevent any future corrosion and water ingress to the structure.
The solution to the challenge was the innovative Sika® CarboDur® UHM carbon fiber-reinforced polymer (CFRP) system. Being only 4.7 millimeters thick, the strengthening plates reduced tunnel headroom only minimally. The plates were installed every weekday night during the so-called engineering hours, between 1 a.m. and 5 a.m. when no trains were running.
Every night, the scaffolding was relocated, the cast-iron beams cleaned and primed, then coated with Sikadur® epoxy adhesive. The Sika® CarboDur® CFRP plates were then fixed to the beams. In all, some 1,300 meters of strengthening plates were installed within five months, at a rate of two beams per night.
Thanks to Sika’s solution, the overall project was completed bang on schedule and without any disruption to services. The 90 tunnel ceiling beams are now well equipped to support the loads of another 100 years.
USA Government, Federal Highways Department
HDR Services Inc. & T. Y. Lin International
Joint Venture of Obayashi Corporation and PSM Construction
Concrete Producer – Ready-mix
Casino Readymix Inc.