By: Jennifer G. Prokopy
During the past 10 years, the design and use of
high-performance concrete (HPC) has become more common, with hundreds of
structures created using the material. Signature bridges, including the
Confederation Bridge in Prince Edward Island, Canada, and the Sagadahoc Bridge
in Maine, often are made possible with HPC.
Increased use of HPC grows from the coordinated efforts of
individuals and organizations to research and develop the best high-performance
solutions throughout the country. Groups like the National Concrete Bridge
Council (NCBC) are joining with other industry associations and the Federal
Highway Administration (FHWA) to provide technical assistance to states and to
secure more funding sources, especially in anticipation of TEA-21
reauthorization. The benefits of HPC for bridges make it attractive for
projects large and small.
Backing the popular demand
The cost savings associated with HPC makes bridge
construction with the technique a good long-term decision, said Shri Bhide, chairman
of the NCBC and manager of the Portland Cement Association's (PCA) bridge
program. "HPC bridges have minimal maintenance needs," said Bhide,
"and the lifespan of HPC bridges is typically twice that of conventionally
built bridges."
HPC makes it possible to create longer components for the
bridges, reducing the number of piers in water and adjacent to roadways,
resulting in lower foundation costs and improved safety. It reduces the number
of expansion joints and bearings and the subsequent need for repair and
replacement. HPC also lends itself to use of precast/prestressed elements,
which reduce construction time and labor costs. "HPC ties in very well
with FHWA's emphasis on reducing congestion through accelerated construction
techniques," said Lou Triandafilou, P.E., high-performance structural
materials specialist for FHWA's Resource Center in Baltimore.
The durability of HPC makes it equally attractive. HPC
bridges are typically designed for 100 years of service life, and advances in
testing and mix design allow designers to create concrete that resists
corrosion in the harshest environments.
Helping the deficient
A prime motivator for increased use of HPC in bridge
building is the high number of deficient bridges in the U.S. According to 2002
National Bridge Inventory data, one in four bridges is structurally deficient
or functionally obsolete. The NCBC and FHWA believe HPC usage can reduce the
number of deficient bridges and create a more lasting infrastructure.
"The FHWA's goal is to get in front of the bridge
deterioration curve and stay there," said Bhide.
Perhaps the biggest challenge, agreed Bhide and
Triandafilou, is to get state DOTs comfortable with the method.
DOTs often hesitate to use new technology, said
Triandafilou: "When it comes to HPC, putting that first foot forward and
taking steps is a challenge."
To that end, FHWA is identifying states with potential for
growth in HPC use and working personally with their administrations to speed
the process. Partnerships with the concrete industry in those states smooth the
way, and Bhide said the NCBC and state organizations have plans to hold
workshops, introduce courses at universities and develop certification programs
to further FHWA's efforts.
So far, the result is a rapid increase in HPC bridge
construction activity. In the last five years, under TEA-21's Innovative Bridge
Research and Construction (IBRC) Program, 40 HPC bridges were constructed using
the cast-in-place bridge deck method and 20 HPC bridges were built using
prestressed concrete girder superstructures. HPC substructures and foundations
were funded on 10 additional bridges. States leading the pack include New York,
New Hampshire, Virginia, Ohio, Washington and Florida.
More super strength
At the Turner-Fairbank Highway Research Center in McLean,
Va., the FHWA is researching what Triandafilou called "the next
generation" in HPC: ultra high-performance concrete. Capable of strengths
more than double that of typical HPC, the material uses prestressing strands
but requires no conventional reinforcing, relying on steel and polypropylene
structural fibers in the concrete mix.
"Because of the higher strength of this material, we
can look at new shapes of girders that take the best advantage of its higher
strength," said Triandafilou. Additional research is focused on creating
HPC with lightweight aggregates and on self-consolidating concrete, which is
gaining popularity among North American precasters based on experiences abroad.
The concrete itself is only a small piece of the puzzle,
said Triandafilou. For HPC bridges to become truly mainstream, he said,
"there is a learning curve, both for materials and for construction,
placement, handling and curing, and that comes back to working with local
industry and getting people comfortable with using it." To increase
dialogue, the FHWA created the HPC exchange
(know-ledge.fhwa.dot.gov/cops/hpcx.nsf/home).
With TEA-21 reauthorization still up in the air, the
industry is hesitant to put a number on predictions for the future of HPC bridges.
Nineteen HPC projects were submitted for review and approval under the last
year of the IBRC Program, and Triandafilou said many states will embrace the
technique no matter what level of funding is available. The FHWA and NCBC both
consider the construction of HPC bridges in every state a key goal.
Examples of successful training in the U.S. are:
Wacker Drive Reconstruction--
Chicago
High-performance concrete played a key role in reviving
Chicago's Wacker Drive, a main artery through the heart of the city's downtown
area. First completed in 1926, the drive in recent years has served as many as
75,000 automobiles a day on its upper and lower levels, not to mention more
than 100,000 cars that cross it daily on intersecting streets and bridges.
After 75 years of service, the drive was severely deteriorated when the Wacker
Drive Viaduct Reconstruction Project began in October 2001.
Wiss, Janney, Elstner Associates Inc., Northbrook, Ill.,
developed an HPC mix that would resist chloride penetration, achieve appropriately
high compressive strengths and work well in both post-tensioned segmental and
traditional cast-in-place construction. A quaternary mix--incorporating
low-alkali cement, class F fly ash, silica fume and ground granulated blast
furnace slag--provided concrete that could stand up to Chicago's vacillating
freeze-thaw cycles and aggressive wintertime salting program, while providing
workability in areas of congested reinforcing steel and post-tensioning ducts
and anchors.
Tim Schmidt, construction manager for Alfred Benesch &
Co., helped oversee the work of contractors and resident engineers hired by the
Chicago Department of Transportation on the project. Schmidt said that while
"the HPC used on the project possessed many good qualities,"
controlling air contents was a challenge that required aggressive quality
control and assurance efforts.
Virginia Dare Memorial Bridge--Manteo, N.C.
Located on U.S. 64/264 over the Croatan Sound at Manteo, the
Virginia Dare Memorial Bridge is the longest bridge in North Carolina,
connecting the mainland to Roanoke Island. The bridge is on a hurricane
evacuation route in a highly corrosive coastal environment, and design had to
accommodate high-level navigable clearances, vessel impact forces and coastal
storm surge and scour characteristics. With allowances for nearby
environmentally sensitive wetlands further complicating the situation, the
design team had a lot of challenges to tackle.
In a unique design partnership, Wilbur Smith Associates
provided structural design, working with the North Carolina Department of
Transportation's plan for durability. Rodger D. Rochelle, state research
engineer with NCDOT, led a research group that used Fick's Second Law of
Diffusion to model diffusion of chlorides and determine the best HPC mix design
for every bridge component, ensuring 100 years of service life for the entire
structure.
"Designing for durability is a rather new
concept," said Rochelle. The model examines input parameters that take
into account the quality of the concrete, the amount of concrete cover over
reinforcing steel and how much chloride exposure each structural element will
experience. For each major component, the law was applied and a mix was
designed. After running between 300 and 400 permutations, the team evaluated
constructability of a narrowed field of choices and determined the best
approach for each component.
The NCDOT team started a new research project in July 2003
that will measure chloride exposures in core samples of existing bridges
throughout the state, using the data to apply the model even more accurately on
future projects.
About The Author: Prokopy is a freelance writer based in Chicago, Ill.
