| Better Bridges
New Techniques Enhance High-Performance Precast Bridges
A variety of ideas being tested and already in the
field aim to enhance
precast concrete bridge components
and offer designers new alternatives.
by Andrew J. Keenan
Bridge designers continually seek new approaches and ideas that will
make their structures more aesthetically pleasing, functionally effective,
and cost efficient. Precast, prestressed bridge components have long
offered a strong option in this regard, due to their speed of construction
and low, long-term maintenance, which minimizes costs.
Today, a variety of new programs, underway in both laboratories and in
the field, are establishing new techniques for designing high-performance
precast concrete bridges that perform even better.
High-performance concrete bridges include two key elements: total
precast bridge systems that can dramatically improve construction speed,
and high-performance concrete that can improve durability and structural
efficiency. In HPC bridges, these improvements are achieved at no cost
premium and often at a reduced initial cost. HPC uses the same materials
as typical concrete, but it is engineered to provide higher strength and
better durability. These traits can be varied to align with the design’s
needs and ultimately will be affected somewhat by weather and temperature
conditions and whether the components involve substructure, beams, or
deck.
HPC educational program
To spread the word about the benefits of high-performance precast
concrete bridges to designers and legislators, the National Concrete
Bridge Council instituted an educational program. The group hopes to
generate more research and application funding to take advantage of the
potential and expand the use of HPC bridges by federal, state, city, and
local jurisdictions. NCBC also hopes to gain federal funding to enhance
technology transfer, research, and creation of additional high-performing
concrete bridges. Work has begun on a strategic plan that is currently
being developed. This program will be used to work with various
legislative, funding, and design groups to advance applications of
high-performing concrete bridges.
HPC components can drastically reduce construction time because the
longer spans allow for fewer girders and/or piers. This minimizes the
number of picks required and speeds the erection process. HPC can be used
effectively in virtually all bridge components, including: piers and pier
columns; pier and pile caps; abutments; decks; rails; and barriers.
The desirability of higher strength and durability must be balanced
with the needs for constructability and cost control. But the longer life
spans possible with HPC designs can dramatically cut maintenance and even
replacement needs over the entire life of the bridge. This means designers
should examine all costs, especially maintenance and replacement needs
over the bridge’s full lifespan, when considering construction options.
HPC criteria
To ensure all designers and suppliers understand the term, the FHWA
created a uniform definition of HPC that consists of four durability and
four strength parameters. These eight criteria are supported by
performance criteria, testing procedures, and recommendations for
adaptations to field conditions. The eight criteria are:
1. Freeze/thaw durability.
2. Scaling resistance.
3. Abrasion.
4. Chloride penetration.
5. Strength.
6. Elasticity.
7. Shrinkage.
8. Creep.
For its definition, the Precast/Prestressed Concrete Institute’s High
Performance Concrete Committee states that HPC must offer a 28-day
compressive strength above 8,000 psi. But defining the material by its
compressive strength focuses its impact solely on the strength parameter,
which is not the only factor that makes HPC such an ideal candidate for
bridge design. Indeed, precasters have systematically been improving their
compressive strength for many years, with many offering higher-strength
concrete than is called for in current specs. This high early strength
offers a win-win situation, as it provides better design capabilities
while allowing casting beds to be turned quicker.
In fact, while much attention has been drawn to HPC’s strength
characteristics, the material’s durability performance creates a strong
argument for its use even where long spans or other strength capabilities
are not required. This durability reduces costs over the life span, just
as its strength can reduce material costs by eliminating a line of piers
or other supports for its in-ground costs. This resistance to the
environment makes HPC a popular choice for those designers creating
bridges in harsh climates or places where it is difficult to maintain
bridges.
In general, HPC components can produce lighter, longer precast pieces
and smaller-diameter columns that creep less. This means span lengths can
be lengthened and underclearances can be maximized. These advantages
accrue especially to HPC decks, as concrete girders long have shown their
ability to withstand the rigors of their environment.
Ultra-high performance concrete
Researchers now are looking to push high-performance concrete to
another level, creating concrete mixes far beyond those in current use.
One of these now being tested — and used in the field — is called
ultra-high-performance concrete. It has been specifically engineered to
produce a highly compacted concrete with a small, disconnected pore
structure, according to researchers with the Federal Highway
Administration.
The material includes fine sand, quartz flour, and steel or organic
fibers measuring 0.5-inch long and 6 mils in diameter. These fibers are
responsible for much of the tensile strength and toughness of the
material. Its compressive strength can reach 30,000 psi, more than twice
that of any high-performance concrete used for bridge components to date.
The material was developed by Bouyges S.A. in Paris in 1995 and has been
produced by LaFarge S.A. in Paris, as well as by a few precasters in the
United States.
Tests are being conducted at the Turner-Fairbank Highway Research
Center Structures Laboratory in McLean, Virginia using an 80-foot-long
girder. Under load testing, the beam deflected 19 inches before failing,
and it deflected 12 inches with no sign of any visible cracking —
including examination under magnification of 300%. The beam’s ultimate
failure resulted from the prestressing strand breaking at the single
location where gross cracking was observed.
Several components have been produced for use in LaFarge’s facilities
in southern Illinois, including a 20-foot I beam and a roof panel for a
concrete silo. The roof panel is so strong that it measures 2-inches thick
at its maximum depth. To make full use of the material, however, batching
and sorting with this material must be handled in new ways by the
precaster, and it can be cast so quickly that precasting operations will
have to adapt by using longer beds and new processes to make the material
cost effective.
The incredible strength of the UHPC mix means component designs also
would have to be altered to take advantage of the material’s abilities,
adapting the stout, compact shapes that allow significant strand spaces to
make them thinner and lighter. The Virginia Department of Transportation
currently is using the test data to consider building a bridge using UHPC
in the next two years.
Spliced-girder study
Another study underway is designed to catalog and unify approaches
being used in various regions incorporating precast concrete
spliced-girder technology. The National Cooperative Highway Research
Program has commissioned Ralph Whitehead Associates Inc. in Charlotte,
North Carolina to conduct a study on these techniques to be completed next
spring. The $240,000 study is intended to develop design recommendations,
standard details, design examples, and proposed design specifications for
achieving longer spans using precast bridge girders.
The team intends to document existing spliced-girder bridges and
construction methods and design approaches, including available software.
It also plans to compare the different approaches, factors, and concepts
being used around the world. It will provide design examples and
procedures for design and fabrication as well as construction
specifications. The team will develop proposed revisions to the AASHTO
LRFD Bridge Design Specifications, with a publication date set for next
April.
New girder designs
Other techniques help designers with specific design challenges. For
instance, precast, prestressed bulb-tee girders post-tensioned for
continuity have been used extensively over the past 10 years. But in
recent years, bridge designers and precasters have been tweaking existing
designs to produce more efficient shapes. A variety of localities, from
Massachusetts to Washington state, are creating options that meet the
needs of bridge designers nationwide.
One of the first bridges using the NEBT girder was the Jetport
Interchange on the Maine Turnpike. The overpass’ location required that
the structure be built over a heavily traveled roadway in an area of poor
soil conditions. Rather than use traditional steel-beam construction,
designers specified the NEBT girder due to its lower cost of construction,
operation, and maintenance. It also provided the aesthetic look desired as
well as the ability to maintain uninterrupted turnpike traffic during
construction. The bridge was erected at night with only one lane of
traffic closed at a time.
The girders permitted the use of integral abutments, which reduced
substructure costs by eliminating bearing devices and roadway joints.
That, in turn, will trim maintenance costs and improve ride quality
throughout the bridge’s life. Wider flanges on the NEBT girder also
permit the use of a thinner deck slab, reducing costs further.
The bridge features two 125-foot-long end spans and an interior span of
106 feet, giving it a total length of 396 feet. Each span contains six
precast, prestressed NEBT girders measuring 71-inches deep with a
7.5-inch-thick ordinary reinforced concrete deck. The deck is 43-inches
wide and carries two 12-foot-wide traffic lanes plus 8-foot shoulders and
concrete parapets. Abutment and pier locations were set to accommodate
future widening of the turnpike.
A variety of states now use the NEBT, with projects in New Hampshire,
Connecticut, Massachusetts, New York, Vermont, and Maine incorporating its
design specs. The girder also was specified for more than eight design
contracts in Boston’s massive Central Artery/Tunnel project, which is
remaking traffic patterns throughout the city.
Another innovative design has been produced in Washington state and was
first used on the Allen Street bridge over the Cowlitz River in Kelso,
Washington. The $14-million structure, completed in January 2001, features
60 precast concrete girders of a design that extends typical span lengths
to 175 feet, at least 20 feet longer than past designs used in the state,
providing a more cost-effective structure.
The girder, called the W83G, has geometry based on metric dimensions
and was developed in accordance with the AASHTO LRFD Bridge Design
Specifications. The girder was modeled after the Nebraska NU girder types.
The bridge’s 60 precast, prestressed girders include 25 that are
164-feet long and 35 that are approximately 150 feet in length. Girders
are 6-feet, 10.625-inches high with a 6.125-inch-thick web; a 4-foot,
1-inch top flange; and a 3-foot, 2.375-inch-wide bottom flange.
One advantage of the longer spans was that they minimized the number of
piers, a key issue for this project and many others. The 1,114-foot bridge
was designed with just three piers in the environmentally sensitive river
instead of four. Adding a fourth pier also might have extended
construction into a second season, which the city wanted to avoid at all
costs.
All of these design techniques can enhance the performance of precast,
prestressed concrete bridges. Bridges already constructed using a number
of these techniques have shown clearly that they provide excellent
durability and cost efficiency. With the precasting industry and those who
work with the material helping to spread the word of these advantages
through studies and in-field demonstrations, designers, and transportation
department officials at the federal, state, and local levels will gain new
weapons in their design arsenals. These approaches can produce
aesthetically attractive, functionally enhanced, and cost-efficient
designs for virtually any bridge environment and situation.
Andrew J. Keenan is vice president of sales, Prestress Engineering
Corp. and chairman of the Transportation Market Segment of the Precast/Prestressed
Concrete Institute.
Reprinted from Better Roads Magazine
August 2002 |
