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New technologies like CFRP help strengthen bridge columns in seismic zones
Twenty-five deteriorated bridges along a 5-mile corridor of
I-80 in Salt Lake City were in need of major repair. Most were showing signs of
advanced corrosion due to prolonged exposure to the harsh environments and
deicing salts. Of the 25 bridges, 12 were considered structurally deficient
indicating that significant elements of each bridge needed repair. There was a
considerable amount of spalling concrete from the decks, pedestals, bent caps
and columns, and beams and bearing units also were in poor condition. Without
repairs, many of these spans might have required weight restrictions, bracing,
shoring or emergency replacement of decks and other components. All of the
bridges did not meet current seismic design standards.
I-80 in this part of Salt Lake City crosses over the Wasatch
Fault that runs along the east bench of the valley. The Wasatch Fault is one of
the longest and most active faults of its type in the world and contributes to
the Wasatch Front having the greatest earthquake risk in the interior western
U.S. The bridge structures along this section of freeway were designed and
built between 1964 and 1971, prior to the seismic design codes in place today.
The violent shaking during an earthquake could collapse a number of these older
and under-designed bridge structures along I-80 between State Street and the
mouth of Parley's Canyon. Since it was not economically feasible to completely
rebuild the bridges to current seismic standards, other options were
considered. Ultimately, it was decided to use simple, low-cost techniques,
including carbon fiber reinforced polymer (CFRP) fabrics and other structural
repairs that could reduce the severity of damage from an earthquake.
The Utah DOT engineers and expert consultants from the University
of Utah discussed and presented new technologies such as CFRP, emphasizing the
importance of performing as much seismic retrofit work as possible. UDOT
expressed its intent to complete as much research as funding would allow (in
partnership with the university) on the rehabilitation techniques and materials
to be used for the I-80 project. UDOT hoped to extend the life of these bridges
for a minimum of 10 years at which time they would rebuild this section of
freeway with new structures.
In 1998 and 1999, University of Utah engineers were
presented with the unique opportunity of conducting full-scale, in-situ tests
of the CFRP system on bridge bents similar to the ones on I-80 scheduled for
demolition. Dr. Chris Pantelides, professor of civil engineering at the
University of Utah, explained that in the tests several concepts and designs on
retrofitting older bridges with FRP were tested and evaluated including:
1. Restoration of columns for confinement, lap splice
control and shear strengthening;
2. Shear strengthening of the bent-cap column joints; and
3. Tensile anchorage of longitudinal column bars ending in
the bent cap.
The test bents
were wrapped with CFRP according to the design and then pushed to failure. The
retrofitted columns and bent caps deflected 201/2 in. before reaching total
failure. Pantelides explained, "All of the design concepts were verified
in this testing, and the goal of improving the ductility of the bridge bent
caps and columns was achieved." These proven design ideas were eventually
implemented on the State Street and I-80 Bridge proj-ects, which needed
flexural strengthening of the bent caps as well as shear reinforcement.
The second part of this research program was the development
of the specifications. The specs were written with input from UDOT, the
University of Utah and outside consultants who created special provisions for
column and bent-cap wrapping. UDOT said that due to environmental concerns only
CFRP systems would be considered. The composite wrap system was required to
meet minimum initial properties for tensile strength, Naval Ordnance Laboratory
(NOL) Ring strength, fiber volume and glass transition temperatures. In
addition, the special provision called for the required design thickness to be
based on an environmental durability rating factor, which would account for
material property losses due to environmental aging over the projected life of
Test the panel, train the group
Another part of the specification required a great deal of
field sampling and testing to ensure quality control on the project. This
included flat panel samples, NOL Rings and core samples of the CFRP to verify
strength, stiffness, fiber volume, resin/fiber ratio, thickness and glass
transition temperature. An independent laboratory tested the panels and core
samples and the results were sent to both the contractor and UDOT. Michael
Fazio, P.E., UDOT, stated, "The benefit of this testing was an
after-the-fact assurance of quality control on the project. The testing was specified
in the beginning because the selection of the fiber and resins was unknown and
testing was required to assure quality and uniformity of the product from our
Gerber Construction, Lehi, Utah, UDOT, the Federal Highway
Administration and Sika Corp., Lyndhurst, N.J., conducted a training seminar
long before the first layer of CFRP was installed to ensure a smooth project.
During the seminar, installation crews, inspectors, quality assurance, quality
control and testing personnel were given training and hands-on experience with
the CFRP. This training helped to coordinate the responsibilities of all
parties and provided a perspective from the installer's point of view while
focusing on how important QC was going to be. Excellent cooperation on the
project was a key benefit of the training seminar and made everyone's job
Five bridges were chosen to receive the CFRP for seismic
upgrade. A total of 76 columns and four bent caps (beams) were wrapped. Some
columns received as many as 17 layers of CFRP. During the project, 124,000 sq
ft of fabric and 1,760 gal of epoxy were used. After weeks of concrete
restoration and repair were completed, surface preparation began prior to the
installation of the fabric.
The bent caps at the State Street location required the most
extensive surface preparation and were the most difficult because of the
horizontal surfaces. The Highland Drive Bridge required the most extensive
concrete repair prior to application of the CFRP.
Traffic control also was a critical part of this project.
Because some traffic lanes could only be closed during certain hours of the
day, the staging of the work by the contractor was very important.
Using a typical installation crew of 10 workers, Gerber
Construction was able to make quick progress on the bridges. Custom-built
platforms lifted in place by a forklift allowed the workers 360° access to
the columns, eliminating the need for scaffolding and providing a time and cost
savings. The installation crews were responsible for making the required daily
test specimens that had to be prepared with great care and precision while
keeping detailed logs of the installation process and test samples.
Special tools and equipment were needed (like the
custom-made NOL Ring forms) to produce the samples and cure and store them
before delivery to the laboratory for testing. Approximately 342 flat panel
specimens were prepared along with several dozen NOL rings and nearly 75 core
samples during the project.
The application of fabric on the bridge columns was
clear-cut, but the bent caps presented a new challenge for the installation
crew. Because the bent caps would require flexural layers of CFRP on the bottom
followed by diagonal wrap layers and then circumferential wraps, the crew spent
a great deal of time cutting and preparing fabric for the special widths and
angles. This part of the installation was further complicated by the fact that
the bridge girders were offset from one another. Following the curing and
testing of the CFRP it was coated with a textured acrylic coating for UV and
abrasion protection, as well as long-term durability.
Not only was field inspection performed by UDOT for quality
assurance, but the contractor also had to provide an independent quality control
inspector to make daily site visits and prepare reports on the installation
process. Every day during construction two site visits were conducted. During
these inspections surface preparation, fabric saturation, application, curing,
sample preparation and coatings were reviewed.
Currently, the State Street Bridge is being monitored by the
University of Utah to determine in-situ environmental characteristics of the
CFRP composite. This includes both non-destructive and destructive testing. NOL
Rings (20-in.-diam. cylinders constructed from five layers of CFRP) exposed to
the environment are currently being stored at the bridge site and will be
tested on an ongoing basis. The data from these tests will provide needed
information about the long-term effects of UV exposure, salt water and
freeze-thaw cycles on the CFRP. Flat panels plus CFRP-wrapped concrete
cylinders and beams will be tested for tensile, direct pull-off, flexural and
axial compression strength as part of this study.
The deteriorated concrete was removed and replaced and the
carbon fiber system did not alter the existing geometry or look of the bridge
columns and bents. The strengthening materials were unobtrusive and were coated
as well to blend in with the existing bridge structure.
In the past, alternative methods would have entailed either
steel jacketing or concrete enlargement, but both would have noticeably changed
the appearance of the bridge and would have been a reminder to the public that
a rehabilitation project had taken place.
The installation of the lightweight, non-corrosive,
high-strength CFRP allowed UDOT to seismically upgrade older bridges for a lot
less tax-paying dollars than building new bridges, especially if the bridges
are damaged by a seismic event.
Full-scale testing provided the needed proof of the FRP
design giving the state confidence in the system they specified. Good planning
and staging by a quality contractor minimized impacts on the driving public.
Follow-up testing will provide needed data for strengthening projects in the
A major fault
When people in the U.S. think of earthquakes, places like
California, Utah and Alaska come to mind. However, not many people are aware
that the largest release of seismic energy in the continental U.S. occurred in
the Mississippi Valley. The New Madrid earthquake of 1811-12 included three
main tremors greater than 8.0 on the Richter scale. There also were two events
equal to 8.0 and five more greater than 7.7. By comparison, the Northridge,
Calif., earthquake of 1994 measured 6.7 and the recent devastation in Turkey
The 150-mile New Madrid Fault crosses five state
lines--Arkansas, Missouri, Tennessee, Kentucky and Illinois--and passes under
the Mississippi River in at least three places. The Illinois DOT began a
program in 1999 to seismically upgrade the bridges in this region.
The first project undertaken was the I-57 Bridge in Cairo,
Ill., which is near the apex of the fault. A total of 50 piers and 158 columns
were strengthened using the SikaWrap Composite System. The number of layers
varied from four to 14 and the columns were then coated with two layers of
Sikagard 670W, an acrylic, protective, anti-carbonation coating. A saturator
was used to install 94,000 sq ft of fabric and 1,820 gal of Sikadur 300, a
high-strength, high-modulus epoxy.