15-05-2012, 11:17 AM
COMPOSITE MATERIALS FOR BRIDGE CONSTRUCTION
Composite-Materials-For-Bridge-Construction.pdf (Size: 1.24 MB / Downloads: 363)
ABSTRACT
The California Department of Transportation (Caltrans) has been engaged in cooperative
research with various researchers, universities, and material suppliers to stay abreast of the latest
developments in engineering materials. The area where Caltrans has pushed the envelope is in
the use of advanced composite materials for bridge repair and new construction. We have been
working with the University of California at San Diego for the past ten years to develop field
applications of advanced composite materials for both repair of older structures and construction
of new bridges. These materials are not new but have been used only by the defense and
aerospace industries until the mid 1990’s.
The most highly developed application to date is the use of advanced composites in repair of
bridge columns and other supporting elements to improve their ductility for seismic resistance.
Both epoxy impregnated fiberglass and carbon fiber materials have been tested in the laboratory
on half-scale models of bridge columns to determine the ductility that can be achieved in an
older, non-ductile concrete column. The tests have confirmed the viability of these materials for
strengthening existing structures and field application quality specifications have been
developed. Since March, 1996 these specifications have been published and included as
alternatives in over 50% of the seismic retrofit strengthening contracts advertised for
construction in California.
The more exciting application of advanced composites is for new bridges and bridge deck
replacement units. The research conducted so far has resulted in the design of a highway bridge
composed of three foot diameter carbon fiber tubular bridge girders and a fully advanced
composite bridge deck. Development of these elements has been underway for six years and
field testing of a prototype bridge is currently underway. The bridge design was utilized on a
state highway bridge in Southern California which is located on a major truck route for vehicles
entering the United States from Mexico. Further development of bridge deck replacement
elements composed of advanced composite materials is continuing, with emphasis now on the
connection details between older steel and concrete girders and the new composite decks.
Although these advanced composite materials are expensive, the long life expected and their
resistance to corrosion makes them competitive if the life cycle cost of a bridge in a highly
corrosive environment is considered.
Future plans in the Caltrans-UC San Diego-Defense Advanced Research Projects Agency
(ARPA)-Federal Highway Administration (FHWA) cooperative research program include the
construction of a fully composite vehicle bridge on the UCSD campus, which will cross over
Interstate 5 north of San Diego. Construction of the smaller bridge is a preliminary step in the
development and testing of the various components which will be utilized on this larger bridge.
1. Advanced Composites in Bridge Applications
Following the October, 1989 Loma Prieta earthquake Caltrans began a research program, in
cooperation with the University of California at San Diego (UCSD), to develop techniques for
utilizing epoxy impregnated fiberglass sheets to wrap around older, non-ductile concrete bridge
columns as an alternative to the already proven steel jacket technique. The jackets provide
sufficient confinement in the concrete to allow them to perform in a ductile manner under
seismic loading. It was known that the Japanese had used high strength carbon strands to
similarly reinforce industrial stacks and chimneys but the use of glass fiber sheeting had not been
used. The major unknown was the durability of the fiberglass materials under cyclic loading and
to what level of ductility the columns could be designed. Caltrans technical staff and the
principal investigator at UCSD visited the Swiss National Laboratory in Zurich to observe the
many durability tests they have performed on composite materials. We also visited several field
sites to observe the repair and strengthening applications. The testing program at UCSD was
conducted under the same conditions that were used in the testing of steel plate jackets. Half
scale models of the prototype bridge columns were constructed, wrapped with the desired layers
of glass fiber sheets and tested through several cycles of loading at various levels of ductility
until the column failed due to shear failure and consequent degradation of its hysteretic
performance. These laboratory tests proved that the epoxy impregnated fiberglass column wraps
could develop nearly the same ductile performance as the steel plate jackets.
Material properties are readily available from the manufacturers but there remained the issue
of adequate quality control specifications for the field application. These early applications were
rather crude, being hand laid in a similar manner as hanging wallpaper. It required some months
to fully develop adequate quality control (QC) specifications so the materials tested in the
laboratory could be replicated with confidence in the field. The application using epoxy
impregnated fiberglass has been approved for two systems and field applications have been in
place for over ten years.
In 1993, following the end of the cold war and reduction of major aerospace and defense
applications, the advanced composites industry began looking for applications of advanced
composites in the civil infrastructure. The Caltrans-UCSD testing program was expanded to
develop similar applications for the higher strength carbon fibers. This testing program has
continued as more manufacturers submit their materials for approval and there are at least five
systems approved for field application in California at this time. The carbon fibers are applied
by automatic wrapping machines which wrap several 1/4 inch strands simultaneously and can
fully wrap a typical four to six foot diameter, 20 foot long bridge column in two hours. Because
of the higher strength to weight ratio these materials are very competitive with the steel shell
retrofit technique, and they can be applied with much less heavy lifting equipment. The
materials are much more resistant to corrosion than the steel jackets and they will require very
little maintenance.
Working in cooperation with the University of California at San Diego research team and the
ARPA and FHWA technology transfer programs we have been testing other applications of
advanced composites in the seismic reinforcing of older bridges. We have also investigated the
construction of major bridge components and ultimately, a complete highway bridge designed
for AASHTO loads. The first applications involve resin impregnated fiberglass or carbon sheets
on non circular bridge members. These include the use of sheets to wrap and confine the
spandrel columns and rib members on several arch bridges where it is difficult to access the
locations with heavy equipment. The second application involves the use of small diameter
carbon fiber tubes, constructed by the same technology as rocket bodies, for bridge girders. This
application has been tested at the laboratory and design details have been developed for a bridge
on the state highway system in southern California. The bridge includes deck units and girders
which are composed entirely of advanced composite materials and construction was completed
in late Fall of 2000. This bridge, the Storm King Channel Bridge, is on a heavily traveled truck
route which handles trucks from Mexico coming into the United States.The testing program for
these bridge components has been underway at UC San Diego for over five years, under the
ARPA grant.
2. Column Strengthening
The most widely used application of advanced composite materials for bridges in California
and other states, to date, is the seismic strengthening of bridge columns to improve their ductile
performance in an earthquake. However, there is a larger market for this technology in the
simple repair and strengthening of columns which have deteriorated from corrosion. It is
relatively easy to clean and repair these columns and encase them with the non-corrosive
composite materials. This application will undoubtedly increase the life of the columns or piers.
Three manufacturers have developed prefabricated resin impregnated-fiberglass shells which can
also be used as the form for concrete in the repair process. Many States have followed Caltrans
lead and are using these composite shells for both seismic strengthening and non-seismic repair.
New York State has been the heavy user to date but other states are following the lead as
their confidence in the materials and field applications increases. Figure 1 illustrates the
prefabricated fiberglass column shell which has been approved and can be used also for repair of
piling below the water line.
Figure 1 Installing Prefabricated Fiberglass Shell
Figure 2a) shows the clamping system utilized to hold the pre-fabricated shell tight until the
adhesive cures. This system is the "Clockspring" system, utilizing an Isothalic Polyester resin.
Several layers of shells are applied to provide the required ductility. Figure 2b) shows the
installation of a full height prefabricated shell. This is the "Du-Pont Hardcore" system, utilizing
a Vinylester resin. These applications were installed in 1996 on the Santa Monica Freeway,
Interstate 10, in Los Angeles.
Figure 2a) Clamping the Pre-Fabricated Shell Figure 2b) Full Height Shell
A third prefabricated system has been developed by NCF which utilizes several layers of four
foot high single shells. The system, known as "Snaptite" is fabricated and heat cured on a
mandrel under controlled curing conditions in the manufacturing plant and shipped to the field
much the same as the two systems illustrated in figures 1and 2. This system appears much less
cumbersome to install than the other two prefabricated systems.
Figure 3 shows the use of epoxy-fiberglass as a confinement membrane to increase column
ductility and toughness. This was the first application to be tested and approved in California.
The material has been used for both circular and rectangular columns. The aspect ratio of the
rectangular columns cannot be more than 2:1 or the longer face will buckle under dynamic
loading and the needed confinement will not be maintained. This material can be applied as a
pre-preg or dry application with the epoxy being applied in the field. One of the initial problems
with these materials was uniformity of the final appearance because they are hand laid sheets
about three feet wide. Final appearance is dependent on the expertise of the field crew.
Attempts have been made to design a machine to improve the application and insure more
uniformity, but we have not seen that machine in use in California yet. Careful quality control of
the field application and material mixing is necessary to guarantee a quality final product.
Figure 4 shows the same material after field application and painting have been completed. The
paint serves two functions; one is protection from ultra-violet light and the second is for
aesthetics. The concrete colored paint does an excellent job for both functions. This
application was implemented in 1991 on the Glendale Freeway (State Route 134) in Los
Angeles. These materials had been tested at the UC San Diego Powell Laboratories in 1990 with
excellent results. Both shear and moment ductility of over eight (8) have been achieved in these
tests.
Figure 3 Epoxy-Fiberglass Column Wrap Figure 4 Field Application
in Laboratory Test
Figure 5 illustrates the application of pre-preg carbon fiber wrapping on bridge columns at
the field test site. The wrapping machine does not require heavy lifting equipment and a later
version now applies more strands simultaneously but can wrap a column of 20 foot height in two
hours. The columns are heat cured under controlled conditions by electrically heated blankets or
enclosures. The columns are painted concrete color for aesthetic purposes, but the coating does
provide protection against the elements. This system has been developed by XXSys
Technologies and a second, similar system is being tested by Mitsubishi Industries. The
thickness can be varied as the ductility requirements dictate. In the field applications on the
Santa Monica Freeway the white paint was also used for the same purposes as on the resin
impregnated fiberglass wraps. Figure 6 shows the field application on a seismic retrofit project
in San Diego. This material has the potential of becoming the most cost effective column
wrapping system because of its high strength to weight ratio. The system does not require heavy
lifting equipment and can generally compete favorably against the steel shell retrofit systems.
Since it is not as labor intensive as some of the other systems being approved, it will ultimately
be the system of choice for most contractors. The controlled heat curing system that is used by
XXSys provides a material that is very reliable and has the best chance of guaranteeing the same
properties as those of the laboratory samples. This reliability is more difficult to achieve with
many of the other systems.