The
typical bridge service life is less than 50 years. Over ¼ of
the US’s 590,000 bridges are currently classified as structurally
deficient or functionally obsolete. Problems in bridge construction
and maintenance include quality control, premature deterioration caused
by corrosion, insufficient staffing and funding resources to maintain
aging bridges, and increasing traffic volumes and load weight.
An American Society of Civil Engineers (ASCE) study of over 500 bridge
failures between 1990 to 2000 found 53% were from flood and scour
(undercutting or exposing foundations). The average age of a failed
bridge was 62.6
years. Bridge overload and lateral impact forces from trucks, barges/ships,
and trains constituted another 20% of the total bridge failures.
The current practice in the United States is to inspect every highway
bridge
at least once every 2 years. Other countries use a risk-based approach
for both frequency and how in-depth the inspection needs to be.
In 1998, the Federal Highway Administration’s (FHWA) created
a program to encourage the development of high-performance materials
and techniques as well as reduce the maintenance and life-cycle costs
of bridges. Some notable improvements have been made in the past ten
years.
Even old standbys, concrete and steel, have been vastly improved
into ‘high
performance’ products. HPS is stronger and tougher than conventional
steel, easier to weld, and offers greater resistance to cracking. It
has now been used in more than 150 bridges in the US.
New versions of concrete provide better quality, improved durability,
and higher strength. HPC has a significantly longer life expectancy
than conventional concrete and it has come into standard usage
in many states. Already UHPC (ultra high performance concrete)
and SCC
(self-consolidating
concrete), which has a high degree of workability and does not
require vibration to achieve full compaction, are beginning to
be used.
SCC can be tricky in cold weather – it is more sensitive to temperature
during the hardening process than vibrated concrete. But eliminating
vibration cuts down on the labor needed and speeds up construction,
resulting in cost savings and less traffic disruption. Studies abroad
have found as much as seven percent project cost decrease with SCC.
It also reduces the noise level at construction sites and reduces aggregate
segregation, honey combing, and voids in the concrete.
Another technology becoming poplular is fiber-reinforced polymer
(FRP) composites. FRP has unique properties, such as corrosion
resistance, high strength, light weight, and fatigue resistance,
which make it
very attractive for the strengthening, hardening, repair, and
seismic retrofit of bridges and structures. FRP composites are
typically
made
of fibers - glass, aramid, and carbon in a polyester or vinyl
ester resin matrix. FRP composite deck systems for new bridges
provide
relatively easy construction and handling and can be pre-engineered
and prefabricated
offsite, facilitating a rapid installation.
The use of prefabricated bridge technologies has proven in Japan
and Europe that it can minimize traffic disruptions, increase
quality, often reduce initial cost, lower life-cycle costs,
and minimize
disruptions to the environment. Pre-casting produces superior
product because
fabrication
tolerances are greater uncer controlled conditions, environmental
effects are lessened, and forms may be used repeatedly.
An essential element of large scale prefabrication is the
use of SPMTs, specialized vehicles to move and place large
bridge
components.
Automated
bridge building machines/robots are also becoming more
sophisticated and will undoubtedly become more common as well.
Computer analysis and design programs are making their
mark in bridge building as well. Programs developed by
Washington
State
to meet
AASHTO Load and Resistance Factor Design (LRFD) Bridge
Specifications (which
must be implemented in all states by late 2007) are available
at http://www.wsdot.wa.gov/eesc/bridge/software.
There is much work to be done to replace and repair our
aging infrastructure but we can hope that product innovations,
new technology, and creative
ideas will help us make big improvements and avoid the
failures
of the past.
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