High-Performance Concrete Case Study
Bridges Home > High
Performance Concrete > Confederation
Bridge
HPC FOR DURABILITY OF THE CONFEDERATION BRIDGE
Laszlo Dunaszegi, Stantec Consulting Ltd., Calgary
 The
Confederation Bridge is an 8.1-mile (13-km) long bridge across the
Northumberland Strait between Prince Edward Island and New Brunswick,
Canada. Opened in 1997, the bridge consists of gravity-based piers
and a single-cell box-girder superstructure. It was constructed
under a design-build-operate-transfer contract in which the developer
operates the bridge for 35 years and then transfers the bridge to
the federal government.
Long-Term Durability
The aggressive environment of the Northumberland Strait includes
significant amounts of annual ice that are constantly moving, high
winds that result in splash and spray zones on the piers, and frequent
cycles of freezing and thawing.
After extensive review and consideration of the various factors
affecting corrosion, it was concluded that the most effective way
to protect the structure against corrosion was to utilize high performance
concrete in combination with increased concrete cover to the reinforcement.
No epoxy-coated reinforcement or corrosion inhibiting admixtures
are used due to a perceived high cost-to-benefit potential. The
HPC specified on this project possesses a low chloride ion permeability
and a high electrical resistivity.
Diffusivity tests conducted on concrete specimens made in the field
using the actual mixture and using field placement techniques yielded
diffusion coefficients as low as 4.8 x 10-13 m2/s at a maturity
of six months. This value is 10 to 30 times lower than the diffusion
coefficients of conventional concretes.
The electrical resistance of the HPC was measured in the 470 to
530 ohm-m range, based on wet specimens with a six month maturity.
This compares to a 50 ohm-m range for conventional concretes.
Since the attainment of long-term durability depends on the quality
of the concrete protecting the reinforcement, curing of the surface
concrete is important. Moist curing was not always practical due
to the size of the components and construction in winter. Therefore,
combinations of water curing, membrane curing, and in-form curing
for five days were used.
Service Life Estimate
As part of a more general evaluation of the corrosion protection
system, a series of theoretical chloride ion profiles were generated
using the test results and Fick’s Second Law of Diffusion.
These calculations provided an order of magnitude estimate of the
increase in chloride concentration with time. For the case of concrete
located in the splash or tidal zones, the generally accepted chloride
ion threshold level of 0.4 percent of the cement content, or 2.7
lb/cu yd (1.6 k g / m3), is not expected to be exceeded
until an approximate age of 60 years with 3 in. (75 mm) of concrete
cover to the reinforcement. Also, since research results indicate
that the diffusion coefficient of HPC continues to decrease with
time, it is anticipated that the time required to actually depassivate
the reinforcement will be longer than the 60 years projected.
Attainment of the theoretical corrosion threshold, however, does
not mean that significant corrosion will occur immediately. The
rate of corrosion in the reinforcement depends on a number of factors
including temperature, oxygen availability, and concrete resistivity.
The high concrete resistivity in itself will result in a rate of
corrosion that is potentially less than 10 percent of the corrosion
rate for conventional concretes. This could extend the duration
from depassivation to initial spalling from three years, which is
typically assumed for conventional concretes, to over 30 years.
Taking all these factors into account, it is felt that the specified
HPC, in conjunction with the inspection and maintenance program,
should efficiently protect the embedded reinforcement from corrosion
during the 100-year design life.
Further Information
More detailed information about this bridge is given in a series
of articles in the Canadian Journal of Civil Engineering,
December 1997.
|
 |
|