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 kg/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.