Rico Fung, Cement Association of Canada
In Canada, the extreme weather conditions and liberal use of deicing chemicals have led to severe deterioration of many concrete structures. In the search to improve durability and extend service life, Concrete Canada was established in 1990 to conduct a coordinated and focused high performance concrete (HPC) program. The technology transfer component of the program included many seminars, workshops, and technology transfer days across Canada as well as demonstration projects to implement HPC on construction sites. As a result, many HPC structures were built in Canada.
The first Canadian HPC bridge was a 56-ft (17-m) long single-span bridge at St. Eustache, Quebec built in 1992. The structure consists of adjacent pretensioned channel girders made with 10,000 psi (70 MPa) compressive strength concrete. The deck was cast with 4300 psi (30 MPa) compressive strength concrete.
In other provinces, HPC applications have been evolving with local expertise and experience. The pre-eminent example of precast HPC is the 8-mile (13-km) long Confederation Bridge connecting Prince Edward Island to Nova Scotia.* Designed for a service life of 100 years in a severe salt exposure and freeze-thaw marine environment, corrosion protection of the reinforcement in the superstructure is provided solely by the HPC.
For short and medium span bridges, precast, prestressed HPC bridge girders have shown significant technical and economic benefits. On the Highway 407 project in Ontario, two prototype bridges were constructed of HPC in 1997. For one of the bridges, precast, pretensioned girders were cast with 8700 psi (60 MPa) compressive strength HPC. The concrete strength at transfer was 7000 psi (48 MPa) and special prestressing strands with a cross-sectional area of 0.167 sq in. (108 sq mm) were used. By these means, the number of girders required in each span was reduced from 4 to 3, realizing a savings of about $19,000 (CAN$30,000). Since then, over 60 bridges with HPC have been constructed in Ontario.
The specifications for cast-in-place and precast HPC tend to include the following common elements:
Cement: ternary Type 10E-SF
Silica fume content: 6.0 to 9.5 percent
Cement content: 590 to 760 lb/cu yd (350 to 450 kg/cu m)
Supplementary cementitious materials (slag or fly ash): 0 to 25 percent
Water-cementitious materials ratio: 0.32 to 0.37 28-day compressive strength: 5800 to 10,900 psi (40 to 75 MPa), with 7250 psi (50 MPa) in most cases
Plastic air content: 5 to 8 percent
Rapid chloride permeability (ASTM C 1202): less than 1000 coulombs at 28 days
Based on experiences in Canada over the last 10 years, several observations can be made. The rapid chloride permeability (RCP) test has proven to be a reliable index of durability. Stable in-situ, air-void systems can routinely be achieved if suitable air-entrainment admixtures are chosen, and the mixes are designed to allow for significant testing variations.
Pre-construction and pre-concreting meetings are essential for the successful implementation of HPC. All those responsible for the supply, installation, and supervision of concreting should participate. Adequate lead-time should be allowed for trial mixes or trial placements.
Fog misting is a must immediately after finishing to prevent premature cracking. This must be followed by 7 days of wet curing.
More Information
Bickley, J. A. and Mitchell, D., “A State-of-the-Art Review of High Performance Concrete Structures Built in Canada: 1990-2000,” Cement Association of Canada, May 2001, 114 pp. available at www.cement.ca.
*See HPC Bridge Views, Issue No. 5, September/October 1999.