Dale Serink, UMA Engineering Ltd.
Located near Edmonton, Alberta, the Sturgeon River bridge incorporates innovative features aimed at achieving a 100-year service life and meeting difficult geometric constraints. Integral abutment design, stainless steel-clad reinforcement, and high performance concrete were employed to achieve these objectives. The selected design consists of a single span 131-ft (40-m) long low profile precast, prestressed concrete girder bridge with high performance concrete in the deck and the girders.
Girder Design
The standard girder shape in Alberta at the time of construction was the bulbtee girder available in three standard depths of 43.3, 55.1, and 66.9 in. (1100, 1400, and 1700 mm). For a 131-ft (40-m) single span bridge, a 66.9-in (1700-mm) deep girder is generally required. However, geometric constraints allowed only 55.1 in. (1400 mm) of girder depth at this site. In order to meet design requirements, a 131-ft (40-m) span high performance concrete and integral abutment design were employed.
Using high performance concrete in the girders allowed 0.6-in. (15.2-mm) diameter strands to be substituted for the standard 0.5-in. (12.7-mm) diameter strands, thereby, increasing the girder’s capacity. Thirty-eight strands were required for each girder. A concrete strength at transfer of 6530 psi (45 MPa) and a design strength of 9430 psi (65 MPa) were specified to meet the requirements.
The girders were fabricated in Calgary and transported more than 185 miles (300 km) to the site by truck. The fabricator was able to achieve a one-day construction cycle for each girder and complete the fabrication ahead of schedule. There were initial concerns over transportation of the slender girders but no problems were encountered during fabrication or erection.
Bridge Deck Design
The most common service failure for bridge decks is corrosion of the top layer of steel reinforcement. Traditional methods of prolonging the onset of corrosion include reducing the permeability of the concrete, coating the steel with epoxy, or coating the top of the deck with a waterproof layer. All these methods have been effective but have not yet provided the desired increase in service life. To assure a 100-year service life, a more innovative deck design was required.
Rather than focusing on methods to protect the top layer of steel reinforcement, use of stainless steel-clad reinforcement was specified. Stainless steel differs from regular steel in that it is essentially neutral in an alkaline environment. The chromium content of the steel allows the formation of a self-healing chromium oxide film on the steel surface. Stainless steel-clad reinforcement combines a stainless steel outer layer with a carbon steel core to provide the benefits of stainless steel at a reasonable cost. This type of reinforcement is produced at a cost about double that of epoxy-coated reinforcement.
The concrete deck consisted of 7250 psi (50 MPa) compressive strength high performance concrete with a watercementitious materials ratio of 0.30 and incorporating both silica fume and fly ash. This provided a high quality deck with greater resistance to carbonation and a minimum 100-year period for chloride ions to diffuse to the unprotected bottom layer of deck reinforcement.
The deck finishing proved to be more challenging than expected. Alberta Transportation’s specifications require the use of magnesium floats to finish the concrete. For this sticky concrete mix, stainless steel floats seemed to reduce surface tearing and provided a superior finish. The deck was prepared for traffic with the application of 1/4-in. (6-mm) deep transverse grooves. Some tearing proved to be unavoidable during this operation, particularly in the last corner of the deck completed where the concrete had cured for a longer period before it was raked.
The new Sturgeon River bridge was completed in 16 weeks at a unit cost lower than the average unit cost for bridges of this category in Alberta. High performance concrete allowed the difficult constraints of the project to be met and will contribute to the long-term durability of the bridge. However, it is not the use of high-tech materials or the innovativeness of any single feature alone that provides an outstanding solution but the careful selection of technologies that complement each other to achieve the challenging goals of this project.
Further Information
For further information, contact the author at [email protected]. or 780-486-7043