The City of Edmonton’s Gold Bar Wastewater Treatment Plant is located along the south shore of the North Saskatchewan River, bound by two scenic city parks, nature trails and the expansive river valley. The plant is a tertiary treatment facility that treats domestic and industrial wastewater from across Edmonton and the surrounding area. (Tertiary treatment is a biological process that reduces or eliminates ammonia-nitrogen and phosphorous.) Serving over 700,000 in the Edmonton area, the plant is among the top five facilities of its kind in Canada. The natural treatment facility currently handles approximately 100,000 million litres annually, which includes wastewater from both the sanitary and combined sewer (sanitary and storm) systems. With the recent plant addition, the plant’s primary treatment capacity will increase from its current 910 million litres/day to 1250 million litres/day. With that added capacity, a much larger volume of wet weather flows from the City’s older combined sewer system will undergo treatment.

In addition to being a leader in wastewater treatment processes, the Gold Bar plant has become the first facility in the world to incorporate clarifier troughs made with “Ductal®” Ultra-High Performance Fibre Reinforced Concrete (UHPFRC).

WINNER
ACI AWARD OF EXCELLENCE 2007
Concrete Design & Construction

Design and Construction
There are two significant areas where the owner (The City of Edmonton) benefits by the designer’s selection of concrete for this project. The first is the use of UHPFRC to construct clarifier troughs for the process area, and the other is the total precast, prestressed concrete solution for the building.

The Clarifier Troughs
Stainless steel was the original material of choice for construction of the clarifier troughs. However, during the design phase it became evident that there were impending cost increases and material shortages facing the steel industry, including supply of specialty products such as stainless steel. As a result, the design engineer (Stantec Architecture Ltd.) was proactive by seeking out alternate materials that would meet their design requirements. In addition to cost, two major technical challenges faced the design team in their quest to find a stainless steel replacement: weight and durability.

The clarifier troughs are supported by filter plate frames which have very limited structural capacity. During the life of the plant, the troughs will have to be removed to allow the plate packs underneath to be serviced. Conventional concrete is too heavy to be practical in this application. The solution that Stantec developed utilized UHPFRC to replace the U-shaped stainless steel clarifier troughs. With a wall thickness of 17 millimetres, the 600 mm by 600 mm troughs would provide a dead weight comparable to the original stainless steel concept. Altogether, 1000 lineal metres of UHPFRC troughs were incorporated into the project.

UHPFRC is extremely mouldable, with high-quality surfaces, low porosity, superior strength and durability properties. It is significantly stronger than conventional concrete, with compressive strengths up to 200 MPa and flexural strengths up to 40 MPa.

The cost to produce the troughs with UHPFRC was significantly lower than the stainless steel solution, even before the expected steel cost escalations were considered. In addition, it was established that the troughs could be manufactured locally (by Lafarge) and therefore, production would not be delayed by lengthy lead times as presented by the steel industry (ie, mill scheduling). With the benefits and material properties clearly identified, the design team was confident in their selection of UHPFRC as the material of choice for production of the troughs.

The Buildings
The buildings that enclose the process areas were initially conceived as steel structures, with exterior concrete masonry walls. The building architect (Stantec Architecture Ltd.) and the structural consultant (Jacobs Techna-West Engineering) quickly realized that the environment in the building would pose significant challenges for a steel framing solution. Considering that the enclosure covers a large area of open water, the building interior can be expected to have a very high humidity. The temperature in the enclosure is relatively low, especially during the winter months. The combination of high humidity and low temperatures creates the potential to generate condensation on exposed interior building surfaces. In this environment, steel requires special coatings and frequent maintenance to prevent corrosion. The building designers found a better construction solution – concrete. Concrete has no adverse reaction to moisture and in fact, as we all know, moisture is beneficial in the concrete curing phase.

Concrete was the material of choice for the foundations, trenches, and tanks. This portion of the work was undertaken by Sure-Form Construction Ltd., who acted as both the forming and general contractor for the project.

In order to use concrete to frame the building, several challenges had to be overcome. The location and operation of the process equipment defines the positioning of columns which in turn, dictates span requirements. The design team elected to use prestressed, precast concrete hollowcore slabs as the main span members, thus providing a smooth, flat surface that would be ready to receive the roofing system. The interior surface of the hollow core slab also provided an attractive, smooth finish that could easily be left "as-is" or coated.

Precast columns and beams were chosen to support the hollow core roof, allowing US Filter essential access for installation of process machinery prior to building construction. In addition, the use of these concrete elements eliminated the need for Sure-Form to install and remove forming in the process area (which would have been difficult with equipment in place). The building walls were constructed with precast, prestressed insulated sandwich panels, providing an extremely efficient, durable wall system that functions as the building envelop. The exterior and interior surfaces were then finished with a coloured stain.

Conclusion
This project exemplifies the versatility of concrete to meet the unique challenges faced by an owner and the designers of a special purpose structure. In addition, the designers have used UHPFRC (Ductal®) to successfully replace a stainless steel solution without sacrificing any of the owners’/designers’ needs. This project could easily become the marquee illustration of progress made in material design that allows concrete to successfully displace stainless steel. Furthermore, it will serve as compelling motivation for future designers to seriously consider innovative concrete construction solutions that provide improved value for their clients.

Credits:
Owner: City of Edmonton, Wastewater Treatment Section, Drainage Services Branch
Architect: Stantec Architecture Ltd.
Engineer: Jacobs Techna - West Engineering
Contractor: Sure-Form Construction Ltd.
Precast Concrete: Lafarge Construction Materials – Precast Division