Concrete production

Other methods used in concrete production to reduce the carbon footprint included using glass sands and recycled concrete fine aggregates, especially for precast concrete. Ready-mix plants used recycled water to reduce the amount of potable water needed to mix concrete. This included concrete truck washout water and collected rainwater, resulting in a 9% reduction. Finally, using polycarboxylate superplasticizers reduced the amount of cementitious content in mixes by 11.6% while still meeting strength requirements. The amount placed in mixes depended on usage.

The primary projects for concrete at the Olympic Park were the Olympic Stadium, the Aquatics Centre, and the Velodrome. Most of this concrete was used for piles, footings, foundation walls, precast pieces, floors, paving, and other structural purposes. The exception is the Aquatics Centre, where Zaha Hadid Architects, London, used concrete in both decorative and architectural ways.

The most beautiful concrete work in the entire Olympic Park is the diving platforms inside the Aquatics building. There are six platforms: 33, 25, 16, 10, and 3 feet above the water. The horizontal run for the 33-foot-high platform is 26 feet and most of this length cantilevers beyond its inclined support.

Sara Klomps, associate at Zaha Hadid, one of the two project architects overseeing the project, says the design they created was inspired by the dynamic movement of divers, providing a background to complement their graceful form.

The fluid shapes of the platforms were designed using Rhinoceros (Rhino) software developed by Robert McNeel & Associates for 3-D modeling. Then a computer numerical control milling machine, guided by the design model, routed the positive shapes from large polystyrene blocks. These were used as mold surfaces to create the fiberglass forms and yokes for concrete placement. Each fiberglass form was about 8 feet, 2 inches high.

“We were able to reuse most of the forms because the shape of each platform was the same. Imagine the tallest platform sinking into the ground to the same height as the next tallest platform. The shape for the next tallest platform is the same,” Klomps says. “This reuse of forms made the project affordable.”

The foundations beneath each platform are substantial. Groups of continuous flight auger piles are topped with load-sharing pile caps that support the foundation pads that each platform is built on.

Congested reinforcement

Reinforcement for the cantilevered structures was very congested, so using standard vibration equipment for consolidation around the reinforcement wasn't an option. In addition to achieving consolidation around rebar, the design team wanted finely-finished surfaces with very few bug holes, so the decision was made to use a self-consolidating concrete mix supplied by Lafarge, from a plant outside the Park. In keeping with the ODA's low-carbon footprint for all concrete, the producer adjusted its proprietary mix to include a 30% portland cement replacement with slag. When used as a replacement, slag lightens the color of the concrete somewhat.

Each platform was cast in 8-foot, 2-inch sections. There were several reasons for this. Forms could be designed for lower form pressures, controlling bug holes was easier, and timely deliveries of ready-mix concrete to a congested location were less risky because fewer trucks were involved. The offsite concrete also had to be checked by security, delaying each truck. Too much time lag between trucks could jeopardize the successful completion of a platform.

Placing forms for each new lift was carefully monitored to minimize the appearance of cold joints. “You can see the marks but they are small and the graceful lines of each structure were maintained between placements. A small ‘shutter' on each form overlapped the hardened concrete from the previous placement to achieve clean lines,” Klomps says. The horizontal cantilevers and the bend or “knuckle” from vertical to horizontal were each completed in one placement. Pumps placed all of the concrete.

The completed platforms had few bug holes, finishes were lustrous, and the concrete color was uniform and light.

This type of concrete work is “as cast” construction because no patching or remedial work is allowed after the forms are removed. It's very risky work for the contractor but Klomps says they were very satisfied with the result.

Reducing the carbon footprint

The ODA addressed the issue of reducing the embodied carbon footprint of the Olympic Park in several ways. Contractors were reluctant but after experience with recycled materials they became more confident and innovative with mixes that contained higher quantities. But some specifications were be modified. Thirty-day strength requirements for mixes with high fly ash or slag content were unrealistic so they were changed to 56-day requirements.

Building a green Olympic Park was the primary ODA goal, accomplished partly through using innovative concrete mixes.

Joe Nasvik is the former Senior Editor of CONCRETE CONSTRUCTION, THE CONCRETE PRODUCER's sister magazine.