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Competing for the Green

Source: | Updated: Feb 25, 2016

In the world of green there are two ways to evaluate the release of carbon compounds into the atmosphere: “embodied carbon,” which defines carbon releases along the complete material supply-chain, and “emissions,” which quantifies the amount of carbon released during the lifetime of a structure. The word green usually focuses on embodied carbon while sustainability focuses on the service life of concrete and the release of both carbon compounds. At the London Olympic Park, reducing the embodied carbon footprint was the primary focus.

When London won the bid for the 2012 Olympics, it decided the site should be built in the Lower Lea Valley, just 5 miles away from the downtown area. The 1.5-square-mile site was dilapidated and contaminated, resulting from over 100 years of heavy industrial use. But the area has access to both rail and waterway transportation and planners are hopeful the revitalization will stimulate growth after the games are over.

The first step was to remove a century’s worth of waste. It was the starting point for a green/sustainable philosophy covering every aspect of the site development. The Olympic Delivery Authority (ODA), decided to incentivize recycling to the greatest extent by keeping the construction carbon footprint as low as possible, and increasing jobsite safety by monitoring and controlling air quality and construction noise.

The program the ODA adopted is impressive in terms of what can be achieved on a construction project. More than 93% of construction waste was either reused or recycled along with 98% of the demolition waste, with 80% of it being segregated at the source. Vehicle movement related to waste management was reduced by 90%, also reducing carbon emissions.

Low carbon concrete

We often call concrete green by replacing some of the portland cement in a mix with pozzolans such as fly ash or slag. But the ODA wanted concrete with the least carbon footprint possible. So they considered all the mix ingredients, the transportation of materials to batch plants, and concrete deliveries.

The London Olympics Park required approximately 654,000 cubic yards of concrete made with 1.1 million tons of coarse aggregate. Recycled aggregate accounted for 188,000 tons of the total, reducing the carbon footprint of the concrete by 33,069 tons and eliminating over 70,000 road vehicle movements.

Reducing the transportation of materials became a major construction focus. The ODA decided to use only one ready-mix producer to provide concrete for all Park needs. It provided space for a ready-mix plant within the Park, located near a rail head so aggregate could be delivered cheaply with a reduced carbon footprint. With the exception of stent aggregates, all recycled materials were delivered from locations near the Park. The much longer distance to transport stent from a vast supply still produced a lower carbon footprint than using mined aggregates.

Recycled material replacements

Thirty mix designs were originally specified, but contractor knowledge of recycled materials, program constraints, and technical requirements grew the list to several hundred. Contractors and specifiers collaborated in an effort to develop mixes that provided concrete with the right requirements for each application and reduced the carbon footprint.

The ODA’s goal was to reduce the concrete carbon footprint by 25%, actually achieving 24% compared to an industry average in London of 18%. This was achieved by replacing the amount of mined coarse and fine aggregates, portland cement, and mix water with recycled materials. Polycarboxylate superplasticizer admixtures were also used to reduce carbon emissions.

Stent is a mined pozzolanic byproduct of the Cornish clay industry, similar to metakaolin clay, which was used as a coarse aggregate substitute. One hundred and ten tons of stent byproduct is produced for each 1.1 tons of china clay. Many contractors were initially reluctant to use stent as an aggregate replacement but the mandate to achieve 25% recycled coarse aggregate overall motivated them to use higher replacements where they could, and lower replacements where the properties of natural aggregates were more important. Stent coarse aggregate replaced 76% of the mined aggregate for the interior wall construction of the Aquatics Centre, still achieving a high quality finish. The podium topping in the Team Stadium as well as the walking surfaces and structural cores of the Media Press Centre used 100% replacements, helping contractors meet their overall contractual obligations.

Crushed recycled concrete was also used as an aggregate source. It came from concrete left on the site over the years and concrete removed from projects within reasonable distances. Also, ready-mix concrete returned to batch plants was recycled for its aggregates.

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 wash-out water and collected rainwater, resulting in a 9% reduction. And, finally, the use of polycarboxylate superplasticizers reduced the amount of cementitious content in mixes by 11.6% while still meeting the concrete strength requirements. The amount placed in mixes depended on usage.

The London Aquatics Centre

The primary projects for concrete use 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 line. 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 (see “Sculptural Formwork” in the March 2012 CC issue for a more detailed discussion of 3-D design modeling for concrete work). Then a computer numerical control (CNC) 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 approximately 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,” she says. “This reuse of forms made the project affordable.”

As you can imagine, 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.

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 (SCC) supplied by LaFarge, from a plant located outside the Park. In keeping with the ODA’s low-carbon footprint for all concrete, LaFarge 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, which the design team considered a plus.

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 off-site 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 in an effort to minimize the appearance of cold joints, Klomps says. “If you look closely 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,” she adds. The horizontal cantilevers and the bend or “knuckle” from vertical to horizontal were each completed in one placement. All the concrete was placed using pumps.

The completed platforms met with everyone’s approval; there were 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 a wide variety of ways. Contractors were initially reluctant but after experience with recycled materials they became more confident and innovative with mixes that contained higher quantities. But some specifications had to be modified. For instance, 30-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 a primary ODA goal for the London Olympics, accomplished in part through innovative concrete mixes.

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