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 Summer Olympics, it decided the site should be built in the Lower Lea Valley, five miles from downtown. The 1.5-square-mile site was dilapidated and contaminated, resulting from more 100 years of heavy industrial use. But the area has access to both rail and waterway transportation and planners are hopeful the revitalization would 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.
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 smallest carbon footprint possible. So it considered all the mix ingredients, the transportation of materials to batch plants, and concrete deliveries.
The London Olympic Park required about 654,000 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 more than 70,000 road vehicle movements.
Reducing the transportation of materials became a major focus. The ODA used 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 aggregate s, 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 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 also reduced 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.