Winning On and Off The Court: The Seattle Storm and Low-Carbon Construction
With four WNBA titles to their name, the Seattle Storm are used to excellence on the court. These winning ways have now extended to off the court as well, as the team’s new $64-million training facility – the Seattle Storm Center for Basketball Performance – was recently named a winner in the National Ready Mixed Concrete Association’s (NRMCA) 2024 Concrete Innovation Awards.
The award, announced in March, was given in recognition of the 50,000-square-foot project achieving significant reduction in greenhouse gas emissions from the concrete: 52.4 percent embodied carbon savings when compared to the regional industry average 28-day standard-cured compressive strength, rising to as much as 68 percent with specific applications in the building. The facility was designed by architects Shive-Hattery and ZGF Architects, built by Sellen Construction, and NRMCA member Stoneway Concrete supplied the low-carbon concrete required to meet the project’s sustainability goals.
For a team that plays in the Climate Pledge Arena (the first net zero carbon certified arena in the world), in a city that passed new building performance standards to reach net zero by 2050, reducing the carbon footprint of the Storm’s training facility was a key consideration from the earliest stages of the project’s conception. In addition, the project targeted LEED Gold certification to qualify for the city of Seattle’s Priority Green expedited program. These stipulations meant emissions reduction was a key driver of decisions throughout the design and construction processes.
A Three-Pronged Approach
To meet this low-carbon need, the architects and Stoneway adopted three key strategies: deploying strength requirements based on concrete compressive strength tests at 56-days instead of a typical 28-day test age for specific applications, using higher slag cement replacement of between 50 and 80 percent, and implementing Type 1L portland-limestone cement (PLC).
The design and construction teams worked together closely to identify parts of the project that could be specified with a design strength at 56-days, rather than the typical 28-days. The longer specification schedule allows for reduced embodied carbon and a lower global warming potential (GWP) of the completed building, by further reducing the need for high amounts of cementitious materials to achieve the required strength. In appropriate locations, specifying strength requirements at 56 days rather than 28 days should have little or no effect on the final design.
Sections of the project, such as the foundations and the tilt-up walls, were identified as appropriate for specifying the design strength at 56-days. The project was fortunate that the Seattle area has extensive experience with working with 56-day design strength, as they have been common in the city since the 1980s. To allow crews to get on the slab quickly, Stoneway adjusted the admixture by minimizing the amount of water reducer and using a super plasticizer as well as a viscosity modifier – producing an admixture that met the approval of the project’s finishers.
The design called for lighter, whiter concrete for the structure. Stoneway considered two initial options, but they were either too expensive or the carbon footprint was too high: The cost of adding titanium dioxide as a color additive was prohibitive, and getting white cement meant transporting a specific cement across the country.
The solution was in using high slag cement contents in the concrete. Stoneway already had mixtures developed with slag and was using 50 percent slag on a regular basis at other construction projects, such as the Microsoft Campus refresh currently underway in Redmond, WA. Project builders Sellen Construction had used slag mixtures of up to 80 percent on The Spheres at Amazon’s corporate office in downtown Seattle and were aware it produced a very light color after drying.
In total, 27 percent of the Storm Center’s concrete utilized 80 percent slag cement; 64 percent of the concrete used 50 percent slag cement. Initial issues with cracking due to set times were resolved quickly by Stoneway, which adjusted the admixture combinations to reduce set time. Tilt walls were designed for 6,000 psi at 56 days and utilized 20 percent cement and were still able to achieve 4,000 psi within three days. To achieve this, Stoneway used Giatec Smart Rock maturity sensors to confirm the strength requirements were met within three days.
The use of slag cement also brought environmental and sustainability benefits that contributed to the overall GWP and embodied carbon reductions of the project. Not only does replacing a portion of portland cement with slag cement hold the potential to significantly reduce the environmental impact of the concrete, but it also requires nearly 90 percent less energy to produce than portland cement. According to the Slag Cement Association, using 50 percent of slag in the cementitious content can reduce greenhouse gases by more than 40 percent, and lower the embodied energy of concrete by more than 30 percent.
GWP requirements were provided, along with maximum cement contents. In addition, all the cement used was Type IL: the project used ASTM C595 Type IL portland-limestone cement produced by Ash Grove Cement, a CRH Company. As PLC utilizes a higher blend of limestone, it can result in carbon savings of up to 10 percent across the life cycle of a project. As Stoneway has been a longstanding user of PLC, the company required no mixture adjustments for this particular project. While there were some adjustments to be made when they transitioned to Type IL, in general Stoneway has been pleased with the overall performance of PLC in its concrete mixtures.
In addition, the project used CHRYSO & GCP’s admixture and fiber technologies to help exceed the specified embodied carbon limits and overall performance. Across the entirety of the design and construction process, the teams collaborated to identify the best low-carbon mixes for each intended application.
Creating a Healthy and Productive Building
As a sports training facility for elite athletes, there was a strong desire to prioritize health and wellness in design, construction, and operation. As a result, rather than just rest at LEED Gold, the client, contractors, and architects were keen to push further and reduce operational carbon emissions resulting from the energy used to operate the facility. Doing so poses less of a challenge in the City of Seattle – and Washington state more broadly – as there are commitments to going all-electric in order to decarbonize the state’s building stock.
Moreover, the local utility that the project is connected to already sources the majority of its power from renewables. More than 80 percent of Seattle City Light’s power comes from hydroelectric projects on the Skagit and Pend Oreille Rivers. In addition, the city required the project to install photovoltaics on the roof of the building. Inside the building, low volatile organic compound (VOC) materials were used to reduce the carbon and wider greenhouse gas impacts on the interiors and the players themselves.
Meeting the Tight Timeline
Speed was also of the essence for this project. Construction of the facility began in Spring of 2023, and the team wanted to be in the building by the start of the 2024 season in late April. The project’s sustainability commitments helped them in this regard: The City of Seattle’s Department of Construction and Inspections runs a program called Priority Green Expedited, which reduces permitting wait times in exchange for meeting a green building certification and other criteria.
As the building was chasing LEED Gold along with similar sustainability goals, the project qualified for Priority Green Expedited, which aims to shorten initial plan review times by at least 50 percent. As a result, the project was given crucial extra time back to focus on construction.
A Collective Commitment to Low-Carbon
The combination of approaches resulted in a project that met and exceeded its GWP and embodied carbon goals. Overall, the project exceeded a 50 percent reduction compared to typical average mixes in the region and achieved 80 percent of the 2030 targeted reductions of the World Economic Forum’s First Movers Coalition for “near zero carbon: concrete.
The achievement was a team effort, and the result of putting into place an early collaborative process with the contractor, architect, and design and project teams that identified what objectives were required and to meet or exceed the client’s sustainability goals. Architects ZGF noted these carbon reductions are the highest they had seen on a project.
Throughout the project, there are examples of attention to detail that raise the profile and results of the finished building as a whole. Whether that is the exposed concrete in the walls acting as a thermal buffer to moderating temperature changes, or the storm drains in the streets around the building having Seattle Storm-branded manhole covers, thoughtful and impactful touches are seen throughout.
The thermal buffering provided by the exposed concrete has a particularly pronounced effect on the building’s energy efficiency and GWP over its lifetime: The concrete absorbs heat in warmer weather to minimize its effect on indoor temperatures and releases that heat when temperatures cool down. Strategic placement of this thermal buffering – such as in the walls, as is the case with the Storm Center – is a strong, long-term step toward reducing the operation carbon and GWP of the building.
Other design decisions, specific to buildings of this type, further helped the project meet its energy efficiency and GWP goals. Because the practice courts require consistent lighting, large parts of the building’s exterior have a very low window-to-wall ratio – in this case, it was approximately 6 percent compared to a more typical 30 or 40 percent. As windows are traditionally the weak link in a thermal seal, this design consideration specific to a basketball practice facility will help the Storm reduce its energy consumption and costs. When taken into consideration alongside the photovoltaic solar array installed on the roof of the building, the full suite of operational efficiencies has enabled the Storm Center to reduce its energy costs by 46 percent when compared to LEED energy baseline.
Keeping Carbon Reduction at the Core
Ultimately, ensuring a project stays true to principles of carbon neutrality rests on the motivations of the client. Understandably, contractors across the industry have tried, tested, and trusted techniques and mixtures they know will work for specific buildings and other infrastructure. While Stoneway’s low-carbon concrete mixtures were not experimental – they had deployed them on other projects – they remained largely outside the industry norms. A client that is invested in keeping the project tied to its low-carbon goals, and partners across the project that are experienced and committed to doing the same, will make sure that design and construction decisions have sustainability as a key metric in addition to the usual pillars of safety, reliability, durability, and cost.
From training in a university sports hall that was only available from 10 a.m. to 2 p.m. to having their own state-of-the-art facility that exceeds LEED Gold, the Seattle Storm are setting new standards both on and off the court. Much of this is due to the commitment of contractors to devise solutions to meet their long-term, low-carbon needs.