Designing with Precast Concrete for Sustainable Offices
By Miles D. Britz, AIA, LEED AP
When the owner of a precast concrete manufacturing facility builds new offices, the material of choice is pretty clear. Similarly, when the owner is an advocate for sustainable building practices, there is no other option than to go for Gold. Fly ash became integral to ensuring this success.
In 2007, a Lino Lakes, Minnesota-based designer, manufacturer, and installer of precast concrete structural products identified a need for additional office space for its engineering, design, drafting, and estimating personnel. The company was determined to use sustainable principles in building the new offices, and also wanted to demonstrate the attributes of precast concrete. The mission was set to achieve a Gold rating under the U.S. Green Building Council (USGBC) Leadership in Energy and Environmental Design (LEED) program. This was accomplished through the project’s earning of 41 credits. The new building provides seven fixed-wall offices, with partitions providing up to 27 workstations. Additional spaces include:
- two conference rooms;
- employee break room;
- combined planning/printing/filing room;
- mechanical and electrical room;
- shower room; and
- janitor closet.
A precast concrete covered walkway was also erected to connect this new engineering/design office to the existing administration building.
Insulated precast walls
To achieve the owner’s goals, the design team used R-26 precast concrete insulated sandwich wall panels (SWPs) for the building to increase thermal efficiency and minimize temperature fluctuations. With a sandblasted, colored finish, the precast SWPs feature plastic wythe connectors and edge-to-edge insulation, greatly reducing thermal bridging in the wall system. The wall system’s thermal mass also lowered HVAC loads and overall energy consumption. The panels comprised a 152-mm (6-in.) interior concrete wythe, 76 mm (3 in.) of polyisocyanurate (polyiso) foam insulation, and a 76-mm exterior aesthetic concrete wythe. Typical SWPs measured 3.1 m (10 ft) wide and 4.9 m (16 ft) tall. Quality control, repetition, speed of construction, and durability are all benefits of precast construction. Cast in a plant, precast product production allows exacting control over concrete curing temperatures, water ratios, and resulting concrete strength. Precast concrete buildings are often designed to maximize repetition of components, such as wall panel units. This significantly reduces in-plant labor and overall building costs. Precast components also install fast on the job site with limited need for onsite trades and without the necessity of tenting or heating in winter. There is less site disturbance and less construction waste. Further, precast products, such as hollow-core planks, provide immediate working surfaces for interior finishing. Highly durable, precast construction greatly reduces long-term maintenance. Beyond aesthetics and thermal control, precast concrete wall panels also provided the flexibility to size large window openings, offering light and exterior views to more than 99 percent of all occupied spaces. High ceilings and side-lite windows at interior doors also transmit daylight to spaces inside. On the building’s south façade, precast sunshades minimize solar heat gain. A precast concrete roof deck, composed of 406-mm (16-in.) deep hollow-core units, spans the building’s entire 17.7-m (58-ft) width. This allowed the interior space to be free of columns, providing flexibility of workstations. The roof is covered with a white membrane for solar reflectance to reduce the urban heat island (UHI) effect and keep indoor temperatures cooler. Tubular skylights are installed on the roof to increase the natural light entering the space; their openings are small relative to the sunlight they let in, limiting the impact on the roof’s structural capacity. In many ways, the office project was used as an opportunity to push the envelope in concrete mixture proportioning. Supplementary cementitious materials (SCMs), such as fly ash and slag cement, commonly comprise 15 to 35 percent of the total cementitious material in a mixture.
Fly ash as a supplement
Used since the 1930s as a partial replacement for portland cement in concrete mixtures, fly ash can offer numerous benefits. It increases concrete durability, decreases permeability, lessens shrinkage cracking, and reduces the potential for alkali-silica reactivity (ASR) and sulfate attack. Fly ash is a by-product of burning coal to produce electricity. Using it in concrete reduces the environmental impact by decreasing reliance on portland cement, which is very energy-intensive to manufacture. It also lowers the amount of fly ash that goes to landfills. (Slag cement provides similar benefits; it is a by-product of steel manufacturing.) The use of fly ash can dramatically affect concrete setting times. For high-fly-ash concrete, strength development is generally much slower, resistance to de-icer scaling can be lower, and entrained air content reduced. Many of these issues can be offset by using chemical admixtures. Highly reactive pozzolans—such as silica fume and metakaolin—can increase the rate at which concrete gains strength and produce ‘high-early-strength’ concrete. In 2010, the U.S. Environmental Protection Agency (EPA) addressed fly ash disposal following the failure of an earthen dam of a pond holding accumulated fly ash slurry from a power plant in Kingston, Tennessee. The failure released slurry downstream. Coal ash contains measurable quantities of heavy metals, such as arsenic, lead, and selenium. If openly contacted, these substances could cause cancer or neurological problems. In the regulatory development process, heated debate raged over whether to label fly ash as a hazardous material—some argued this would limit its recycled uses. There is also debate over fly ash’s level of toxicity, with some claiming it is no worse than background material such as dirt. As a result, EPA focused its regulatory actions on fly ash disposal. The agency has encouraged use of coal ash waste in products, like concrete, where the substance is encapsulated. Two options were proposed:
- label fly ash as ‘special waste’ with federally enforced disposal regulations; and
- have EPA set performance standards with enforcement by lawsuits. Both would unfortunately discourage its reuse.
As noted, concern has been raised by some building health experts about the presence of trace heavy metals in fly ash. However, others maintain the metals are effectively locked into the cementitious matrix, preventing their release.
Specifications for fly ash
Two types of fly ash—Classes F and C—are used in concrete. The former is a by-product of burning bituminous coal; the latter is from burning lignite and sub-bituminous coal. Concrete with Class C fly ash develops strength faster than concrete with Class F fly ash. What percent of fly ash is appropriate depends on:
- project and properties requirements;
- type of fly ash selected;
- climate conditions at the building site; and
- use of admixtures.
For architectural precast, fly ash is not commonly employed because of potential impact to color. However, if used, such as with darker or gray-colored concrete, the amount of fly ash is normally limited to 15 to 20 percent. The Precast/Prestressed Concrete Institute (PCI) suggests limiting the use of fly ash for structural precast concrete to 35 percent; ASTM C 595, Standard Specification for Blended Hydraulic Cements, limits the fly ash content of blended cement to 40 percent. Under the American Institute of Architects (AIA) MasterSpec, published by ARCOM, fly ash should conform to ASTM C 618, Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete, while slag cement should conform to ASTM C 989, Standard Specification for Slag Cement for Use in Concrete and Mortars, requirements. Typically, the amount of fly ash specified has been in the range of 15 to 25 percent by weight of the total cementitious materials in the mixture. However, higher amounts are often used. Limits commonly range from 15 to 30 percent for Class F fly ash and from 15 to 40 percent for Class C fly ash. American Concrete Institute (ACI) 318, Building Code Requirements for Structural Concrete, limits the amount of fly ash content to 25 percent for Exposure Class F-3 (i.e. concrete in continuous contact with moisture, exposed to freezing and thawing, and exposed to de-icing chemicals).
For these applications, it also recommends specifying fly ash with low levels of unburned carbon. Unburned carbon in fly ash can interfere with concrete’s ability to reliably entrain correct amounts of air. High-carbon fly ash can greatly decrease concrete durability and performance.
While the office building described in this article uses fly ash and slag concentrations as high as 50 percent in the hollow-core components, a recent LEED Platinum structure went even further. The Russell Branch of the Missoula Federal Credit Union in Montana is being touted as the first building in the world where all concrete was made of 100 percent fly ash and recycled glass aggregate, entirely eliminating the need for portland cement. This mixture is slightly more expensive than using portland cement. It also takes longer to gain strength and, therefore, requires careful testing to ensure proper strength is reached to meet erection stress. The mix used a borax retarder to prevent flash setting. Since the mix generates a large amount of heat during curing, live steam was not needed.
Special concrete mix
For the Lino Lakes offices, an average of 36 percent fly ash was specified for the precast concrete SWPs, reducing the amount of portland cement used and the related amount of carbon dioxide (CO2) off-gassing during cement manufacture. The precast concrete insulated SWPs consisted of:
- 76-mm (3-in.) exterior wythe with an architectural finish;
- 76-mm insulation; and
- 152-mm (6-in.) interior structural wythe.
For color retention requirements, the exterior wythe does not use SCMs. The mixture proportion for the hollow-core units consisted of 25 percent fly ash, 25 percent slag cement, and 50 percent Type III cement. In a precast fabrication plant, the concrete mixing process can be controlled with a great degree of accuracy, resulting in a consistent and highquality concrete. A small amount of super-plasticizer was used in the mixture for both the hollow core and the structural wythe of the wall panels. Curing time was increased by only a few hours, allowing the forms in the plant to be stripped the next day. Chemically, fly ash is a pozzolan that, when mixed with lime (i.e. calcium hydroxide), forms a cementitious compound. The amount of cementitious material in an average concrete mixture is only about 15 percent by weight. However, increasing the proportion of SCMs reduces the amount of CO2 associated with the concrete. On this project, average portland cement content was reduced by one-third, which has a positive effect on the environment. In addition to diverting these by-products from landfills, specifying fly ash and slag cement increases the recycled content and lowers the concrete’s embodied energy. Properly proportioned and cured concrete containing fly ash and slag cement becomes more durable and resistant to chemical attack. Mechanically, fly ash and slag cement also pay dividends for concrete production. As the particles are small, they effectively fill voids. Further, fly ash particles are hard and round, resulting in a ‘ball-bearing’ effect that allows concrete to be produced using less water, while still maintaining workability.
Beyond fly ash, the designers for the office project were careful to specify sustainable products and high-performance systems to enhance the building’s energy efficiency, resource use, and indoor air quality (IAQ). The fabrication/office campus is situated within Minnesota’s Rice Creek Watershed District. Included in the scope of the new office project was restoration and enhancement of an existing wetland area using native vegetation. The project also includes water-efficient landscaping and a stormwater management system. The system reduces impervious cover, promotes infiltration, and treats runoff; it is capable of removing 80 percent of total suspended solids (TSS), improving water quality. Low-flow plumbing fixtures reduce water consumption by 34 percent. Also included are bicycle storage facilities and parking spaces for low-emitting and fuel-efficient vehicles. (No additional parking was needed for the structure.) Aside from the wall panels, the structure boasts precast sunshades, columns, and hollow-core roof planks. The concrete’s inherent thermal mass, acting as a heat sink, helps stabilize heating and cooling swings and was a contributing factor in reducing HVAC loads. High-efficiency mechanical condensing units and air-handling units (AHUs) were installed for a 28.4 percent reduction in energy use and, ultimately, a 23.7 percent energy cost savings. HVAC equipment consists of an in-floor heating system with heat recovery units and multiple thermal zones to increase employee comfort.
On the building’s south façade, sunshades over exterior windows minimize solar heat gain and reduce the cooling load. A white thermoplastic polyolefin (TPO) roofing membrane was installed on the building’s precast roof, as well as the covered walkway. The material has a solar reflectance of 0.99—well over the required value of 0.64 for low-sloped roofs—to minimize the heat island effect. Sensors control the building’s lighting system. Tubular daylighting devices (TDDs) are fitted into the ceiling to bring in natural daylighting.1 Daylight is accessible to 99.8 percent of occupied areas, and line-of-sight views are available to 99.4 percent of occupied spaces.
Precast concrete components used on the structure contain 42 percent post-consumer and 25 percent pre-consumer content, including the fly ash and slag cement, both of which are eligible for LEED’s recycled content credits. Overall, building materials specified contain 21.5 percent combined recycled content. Seventy-two percent of the materials in the precast concrete components were extracted, processed, and manufactured within an 800-km (500-mi) radius of the site. Overall, the project contained 27.7 percent regional materials, reducing carbon emissions during delivery.
The precast concrete was produced in a plant within a controlled environment, to tight tolerances, virtually eliminating concrete waste—any that remained was separated out for recycling. Overall, 64.7 percent of the construction waste from the project’s other building materials were diverted from landfills.
The precast concrete insulated sandwich wall panels were left exposed in several areas, reducing the amount of gypsum wallboard needed. This helped lower total waste, increase the effectiveness of the concrete’s thermal mass, and provided a durable interior finished surface that will reduce maintenance and lifecycle costs. Precast concrete contains no volatile organic compounds (VOCs) and is a mold-resistant material. Low-emitting paints, coatings, adhesive, sealants, and carpeting were also specified for the building.
Another kind of ‘green concrete’
The U.S. concrete industry has worked hard to reduce its environmental footprint. In 2003, it committed to lower—by the year 2020—CO2 emissions by 10 percent below 1990 levels. According to a 2004 Portland Cement Association (PCA) statement:
the industry has taken significant, voluntary steps throughout the past decade to improve its manufacturing process and minimize adverse effects on the global environment. Manufacturing facilities have implemented new technologies and processes that use energy more efficiently and minimize emissions and waste, while continuing to produce a quality product.
In the fall of 2002, the industry proposed a change to ASTM C 150, Standard Specification for Portland Cement, to permit use of up to five percent ground limestone. Approved, it was published in ASTM C 150-04 and the provisions were incorporated into the American Association of State Highway and Transportation Officials standard of the same name— AASHTO M 85—in 2007. According to PCA, this material use should significantly reduce emissions and provide environmental benefits such as lowering carbon dioxide emissions by approximately 2.6 percent per ton of cement produced, with an annual reduction of 2.5 million tons.2
In 2010, PCA introduced a proposal to ASTM and AASHTO to include provisions in ASTM C 595, and AASHTO M 240, Standard Specification for Blended Hydraulic Cement, for portlandlimestone blended cement that would contain more than five percent (but less than 15 percent) limestone. ASTM Committee C01 on cement and AASHTO Subcommittee on Materials supported the concept, and these proposed changes are now being balloted. Employing limestone in cement is a proven technology, used for decades in Europe and Canada. It is not a replacement for slag or fly ash, but is compatible with those materials.
According to PCA, cement manufacturing accounts for less than 1.5 percent of the country’s CO2 emissions—well below other sources such as electric generation plants for heating and cooling (33 percent), transportation (27 percent), and industrial operations (19 percent). As exemplified by the offices profiled in this article, when used with other building materials and components, precast concrete can be an integral part of green projects pursuing sustainable design goals.
For more on TDDs, see the article, “Daylighting Goes Tubular,” by Kate DeBellis, CSI, CDT, LEED AP and Neall Digert, PhD, MIES, in the April 2008 issue of The Construction Specifier. Visit www.constructionspecifier.com and select “Archives.” 2 Documentation for the reduction of carbon dioxide (CO2) emissions from the use of limestone in portland cement is contained in M. Nisbet’s “The Reduction of Resource Input and Emissions Achieved by Addition of Limestone to Portland Cement,” which is PCA R&D Serial No. 2086, November 1996. The percentage reduction assumes an average use of limestone in portland cement of 2.5 percent. The annual reduction is consistent with 2001 annual cement usage of 108 million tons.
Miles D. Britz, AIA, LEED AP, is a senior project architect/manager of the Professional Design Group (Northfield, Minnesota). With 34 years in architecture, he has diverse experience with municipal facilities, corporate office and industrial facilities, healthcare projects, and commercial retail spaces. As a project architect, Britz has supervised, managed, and coordinated interdisciplinary work efforts on design development and construction document production. His responsibilities also include space needs analysis, space planning, site development, and sustainable design solutions. Britz can be contacted via e-mail at firstname.lastname@example.org.
Using a LEED Gold project from Minnesota as a case study, this article explores how to design with precast concrete for office facilities. The article specifically focuses on issues like fly ash, portland cement, and insulated wall panels, taking an in-depth look at lessons learned with sustainable design.
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[CREDIT] Photo © Erik Molin. Photo courtesy Molin Concrete Products Co.
[SIDEBAR] Project Team Architect: Professional Design Group (Northfield, Minnesota) Mechanical/electrical design engineer: Karges-Faulconbridge (St. Paul, Minnesota) Precaster: Molin Concrete Products Co. (Lino Lakes, Minnesota) Contractor: Kraus Anderson (Minneapolis, Minnesota) Landscape architect: Buells Landscape Center (Hastings, Minnesota)