Designing for Sustainability:  

As a core building material with far-reaching sustainable applications, concrete plays a critical global role in providing environmental, social and economic benefits. Concrete components can be used to contribute to the achievement of LEED credits in uses ranging from stormwater management to the improvement of indoor air quality. They do so, while also offering intriguing possibilities for versatile design innovations using shape, color and texture.

What is Concrete?

Concrete is the oldest engineered building material and one of the most widely consumed materials on earth. In its basic form, it comprises a mixture of portland cement, aggregates and water.  Historically, portland cement has been the principle cementitious material at approximately 10 to 15 percent by mass weight.

Proponents of sustainable design have criticized the use of concrete because of the energy-intensiveness and generation of carbon dioxide (CO²) in portland cement manufacture.

Through the cement industry's aggressive efforts to reduce
emissions through innovations in manufacturing, usage of waste-derived raw materials and the extraction of energy from industrial waste fuel or biofuels, attributable man-made CO² emissions have been reduced by 33 percent, while energy efficiency has increased by the same amount.

Yet, despite such innovations, portland cement remains an energy-intensive building material. The increasing use of alternative raw materials as a partial replacement for portland cement in most concrete mixtures today, though, has had a positive environmental impact. Known as Supplementary Cementitious Materials (SCMs), they enhance the strength and versatility of concretes and increase the many ways in which concrete can contribute to LEED credits.

The three most commonly used SCMs are slag cement, fly ash and silica fume. 

 

National Museum of the American Indian, Washington, D.C. by Douglas Cardinal (Blackfoot) of Ottawa, architect and project designer and GBQC Architects, Philadelphia, and architect Johnpaul Jones (Cherokee/Choctaw) design architects. Slag cement was used in the concrete mix. Photo by Max Mackenzie.

 

Slag cement is a reclaimed, recyclable industrial non-metallic byproduct from an iron blast furnace. Fly ash is a byproduct of coal-fired furnaces at power generation facilities. Silica fume is a byproduct of producing silicon metal or ferrosilicon alloys.

SCMs are proportioned within concrete and cement-based building materials as individual components or blended, interground or a combination thereof, with portland cement. Since they are recycled industrial materials, they enable the concrete industry to employ thousands of millions of tons of byproduct that would otherwise be landfilled. Moreover, their use reduces the volume of portland cement required to make concrete, thereby decreasing the amount of energy associated with cement production, lowering emissions of greenhouse gases and reducing the amount of virgin material required for the manufacture of concrete.

A life cycle inventory performed in 2006 by Construction Technology Laboratories, Skokie, IL, found that when slag cement replaces 50 percent of the portland cement in 7,500 psi concrete, the energy required to produce one cubic yard of concrete is reduced by 37 percent; carbon dioxide emissions are reduced by 46 percent; and virgin material used is reduced by 15 percent.

Characteristics of Concrete Today

Workability. In general, SCMs will enhance concrete's plastic properties such as workability and placeability. Designers like to control concrete finishes and SCMs reduce surface imperfections and segregation in stripped formwork. More importantly, SCMs enhance the hardened properties of concrete by increasing ultimate compressive strength, decreasing permeability, and enhancing long-term durability.

 

The Verdesian Opened in 2006, the LEED Platinum-certified 24-story, 299,000 ft2 Verdesian in Battery Park, New York City, designed by Pelli Clarke Pelli Architects and developed by the Albanese Organization, exceeds New York City energy efficiency standards by almost 40 percent.  Concrete contributed to the Platinum certification in two major areas: namely the recycled content and regional materials credits of the Materials and Resources (MR) category. The material costs of the concrete used in the foundation  and superstructure amounted to approximately $1.5 million; the concrete contained on average 10 percent  pre-consumer recycled  content in the form of slag cement (45 percent of the cement was replaced with ground granulated blast-furnace slag). Of the $17 million total cost of materials permanently installed in the building, the concrete component contributed about one percent of the cost. Even though one percent sounds low, it was a sizeable contribution since only 10 percent was required to earn one LEED credit. With the addition of 100 percent recycled steel for rebar and other recycled materials, such as wood used in millwork, the project received one point for the MR Recycled Content Category. In the category of Regional Materials, the concrete helped earn two points because all components of the concrete mix were manufactured and extracted within 500 miles of New York City. Photo © Jeff Goldberg@ESTO

 

Permeability is the measure of ease by which water, air and other substances such as chloride, sulfate and other deleterious ions enter concrete through pores in the cement paste fraction. SCMs' smaller particle sizes and chemical activities greatly reduce permeability. Chemical processes such as the corrosion of embedded steel, sulfate attack, and alkali-silica reaction are  greatly reduced, preventing the premature deterioration of concrete structures.

Color. The surface color of hardened concrete may be enriched by SCMs.  Surface color and texture of cementitious building materials help the designer control glare and reduce or improve heat absorption of surfaces. Some silica fumes may give concrete a slightly bluish or dark gray tint and fly ash may impart a tan color when used in large quantities. Slag cement can make concrete lighter and pigmented concretes brighter in color. It may impart an initial bluish or greenish undertone that disappears over time as the concrete surface oxidizes. The designer's control of color can impact sustainable design initiatives.

Life cycle assessments. A life cycle assessment (LCA) is a tool for the systematic evaluation of the environmental impacts of a product or system through its lifespan. "Life cycle" refers to the analysis of raw material production, manufacture, distribution, use and disposal including all intervening transportation steps. This analysis extends from the extraction and processing of raw materials through to manufacture, delivery, and use, and finally on to waste management. The goal of LCA is to compare the environmental and economic performance of products and services, to select the most sustainable system.

Cradle-to-cradle is another way of thinking about life cycles. "If the grave of one cycle can be the cradle of its own or another's, the life cycle is called cradle-to-cradle," says Julie Buffenbarger, LEED AP, engineering and architectural specialist, Lafarge, Cement Division. "Such is the case with concrete when the end of its life finally arrives; it is recyclable and can be turned into new concrete by crushing it into aggregate."

 

Concrete rates highly when the architect reviews both the LCA and LCC factors. When concrete buildings are designed appropriately, they offer much lower predictable operational energy (heating, cooling, and lighting) and maintenance costs per year over other building materials throughout the life of a building.

 

The embodied energy is reduced by replacing portland cement with supplementary cementitious materials like fly ash and slag cement.

 

How Concrete Contributes to Sustainable Design and LEED Credits

There are several sustainable initiatives, which have established performance and rating models for building materials.

The most widely adopted green building rating system in the United States is the U.S. Green Building Council's (USGBC) Leadership in Energy and Environmental Design (LEED) rating system, which provides a suite of standards for environmentally sustainable construction. For the purposes of this continuing education course, concrete and cementitious building materials' contribution to sustainable construction will focus upon LEED Version 2.2 for New Construction and Major Renovation.

Sustainable Sites (SS)

Brownfield Redevelopment (SS Credit 3; 1 point)
Intent: Rehabilitate damaged sites where development is complicated by environmental contamination, reducing pressure on undeveloped land.

Cement and SCMs  are  often used as construction components for brownfield redevelopment. Solidification and/or stabilization of damaged sites can be achieved by mixing cementitious materials with contaminated media or encapsulating waste particles with an impermeable coating.

 Site ­Development Protect or Restore Habitat (SS Credit 5.1)
Intent: Conserve existing natural areas and restore damaged areas to provide habitat and promote biodiversity.

Site Development Maximize Open Space (SS Credit 5.2) 
Intent: Provide a high ratio of open space to development footprint to promote biodiversity.

The use of concrete and cementitious-based building materials  can contribute to maximizing open space and limiting site disturbance by design and during construction. Underground or under-building concrete parking structures reduce the amount of land needed for parking lots.

Stormwater Management: Quantity and Quality Control (SS Credit 6.1 and Credit 6.2; 1-2 points)
Intent: Limit disruption of natural water hydrology by reducing impervious cover, increasing on-site infiltration, reducing or eliminating pollution from stormwater runoff, and eliminating contaminants.

Surface run-off causes flooding, damage to waterways and diminishes groundwater levels. Because it is not filtered through the ground on its way to rivers and coastal waters, it also carries pollutants such as pesticides, toxic chemicals and, in some older cities with combined sewers, sewage.

Local agencies, responding to growing federal and state regulatory pressure, are already instituting tough requirements and publishing Best Management Practice (BMP) guides. The Environmental Protection Agency's (EPA) BMP manual lists an array of structural and green approaches such as green roofs, porous and pervious pavements, and retention and detention basins.

 

Pervious concrete reduces stormwater runoff. Photo courtesy of Portland Cement Association.

 

Concrete offers a number of applications for draining and filtering stormwater:

Pervious or permeable concrete is formed by removing all or a major portion of the fine aggregate and binding the remaining larger aggregate by a relatively small amount of cement paste.  When hardened, typically between 15 to 35 percent of the concrete volume are voids, allowing water to drain at a rate of 5 gal/ft2/min or 200 L/m2/min through the concrete. Designing pervious concrete with detention systems will allow for higher amounts of rainfall to be collected. Pervious concrete must be suitably designed for freeze/thaw climates.

Permeable and open grid pavement systems. Another option is to use concrete pavers with large voids through which vegetation can grow. Water infiltrates through a joint spacer arrangement filled with soil, grass or stone; an open grid allows for growth of vegetation. They are appropriate for light load parking, plazas and walkways in any climate. They are usually more cost-effective than a separate detention facility in some urbanized areas where the existing impervious area is greater than 50 percent and space is limited. Permeable interlocking concrete pavements can reduce runoff to zero for the most frequent storms. Pavers may be modular pre-cast or cast-in-place.

Rainwater harvesting systems. For irrigation and for holding captured stormwater prior to release back into the landscape or reuse within the site are often constructed of precast concrete.

Heat Island Effect: Non-Roof and Roof (SS Credit 7.1 and Credit 7.2; 1-2 points)
Intent:Reduce heat islands (thermal gradient differences between developed and undeveloped areas) to minimize impact on microclimate and human and wildlife habitat.

Many U.S. cities and suburbs have air temperatures up to 10 °F (5.6 °C) warmer than surrounding natural land cover. Heat islands form as cities replace natural land cover with impermeable pavement, buildings, and other structures. Built areas absorb more of the sun's heat than do natural surfaces, causing surface and air temperatures to rise.

Under its Green Alley initiative, Chicago has embarked on retrofitting its 1,900 miles of alleys with environmentally sustainable permeable concrete or porous road-building materials. Goals are to reduce the heat island effect, reduce flow and pollution from runoff,  recharge the underground water table and recycle materials.

Heat island effect is recorded by a composite index called the Solar Reflectance Index (SRI), a measure of a surface's reflectance (albedo) and its emissivity (release of heat) on the surface temperature. SRI is defined with a standard black surface of 0 and a standard white surface of 100.

McMaster Universtiy Engineering and Graduate Studies Building Located in Hamilton, Ontario, the new 125,600 ft2 McMaster University Engineering and Graduate Studies Building (designed by Vermeulen/Hind Architects, Dundas, Ontario), exemplifies current building engineering and sustainable design trends. With its reinforced concrete flat slab and column structure, one of the project's greatest potential environmental impacts was due to cement manufacturing's typically high green house gas (GHG) emissions. Structural engineers Halcrow Yolles, Toronto, Ontario, developed a concrete design utilizing slag cement, which was used to replace 20 percent of the cement required for floors and columns and 50 percent of the cement for the elliptical walls in the teaching presentation rooms. Total reduction in GHGs was calculated at over 300 metric tons. The project is scheduled for completion in spring 2009. The building is targeting LEED Canada NC Gold certification (LEED Canada is a close adaptation of the USGBC LEED system). While using slag cement as a replacement for regular cement does not in itself result in a scorecard point, this strategy does contribute to the following credits: MRC4 Recycled Content; MRC5 Regional Materials; EQ4.2 Low emitting paints and coatings (using water-borne sealers for exposed concrete finishes). In addition, concrete is expected to contribute towards ID credits for exceptional regional content and green education innovation. Photo courtesy of Vermeulen/Hind Architects.

 

LEED credit requirements offer two options for non-roof areas: Provide any combination of several strategies-shade, paving materials with an SRI of at least 29, and/or an open grid pavement system for 50 percent of the site hardscape. Or, place a minimum of 50 percent of parking spaces under cover.  Any roof used to shade or cover parking must have an SRI of at least 29.

 

Cool paving materials minimize the absorption of solar heat and the subsequent transfer of this heat to the surroundings. One type of cool paving material is a lighter-colored concrete product (manufactured with light-colored cementitious materials and light-colored aggregates) with a high ability to reflect sunlight. Solar reflectance or albedo is a measure of a material's ability to diffusely reflect light from the sun (unlike SRI which incorporates both reflectivity and emissivity). It is expressed on a scale of 0 to 1.0, where an albedo value of 1.0 represents total reflectivity. Light-colored surfaces in shades of white, beige, light gray and terra cotta generally have higher albedos than dark-colored surfaces and therefore help achieve the minimum SRI of 29. Proper maintenance, including washing and resealing, is required to maintain a high albedo level and subsequent SRI value.

A second type of cool paving is porous or permeable pavements, which allow water to filter into the ground, keeping the pavement cool when moist. Compared with asphalt, pervious concrete and grassed grid pavements through evapotranspiration, will reduce surface air temperatures by 2 to 4 °F (1 to 2 °C).

For LEED credit requirements for roofs, options include using concrete roof tiles or cement shingles having a specified SRI (equal to or greater than 78 for a low-sloped roof or 29 for a steep-sloped roof) and a high albedo.
The Portland Cement Association (PCA) found that 135 concretes tested for solar reflectance had an SRI of least 29(Solar Reflectance of Concretes for LEED Sustainable Sites Credit: Heat Island Effect, 2007). All products tested, therefore, met LEED Heat Island Effects requirements for Non-Roof and for steep-sloped roofs in the Roof categories (segmental concrete paving would typically be used on low-slope roofs.) The lowest reflectance values were from concretes composed of dark gray fly ash.

The study reports that solar reflectance of the cement had more effect on the solar reflectance of the concrete than any other constituent material. The solar reflectance of the supplementary cementitious material (fly ash or slag cement) had the second greatest effect. The solar reflectance of the fine aggregate had a small effect, while the solar reflectance of the coarse aggregate had no significant effect.  Surface finishing techniques also have an effect as smoother finishes typically have higher solar reflectance. "With
most concretes having an SRI of at least 29, this is an easy credit to earn that most designers ignore," says building science engineer Medgar Marceau, PE (Illinois), LEED AP, CTL Group. 

Precast concrete pavers with an SRI of at least 29, offer freedom of design with a variety of colors, textures, patterns and sizes. They enable designers to create garden terraces, lunch areas and architectural plaza decks from rooftop spaces that would otherwise be unattractive and inaccessible.

Concrete is the structural system of choice for vegetated roofs because it provides a continuous load-bearing surface. Projects constructed using waterproof concrete made with proprietary admixtures allow for the elimination of membranes, and therefore simplify design, construction and maintenance.

Water Efficiency (WE)

Water Efficient Landscaping (Credit 1.1 Reduce by 50 percent; 1 point. Credit 1.2 No irrigation or No potable water use; 1 point)
Intent:Limit or eliminate the use of potable water, or other natural surface for landscape irrigation.

Innovative Wastewater Technologies (Credit 2; 1 point)
Intent: Reduce generation of wastewater and potable water demand, while increasing the local aquifer recharge.

A growing number of urban projects are capturing runoff and recycling water for irrigation. Concrete pervious, permeable and grid pavement systems with rainwater harvesting systems help contribute points in this category.

Energy & Atmosphere (EA)

Minimum Energy Performance (EA Prerequisite 2)
Intent: Establish the minimum level of energy efficiency for theproposed building and systems.

This prerequisite requires compliance with ASHRAE/IESNA Standard 90.1-2004 or the local energy code, whichever is more stringent.

Optimizes Energy Performance (EA Credit; 1 to 10 points)
Intent: Achieve increasing levels of energy performance above the baseline in the prerequisite standard to reduce environmental and economic impacts associated with excessive energy use.

Ninety percent of the environmental impact of CO² associated with buildings arises from heating, cooling and lighting during their operational lifetime. Concrete buildings have the ability to absorb and release heat, via thermal mass effect, which means that less energy is needed for heating or cooling.

A PCA study that modeled energy performance of concrete buildings (www.concretethinker.com) shows that the effect of thermal mass in concrete-framed buildings combined with thermal improvements to the building envelopes lowers energy cost up to 23 percent relative to baseline steel-framed insulated buildings. This is an energy savings that qualifies for up to four (4) LEED-NC v2.2 points.

The concrete thermal mass of a building contributes to EA credits in three ways. First, thermal mass moderates indoor temperature fluctuations, thus reducing spikes in temperature. Second, massive wall and roof elements slow the transfer of heat through the building envelope. Third, concrete thermal mass can store energy, thus shifting demand to off-peak periods.  In addition, insulated concrete wall systems reduce air infiltration. Their solid wall assemblies and high R-values from rigid insulation provide airtight building envelopes that increase energy efficiency.

A number of concrete construction methods may contribute to thermal mass. Lafarge's Buffenbarger lists tilt-up walls, insulating concrete forms (ICFs), precast sandwich panels, insulated concrete masonry unit walls, cast-in-place concrete with removable formed walls, autoclaved aerated concrete panels, insulated cement siding, shotcrete (sprayed concrete), and stucco.

Materials & Resources (MR)

Building Reuse: (MR Credit 1.1, Credit 1.2 and Credit 1.3; 1-3 points) Maintain 75 percent or 95 percent of Existing Walls, Floors & Roof; 50 percent of Interior Non-Structural Elements.
Intent:Extend the life cycle of existing building stock.

Well-constructed building envelopes and structures of concrete, concrete masonry units; concrete roofing elements; interior floors and walls and other structural materials that contain concrete and cementitious building materials can be left in place during major refurbishment. The reuse of existing structures provides multiple benefits including land reuse and preservations of greenfields, raw material conservation, reduced strain on landfills, as well as lowered embodied energies in their deconstruction.

Construction Waste Management (MR Credit 2.1 and Credit 2.2; 1-2 points) Divert 50 percent or 75 percent in weight or volume from disposal.
Intent: Divert construction, demolition and land-clearing debris from disposal in landfills...Redirect recyclable recovered resources back to the manufacturing process.

Concrete and concrete masonry materials may be recycled, crushed and reused on site.

Materials Reuse (MR Credit 3.1 and Credit 3.2; 1-2 Points) 5 percent or 10 percent materials reuse, based on cost. 
Intent: Reuse building materials and products in order to reduce demand for virgin materials.

Credits apply to individual components and include incoming material from the marketplace and materials salvaged from on-site demolition or renovation.

Generally, concrete and cementitious building components have low waste content as concrete is ordered and mixed for each job, thus minimizing waste. Any leftover or waste concretes or concrete masonry unit materials may be recycled by crushing and then reused onsite for roadbase, trench or structural fill or crushed and reused as concrete aggregate for new concrete.

Recycled Content (MR Credit 4.1 and Credit 4.2; 1-2 points) 10 percent or 20 percent based on cost (post-consumer + 1/2 pre-consumer).
Intent: Increase demand for building products that incorporate recycled content materials.

Pre-consumer recycled content is based on waste from industrial and manufacturing processes and does not include scrap or trimmings.

Post-consumer material is defined as waste material generated by households or by commercial, industrial and institutional facilities in their role as end-users of the product, which can no longer be used for its intended purpose.

The use of SCMs as a replacement for portland cement in concrete mixtures, concrete masonry units, and other cementitious-based building materials provides easy inclusion of pre-consumer recycled material content into sustainable building projects. SCMs, such as fly ash, slag cement and silica fume are recycled materials. They may replace portland cement in cementitious-based building materials and reduce CO² production and energy requirements needed in the manufacture of cement. Fly ash is commonly used at replacement levels up to 40 percent; slag cement up to 70 percent; and other byproduct materials such as foundry sand, fiberglass, polystyrene, bottom ash and slag aggregate may also be used at various levels.

Post-consumer material includes recycled concrete aggregates produced by crushing concrete, glass, carpet, and returned crushed concrete. Concrete made with pre-consumer and post-consumer materials should undergo testing to confirm performance, durable properties and compliance to local building codes.

Concrete and cementitious containing building materials containing multiple pre-consumer and post-consumer materials will contribute more in this category because the credit is based on cost of replaced mass of virgin materials with recycled content.

Regional Materials (MR Credit 5.1 and 5.2; 1 to 2 points)
Intent: Increase demand for building materials and products that are extracted and manufactured within the region.

LEED credits require that 10 or 20 percent of building materials or products, based on cost, is extracted or recovered, as well as manufactured, within 500 miles of the project site.

Concrete, concrete masonry and other cementitious-based building materials are generally produced nearby as their primary constituents, i.e. aggregates, tend to come from local sources. Most concrete, concrete masonry units, and cementitious-based building materials are manufactured locally.

Indoor Environmental Quality (EQ)

Most concrete and cementitious-based building materials require no coatings or finishes in interior applications, and can be used as a structure/finish combination. Such materials include concrete masonry, glazed masonry, decorative concrete floors, stucco, cementitious siding and cement wallboard.

Poor indoor air quality (IAQ) conditions in buildings, including dampness and mold, particulates, and chemicals, are associated with a host of health problems. Exposure to health risks can be lowered through the use of concrete as a finish material. Construction IAQ Management Plan, During Construction (EQ Credit 3.1; 1 point)
Intent:Reduce indoor air quality problems resulting from the construction/renovation process in order to help sustain the comfort and well-being of construction workers and building occupants.

Applications using precast concrete help meet these requirements as concrete assemblages are delivered to the site in pieces that do not require fabrication, processing or cutting, thereby reducing dust and airborne contaminants on the construction site

Low-Emitting Materials: Adhesives & Sealants  (EQ Credits 4.1; 1 point); Paints & Coatings (EQ 4.2; 1 point)
Intent: Reduce the quantity of indoor air contaminants.

Today's interior concrete walls, concrete and cementitious overlay floors and concrete masonry units are offered in nearly limitless colors and textures. Interior applications include stamped, stained, engraved, ground and polished concrete floors, walls and countertops, glazed concrete masonry unit walls, cement fiberboard, and stucco with aesthetic appeal, long-term durability and low maintenance. Unlike other interior finish materials, concrete, concrete masonry units, cement-fiberboard and cementitious-based building materials emit very low or no volatile components, do not harbor dust allergens or contribute to mold and mildew growth.

Innovation in Design Process (ID) (ID Credit 1.1 − 1.4); 1 − 4 points
Intent: To provide design teams and projects the opportunity to be awarded points for exceptional performance above the requirements set by the LEED not specifically addressed by the LEED-NC Green Building Rating System.

At present, most ID points are awarded for exceptional performance above LEED requirements in the MR category. In 2001, Mithun, Inc., a design firm in Seattle, WA, worked with structural engineers Magnusson Klemencic Associates to help obtain an ID point by utilizing a 50 percent fly ash containing concrete mixture for IslandWood, an environmental education center in Bainbridge Island, WA.

 

 

Another example is the Gold-certified Clearview Elementary School in Hanover, PA., designed and engineered by L. Robert Kimball & Associates. The school was built with ICFs filled with concrete containing as much as 60 percent slag cement replacement.   To help designers brainstorm opportunities for earning ID points, the USGBC (www.usgbc.org) publishes anInnovation & Design Credit Catalog. One recent ID point relates to preserving open space by locating a building on top of the campus' chilled water reservoir tank constructed with 30-inch concrete walls and steel superstructure. Another ID point applicant minimized the quantity of materials by using the underside of the concrete floor and roof deck as the ceiling on all levels.

The USGBC has granted an ID credit for concrete walls and ceilings with no paints or coatings on the interior. It has also approved that a durability credit allowed in LEED Canada-NC 1.0 can be used as an ID credit in LEED NC 2.2 (Guide to Sustainable Design with Concrete by the Cement Association of Canada (www.cement.ca) is a useful source for Canadian LEED credits, which could be allowed by the USGBC).

LEED Accredited Professional (ID Credit 2; 1 Point)
Intent: To support and encourage the design integration required by LEED.

One principal participant of the project team shall be a LEED Accredited Professional (AP). The LEED professional's knowledge of the properties of concrete will help determine additional
credit possibilities.

Conclusion

Complying with sustainable design requirements and employing integrated, whole building design practices is the first instrumental step towards minimizing the detrimental impact that construction imparts on our nation's resources. The use of concrete, concrete masonry and cement-based building materials offer extensive sustainable benefits. An increased understanding and awareness of the environmental impacts of using concrete is necessary to understand its extraordinary delivery of sustainable benefits to the environment.

 

Lafarge is the largest diversified supplier of construction materials in the U.S. and Canada. We produce and sell cement, ready-mixed concrete, gypsum wallboard, aggregates, asphalt, and related products and services. Our products are used in residential, commercial and public works construction projects across North America.lafarge-na.com