Building Today to Conserve the Future

Excerpted from an article in Environmental Design and Construction, September 2007 by William D. Palmer Jr.

Concrete is strong—at least in compression. And concrete reinforced with steel can handle the worst abuse--we build bomb shelters and prisons with concrete. It doesn’t rot, even under water or in soil. It can be molded into nearly any shape imaginable.

Concrete’s thermal mass can contribute to a building’s energy efficiency by reducing interior temperature swings, thereby reducing the size of heating and cooling equipment. It can incorporate recycled material like fly ash and blast furnace slag. And it’s mold proof and does not off-gas toxic substances.

In addition to its structural and architectural advantages, concrete is an essential ingredient of sustainable construction. Concrete’s greatest contribution to sustainability lies in is its durability. Concrete structures last a long time. When a concrete building is designed properly, it will perform admirably for many years and will offer its residents energy efficiency and safe shelter from nearly any natural or manmade disaster. And when the end of its life finally does arrive, concrete is eminently recyclable and can even be turned into new concrete by crushing it into aggregate.

But aren’t concrete structures more expensive? The same question is often asked about sustainable construction in general and that of course depends on how you measure it. The initial cost for concrete buildings is typically a bit higher than for buildings using other materials. But we all know that is only a small part of the story. The true way to look at the cost of a building is through life cycle assessment, including everything from extracting the raw materials (and reclamation of mine sites) through operating and maintenance expenses until the end of the building’s life. This is where concrete shines, since operating expenses are typically much less for a concrete structure—a structure that can easily have a 100-year life. Consider the Roman Pantheon’s concrete dome, which is still standing after 2000 years. More about life cycle costing and concrete.

Typical cast-in-place concrete construction is air tight and has thermal mass, although without added insulation its R-value is low. Many modern concrete building techniques, though, incorporate insulation in a variety of ways to result in energy-efficient, quiet, disaster-resistant homes:

• Precast panels—Sandwich panels incorporate insulation into the wall cross section. Dukane Precast in the Chicago area, for example, manufactures double-wall panels with 3 ¼ inches of insulation sandwiched between layers of concrete to achieve an R-22 wall.


• Insulating concrete forms—Once relegated to residential basements, highly energy-efficient ICFs are now also being used for above-grade walls for homes and even for multistory buildings, especially hotels and schools.


• Autoclaved aerated concrete—Blocks and panels made from AAC are light weight and a 10-inch wall has equivalent R-values in the low 20s.


• Tilt-Up—Several systems are in use today to insulate tilt-up panels for homes, including T-Mass from Dow Chemical’s Styrofoam line. The New American Home (TNAH) at this year’s International Builders Show used T Mass panels (see sidebar) but with plant precast panels rather than site-cast tilt-up panels.


• Concrete masonry—CMU construction can achieve high R values by injecting foam into the cores. The majority of Florida homes today are built with CMUs.


• Sprayed concrete—Several systems have emerged recently where polystyrene insulation is erected and then concrete is applied to both sides by spraying or shotcreting. A recent 9200 square foot home in Illinois built with this method has an expected 100 year life.

Concrete’s contribution to sustainable construction doesn’t end with walls. Over the past 10 years, we have seen a revolution in decorative concrete, especially for interior floors and countertops. Durable decorative surfaces have many advantages both in commercial and residential construction.

Streets, driveways, and parking lots have been concrete for many years, and contribute to sustainability by reducing heat island effects with their lighter, more reflective surfaces. But pervious concrete takes this one step further, by draining rain and snowmelt directly into the subsurface and eliminating the need for runoff retaining ponds.

One criticism of concrete has always been that cement manufacturing is energy-intensive and produces large quantities of CO₂. While at one time those points may have been valid, the cement industry has taken an extremely aggressive approach to reducing these impacts. Start with the fact that today cement manufacturing accounts for only 1.5% of U.S. CO₂ emissions.

 
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