Concrete Ideas for Reducing a Structure’s Carbon Footprint: Lightly Loaded Foundations

September 2014 » Feature Articles » Sustainable Design
Jim D’Aloisio, P.E., SECB, LEED AP BD+C

If it is important for people to reduce carbon-dioxide equivalent (CO2e) atmospheric emissions, then structural engineers have a tremendous amount that they can — and should — do.

Materials such as concrete, steel, and masonry used in a building or infrastructure project create a large “bloom” of CO2e emitted from the construction. We need to come to terms with using these important and necessary materials in a responsible way, making adjustments in their design, specification, and selection, based on an awareness of these emissions.

Many people in the building design industry believe that CO2e emissions from construction of a building are relatively small and insignificant, compared with the CO2e emissions of burning fossil fuel for heating and cooling that will occur over the lifetime of the building. I consider this reasoning to be a red herring. The amount of CO2e emitted from both the construction and operation of buildings varies greatly between building types, material selection, climate zones, and energy and envelope efficiencies, and comparing the construction-versus-operation emissions is no simple task. And what, really, is the point in comparing these two different sources? If we need to reduce emissions, let’s do so for both construction and operation. The reality is that in all but a few very rare cases, the amount of CO2e released during, or resulting from, construction of a building is many times greater than the annual CO2e emitted from operations.

Let’s start with concrete. Outside large cities, much of the concrete used in buildings is for foundations of lightly loaded low-rise buildings. The stress levels in these elements are often much lower than the strength of the concrete used; their geometry is dictated by wall dimensions, frost depth, and soil bearing capacity. Reducing the cross-sectional area of concrete by, say, 20 percent, obviously reduces the CO2e of the foundations by 20 percent. In real numbers, this might reduce the emissions due to the perimeter foundations of a 50-foot by 100-foot building from 45,000 pounds to 36,000 pounds of CO2e. That’s 4-1/2 tons of CO2e emissions prevented.

Another source of low-hanging fruit for concrete CO2e reductions is strength and portland cement content. The technique of substituting a moderate amount of fly ash and slag for cement is a well-known practice that has been accepted in most (but not all) construction circles. However, the construction industry is not (yet) prepared to move out of its comfort zones.

Figure 1: Lightly loaded perimeter foundation wall sections.

Last summer, I specified 2,500 psi and 50 percent fly ash or slag for a lightly loaded strip footing on a small commercial project, hoping to get the amount of cement down as low as possible. The batch plant supervisor became uncomfortable, and we agreed to a compromise. In the end, the cylinders all broke above 6,500 psi — more than 2-1/2 times the specified strength.

The frost-protected shallow foundation methodology holds tremendous promise to reduce the volume, and the corresponding CO2e, of foundations if conditions are favorable. In Figure 1, the two lightly loaded perimeter foundation walls provide the exact same function, yet the amount of concrete in the right section is about one-third the volume of concrete in the left section. For a 50-foot by 100-foot building, this technique could eliminate about 15 tons of CO2e.

Other benefits of frost-protected shallow foundations include lower construction costs, fewer materials, a shallower excavation, and faster construction time. So why isn’t this system being used more frequently?

  • It takes a little more effort to design and detail a proper frost-protected shallow foundation than a standard foundation. If the engineer’s fee is fixed — possibly the engineer was selected based on lowest fee — there is a disincentive to choose the more time-consuming design path.
  • Some people in the industry — contractors, designers, owners, and developers — have been burned by problems that have arisen by the misuse of this design methodology. Design standards and guidelines should be carefully followed, especially ASCE 32-01 Design and Construction of Frost Protected Shallow Foundations.
  • More care must be taken to coordinate with subsurface elements and conditions. For example, one of my projects suffered change orders that resulted from the plumbing engineer not having laid out the piping inverts until after the foundation contract was awarded, which required lowering footings in several locations.
  • Some contractors are unwilling to do anything that they have not done before. Of course, this approach would preclude us from making any changes in construction, no matter how beneficial or necessary the change!

Ironically, although it may be important — and even part of our ethical duties as engineers — to actively reduce the CO2e of our building structures, there is currently an implicit disincentive to do so. Expending the effort to consider this aspect of structural design takes time. In this world where lowest construction cost is valued more than any other measure of a project, and where engineering fees are all too frequently price-compared (rather than value-compared), practitioners who pursue this path may put themselves at an economic disadvantage. Since economics governs, until a financial value is given to CO2e, such as by adoption of a revenue-neutral carbon tax, this consideration — that just might be of vital importance to everyone (i.e. the public, whom we are called as engineers to protect) — will continue to be marginalized.

Jim D’Aloisio, P.E., SECB, LEED AP BD+C, is a principal at Klepper, Hahn & Hyatt, based in East Syracuse, N.Y. He is the current chair of the ASCE/SEI Sustainability Committee, and can be reached at jad@khhpc.com.


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