August 12, 2011
The Concrete Sustainability Hub at the Massachusetts Institute of Technology (MIT) has released two studies looking at the life-cycle assessment (LCA) of concrete pavements and buildings. A third assessment looked at life-cycling costing for streets and highways.
Both studies signify major advancements for construction-related life-cycle assessments. They thoroughly examine the cost and environmental impacts for the full life of pavements and buildings — including the use and operations phase — not just the costs and embodied CO2 that occur at initial construction. Currently, most LCAs in use do not fully account for these impacts, which can include traffic delays, energy consumption, and maintenance.
MIT also used this life-cycle approach to evaluate the real cost of pavement throughout a 50-year lifetime, beyond initial construction costs.
Researchers started with the Federal Highway Administration’s (FHWA) Life- Cycle Cost Analysis in Pavement Design Interim Technical Bulletin, a process that accounts for both initial construction and future rehabilitation.
What the FHWA procedure fails to account for, however, are changes in the prices of building materials over the life of the pavement. MIT’s research showed that during a 50-year timeframe, the mean real price of concrete decreases by 20 percent, while the mean real price of asphalt increases b 95 percent. To allow states to address this, MIT developed a paper and a procedure that departments of transportation can readily adopt to account for inflation.
In its environmental assessment, MIT researchers found that while concrete pavements are already sustainable in many ways, their carbon footprint can be further reduced. First, MIT developed a comprehensive methodology outlining the best-practice concepts that should be followed when conducting any pavement LCA. Specifically, any complete LCA should include the use and rehabilitation phases, which can account for between 33 and 44 percent of the CO2 emissions for interstate highways.
Next, the Hub researchers applied these concepts to evaluate strategies to lower a concrete pavement’s carbon footprint and overall environmental impact. A major advancement was the incorporation of a cost-effective analysis to determine whether or not a given environmental reduction strategy made sense economically. Among the strategies evaluated, the two that reduced embodied emissions — increased fly ash and reduced overdesign due to better designs — were found to lower the CO2 emissions by about 10 percent and 17 percent, respectively, while also saving upfront costs.
Finally, researchers reviewed fuel economy from a unique perspective.
Instead of focusing on the efficiency of cars and trucks, they analyzed how pavement properties affect fuel economy. MIT developed the first-ever mechanistic pavement-vehicle interaction (PVI) model that relates fuel consumption to pavement material and structural properties. This model provides realistic estimates of changes due to deflection.
Pavements that deflect or bend slightly under traffic loads cause cars and trucks to run in a slight depression that increases fuel consumption.
Pavements with greater stiffness, MIT found, mean better fuel economy for the vehicles that travel on them. As an example of the initial results, MIT looked at typical material properties for concrete and asphalt pavements and found that, for the same stiffness and fuel consumption, an asphalt pavement had to be up to 60 percent thicker than the concrete pavement. With fuller development of this model, it will be possible to include the impacts of pavement properties on fuel usage in both the environmental and cost analyses.
Furthermore, the research conducted by MIT has quantified the relative CO2 contribution from buildings across all phases of a building’s life cycle.
This rigorous analysis, with a similar study of whether the best environmental strategy was beneficial economically, will allow the construction industry to improve the accuracy and transparency of existing and future life-cycle assessments, providing legislators, code making bodies, and building design professionals with a comprehensive and unbiased LCA.