CONCRETE PAVEMENT PROGRESS 16 WWW.ACPA.ORG » continued from page 15 the Massachusetts Institute of Technology (MIT CSHub) are pavement-vehicle interaction (PVI) and albedo. PVI describes the excess fuel emissions/energy from vehicles due to excess rolling resistance between the pavement and the vehicle. Albedo is the fraction of solar energy reflected by the Earth's surface; lighter-color, higher-albedo surfaces reflect more energy than lower-albedo, darker surfaces.1 The less bumpy a road is, the less energy is needed for a vehicle to travel the pavement, which means less fuel needed, and fewer emissions explained Ferrebee. Concrete is an inherently stiff pavement which reduces deflection-related PVI. “There are three pavement vehicle interactions that impact excess fuel usage,” Mack says. “Researchers at MIT CSHub have now quantified that data and demonstrated how emissions are affected by these factors.” The three factors include: 1. Smoothness/Roughness (IRI): The excess energy needed to overcome the frictional forces in the vehicle suspension due to the bouncing of the car.2 2. Structural/Deflection: The excess energy required to traverse or drive uphill due to the deflection or bending of pavement from the weight of the vehicle.3 3. Surface Texture: The excess energy required to overcome the abrasiveness or macro texture at the tire-pavement contact area—a factor that is critical for safety. The measurement of PVI is always important no matter what type of traffic is common on the road, says Mack. “PVI from roughness is the one everyone pays attention to now because it affects cars and trucks. It evolves over time and is a function of how well we maintain our pavement smoothness,” he says. “PVI deflection is a measurement that depends on pavement type/ design. Because concrete is naturally stiffer, it has lower PVI deflection, and this is what matters—especially on corridors with medium to high truck volume.” “Another important factor is the albedo effect— the reflection of sunlight back into the atmosphere,” says Mack. “Dark pavements absorb the heat, then release it later as the atmosphere cools at night, creating ‘heat islands.’ Lighter pavements, such as concrete, reflect light and heat, so it dissipates throughout the day.” “Concrete’s albedo is closer to the overall average of Earth’s albedo,” Mack explains. “Earth’s average is about 0.33, and concrete is 0.2 to 0.35, depending on the age and mix components. Asphalt pavements’ albedo starts out low, around .1, because they are black and lighten over time.” After 10 to 15 years, asphalt pavements are rehabbed, which makes them black again. Concrete starts out at a much higher albedo, and over time, it darkens, but it never darkens to the point that it is darker than asphalt—it is always lighter. When concrete is rehabbed with diamond grinding, it returns to its original, higher albedo. “Studies have shown in some cities, such as Phoenix, temperatures could be reduced by a few degrees when using a lighter pavement material such as concrete,” says Ferrebee. Understanding the effect of pavement that absorbs and holds heat is important to understanding how urban heat islands create greater greenhouse gas emissions. At this time, no state agencies are making decisions based on greenhouse gas emissions, but Credit Jacquelyn Wong, Caltrans “The concrete pavement industry has been talking about use-phase impacts for more than a decade, but it’s important to get our message out that including use-phase emissions is valuable data that provides a better view of environmental impact over the pavement’s life.” Eric Ferrebee, P.E., Senior Director of Technical Services, American Concrete Pavement Association
RkJQdWJsaXNoZXIy Nzc3ODM=