Concrete Products

MAR 2018

Concrete Products covers the issues that attract producers of ready mixed and manufactured concrete focusing on equipment and material technology, market development and management topics.

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54 • March 2018 TECHNICAL TALK BY TOM KUENNEN The 97th annual meeting of the Transportation Research Board in January drew 13,000-plus transportation engineers and specialists from across the country, and around the world to Washington, D.C. More than 5,000 presentations in over 800 sessions addressed topics in multimodal transportation, including materials and design involv- ing ready mixed concrete and precast/prestressed products. Concrete Products was there and this month presents a report on new research findings in cast-in-place concrete. We'll look at precast/prestressed related research from TRB 2018 in an upcoming issue. For more infor- mation, visit PCC ALBEDO CHANGES VARY URBAN HEAT ISLAND EFFECT Quantifying radiative forcing and building energy demand impacts due to pavement albedo changes is essential to understanding the role of concrete pavements in keeping urban areas cooler and forestalling global warming, say Xin Xu, Dr. Jeremy Gregory and Dr. Randolph Kirchain in their TRB paper, The Impact of Pavement Albedo on Radi- ative Forcing and Building Energy Demand: Comparative Analysis of Urban Neighborhoods. Albedo is a ratio expressed on a scale from zero to one that describes how much solar radiation any given surface reflects: a sur- face with an albedo of "zero" would be impossibly dark, taking in 100 percent of the sun's energies, where a surface with an albedo of "one" would be completely reflective. This should benefit concrete as its lighter color is more reflective than other building materials, for example, giving it an advantage over dark asphaltic concrete in keep- ing urban areas cooler, and even deferring presumed climate change. The amount of solar energy absorbed or reflected by surfaces in urban environments impacts the energy demand of surrounding build- ings and also has an impact on the climate—locally, regionally, and globally. Researchers developed an approach to quantify the impacts of changes to pavement albedo in different locales, and then translate those changes into global warming potential. "The albedo effect applies to any type of urban surface, not just concrete," says Gregory, author and executive director, Concrete Sus- tainability Hub [CSHub] at Massachusetts Institute of Technology. "Considering albedo, there are differences in how that can impact the climate. A change in albedo can change the radiative forcing, that is how much radiation enters the atmosphere versus how much leaves. Deforestation causes albedo changes too, for example. With this paper we quantify that radiative forcing impact." He and fellow authors also quantify the impact of changes in albedo in the energy consumptions of buildings near those surfaces. That's because, they say, benefits of a high albedo material like con- crete, depending on the location, actually can force adjacent buildings to consume more energy for air conditioning. "In a hot climate, if the [pavement] albedo is increased, it can increase the amount of radiation that strikes nearby buildings," Greg- ory tells Concrete Products. "It can increase the cooling demands on the building, which leads to more energy generation, which can impact the climate. We're quantifying both those mechanisms and looking at their net effects." Like white concrete pavements, use of white roofs to mitigate urban heat island effects is all about balancing net effects, he adds, noting: "If you do that in a northern climate it decreases the amount of cooling air required, but increases the amount of heating required. You have to look at what the net benefit will be and where those applications make the most sense. "It's the same with pavements, but the other complicating factor with concrete pavements—particularly in the urban environment—is there are 'canyoning' effects [of building shadows] that change the amount of shading there is on the pavement. In this paper we have quantified the impact of the change of albedo on different locations, and the geometry of different urban configurations." Thermal mass of a building—optimized by cast-in-place or precast concrete construction—can mitigate urban heat island effects. That's why the design of the building and how much energy is absorbed by the reflection of the pavement both have an impact on the energy consumption of the building, dubbed the building energy demand, or BED. "If it's a well-designed, energy-efficient building, and certainly one with thermal mass, that can mitigate any effects of energy being reflected off horizontal surfaces onto the building's vertical surfaces," Gregory observes. To be fair, a nonreflective structure can be made reflective just by cladding it in a reflective surface. "Knowing the reflectivity of the building is very important," he says. "Any surface—regardless of its natural color—can be made reflective with a coating. It's not just about concrete or asphalt, it's about anything that is reflective. Concrete is being made with lighter aggregate, and in some places, asphalt is being coated to increase its albedo. There are a lot of different solutions." This research's benefit for the industry is that for the first time practitioners can understand quantitatively what role a concrete structure will play in its urban context. "A big challenge associated with this is measuring the albedo of different surfaces," Gregory explains. "We know initially concrete has a higher albedo than asphalt, but over time, concrete darkens, while asphalt lightens. We need better research on how that process unfolds over time, and how that depends on their locations. But for researchers, the starting point is very clear: Concrete starts out with a naturally higher albedo, which depending on its location or urban context, can benefit the environment. TRB: Albedo change study quantifies role in climate change; PCC overlays perform PHOTO: Tom Kuennen MIT CSHub researcher Xin Xu weighs the result of changes in albedo of pavements and structures in different urban contexts.

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