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In Berkeley, we are fortunate to have such events as Science at the Theater, where Lawrence Berkeley National Lab researchers give talks on their work at the Berkeley Repertory Theatre. The lectures are free and get a pretty sizeable audience.
On Monday, October 11, I was in the audience as researchers from Lawrence Berkeley National Laboratory (and the beloved Art Rosenfeld) gave a presentation titled “Cool Roofs, Cool Cities.” The post below consists of Part 3 of my record of the presentation – Melvin Pomerantz gives an overview of cool pavements. All portions are included in chronological order.
An ellipsis (…) indicates that I was not able to capture the words or thoughts skipped. The presentation is transcribed as accurately as possible – punctuation choices are mine. I also added any images.
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Now we’re talking about cool pavements … because they are a significant fraction of a city … including the streets, parking lots, driveways, and sidewalks.
We need to understands that they are a composite material … they are aggregate, an array rocks of different sizes … We then have to hold them together. So we have to glue the rocks and sand together by coating each rock with a binder of some sort. For “asphalt”, the binder is asphalt, which is a petroleum product … We can also bind the rocks together with cement, which is a mineral product, and we call that concrete … The key thing is that because you’re coating the aggregate, you mostly see the color of the binding … Our target is the binder.
Of the third of the city that is covered with pavements of various kinds, 50% is streets, 40% are exposed parking, and about 10% are sidewalks … Because it covers about 90% of the paved city streets, our target is the asphalt concrete.
Image credit: Wikimedia Commons
Start with fresh asphalt concrete … which is very black and has a solar reflectance of about 5%. As it ages, sunlight breaks it downs … Typically, its solar reflectance goes up to about 15% … The question is can we do any better … If you use a light-colored aggregate, it will show as the binder wears off … can use seashells, or porcelain … For old pavement, which required resurfacing periodically … we can put on a layer of asphalt emulsion, and put light-colored aggregate on top. That is called a “chip seal” … One issue is that aggregate is heavy, so it’s expensive to ship. So we want to use rock that’s nearby, and it may not be white … If the road stay cool, it doesn’t deform as much …
An example is from San Jose … They happen to have a quarry nearby that has white rock, and they’ve used a chip seal …
The other type of road, a little less common, is cement concrete … Fresh cement has a solar reflectance of about 35% … as it ages, it gets darker and reflectance drops to about 20% … The fine aggregate tends to float to the top … If you have light-colored fine aggregate, you can get an initial solar reflectance of about 40% …
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