Wednesday, July 29, 2009

Working in the heat

When it comes to the weather, sometimes we just can’t win.

During the winter, it’s hard to get much road work done because it’s either too wet, too cold or both. And if you look back at our winter blog posts, we were waiting impatiently for the warm, dry summer weather.

Well, it’s summer. And, depending on the day, it’s either been warm and dry or painfully hot and humid. This week has been mostly the latter. For the most part, we’d rather be working in this weather. But the heat and humidity come with their own set of challenges for our crews.

The most obvious challenge for crews is simply working outside for eight to 10 hours every day on the hot pavement. If it’s 90 degrees outside, it feels even hotter for crews standing on a road that’s just soaking up the heat. And for paving crews…well, you can imagine how scorching that must be.

Needless to say, keeping hydrated is key for everyone. Crews are drinking tons of water before they start their day, during the heat of the day and after they’ve finished work. And everyone stays on high alert for signs of heat exhaustion: disorientation, profuse sweating, nausea or dizziness.

Another less-apparent challenge for crews is compensating for the effect the heat and humidity have on asphalt and concrete. We know what you might be thinking: Aren’t you always worried about bad weather? Isn’t this hot weather a good thing? Yes … and no.

When we pave with asphalt, that asphalt goes down at 300 degrees. And at that temperature, it’s a lot like cookie dough when it gets taken out of the oven: soft and gooey. Naturally, we have to wait until it cools down before we can let traffic drive on it. That’s where the heat and humidity come into play.

When we put down hundreds of tons of 300-degree asphalt, it’s a very concentrated mass of heat. Consequently, it takes a long time for that heat to dissipate and the asphalt to reach a temperature suitable for traffic – 150 degrees or cooler. When the surrounding air is 80 or 90 degrees, it’s much harder for that heat to dissipate. Add in the humidity, which makes the ambient air temperature feel even warmer, and the asphalt will take even longer to cool.

Concrete has exactly the opposite problem: It cures too quickly in the heat. Unlike asphalt, concrete needs humidity to cool and cure properly. When it’s too hot out, water evaporates out of the concrete too quickly and can lead to cracking. Typically, crews blanket the fresh concrete with visqueen (they look like giant white tarps) to help trap the moisture. When it’s extra hot, crews have to work at warp speed to cover up the concrete before it gets too hot. They’ll also layer burlap rags soaked in water atop the fresh concrete and then cover it with visqueen to trap even more moisture.

So while we may have to do a little bit of extra planning when it’s this hot out, we’re taking advantage of the summer weather to get as much work done as possible.


Libby said...

Sounds seriously intense! Thanks for all of your hard work even when the weather is less than favorable.

Phil said...

I thought humidity in the air increased heat transfer, making hot air feel hotter and cold air feel colder to us because the heat transfers from the air to us or from us to the air more rapidly. It would then seem that moist 80- to 90-degree air would cool 300-degree asphalt faster than dry air would. To the asphalt, that air -- which is relatively cool compared to the asphalt -- should "feel" cooler than dry air, shouldn't it?

Bronlea said...

I'm no engineering genius, but I asked one of our engineers that question. His answer was that the wet air prevents the heat in the asphalt from escaping and dissipating. Basically, it acts like a wet blanket and helps keep the heat trapped in the asphalt for longer.

Phil said...

Huh. I don't really get the blanket analogy. The heat in the asphalt doesn't just disappear, it transfers to the air: the asphalt cools as it warms the air next to it. My body would warm a wet blanket that is about 1/4 my temperature faster than it would warm a dry blanket that is about 1/4 my temperature.

It's odd that with humans, humidity increases heat transfer to/from the air, and with asphalt, it decreases the transfer. It would be interesting to hear your engineer's explanation of that.

Bronlea said...

You can contact him and ask. He's great at explaining things like this. If you e-mail me (, I'll send you his contact info.

Cash Games said...

I'm very appreciative of the work. When I was in the military one of the hardest things to get past was heat exhaustion and heat stroke over in the Middle East. It can be extremely dangerous too, definitely keep hydrated guys!

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