In the paving industry, hot mix asphalt (HMA) has set the standard for building high quality, long-lasting asphalt driving surfaces. HMA goes down steaming hot and is spread in a thick black coating to preserve and transform cracked, rutted roads into smooth driving surfaces. We use millions of tons every year to maintain thousands of miles of asphalt-paved highways, ramps and bridges across the state.
HMA may become a thing of the past, though, as we start using lower-temperature warm mix asphalt (WMA) in a greener approach to paving.
Asphalt mixes are usually manufactured at hot temperatures because the adhesives that make the asphalt stick to the roadway needs heat to stay pliant. WMA is heated anywhere from 35 to 100 degrees cooler than HMA. To keep it malleable at lower temperatures, a special lubricant is added at the asphalt plant, sort of like adding canola oil to butter to make it spreadable.
Heating asphalt mix to lower temperatures reduces energy use and cuts costs for manufacturers. It also reduces emissions, both at the plant and on the highway; when WMA is spread, it doesn’t steam like HMA does. “On the job site, warm mix asphalt creates less volatile gases – it’s safer on the environment and safer on workers,” says project manager Allen Hendy. “And we get the same end product.”
In national and WSDOT studies, WMA appears to holds up just as well under traffic as HMA. We’re continuing tests to see if WMA will have the same life as HMA, which typically lasts about 15 years before it needs resurfaced. So far, all signs point to WMA being just as durable.
There are a lot of financial and environmental reasons to use WMA, and there’s something in it for drivers as well. WMA cools more quickly, which means crews can open lanes and put traffic on the new pavement faster than with HMA.
Drivers will get a first-hand look at this during the I-5, Interstate Bridge to 39th Street paving project this summer. Crews on this project plan to use WMA to pave more than four miles of deteriorating pavement on I-5 in Vancouver. They began night time double-lane closures on July 5 to perform utility work. Paving work with continued night time lane closures begins next week.
With WMA, the closures on I-5 may be shorter in duration and crews should be able to get more paving completed each night than with HMA.