Designing Better Asphalt

Published on February 19, 2014, 2:53 pm
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Dr. Haifang Wen grew up in a rural area of Shandong province, in eastern China. In his youth there were not many paved highways in the Chinese countryside.

“Lots of the roads were gravel,” he told me recently. “They were muddy when it rained. I remember riding a cow on them, or going along in a wagon pulled by a donkey.”

Living in those conditions, Wen could see quite a bit of room for improvement in road materials.

“I thought, we can do better,” he said with a smile.

Thus was born Wen’s interest in asphalt, the cheapest material that can be used to pave highways. That interest propelled him through a university education, ultimately capped by earning a doctorate in engineering at North Carolina State University. Now a professor in Civil and Environmental Engineering at Washington State University, Wen uses his education to research new ways of making asphalt better and cheaper.

The asphalt used in roads has traditionally been made from aggregate – small particles of rock – and products made from crude oil. When crude oil is refined, it produces a variety of products including light fuels like gasoline, heavier plastics, and also dense asphalt.

“But the price of asphalt made from crude oil is pretty high, about $700 to $800 per ton,” Wen told me. “That really adds up. One lane of a highway, paved for one mile, costs about $1 million. Now you know where your taxes go!”

One alterative to traditional asphalt that Wen and the people in his lab are looking into is bioasphalt. Instead of using petroleum, waste cooking oil can be processed into asphalt. Restaurants can be spared having to pay to have their waste fat hauled away, and with free raw materials the asphalt made from the waste oil can be more economical.

Bioasphalt is grey, rather than black, and after sticking my nose into a little jar of it, I can testify that it smells better than asphalt made from crude oil.

“Sometimes you can even smell what the restaurant was frying in the oil,” Wen said with a laugh. “Corn oil or peanut oil, it all works.”

The name of the game when it comes to designing asphalt is to balance the properties of the material so that it’s not too stiff (or rigid) but also not too soft (or ductile). If it’s too stiff, the material will crack in the cold of winter. If it’s too soft, a truck driving over the asphalt on a hot summer day will make ruts in the pavement.

“We fine-tune our recipes,” Wen said. “First we make a very small sample and measure its properties. Then we make a slab about the size of a conference table and run a wheel over it. Then we make more pavement and put it outside in the elements.”

Another research area Wen’s group is looking into uses solid waste in place of rock aggregate. Sometimes crushed glass can be used as aggregate, as can broken-up concrete. Even crushed steel slag is being tested. By recycling these solids, large volumes of materials can be diverted from the nation’s waste stream.

Yet another part of Wen’s research involves the temperature to which asphalt must be heated to be used for paving roads. Traditionally the material has been heated to 300 degrees. That is very hot, and accounts for the blue smoke you can see wafting up from paving operations along a road in the summertime.

“That means a lot of energy is required for major paving operations. And the smoke is not good for the environment or the workers,” Wen told me.

Using different mixtures, Wen’s group is researching materials that need only be heated to 200 or 220 degrees. That is a significantly lower temperature that allows for real energy savings. As an added bonus, the blue smoke is not produced at those lower temperatures.

Wen’s research is funded both by government agencies and industry. And although he is glad to be working in this country, he still goes back to China to collaborate with engineers there.

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