Pavement Science

Bill Lindeke, writing for MinnPost:

Barr Engineering

Barr Engineering

“There are two types of pavement, concrete and asphalt, and these industries compete with each other,” explained Lev Khazanovich, an engineering professor at the University of Minnesota. “Each is always trying to improve their product, and so asphalt and concrete in Minnesota are much better than what was built 30, 40 years ago.”

The difference between the two pavements comes down to cost and lifespan. Concrete is harder and more durable, but asphalt is cheaper. In fact, considered philosophically, these two technologies might even be diametrical opposites.

“Fundamentally, asphalt is a mechanical compaction of the paving products,” Steve Lund explained to me. “It’s a physical compaction and pressing, whereas on the concrete side it’s more of a chemical reaction and hardening of the paving product.”

Concrete is composed of water, aggregate and cement (with occasional other kinds of material, such as coal ash from coal power plant filters). The chemical reaction between cement and water causes it to harden. It can thus be molded, like plastic, into a seemingly infinite variety of shapes. Asphalt, on the other hand, is a form of petroleum mixed with aggregate (mostly sand and gravel), heated up and then pressed down to form our roads, bike paths and parking lots...

“There are several different types of pervious concrete,” explained Nathan Campeau, another engineer at Barr. “It has been around for quite a while now, 10 to 12 years, and installed in several locations around the state. And there’s a corollary in porous asphalt, special mixes that have a lot of void spaces in them. They’re basically like a sponge, and allow water to go from the surface down to the sub-grade below the pavement.”

While the benefits of new pervious pavements are clear, they also present technological challenges. First, it can be tricky to install them, and they require specific equipment and time-consuming construction processes. Many contractors struggle with pervious technologies when using them for the first time.

Second, cities like Shoreview have to carefully maintain their pervious pavements, so that sediment and grit does not clog up its “void spaces.” To do this, cities and property owners have to actually vacuum the sediment out of their pervious roads and parking lots.

Use pervious surfaces, we must. They are our city rainwater management  hope.