Pavement: the ins and outs of what's under your feet

U.C. Pavement Research Center Director Carl Monismith and Assistant Director Jim Signore. Photo by Casey Miner.

If you’re driving right now, or riding a bus, trolley, taxicab or your bike after a long day at work, your mind might be on the traffic, or what to cook for dinner, or – why won’t that guy turn off his blinker already? But have you ever thought about what’s underneath your wheels? The actual road?

It’s the kind of thing we only really notice when it’s not working – ahem, potholes – but pavement is everywhere: the U.S. has four million miles of paved roads; close to 10% of them are in California. So, yes, it’s something that affects each and every one of our lives.

We sent transportation reporter Casey Miner to the U.C. Pavement Research Center to find out just what goes into making what’s beneath our feet.

*     *     *

CASEY MINER: The guys at the pavement research center are basically the brains of California’s roads. There’s Carl Monismith…

CARL MONISMITH: Director of the Pavement Research Center at U.C. Berkeley, retired professor of civil engineering.

He’s like the frontal lobe. The memory. And there’s Jim Signore…

JIM SIGNORE: Assistant director of the Pavement Research Center, been here about six years.

He’s like the parietal lobe – controls movement. Signore runs a lot of the day-to-day lab and fieldwork; Monismith teaches and runs the whole center. Signore calls him “the Professor.”

MINER: What are we walking on?

SIGNORE: This is very aged asphalt. You can see how aged it is because it’s very gray, very toothy and bumpy. Ages and oxidizes, turns gray, fine materials start to wear away. End up with rock surface. See how brittle it’s gotten, how cracked it’s become?

MONISMITH: That’s been here since the 1950s.

SIGNORE: You remember when they paved it?



Monismith is 84 years old; he’s been at the Pavement Research Center since 1953. For a little perspective, that’s before we even started building the interstate highway system, more than 46,000 miles of pavement. These days, the Pavement Research Center works with Caltrans to study, design and improve the state’s roads. Signore says it’s complex work.

SIGNORE: Pavements are complicated, they’re not the easiest thing in the world to build, believe it or not. It may look simple, but really is an engineered structure.

But Monismith says ignoring pavement is a mistake.

MONISMITH: We spend about $100 billion a year on pavements in the U.S. $100 billion. You have to have people in the field that are going to build these pavements properly, because if they’re not built properly they’re not going to last.

So what does it take to build a great pavement? Well, that’s what I’m here at the Pavement Research Center to find out. I solicited your most burning questions and brought them with me to ask the Professor and Mr. Signore. Let’s get started:

QUESTION 1: What’s pavement made of, anyway?

For the answer to this one, we’re going to have to really get into it:

SIGNORE: That’s actually fairly quiet. Normally it’s much louder.

What you’re hearing is a giant saw slicing up piece of pavement – well, technically it’s called asphalt concrete. It’s a mix of rocks, mostly from different places in California, and asphalt, which is an oil byproduct that glues the rocks together. Signore is standing at a research table where long vertical slabs are laid out.

SIGNORE: Those are pavement cores either drilled out of the pavement or we made them in the lab. We have thousands. We test them all, may look the same but have subtle differences.

Pavement is actually very layered – you don’t see it, but there’s big chunks of rock in there. You can see the top layer, which looks like any road, then all the layers underneath, which are filled with different kinds of rocks.

SIGNORE: That’s pure granite. Almost seems wasteful doesn’t it? You could make a lot of nice countertops out of this.

Alright. Next question.

QUESTION 2: How do you know if you made a good batch of pavement?

MONISMITH: When you mix asphalt and aggregate together and come out of mixer, it looks like maggots in the garbage can. It’s an indication that the mix is pretty good.

Gross. Okay. Next question.

QUESTION 3: So why does my road have so many potholes?

SIGNORE: Garbage trucks and buses do all the damage. Cars do next to nothing.

MINER: So does one pothole spawn another?

SIGNORE: They sure do. There’s more places, more space for water to enter. Consequently larger areas of pavement get wet, get softer, and as the pavement cracks they want to spread. So sure. In a nutshell.

Alright, and speaking of road repairs:

QUESTION 4: Why doesn’t my city fix all those potholes? 

SIGNORE: If you try to fix the worst pavements first, you won’t have enough money. The trick is to try and maintain your network at a high level. Once a pavement at the end of its life, letting it go a little longer isn’t going to make a difference. Try and improve while you can.

Right. Okay. Here’s a regional question:

QUESTION 4: We have earthquakes in California. Does that mean you use different kinds of pavement?



QUESTION 5: Do you ever use different kinds of pavement in different places? 

SIGNORE: Here we go, put your finger in that. It won’t stick to you, feel how soft that is?

“That” is a metal can filled with a material that looks kind of like rubber cement.

SIGNORE: This is a material they’d use up in say Lake Tahoe, or Northern California.

MINER (on tape) That’s so easy, it feels like I could stick my thumb right through here.

SIGNORE: You need something soft in cold temperatures so it doesn’t crack and become brittle. There’s a balance you’re trying to achieve between stiffness, so it doesn’t rut, and softness so it doesn’t crack. It’s quite the science.

Alright. So that answers that. My time at the Pavement Research Center was over, and Mr. Signore and the Professor sent me home with two souvenirs: A moisture-resistant chunk of road, and an asphalt business-card holder. That thing’s indestructible.

In Richmond, I’m Casey Miner for Crosscurrents.

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This story originally aired on December 7, 2010.