Transportation Defined: Pavement Design Life

After 300-plus blog posts, you’d think that by now we’d have covered everything there possibly is to know about pavement...

But you’d be wrong.

Certainly, it’s not from a lack of trying – we’ve blogged about quiet pavement, pavement at airports, pavement materials testing, pavement markers and even the impact weather and climate can have on pavement. Oh, and remember when we explained how our crews are able to make sure pavement is as smooth as it can be (bonus points if you can recall how a profilograph works)?

It’s just that there’s so much to know about pavement (we’re not kidding – there really is) that we haven’t even scratched the surface.

Which brings us to today’s topic: pavement design life.

Pavement design life is a term that engineers use when they’re planning to build a new road or maintain an existing roadway. They’ll also use a number of years to go along with it, for example: 10-year pavement design life, 20-year pavement design life, etc.

The phrase should not be taken to imply that a road is only being built to survive for a set number of years. What it does represent is the road’s age at which some preventative maintenance or reconstruction will be considered so the road can continue to be durable and useful for the traffic it’s serving.

“For a typical highway, we generally will design an asphalt pavement for 20 years,” said ADOT Pavement Design Group Manager Paul Burch. “It does not mean that the road’s going to be falling apart and rubble in 20 years.”

A little more on pavement design…

Burch says that when engineers want to build a road, they take a lot into consideration, including soil condition, location, expected traffic levels and the area’s climate. Once those factors are determined – and the engineers know the pavement design life they want to build for – design begins.

All those conditions play a role in how the pavement is designed. Say, for example, the road’s being built in an area that gets very cold weather. If that’s the case, engineers will adjust the asphalt pavement mix to account for the temperature extremes.

Now, what if there’s a road that was constructed decades ago that’s not serving the traffic levels very well … can it be built up to a 20-year pavement design life?

Of course it can!

“What we would do if we were to inherit a road like that is we’d start by getting soil samples to test so we can determine what the strength of the soil is. Or, we could do Falling Weight Deflectometer testing so we could determine the strength of the existing roadway and the underlying soils through a method called 'Back Calculation.' From there, we would estimate what our traffic level would be and what design life we would want to design for,” said Burch, adding that the next step would be to create a design that takes all those important factors into account.

For more on pavement, check out our previous blog posts. And, stay tuned – we promise there are more pavement posts to come!

Transportation Defined is a series of explanatory blog posts designed to define the things you see on your everyday commute. Let us know if there's something you'd like to see explained ... leave a comment here on the blog or over on our Facebook page!