How Thick Should Your Pavement Be?
BY AsphaltPro Staff
Asphalt professionals know the best practices to follow to avoid common asphalt distresses. We avoid stripping by washing our aggregate of dust and clay so the asphalt cement (AC) can bond to the aggregate. We apply a tack coat between pavement layers to avoid slippage cracking. We address drainage issues to mitigate future potholes. We achieve the correct density to avoid raveling and minimize cracking.
Asphalt professionals also know that asphalt is designed to be flexible. Normally, this flexibility is a benefit, allowing the asphalt to flex under heavy loads and repair itself in between these loads. But when a pavement isn’t properly designed to hold up to the traffic load upon it (or if that traffic load increases more quickly than the owner or agency can keep up), this flexing can result in the development of microscopic cracks.
“As the pavement ages and oxidizes, or if there’s not enough time between each load, those microcracks can’t heal themselves,” said Wayne Jones, senior regional engineer with the Asphalt Institute, during his presentation “Introduction to Thickness Design” at the 2024 National Pavement Expo in Tampa. “Eventually, they’ll accumulate and lead to fatigue cracking. And that fatigue cracking will work its way up through the pavement and you’ll see cracks in the wheel path.”
We can follow all the best practices of paving and maintain and rehabilitate the road perfectly, but time will pass and asphalt will age. The pavement will likely experience more traffic over time than it was initially designed for. These things are not in our control. So, how can we avoid fatigue cracking from the outset?
Jones suggested considering what kind of maintenance and rehabilitation the owner or agency has planned for the pavement during the design phase. “If you don’t do anything after the first day you open a new pavement to traffic, that’s the best condition it will ever be in,” Jones said. “If you can convince the owner to spend his maintenance dollars earlier on with seal coats and chip seals, then overlays, he’s going to get more bang for his buck. If he waits until the pavement is so distressed that it needs to be rehabilitated, it’s going to cost major dollars.”
From First Principles to Forever Pavements
A pavement consists of several layers. The base, sub-base, intermediate layer and wearing course each play a role in structure and drainage—both key components to a pavement’s longevity. “All those layers together are there to ultimately hold up to the future traffic loading on that pavement,” Jones said.
The pavement surface serves four functions: It must have the structural strength to hold up to traffic. It must provide friction resistance for braking and steering for safe operation of vehicles. It must provide a smooth ride. And it must be a moisture barrier to prevent water from reaching the rest of the pavement layers.
When determining how thick each layer should be, the entire system must be taken into account. For example, will the pavement design be full-depth asphalt, or asphalt plus aggregate, Jones said. Thickness design must also factor in the subgrade, material selection, maintenance and rehabilitation, the environment/weather conditions, and traffic. “We need a thickness design program that addresses all these factors when we’re trying to design the thickness,” Jones said.
Perpetual Pavement Performs for “Peanut Capital of the World”
His preferred tool to account for these factors in pavement design is Asphalt Pavement Alliance’s tool called PAVEXpress. PAVEXpress is a free web-based pavement design tool to quickly determine the necessary pavement thickness for a given section of roadway or project. “You can create an account and store all your design data in there, and use past designs for future projects,” Jones said. After you input your data, it provides the recommended cross section.
Although the tool makes it easy to establish the correct minimum thickness of the cross section, PAVEXpress can also be used to determine how thick a pavement needs to be to be considered a perpetual pavement.
“About 20 years ago, we noticed that some interstates with really thick asphalt never experienced fatigue cracking,” Jones said. “What was happening there?”
Researchers discovered, as traffic increased, more and more layers of asphalt had been applied to the pavements until they were so thick that the tensile strength of the traffic never exceeded the internal strength of the asphalt.
“You don’t get that bottom up cracking,” Jones said. “If I never get those cracks from the bottom up, all I have to deal with is oxidation and maybe some block cracking—neither of which affect the entire body of the pavement—so I can get by with much less maintenance and repair. That’s what we call a perpetual pavement.”
He estimates this can be accomplished by building an asphalt pavement that’s 20 to 25% thicker than what the typical design would be. PAVEXpress can give you this number.
“You can print out that recommendation, supported by all the research and science behind PAVEXpress, with your company name and logo on it and present that to your client,” Jones said. “You can upsell your services by maybe an inch or two [of asphalt] and give your customers a much longer lasting pavement than the typical design their architect might use. You can deliver them a pavement that acts like a perpetual pavement.”