Going Beyond the Surface of Asphalt Pavement
Amy J. Born | October 21, 2017Asphalt has been in use since ancient times. The La Brea Tar Pits are actually naturally occurring asphalt in a semi-solid state. During President Ulysses S. Grant's administration, Pennsylvania Avenue was paved with asphalt from Pitch Lake in Trinidad, the world's largest deposit of natural asphalt.
Refined asphalt (the product used today) came along as the oil industry developed. Asphalt pavement starts with aggregates, typically stone, sand or gravel, that make up 95 percent by weight of the mix. Next, a binder, called asphalt cement, is mixed with the aggregate, and makes up the other 5 percent by weight. Asphalt cement is engineered to meet the performance characteristics encountered during use, such as high and low temperature ranges and traffic loading. The cement may be polymer-modified to be, for example, more rut resistant.
"Generally asphalt pavement is a fairly local product that is made from the aggregates available in the area," says T. Carter Ross, spokesperson for the National Asphalt Pavement Association. "In some places limestone, for example, may be the most commonly quarried aggregate, while in other places trap rock or granite may be more common. The mix design — which materials and at what proportion — also varies by climate, traffic loading and expected use. You wouldn't find the same mix on a highway, as on a city street or a bike path."
Different Asphalt Pavement Mixtures Meet Different Needs
Stone matrix asphalt (SMA) is a specific type of asphalt mixture that gains durability from stone-on-stone contact. SMA is typically used in high-traffic applications.
Open graded friction courses have a large percentage of air voids that allow water to seep through the surface layer of the pavement. This prevents puddling — a cause of hydroplaning, road spray and glare, as well as refreezing in winter.
Full-depth porous asphalt allows water to seep all the way through the pavement and into the ground below. Once used mainly in parking lots, it is gaining wider appeal as a low-impact development option in communities where runoff from roadways is a major issue.
When water is able to seep back into the ground rather than flow directly into waterways, naturally occurring microbes break down the contaminants that pose a threat to the environment and people's health before they leach into the water supply. Provincetown, Massachusetts, on the tip of Cape Cod, solved the problem of frequent beach closures due to high levels of bacteria by paving its main beachfront street with porous asphalt.
Ocean City, New Jersey, began replacing roadways with porous pavement after Hurricane Sandy to reduce the amount of stormwater that collects on them. Porous asphalt is also useful in preventing flash floods.
Asphalt Pavement Innovations for Performance and Sustainability
"The industry is always looking at innovations to improve driving performance and performance for road owners (departments of transportation, public works and toll authorities)," says Ross. One of the biggest changes has been the introduction of warm-mix asphalt.
Warm-mix asphalt is a combination of technologies that significantly reduces the temperature of asphalt pavement material during production, transportation, mixing and compacting, and application. Hot mix asphalt requires temperatures above 300 degrees Fahrenheit for production, while warm mix has been shown to lower that temperature as much as 30-120 degrees Fahrenheit. Warm-mix asphalt provides several important engineering, construction and environmental benefits.
In cold weather, warm mix cools more slowly, giving workers flexibility and time to work with the asphalt at lower temperatures. Paving projects can, therefore, take place during the cooler months of the year. Warm- mix asphalt has a lower viscosity that makes manipulating and compacting the mix easier and helps achieve a more uniform density in pavements. Because warm mix performs better at cooler temperatures, asphalt paving mixtures can be hauled further distances than hot mix.
The lower temperatures benefit both the environment and the bottom line. Less energy is needed from burning fuels to heat the aggregate and asphalt cement mixture to production temperatures, which translates to 30-35 percent savings in fuel consumption. Emissions from fuels, fumes and odors at the plant and paving site are also reduced.
In general, the energy required for the production of asphalt pavement creates a smaller amount of greenhouse gases than for the production of alternative paving materials. Asphalt pavement is 100 percent recyclable: milled up pavement is transformed into new pavement, and other recycled materials, such as roofing shingles, slags and ground tire rubber, find their way into the mix.
Because asphalt pavement is smooth and flexible, tires encounter less resistance than on other types of pavement. This improves fuel economy and reduces road noise.
Learn more about asphalt (yes, there is more to know than you might have imagined) at asphaltpavement.org.