Everyone is interested in extending the lifetime of highway pavements. What are the advantages of asphalt in this regard?
Well-designed and well-built asphalt pavements last many years. There are a number of case studies which support this conclusion. For instance, the asphalt portions of Interstate 90 in Washington State have been in place since their original construction up to 35 years ago with no rehabilitation for structural reasons. These pavements have only required maintenance and periodic replacement of their surface layer. The New Jersey DOT found the same to be true on I-287 on a 26-year-old 10-inch asphalt pavement; the original structure has remained intact and only a surface profiling followed by an overlay was necessary to restore the pavement. The entire New Jersey Turnpike is asphalt. It was built in 1951. They have never had a structural failure in the pavement. The only maintenance they've done is surface treatments and overlays. In a recent interview, the chief engineer for the Turnpike said that expected the pavement to last another 50 years. It was very well-designed and well-built. The designers put a lot of thought and care into the pavement structure and how they built it, they used top-quality materials, and they got a pavement which has held up extremely well.
The National Center for Asphalt Technology in Auburn, Alabama, is currently conducting experiments on its test track directed toward improving asphalt pavement performance. They are putting 10 million ESALS (Equivalent Single Axle Loads) on the pavement in two years. They are now about halfway through the tests, and they have found very little distress in the pavement. They are using a wide range of asphalt mixes, including conventional, Superpave, Stone Matrix Asphalt, and Open Graded Friction Courses. The tests show that asphalt mixes which are well-designed and well-built will give very good service. They will last a long, long time.
Is that happening on a very wide scale?
Absolutely. Arkansas, West Virginia, Michigan, Wisconsin, and North Carolina, as well as many other states, also use rubblization extensively.
Can you design an asphalt highway in different ways?
Pavement design is continually evolving. Empirical processes were developed 40 to 60 years ago. These involved observations of how pavements interacted with soils, climates and levels of traffic, and then basing the pavement thickness on these observations. While empirical methods are still widely used, the industry is moving toward mechanistic-empirical procedures. In these newer design methods, the pavement is treated like a structure such as a building or bridge. The reactions of the pavement to different traffic loads are modeled mathematically, giving designers more information concerning options on materials and design factors. This could result in more efficient utilization of materials to obtain long-lasting pavements.
How widely is asphalt used?
About 94 percent of the nation's roads and highways are surfaced with asphalt.
As far as the future is concerned, the Federal Highway Administration is paying a lot of attention to extending the lifetime of pavements. Is extending lifetimes a general trend and focus?
Absolutely. Certainly in the industry we've made a huge investment into improving the product, improving the pavement, and getting more life out of the pavement. People don't want to build a pavement and then have to go back and completely replace it in 30 years.
Today we have a lot of interstates and other heavily traveled pavements that are in need of rehabilitation. Carrying traffic the way they do, they are very necessary, part of our daily lives, and the challenge is how to keep traffic on the interstates when the pavement needs extensive rehabilitation.
Often times, on thick asphalt pavements, only the top few inches need to be replaced after 10 or 15 years of service. Milling this material off and replacing it with a new wearing surface can be done with a minimum of traffic disruption.
An alternative to the reconstruction of failed concrete pavement is to rubblize it. There are two different kinds of machine which can go in and do rubblization: a resonant breaker and a multiple head breaker. The machine breaks the existing pavement into fragments, with smaller pieces on the top and larger pieces toward the bottom. Then they use a compactor to seat the particles. Then it is overlaid with asphalt. With rubblization, an agency can rehabilitate a pavement more quickly than to go in, dig up all the old concrete, haul it off to the landfill and build a new pavement. People just won't stand for that.
Does concrete last longer than asphalt?
The New Jersey Turnpike provides a good example of the life cycle of an asphalt pavement. That pavement has been in use for 60 years under very heavy traffic. That's what we call the Perpetual Pavement. Asphalt pavements can last a lifetime because it's possible to maintain them just with milling and overlays, and the deeper portion of the pavement structure remains sound. If you design a pavement correctly for the amount of traffic it will have to stand up to, you can have actually a permanent pavement structure. With our new heavy-duty surface pavements, it is possible for overlays to last more than 20 years. It just makes sense to design a pavement so that it will serve you long-term, not so that it has to be replaced at a given point in time.
Is asphalt generally considered to be less expensive than concrete?
There have been numerous studies within the U.S. and Europe which have shown that asphalt pavements generally have a lower cost over their life cycle.
Are concrete pavements more expensive to build than asphalt?
The initial cost of asphalt pavement construction is usually less than concrete. But, in addition to construction cost, an increasingly important factor is the traffic delay cost incurred by the public during construction or rehabilitation. You can't close down a busy road and spend weeks repairing it without costing businesses and individuals potentially millions of dollars. With asphalt, you can usually perform construction and rehabilitation operations at night. Some of our contractor members have done jobs where the public never even sees an orange barrel unless they're there at night when the paving is going on. The contractors will go out and set up their traffic control and do the milling and the overlay, and everything is gone by 5 a.m. The average commuter goes to work the next day, drives on a whole new pavement, and has never even seen an orange barrel.
How about recycling asphalt?
Asphalt pavement is the most recycled material in America. We recycle more than 70 million tons of asphalt pavements every year, more than the combined total of glass, paper, plastic, and aluminum combined.
What is the basic technology of asphalt pavement?
Asphalt pavement is usually approximately 95 percent aggregate, which could be stone, sand, or gravel, and 5 percent asphalt cement as a binder. The binder is a product of oil refining and acts to glue the aggregates together.
Is that heated?
The aggregate and asphalt are heated, combined with the recycled material and mixed together. Then we load the hot pavement material into trucks and take it out to the site.
Do states add crumb rubber to the binder?
The states that use asphalt rubber most extensively are Florida, Arizona, Texas, and California.
How thick do you lay the asphalt?
All that is engineered. You have to look at what kind of stresses you put on the pavement, trucks vs. cars, and other factors such as soil conditions and climate. It also depends on what materials you intend to use in the asphalt and what materials might be present in the lower layers of the pavement. All these considerations are combined to obtain the pavement thickness.
The types of materials used in the HMA are also chosen for specific applications. For example, an intersection may require a different mix from a city street. An intersection requires more strength because cars and trucks are going to be standing on it, as well as starting, stopping, and turning. This puts a great deal of stress on the pavement surface. You have to use a stiffer mix. Asphalt pavement is very much an engineered product.
Can you design an asphalt pavement that is safer during rainstorms?
One of the great things about asphalt pavement surfaces is that you can engineer them for different types of conditions. If you are in an area which gets a lot of rain, you can use an open-graded pavement. That will actually let the water drain out through the pavement structure. It can be called a porous pavement. Because the water can drain off the surface, the risk of hydroplaning is reduced which enhances safety. Another important safety feature of an open-graded asphalt surface is that it reduces the splash and spray that you would see with other types of pavement. These pavements are also frequently used to reduce road noise in urban areas. This is an advantage of asphalt pavements.
Could you name some other types besides open-graded?
One of the mixes that is getting a lot of attention is stone-matrix asphalt (SMA). It's a very tough and durable mix. In addition to providing a long-lasting pavement surface, it is used to reduce pavement noise.
Are asphalt pavements quieter than other types of pavement?
Yes, there is considerable research that shows that asphalt pavements tend to be quieter than concrete pavements on the whole.
How have asphalt plants changed over time?
Our industry has come a long way. For example, in the early 20th century, there were asphalt plants on railroad cars. They would take them from town to town, set up in a little town, pave all the roads, and go on to the next town. Today, many modern asphalt plants are a permanent part of communities all over the country. The plants need to be near where roads are built, because the paving material has to be delivered to the paving site while it is still hot. Portable asphalt plants are normally used in more remote areas, and they can temporarilybe set up near a paving site. Whether asphalt plants are on permanent or temporary sites, they are environmentally friendly, and are good neighbors within their communities.
What equipment is used at a typical asphalt paving site?
A milling machine is typically used to remove the surface material from an existing roadway. That material is loaded into a truck and carried back to the plant for recycling. A brooming machine then comes to clean the surface, followed by a distributor truck which puts down the tack coat which helps glue the new pavement to the existing surface. A truck carrying HMA paving material from the plant backs up to the paver and dumps the material into the hopper or material transfer device, or places the material in windrow so that it may be picked up and put into a paver. The paver lays a smooth mat. Then a series of compactors comes after the paver to densify the material. These compactors may include vibratory or static steel wheel rollers or rubber tire rollers.
The material transfer vehicle is gaining popularity with contractors and agencies. Basically it keeps the paver moving at a constant speed to improve smoothness and remixes the asphalt mix to keep the aggregate from segregating and to help insure a uniform temperature.
In conclusion, how would you describe the future of asphalt for highway construction?
Through programs of research and engineering, we have created solutions that meet every pavement need. The market is larger than ever before. In short, the asphalt pavement industry is on a roll.