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Frequently Asked Questions
- What is Hot Mix Asphalt (HMA)?
- How is HMA made?
- Is asphalt an environmentally sound product?
- Everyone is interested in extending the lifetime of highway pavements. What are the advantages of asphalt in this regard?
- Can you design an asphalt highway in different ways?
- How widely is asphalt used?
- 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?
- Does concrete last longer than asphalt?
- Is asphalt generally considered to be less expensive than concrete?
- Are concrete pavements more expensive to build than asphalt?
- How about recycling asphalt?
- How thick do you place the asphalt?
- Can you design an asphalt pavement that is safer during rainstorms?
- Are asphalt pavements quieter than other types of pavement?
- What are the benefits of HMA pavements?
Q: What is Hot Mix Asphalt (HMA)?
A: Hot Mix Asphalt (HMA) is a combination of approximately 95% stone, sand, or gravel bound together by asphalt binder, a product of crude oil. « Back to top
A: HMA pavement refers to any paved road surfaced with asphalt. The asphalt binder is heated, combined and mixed with the aggregate at a HMA facility. HMA is loaded into trucks for transport to the paving site. The trucks dump the HMA into hoppers located at the front of paving machines. The asphalt is placed, then compacted using a heavy roller which is driven over the asphalt. Because the temperature of the HMA drops rapidly after spreading and because compaction of the HMA to achieve a specified density must take place at temperatures above 175 degrees Fahrenheit, compaction usually takes place within a matter of minutes after the paver spreads the HMA. Traffic is generally permitted on the pavement immediately following compaction. « Back to top
Q: Is asphalt an environmentally sound product?
A: Yes, asphalt is basically an inert material which is not soluble or harmful in a water environment.
Environmental Liners
Asphalt paving materials have been used to line the surfaces of everything from fish hatcheries to industrial retention ponds. In the case of fish hatcheries, an inert material was needed that would not chemically react with the water. This allows fish and game experts to precisely monitor and control the environment during the delicate stages of incubation and early growth for various fish species.
For example, Oregon and Washington state fish and wildlife agencies began using HMA to line their fish rearing ponds in 1987. Typically 1/2-acre in area, the ponds are home to chinook salmon and other fry for about 18 months before the fish are released into rivers and streams. Both the Washington and Oregon state agencies are pleased with the effectiveness of the HMA liners and plan to use them for additional fish hatcheries in the future.
For decades asphalt paving materials have been used as liners for water reservoirs.
For example, The Metropolitan Water District of Southern California (MWDSC) has been using HMA lined water reservoirs for more than four decades. Currently, the California Water Resources Board is completing construction on the Devil’s Canyon Reservoir to store MWDSC drinking water. The 19-inch thick asphalt liner for the huge facility will hold 800-acre-feet of water.
« Back to topQ: Everyone is interested in extending the lifetime of highway pavements. What are the advantages of asphalt in this regard?
A: 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 1-287 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 he 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.
An example of this strategy can be found in California on a portion of I-710 between Pacific Coast Highway and I-405. Known in California as “Longer Life Asphalt”, this strategy was researched by the University of California Berkeley together with Caltrans and the HMA industry. This pavement was designed for 200 million ESAL’s and is expected to perform for over 35 years. Additional projects using this technology are currently planned for the state.
« Back to topQ: Can you design an asphalt highway in different ways?
A: 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.
A: About 94 percent of the nation’s roads and highways are surfaced with asphalt.
« Back to topQ: 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?
A: 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 crack and seat it or rubblize it. California has had a very successful crack, seat & overlay program for many years. The crack & seat strategy takes advantage of the existing structural capacity of the broken PCC pavement; then creates a superior pavement with a combination of leveling courses and overlay courses of HMA to increase the structural capacity of the roadway and improve the smoothness at the same time. Rubblization is another alternative that uses the existing pavement as a high quality base material under the new HMA pavement. There are two different kinds of machine which can go in and do the 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. It is then overlaid with asphalt. With both the crack & seat and rubblization strategy, 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.
A: The New Jersey Turnpike provides a good example of the life cycle of an asphalt pavement. That pavement has been in use for 50 years under very heavy traffic, and the chief engineer says he thinks it can last another 50 years. 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 15 to 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.
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Q: Is asphalt generally considered to be less expensive than concrete?
A: There have been numerous studies within the U.S. and Europe which have shown that asphalt pavements generally have a lower life cycle cost.
« Back to topQ: Are concrete pavements more expensive to build than asphalt?
A: 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 or within a very narrow construction window. Some of our contractor members have done jobs where the public never even sees an lane closure or orange cone 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 traffic cone.
A: 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.
A: 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.
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Q: Can you design an asphalt pavement that is safer during rainstorms?
A: 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.
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Q: Are asphalt pavements quieter than other types of pavement?
A: Yes, there is considerable research that shows that asphalt pavements tend to be quieter than concrete pavements on the whole.
A: User Benefits
HMA is really the only paving material for our roads that really makes sense. It is the best buy for taxpayers. It is safe, economical, and it is the most long-lasting and durable paving material. In addition, asphalt pavement is 100% recyclable (ie., old HMA pavements can be remixed with a portion of fresh materials, and used again and again).
HMA also makes sense because it is the paving material of choice on roads in this country. Of the 2.27 million miles of paved road in the US, 94% is surfaced with asphalt, including 65% of the interstate system.
HMA pavements provide the user with a smooth, quiet, skid-resistant ride surface. A smooth pavement maximizes tire contact, providing more traction, and saves wear and tear on vehicles.
Economic/Engineering Benefits
HMA pavements can be built very quickly thereby reducing costs due to traffic delays, which save the traveling public time and money.
Asphalt pavements save taxpayers dollars. In a 3-year analysis conducted by the Florida Department of Transportation, initial construction of an asphalt pavement cost $544,981 to concrete’s $765,729. After wear and tear and maintenance, the residual value of the asphalt pavement was 203% more than the concrete road.
While each road needs to be analyzed based on its own merits, for maintaining roads, the Florida Department of Transportation calculated their routine annual maintenance costs as $132 per lane mile for asphalt pavement and $261 per lane mile for concrete pavement.
HMA gives the best return on investment of any paving material. HMA pavements can be cold milled and recycled which saves costly readjustment of manholes, curbs and sewer drains when rehabilitating urban streets.
Simply put, HMA pavements are the most cost effective pavements in the World, PERIOD!
ENVIRONMENTAL BENEFITS
HMA pavements are 100 percent recyclable and are America’s most recycled product. Approximately 80 million tons of HMA are recycled each year. Compare this to other recycled
products below. HMA pavements are truly environmentally friendly.
HMA pavement recycling is more cost effective than using only virgin or new materials. The quality of a recycled asphalt pavement is as good, or even better, than brand new pavement.
HMA pavement recycling not only conserves our precious natural resources and speeds construction, it also saves American taxpayers over $300 million a year.
HMA pavements which are not recycled must be disposed of, usually in a landfill. Transporting old asphalt pavements to landfills does not make best use of a product that can be recycled. HMA pavements can be recycled numerous times.
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Wednesday, May 14, 2008
