Construction Equipment Guide recently asked two experts--one from the National Asphalt Association and one from the Portland Cement Association--what are the pros and cons today when using either concrete or asphalt. Here is what each had to say:
Terry Collins, concrete construction engineer with the Portland Cement Association in Skokie, IL, provides new insights on concrete in the following interview.
Is there a general trend toward longer-lifetime highway materials and longer-lifetime concrete?
There is always a push for identifying combinations of materials and methods that will extend the life of paving materials. That’s always going to be an on-going process.
What’s happening in the world of concrete? How about ultra-thin concrete?
Well, ultra-thin whitetopping is certainly an interesting newer concept in the battle for durable pavements. It’s typically an application of a thin concrete overlay over an existing asphalt pavement. It was identified many years ago that asphalt pavements at or near intersections tend to rut more severely and be more susceptible to curling damage, when large vehicles break etc. on that surface. They have found that by adding a 2-in. overlay of concrete to that surface, they can minimize or eliminate that kind of curling and rutting problem at intersections. This is certainly being done a lot more frequently for repairs. It’s also an interesting application because quite frequently it greatly reduces the preparation process. You don’t have to take all the existing material out and start replacing. You can treat the surface to that overlay and you’re ready to put it back to its service life.
Is ultra-thin a new material?
It’s actually a more traditional concrete material than one would suspect. It’s just a new application of that same material. They are obviously always trying to optimize every concrete mixture to perform better, given the nature of concrete materials, where large weights are involved.
To make it more economical, you are always using fairly closely available local materials. There’s always an optimization process going on with those local materials, to obtain the best combination for performance.
How long does a typical concrete highway last?
That’s a difficult question. What’s typical? Defining that word by itself is nearly impossible. Every highway is an individual case with different traffic loading.What would be typical of Chicago certainly wouldn’t be typical of rural South Dakota. There’s no way to compare those.
Would you say concrete pavement can last 20, 30 or 40 years given that it has been correctly maintained?
I think a service life from 25 to 40 years with reasonable maintenance is not an out-of-bounds expectation, depending of course again on the actual service conditions. If you have a million vehicles a day on that pavement, there’s certainly no surface that’s going to guarantee that kind of service life.
Looking down the road, might concrete last 50 years or longer?
There are certainly projects in the country designed with the idea of creating 50, 75 or 100-year service lives. I guess only time will tell whether or not those predictions are valid. I would hope that would be possible.
When a concrete pavement breaks down, don’t you have to rubbilize it, break it up?
There are preservation methods. At some point in the life of the concrete, there will be some maintenance required to return it to near its original condition. If you let concrete pavement go too long and it has deteriorated to such a state that maintenance methods are going to be ineffective, I guess you would have to remove and replace, but, if proper maintenance is done, you can bring that concrete pavement typically back up to a high level percentage of quality compared with when it was new and then you have breathed additional life into that pavement.
How long will concrete last compared with asphalt?
There is a concrete pavement that was placed in, I believe, 1921 that’s still in service. Given specific cases, you can point towards specific pavements that have had great durability. I’m pretty confident that a concrete pavement will always outlast an asphalt pavement when you compare them under equal service conditions.
Are there certain areas where concrete is better and certain areas where asphalt is better?
I would think that for high-traffic public access, concrete is probably a better choice because of that high level of use. It’s not going to break down as soon as an asphalt pavement and it’s not going to require the same level of maintenance. Areas with significant freeze-thaw conditions are also good candidates for concrete pavements because these rigid pavements will withstand the movement of the soils beneath them more readily. As soils freeze and expand, you have a basically frost-heave situation in the soil.
A rigid pavement from my perspective will give a better service life than a flexible pavement like asphalt in those kinds of conditions.
Isn’t it true that approximately 94 percent of U.S. highways are asphalt?
It’s very possible. Asphalt is the cheapest building method as far as initial cost. People who have to justify what they’ve spent on a roadway are in the position of building with the cheapest method, or justifying the life-cycle costs, or proving that using the more expensive method will be less expensive over the extended life of that pavement. When you look at the life-cycle costs, the concrete pavement is likely less expensive throughout its life. It lasts longer and it requires less maintenance.
How do you produce the concrete? Is it right there at the site?
That varies pretty widely. If you are close to a city with an established ready-mixed concrete producer, that might be the most viable way of producing that concrete.
If you are at a remote site, say 25 to 100 miles from the nearest local city, at that point bringing in materials and setting up a portable batch plant may be the most economical way to approach that type of project.
It really varies quite a bit depending on what resources are available to you and where the project is taking place.
How much do concrete batch plants cost?
That can vary quite widely, but the modern large-size batch plants that are required for large paving jobs could cost several million dollars.
What equipment do you use in with the plant?
If you’re setting up a portable plant, why you’re going to have the plant itself, a fleet of trucks. These may be a couple of different types. You may have end dumps to haul the concrete from the batch plant to the placement site. You may have belly dump trucks and pneumatic trucks to haul aggregates and cement to the plant site. You will also have grading and compaction equipment. On a large paving project, you will in all likelihood have a slip form paver. Probably also a wheeled bridge unit for finishing and curing and, following that operation, you are going to need saws to place contraction joints and sealing equipment to seal.
How about repairing older pavements?
The first step is generally to do spall repairs and then diamond grind the surface to provide a nice clean riding plane. You don’t put a new layer on it at all. With spall repair, you would cut around a joint where it has deteriorated and remove the concrete to a certain depth and then go ahead and replace the concrete in that area. After the spall repairs are in place you may or may not grind the surface to improve the riding plane.
In conclusion, how would you describe the future of concrete for highway construction?
Concrete has been and will continue to be a dependable material to be used for paving applications. The quality and value of a concrete pavement will always have a place in the construction of our infrastructure.
In this interview, Margaret Cervarich, vice president, marketing and public affairs of the National Asphalt Paving Association (NAPA) in Lanham, MD, describes exciting developments in asphalt for highway construction.
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-in. 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. I interviewed the chief engineer for the turnpike a couple of weeks ago and he said that he didn’t see why it couldn’t last another 50 years. It was very well-designed and well-built. The designers put a lot of thought into 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, AL, 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.
You can 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 is 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 rubbilize 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?
I would go back to the New Jersey Turnpike. That pavement has been in use for 50 years under very heavy traffic, and the chief engineer says we think 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 maintenance 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.
Could we discuss Superpave technology?
Superpave was developed during the Strategic Highway Research Program (SHRP) (1988-1993). It’s an asphalt mix-design system, an improved way of selecting materials and designing HMA mixtures.
Is asphalt generally considered to be less expensive than concrete?
There have been a number of studies within the U.S. and Europe which have shown that asphalt pavements generally have a lower life cycle cost.
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.
Have states stopped adding 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 heftier mix. Asphalt pavement is very much an engineered product.
You must also determine what it will do when it’s wet?
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, which is also called Gap Graded Superpave. It’s a very tough and durable mix. It may be used to reduce splash and spray, and to reduce noise to some extent. But, its main advantage is providing a long-lasting pavement surface.
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 be set up near a paving site temporarily. 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. 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 steel wheel rollers or rubber tire rollers.
The material transfer vehicle is gaining popularity with contractors and agencies. Basically it agitates 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 a word, the asphalt pavement industry is on a roll. CEG
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