Maintaining Asphalt Pavements

By Aram Kalousdian, Editor | September 28, 2010

It is important to know what types of pavements exist on a road before an appropriate treatment can be applied. That was a message that David Peshkin, P.E., vice president and principal of Applied Pavement Technology, Inc., delivered at the 2008 Michigan Seminar on Maintenance of Asphalt Pavements held in Mt. Pleasant in January. The program was presented by Michigan's Local Technical Assistance Program (LTAP), the National Center for Pavement Preservation, and the Asset Management Council.

"Do you have asphalt over concrete? Do you know what pavements have surface treatments or chip seals that have bituminous products but aren't hot mix asphalt? These all perform differently, and when we go into project selection and treatment selection, we're going to want to know exactly what type of pavement it is, because that's going to dictate in part what sort of treatments are going to be appropriate for that pavement," Peshkin said.

"Inventory information is also important. That includes things like the materials that went into the pavement, when it was built, and what the different layers are in the pavement structure. This information serves a number of different purposes including helping you to group together different types of pavements when you don't have to treat each individual section of pavement as its own entity."

If, for example, there are a group of pavements that consist of 7 inches of concrete and 3 inches of asphalt on top, those will perform fairly similarly, so those could be grouped together.

"History is also important. How has the pavement performed in the past? One source of knowledge about the history of pavements is the people who are involved in maintaining those pavements. Knowing how a pavement has performed in the past is a very important step for seeing how it's performing now and predicting how it's going to perform in the future," Peshkin said.

"Of course, the measure of current conditions is essential. What does the pavement look like today? How does it compare to other pavements that you're responsible for? This is all going to be essential information for selecting pavements and selecting treatments for those pavements. This is also the framework for pavement management. All of this information should be captured in a pavement management system.

"I think the next step is to figure out what you want or what you need from your pavement program," Peshkin said. Objectives include extending the life of the pavement, providing a good ride, making the most use of available funds, and maintaining the existing pavement instead of reconstructing.

"Learn from your pavements what works and what doesn't work and why what doesn't work, doesn't work," Peshkin said.

"Quite often you hear that by spending one dollar here you'll save more dollars later in the life of the pavement. I don't think that's entirely accurate. But what you do is delay the spending of that money. By spending money on pavements that are in better condition, we're delaying the need to spend more money on pavements that are in worse condition. So, when you talk about treating as many miles as you can with the dollars that you have available, I would suggest that if you look at the relative cost of what treatments are going to run you at different pavement conditions, you're going to get a lot more miles treated when you put those treatments on pavements that are in better condition." Peshkin pointed out that safety is also an important goal of road maintenance.

"There are over 40,000 fatalities per year on our nation's roads and highways," Peshkin said. He explained that according to some estimates, fatalities could be reduced by as much as 25 percent if safety improvements were made to the nation's roads and highways. "From work zone safety through better surface friction characteristics and better selection of treatments, we can improve safety," Peshkin said.

"Over the first 75 percent of a road's life, it drops 40 percent in quality. But it only takes another 17 percent of its life to drop another 40 percent inquality," Alan Gesford, P.E., said. Gesford was a senior research assistant at Penn State University. He is retired. Gesford explained that that is why it is important to do preventive maintenance work early in a road's life.

"You need to take care of all of your water and drainage problems first. If you don't take care of the drainage, the pavement is not going to last as long as it should," Gesford said.

Gesford discussed asphalt patching materials for potholes. He explained that a manual published by the Federal Highway Administration (FHWA) examined what could be done in emergency adverse weather conditions.

"The statement up front in the manual says 'materials and procedures are for cold mix stockpiled materials and spray injection patching. The goal is to restore ride quality and safety as quick as possible; not to repair the distress permanently,'" Gesford said.

"They talk about conventional cold mixes, primary cold mixes and spray injection. They say that 'the agency must verify the quality of the material used.' Then they come up with three procedures that they say can be used: place the material in the hole, compact it with truck tires, and open it to traffic. That's a good emergency procedure.

"Then they look at semi-permanent repair where they are doing some preparation, including removing water, debris, squaring the sides, placing the mix, and compacting with a device. They also looked at spray injection. It involves blowing out the water, debris, spraying the tack, placing the asphalt, and opening the road to traffic." Gesford also discussed conventional patching.

"Cold mix was never meant to be a permanent patch. Many use it that way. Hot mix is asphalt and aggregate heated to a high temperature mixed in a plant. Cold mix is asphalt and aggregate heated to not quite as high a temperature and is usually in the form of an emulsion. There is solvent in it. It can take up to one year for the solvent to fully evaporate from the patch. During that time, that cold patch is going to be a lot more flexible than hot mix asphalt. We want cold patch to be pliable, but we don't want it to drain. We want it to be stable under traffic. A well-graded aggregate from fine to coarse is going to lock up better and stay stable under traffic, but it's probably going to make the material a little more difficult to handle. Rounded aggregate is easier to shovel, but is it going to be stable under traffic? A softer asphalt binder is easier to handle, but is it going to be stable under traffic? The more asphalt and binder we have in the mix, the easier it is to handle, but it will be less stable under traffic. So we have all of these opposing demands on cold mix asphalt. Think about them when you are using it."

Traffic has a weight factor and a force in the direction it's moving. Gesford said that for this reason, the patch must have vertical sides in order to prevent traffic from pushing it out. Vertical sides are also important for compaction.

Gesford said that the tack coat should be sprayed or brushed on, not poured. "All you want is a thin coat on the vertical wall in particular. This is the glue that's going to hold the patch in the hole," Gesford said. He added that a blacktop rake should be used, particularly with hot mix asphalt. Garden rakes tend to segregate the aggregate. Large patches demand large equipment in order to get good compaction and good ride quality.

"Infrared patching is for surface distresses only. You soften the existing asphalt area with infrared heat. You can add new material to bring it up to level. It eliminates cutting out material. You're using the existing material, so there is nothing to haul away. It eliminates tack coating, and, depending on what the pavement is like, the repair can look like a seamless repair," Gesford said.

"Spray patching was originally developed for emergency patching. We weren't looking at it as a permanent patch. What we've found out is that the patches are lasting as long as permanent patches."

Kevin Kennedy, capital preventive maintenance engineer for the Michigan Department of Transportation (MDOT), discussed a relatively new technology called Fibermat.

"There are two types of applications, Type A and Type B. Type A is a stress-absorbing membrane wear course. It is also used as a stress-absorbing inner layer membrane," Kennedy said.

"The process is going to look very similar to a chip seal. What's different about Fibermat is in the binder. It's put down by a spray bar so it goes down in two courses. We introduce fiberglass between those courses. The intent of that is to reinforce the binder and give it a little more tensile strength," Kennedy said. With this application, the road can be opened to traffic quickly.

Kennedy said that tests have been conducted on Fibermat at Penn State and Texas A&M University. Tensile testing, fatigue testing and wheel track cracking programs were conducted. All of these tests have shown that when Fibermat is adhered to a non-Fibermat section, it provides increased strength and delays cracking. MDOT's first Fibermat project will be done this year on M-72 in the Grayling area. It will be used as an inner layer with a micro-surface on top. "In 2009, we'll probably be looking at some other surface course applications," Kennedy said. Fibermat has been used on other projects in Michigan.