"The International Agency For Research On Cancer has said that they are going to write a monograph with respect to the effects of asphalt fumes on workers. They expect that that will come in 2010," David E. Newcomb, P.E., Ph.D., vice president of Research & Technology at the National Asphalt Pavement Association (NAPA), told attendees of the Asphalt Pavement Association of Michigan's 53rd Annual Asphalt Paving Conference in Kalamazoo, MI, in February.
"It will rely very heavily on human studies. So, along those lines, NAPA, in cooperation with state asphalt pavement associations, formulated an environmental survival fund to fund research so that data is available and that scientists are looking at the correct things." Other studies regarding the effects of asphalt fumes on worker health are expected to be concluded this year.
"Silica milling is something that has come up in the last couple of years. A partnership was formed between equipment manufacturers, the National Institute for Occupational Safety and Health, organized labor, and NAPA. The idea there was to take a look in a proactive fashion at what we could do to reduce the amount of dust in milling operations.
"With respect to that, there are prototypes being tested and, in the end, we want to bring this to a successful conclusion." Newcomb also discussed warm mix asphalt.
"Warm mix asphalt is the future of asphalt. This is where we need to go with our product. Lowering the temperature does a lot of things that are good for our operations in terms of lowering emissions, being better neighbors and lowering energy consumption. From an operational/performance standpoint, we can get better and more consistent density out of warm mix asphalt.
"In terms of challenges and opportunities, there are a number of things that are out there on the horizon that need to be addressed. One is developing a good mix design procedure. That's being done. We need a methodology for product improvement. How do you know that warm mix technology that's being used is something that will perform well? Well, there are ways of looking at that in terms of having a track record. We see a lot of states coming up with permissive specifications for warm mix asphalt. I believe that Michigan is working on one at the current time.
"We need to continue to monitor the long-term performance of warm mix asphalt. The first project in the United States was done in 2004. We also need to quantify the environmental and health benefits. The number of warm mix units that are being sold to place on asphalt plants suggests that folks are moving in the direction of warm mix asphalt. There are a lot of advantages to it."
Newcomb also discussed the importance of reclaimed asphalt pavement (RAP). "We've been doing RAP since the early 1980s, but we got comfortable at relatively low levels of RAP. We need to be pushing for higher and higher levels of RAP usage and in order to do that, we need to do it correctly. We need to make sure that we characterize the material in the same way that we would any other ingredient in our mixes. We need to make sure that we don't do anything to harm the performance of the product in the end.
"We need to encourage milling, particularly in rural areas where RAP is readily available. One of the things that often happens is that the milling portion of the project is ignored, so we wind up paving overlays on cracked and rutted pavement. Well, if you don't prepare the surface, many times you will be buying a shorter pavement life. Milling is something that is crucial to pavement performance and, I think, in the end, provides us with materials that we can recycle."
Andrea Kvasnak, Ph.D., lead research engineer for the National Center for Asphalt Technology, explained the benefits of using RAP.
"If you use RAP you're going to reduce the cost of your mix, because you are replacing part of your virgin material. You're also going to decrease the cost of transporting in materials. In today's environment, you can't ignore being 'green.' That is what the public wants. They want to hear that you are doing things in a green-friendly way," Kvasnak said.
"You're cutting down on the amount of space you need for stockpiles. You're not disposing of your RAP, so you're not taking up space at the dump."
Kvasnak pointed out that if a contractor was using RAP as 20 percent of its mix in the summer of 2008, they could've realized a savings of close to $10 per ton of asphalt. She pointed out that with the current price of asphalt being somewhat lower, the savings will be somewhat less, but there is still a benefit to including RAP into a mix.
Kvasnak said that barriers to using a higher percentage of RAP include concerns over quality, the consistency of RAP, durability of the mix, the ability to meet volumetric requirements, stiffness of the binder, and use with polymer-modified mixes.
"The reason that some people think that RAP is a waste product is that they've seen some bad examples of RAP stockpiled. Many times they will see an unprocessed stockpile, or they will see a stockpile that has everything in it from the kitchen sink to virgin aggregates. I have seen stockpiles that had garbage in them. This is what state agencies see. You need to make sure that you have good stockpiling techniques. Treat your RAP the way you treat your virgin materials. Make sure that there is no contamination in the stockpile," Kvasnak said.
"Another concern I hear is that the aggregate is not as good. The good quality aggregate has been used. So, in many cases, you're going to get a higher quality aggregate in your RAP.
"The next thing is consistency. Many people are worried about the variability of RAP. I have looked at a lot of data about virgin aggregate and RAP and almost without fail, the RAP is less variable than the virgin aggregate. Why is this? The virgin aggregate has not been processed as much as the RAP has. The aggregate that goes into your pavement must meet certain criteria and it must be within certain limitations for variability. When you mill it, just make sure that you don't overprocess it and you will have a nice product.
"If you have a stockpile made up of multiple source RAP material, you need to blend it.
"People are also concerned that since you have a harder binder with RAP, the material will be more prone to cracking. They're also concerned that asphalt on the RAP is not getting activated, so you're not getting the proper amount of binder around the aggregate in your mix."
Kvasnak said that one study looked at several different RAPs to see how much asphalt is contributed to the mix. The study concluded that there may not be a 100-percent contribution of asphalt from the RAP, but a significant amount of asphalt does come from the RAP. The quantity varies depending on the type of mix.
"We don't have a lot of information about the performance of high-RAP mixes, because people have not documented them. There are a lot of high-RAP projects out there, we just haven't heard of any major failures," Kvasnak said.
"The few failures that have been well documented were more attributable to construction practices. The mix was not designed with RAP in it.
"You tend to have a little more dust with RAP, so it's going to make meeting volumetrics and the dust-to-asphalt ratio a little tricky to meet in some cases. Some of us have materials that break down very easily. So, using high percentages of RAP is great, but given your regional materials, it might not be achievable if you have too much breakdown. One way to get around this is to not overprocess the RAP. The more you process it, the more it's going to cost you and, in addition, you're going to create more fines.
"You can think about fractionating. This splits out your coarse from your fines. When you fractionate, if you have an issue with a lot of fines, you can use a higher percentage of coarse aggregate and still see some savings.
"Fractionating is great if you're going to do high percentages of RAP on a regular basis. If you think that you're only going to have one high-percentage RAP project in the future, this is probably not an investment that you want to make. You also need to look around your yard and make sure you have space for this. You're going to need space for the equipment and the multiple stockpiles."