Asphalt Rehab: Cold Gains Ground

Dec. 2, 2011

Cold in-place asphalt recycling (CIR) has been around for over 50 years as a way to deal with decaying roadways, but because of its environmental and financial qualities, it is becoming more and more popular. As its name implies, the technique involves using the existing road material rather than bringing in new asphalt. The “cold” refers to the fact that a hot asphalt mix does not have to be brought onsite.

The Asphalt Recycling and Reclaiming Association (ARRA), a trade association of recycling contractors, sponsors the Pavement Recycling and Reclaiming Center (PRRC), which is also supported by Caltrans. Steve Cross is the executive director of PRRC at Cal Poly, Pomona CA, though Cross’s headquarters are at Oklahoma State University in Stillwater, OK.

The center has been in existence for one year. “Right now our emphasis has been on education and outreach. Once we get up and going, we plan to get more into research-and-innovation-type work,” says Cross. “We are working on putting together several two-day courses on the different phases of CIR.”

In addition to the west, CIR is used extensively in New England and New York State, according to Cross. “Though Oklahoma has not done a lot of CIR, Kansas, Nebraska, Missouri, and South Dakota have. There are states that do CIR and know all about it and other states that don’t. But interest is picking up everywhere. Part of it is the sustainability and the other is the economics.

“What we are trying to let people know is that CIR is a tool in your toolbox. It won’t work everywhere. But in the right situations it works well; it’s cost effective and environmentally desirable. It’s like any other maintenance technique: You need to perform the right procedure at the right time. There are many instances where this is the right procedure at the right time. But not everybody knows about it.”

Pavement recycling and reclaiming covers a whole host of rehabilitation procedures, including hot in-place recycling (HIR), CIR, and full-depth reclamation (FDR). HIR is generally done higher up on the pavement condition curve; the pavement’s in a little better shape. If it deteriorates further it may be a candidate for CIR. Full depth reclamation is usually done on pavements that are near the end of their life. “If you do CIR and you have serious base issues, it’s not going to perform; you would be better to do full-depth reclamation,” says Cross. “CIR goes down 4 to 5 inches, but with FDR you can easily go down a foot.

“Each of these methods is still more environmentally friendly when they are compared to remove-and-replace. Each of these is applied at different times in the pavement’s life. You wouldn’t compare them to each other; you would compare them to alternatives. HIR can be quite competitive with mill and fill, thin overlays, and CIR can be compared with thicker overlays. Full-depth reclamation is compared to remove and replace.

“If anyone has questions about these technologies or is not familiar with them, our center is out there to help. We can give help on construction procedures, specifications and we are a clearinghouse of information and technology.”

How to Do CIR
It’s generally agreed that there are three main ways to do CIR. The first way is to grind up the existing road surface and mix it with asphalt emulsion pumped from a tanker, which mixes it before it is left in place to be compacted by a roller. Once the water has evaporated, usually a mater of hours, the surface can be paved over with a high-quality friction surface course. So the existing road is being used to create a base course on which a fresh new course is placed.

Another way is to put powdered lime or cement on the road and then go over it with a stabilizer or recycler machine that grinds it up and mixes it. Water is applied. A variation of this method is for the lime and cement to be made into slurry in a tank truck that mixes it into the material as it crushes it. After it sets up, workers go through, compact it, and place a surface course above it.

A relatively new technique called foamed asphalt is the third way to do CIR. Other terms for this are expanded asphalt, foam bitumen, and standard bitumen. (Bitumen is another name for asphalt.) In this technique, as the machine goes forward, on one end is an asphalt tank or water truck and on the other end is a water truck or liquid asphalt truck for the hot liquid asphalt that’s kept heated.

The machine comes along and crushes the road into small pieces. That is put up into a mixing chamber. Inside the mixing chamber from one end there is cold water and at the other hot liquid asphalt. When the water hits the asphalt it foams and spatters, expanding into all of the material that’s being crushed up. The mixture then comes out of the back of the truck, and that recycled foamed asphalt is compacted into the road. Traffic can proceed over this surface very soon after.

When to Use CIR
CIR is most effective for roads with rutting, corrugations, shoving, patching, thermal cracking, fatigue cracking, slippage cracking, block cracking, longitudinal cracking, transverse cracking, reflection cracking or discontinuity cracking, according to ARRA. If there are serious problems with the ride quality of the pavement, both HIR and CIR are very appropriate treatments to consider.

In order to determine if CIR is the proper treatment route to go, a condition survey, sampling by coring, trench milling, or a falling weight deflectometer will help determine the cause of the pavement distress. If the distress has any of the functional causes listed above, it is a good candidate for CIR. If it has structural distress, full-depth reclamation and overlay would be a better option.

CIR provides a crack barrier when used with overlay just above it. (If only an overlay is done, cracks and ruts tend to reflect through.) CIR conserves energy and natural resources, reduces carbon emissions, improves mix characteristics, removes cracks, is cost effective, saves time, may be performed under traffic, and it simplifies logistics. One study cited in Sustainable Green Paving Practices (AMEC Earth and Environmental) found a 37% drop in CO2 emissions along the route being studied. According to the AASHTO-MTO Pavement Design Guide, CIR has 66% to 90% of the strength of hot mix asphalt (HMA) and the Ontario Ministry of Transportation found CIR averages a structural coefficient of 0.35 or 83% of the strength of HMA.

According to Kansas DOT, Kansas has been a longtime user of CIR, with 29 years of CIR experience. Its DOT typically uses a CIR mix design with 4-inch (100-mm) using some 3% engineered emulsion (PG 58-28), 1.5% lime and 1.5- to 2-inch (38- to 50-mm) HMA overlay. They’ve found the expected service life of CIR to be five to 10 years with little standard maintenance and that it costs 45% less than 4-inch HMA. In Nevada, the DOT compared CIR of 3 inches in depth with the use of 2-inch HMA overlay and found a savings of $104,000 per centerline mile.

Where to Use CIR
Asphalt paving is among the main things that Trevor Moore, P.Eng., GSC, is involved with as manager of recycling and design at Ontario Pavement Products/Miller Paving Ltd. in Toronto. Ontario Pavement is involved with preservation treatments, soil surfacing, micro-surfacing, and seal coating in addition to civil construction of things such as bridges and any concrete supply or waste management of residential waste.

“I think one reason the CIR method hasn’t caught on sooner may be that people get stuck in doing things a certain way and don’t want to change,” says Moore. “But certain areas of the continent have been able to change a lot quicker than others because I know in the west it’s been used for a long time, probably since the mid-1980s.” Miller Paving was the first to bring these technologies to Canada, according to Moore. It was the first to bring full-depth reclamation in 1983 and cold in-place recycling in 1988.

For cold in-place recycling, a user needs a binder supply that is either asphalt or emulsion, so a tanker is needed. In addition to this, a grinder, a crusher, a paving unit, a series of rollers, some water trucks, and a grader are needed. For regular paving operations, a hot-mix asphalt plant is needed for making the asphalt, and a paver, rolling equipment, water trucks and typically a grader onsite for doing the shoulders.

“We work with manufacturers to get the equipment, and many times we will make our own modifications to suit what we’re doing here, which might be something small or a little bit larger-it all depends.”

The biggest benefit from CIR is the environmental benefit, according to Moore. “You use fewer new natural resources because you don’t use the aggregates and you don’t use asphalt cement or bitumen. You’re not using the heat that’s required for hot-mix asphalt, not spending as much money for all those trucks required for mill-and-fill or a shave-and-pave operation, and you’re not taking away all the asphalt from the site or bringing all the new asphalt back, so there’s a big environmental savings with CIR. User costs will be lower.

“Doing the proper engineering is one of the biggest factors in the job, and understanding that you’re not doing hot-mix asphalt. A lot of people try to take their existing hot-asphalt specs and then massage them a little bit for recycling, but it doesn’t always work out that well; it’s a totally different material.

“CIR is 100% RAP [reclaimed asphalt pavement], 100% asphalt-coated particles that you recycle. Full-depth reclamation could be 100% granular, clay-type materials or partial pavement or partial granular-any mix-and-match-but CIR is strictly 100% RAP.”

Each CIR Project Presents Opportunities to Learn
“Growth in use of CIR is occurring all over, a lot in the west. It continues to grow in the Northeast and the South is starting to look at it,” says Don Matthews, technical engineer with Pavement Recycling Systems, Mira Loma, CA. “And though it’s been around awhile, we still learn things on CIR projects. The whole industry needs to get a better characterization of the existing aggregate, pavement and materials at each job site and we’re starting to understand that in order to work up new specifications and mix designs. That’s the real key, getting a better characterization of the road, not just assuming what works for one project will work for another. The key to innovation is figuring out what’s coming next and being sure that there is the right technical background behind it.”

Pavement Recycling Systems customizes their process, emulsions and top coats to the specific parameters of the project, including how soon the road needs to be back in service. Normally traffic can resume in a few hours, but with adjustments to the binder in the mix, cars can be on the road in minutes.

The majority of Pavement Recycling Systems work is at the municipal level. The company uses a variety of different manufacturers for its equipment needs. One of the recyclers it uses is the Nescon CRMX-2 cold in-place Recycle Train, a very popular one, according to Matthews. This is manufactured by Nesbitt Contracting Co. and its sister company, Arizona Pavement Profiling. “We have two trains, both made by them,” says Matthews.

“Normally, a recycle train consists of a mill that is made by Caterpillar or Roadtec. Then there’s a middle unit-or middle two units-of a recycle train that has a screening deck, pug mill, and the crusher.

“Wirtgen makes a combined piece of equipment where the mill is in combination with the recycle portion of the train. We have a lot of Wirtgen mills but don’t use their recycle mill yet. We also have a lot of Roadtec mills and haven’t used their recycler either. We’re not in the market for another recycler right now. Our company has a large number of mills and pulverizers. In addition to Wirtgen and Roadtec, we have CMI and Caterpillar mills. We use different ones for different applications. We like them all.”

Evaluating Is Key to Doing the Job Right
Road Science develops new products for pavement recycling, pavement preservation, and hot-mix asphalt work. The company does partial-depth CIR, (its CIR brand is called Reflex), full-depth in-place recycling, and hot in-place recycling. Recycling is a substantial part of Road Science’s product portfolio, according to Todd Thomas, product commercialization director.

“When the pavement is evaluated, we want to understand variability and determine that it’s strong enough to support a recycling train, including getting representative samples from the road, bringing those back to the laboratory and doing an engineering design on them.

“After we’ve gone through this extensive pavement survey, and gone through a detailed sampling plan, we do a mix design. Road Science goes through the laboratory process and does a fair amount of evaluation requiring quite a bit of material. But we think from a reliability standpoint understanding the material very well is important in order to build it the right way.”

The criteria for evaluating pavement includes short-term raveling (can it withstand the abrasion of traffic tires?) and strength test of samples that have been cured a longer period of time. “It’s a pretty strict criteria: The strength has to be at least 70% of our dry strength,” says Thomas. “A thermal cracking test is done to ensure we’re not putting in too hard of an asphalt. A strength test helps us know we’re not getting too high, and a thermal cracking test helps us know if we’re getting too low.

“We do field support where we help the contractor make sure the roller patterns look right using nuclear-density gauges, and we do particle-size analysis of the material in the field to see if we’re getting close to what we did in our lab and make adjustments if necessary either on compaction or anything in the operation that may need to be changed to help improve compaction.”

Thomas feels there is always some customization to do with each project. “CIR’s been done on interstate pavement all the way down to county roads. If it’s engineered right in terms of the actual pavement design and we’ve gone through what the thickness of each layer needs to be and the truck traffic involved, it can be applied to any pavement, reasonably speaking. That’s making sure you have a pavement subgrade or base failure. Our field engineers make the first contact to make the determination of what needs to be done.”

Diversified Asphalt Paving
The Dunn Co., of Decatur, IL, does hot-mix asphalt, pavement maintenance, and pavement recycling-both full-depth and partial-depth CIR. In the 1980s the company saw the benefits of CIR and this work being a growth market as budgets continue to tighten. In 2009 it purchased a Wirtgen W-3800 CR recycler.

“We see CIR as a reliable alternative to some of the other options out there,” says Jim Schwarz, vice president of the rotomilling, stabilization, and recycling group with Dunn Co. “Agencies today, whether public or private, are looking for cost effective ways to rehabilitate pavement. It’s a lot better deal than taking it out and replacing it.”

Dunn Co. has projects in Richmond, VA, central Michigan, and Indiana, and continues to look at other markets. Most of the work is governmental, according to Schwarz. “You need to do your homework and make sure it’s a viable project and look at the different alternatives. Even in CIR there are alternative methods to do it. Some work better with certain materials than others do; we can do foamed asphalt, emulsified or engineered emulsions.”

Taking a Cue From the Contractor
Roadtec, part of Astec Industries Inc. of Chattanooga, TN, builds asphalt road-building equipment. For CIR, Roadtec’s RX-110 consists of a two-unit train, screen, crusher, weigh bridge, pug mill, and the emulsion tank. The company’s RX-120 is similar in its layout but has higher production amounts, according to Eric Baker, Roadtec marketing manager.

“Most of HIR equipment is built by the contractors themselves, kind of a homegrown effect as equipment is part of their process and kind of their trade secret,” says Baker. “They just build the HIR equipment themselves and do it themselves, for the most part. As far as CIR, what we do is build milling machines, which are kind of the core or heart of a cold-in-place job-milling up the asphalt.”

There are two different segments to the CIR market, according to Baker. If a user is going to do DOT or interstate-type work involving high traffic volumes and more high-profile roads, then you’re going to have to guarantee sizing of the material you are milling and weigh the material as accurately as possible, including adding additives at the right percentages. This involves a milling machine feeding into a trailer unit which sizes it, weighs it, and mixes it prior to its being laid down with a paver.

“On lower-traffic-count roads and more county, municipal type work, we inject the additive right in the cutter housing of the milling machine,” explains Baker. “It’s all mixed in there; we rely on the forward speed of the mill, and we also down-cut sort. The drum is turning in a direction so it’s using the ground to brace the material; that gets us better sizing. We don’t run it through a screen or anything like that, we just visually inspect it to make sure we’re getting the right sizing for municipal-type work. Then we mix in the cutter housing. It’s fed out into a paver and laid by the paver.

Roadtec is trying to use pieces that people already use and can use for other purposes. The milling machine used for CIR can be taken and used for conventional milling the next day if desired. It doesn’t have a piece of equipment that’s only for one application, but instead tries making it as versatile as possible so it’s a better investment. The company does build equipment for stock, but it has options enabling machines to do CIR. Some are bolt-on, while some can become more custom-built machines. “All of our manufacturing is done here in Chattanooga, so we’re not outsourcing much,” says Baker. “This provides us the flexibility of being able to customize for someone who wants more.

“We’ve been in the CIR market for 10 to 20 years now; but it tends to go with oil and asphalt prices. As asphalt gets more expensive, it becomes more of a hot topic. Now with both asphalt and oil prices being high, plus the environmental concerns that have come to light, it’s really a hotter topic and more of a growing market than we’ve ever seen before in all the years we’ve been involved with this method. There’s been more interest in it than ever before.”

Baker says that what has driven CIR’s popularity out in the west is a lot more roads with less population and smaller budgets. “They’re trying to really stretch their dollars out there. By proximity it’s started to catch on in California, and there are contractors who have really promoted it and gotten it going in the west. CIR makes much sense when life cycle costs are studied. You’re paying less for more years.”

A lot of CIR is done in the Northeast, just not at the DOT level. Much of it is municipal, county work, somewhat under the radar, according to Baker. “People have been doing CIR for a long time, but it doesn’t get quite the attention. CIR works opposite of some of the other technologies. Contractors are doing this for smaller agencies and they go to state and larger agencies, point out how well the roads are doing and ask them to look at this for their roads.”

Advances in CIR
Bomag has been building recyclers for many years, starting with its MPH-100 nearly 40 years ago. Bomag offers three different asphalt recyclers that range anywhere from a 350-horsepower rear-mount, which is more for use in small municipal application, up to its 600-horsepower production machine.

“With Bomag’s recycling line, we can do stabilizing recycling with the same equipment,” says John Hood, product manager for milling and reclamation equipment in North America. “Universal features include various sizes to fit any need and application. Most of the systems are very simple to use, and the learning curve to educate operators is relatively short.

“Probably the biggest advances that Bomag has made in the past few years have been with the foamed bitumen injection systems. These give contractors the ability to inject asphalt bitumen into the mixture to adhere it.

“This technology has been around for several years,” says Hood. “But it’s really been growing in the last two to three years. With the advent of injection systems we believe it’s definitely a move toward the future.”

About the Author

Peter Hildebrandt

Peter Hildebrandt writes about construction, technology, and industry.

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