Cold In-Place Recycling (CIR)

Cold in-place recycling (CIR) is an emerging [[#|rehabilitation]] practice [[#|asphalt pavement]] roads. This paper focuses on the cold central plant recycling process in which the recycled material does not leave the site and an offsite mixing plant is not used. The CIR process happens in one chain process beginning with the milling of existing pavement. The milled pavement is the screened, and then mixed with an asphalt emulsion. The emulsion is then laid on to the milled pavement surface and compacted as emulsion begins to break. This process will be covered in greater detail later. The benefits of CIR include: minimal material transportation costs, no hot-mix plant required, one-time traffic impact, ability to be done at colder temperatures. Shortly after the CIR, a surface treatment of treatment of a thin hot mix asphalt overlay, chip seal, or slurry seal is applied to reduce moisture penetration.

Ideal Locations

CIR is ideal in locations where there is a structurally deteriorated pavement that is only subjected to limited heavy vehicle traffic. These locations are often rural two-lane highways that are a large distance from hot mix asphalt (HMA) plants. An additional benefit of CIR is that it is cold and does not need a HMA plant and is not sensitive to loss of temperature during transportation.

Chain Process

A CIR train can be seen in image 1 [1]. At the start of the train (left side of picture) there are tankers carry the lime slurry and emulsified asphalt. Immediately following the tankers is the [[#|milling machine]]. The conveyor belt carrying the milled material to the crusher can be seen in the middle of the picture. At the end of the train, on the right side of the picture, the pug mill can be seen leaving the new material on the road ready to be compacted. Pneumatic compactors, not shown in the picture, are [[#|last]] in the train.

1.jpg

The existing pavement is generally milled at-least two inches in CIR. The depth of the milling depends on the existing pavement. In general it has been found that milling 50-70% of the existing pavement is effective. CIR effective limits reflective cracking by disturbing the crack propagation path of the existing pavement. The depth of milling is a critical design aspect that must be carefully controlled in the design process. Milling needs to remove enough material to create a stable CIR layer but leave enough existing pavement to provide sufficient structure. Also the cost of milling increases with depth, therefore the least amount of milling possible is the most cost effective. The image [2] below illustrates the cross section of CIR pavement. In a typical [[#|rehabilitation]] or [[#|repair]] project the existing pavement that was milled away would be need transported and stockpiled at another location. In CIR this milled material is dumped directly into a crusher.
2.jpg
The milled material is the crushed and dropped over a screen (usually about 0.5”) to limit the max size. The screen is placed on an angle, so that material that does not pass through will be dropped back into the crusher. The level of crushing is also a critical piece in the design process. Crushing material smaller increases [[#|energy]] demand and will likely require more emulsion to re-coat the aggregates because smaller aggregates have a larger surface area. Both the increase in [[#|energy]] and in emulsion will increase the cost so it is imperative for the [[#|design engineer]] to crush the material as little as possible. Excessive crushing can result in increased dust which increases the rutting potential of the pavement. [7]
Depending on the exact system being used, a lime-slurry will be added to the milled material before or after the crusher. The purpose of the lime-slurry is to help create better adhesion between the aggregates and the binder. Lime slurries have also been found to reduce the moisture susceptibility of the mix which is critical in CIR because there are 15 – 20% air voids in the mix.
The most critical piece of a CIR project is the emulsion. An emulsion is an asphalt binder that is mixed with water, see image below [3]. The water in the emulsion will then evaporate and the residue that is left acts is the binder. The evaporation and hardening of the residue is called “breaking” of the emulsion.
3.png
The amount of emulsion required for a given CIR mix is dependent on the previously mentioned factors of: existing road material and condition, and size of crushing. The amount of emulsion is also dependent on the type of emulsion used. The dosages of emulsion ranges from 1-4% of the total mix weight. This is significantly less than typical HMA mixes which are usual 4% binder and above. Too much emulsion is uneconomically and will likely lead to increased rutting potential of the pavement.
The emulsion is mixed cold with the lime-slurried aggregates in the pug mill. After the materials have been thoroughly mixed they are left on the base to be paved. The paving process is similar to HMA however because temperature control is not an issue the material is typically dropped on the road before paving instead of being dumped into a hopper. The image[4] below, shows CIR material waiting for the paver. Notice the smooth surface behind the paver.

4.jpg

Following the paver will be the final [[#|step]] in the CIR train, the pneumatic rolling compactors. It is important to notice in all of the CIR train pictures shown in the report the rollers are never pictured. This is because there is a narrow time window for compacting. In order to compact the emulsion must be beginning to break so that is stiff enough not to flow out from under the rollers. It must also be soft enough that it has not already set. The rollers are compacting to a relatively high air voids in CIR, typically 15-20% air. After the pavers have sufficiently compacted the material, the roadway may be opened to traffic (depending on emulsion), however, it is more likely that a surface treatment will be applied. Some emulsions may take up to two weeks before the emulsion is fully cured and a surface treatment can be applied. Surface treatments are outside the scope of this paper and will not be discussed in detail.

Unique Challenges

Some of the challenges of CIR have been presented in the description each piece of the train. Optimization of the mill depth, crushing size, and emulsion content must be done to create an economical road that is still structurally sufficient. These challenges are difficult in any road cold recycling construction process, however they are substantially more difficult to address in real time as the CIR train is working. Since the train is constantly moving and stopping reduced efficiency and increases costs, there is very little time for an engineer to adjust the initial mix. This requires the designer to have a good idea of pavement depth and binder content before construction because there is considerably less margin for error in CIR than conventional HMA.
Any project that utilizes recycled asphalt pavements (RAP) has additional challenges that construction with new materials. There is still not an industry accepted way to determine the amount of binder that RAP contributes to the mix. It is accepted that some of the existing binder is rejuvenated and incorporated in the new binder material and some of the existing binder will act as a “black rock” in the mix. RAP also is subject to the quality of the original materials, which is often unknown since construction history is often difficult to obtain, especially for rural type roads that are likely to use CIR.
There currently are no industry accepted test methods for CIR roadways or emulsions. The test methods used in the industry have been developed specifically for HMA and do not apply well to emulsions. Adding to the challenges with emulsions are the proprietary nature of the products; that is manufactures that develop emulsions will not provide detailed information on the chemistry of their product. From the laboratory, there is no good way to predict the amount of residue left by an emulsion in the field because emulsion are extremely sensitive to environmental issues, such as sunlight, temperature and humidity. Laboratory binder grading is also not applicable to the emulsion residues. Emulsions have been known to behave differently with different aggregates.

Benefits

Despite the challenges, CIR has numerous benefits that make it an attractive option for road rehabilitation. The primary benefits of a properly constructed CIR project are listed below:
  • Cost Savings
  • Energy Savings
  • Non-Renewable Material Savings
  • Disposal Problems Eliminated
  • Profile Irregularities Eliminated
  • Effective Roadway Provided
The cost [[#|savings]] are a result of eliminating transportation of old roadway material, eliminating transportation and purchase of the new construction material, and eliminating the energy required for heat in HMA. A typical reduction in transportation demand for a CIR project can be seen in the image below [5]. It is estimated the total energy savings from using cold mix is 40-50% [6]. Virgin asphalt binder is a non-renewable fossil fuel that is used significantly less in emulsions. [7]
5.jpg

Conclusion

It is apparent by the increasing use of CIR projects that the advantages of CIR outweigh the challenges. The image below shows CIR usage by state in 1998. It has been over 15 years since this map was produced and therefore it is likely that all states have at least tried a CIR project since. The best case for a CIR project is a well performing roadway that had costs and environmental impacts reduced about 50%. The worst case for a CIR project is a material that must be removed, disposed of, and then replaced with new materials. The total cost of a failed CIR project is likely to be the full cost of the failed CIR, plus the cost of removing and disposing of the failed CIR material, plus the cost of complete new construction, in addition to the cost of user delays. Therefore when using CIR it is imperative that the designer pay careful attention to all inputs and be able to make adjustments on the fly.
CIR-use-map.jpg

Works Cited


[1]
[Online]. Available: http://www.valentinesurfacing.com/JPGs/cold_in_place_recycling.JPG.
[2]
[Online]. Available: http://reality2.com/prs/prs1/img/structual_img.jpg.
[3]
[Online]. Available: http://2.bp.blogspot.com/-k3bD9PHTCd4/Ty_SnI-D7SI/AAAAAAAAAtc/X7Hlq-hcSuQ/s1600/asphalt+emulsion.png.
[4]
[Online]. Available: http://www.clrp.cornell.edu/q-a/77_CIR/images/paver.jpg.
[5]
[Online]. Available: http://www.coughlincompany.com/uploads/fb/Truck-Chart.png.
[6]
M. Calkins, Materials for Sustainabile Sites, Hoboken, NJ: Wiley, 2009.
[7]
Recycled Materials Resource Center, "Recycled Asphalt Pavement - Asphalt Concrete (Cold Recycling)," [Online]. Available: http://rmrc.wisc.edu/ug-mat-reclaimed-asphalt-pavement/. [Accessed 9 April 2013]
[8]
"Dr. Pedro Romero Course Notes" University of Utah Civil and Environmental Engineering, CVEEN 7570, Spring 2013