Roughly 30 years ago, Bomag pioneered some of the first cold in-place asphalt recyclers in the roadbuilding industry. This was not only a new piece of equipment, it was the introduction of a new concept. The idea was intriguing enough. Instead of investing the time, effort and materials in totally reconstructing an asphalt road from the ground up, cold in-place asphalt recycling would permit the reuse of the previous road surface and sub-base to create a strong foundation for a new roadway. Though cold in-place recycling offered a revolutionary alternative that promised a tremendous cost savings, contractors, county engineers and state officials alike were skeptical. How could something that costs less be just as effective as the established reconstruction method? The task of selling the concept proved to be an uphill battle.
Today--30 years later--several manufacturers have introduced cold in-place asphalt recycling equipment. Though credibility has been established through education and successful applications in many states and counties, understanding the process and its benefits remains the key challenge to its acceptance as a viable road reconstruction option.
Before you begin . . .
It's difficult to describe a "typical" cold in-place recycling application since there are so many variables to consider. What materials currently make up the structure of the road? Is the current subgrade satisfactory, or will the road require full-depth reclamation? Does aggregate or asphalt need to be added to the mix? What about additives? All of these questions, and several others, need to be answered before recycling can take place. But there are basic steps that should be conducted with each application.
Core samples
In order to improve a road through cold in-place recycling, one first needs to know what materials make up the current road surface and base. This is why core sampling is the single most important step in the recycling process.
Core sampling takes place before any equipment is on site. It determines the composition of the asphalt in the road, the percentage of asphaltic concrete in the mix and the amount and type of aggregate present, as well as pinpointing any potential problems that can't be seen on the surface. Though many will claim to know enough about road construction or a particular road surface to forgo taking core samples, sampling is truly the only way to successfully determine the recycling needs. Since the success or failure of a road relies on the makeup of the base materials, such information should not be left to speculation.
When taking core samples, it's best to take samples from both the best and worst portions of the road. As those in the road construction industry know, a core sample taken from one area and another taken a mile down the road can yield two completely different results. One must analyze all the core samples and recycle based on an average of the measurements.
Grading
Once the core samples have been taken and the asphalt and base makeup is determined, it's time to put the recycling plan into action. But before beginning to grind the road, the contractor should take time to evaluate the current road width. Many of today's state highways are simply not built to the exact specifications. When embarking on a recycling project, it may make sense to grind a bit wider than the existing road to effectively stabilize the base under what will be the new road. In exercising this option, first utilize a motor grader to scrape the shoulders free of any organic material. This step should not be taken lightly. Any organic material that gets into the mix will eventually die. Following decay of the material, a void is left in the road's substructure resulting in a possible collapse and destruction of the surface. By simply making a few passes with a motor grader to skive off any organic material, much of this concern can be eliminated.
Recycling
Once the road's needs have been determined and all organic material has been removed, it's time to grind, or recycle, the road. This process involves grinding both the asphalt and aggregate base together. Again, much of the specifics of this process depend on the core samples. If it is found that the established base is in good shape, one may decide that grinding only a minimal amount of the base material is necessary. On the other hand, if the subgrade is weak, aggregate can be added prior to recycling to establish a more effective material gradation. Since the most common cause of road failure is a faulty base, full-depth reclamation may be a consideration.
Basically, the cold in-place asphalt recycler uses a spinning rotor with cutting teeth to pulverize the road and base material, leaving the ground-up material in its place. The time it will take for this initial cut will depend on the makeup and hardness of the material, but the core samples should provide a clue. If core samples show that the base is made up of fine materials, aggregate should be added to the surface of the road prior to grinding. When recycling, material should be pulverized until the proper grade is established. Sometimes this is achieved with the first pass; other times multiple passes may be necessary.
With basic mechanical stabilization, a water truck is utilized following the recycler to maintain moisture in the ground-up material, acting as a lubricant to facilitate the best compaction. In other cases, the use of an additive may be necessary to help stabilize the material.
Additives
Additives serve many purposes. As with water, some additives act as a lubricant to facilitate compaction. In other cases, they may act as a binder for the material, helping to make the road impermeable to water. Since water is the natural enemy to the structural integrity of any road, impermeability is a key objective.
Compaction
Following the establishment of a proper grade through grinding and the addition of water or additives, the material is ready for compaction. The best tool to use for this step is a padfoot roller.
Padfoot rollers are ideally suited for the compaction of cohesive soils. Because of the basic properties of asphalt and its presence within the mix, the material tends to have the properties of a cohesive soil. Padfoot rollers work well because they compact unevenly, preventing material layering and establishing a good mix of material.
During the first pass, the pads of the roller typically sink all the way down to the drum. But as compaction increases, the pads begin to "walk out" or rise up out of the mix as the material achieves density. Eventually, the roller will be standing near the tips of the pads, indicating that proper compaction has been established. It is important to remember that if the material becomes mis-shapen during the compaction process, the use of a motor grader is advised for any required reshaping. Additionally, when compaction is established, the grader should be used to remove any of the "uncompactable" materials, or ravel, from the surface. Finally, a rubber-tired or steel-drum roller is used to smooth and finish the recycled base.
Sealing
After the road has been mixed, shaped and compacted, it should then be protected by some form of temporary seal. A fog seal is an asphalt emulsion that is applied to the top of the base and allows water to shed off the top instead of soaking in and compromising the integrity of the compacted base.
After the sealant has been applied, the newly established base is ready for an overlay, chipseal or whatever is specified for the job. If an additive was not used during recycling, the finishing treatment can be applied shortly after completion. If an additive was used, time must typically be allowed for the substance to cure. This extra time needed between recycling and the finishing treatment makes the sealing step even more important to maintain the integrity of the compacted material.
Repeat savings
So if one is already familiar with conventional reconstruction, why even consider a "new" process, such as cold in-place recycling? The answer is simple--money.
Cold in-place recycling, which is essentially total reconstruction of a road, encounters few cost-prohibitive problems. Material costs are less because the existing material is recycled and reused. Though many times additional aggregate or asphalt must be added to create a proper base, this is much more cost-effective than removing the road bed. Additionally, the material is recycled "in-place," meaning there is little need for excavation or hauling.
In the end, cold in-place recycling costs anywhere from one-third to one-half of the total cost incurred for conventional reconstruction. And when done properly, cold in-place recycling has offered better results than conventional reconstruction.
Don't miss the concept
So if it yields equivalent or better construction quality, yet costs substantially less, one would think that cold in-place recycling would be mainstream. The fact is that many still do not even consider the process.
One obstacle is the perception that cold in-place recycling only involves the road's top layer of asphalt and, thus, cannot be directly compared to conventional construction methods. Though this misconception has been mostly overcome in the last 30 years, it should be reiterated that cold in-place recycling is total reconstruction of a road. Because the process goes down to the subgrade, the two methods are quite comparable.
Other misconceptions stem from the lower cost of recycling. Many contractors and government agencies don't believe that something that costs less can actually do the same or better job than conventional methods. The belief that spending more money results in a better quality product is still quite prevalent.
Additionally, the fact that recycling is still perceived as a new process is another problem. There is nothing more difficult than trying to convince someone that something new could be better when an established method is the norm.
Additives also have been a point of contention in the acceptance of recycling. There are a large number of additives on the market--some good, some bad--and many times they are sold as being able to do something they cannot. When the wrong additive or incorrect dilution is used, the entire recycling process is jeopardized. If this occurs, it's often the process that receives the blame, not the additive.
The only true limitations to recycling are concrete and brick. In some cases, asphalt roadways have bases constructed of concrete or are overlays of previous brick streets. A cold in-place recycler cannot handle such materials. Again, a core sample would indicate the presence of such materials, but this would be the one situation where excavation and conventional reconstruction would be the only option.
Accepted everywhere?
Though the cold in-place recycling process may appear too good to be true--offering the benefits of conventional reconstruction at a fraction of the cost--many counties and states have experienced tremendous success with the recycling option. Acceptance is still a work in progress, but when twice as many roads are being rebuilt for the same taxpayer's dime, the results are becoming harder to ignore. As more and more decision makers learn about and gain confidence in the process, cold in-place recycling will steadily win acceptance.
