How contractors are using cold in-place recycling to mine the urban quarry and build resilient infrastructure

When a road begins to fail, the road owner has a decision to make: cover over the deficiency or replace the road? These decisions often come down to budgets, not always what's best for the travelling public and taxpayers in the long-term, and budgets today aren't getting bigger.

Cold in-place recycling (CIR) is a repaving method that is both cost efficient and long lasting. By taking advantage of the wealth of material already on the ground in the form of the failing road, this paving technique cuts material costs, saves time, and reduces job site emissions. Utilizing the urban quarry reduces the amount of virgin aggregate that needs to be mined and processed and is a win across the board for road owners, taxpayers, contractors, and the environment.

Many roads in North America carry far more traffic than they were designed for, and milling and filling the top couple of inches to overlay deeper cracks and fatigued pavement doesn't address the problems that will cause these roads to continuously fail. Cold in-place recycling is proven to produce long-wearing road infrastructure that can end the cycle of continuous repairs that only cover up deeper foundational issues.

How cold recycling works

I spoke with Mark Stahl, VP of Wirtgen North America, about the incredible opportunity available to road owners and contractors who embrace cold in-place recycling as their method of choice for road rehabilitation.

According to Stahl, cold recycling, while not new, is gaining renewed attention in North America for its sustainability, cost, and efficiency advantages. First introduced in the 1970s, cold recycling involves recycling pavements at ambient temperature for road repaving and resurfacing projects. The construction crew mills the asphalt roadway, downsizing it into its original size of aggregates, then adds a binding agent. Once the material is re-laid, a wearing course is laid on top; the wearing course could be another lift of asphalt on high-volume roads, or a slurry seal or a chip seal on low-volume roads.

Depending on the condition of the road, cold recycling can be done as either a partial depth replacement or a full-depth reclamation (FDR). There are multiple cold recycling methods, including cold in-place recycling, where the RAP is processed within the paving train on the job site, and cold central plant recycling (CCPR), where stockpiled RAP is processed off-site.

Compared to CCPR, cold in-place recycling offers greater efficiency, cost savings, and emissions reduction as all material is extracted and reused on the job site.

The goal of cold recycling is to produce a perpetual pavement, described by the Asphalt Pavement Alliance as, "A deep-strength flexible pavement that can resist structural fatigue distress for a long time (at least 50 years), resulting in a long-life pavement. Because there are no bottom-up distresses, structural integrity remains intact indefinitely. These long-lasting asphalt structures can be economically maintained by preserving and renewing just the top layer to address near surface distresses, never needing significant rehabilitation or reconstruction."

Cold in-place recycling has found success on Canadian job sites, including in B.C., Alberta, and Ontario. In the U.S., Stahl says, there are regional hot spots — places where local champions of cold recycling raise awareness of the method and its benefits — including in California, the Midwest, and eastern states.

The cold in-place recycling paving process

While there are a few different methods to produce the cold recycled material, the paving processes are similar to a typical mill and fill.

In a traditional mill and fill, the paving train starts with a milling machine feeding the milled material into trucks. The waste material is hauled away, and a crew follows the mill to clean up remaining dust and debris. Then the paving crew follows in to lay down new asphalt. Potentially hundreds of truck loads are needed to haul material back and forth between the job site, the asphalt plant, and the landfill or RAP stockpile.

When using cold in-place recycling methods, almost all of the material stays in place on the job site. The recycler downsizes the asphalt, then injects the binding agent into the material which is conveyed to the paver. The paver runs right behind the recycler to lay the material back in place and two or three rollers follow to provide compaction. Once the recycled material is compacted, a wearing course — an inch and a half to two inches of hot-mix asphalt surface — is laid by a traditional paving train.

The binding agent, foamed bitumen, is the secret sauce that creates flexible and long-lasting base layers from the RAP material. It is produced from normal bitumen heated to 175 degrees Celsius and injected with small amounts of water and air while under high pressure, causing it to foam and expand. The foamed bitumen is injected into the RAP inside the cold recycler, and the end product is referred to as bitumen-stabilized material (BSM).

This process is substantially more efficient and eliminates the transportation and material costs associated with the traditional paving process. Stahl's enthusiasm is clear, "When we do recycling, the beauty is that almost everything, 95 percent of all the material, stays in place."

Wirtgen offers CR Series and WR Series cold recycling machines. The CR Series machines require a traditional paver to follow, while the WR Series cold recycler and soil stabilizer machines don't. According to Stahl, for a typical FDR application of 8 to 12 inches depth where the aggregate is recycled into the asphalt, the equipment train will use a rubber-tire WR Series machine along with a padfoot compactor and motor grader followed by two more compactors. The padfoot compactor is needed to achieve the appropriate compaction with that thickness of depth.

CR machines are ideal for partial depth replacement of three to five inches. Less overall compaction is needed and can be achieved with a standard vibratory screed on the paver which will be followed by rollers. There is no need for a padfoot roller or motor grader in this scenario.

CR machines can be used for FDR when paired with a high-density screed, but it is not common. Stahl calls this application "paver-laid FDR."

The advantages of cold in-place recycling

It's rare that a construction process delivers so many wins to all of the stakeholders involved.

The re-use of the materials already on-site is the biggest cost, time, and emissions saving opportunity and has additional spillover advantages, including improved safety and reduced waste. 

Stahl predicts a total cost savings of 40 to 60 percent on any given cold recycling project in North America, and the cost and emissions savings largely result from eliminating the incredible volume of trucks that are typically needed to transport materials to and from the job site.

Compared to a standard remove and replace project, trucking emissions are reduced by 90 percent when using cold in-place recycling, and the total job site CO₂ emissions reduction is about 70 percent, estimates Stahl.

Reducing the number of trucks coming and going from the job site has safety benefits for both the travelling public and the workers on the ground. Less overall traffic and fewer large vehicles that need to manoeuvre on a potentially congested job site reduce the risk of accidents. The faster timeline also reduces the amount of time that workers are exposed to safety risks posed by moving traffic.

Shorter road closure times are an added convenience for the public. A cold recycling project doesn't have to be finished for cars to drive over it right away, resulting in shorter lane closures as the road can reopen at the end of each shift.

All of these advantages are enticing, but the ultimate test of cold recycling's viability is its longevity and whether this cheaper, more efficient, emissions reducing solution also provides what the travelling public and the road owners need — a reliable road that lasts.

Cold recycled road longevity proven through research and testing

The true value of cold recycling is found in the proven longevity of the roads. 

Hot-mix asphalt and concrete are bound materials that don't provide a lot of flexibility when under continuous stress. Cold recycled material is non-continuously bound material which prevents cracking. It provides a lot of flexibility which makes it difficult for cracks to propagate from continuous use.

There has been robust academic investigation into the viability of cold recycling, including research studies devoted to improving the design, construction, and maintenance of these roads. Researchers study samples from construction sites and roads that have been in service to analyze how they perform and test the longevity of the material. They build their own experiments at research labs as well.

Perpetual pavements are researched at several institutions across the U.S., including the National Center for Asphalt Technology (NCAT) Pavement Test Track at Auburn University in Alabama. At the test track, researchers compare differently paved sections to determine the longevity of different repaving methods. Accelerated pavement testing at multiple research institutions involves running continuous high volumes of simulated traffic over test sections, rapidly loading the test tracks with years — or even decades — of typical traffic over a span of months. The researchers study the track samples to learn how quickly the road degrades, how it degrades, and what longevity can be expected.

According to Ben Bowers, assistant professor at Auburn University, NCAT researchers loaded an FDR section at the test track with nine years worth of traffic. The FDR held up so well that it showed no sign of change and could be considered a perpetual pavement that would only need a periodic overlay.

This longevity has been found at multiple research institutions.

"Our research testing, our knowledge, our modelling of the pavements, our understanding of the behaviour of the materials means we can confidently predict pavements that could last up to 80 years," says David Jones, associate director, UC Pavement Research Center, Davis California.

To give road owners further confidence in the longevity of their specific project, Wirtgen makes a laboratory-scale foamed bitumen plant and laboratory-scale twin-shaft compulsory mixer that can simulate cold recycling in the lab, producing test specimens before the construction project starts, to ensure that the final result will perform as expected.

Caltrans has more than 25 years of experience with cold recycling

Reduced costs and increased longevity are an appealing pitch to taxpayers, and the California Department of Transportation (Caltrans) has been using cold recycling to revitalize roads across the state for more than two decades, including hundreds of lane miles around San Jose.

It started as a way to speed construction and save money on low-volume highways, but Caltrans now uses cold in-place recycling to reduce greenhouse gas emissions and improve sustainability, according to Thomas Pyle, state pavement engineer, Caltrans.

And, according to the pavement design engineers, the cost of cold in-place recycling is roughly $250,000 to $300,000 USD per lane mile for FDR plus an asphalt overlay, compared to more than $500,000 per lane mile for reconstruction, which is essentially a thick overlay that doesn't address the root causes of road failure.

Long-term solutions benefit all stakeholders

Cold in-place recycling presents an incredible opportunity to mine the urban quarry for material that can be reused in the construction of more resilient infrastructure. Adoption, however, requires long-term thinking and an openness to trying new methods. Cold in-place recycling's proven longevity should make contractors and road owners take notice and consider that it saves money, reduces emissions, improves safety, and builds long-lasting roads that will serve generations to come. 

"We can get bogged down in the details of the processes and the benefits, but at the end of the day we have processes that are proven for decades," says Stahl. "The success of recycling pavements shows that it's not really a risk at all. It's just a different way of doing it."