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

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.