Researchers examined the practice of reducing the binder content of cold in-place asphalt recycling mixtures in the field on especially hot days to improve workability. Laboratory testing of mixtures at various temperatures and binder levels found the practice keeps mixtures workable, improves compaction and does not significantly diminish performance.

What Was the Need?

Cold in-place recycling (CIR) allows crews to reuse existing asphalt in a new layer that is mixed on the construction site. Asphalt is crushed into less than 1.25-inch pieces that are mixed with asphalt binder on-site and then placed back on the surface. While hot-mix asphalt may be near 300 degrees Fahrenheit (F) when it arrives at a work site, CIR emulsion is generally held at temperatures between 100 and 120 degrees. This reduction in required energy coupled with the reuse of aged pavement makes CIR a sustainable and less-expensive option for road agencies improving their network by repaving asphalt.

During summer heat on a Minnesota roadway where crews place recycled asphalt concrete, temperatures can rise to 125 degrees F, and constructability of CIR becomes challenging. Recycled mixes with carefully designed binders can get warm enough for aged binder in the recycled material to become more effective and thus contribute to the overall bonding of the mix. In these conditions, local crews typically reduce the amount of added binder they use to maintain a more workable product.

“Some local agencies have been hesitant to use CIR because of this problem with asphalt content. We will keep doing it and can now say more confidently that we’re getting a good product,” said Wayne Stevens, county engineer, Brown County.

Local road agencies in Minnesota have observed little or no decrease in CIR durability and performance despite the diversion from recommended design levels of binder. Yet engineers and crews lack hard data to support the practice. If reducing binder quantity can improve working with CIR mixtures in summer heat without compromising asphalt performance, then validation of the practice through research will support a method that makes roadwork easier and stretches local agency funds for materials and construction.

What Was Our Goal?

MnDOT and the Local Road Research Board sought to examine the performance of CIR mixtures with reduced added asphalt emulsion binder content. The investigation team would analyze recycled asphalt mix performance at temperatures representative of field conditions and compare performance to standard mixing temperatures, practices and asphalt levels used in the lab during the mix design.

What Did We Do?

The research team gathered millings from a Brown County, Minnesota, asphalt pavement project for laboratory testing, where the short-term and long-term performance properties of mixes were evaluated at various temperatures and binder content levels.

Mixtures were prepared and tested at three temperatures—room temperature, 110 degrees F and 125 degrees F—to examine optimum asphalt content using standard design measurement and performance tests, as well as a new low-temperature, semicircular bending test developed in CIR fracture performance testing. 

Researchers examined results and developed recommendations for binder content levels in CIR mixtures at the three temperatures, and also suggested potential further research.

What Did We Learn?

Analysis showed that binder levels can be reduced at increased temperatures with little impact on tested performance characteristics of CIR mixtures. At higher temperatures, some of the asphalt binder in the millings may activate and contribute binder to the mixture.

At room temperature, the standard, MnDOT-specified CIR binder level is typically in the range of 2.5 to 3 percent of the mix. Mixtures at 110 degrees F met performance tests with 1.75 percent binder. At 125 degrees F, 1.5 percent binder appeared to be sufficient for good performance in this research study.

“I did not expect that these performance test results would be as good as they are. Even though they show some slightly lower performance levels, they still meet specs and are very close in performance statistically,” said Daniel Wegman, principal engineer, Braun Intertec Corporation.

Mixing at higher temperature improved compaction. At the same compaction level, the high-temperature mixture had higher density, lower air void content, higher dry stability and higher percentage of retained stabilities. Low-temperature strengths were greater for room temperature mixtures than for higher temperature mixes with reduced binder content.

Raveling percentages were higher for the warmer mixes than the room temperature mixes, though still within allowable limits. The fracture energy was also slightly lower for the high-temperature mix.

Ultimately, reducing the emulsion binder content in CIR mixtures used on hot days does not appear to significantly deteriorate mixture performance properties.

What’s Next?

This study validates the local agency practice of lowering binder levels in CIR during hot paving conditions. At $500 per ton of emulsion, binder can be a considerable expense. Lowering binder levels to 1.5 percent on a two-lane rural highway for 5 miles could save roughly $200,000 in materials.

The impact of reductions in content with softer binders sometimes used in CIR and on performance and constructability of chip seals warrants examination, as does the use of stabilizers in high-temperature performance. Low-temperature performance should also be followed closely to determine if this performance change has any long-term impact on pavement performance. Finally, procedures for reducing binder levels in field conditions may need to be developed and standardized.  

This post pertains to Report 2019-43, Evaluating Effective Asphalt Content in CIR Mixtures, published November 2019.