Introduction
“Of all the materials recycled in the United States, nothing is recycled as consistently as asphalt pavement mixture,” according to a National Asphalt Pavement Association press release. “The latest survey of asphalt mix producers finds that more than 99 percent of the asphalt pavement material removed from roads and parking lots in 2017 was put back to productive use, primarily in new asphalt pavement mixtures.”
Most asphalt mixes use reclaimed asphalt pavement (RAP) in their designs to help reduce costs during production. Unfortunately, too much RAP can make the asphalt mixture stiff and brittle, which will result in poor pavement performance and shorter lifespan.
In Florida, styrene-butadiene-styrene (SBS) modified binders are proven to have superior performance compared to non-polymer modified binders. Currently, the Florida Department of Transportation (FDOT) allows a maximum of 20 percent RAP in any mixture using a PG 76-22 to capitalize on the RAP usage’s cost benefit and the polymer’s performance advantage.
A concern with higher RAP content is the recycled binder replacement, which results in less virgin asphalt, and consequently, a diluted polymer content. To combat this, Kraton recommended using PolyRejuvenator™ technology. This solution combines the use of a rejuvenator – which activates and “rebalances” the RAP binder to a softer, more usable product – and a HiMA binder (designated as “High Polymer” in the FDOT Specifications). PolyRejuvenator thus increases the RAP amount used in the mixture while compensating for the diluted polymer content.
A balanced mix design concept optimizing the asphalt mixture’s cracking and rutting performance allows an asphalt producer to find innovative ways to use RAP while maintaining performance. During the design process, if performance test results for cracking resistance are low, a common fix would be to decrease the RAP content or increase the asphalt binder content – with the latter being more expensive. An innovative alternative is to add a rejuvenator to the asphalt mixture, which activates the RAP binder and enhances the overall performance.
Kraton completed a cost analysis calculating the PolyRejuvenator™ technology’s performance benefit and increased binder content versus the cost-benefit of RAP. They found the breaking point to be approximately 40 percent RAP. Kraton then evaluated a 40 percent RAP granite mixture using its PolyRejuvenator™ technology at a traditional 4.0 percent air voids per FDOT requirements. They also measured the mixture’s rutting and cracking performance compared to the more conventional mix of 40 percent RAP using a PG 52-28. Kraton then decided to use a balanced mix design approach by designing at 3.5 percent air voids, which increased the optimum binder content but ultimately balanced the mixture’s performance.
Project Scope
Three mixtures were produced at the Asphalt Testing Solutions & Engineering (ATS) laboratory in Jacksonville, Florida, using components outlined in Table 1. PG 52-28 binder was used in one mix designed with 4.0 percent air voids, while PolyRejuvenator™ was used in two mixes: one designed with 4.0 percent air voids and one with 3.5 percent air voids. The PolyRejuvenator™ mixes included the HiMA binder as well as a dosage of SYLVAROAD™ RP 1000 Performance Additive.
Table 1 – Mix Design Components
| Sample | Description |
|---|---|
| RAP | Crushed RAP |
| #78 Stone | Vulcan Materials |
| #89 Stone | Vulcan Materials |
| W-12 Screenings | Vulcan Materials |
| HiMA (High Polymer) | FDOT Approved |
| Rejuvenator | SylvaroadTM RP 1000 Performance Additive |
| PG 52-28 | FDOT approved |
| Liquid Anti-Strip (0.5% by weight of binder) | FDOT approved |
Performance Testing Evaluation
Each mixture was evaluated using the Illinois Flexibility Index Test (I-FIT) per AASHTO TP 124 and the Hamburg Wheel Tracking Test per AASHTO T 324. A summary of the results can be found in Table 2.
Table 2 – Summary of Performance Testing Results
| Properties | Results | |||
|---|---|---|---|---|
| Air Voids, % | 4.0 | 4.0 | 3.5 | |
| PG Binder Grade |
52-28 |
PolyRejuvenator™ |
PolyRejuvenator™ |
|
| Total Asphalt Content, % |
52-28 |
PolyRejuvenator™ |
PolyRejuvenator™ |
|
| Recycled Binder Ratio, RBR |
0.45 |
0.45 |
0.41 |
|
| SylvaroadTM RP 1000 Dosage, %, by weight of total asphalt binder (RAP+Virgin) |
n/a |
2.91 |
2.91 |
|
| VMA, % |
14.3 |
14.4 |
14.4 |
|
|
Hamburg Wheel Tracking, 20,000 passes @ 50oC
|
Max. Deformation, mm |
2.46 |
2.13 |
3.74 |
| I-FIT @ 25oC | Flexibility Index |
1.98 |
2.73 |
11.2 |
| Fracture Energy (J/m2) |
4,618 |
2,046 |
4,950 |
|
Resolution
The traditional FDOT approved mix design with 40 percent RAP and PG 52-28 at 4.0 percent air voids versus 40 percent RAP with PolyRejuvenator™ technology had comparable volumetric and performance-related test results.
Using the balanced mix design approach, the asphalt binder content was increased by 0.4 percent, reducing the air void content by 0.5 percent while maintaining all FDOT volumetric requirements. The Hamburg Wheel Tracking Test result increased slightly (1.61 mm) for the modified PolyRejuvenator, while the Flexibility Index was more than four times greater.
In conclusion, the adjusted PolyRejuvenated balanced mixture design has a much greater cracking resistance while maintaining excellent rutting resistance.
For more information from Asphalt Testing Solutions & Engineering (ATS) contact:
Tanya Nash, P.E.
Pavement Materials Engineer
(904) 510-3072
tnash@ats.consulting
Resources
- European Rubber Journal: Case Study- Kraton Technology Enhances Recycled Asphalt
- Asphalt Testing: Case Study – Testing Kraton Polymers/