ALDOT Field Evaluation of Rejuvenators

Asphalt pavements have been known as America’s most recycled product. As asphalt plants use more reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) in the process of producing asphalt mixtures, there is concern that mixtures with high percentages of these recycled materials may be more susceptible to cracking, as recycled binders are aged and stiffer than virgin asphalt. Thus, most states limit the recycled binder content of asphalt mixtures with percentages varying by pavement layer. To address the concern, rejuvenators, or recycling agents, have been developed to potentially improve the cracking resistance of recycled asphalt mixtures and to more effectively use recycled materials in asphalt mixtures.

Rejuvenators have been used since the 1960’s for pavement preservation treatments (such as fog seals) and have also been used in cold and hot in-place recycling. Recent laboratory studies have shown that rejuvenators or recycling agents can improve the laboratory properties of recycled mixtures, but as yet, the U.S. has little documented experience in their use with hot mix asphalt (HMA) or warm mix asphalt (WMA). A 2014 NAPA study tour of Japan, however, showed that recycling agents are used successfully there to produce mixtures with high recycled contents.

This study, conducted for the Alabama Department of Transportation (ALDOT), is evaluating the results of laboratory tests and the field performance of recycled mixtures with and without rejuvenators. Three 12.5-mm nominal maximum aggregate size (NMAS) mixes, each produced with PG 67-22 virgin binder, were placed on U.S. 31 near Prattville, Alabama. The control mix (no rejuvenator) contained 20% RAP, while the mixes with rejuvenators contained 25% RAP and 5% RAS. Two rejuvenators were evaluated in this study. The respective suppliers recommended the dosages used in the experimental mixes. The first experimental mix used 4.8% of Rejuvenator 1 by weight of total binder while the second experimental mix used 8% of Rejuvenator 2. Both rejuvenators were blended with the virgin binder prior to mixing with the aggregate. Each mix was sampled during production for laboratory testing (binder and mix). Field performance of the test sections continues to be monitored.

To evaluate the effect of the rejuvenators on asphalt binder, the following asphalt blends were graded: virgin binder with and without each rejuvenator (at the recommended amounts used for mix production), recovered binder from each plant-produced mix, and recovered RAP and RAS binders with and without each rejuvenator (at a rate of 10% by weight of RAP/RAS binder). For the recovered RAP/RAS binders, both rejuvenators decreased the high- and low-temperature grades. While Rejuvenator 2 improved the PG grade of RAP binder more effectively than Rejuvenator 1, both rejuvenators had a similar effect on decreasing the high-temperature grade of RAS binder. Compared to RAP, RAS was less sensitive to both rejuvenators. Performance grading of the recovered binders from the plant-produced mixes showed that both rejuvenated 25% RAP/5% RAS mixes were PG 94-10 and the control 20% RAP mix was PG 88-10.

Results of laboratory testing of the three plant-produced mixes were as follows:

All three mixes met ALDOT specifications for gradation and asphalt content (determined using both solvent extraction and ignition oven).

Moisture susceptibility was assessed according to AASHTO T 283. While the control mix had a satisfactory tensile strength ratio (TSR) greater than 80%, both rejuvenated mixes had TSR values less than the recommended minimum of 80%. Thus, both rejuvenated mixes may show greater susceptibility to moisture damage than the control mix.

Rutting resistance was evaluated using the Asphalt Pavement Analyzer (APA). Numerically, the control mix had greater rut depths than the rejuvenated mixes, which both had similar rut depths. Based on a statistical analysis, the control mix had statistically greater APA rut depth results than either rejuvenated mix. Thus, both rejuvenated mixes showed better rutting resistance than the control mix.

To evaluate mix stiffness, dynamic modulus (E*) was determined at three temperatures (4°C, 20°C, and 40°C) and four frequencies (0.01, 0.1, 1, and 10 Hz). Master curves at a reference temperature of 20°C were constructed for all three mixes. As shown in the E* master curves (Figure 1), both rejuvenated mixes had slightly lower moduli than the control mix at higher reduced frequencies (which correspond to lower temperatures) but were stiffer than the control mix at lower reduced frequencies (which correspond to higher temperatures). Thus, the two rejuvenated mixes show higher stiffness at high temperatures, concurring with the APA test results.

Dynamic Modulus Master Curves
Figure 1: Dynamic Modulus Master Curves

The semi-circular bending (SCB) test developed at the Louisiana Transportation Research Center was used to evaluate resistance to intermediate temperature cracking or fatigue cracking. For fracture-resistant mixes, a higher critical value of J-integral (Jc) is desired. The experimental mix with Rejuvenator 2 exhibited the highest Jc value (0.479 kJ/m2), followed by the control mix (0.458 kJ/m2) and the experimental mix with Rejuvenator 1 (0.376 kJ/m2). Thus, the experimental mix with Rejuvenator 2 would be expected to have the best fatigue cracking resistance.

Resistance to reflective cracking was evaluated using the Overlay Tester (OT) in accordance with the Tex-248-F test method used by Texas DOT. The control mix had significantly higher cycles to failure than the rejuvenated mixes, which had statistically equivalent cycles to failure. These results indicate that both of the experimental mixes with 25% RAP/5% RAS could show less resistance to reflective cracking than the 20% RAP control mix.

The U.S. 31 test sections have been monitored for field performance for approximately two years. No distresses (rutting, cracking, moisture damage, or bleeding) were visually evident in any of the three test sections. Field performance of these test sections continues to be monitored to validate the laboratory test results.