THE KEY TO UNLOCKING RAP’S POTENTIAL

By: Dr. Krishna Srinivasan

In recent years, our industry has made great strides toward embracing reclaimed asphalt pavement (RAP). Many authorities now encourage or mandate its use, and the number of RAP-bearing roadways around the world increases every year.

The reason for this enthusiasm is clear: asphalt recycling is a rare example of a practice that’s truly mutually beneficial for business and sustainability. For mix producers, incorporating reclaimed pavement allows them to save on their biggest expense — raw materials. For the environment, it prevents old roads from contributing to waste and reduces emissions by minimizing demand for carbon-intensive virgin materials.

However, there is still a lot of progress to be made. A staggering 750 million tons of RAP are generated globally each year, but less than 20 per cent of it is recycled. Most of the rest is shipped to landfills.

One obstacle to adoption is uncertainty surrounding the performance of highRAP roadways. Most experts agree that pavements with RAP levels above 25 per cent require a rejuvenator or recycling agent to rebalance the properties of the aged RAP bitumen. From cooking oil to soft bitumen to petroleum products, a wide range of options promise to optimize aged bitumen for new pavements. Yet, their outcomes are equally diverse.

Given this variability, it’s useful to define what makes an asphalt rejuvenator truly effective. Unlike a softener, which simply reduces the viscosity of bitumen, an asphalt rejuvenator (AR) or recycling agent also restores its rheological properties and aging behaviour to that of virgin bitumen. An effective rejuvenator must do all of the following:

1. Soften the bitumen;

2. Improve the properties of the bitumen; and

3. Deliver excellent roadway performance and durability.

More than a decade of research and development at Sripath Technologies led to ReLIXER, an asphalt rejuvenator made from bio-based oils. Through the lens of ReLIXER, we can see the many ways that an effective rejuvenator impacts high-RAP mix properties. We can also compare available rejuvenators and view practical examples that showcase the incredible potential of high-RAP mixes when combined with an effective rejuvenator.

Balancing Rutting Resistance And Fracture Toughness

Balanced Mix Design (BMD) is an approach to mix formulation that seeks to balance qualities like rutting resistance and fracture toughness while maintaining cost efficiency. The results in Table 1 demonstrate how an effective rejuvenator can be a useful tool for BMD in RAP mixes. In a study conducted on a U.S. roadway, two high-RAP mixes were tested as wear layers: one with 25 per cent RAP and another with 50 per cent RAP. The 25 per cent RAP mix contained 0.5 kg per ton of ReLIXER rejuvenator and 3.4 per cent virgin bitumen PG 64-22. The 50 per cent RAP mix incorporated 1 kg per ton of ReLIXER and 2.4 per cent virgin PG 64-22. The target for total liquid volume was around 5.6 per cent. By properly adjusting the amount of rejuvenator in each mix, we achieved an excellent balance between rutting resistance and fracture toughness.

Restoring Low Temperature Properties

One of the most important features of an effective rejuvenator is the ability to restore low temperature properties to reclaimed binder. This is a capability that varies widely among commercially available rejuvenators. In a comparative study, samples of a PG 58-20 base binder were combined with three different rejuvenators: Sripath’s bio-based ReLIXER and two other petrochemical-based, binder-softening oils from India and Australia. ΔTc values were measured for each sample. As shown in Figure 1, the ΔTc value for the binder with 5 per cent ReLIXER was comparable to that of the base binder, whereas the ΔTc value for the petrochemical-oil based rejuvenators was significantly lower. This indicates that ReLIXER imparts much greater durability in the face of temperature stress.

DURABLE HIGH-RAP PAVEMENT

The aging behaviour of RAP pavements is highly dependent on the type of rejuvenator used in the mix. Our results show that ReLIXER-dosed mixes maintain exceptional long-term durability, outperforming mixes made with alternative rejuvenators and even matching noRAP mixes made with entirely virgin binder. Figure 2 illustrates a study which assessed fracture toughness after aging for mixes containing 0 per cent, 30 per cent, or 50 per cent RAP. The RAP mixes were dosed with either 5 per cent ReLIXER or 10 per cent petrochemical oils. All groups underwent loose mix oven aging for 4, 8, or 16 hours. The aged samples were then evaluated using the IDEAL-CT Index, showing that ReLIXER-enhanced mixes had superior fracture toughness at both 30 per cent and 50 per cent RAP relative to mixes dosed with petro-oils. Remarkably, the high-RAP ReLIXER mixes performed on par with no-RAP mixes, demonstrating immense restorative capability.

Moisture Resistance

ReLIXER imparts moisture resistance to pavement, which improves its longevity. As summarized in Figure 3, the Tensile Strength Ratio (TSR) was measured to compare the moisture resistance of two asphalt mixes. The control mix, based on the Australian multi-grade M1000 binder with 15 per cent RAP, had a TSR value of 70 per cent. In comparison, a mix based on the M1000 binder with 40 per cent RAP and dosed with 3.2 per cent ReLIXER had a TSR value of 85 per cent.

Weight Loss

Stripping Resistance

Figure 4 shows the percentage Cantabro Weight Loss (CWL) for three asphalt mixes: (1) a base mix containing 20 per cent RAP; (2) a mix with 40 per cent RAP and 5 per cent AR-B6 rejuvenator; and (3) a mix with 40 per cent RAP and 5 per cent ReLIXER. All three mixes were based on a PG 58-28 grade binder. The base mix and the AR-B6 dosed mix had a similar stripping resistance, showing a 7.6 per cent and 7.7 per cent CWL, respectively. The ReLIXER-dosed mix held on more

Comparison To Soft Grade Bitumen

While “softer” grades of bitumen are commonly used to rejuvenate high-RAP mixes, the methods used to achieve this softening can substantially affect the mix’s final properties. Figure 5 depicts the aging behaviour of a base bitumen, a rejuvenated bitumen dosed with ReLIXER, and a softer grade bitumen obtained from the same source as the base bitumen. Samples of each were aged in a Pressure Aging Vessel (PAV) and subjected to Bending Beam Rheometer (BBR) tests to determine their ΔTc values. As seen in the figure, the rejuvenated sample closely maintains the ΔTc characteristics of the base bitumen, while the softer grade bitumen has an unacceptably high ΔTc value after aging, demonstrating that the ReLIXER-rejuvenated binders had better aging characteristics than this softer grade binder. ››

Better than “Soft” Bitumen

high-RAP mixes along with ReLIXER for base-course paving applications, not just in the north but throughout India.

In Dublin, Ireland, Roadstone Ltd. conducted a field trial on a roadway to evaluate ReLIXER’s impact on a 60  per cent RAP mix. This trial considered two dense base mixes with 40/60 grade bitumen: AC20 and AC32. A control mix containing no RAP or rejuvenator and a ReLIXER-dosed mix with 60 per cent RAP were prepared for each mix type.

Case Studies

One case study on a 100 per cent RAP roadway in New York City exemplifies the incredible potential of high-RAP mixes when paired with an effective rejuvenator. A stretch of road in New York City that is subject to very high traffic volume, stopand-go traffic, and extreme weather patterns was paved using a 100 per cent RAP mix. ReLIXER was sprayed directly onto RAP particles at a dosage of 0.55 per cent by weight of the mix, with no virgin binder or aggregates added. Field core samples were obtained nine months after paving. The results of the laboratory tests and field core sample tests from this trial are summarized in Table 2 and Table 3.

Despite using entirely reclaimed materials, the road met all specifications established by local authorities. The trial mix had 3.9 per cent Marshall Air Voids, stability of 2110 kN, and flow of 9.9 mm. Rutting resistance was 10.9 mm at 10K cycles and 14.6 mm at 20K cycles. The Semi-Circular Bend (SCB) fracture toughness of cored specimens after one year of service had a JC value of 0.71 kJ/m 2 at 25°C.

Meanwhile, a contractor in northern India laid down a DBM II base course with two RAP mixes: one with 50 per cent RAP and 0.2 per cent ReLIXER and the other with 60 per cent RAP and 0.3 per cent ReLIXER. Respectively, the results showed Marshall Air Voids of 3.1 per cent and 3.23 per cent; Marshall Stability values of 15.2kN and 15.1 kN; and flow measurements of 3.1 mm and 3.2 mm. The customer now routinely uses

Shown in Table 4, mixes with 60 per cent RAP and ReLIXER had improved compaction levels for both AC20 and AC32. Bulk density and air voids also stayed at sufficient levels after the addition of rejuvenated RAP. Indirect Tensile Strength Ratio (ITSR) values were very similar for high-RAP and no-RAP mixes, indicating acceptable moisture resistance in the rejuvenated mix. HighRAP mixes displayed acceptable Wheel Tracking Slope (WTS) and stiffness results compared to no-RAP control mixes. The addition of the rejuvenator restored penetration of the bitumen extracted from high-RAP mixes to values close to the no-RAP extracted bitumen. The virgin 40/60 bitumen showed a penetration of 51 dmm. By using fewer virgin materials, the contractor was able to make a substantial dent in their CO 2 emissions. All mixes tested met specifications established by Ireland’s public transport agency.

Summary

These data underscore the importance of effective rejuvenation in harnessing the full potential of RAP. Through laboratory evaluations and field studies, one can observe the tangible benefits that can be derived from high-RAP mixes when paired with an effective rejuvenator. From improved mix properties to enhanced road performance, these findings provide a clear roadmap. As the industry continues to lean towards sustainable practices, it’s crucial to base decisions on robust technical evaluations. With a focus on maximizing RAP use and ensuring optimal rejuvenation, the path forward promises both economic and environmental advantages.