Asphalt producers and paving contractors face a persistent challenge: how to increase recycled asphalt pavement (RAP) content without compromising pavement performance. State specifications typically cap RAP at 25 to 30 percent binder replacement for surface courses, largely because aged, oxidized RAP binder creates a brittle mix prone to cracking. A recent field trial by E&B Paving in Indianapolis, working with researchers from Iowa State University and Colorbiotics, demonstrated that a soybean oil-derived rejuvenator called Invigorate can push RAP content to 40 percent while maintaining workability and durability. For contractors seeking to maximize material reuse and reduce costs, understanding the approach behind this trial offers practical lessons in advanced mix design. Those involved in similar projects may also benefit from reviewing Paving Utility Cuts Paths and Parking Lots Best for broader small-commercial paving strategies.
The Economics and Chemistry of Higher RAP Mixes
The push for increased RAP usage is driven by both economics and sustainability. According to the National Asphalt Pavement Association (NAPA), using RAP saves American taxpayers and consumers more than $2.8 billion annually. Every ton of RAP used in place of virgin materials reduces the demand for virgin aggregates and liquid asphalt binder, lowering project costs and conserving natural resources.
Why RAP Content Is Capped
Despite these benefits, increasing RAP content introduces technical limitations. RAP binder is aged, oxidized, and brittle. When used at high percentages without treatment, it produces a stiff mix that is difficult to work with during placement and prone to premature cracking under traffic and thermal cycling. Elizabeth Pastuszka, executive director of the Asphalt Pavement Association of Indiana (APAI), explains that in Indiana, mixes are limited to a maximum of 25 percent binder replacement. This creates a surplus problem: while 100 percent of the existing road surface can be milled and returned to the plant, only a fraction can be put back into new mixes.
The result is a growing stockpile of RAP at many plants. Producers must find ways to either incorporate more RAP into their mixes or manage excess inventory through other channels. The key barrier remains binder performance: without effective rejuvenation, high-RAP mixes simply do not meet the cracking and rutting requirements specified by transportation agencies.
Understanding Binder Chemistry
Asphalt binder consists of two primary components:
- Asphaltenes – high-molecular-weight compounds that provide structural integrity and resistance to rutting
- Maltenes – oily and resinous components that give the binder its liquid properties, enabling workability and aggregate coating
As binder ages, the ratio of asphaltenes to maltenes increases. The asphaltene molecules aggregate into larger clusters, making the binder stiff and brittle. Traditional rejuvenators address this imbalance by adding maltenes back into the blend, essentially diluting the aged binder with softer material. While effective to a degree, this approach has limits: adding enough maltenes to fully restore workability at high RAP contents can soften the binder too much, compromising rut resistance.
Invigorate: A Chemically Active Rejuvenator
Researchers at Iowa State University (ISU), in collaboration with Ames-based Colorbiotics (a brand of the MBCC Group), developed a new type of rejuvenator that works differently from traditional softening agents. Rather than simply adding maltenes, Invigorate triggers a chemical reaction that breaks down asphaltene aggregation at the molecular level, reversing the aging process rather than compensating for it.
How Invigorate Works
The soybean oil-derived formulation penetrates the aged binder and disruptes the molecular forces holding asphaltene clusters together. This restores the balance between asphaltenes and maltenes without over-softening the binder. Ken Staebell, director of new technology development at Colorbiotics, describes the mechanism: the rejuvenator is highly compatible with asphalt binder, so blending and homogeneity are achieved easily. The result is a rejuvenated binder that retains its structural integrity while regaining the workability characteristics of virgin binder.
Key advantages of this approach include:
- Higher RAP content without proportional loss of low-temperature cracking resistance
- Improved workability during placement and compaction
- Compatibility with standard plant equipment and blending procedures
- Reduced dependency on virgin binder, lowering material costs
Application Methods
Invigorate can be incorporated into mixes through multiple methods. Because of its high compatibility with asphalt binder, it can be blended directly into the liquid asphalt supply at the plant, added at the mixer, or introduced via a separate metering system. The recommended addition rate depends on the RAP content and the target performance grade of the final binder. For the E&B Paving trial, the rejuvenator was added at 4.2 percent by volume of total binder.
The Indianapolis Parking Lot Field Trial
E&B Paving, a contractor based in central Indiana, partnered with Iowa State University and Colorbiotics to test the Invigorate rejuvenator on a commercial parking lot application in Indianapolis. The project was designed to evaluate whether a 40 percent RAP surface mix could meet the same performance standards as conventional lower-RAP mixes under real-world paving conditions.
Mix Design and Production
The trial involved approximately 300 tons of a 9.5 mm surface mix containing 40 percent RAP with Invigorate, plus a second experimental biopolymer mix for comparison. The target gradation and asphalt content were developed through standard mix design procedures with the goal of achieving a balanced mix that included proper aggregates, baghouse fines, liquid AC, and RAP material meeting Superpave specifications.
| Parameter | Value |
|---|---|
| Mix type | 9.5 mm surface course |
| RAP content | 40% |
| Virgin binder grade | PG 64-22 |
| Invigorate dosage | 4.2% by volume of total binder |
| Production temperature | 285-300 °F |
| Plant type | Counterflow drum plant |
| Production location | E&B Stoney Creek plant, Noblesville, IN |
| Haul distance | Approximately 30 minutes |
| Total tonnage | ~300 tons per mix |
Site Conditions and Paving Challenges
The parking lot surrounding a multi-tenant office building presented several challenges typical of tight commercial paving projects:
- Short pulls – the paver had to stop and start frequently due to the U-shaped lot layout
- Varying paving widths – the lot width changed along the alignment, requiring constant screed adjustments
- Inverted drains – drainage structures required careful hand work around utilities
- Limited working room – the lot was bounded by the building and adjacent parking areas
Prior to paving, the top 1.5 inches of the existing asphalt surface was milled and recycled. The crew then placed a 1.5-inch overlay of the three different mix designs, achieving target breakdown, intermediate, and finish rolling patterns for compaction.
Crew Observations on Workability
Stephen Perkey, paving foreman in his 12th season with E&B Paving, noted that tight lots are among the most demanding paving applications due to offset paving, inverted drains, short pulls, and extensive hand work. Despite the 40 percent RAP content, the Invigorate mix handled remarkably well.
Perkey observed that high-RAP mixes often turn stiff and lose temperature rapidly, becoming difficult to work before achieving proper compaction. With the Invigorate mix, the material stayed workable throughout the placement window. The crew achieved densities exceeding 95 percent using their standard rolling pattern, indicating that the rejuvenator restored adequate binder flexibility even at the elevated RAP content.
Performance Testing and Quality Verification
Laboratory testing of the field-produced mix confirmed that the 40 percent RAP mix with Invigorate met or exceeded standard performance benchmarks for both rutting and cracking resistance.
Hamburg Wheel Tracking Test
The Hamburg Wheel Tracking Test evaluates rutting resistance and moisture susceptibility by rolling a steel wheel across compacted specimens submerged in hot water. The Invigorate mix performed well in this test, showing minimal rut depth and good resistance to moisture damage. This confirms that the rejuvenator did not over-soften the binder, preserving its high-temperature stability.
Cracking Tests: CT Index and I-FIT
Two cracking tests were used to evaluate the low-temperature and fatigue performance of the mix:
- CT Index – measures cracking tolerance at intermediate temperatures
- Illinois Flexibility Index (I-FIT) – evaluates flexibility and cracking resistance of compacted specimens
The Invigorate mix achieved strong results in both tests. Pastuszka noted that the CT Index values were encouraging, while the I-FIT results were more than acceptable, providing confidence that the 40 percent RAP mix would perform well in the field over the long term. Researchers at Iowa State University also observed that these results confirm Invigorate offers producers a viable solution for running more RAP without sacrificing pavement performance.
Performance Comparison Summary
| Property | Test Method | Result |
|---|---|---|
| Rutting resistance | Hamburg Wheel Tracking | Passed – minimal rut depth |
| Cracking tolerance | CT Index | Strong values achieved |
| Flexibility | I-FIT | Acceptable performance |
| Field density | In-place compaction | >95% achieved |
| Workability | Crew observation | Good – no premature stiffening |
APAI’s Pauley, who observed the paving operation firsthand, commented that the finished surface looked no different from standard mixes. This visual assessment, combined with the laboratory and field data, supports the conclusion that chemically active rejuvenators can enable higher RAP content without compromising the quality that contractors and agencies expect.
Practical Implications for Producers and Contractors
The E&B Paving trial demonstrates that 40 percent RAP content is achievable in surface course mixes when the right rejuvenation chemistry is applied. For asphalt producers managing growing RAP stockpiles, this opens the door to higher utilization rates and reduced virgin material consumption. For contractors, the improved workability observed during the trial means less risk of compaction issues and smoother ride surfaces.
Key Considerations for Adoption
Contractors and producers evaluating high-RAP mixes with rejuvenators should consider the following factors:
- RAP variability – aged binder properties vary by source; mix designs should be verified for each RAP stockpile
- Rejuvenator dosage – optimal addition rate depends on target RAP content and desired binder performance grade
- Plant integration – ensure the rejuvenator can be metered accurately into the mix at the required rate
- Quality control – implement Hamburg and I-FIT testing to verify that high-RAP mixes meet specification requirements
- Agency approval – work with local DOT or agency representatives to obtain approval for RAP content above standard caps
The role of aggregate quality in pavement performance is also critical. Asphalt producers should ensure that all materials, including sand fractions, meet specification limits for cleanliness. For guidance on this topic, see Silt Content Costruction Sand, which covers the impact of excessive fines on mix durability.
For mix designers aiming to push RAP content further, understanding how rejuvenators interact with aged binder is essential. The Iowa State University research demonstrates that chemically active rejuvenators such as Invigorate can restore aged binder properties more effectively than traditional softening agents. Contractors interested in this technology can review Reviving Recycled Asphalt How Additives Enable Higher Rap for a deeper look at additive-based approaches to RAP mix design.
Moisture and material testing are also relevant for quality assurance in paving projects. When compaction testing involves moisture-density relationships, the torsion balance method provides reliable results for determining water content in soils and aggregates. See Determination of Water Content in Soil By Torsion for more information.
Future Outlook
As state DOTs and federal agencies push toward greater sustainability in pavement construction, the demand for high-RAP technologies will continue to grow. Chemically active rejuvenators represent a significant advancement over traditional approaches, offering a path to higher recycling rates without compromising pavement performance. The E&B Paving trial provides real-world evidence that 40 percent RAP content is not only possible but practical for commercial paving applications.
With continued research and field validation, the 25 percent RAP cap that many producers currently work within may become a thing of the past. For now, the combination of chemically active rejuvenators, sound mix design, and quality control testing offers a reliable framework for contractors ready to push beyond current limits.