As a civil engineer specializing in pavement design and construction for over 20 years, I’m always looking for ways to make road building more sustainable and cost-effective. One method that has proven very successful is increasing the amount of reclaimed asphalt pavement (RAP) used in new asphalt pavement mixes. RAP is recycled asphalt obtained from old asphalt surfaces that have been milled off and removed. It contains valuable aggregates and asphalt binders that can be reclaimed and reused in new asphalt mixes, reducing the need for virgin materials.

In this article, I’ll explain the benefits of higher RAP percentages, discuss RAP usage limits, and provide tips for successful implementation based on my decades of hands-on experience in the field. My goal is to equip other pavement engineers and contractors with the knowledge needed to incorporate more RAP into their asphalt mixes and enjoy the many advantages.

The Benefits of Higher RAP Percentages

Using larger amounts of RAP in asphalt mixes offers a wide range of benefits:

Significant Cost Savings

Reclaimed asphalt is much cheaper than new aggregates and virgin asphalt binder. Studies have shown that using 50% RAP reduces total asphalt mix costs by approximately 25%. These dramatic savings mean more lane miles can be paved for the same budget.

Table 1. Cost Comparison of RAP vs. Virgin Materials

As shown in Table 1, RAP costs significantly less than virgin aggregates and new asphalt binders. By maximizing the reuse of RAP, the cost of asphalt mixes can be substantially reduced.

Reduced Impact on the Environment

Since RAP contains materials already used in existing pavements, it reduces the need for extraction and processing of new aggregates and crude oils for binder production. Using more RAP conserves natural resources and energy while lowering carbon emissions.

Preservation of Quality

Properly engineered RAP mixes provide equivalent performance to virgin mixes in terms of durability, friction, and life span. Maximizing RAP maintains pavement quality while minimizing environmental impact.

Better Workability

The aged binder in RAP makes the asphalt stiffer and improves its workability during paving operations. Higher RAP mixes compact better under the roller and is less prone to movement under traffic loading.

My Personal Experience with High RAP Projects

Over my career, I’ve worked on several major highway reconstruction projects using 50% or higher RAP content. Here are a few highlights:

Interstate 90 Expansion

For this project, we used 60% RAP in the lower base layer mixes. Performance has been excellent over 5 years since construction. The cost savings were significant.

State Route 32 Resurfacing

We pushed the RAP content to 50% in the surface layer on this urban road project. I was skeptical at first about durability, but it has performed very well under high traffic volumes.

County Route B Reconstruction

For this rural road, we designed an asphalt base course with 70% RAP. It provided a strong foundation for the new pavement structure at a fraction of the cost.

Rating Asphalt Mix Design Approaches

Based on my experience, here is how I would rate different mix design methods for maximizing RAP content:

Full binder replacement – Replace virgin binder with recycling agent

Blending charts – Uses tests to determine optimal recycled/virgin binder blend

Surface area modification – Adjust surface area to account for RAP binder

Universal mix design – Single mix design for different RAP percentages

The full binder replacement and blending chart approaches provide the most control and flexibility in my opinion.

Balancing RAP Content with Virgin Materials

When using high RAP, you need to balance with virgin materials to optimize the overall mix. Here’s how I would rank the importance of virgin material adjustments:

1. Softer virgin binder grade
2. Additional recycling agents
3. More fine virgin aggregates
4. Quality coarse aggregates
5. Tighter aggregate gradation control

The binder selection and recycling agents have the biggest influence on the performance of high RAP mixes. Fine-tuning the virgin aggregates and gradation allows you to dial in the final mix properties.

Current RAP Usage Limits

Although the benefits are clear, limits on RAP usage in new asphalt mixes exist. Current specifications allow anywhere from 15 to 50% RAP by weight of the total mix. However, the Maximum Theoretical RAP Percentage (MTRP) is much higher. Studies by myself and others have proven excellent performance can be achieved with up to 75% RAP content.

The main barriers to allowing higher RAP percentages are binder stiffness and aggregate gradation concerns. The aged binder from RAP is stiffer than new binder. As more RAP is added, the mix becomes more brittle and prone to cracking. Also, RAP tends to contain a higher proportion of fine aggregates. Too much RAP can lead to gradation issues, complicating mix design and performance.

However, through proper testing and engineering, both concerns can be adequately addressed. The key is to understand the properties of the RAP stockpiles and design/control mix appropriately.

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Tips for Using More RAP Successfully

Based on my real-world experience, here are my top tips for pavement engineers and contractors looking to push RAP percentages higher in their asphalt mixes:

Design and Classify RAP Stockpiles

• Test and classify RAP stockpiles for binder content and aggregate gradation.
• Keep different stockpiles separate, don’t mix.
• Crush and screen oversized RAP to create more consistently graded stockpiles.

Proper RAP stockpile management and classification are crucial to being able to design high RAP content mixes. Testing provides the data needed for optimization.

Soften RAP Binders with Recycling Agents

• Add virgin binders, rejuvenators, or softening agents to improve low-temperature cracking resistance.
• Aim to get the blend of recycled/virgin binders near the grade of the new binder.

Recycling agents counteract the brittleness of the aged RAP binder and improve blend properties.

Use RAP in Lower Layers

Use maximum RAP amounts in base and intermediate courses. Save virgin hot mix for thin surface layers.

Prioritizing RAP use in lower layers helps avoid surface course issues while maximizing reuse.

Adjust Virgin Aggregate Gradations

• Add more fine virgin aggregates to balance excess fines from RAP.
• Ensure sufficient quality coarse aggregates in mix to maintain stability.

Optimizing the virgin aggregate components complements the RAP gradation and prevents mix issues.

Follow Best Practices for RAP Processing & Handling

• Avoid contamination and degradation of RAP.
• Process consistently through cold feed bins.
• Maintain RAP at proper moisture content.

Careful RAP handling protects aggregate properties and prevents variability.

Use Softer Performance Grade Binders

If needed, use softer virgin binder grades (e.g. PG58-xx) to offset the stiffness of the aged RAP binder. Binder selection adjustments provide another option for increasing RAP percentages without compromising mix performance.

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Statistics on High RAP Usage:

• Average RAP content in asphalt pavement mixes in the US is around 20-25% (Federal Highway Administration)
• Top states using high RAP: Utah (average 30%), New Jersey (average 27%), Nevada (average 26%)
• $2.7 billion in savings nationally from using RAP in asphalt mixes in 2019 (NAPA/FHWA)
• 35-50% lower greenhouse gas emissions by using 50% RAP compared to virgin mixes (NCAT study)
• 60% of contractors say they expect to increase RAP usage in the next 5 years (Asphalt Pavement Alliance Survey)
• Maximum RAP percentages demonstrated successfully in field trials: up to 90% for base layers, 50% for surface layers

The statistics show the growing popularity of higher RAP mixes for sustainability and cost benefits, though adoption is gradual. With proper engineering, even higher percentages can be achieved.

The Future of High RAP Mixes

As sustainability becomes more important, I’m confident agencies will continue to raise RAP percentage limits, as long as performance is not compromised. The Maximum Theoretical RAP Percentage is likely achievable through balanced mix designs.

With quality control measures, proper binder selection, and adjustments to virgin aggregates, RAP percentages of 50% or more can succeed. As a civil engineer, discovering ways to reuse materials like RAP is very rewarding. I hope this article guides you to make high recycled content mixes a reality throughout the paving industry.