As state departments of transportation and paving contractors face ongoing pressure to stretch budgets further without compromising road quality, innovative material solutions are becoming essential. One approach gaining significant traction is the use of granulated tire rubber (GTR) in asphalt mixes. The Georgia Department of Transportation has emerged as a leader in this area, demonstrating that smart strategies to reduce costs without sacrificing quality can be applied to infrastructure as effectively as they can to residential construction. By replacing expensive polymer modifiers with recycled tire rubber, agencies are achieving durable, high-performance pavements at a fraction of the traditional cost.
Following sharp price fluctuations in asphalt and polymers tied to crude oil prices in 2007, the Georgia Department of Transportation began evaluating granulated tire rubber as a modifier in dense-graded asphalt mixes on low-volume highway projects. As the cost of virgin asphalt remained elevated while recycled tire rubber prices stayed stable, the economic case for GTR became increasingly compelling. To date, more than 500,000 tons of GTR-modified asphalt have been placed across Georgia, representing one of the most extensive real-world deployments of rubberized asphalt technology in the United States. As covered in the original GA-DOT case study on For Construction Pros, the agency has proven that rubberized asphalt can perform as well as, or better than, conventional polymer-modified mixes.
The Economics Behind Rubberized Asphalt
The primary driver behind GTR adoption is simple economics. Polymer modifiers such as styrene butadiene styrene (SBS) are petroleum-derived products whose prices fluctuate with the global oil market. When crude oil prices spiked in 2007, the cost of SBS-modified binders rose sharply, straining agency budgets. In contrast, granulated tire rubber is manufactured from recycled scrap tires, a waste stream with stable, lower pricing independent of oil markets.
Cost Comparison at Scale
| Modifier Type | Source Material | Cost Volatility | Relative Cost per Ton of Binder | Supply Chain Risk |
|---|---|---|---|---|
| SBS Polymer | Petrochemical-derived | High (tied to crude oil) | Baseline (reference) | Subject to petrochemical shortages |
| GTR (Granulated Tire Rubber) | Recycled scrap tires | Low (stable pricing) | 20-40% lower | Consistent supply from recycling stream |
| SBS + GTR Hybrid | Blended approach | Moderate | 10-25% lower | Diversified sources |
Peter Wu, P.E., Ph.D., from the Office of Materials and Research at the Georgia Department of Transportation, summarized the agency rationale succinctly: “Granulated tire rubber is a product that provides long-term supply and presents a more cost-effective and competitive way to modify asphalt. The department is always looking for ways to provide taxpayers with a greater value. Rubberized asphalt may be a sustainable solution.” This cost advantage does not come at the expense of performance. In fact, GTR-modified mixes have demonstrated superior crack resistance and durability in laboratory testing and field performance evaluations.
Stable Pricing Through Recycling Streams
The environmental and economic benefits of GTR are closely linked. Each year, millions of scrap tires are generated across the United States. By diverting these tires from landfills and processing them into granulated rubber, the asphalt industry gains access to a modifier that is both sustainable and cost-stable. Companies like Liberty Tire Recycling collect and process scrap tires, while Lehigh Technologies further refines the material into micronized rubber powder (MRP) tailored for asphalt formulation. Ryan Alleman, director of asphalt with Lehigh Technologies, noted: “If Georgia starts using GTR more routinely, it will be a great example for others on how joint technical collaboration throughout the supply chain can lead to better and more cost effective roads.”
How GTR Modification Works in Asphalt Mixes
Understanding the technical process behind GTR modification helps contractors and specifiers evaluate whether this approach fits their project needs. Unlike traditional polymer modification, which requires pre-blending at an asphalt terminal, GTR can be added directly at the hot mix plant using standard equipment. This flexibility reduces logistics costs and allows for on-demand production of modified asphalt. The approach mirrors broader trends in the construction industry, where manufacturers continue to deliver improved performance without sacrificing core capabilities, whether in vehicle design or material technology.
Two Methods of GTR Addition
The Georgia DOT test section on Route 247 in Bibb County evaluated two methods of incorporating GTR into the mix, alongside a control section using conventional SBS-modified binder:
- Fiber distribution equipment method: The GTR and trans-polyoctenamer (TOR) additive are fed into the mixing drum using the fiber distribution equipment already attached to the hot mix plant. This method requires no additional capital investment beyond what most plants already have for fiber addition.
- Mobile blending unit method: The base asphalt is blended with GTR and TOR using a mobile blending unit before being introduced to the mixing drum. This approach provides more controlled blending and may be preferable for projects requiring tighter quality control.
Both methods use a PG 67-22 base asphalt upgraded to a PG 76-22 performance grade. The GTR is added at approximately 11 percent of the weight of the liquid asphalt, while TOR is proportioned at 0.45 percent of the rubber weight. TOR, or trans-polyoctenamer, plays a critical role in the process by improving workability and reducing fumes during production.
The Role of TOR Additive
The inclusion of TOR as a processing aid is one of the technical innovations that makes GTR modification practical for hot mix plants. Wayne Marshall, an asphalt engineer with Reeves Construction Company, which produced the OGFC mix for the Route 247 project, emphasized the operational advantages: “The material is placed with standard paving equipment and is easy to handle. The additive TOR helps workability and reduces fume. The advantage of the plant mix is that we can make a high-quality modified asphalt on site, on-demand, reducing costs to the contractor and ultimately to the owner agency.”
This on-demand production capability eliminates the need for terminal-blended modified binders, which require advance ordering, transportation, and storage. Contractors can adjust the mix design at the plant in real time, responding to changing conditions or performance requirements without waiting for a new batch of pre-blended binder.
Field Performance and Laboratory Validation
The five-mile test section on Route 247, located about 80 miles outside Atlanta, serves as a proving ground for GTR-modified open-graded friction course (OGFC) mixes. OGFC mixes are designed with high air void content to allow water to drain through the pavement surface, reducing hydroplaning risk and improving wet-weather traction. However, OGFC mixes require strong, durable binders to resist raveling and cracking. Early results suggest that GTR-modified binders meet these demanding requirements, much like additives that enable higher RAP content without sacrificing pavement performance in recycled asphalt applications.
Laboratory Testing at NCAT
Material collected from the Route 247 project site was delivered to the National Center for Asphalt Technology (NCAT) at Auburn University for comprehensive testing and evaluation. Richard Willis, assistant research professor at NCAT, described the importance of the research: “NCAT is proud and excited to be a part of this project. It is our goal to help the industry develop asphalt mixtures which are not only environmentally sustainable, but also durable under trafficking. This project is an opportunity for us to get a better understanding of how ground tire rubber truly influences asphalt mixture properties.”
The laboratory study, which was completed by spring 2012, evaluated key performance indicators including:
- Rutting resistance: GTR-modified mixes were tested for their ability to resist permanent deformation under heavy traffic loads.
- Fatigue cracking: Researchers assessed how well the rubberized binders resisted cracking from repeated loading cycles.
- Thermal cracking: Low-temperature performance was evaluated to ensure the mixes could withstand winter conditions without cracking.
- Moisture susceptibility: Tests measured the resistance of GTR mixes to water damage and stripping.
The study found that GTR-modified OGFC mixes performed comparably to, and in some metrics better than, conventional SBS-modified mixes. The rubber particles in the binder provide enhanced elasticity and crack resistance, particularly valuable in OGFC applications where the open-graded structure places more stress on the binder film.
Long-Term Field Monitoring
Beyond the laboratory, the Georgia DOT committed to long-term monitoring of the Route 247 test sections to track real-world performance over multiple years. This dual approach of laboratory validation followed by field verification is a best practice for agencies considering new material technologies. The results from the Georgia program align with findings from other states that have experimented with rubberized asphalt, reinforcing the case for broader adoption. The durability and crack resistance that GTR provides makes it particularly well-suited for OGFC mixes, which demand a binder that can withstand material durability and aesthetic performance that meets the demands of modern construction standards.
Implementation Strategies for Contractors and Agencies
For paving contractors and transportation agencies considering GTR-modified asphalt, the Georgia DOT experience offers practical guidance. The agency methodical approach from low-volume pilot projects to high-traffic interstate applications provides a replicable roadmap for adoption. Key considerations include material sourcing, plant modifications, quality control protocols, and performance monitoring.
Material Sourcing and Supply Chain
Granulated tire rubber is available from multiple suppliers across the country, with established recycling networks ensuring consistent supply. Agencies should evaluate potential suppliers based on rubber particle size distribution, consistency, and the presence of fiber or steel contaminants. The GTR used in the Georgia project was sourced through a supply chain that includes Liberty Tire Recycling for scrap tire collection and Lehigh Technologies for processing into micronized rubber powder.
Plant Equipment and Modifications
One of the most attractive features of GTR modification is the minimal capital investment required. As demonstrated on the Route 247 project, existing fiber distribution equipment can be used to add GTR and TOR into the mixing drum. Contractors who do not have fiber distribution equipment can opt for the mobile blending unit method, which adds GTR and TOR to the base asphalt before it enters the mixing drum. Both approaches use standard paving equipment for placement, meaning no specialized laydown machinery is required.
Quality Control and Testing
Effective quality control for GTR-modified mixes requires attention to several parameters:
| Parameter | Target Range | Testing Frequency | Impact on Performance |
|---|---|---|---|
| GTR content (% of binder weight) | 10-12% | Per production shift | Modification level and elasticity |
| TOR content (% of rubber weight) | 0.4-0.5% | Per production shift | Workability and fume reduction |
| Base binder PG grade | PG 64-22 to PG 67-22 | Per binder delivery | Starting point for grade bumping |
| Final binder PG grade | PG 76-22 (target) | Per project | High-temperature rut resistance |
| Mixing temperature | 300-340 F | Continuous | Rubber incorporation and workability |
Contractors should establish a quality control plan that includes regular sampling and testing of both the raw materials and the finished mix. The laboratory results from NCAT provide a benchmark for expected performance, but local materials and conditions may require adjustments to the mix design.
Performance Monitoring and Lifecycle Analysis
Agencies adopting GTR-modified asphalt should plan for both short-term quality assurance testing and long-term performance monitoring. The Georgia DOT approach of combining initial laboratory testing with ongoing field evaluation provides a model for data-driven decision making. Performance indicators to track include rut depth, cracking extent, ride quality, and friction numbers over time. When these metrics are compared against conventional sections, agencies can calculate the true lifecycle cost benefit of GTR modification.
Conclusion
The Georgia Department of Transportation experience with granulated tire rubber modified asphalt demonstrates that cost savings and pavement performance are not mutually exclusive. By leveraging recycled tire rubber as a binder modifier, the agency has placed more than 500,000 tons of durable, high-performance asphalt while reducing dependence on expensive, petroleum-derived polymers. The Route 247 test section and supporting NCAT research provide the technical validation needed for other agencies and contractors to adopt this approach with confidence. For construction professionals seeking materials that balance budget constraints with performance requirements, rubberized asphalt represents a proven solution that continues to gain momentum. As the industry pushes toward more sustainable practices, the ability to achieve enhanced capability without sacrificing core functionality will define the next generation of pavement technology. The GA-DOT model, built on collaboration among state agencies, contractors, suppliers, and research institutions, offers a replicable path forward for the entire industry.