📄 chemistry

Rheological Benchmarking of GTR-Modified Asphalt Binder as a Sustainable Alternative to SBS Modification

This study demonstrates that incorporating 10% ground tyre rubber (GTR) into AC-20 bitumen yields rheological performance comparable to 1% SBS-modified binder, establishing GTR as a viable and sustainable alternative to the more expensive proprietary SBS modification.

Original authors: Michael Suma Appiah, Kenneth Adomako Tutu, Opeyemi Antoinette Gbadewole, Ahmed Kudu Mohammed, Yaw Adubofour Tuffour, Daniel Atuah Obeng

Published 2026-06-25

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CC BY 4.0

Original authors: Michael Suma Appiah, Kenneth Adomako Tutu, Opeyemi Antoinette Gbadewole, Ahmed Kudu Mohammed, Yaw Adubofour Tuffour, Daniel Atuah Obeng

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are baking a special kind of "road cake" (asphalt) that needs to be tough enough to handle heavy trucks without melting in the summer sun, yet flexible enough to crack in the winter without breaking apart.

For a long time, the secret ingredient to make this road cake super-strong has been a fancy, expensive plastic called SBS (Styrene-Butadiene-Styrene). It's like the "premium chocolate" of the road world: it works great, but it costs a lot of money because it's a proprietary product.

On the other hand, there's a waste product called GTR (Ground Tyre Rubber), which is just old tires chopped up into tiny bits. It's free (or very cheap) and helps the environment by keeping tires out of landfills. But, road builders in some places haven't been sure if GTR is actually as good as the expensive SBS. They've been hesitant to use it because they didn't have a clear "recipe" to compare the two.

The Big Experiment
The researchers in this paper decided to play the role of "road chefs" to find the perfect recipe. They took a standard road binder (AC-20) and started adding different amounts of chopped-up tire rubber (from 2% up to 12%). They then compared these "tire-rubber cakes" against the "premium chocolate cakes" (SBS-modified binders) to see which one performed better.

Here is what they found, broken down simply:

1. The Heat Test (Rutting)

The Problem: In hot weather, soft asphalt can get squished by heavy trucks, leaving permanent ruts (like a tire track stuck in mud).
The Test: They heated the binders up to see how well they held their shape.
The Result: Adding more tire rubber made the binder stiffer and better at resisting heat.
- The Sweet Spot: When they added 10% tire rubber, the binder became just as tough against heat as the binder with 1% of the expensive SBS.
- The Warning: When they added 12% tire rubber, it got too sensitive. It was like a rubber band that snaps if you pull it too hard; it reacted too wildly to different weights. So, 12% was too much, but 10% was perfect.

2. The Fatigue Test (Cracking from Traffic)

The Problem: Over years of traffic, roads get tired and develop cracks, similar to how a piece of metal bends back and forth until it breaks.
The Test: They simulated years of traffic aging and checked how well the binder resisted cracking.
The Result: The 10% tire rubber binder was a superstar here. It was actually better at resisting fatigue cracks than the binder with 1% SBS. In fact, it performed so well that it beat even the binders with 2% and 3% SBS in some tests. It's like the tire rubber gave the road an extra layer of "elasticity" that helped it bounce back from stress.

3. The Cold Test (Thermal Cracking)

The Problem: In freezing weather, asphalt can get too hard and brittle, cracking like a frozen cookie.
The Test: They cooled the binders down to see if they stayed flexible.
The Result: The 10% tire rubber binder stayed more flexible in the cold than the SBS binders. The SBS binders got stiffer (harder) in the cold, making them more likely to crack. The tire rubber kept the road "soft" enough to handle the freeze without breaking.

4. The "Magic" Check (Chemistry)

The Question: Did the tire rubber chemically change the asphalt into something new, or was it just mixing in physically?
The Tool: They used a special scanner (FTIR) to look at the molecular structure.
The Result: The scanner showed no chemical changes. The tire rubber didn't turn into a new chemical compound; it just physically mixed in, like adding chocolate chips to cookie dough. The improvement came from the rubber particles physically interacting with the asphalt, making it stretchier and stronger without changing its fundamental recipe.

The Final Verdict

The study concludes that you don't need the expensive "premium chocolate" (SBS) to get a high-performance road. If you take a standard road binder and mix in 10% ground-up tire rubber, you get a product that:

Handles heat as well as the expensive option.
Resists traffic fatigue even better than the expensive option.
Handles cold weather better than the expensive option.
Is much cheaper and helps the environment by recycling waste tires.

In short: The researchers found the "Goldilocks" amount of tire rubber (10%) that makes a road binder perform just as well as, or better than, the costly commercial alternative, proving that waste tires can be a sustainable and effective super-ingredient for our roads.

Technical Summary: Rheological Benchmarking of GTR-Modified Asphalt Binder

Problem Statement
Styrene-butadiene-styrene (SBS) is a widely used petroleum-based thermoplastic elastomer for modifying bitumen to enhance rheological properties, specifically improving resistance to cracking and rutting under heavy traffic and challenging environmental conditions. However, SBS is a proprietary product with high costs that can significantly increase construction project expenses. Conversely, Ground Tyre Rubber (GTR), derived from waste tyres, offers a sustainable, cost-effective alternative that improves asphalt performance and reduces lifecycle greenhouse gas emissions. Despite these potential benefits, GTR-modified binders have not seen widespread application in certain regions, partly due to a limited empirical understanding of how their performance characteristics compare to established SBS-modified binders. There is a lack of comparative research benchmarking GTR against SBS to determine if GTR can serve as a viable substitute in high-performance pavement applications.

Methodology
The study aimed to identify an optimum GTR dosage for an AC-20 graded asphalt binder that yields rheological characteristics comparable to commercial SBS-modified binders.

Materials: The researchers used an AC-20 base bitumen. GTR was produced from a six-year-old scrap Yokohama tyre, processed into particles ranging from 75 µm to 300 µm. Commercial SBS-modified binders containing 1%, 2%, and 3% SBS (corresponding to Superpave grades PG 70-22, PG 76-22, and PG 82-22) served as controls.
Preparation: GTR was blended into the AC-20 bitumen at dosages from 2% to 12% (in 2% increments) using a high-speed shear mixer (5000 rpm for 45 minutes), followed by a digestion period at 165°C for 2 hours.
Conditioning: Samples underwent Superpave aging conditioning, including Rolling Thin Film Oven (RTFO) for short-term aging and Pressure Aging Vessel (PAV) for long-term aging.
Testing: Rheological characterization was conducted according to AASHTO and ASTM standards:
- Rutting Resistance: Evaluated using the Dynamic Shear Rheometer (DSR) for complex shear modulus ( $|G^*|/\sin\delta$ ) on unaged and RTFO-aged samples, and the Multiple Stress Creep Recovery (MSCR) test (AASHTO M332) to determine non-recoverable creep compliance ( $J_{nr}$ ) and stress sensitivity.
- Fatigue Cracking: Assessed via DSR on PAV-aged samples using the fatigue parameter ( $|G^*|\cdot\sin\delta$ ).
- Thermal Cracking: Evaluated using the Bending Beam Rheometer (BBR) to measure stiffness ( $S$ ) and rate of stiffness change ( $m$ -value).
- Chemical Analysis: Fourier Transform Infrared (FTIR) spectroscopy was performed to detect molecular structural changes or chemical interactions.

Key Results

Rutting Resistance: Increasing GTR dosage improved rutting resistance. At 70°C, the 10% (G10) and 12% (G12) GTR-modified binders met the Superpave rutting criterion for unaged binders, performing comparably to the SBS-modified binders. In MSCR tests on RTFO-aged samples, G10 and G12 achieved "Extreme" (E) traffic loading grades, similar to all SBS controls. However, G12 exhibited a $J_{nr}$ difference of 99.57%, exceeding the 75% threshold for stress sensitivity, indicating potential instability. G10 maintained an acceptable $J_{nr}$ difference (50.60%) while meeting Extreme traffic requirements.
Fatigue Cracking: GTR modification consistently reduced the fatigue parameter ( $|G^*|\cdot\sin\delta$ ), indicating improved fatigue resistance. At 31°C, the G10 binder showed a 124.9% reduction in the fatigue parameter compared to the 1% SBS binder (PG 70-22). G10 successfully met Superpave cracking criteria at 22°C, whereas the 1% and 2% SBS binders failed.
Thermal Cracking: The G10 binder demonstrated lower stiffness (98 MPa) and a favorable $m$ -value (0.36) compared to the 1% SBS binder (156 MPa stiffness, 0.34 $m$ -value). This combination suggests enhanced flexibility and stress relaxation, leading to better thermal cracking resistance than the SBS controls.
Chemical Interaction: FTIR analysis revealed no detectable chemical alterations or new functional groups in the GTR-modified binders compared to the base binder. The performance improvements were attributed to physical interactions, such as particle dispersion and swelling, rather than chemical modification.

Conclusions and Significance
The study concludes that a 10% GTR dosage (G10) represents the optimum content for modifying AC-20 binder. This dosage provides a balanced improvement in rutting, fatigue cracking, and thermal cracking resistance, benchmarking it effectively against a 1% SBS-modified binder (PG 70-22). While 12% GTR offered similar rutting resistance, its high stress sensitivity makes 10% the practical upper threshold.

The significance of this research lies in demonstrating the feasibility of utilizing GTR as a sustainable alternative to SBS. The findings suggest that GTR modification can achieve rheological characteristics comparable to or better than low-dosage SBS modification without chemical alteration of the binder. This offers a pathway to reduce construction costs and divert waste tyres from landfills while maintaining high pavement performance standards. The authors note that while 10% was determined as optimum in this specific study, practical applications may require experimental determination based on local material sources.