The research, led by Baha Vural Kök and Erkut Yalçın of Fırat University’s Department of Civil Engineering together with Furkan Yıldırım of Bursa Technical University, centered on a simple but consequential question: can two waste-derived or lower-cost additives working synergistically replace a premium synthetic polymer in hot mix asphalt? To answer it, the team prepared four sets of bituminous mixtures using the same base bitumen. The first served as a control with no modification at all. The second incorporated 8 percent crumb rubber by weight of the binder, a dosage typical of so-called “dry process” rubberized asphalt. The third combined the same 8 percent crumb rubber with 2 percent B2Last, a methylene diphenyl diisocyanate (MDI)–based reactive polymer. The fourth used 4 percent SBS, the block copolymer that has dominated high-performance asphalt modification for decades. Each mixture was then subjected to a battery of standardized mechanical tests probing strength, moisture sensitivity, stiffness, fatigue resistance, and resistance to permanent deformation—five failure modes that govern how long a pavement survives in service.

The chemistry underlying the hybrid system is what distinguishes it from simple blending. Crumb rubber alone modifies bitumen largely through a physical process: at elevated temperatures, the light fractions of the bitumen—its maltenes—swell the rubber particles, softening them and producing a gel-like network that increases binder viscosity and elasticity. However, the interaction remains physical rather than chemical, and the dispersion of rubber particles can be inhomogeneous, limiting how much of the rubber’s intrinsic elasticity is actually transferred to the mixture. B2Last changes that equation. Its reactive isocyanate (–NCO) functional groups react with hydroxyl and amine-containing components—both in the bitumen’s polar fraction and on the surface of the rubber particles—to form urethane linkages. In effect, the MDI-based polymer acts as a molecular bridge, chemically anchoring the crumb rubber to the bitumen matrix and creating a crosslinked polyurethane network within the binder. The researchers confirmed that B2Last successfully realized this polyurethane reaction within the crumb rubber–modified bitumen, transforming what was previously a loosely associated suspension into a structurally reinforced composite.

The consequences of that chemistry showed up immediately in the strength measurements. Marshall stability, a classic index of a mixture’s resistance to structural deformation under load, increased by 36 percent in the hybrid 8C2B mixture (8 percent crumb rubber plus 2 percent B2Last) relative to the crumb rubber-only mixture, and by 47 percent relative to the 4 percent SBS mixture. In other words, the recycled-rubber hybrid not only beat the unmodified rubber system, it substantially surpassed the synthetic polymer benchmark that many highway agencies treat as the reference standard. Indirect tensile strength (ITS) tests told a consistent story. Adding 2 percent B2Last to the 8 percent crumb rubber binder raised the dry ITS by 20 percent and, more strikingly, the wet ITS—measured after moisture conditioning—by 24 percent, indicating that the chemical network is particularly effective at protecting the mixture against water-induced damage, one of the most common early-life distresses in asphalt pavements.

Moisture sensitivity is quantified through the tensile strength ratio (TSR), the ratio of wet to dry tensile strength, and here the hybrid system again distinguished itself. The 8C2B mixture achieved a TSR of 86 percent, comfortably above typical specification thresholds and well ahead of what crumb rubber modification alone usually delivers. This matters because stripping—the loss of adhesive bond between bitumen and aggregate in the presence of water—is a primary trigger for raveling, potholing, and premature structural failure. The authors attribute the improvement to the polyurethane network’s ability to stiffen the binder film around aggregate particles and resist water penetration at the asphalt–aggregate interface, an effect that a purely physical rubber dispersion cannot replicate to the same degree.

Stiffness measurements reinforced the picture of a fundamentally upgraded material. Using the indirect tensile stiffness modulus (ITSM) test, the team found that the 8C2B mixture was 134 percent stiffer than the unmodified control and 68 percent stiffer than the crumb rubber–only mixture. Increased stiffness at intermediate and high temperatures translates directly into better load-spreading capacity across the pavement structure and reduced strain at the bottom of the asphalt layer, which in turn delays fatigue cracking. That prediction was borne out dramatically by the indirect tensile fatigue test. While the control mixture survived an average of 914 load cycles before failure, the 8C2B mixture endured 10,408 cycles—an elevenfold increase in fatigue life, and a figure that comfortably exceeded the performance of both the crumb rubber and SBS mixtures.

Resistance to permanent deformation—the rutting that plagues heavily trafficked highways, intersections, and bus lanes in hot climates—was assessed through the dynamic creep test, arguably the most demanding trial in the study. After 14,400 load cycles, the 8C2B mixture accumulated 7,088 microstrain of permanent deformation, a value the authors identify as evidence of superior rutting resistance. Perhaps more telling is the flow number, the number of load cycles a mixture can sustain before entering the tertiary creep stage of accelerating deformation. The hybrid mixture’s flow number was 14.9 times higher than that of the unmodified control, meaning the material withstood nearly fifteen times more loading before its resistance to deformation began to collapse. This resilience stems from the combined contribution of the elastomeric rubber particles, which provide elastic recovery, and the crosslinked polyurethane network, which provides a load-bearing skeleton that prevents the binder from flowing under repeated stress.

When the researchers compared the two crumb rubber–based systems side by side with the SBS mixture, a generally comparable performance emerged between the 8 percent crumb rubber and 4 percent SBS mixtures—consistent with a body of earlier literature suggesting that well-designed rubberized asphalt can match moderate SBS dosages. But the hybrid 8C2B system was in a class of its own, outperforming both on nearly every metric. The finding carries significant practical implications. SBS is a petroleum-derived polymer whose price is volatile and whose production carries a substantial carbon footprint; it also demands high-shear blending equipment and careful storage management because SBS and bitumen can phase-separate at storage temperatures. Crumb rubber, by contrast, is a waste stream—scrap tires that would otherwise occupy landfills or be burned—while reactive isocyanate additives like B2Last can be dosed at low percentages. A modification strategy that pairs a recycled material with a small amount of reactive chemistry thus offers a route to high-performance pavements with lower cost, reduced reliance on virgin polymers, and a meaningful contribution to circular-economy goals in road construction.

The Turkish study builds on a growing international research effort into isocyanate and polyurethane chemistry in asphalt. Previous work has shown that MDI-based prepolymers can dramatically stiffen bitumen through urethane crosslinking, and earlier investigations by the same Fırat University group demonstrated that combining crumb rubber with an isocyanate-based additive substantially improves the binder’s conventional and rheological characteristics. What sets the new study apart is its shift from the binder scale to the mixture scale: rather than measuring binder rheology alone, the team evaluated full hot mix asphalt specimens under Marshall, tensile, stiffness, fatigue, and creep protocols, providing the kind of performance-based evidence that pavement engineers need before adopting a technology in the field. Funding for the research came from the Scientific and Technological Research Council of Turkey (TÜBİTAK, project 124M464) and Fırat University’s Scientific Research Unit (project MF.26.05).

The authors are careful to frame the results as laboratory evidence rather than a finished field prescription. Real-world adoption will require validation of workability and compaction behavior at plant scale, long-term aging and low-temperature performance assessment, and confirmation that the polyurethane reaction proceeds reliably under industrial mixing conditions. Nonetheless, the magnitude of the improvements—11-fold fatigue life extension, a nearly 15-fold increase in flow number, and moisture damage resistance that improves rather than degrades with the reactive additive—suggests a genuine step change rather than an incremental one. As governments worldwide grapple with mounting stockpiles of end-of-life tires and simultaneous pressure to build longer-lasting, more sustainable road networks, the prospect of turning shredded tires into pavements that outlast those made with premium synthetic polymers is exactly the kind of elegant industrial symbiosis that materials science promises. If field trials confirm these laboratory numbers, the humble scrap tire may soon be doing far more than filling landfills—it may be holding the world’s highways together.