Crumb-Rubber Asphalt Releases Five Times More Fumes Than Regular Asphalt at Paving Temperatures

Every year, hundreds of millions of end-of-life tires are ground into crumb rubber and blended into asphalt, a recycling strategy that keeps waste out of landfills while producing pavements that resist cracking and deformation. But the process comes with a hidden cost: making crumb-rubber-modified asphalt, known as CRMA, requires sustained heating at temperatures that can approach 200 degrees Celsius, hot enough to drive a complex cocktail of volatile compounds, sulfur gases, and rubber-derived chemicals out of the binder and into the air around production plants and paving crews. A new study from researchers at Guangxi University and the Guangxi Forestry Inventory and Planning Institute in China has now tracked these emissions second by second, revealing just how dramatically the fume burden changes with temperature and with the addition of tire rubber.

The research, published in Frontiers in Materials, addressed a persistent gap in asphalt emission science. Most previous studies relied on discrete sampling, capturing a single snapshot of fume composition at one moment in time. Yet asphalt fume emission is inherently dynamic: volatile components surge when heating begins, peak, and then decay as the most readily releasable fractions are depleted. To capture this behavior, the team built an integrated experimental system combining controlled fume generation in a sealed, thermostatically heated flask with mechanical stirring, real-time monitoring of volatile organic compounds and sulfur-containing gases, and offline collection of fumes for detailed chemical analysis by gas chromatography–mass spectrometry.

Repeatability was a central concern. The researchers standardized the thermal history of every sample, heating the asphalt at 140 degrees Celsius for four hours before dividing it into test portions, then reheating each portion for two hours before monitoring. Each test used 500 grams of asphalt, stirred at 300 revolutions per minute, with VOC concentrations recorded every 10 seconds for 90 minutes. Duplicate tests at five temperatures showed relative deviations below 10 percent during the principal response stages, well within the acceptance criterion of 20 percent, confirming that the system could reliably distinguish genuine differences between materials from experimental noise.

The team first compared four penetration-grade 90 asphalt binders from different sources, designated ZH90#, SK90#, JB90#, and GX90#, across temperatures from 120 to 200 degrees Celsius. All four showed VOC responses that increased with temperature, generally peaking before settling into a slowly varying stage. But the magnitude of the response depended strongly on the crude-oil source of the binder. At 200 degrees Celsius, the peak VOC concentration of ZH90# exceeded five times that of GX90#, even though the four binders had similar penetration and softening-point values. The finding carries a practical warning: conventional physical properties alone cannot predict the fume-emission potential of an asphalt binder, and temperature-control strategies should account for the specific source of the material rather than assuming a uniform response across all binders.

The most striking results emerged when crumb rubber entered the picture. The researchers prepared CRMA by incorporating 20 percent by weight of 40-mesh crumb rubber, derived from end-of-life tires with no desulfurization or activation pretreatment, into the ZH90# base binder. At 200 degrees Celsius, the peak VOC concentration of the modified asphalt reached 3,719.1 milligrams per cubic meter, approximately 5.5 times the 672.9 milligrams per cubic meter recorded for the base asphalt alone. The late-stage mean concentration showed a similar ratio of 5.63, and the cumulative response, calculated as the area under the concentration–time curve, was nearly five times higher. The peak also arrived earlier, at 110 seconds rather than 570 seconds, indicating that the rubber-modified binder released its volatile burden more rapidly.

Temperature proved to be the dominant amplifier. Raising the temperature of the modified asphalt from 180 to 200 degrees Celsius increased the peak VOC concentration by 253 percent, the late-stage concentration by 286 percent, and the cumulative response by 275 percent. The researchers attribute this nonlinear surge to three coupled processes: volatilization of light fractions from the base asphalt, enhanced thermal desorption of low-molecular-weight components bound within the rubber matrix, including residual monomers, processing oils, and vulcanization additives, and swelling-induced mass transfer, in which the rubber absorbs aromatic fractions from the binder, expands its interfacial contact, and opens pathways for volatile substances to migrate from the material interior into the gas phase. Notably, the response was not strictly monotonic at every temperature: the modified asphalt showed lower monitored emissions at 160 degrees Celsius than at 140 degrees Celsius, a local anomaly the team interprets as the combined effect of components released before formal monitoring began and temperature-dependent swelling that altered how light components partitioned among the binder, rubber phase, and gas phase.

Sulfur-containing gases added another dimension to the emission profile. The maximum recorded hydrogen sulfide concentration increased from 10 parts per million at 120 degrees Celsius to 31 parts per million at 200 degrees Celsius, while the cumulative H2S response rose from 265 to 1,035 parts per million·minutes over the same interval. Sulfur dioxide responses were consistently higher, with cumulative values increasing from 3,785 to 5,715 parts per million·minutes as temperature climbed from 120 to 200 degrees Celsius. At every temperature tested, the maximum recorded values for both gases occurred at the onset of formal monitoring, followed by an overall decline, suggesting that sulfur species are released most intensely during the earliest stages of sustained heating.

Gas chromatography–mass spectrometry revealed the chemical fingerprints behind these dynamic responses. Fumes from the base asphalt were dominated by petroleum-derived hydrocarbons, including heptane, octane, decane, styrene, naphthalene, and polycyclic aromatic compounds such as phenanthrene and fluoranthene. The modified asphalt fumes retained these aromatic signals but added a distinctive set of rubber-associated compounds: isoprene and its dimer dipentene, which trace to the polyisoprene structure of tire rubber; benzothiazole and methylbenzothiazole, associated with vulcanization accelerator systems; benzothiophene and dibenzothiophene, sulfur-containing heterocycles; carbon disulfide; and tetramethylthiuram disulfide, a typical sulfur-bearing rubber additive. After baseline correction, the cumulative chromatographic response of the modified asphalt fumes was 18 percent higher than that of the base asphalt, with the largest increase, 40.5 percent, concentrated in the 10-to-14-minute retention window. The researchers emphasize that no single compound serves as an independent marker of crumb-rubber origin; the diagnostic value lies in the simultaneous occurrence of this multicomponent fingerprint.

The authors are careful about what their numbers mean. The real-time concentrations are laboratory-specific sensor-equivalent values, determined by the particular reactor scale, gas-flow configuration, and instrumental calibration, intended for comparative assessment among samples and temperatures rather than direct estimation of field emissions or occupational exposure. Laboratory binder tests differ from plant-scale production in processing scale, adsorption by mineral aggregates, ventilation conditions, and thermal history, and field studies have shown that paving temperature, binder content, task, and work rate all significantly affect worker exposure. The findings therefore provide a basis for optimizing processing temperatures and designing targeted fume control, not regulatory limits.

The engineering implications are nonetheless concrete. Because emissions intensified most sharply in the 180-to-200-degree regime, the researchers recommend adopting the lowest temperature that achieves the intended dispersion, swelling, and pavement performance, minimizing high-temperature storage, recirculating heating, and repeated reheating, and using multipoint temperature measurements to prevent localized overheating near 200 degrees Celsius. Fume capture should cover the entire processing sequence, from crumb-rubber addition through mixing, swelling, transfer, and storage, with local exhaust ventilation activated before processing begins, since the highest sulfur-gas responses occurred at the onset of monitoring. Collected fumes should be treated in stages, removing particulates and condensable droplets first, then applying VOC adsorption, condensation, or oxidation according to composition, with dedicated units for H2S and SO2. And because the unresolved VOC maximum at 120 degrees Celsius showed that even lower-temperature operation does not eliminate prolonged fume release, ventilation and capture must remain active throughout both heating and holding. For a recycling technology poised to expand worldwide, the study delivers a clear message: the greenest binder is only as clean as the temperature at which it is processed.

Subject of Research: Dynamic fume emission behavior and chemical composition of crumb-rubber-modified asphalt compared with base asphalt across processing temperatures

Article Title: Characteristics of fume components and dynamic emission behavior of crumb-rubber-modified asphalt

Article References: Luo, L., Dai, J., Zhu, W., Li, C., Yu, L., Liu, Y., Li, B., & Zhang, Z. (2026). Characteristics of fume components and dynamic emission behavior of crumb-rubber-modified asphalt. Frontiers in Materials, 13, Article 1979048. https://doi.org/10.3389/fmats.2026.1979048

Image Credits: AI Generated

DOI: 10.3389/fmats.2026.1979048

Keywords: crumb-rubber-modified asphalt, asphalt fumes, volatile organic compounds, hydrogen sulfide, sulfur dioxide, gas chromatography–mass spectrometry, tire recycling, thermal desorption, emission control, pavement materials, Frontiers in Materials, dynamic emission behavior