Environmental and functional role of recycled crumb rubber in hot-applied joint sealants based on rheological and microstructural evaluation

Abstract

Hot-applied joint sealants are widely used to seal cracks and expansion joints in road and bridge pavements. Recycled Crumb Rubber (CRM) derived from end-of-life tires is frequently incorporated to enhance flexibility and deformation resistance while supporting circular economy strategies through waste valorization. However, conventional specification tests (e.g., EN 14188-1) may not fully reflect the influence of rubber dispersion and microstructure on long-term performance. This study evaluates the functional and environmental relevance of CRM in seven commercially available hot-applied joint sealants obtained from five European manufacturers. The materials, including five rubber modified sealants and two comparative sealants, were characterized using fluorescence microscopy and Scanning Electron Microscopy (SEM) to assess phase morphology and rubber distribution. Rheological performance was investigated using dynamic shear rheometry, including Multiple Stress Creep Recovery (MSCR) at 60 °C and Large Amplitude Oscillatory Shear (LAS) at 10 °C. The results demonstrate that sealants compliant with the same specification types (N1/N2) may exhibit markedly different microstructures and viscoelastic responses. Variations in crumb rubber content and filler fraction significantly affected resistance to permanent deformation at high temperature and fatigue performance at low temperature. Materials with a more homogeneous rubber dispersion generally showed improved rheological performance. From an environmental perspective, the effective use of CRM not only diverts waste tires from landfill, but may also improve sealant durability, potentially reducing maintenance frequency and material consumption over the service life of infrastructure systems.