Investigation of the Interfacial Transition Zone (ITZ) in asphalt mastic and its effect on the integrity of the mix: A Raman hyperspectral approach

In the context of a circular economy, increasing the durability of existing infrastructure is of paramount importance. Roads are the lifelines of the modern economy, and it is imperative to increase their lifespan as part of sustainable growth. Most conventional roads are a mixture of rock aggregate and bitumen. The region where bitumen meets aggregate in any asphalt mix experiences a complex interaction between electrostatic, chemical, and mechanical forces resulting in the formation of an Interfacial Transition Zone (ITZ) (Zhu et al. 2017). More specifically, there is a complex behavior of materials that could result from three different types of interaction between the aggregates, fillers, and bitumen: i) physisorption ii) chemisorption iii) mechanical interlocking (Pasandín and Perez 2015).

The integrity of the ITZ is one of the crucial factors for the overall durability of any mixture containing aggregate and binding materials. Due to the complex interaction of different minerals, present in the rock aggregate, with the bituminous binder, this junction is considered a weak link that dictates the overall structural durability of the mixture (Zhu et al. 2021). Most of the studies concerning the ITZ are based on mechanical tests at the aggregate-bitumen contact and generally do not include mineralogical observations.

The present study focuses on the ITZ within laboratory-manufactured asphalt samples utilizing rock aggregate varieties typically used in surface courses in the Republic of Ireland. Structural and petrographic observations are made using a combination of digital microscopy and Raman spectroscopy. Early observations suggest that there is a strong control exerted by aggregate mineralogy on the formation of features within the ITZ, including the linear alignment of iron oxide particles in the bitumen. Iron-oxide particles could potentially create a zone of weakness in a stone-mix asphalt, especially where the key aggregate is greywacke.

References

• Zhu et al. 2017; Identification of interfacial transition zone in asphalt concrete based on nano-scale metrology techniques, Materials and Design. 129, 91-102.
• Pasadin and Perez, 2015; The influence of the mineral filler on the adhesion between aggregates and bitumen, International Journal of Adhesion and Adhesives.