Repeatability and sensitivity of FTIR ATR spectral analysis methods for bituminous binders

Bituminous binders as organic materials are prone to aging mainly by oxidation. Aging changes the viscoelastic behavior of the material over time towards higher stiffness and brittleness. FTIR has been increasingly used lately to investigate impacts of oxidative aging on the chemical structure of bitumen. Especially the carbonyl and sulfoxide bands are affected by aging and commonly used to describe changes due to evolving oxidation of a binder. However, spectra obtained from FTIR can be analyzed in fundamentally different ways and the analysis method applied to a spectrum has an impact on the gathered results and especially on the repeatability and sensitivity with regards to oxidative changes. For the presented study, two bituminous binders, a PG 64-16 and an SBS modified PG 76-22 PM were aged by RTFO and PAV and subsequently measured by FTIR. The obtained spectra (up to 90 individual spectra per aging state) were analyzed employing various methods using either the original or a normalized spectrum, a band maximum or integration based calculation of indices from an absolute or tangential baseline. By analyzing the coefficient of variation of the different analysis methods, it was found that not all analysis methods exhibit the same repeatability and sensitivity. From the findings of the study it can be recommended to work with normalized spectra, use an absolute baseline and work with integration of areas for index production in favor over band maximum based methods.