The potential of Attenuated Total Reflection imaging in the mid-infrared for the study of recycled asphalt mixtures

Sustainable development policies currently in effect in civil engineering serve to promote techniques capable of saving both energy and raw materials; such techniques use a combination of warm mix asphalt (WMA) concretes with reclaimed asphalt pavement (RAP). One issue raised when combining WMA and RAP involves determining the potential lack of RAP binder remobilisation during blending with the new binder, i.e. the so-called double-coating phenomenon. This paper aims to present the results of an Attenuated Total Reflection (ATR) imaging by means of infrared microspectrometry on asphalt mixtures featuring high RAP content. In the context of a Ph.D. thesis program, this analysis is combined with a study of the mechanical performance of these materials (not presented herein). The main objective of this research work is to assess the blending and coating quality of the materials at the microscopic scale. The experimental program consists of evaluating raw aggregates, binders, RAP and asphalt mixtures by ATR-FTIR microspectrometry in order to generate pictures for analysis and interpretation according to two distinct methods, namely Principal Component Analysis (PCA) and Independent Component Analysis (ICA). This analysis protocol introduces markers inside asphalts, like SBS or EVA copolymers or carbonyl and sulfoxide indices. The first main result of this study is improved control over the ATR imaging tool and its enhanced potential for analysing bituminous mixtures. A protocol for preparing samples and interpreting their results can now be proposed; an actual asphalt concrete has been analysed, not merely a laboratory-produced sample, as indicated in the literature. As a second result, tracers have been set up to distinguish raw and RAP materials (aggregates and binders), thus representing an initial step to studying the double coating, although it would seem difficult to generalise this study to a wide array of mixtures. Lastly, let’s point out that ATR-FTIR imaging is a “localised” study, making the determination of bituminous material homogeneity at specimen or even stockpile scale a real challenge for the future.