Nano-indentation and SEM analysis of the interfacial transition zone in RAP-concrete

Weak and porous interfacial transition zone, ITZ, largely accounts for the mechanical strength reduction and low durability in RAP (reclaimed asphalt pavement; recycled materials obtained from distressed asphalt pavements)- based concrete mixtures. A fundamental approach to elucidate the role of hydration products, porosity, elemental distribution, and elastic modulus on the ITZ characteristics of RAP-concrete mixtures was studied through SEM-BSE, SEM-EDS and Nano-Indentation techniques. Two types of RAP aggregates were utilized – high oxidized RAP (denoted as R1) and low oxidized RAP (denoted as R2). Two concrete mixtures were formulated at 100 % of RAP replacement levels and water-cured till 3, 7, and 28 days – these mixtures were compared with a control concrete. SEM-BSE image analysis by strip delineation method reveals higher porosity with lesser hydration products in the ITZ region of RAP-concrete compared to control concrete. The average ITZ thickness determined through SEM-BSE, SEM-EDS (elemental distribution), and nano-indentation techniques were found to be typically in the range of 20 – 50 µm, 22 – 50 µm, and 29 – 80 µm for control, R1, and R2 samples, respectively. From volumetric phase distribution analysis, R1 concrete sample was found to consist of lesser concentration of CH and low-density C-S-H, high concentration of high-density C-S-H, and reduced porosity in the ITZ region compared to R2 concrete samples; thereby indicating the former’s higher affinity for interaction with the cement matrix. The correlation between ITZ thickness, hydration products in the ITZ region, and the compressive strength is also elucidated.