Microstructural and Micromechanical Study of the Interfacial Transition Zone of Recycled Concrete by Combined Nanoindentation and SEM-EDX Technique

The macroscopic properties of concrete are largely determined by its microstructure and micromechanical properties. As the most vulnerable structural element in concrete, the interfacial transition zone (ITZ) has a significant influence on the macro-mechanical behavior of concrete. The use of recycled aggregates, which exhibit higher porosity, lower density and higher water adsorption, results in concrete with distinct properties compared to natural concrete. The aim of this study is to investigate the ITZ in both natural and recycled concrete using a combination of nanoindentation and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX). Firstly, the concrete samples were analyzed using SEM to identify areas with a typical ITZ. Backscattered electrons (BSE) are employed to resolve the microstructure of these regions and the thickness of the ITZ, taking into account the porosity distribution. Subsequently, the indentation modulus and the hardness of the ITZ were determined. According to the distribution of the porosity and modulus, the thickness of the ITZ is determined to amount to 50–70 μm. The results show that the ITZ in recycled concrete has higher porosity and lower indentation modulus than that in natural concrete. The obtained results were further analyzed using clustering method (i.e. k-means). The micromechanical investigations allow for the identification of five different material phases with spatially varying properties.