Properties of interfacial transition zones in recycled aggregate concrete tested by nanoindentation

The properties of new Interfacial Transition Zone (ITZ) and old ITZ in Recycled Aggregate Concrete (RAC) were investigated by Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) and nanoindentation. From the SEM images, obvious voids and high concentration of calcium hydroxide can be found in both old ITZ and new ITZ in RAC. Based on the nanoindentation study, it is indicated that the thicknesses of old and new ITZs are in the range 40–50 μm and in the range 55–65 μm, respectively. It is also found that the average indentation modulus of old ITZ is 70–80% of that of old paste matrix, while the average indentation modulus of new ITZ is 80–90% of that of new paste matrix. Additionally, the influences of mix proportion, aggregate types and hydration age on the properties of ITZs in RAC are discussed in this study.