New Insight into the Interfacial Transition Zone in Concrete Based on Fluorescence Microscopy

The interfacial transition zone (ITZ) in the aggregate periphery is often regarded as the weakest area in concrete. In this study, the results of extensive image analysis provide a new insight. First, fluorescence microscopy (FM) was adopted to obtain the microstructure of “complete ITZ”, which overcomes several limitations of the scanning electron microscope method. Then, an ITZ recognition interactive algorithm was proposed, which quantitatively characterizes the pore distributions in the ITZ and cement paste in both lateral and longitudinal directions. Finally, based on the experimental and statistical results, the pore distributions around aggregates, coarse sand and fine sand were characterized. Along the lateral direction, a high non-uniformity was observed in the porosity between units, taken at the same distance from the aggregate/sand surface. On the contrary, along the longitudinal direction, statistical results show minimal increases in the porosity within the ITZ. This is also applicable even in the innermost ITZs.