Nano-mechanical behavior of the interfacial transition zone between steel-polypropylene fiber and cement paste

The fiber-matrix interfacial transition zone (ITZ) at nanoscale plays an important role in determining the mechanical performance of hybrid steel-polypropylene fiber reinforced concrete at upper scales. This paper investigates the elastic behavior of the ITZ between steel/polypropylene fiber and pure cement paste through nanoindentation for different water/cement ratios. The microstructure mapping and gradient of ITZ were analyzed based on the elastic modulus (E) and hardness (H) distributions. The morphology of ITZs was observed with the help of Scanning Electron Microscope. In addition, to probe into the formation mechanism of ITZ, distinct phases in ITZ were statistically determined from the frequency density distributions of E and H values through the deconvolution technique. Results show that the thickness of fiber-matrix ITZ mainly depends on the fiber type, where the steel fiber and polypropylene fiber ITZ are about 30 μm and 15 μm in thickness respectively. It is also found from E distribution that a trough in the steel fiber ITZ exists at lower w/c ratios, whereas a gradual growth of E and H values is demonstrated away from polypropylene fiber surface for all cases shown. The ITZ's chemical compositions at nanoscale suggest that the volume fractions of HD C-S-H, CH/C-S-H and porosity are noted to be the main determinants for the overall properties of ITZ. In comparison with steel fiber samples, a larger amount of porosity exists in the vicinity of polypropylene fiber surface. Finally, based on the test data, the ITZ formation mechanism was discussed with water/cement ratio effect considered.