Micromechanical properties of interfacial transition zone between carbon fibers and UHPC matrix based on nano-scratching tests

Ultra high performance concrete (UHPC) can be prepared by using carbon fiber (CF), instead of steel fiber, as reinforcement and indeed avoid the corrosion of steel fiber. The properties of CF/matrix interface play a key role on the composite performances. However, the properties of interface between CFs with different surface treatments and UHPC matrix are not yet clear. The denser matrix of UHPC makes the CF/matrix interface obviously different from that of ordinary concrete and more difficult to be characterized quantitatively. In this study, micromechanical properties of CF/matrix interface in UHPC were quantified using nano-scratch technique. Fracture toughness of the interface between UHPC matrix and CFs modified with electrochemical oxidation technique was higher than that of unmodified CF by about 51%. Moreover, the thickness of the interface transition zone decreased by more than 50%. Compared with the UHPC matrix without adding silica fume, 15% (by weight of cement) silica fume added to the UHPC matrix make the fracture toughness of CF/matrix interface increased by 17% and the interfacial thickness decreased by 37%.