Macro-micro investigation on the coefficient of friction on the interface between steel and cast-in-place UHPC

The interface friction force between steel and Ultra-high performance concrete (UHPC) can significantly enhance the shear loading capacity and fatigue performance of steel-UHPC composite structures. Current research on interface friction primarily focuses on the coefficient of friction (COF) on steel-concrete interface. However, using the COF of steel-normal concrete (NC) interface to assess the steel-UHPC interface performance might be debatable and lack accuracy, due to the substantial differences in material properties and compositions between UHPC and NC. In this study, the COF on the interface between the steel plate and cast-in-place UHPC was investigated, and the effects of the steel plate roughness and interface normal stress on the COF were considered. The COF of steel-UHPC was compared with those of steel-NC. The steel plate surface microstructure was also tested using a microscope after experiments. It is recommended that the COF between UHPC and NC and a natural steel plate should be considered as 0.803 and 0.704, respectively, under interface normal stress levels ranging from 0.5 MPa to 9 MPa. The adopted COF in the existing numerical analyses for steel-UHPC structures was conservative, underestimating the value by 37.73 % to 62.64 % compared to the recommended value in this paper. Moreover, when the steel plate roughness of steel-UHPC specimens increased by 1.324 times, the maximum increase in the COF was 11.75 %. However, when the interface normal stress increased by 17 times, the COF on the steel-UHPC interface decreased by a maximum of 12.57%. Compared to steel-NC specimens, the COF of steel-UHPC showed a maximum increase of 14.25 % under an interface normal stress of 0.5 MPa and 12.98 % under an interface normal stress of 9 MPa.