This study investigated the electromechanical response of smart ultra-high-performance concretes (smart UHPCs), containing fine steel slag aggregates (FSSAs) and steel fibers as functional fillers, under external loads corresponding to different measurement methods. Regardless of different measurement methods of electrical resistance, the smart UHPCs under compression showed a clear reduction in their electrical resistivity. However, under tension, their electrical resistivity measured from direct current (DC) measurement decreased, whereas that from alternating current (AC) measurement increased. This was because the electrical resistivity, from DC measurement, of smart UHPCs was primarily dependent on fiber crack bridging, whereas that from AC measurement was dependent on tunneling effects.