PSI - Issue 83

Henry Ssenyonjo et al. / Procedia Structural Integrity 83 (2026) 47–56 and Ro represent the resistances before and after load application and ε is the measured strain. = ( − )

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(1)

= ε

(2)

Recent works have shown that UHPC paired with optimized electrodes can exhibit very high sensitivity under compression, with large FCR and GF values reported in short-term cycling and monotonic tests (Yang et al., 2025) while other studies show repeatable strain sensitivities in UHPC containing stainless or coated steel fibres within elastic loading ranges (Qiu et al., 2021). For hybrid systems, combining steel fibres with CNTs or conductive particles can enhance conductivity and enable self-sensing under tension and crack opening, where the signal tracks damage evolution (B. Han et al., 2011; Yoo et al., 2018). 3.3. Piezopermittivity and piezocapacitive mechanisms. Resistance-based sensing is not the only intrinsic modality. Piezopermittivity (piezocapacitive response) describes load induced changes in effective permittivity, which can be monitored via capacitance or complex impedance. Cementitious materials can exhibit strong piezoresistivity and piezopermittivity even without functional admixtures; compression decreases resistivity while increasing permittivity in the elastic range, linking resistance based and capacitance based self-sensing concepts (Kim et al., 2024). For SS-UHPC, permittivity-based sensing is attractive because it can be measured using alternating Current (AC) excitation and can be less sensitive to certain contact resistance issues. However, the signal is affected by electrode geometry, fringing fields, and moisture dependent dielectric polarization, requiring careful measurement design and compensation strategies (Kim et al., 2024; Zhang et al., 2022). AC impedance spectroscopy (ACIS) captures both resistive and reactive components and is increasingly used to diagnose microstructural contributions, electrode polarization, and moisture effects in cement-based sensors (Elseady et al., 2023; Zhang et al., 2022). By using equivalent circuit models, researchers can distinguish bulk transport from interfacial processes such as electrical double layer polarization. This is valuable for SS-UHPC because the dense microstructure of UHPC can alter the balance between bulk and interfacial contributions compared with normal concrete (Yang et al., 2025). Electromechanical impedance (EMI) techniques using embedded piezoelectric cement sensors (e.g. PZT cement composites) provide an additional self-sensing pathway where changes in mechanical impedance or strength manifest as shifts in electrical impedance of the sensor element (Pan & Huang, 2020). While not exclusive to UHPC, EMI sensors can be integrated with UHPC elements to provide complementary information to piezoresistive networks. 4. Sensing Techniques and Measuring Parameters. Sensing in concrete can be performed using either two-probe or four-probe electrode configurations, as illustrated in Fig. 2. While the two-probe method is the simplest and most widely used technique for measuring material resistance, the four-probe method provides more accurate results by eliminating the contact resistance between the electrodes and the composite (Gopalakrishnan et al., 2011). In SS-UHPC research, embedded electrodes can further reduce contact instability and improve durability, and dense electrode systems have been shown to substantially improve piezoresistive signal quality (Yang et al., 2025). In practice, electrode material, embedment depth, spacing, and curing compatibility must be selected to balance signal fidelity with constructability. Standardized resistivity testing in concrete commonly uses AC excitation and prescribed spacing to control polarization and saturation effects. 3.4. Impedance based and electromechanical impedance approaches.