PSI - Issue 83

Henry Ssenyonjo et al. / Procedia Structural Integrity 83 (2026) 47–56

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Nomenclature I

electrical current V electrical voltage R electrical resistance R0 initial / baseline resistance ΔR change in resistance ε strain ρ electrical resistivity ρ0 initial electrical resistivity R² coefficient of determination PZT piezoelectric cement sensor EMI electromechanical impedance FCR fractional change in resistance GF gauge factor

1. Introduction Progressive deterioration of civil infrastructure worldwide is attributed to aging, increased service loads, and exposure to aggressive environmental conditions, which poses significant risk to public safety and overall economic stability. Ensuring the structural integrity of critical assets such as bridges, high-rise buildings, and marine structures requires the implementation of robust and reliable SHM systems (Cheng et al., 2018, 2020; B. Han et al., 2015; Z. Li et al., 2018). While traditional SHM relies on external sensors (such as strain gauges and accelerometers), these devices have significant limitations which include high installation and maintenance costs, long-term durability issues, and incompatibility with the concrete matrix, often leading to performance degradation or corrosion-related failure over time. To overcome these challenges, the field of smart materials has seen rapid growth with the emergence of intrinsic SS-UHPC. SS-UHPC is an advanced composite material that integrates the sensing capability directly into the structural component itself. By turning the structure into its own sensor, SS-UHPC offers a cost-effective, durable, and highly integrated alternative to conventional sensors (Lian et al., 2021). The core mechanism leveraged in most self-sensing composites is piezoresistivity, where the bulk electrical resistivity of the material changes measurably in response to mechanical stress or strain (Ramachandran et al., 2022). This phenomenon is typically achieved by incorporating small quantities of metallic fibres and conductive fillers, such as carbon nanotubes (CNTs), graphene (G), or carbon fibres (CF), into the cementitious matrix to establish a functional, stress-responsive conductive network. Steel fibres are traditionally used in UHPC systems to enhance post-cracking strength, ductility, and energy absorption. Beyond their mechanical benefits, steel fibres also contribute to electrical conductivity and facilitate the formation of robust conductive pathways essential for reliable piezoresistive behaviour. When combined with conductive fillers such as CNTs, graphene, or carbon black, steel fibres can significantly improve the continuity and stability of the conductive network, thereby enhancing the sensitivity of the material for self-sensing applications. The synergistic interaction between steel fibres and conductive fillers enables the development of multifunctional UHPC capable of simultaneously carrying load and sensing internal strain. This review provides a critical and comprehensive analysis of the state-of-the-art in SS-UHPC. It begins by examining the constituent materials of UHPC, focusing on the role of metallic fibres as both mechanical reinforcement and primary conductive elements. The review then delves into the theoretical and physical mechanisms of piezoresistivity in these hybrid systems and the electrical measurement techniques required for accurate data acquisition. Finally, the paper critically discusses the key factors influencing sensing performance and outlines the significant challenges that must be addressed to transition this promising smart material from the laboratory to widespread civil engineering applications.