PSI - Issue 78

Paolino Cassese et al. / Procedia Structural Integrity 78 (2026) 607–614

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available on the market, some issues have been highlighted in the literature regarding their durability, cost, capability of ensuring continuous monitoring, and limited compatibility with concrete structures, the latter representing a significant portion of existing constructions ( Yıldırım et al., 2018). In this regard, ever-increasing attention has been paid to developing advanced self-sensing cementitious composites (SSCCs), i.e., smart materials able to monitor changes in strain without requiring the installation of external sensors because of the enhanced piezoresistivity given by the addition of conductive micro- or nano-fillers to the mix design (Puentes et al., 2025). Despite the significant research interest and the exciting progress reported in the literature, further efforts are still required towards the real on-field application of SSCCs for civil SHM. In particular, relevant critical issues are filler typology, mix design, dispersion techniques, electrical system, elaboration methods, installation mode, durability, and stability over time of the sensing capacity (Cassese et al., 2021; Hao et al., 2023; Qin et al., 2024). SSCCs obtained by adding carbon nanotubes (CNTs) in the mix design are characterized by high self-sensing sensitivity. Nevertheless, the hydrophobic nature of CNTs represents a production challenge because they tend to agglomerate, compromising their performance (Han et al., 2020). The latest scientific evidence recognizes the use of some physical and chemical treatments as highly promising: specifically, the sonication of the water-CNT solution, on one hand, and the use of surfactants and/or the functionalization of conductive fillers, on the other (Li et al., 2023). Some authors obtained SSCCs with relevant sensing performance by using pure multi-walled CNTs (MWCNTs), more sensitive to deformation than single-walled nanotubes (Mesquita et al., 2023), dispersed by combining surfactants and dispersants in the mix design with sonication (D'Alessandro et al., 2021). They did not use chemical functionalization because of the potential strong modification to composite constituents (D’Alessandro et al., 2022). Conversely, other authors have produced high-performance SSCCs by using functionalized MWCNTs, sonication, a dispersant, and a defoamer agent. They proved that carboxyl acid (−COOH group) functionalization was associated with high sensitivity (Rao and Sasmal, 2020). Concerning the mix design, the water-to-cement (w/c) ratio is definitely a relevant parameter affecting the electrical response of SSCCs in addition to filler concentration. The amount of water in the mix mainly affects: (i) the workability of the cementitious matrix; (ii) the dispersion of the functional filler; (iii) both mechanical and sensing performance of the SSCCs (Han et al., 2015). Regarding self-sensing, the observations reported in the literature are not unambiguous (Dong et al., 2019). A higher w/c ratio is generally associated with an increase in sensitivity (Li et al., 2023). Nonetheless, an improvement in the sensitivity and stability of the piezoresistive response has also been found at low w/c ratios (Kim et al., 2014). Typical w/c ratio values used for SSCCs are within the range 0.4-0.6 (Rainieri et al., 2013). About the electrical input, Direct Current (DC) is generally preferred due to its practical convenience and to the higher degree of current penetration. However, DC may cause severe errors in measuring electrical resistance due to considerable polarization in the composite (Elseady et al., 2023). Alternating Current (AC) allows for more accurate measures of the electrical properties of SSCCs, but it is more complex, and a proper measurement system and procedure are necessary (Dong et al., 2019). An innovative electrical measurement technique was proposed by Downey et al. (2017) to solve the polarization issue. Such an approach used a periodic square wave measure discharge DC signal and measurements taken during the discharge phase, with promising results. In the previously described framework, this paper aims to contribute to the investigation of SSCCs' performance by carrying out an experimental campaign on SSCC specimens, obtained by introducing functionalized MWCNTs into cement paste. The main objective is to assess the influence of the sole w/c ratio on the piezoresistive performance. Therefore, no chemical additives, such as dispersant or surfactant, were considered in the mix design. Mechanical tests were performed in the regime of pure compression cyclic excitation on the specimens, which were electrically monitored by using an AC measurement approach. The testing program, concerning materials, specimens, setup, and procedure, is described in Section 2, whereas the main experimental results are described in Section 3.