PSI - Issue 67

Oscar Aurelio Mendoza Reales et al. / Procedia Structural Integrity 67 (2025) 8–16 Mendoza Reales et. al./ Structural Integrity Procedia 00 (2024) 000 – 000

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1. Introduction The proper maintenance of concrete structures is crucial for ensuring structural integrity and safety during their operation Li et al. (2016), providing essential information to pinpoint the need for preventive maintenance and ultimately extending their service life, Clemente (2018). Within this framework, self-sensing cementitious composites (SSCC) have emerged as a promising monitoring solution due to their compatibility with concrete and potential higher durability, Wang et al. (2023). SSCC are composed of cement blended with a conductive filler, Han et al. (2015). Materials like carbon nanotubes (CNT), carbon fibers, carbon black, graphene, graphite, and steel fibers can reduce the electrical resistivity of the matrix, resulting in a new material with enhanced piezoresistive properties, Tian et al. (2019), i.e. when a SSCC is strained, its electrical resistivity changes proportionally, D’Alessandro et al. (2014) . Carbon-based nanomaterials, particularly CNT, stand out as commonly used conductive fillers due to their superior electrical performance, Han et al. (2020). Literature reports indicate that carbon fiber/cement composites exhibit thermistor-like behavior, where their electrical resistivity decreases as temperature rises, Wen et al. (1999). A similar behavior is expected for other SSCC since temperature variations cause changes in the activation energy for the electrical conduction process through the conductive fillers, Cerro-Prada et al. (2021). These changes come from adjustments in the distance between conductor particles, increases in molecular motion, and variations in internal moisture, Adresi and Pakhirehzan (2023). Additional factors, including the presence of the Seebeck effect, Wen and Chung (1999), and thermal expansion, should also be considered for the appropriate application of SSCC in structures prone to daily temperature fluctuations. This work explores the electrical resistivity and piezo resistivity of two different SSCC exposed to temperature in stationary and transient temperature regimes. The SSCC were fabricated using carbon nanotubes as conductive filler in a cement mortar matrix. Sand content was varied to understand its effect on the observed behaviors. The observed changes in resistivity due to mechanical loading and temperature variations were used to decouple the effects of temperature and deformation on the electrical resistivity of the SSCC studied.

Nomenclature MWCNT

Multi walled carbon nanotubes

CNT SSCC VMA

Carbon nanotubes

Self-sensing cement composite Viscosity modifying agent

T ε α

Temperature

Strain

Thermal expansion coefficient Fractional resistivity change Electrical resistivity at a given time

FRC

R i R 0

Initial electrical resistivity Modulus of elasticity

E

GF ∆ T

Gage Factor

Temperature variation

2. Methodology 2.1. Materials

The SSCC were manufactured using industrial grade multi walled carbon nanotubes (MWCT) acquired from Nanocyl S.A. in Belgium. Additionally, early strength development Portland cement was provided by Lafarge-Holcim in Brazil, while a viscosity modifying agent (VMA) was sourced from BASF Chemicals, also in Brazil. Silica flour with a #325 mesh size was obtained from Mineração Jundú in Brazil, along with a mixture of standardized quartz