PSI - Issue 42

E. Tziviloglou et al. / Procedia Structural Integrity 42 (2022) 1700–1707 Tziviloglou et al. / Structural Integrity Procedia 00 (2022) 000 – 000

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1. Introduction Cementitious composites have been used in a wide variety of applications, despite demonstrating low tensile strength and fracture toughness, attributed to their brittle nature. According to Parra-Montesinos et al. (2012), essential scientific work has been undertaken to alleviate the brittleness of cementitious composites by introducing various natural and synthetic reinforcing materials. Among them, the use of carbon-based nanomaterials (CBNs), which exhibit exceptional mechanical and electrical properties, meets the demand of creating multi-functional smart cementitious composites with improved crack-bridging and sensing properties (Lu et al. (2022)). Graphene nanoplatelets (GnPs) are commonly used as nanofiller in cementitious composites. GnPs are exfoliated from pristine graphite and consist of several layers of graphene sheets. Furthermore, GnPs are known to promote the mechanical, thermal and electrical properties of various materials, including cementitious composites, as indicated by Pan et al. (2015), while exhibiting reduced production cost compared to other CBNs according to Lu et al. (2008). Chougan et al. (2019) also showed that due to their characteristics, GnPs can refine the microstructure of the cementitious composites by creating a denser matrix, as they provide nucleation sites for cement hydration products to deposit and grow. Therefore, the addition of GnPs in cementitious materials can optimize the pore structure, and enhance the mechanical performance, as it was shown by Baomin et al. (2019). Moreover, the GnPs-modified cementitious mixtures exhibit conductive and piezo-resistive properties, which can give useful information regarding the stress/strain condition of the material or detect potential internal damage, as suggested by Tao et al. (2019). Agglomeration of GnPs, due electrical interactions, may negatively affect the cementitious matrix by introducing flaws that can attract cracking, causing adverse effects by reducing mechanical strength and influencing electrical conductivity. Thus, Metaxa (2015) and Papanikolaou et al. (2021) proposed an efficient dispersion procedure, which includes application of ultrasonic energy to disperse graphene in water, in the presence of superplasticizer. In addition, various laboratory studies, such as Chougan et al. (2019), Dela Vega et al. (2019) and Dalla et al. (2021), investigated the effect of the incorporation of GnPs in the cementitious matrix through mechanical testing, which often includes compressive and/or flexural strength tests. The studies revealed enhanced compressive and flexural strength up to specific GnP dosages, attributed to the density enhancement, accompanied by inhibition of crack propagation caused by the incorporation of the GnPs in the cement paste. On the contrary, above a certain nanofiller concentration, the mechanical response was degraded, due to the excessive agglomeration of the nano particles. Furthermore, electrical impedance spectroscopy (EIS) has also been applied by Wansom et al. (2006), Danoglidis et al. (2016), Li et al. (2016) and da Silva et al. (2021) in CBN-reinforced cementitious composites to study various aspects; such as the dispersion state of CBNs, the hydration process and the microstructure changes, and for the assessment of mechanical response. The current work investigates the effect of GnPs on the fracture toughness and electrical resistivity of the hardened cementitious matrix with the intention to explore a correlation between those two parameters. Consequently, EIS measurements are examined to be used as a non-destructive tool for evaluating fracture toughness of the hardened cementitious matrix. Nomenclature CBNs carbon-based nanomaterials GnPs graphene nanoplatelets EIS electrical impedance spectroscopy SP superplasticizer

2. Materials and Methods 2.1. Preparation of cement specimens

Cementitious nanocomposites were cast by using Type I ordinary Portland (42.5 R) and Grade M graphene nanoplatelets with an average particle diameter of 5 μm and a thickness of 6 - 8 nm, provided by XG sciences Inc.,