PSI - Issue 47

Domenico Ammendolea et al. / Procedia Structural Integrity 47 (2023) 488–502 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Keywords: Ultra-High-Performance Concrete; nanofillers; mixed-mode crack propagation; moving mesh technique; Interaction Integral method

1. Introduction Ultra-High-Performance Fiber-Reinforced Concrete (UHPFRC) is known to be able to overcome the main limitations of ordinary concrete, especially in terms of compressive and tensile strength, ductility, energy-absorbing capacity, and durability (see, for instance, (Habel et al., 2006; Habel and Gauvreau, 2008; Kang et al., 2010; Máca et al., 2013; Yu et al., 2014a; Yoo and Yoon, 2016; Krahl et al., 2018; Penna et al., 2022)). Such enhanced properties are strictly related to a mix composition characterized by high cement content, small aggregates, high binder content (silica fume, fly ash, or reactive powder), and low water/cement ratio, as well as to the incorporation of randomly dispersed short fibers (made of steel, glass, polyethylene, carbon, etc.), which are responsible for sensible toughness increments (see, for instance, (Thomas and Ramaswamy, 2007; Hsie et al., 2008)). Despite these advantages, the presence of higher volume fraction values of embedded fibers usually negatively affects the flowability properties of the resulting UHPFRC, as demonstrated, for instance, in (Nguyen Amanjean et al., 2019). Therefore, new-generation concretes with improved workability have been recently proposed, by incorporating micro- and/or nano-reinforcements in the cement matrix (see, for instance, (Park et al., 2012; Yu et al., 2014b)). In particular, nano-filled UHPFRC has been proven to be characterized by excellent strength and fracture toughness properties without any sensible decrease in workability (see (Chuah et al., 2014; Gdoutos et al., 2016)). Several kinds of nano-fillers are currently adopted as a reinforcement for UHPFRC mixtures, such as silica/iron/clay nanoparticles, carbon fibers, graphite nanoplatelets, carbon nanotubes, and graphene sheets (see, for instance, (Meng and Khayat, 2016; Wang et al., 2016; Lu and Ouyang, 2017; Sun et al., 2020; Huang et al., 2022)). Several experimental studies have clearly shown that the overall mechanical behavior of nano-filled UHPFRC strongly depends on the shape and content of embedded fibers as well as on the content of dispersed nano-fillers (see, for instance, (Wu et al., 2016; Meng and Khayat, 2016)). In particular, in (Meng and Khayat, 2016) it has been found that the incorporation of 0.3% graphite nanoplatelets leads to increments of 56% and 187% in terms of tensile strength and energy absorption capacity, respectively. Nevertheless, only a few numerical works have been devoted to the simulation of the mechanical behavior of nano-filled UHPFRC (see, for instance, (Eftekhari et al., 2014) and references therein). This is mainly due to the difficulty of capturing by a single-scale model the complex toughening effect induced by the interaction between micro- and nano-reinforcements. As a practical approach, such an effect could be simulated in an approximate manner by considering properly calibrated constitutive laws at the bulk/interface levels, in the spirit of the cohesive finite element method (see also (Xu and Needleman, 1994; Greco et al., 2020a; De Maio et al., 2021; Gaetano et al., 2022; Greco et al., 2022; Pascuzzo et al., 2022a; De Maio et al., 2022, 2023a; Pranno et al., 2022b), for more general information about diffuse cohesive interface approaches). In the present work, a novel Moving Mesh (MM) approach for the failure analysis of nano-filled UHPFRC structures is proposed. Such an approach, which can be regarded as a refined version of similar modeling approaches recently introduced by some of the authors (Ammendolea et al., 2021; Greco et al., 2021; Pascuzzo et al., 2022b), derives from the synergistic combination of three methodologies:  The Moving Mesh (MM) technique, which can capture the geometric variations caused by arbitrary crack propagation, thus avoiding excessive remeshing operations near the moving crack tips during the simulation.  The Interaction Integral ( i.e. , the M -integral proposed in (Yau et al., 1980)), used to extract the mixed-mode Stress Intensity Factions (SIFs), being required for determining the current crack orientation under general loading conditions.  The R -Curve approach, here proposed as an objective measure of fracture toughness of nano-filled UHPFRC, being properly calibrated from the experimental outcomes of notched beams with different nano-filler contents and subjected to a three-point bending test.