PSI - Issue 64

1034 Veronica Bertolli et al. / Procedia Structural Integrity 64 (2024) 1033–1040 2 Veronica Bertolli, Lesley H. Sneed, Francesco Focacci, Tommaso D’Antino/ Structural Integrity Procedia 00 (2019) 000–000 1. Introduction Among different strengthening techniques for reinforced concrete (RC) and masonry structures, the use of externally bonded (EB) reinforcement gained large popularity during the past decades. Nowadays, the use of fiber reinforced polymer (FRP) composites and of their inorganic matrix counterpart, i.e., fabric-reinforced cementitious matrix (FRCM) composites, is becoming increasingly popular (Brückner et al. (2006), D’Ambrisi et al. (2013), Carloni and Focacci (2016), Cholostiakow et al. (2020)). FRCMs overcome some of the drawbacks related to the use of epoxy resin, such as poor resistance to UV light and irreversibility of the application. They have good resistance to (relatively) high temperature and good compatibility with the substrate (Papanicolaou et al. (2008), Bertolli et al. (2023)). FRCMs comprise high-strength and high-stiffness fibers embedded in cementitious- or lime-based inorganic matrices (Arboleda et al. (2016), Koutas et al. (2019)). The most commonly used types of fibers are glass, basalt, carbon, polyparaphenylene benzobisoxazole (PBO), and steel, and are usually arranged in unidirectional or bidirectional open-mesh textiles. When bidirectional textiles are used, fibers are organized in the textile primary direction (PD) and secondary direction (SD). When steel fibers are used, the inorganic-matrix composite is usually referred to as steel reinforced grout (SRG) (Santandrea et al. (2020)). FRCM tensile mechanical properties can be determined with clamping- or clevis-grip tensile tests. Along with the tensile properties of fiber and FRCM, the effectiveness of an EB FRCM strengthening system relies on its bond capacity. Indeed, experimental results showed that failure of EB FRCM reinforcement is usually due to loss of adhesion of the composite, either from the substrate or between fibers and matrix (D’Antino et al. (2014)). The FRCM bond behavior was widely investigated in the literature (Malena (2018), Misseri et al. (2021), Bertolli and D’Antino (2022)). FRCMs can effectively be used for shear, flexural, and torsional strengthening of RC beams and confinement of axially loaded members (Triantafillou and Papanicolaou (2006), Thermou et al. (2015)). FRCM was proven to be an effective solution for the shear strengthening of RC beams (Brückner et al. (2008), Gonzalez-Libreros et al. (2017a)) in three different configurations, namely side-bonded, U-wrapped, and fully-wrapped to the flexural member cross-section. In the side-bonded configuration, the FRCM is applied to the lateral faces of the beam, whereas in the U-wrapped configuration the strips are wrapped around the cross-section bottom and sides. In fully-wrapped members, the FRCM is applied around the whole cross-section (Escrig et al. (2015), Ombres and Verre (2018)). In the U wrapped configuration, mechanical or textile-based anchorages can be used to delay/prevent composite debonding (Trapko et al. (2021)). FRCM can have a continuous or discontinuous (i.e., strips) layout, and fibers can be applied to the RC beams with different inclination angles, with  =90° being the most used. The shear strength of RC beams strengthened with EB composites (FRP and FRCM) is usually computed as the sum of concrete, V c , internal transverse steel, V s , and EB reinforcement, V f , contributions:

(1)

u s f V V V V    c

where the simultaneous attainment of V c , V s , and V f is assumed (CNR-DT 200 R1 (2013), ACI 440 (2017), CNR-DT 215 (2018), ACI 549 (2020)). However, an adverse interaction between internal ( V c and V s ) and external ( V f ) shear resisting mechanisms was observed in some experimental investigations (Ombres (2015), Gonzalez-Libreros et al. (2017b)). V c and V s are provided by guidelines and codes for the design of RC members (e.g., EN 1992-1-1 (2004), ACI 318 (2019)), while V f is usually computed with the Mörsch truss analogy:

w





f

2 σ f fe V n 

cot θ cot β sin β 

(2)

d

t

f

e

f

i

f

In Eq. (2), t f is the equivalent thickness of the textile, w f the width of the strips (measured orthogonally to the fiber PD), i f the center-to-center spacing of the strips (measured along the beam longitudinal axis), n the number of textile layers,  the angle between the fiber PD and beam longitudinal axis, and  is the inclination of the concrete strut. d fe is the effective depth of the FRCM reinforcement, usually assumed as the minimum between d f and z =0.9 d (Fig. 1).  fe is defined as the maximum average axial (i.e., fiber PD aligned) stress in the composite reinforcement bridging the main shear crack. Analytical models available in the literature give different definitions of  fe (Ombres (2015), Tetta et al. (2018), D’Antino et al. (2020)) and are based on different basic hypotheses related to the tensile and bond