PSI - Issue 64

Salvatore Verre et al. / Procedia Structural Integrity 64 (2024) 1508–1515 Salvatore Verre / Structural Integrity Procedia 00 (2019) 000 – 000

1509

2

impregnates the fibers and glues them to the substrate. Despite several advantages, such as low weight and high strength and stiffness, FRP composites have important drawbacks, such as the lack of permeability, low transition temperature of the resin, and the impossibility of being glued to wet surfaces. To overcome these disadvantages, FRCM (fabric reinforced cementitious matrix) and SRG (steel reinforced grout) composites were introduced at the beginning of the 2000s (CNR 2018 and ACI 2020). FRCM-composites consist of open mesh textiles embedded in an inorganic matrix. Similarly, in SRG composites, high-strength steel fibers are embedded in a cementitious or lime based grout. Steel fiber textiles for SRG composites are made of steel cords arranged at different spacings, resulting in different densities. Typical densities are 670 g/m 2 , 1200 g/m 2 , 2000 g/m 2 , and 3300 g/m 2 . The corresponding textiles are usually referred to as low density (LD), low-medium density (LMD), medium density (MD), and high density (HD), respectively. Both for FRP and FRCM/SRG composites, the bond-to-substrate behavior plays a fundamental role in design applications since the failure of strengthened structural members is often caused by the loss of composite action due to the debonding of the composite material (Ombres 2011, Napoli and Realfonzo 2015, Sneed et al. 2016, D’Antino et al. 2020, Ombres and Verre 2021). A great research effort was devoted to studying the bond behavior of FRCM and SRG composites (D’Antino et al. 2014, Carloni et al. 2022). In addition, the design procedures defined in CNR (2018) are based on the effective strain determined with bond-to-substrate shear tests. According to ACI (2020), the effective strain is determined based on the results of clevis-grip tensile tests, which depend on the fiber-matrix bond properties (Focacci et al. 2020, Focacci et al. 2022). The bond behavior of SRG-composites is generally evaluated with single- or double-lap direct-shear tests. These tests allow to identifying the debonding mechanism and determining design parameters such as the debonding load (or bond capacity), i.e., the maximum force that can be transferred to the substrate, and the effective bond length, i.e., the length of the bonded area needed to obtain the debonding load (Santandrea et. al 2016). In addition, the results of single- or double-lap direct-shear tests can be used to determine the interfacial CML (cohesive material law) (Santandrea et al. 2020). The influence of several geometrical and mechanical parameters on the results of single-lap shear tests was investigated by researchers in recent years. The main parameters investigated were the bonded length and width, density of the textile, roughness of the substrate, concrete substrate grade, and number of textile layers. The role of the bonded length was investigated with the aim of estimating the effective bond length L eff . L eff equal to 200 mm was determined by Bencardino et al. (2017) and Ombres and Verre (2020), whereas Ascione et al. (2020) estimated L eff in the range 200mm÷300mm, Santandrea et al. (2020) estimated L eff in the range 175÷200mm, and Thermou et al. (2021) estimated L eff in the range 100÷200mm. The role of the bonded width was investigated in Ascione et al. (2020), where a maximum transferable load proportional to the bonded width was observed. Ascione et al. (2020) and Thermou et al. (2021) observed different debonding mechanisms depending on the density of the textile. It was observed that the debonding mechanism and bond capacity depend on the amount of mortar passing through the voids between the cords. According to Ascione et al. (2020), the mechanical properties and the roughness of the concrete substrate play a minor role in the results of single-lap shear tests, whereas different debonding mechanisms were observed by Thermou et al. (2021) depending on the compressive strength of the substrate. The role of the number of textile layers in the SRG composite was investigated by Ascione et al. (2020) (1 and 2 layers) and Thermou et al. (2021) (1, 2, and 3 layers). In the case of LD textile, a change in the failure mode from fiber failure to debonding at the substrate-matrix interface was observed by Thermou et al. (2021) when the number of layers increased from 1 to 2 and 3. The same authors, in the case of LMD textile, observed a change in the failure mode from debonding at the fiber-matrix interface to debonding at the substrate-matrix interface when the number of layers increased from 1 and 2 to 3. In this paper, the results of single-lap shear tests of a SRG composite bonded to a concrete substrate are presented. Specimens with 1 and 2 layers of LD textile were tested to assess the influence of the number of textile layers on the debonding mechanism and bond capacity. A CML of the fiber-matrix interface obtained from the experimental data is also presented. 2. Experimental investigation 2.1. Materials and specimens Nine single-lap shear tests were performed at the University of Calabria laboratory using a push-pull configuration