PSI - Issue 44

Guadagnuolo M et al. / Procedia Structural Integrity 44 (2023) 942–949 Guadagnuolo et al. / Structural Integrity Procedia 00 (2022) 000–000

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bond tensile strength compared to traditional polymer pultruded pulleys (FRP) (Chen et al. 2022). Anyway, it is common that the increase in bar diameter is related to lower bond strengths (Achillides 1998; Achillides and Pilakoutas 2004; Benmokrane et al. 1996; Chen et al. 2021). The influence of embedment length, type, shape, surface characteristics, and bar diameter as well as the concrete strength on bond strength are being investigated (Achillides and Pilakoutas 2004; Ahmed et al. 2021). For example, the interlaminar shear strength just below the resin-rich surface layer of the bar influences it more than the concrete strength; the shape of the cross-sectional area of the bar also plays a role in the bond strength, and the square bars develop a higher bond strength than round bars probably due to the sharp edges (Achillides 1998). Therefore, it is complex to predict the actual behavior of the FRP’s interface bond. ACI 440.3R-04 (2017) was the first attempt to share a unified method for testing FRPs for use in strengthening R/C structures. The FIB standards (Tepfers et al. 2000) consider the accuracy of models proposed by some authors such as the Bertero-Popov-Eligehausen (BPE) and Cosenza-Manfredi-Realfonzo (CMR) models (Cosenza et al. 2002; Eligehausen et al. 1983; Malvar 1994). In any case, the proposed bond-slip analytical relationships always depend on several parameters that can only be calibrated through experimental data rather than average values (Baena et al. 2009; Gao et al. 2019; Pepe et al. 2013), as well finite element analyses also requires the same parameters (Lu and Sonoda, 2021 Pepe et al. 2013; Zhao et al. 2014). 2.1.2. Masonry The lack of knowledge about FRP bond behavior is present not only when the existing structure is a concrete structure, but also for other materials such as masonry. In all cases, it is necessary to know the actual parameters influencing the strengthening. For example, injection anchorages as additional reinforcement within historic masonry (Gigla 2004) depend on the water absorptive capacity of the surrounding stone material. By comparing theoretical values of maximum pull-out forces with experimental ones (Ceroni et al. 2016), strength models all fail to predict experimental results. A reason is that they do not consider the surface properties of the bars that influence the shear strength. The gap between different approaches for determining anchoring behavior is huge. A way is to choose the smallest value of the force provided by formulas or models that best fit the experimental data and modify this value using appropriate design safety factors (Melatti and D'Ayala 2021). But the scaling effect is one of the main influencing factors, and when it is not considered, the results of experimental tests, analytical models, and numerical analyses often appear to be comparable to each other, in terms of maximum strength and displacements achieved before the anchorage failure (Bertolesi and Galli 2012). 2.2. Grout During the post-installation process, reinforcing bars are inserted in holes (mechanically drilled) of the support and solidified with a binder or adhesive material. The bonding with existing concrete plays a key role in determining the actual improvement in structural performance (Ahmed et al. 2021). In concrete, most of the strength models in the literature do not allow for realistic predictions of the type of failure and the associated slip force, mainly because the real contribution of grout is not available. Current standards and design guidelines impose a uniform bond model that is a good approximation for short-term testing, but not for long-term testing. Using a numerical approach, it is possible to establish the strong adversarial dependence of the time scale on deformation. However, in concretes with strengths of about 20 MPa, this assumption is an acceptable approximation for the vinyl-ester-based anchorage system (Boumakis et al. 2022). In masonry, cement-based grout yields the best performance for both steel and GFRP bars compared to pozzolan based grout (Ceroni et al. 2016). This parameter is all but involved in the common formulas for predicting the failure mode known as "masonry break-out" or "cone-shaped masonry." They refer only to masonry compressive strength and anchorage length: maybe because they were originally assessed for steel bars embedded in concrete elements. The formulas provided in ACI 318 and fib 58 significantly underestimate the average experimental failure loads, while almost all other theoretical formulations significantly overestimate them. However, none considers the surface properties of the bars that play a key role in the efficiency of injected anchorages (Ceroni et al. 2016).