## 3 Numerical Model Solution
When varying stiffnesses between two elements, it is necessary to designate a \"Target\" element for the higher-stiffness element while allocating a \"Contact\" element to the lower-stiffness element. To fulfill this objective, Targe170 and Conta174 were chosen due to their compatibility with the surface intended for connection. The geometric properties of the \"Contact\" element are illustrated in Figure 17. The elements in question are characterized by four nodes; they also possess equivalent degrees of freedom to the SOLID65 and SHELL181 elements and lack intermediate nodes. The real constant set, a single-element constant, is defined by the process of matching the surface normals. Values such as the Coulomb friction coefficient (\(\upmu\)), cohesion (C), normal and shear stiffness, and maximum stress between two surfaces (TMAX) can thus be defined. These parameters were of utmost significance for the present investigation. The compatibility between the wrapping materials and concrete in terms of their interactions does not adhere to a conventional Coulomb friction model. This phenomenon arises due to the relationship between the fraction of the normal force exerted on the surface and the friction coefficient, which might generate a lateral force. Given the absence of  
Figure 21: Contact elements. a normal force effect and a friction coefficient between the wrapping material and the concrete, it becomes necessary to establish a maximum stress value [54]. The degree of slippage occurring between these contacting surfaces is directly correlated with the lateral stiffness. Increased lateral or vertical stiffness decreases slip occurrences. However, a decrease in slip quantity also gives rise to convergence issues. A lack of rigidity might result in slippage and potentially cause inaccurate measurements. To minimize slip, it is advisable to select a very high stiffness value, although opting for a relatively low stiffness value can help prevent convergence issues. These behaviors inside ANSYS can be defined using element constants and KEYOPT. As noted, through a series of experimental evaluations conducted, we found that alterations in TMAX values substantially impact stress and strain distributions [13; 49; 53; 55]  
The geogrid model was enveloped with repair mortar in the tension zone of the beam, incorporating experimental data on friction angle, shear, tensile, and compressive stresses, as well as information on the contact between the concrete surfaces. In the model, 5592 nodes, 4800 (25 \(\times\) 25 \(\times\) 25 mm\({}^{3}\)) solid elements, 791 link elements, and 6236 shell (25 \(\times\) 25 mm\({}^{2}\)) elements were used (Figure 22)  
According to their stiffness in ANSYS, the contact elements of the adhesives specified in the experiment empirically connect the concrete surface and the coating materials [49]. The concept of cohesive zone material (CZM) is applied between the concrete and fiber, where shear stress (\(\tau\)), interfacial separation (s), and stiffness (k\({}_{\text{s}}\)) are defined. \[k_{s}=\frac{1}{\frac{t_{1}}{C_{\text{s}}}+\frac{t_{c}}{C_{\text{c}}}} \tag{2}\]  
Conta174 is the contact element, and Target170 is the target element (Figure 21). The fiber interface, Conta174, is also used in rigid-flexible and flexible-flexible contact analysis [13; 54; 56; 57]. The contact between the concrete and mortar is established based on Equation (2).