# Concrete Silos: Failures, Design Issues and Repair/Strengthening Methods
## 4 Repair and Strengthening Methods for Reinforced Concrete Silos
### Strengthening of RC Silos
#### 4.2.2 External Pre-Stressing for Repair/Strengthening  
The reinforcement of degraded silo structures is a key aspect of the repair process [73]. The external pre-stressing methods are ideal for repairing corroded steel reinforcement bars [78]. The process is characterized by the introduction of tendon pre-stress on the exterior of the concrete structure. The tendons are interlinked using end anchorage and deviators, which occupy small cross-sectional areas. Additionally, the repair techniques ease the inspection and repair processes, and there are marginal friction forces. The pre-stressing can be applied to both new and old/existing structures, subjected to structural degradation, or those that have observable construction and design deficiencies and errors. From a practical point of view, external pre-stressing is suitable to enhance the sustainability of silo structures because fiber-reinforced polymers (FRP) and carbon fiber composite cable are alternatives to steel cable [78]. The FRPs are suitable, considering there is no risk of corrosion. The risk of corrosion is significant in pre-stressing tendons because of the narrow cross-sectional area. Moreover, high-grade stills are highly susceptible to corrosion compared to the low-grade steels. However, if the utilization of steel alternatives is unfeasible, the risk of corrosion can be mitigated by coating with oil, grease, asphalt, plastic tubing, and other anti-corrosion agents. The structure and cross-section of selected composite cable strands are presented in Figure 6.  
The non-steel tendons are ideal given they exhibit \"first-rate behavior in creep and relaxation,\" and high rates of durability and available casting methods are efficient and result in stronger materials [78]. However, similar to other materials, the integrity of the FRP rods used in pre-testing is dependent on the manufacturing process. The commercially available pre-testing rods have different tensile strength, poison's ratio, density, elastic modulus, and cross-sectional area. Each of the listed parameters determines the structural integrity of the silo. Following the comparison of different brands of tendons, the Leadline brand is best suited for silo repairs considering it has the highest mean tensile strength and elastic modulus (2550 Mpa and 150 GPa) relative to other listed brands [78]. On the downside, the tendon has a lower cross-sectional area, which increases susceptibility to corrosion. Additionally, the %strain is low in the Leadline compared to Technora [78]. From an engineering point of view, a low ultimate strain value shows that the material has limited ductility and can fail with minimal warning [79] due to the absence of necking and plastic deformation, which are the hallmarks of ductile materials such as stainless steel [80; 81]. Beyond the mechanical properties, there are other parameters that should inform the selection of laminates, sheets, and near-surface mounted reinforcement (NSMR) for concrete structures tendons are reviewed by Nordin [78]. Similar to other repair/reinforcing method, the external pre-stressing method has its benefits and limitations. First, there is a pronounced risk of corrosion due to the exposure of external tendons to the environment [79]. Second, the installation of the external tendons elevates vibrations, which may cause structural damage. The risk vibrations can be mitigated by reducing the length of the tendons [79]. Another key limitation attributed to FRPs is insufficient ductility, which can result in sudden catastrophic failure considering that the eccentricities of external tendons are lesser than the internal. Even though the FRP tendons have ideal mechanical properties, the structures may not support transverse and large longitudinal forces during loading and unloading.