# Concrete Silos: Failures, Design Issues and Repair/Strengthening Methods
## 4 Repair and Strengthening Methods for Reinforced Concrete Silos  
### Repair of RC Silos  
From an abstract point of view the repair of RC concrete silos should focus on variables that engineers fail to anticipate/design errors [73] construction errors (use of inappropriate or poor quality materials), the maintenance issues. [PERSON] [48] notes that the main design errors encompass eccentric withdrawals that result in the bending of circular walls due to non-uniform increasing hoop pressure exerted on the circumference of the silo. The design-related error can be corrected by ensuring that the withdrawal point of the material as aligned with the vertical center-line [48]. Additionally the pressure risks can be mitigated by reducing the number of outlets group configurations--many outlets are a risk factor to pressure accumulation. Another key issue that should be taken into account during the design of silo hoppers is the non-symmetrical pressure increase induced by the incorporation of inserts [48]. Considering that inserts are incorporated to mitigate the over-pressures associated with extended flows funnel flows [66] the removal of the inserts would have mixed effects that might compromise future repair processes. On the one hand the non-symmetric pressures would be reduced. On the other hand overpressures would increase. The risk factors for self-induced vibrations during loading and unloading should be addressed to reduce the frequency of the shocks and structural damage. Various interventions have been proposed in an attempt to mitigate the vibration related risks. For instance, [PERSON] and [PERSON] proposed the installation of the discharge pipes [48]. Alternatively, the risks associated with the vibrations could be mitigated by the addition of a structured interface on the hopper wall, which in turn facilitates the creation of the shear zone. The creation of a shear zone is integral to avoiding slip-stick intermittent flow [48]. However, the practicality of the proposed design solutions remains a challenge considering the fact that any changes made during repair have to be aligned with the design code requirements, structural designs, and foundation loads. In brief, the repair of silo structures involves a delicate balance between reinforcing the structure and adhering to the load requirements. In rare cases, the moisture may accumulate within the hoppers, and bulk grain flows may oscillate between funnel and mass flows, which is detrimental considering that the latter has been associated with arching and ratholing [65]. The moisture accumulation can be corrected during silo repairs by the installation of dehumidifiers and ventilators, as proposed by MIP-NV Company [74] or by draining the bulk solids [46]. Other studies have proposed the placement of sorbent materials and electrical aeration of the silo before loading with the bulk solids [58]. On the downside, the installation of dehumidifiers and ventilators introduces an element of cost and energy demand, which limits the sustainability of the silos. The aeration of the silo can lead to grain swelling and higher pressure and boundary layer effects on the silo walls [75]. The methods for rehabilitating deteriorated concrete are listed in Table 8. The role of the equation of silo cracking on the design of silo structures is reviewed in the next section. Apart from the design-related solutions, the repair of silo structures is influenced by the equation of silo quaking, which provides a theoretical foundation for the repair of damaged silos. However, the equation is only relevant for silos exposed to quaking, and the theoretical assumptions are limited. For example, the associated dynamic forces and the flow rates are not accurately predicted, and new computational methods are necessary. Quaking can be addressed by adjusting the dampening