Silo Design Best Practices 3: Exploring Repair Options
Several options are available for repairing silos with overstressed silo walls. The most common, and most robust, long term solution is the design of a reinforced concrete liner. This liner is a replacement wall that is installed on the interior or exterior of the existing silo wall.
If the silo in need of repair is a single, standalone silo, an exterior liner is a viable option. However, if the silo is part of a group of two or more silos, with a shared common wall area, then an exterior liner will not provide the circumferential continuity that is needed. While in some cases an exterior liner may be an option for this situation, by and large the optimal solution for groups of silos is an internal liner.
A new concrete liner would be designed accounting for the actual effects of how the silo will be operated in the future and will account for load transfer down to the foundation for any gravity loadings. An internal liner may change the locations of air pads, and may require doweling into the existing wall to transfer gravity loads.
Carbon fiber is generally not a viable solution for situations such as these, even for a single, standalone silo. Carbon fiber only provides tensile resistance on one face in bending. As the effects of the blending and homogenizing process require tensile resistance on both the inside and outside face of the silo, carbon fiber would need to be installed on both the interior and exterior faces of the silo wall. Many times, this is impractical at best, and impossible in many ways. Additionally, the temperature of the material inserted into the silo can exceed 200 degrees Fahrenheit. Many epoxies used to adhere to the carbon fiber have an upper temperature limit of approximately 180 degrees Fahrenheit.
External post tensioning, where high strength steel cables are wrapped around the perimeter, is a temporary option for repair. However, this is not a permanent solution. This repair compresses the silo wall, by the use of the post tensioning. However, similar to the carbon fiber material above, post tensioning alone will not provide the resistance to bending. It will, however, slow the rate of deterioration and should prolong the life of the silo. But the damage will continue at a slower rate, and the post tensioning should prevent catastrophic collapse for some time.
Post tensioning may be a sound repair option in some cases. It may be cost effective to choose this option for a standalone silo, where the post tensioning can relatively easily be installed around the perimeter of the silo. If repairs are needed for a silo group and silos that share a common wall, then the post tensioning would have to extend through the common wall. This would entail costly core drilling the entire length of the common wall, heavily reinforced concrete encasements to transfer forces of post tensioning strands.
In summary, blending and homogenizing silos are subject to forces that “typical” silos are not. The operational requirements of producing a homogeneous material has overstressed a generation of homogenizing and, more recently, blending silos. There have been isolated cases of catastrophic failure, and there are many silos across the world with challenges that are in dire need of repair. The root cause is a lack of understanding, or miscommunication, of what the blending and homogenizing process is and how it affects the structure.
Repairs are needed, and there are options for such. However, typically the best option is to install a reinforced concrete liner, essentially a second silo wall, sized and reinforced appropriately for the loads imparted to it by the process inside.
This blog is part of a three part series. Read part one, Silo Design Best Practices 1: Homogenizing and Blending Silos, and part two, Silo Design Best Practices 2: Structural Challenges.
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