Material Stored: Clean Coal
Silo Size: 70' diameter by 200' tall, Quantity (2) with Interstice
Issue: Asymmetric flow caused concrete delamination.
Pictured above are two Slipform concrete silos with an interstice connecting them. Designed in the 1970s, these silos were originally constructed to be internally reinforced, however, due to asymmetric material flow within, the exterior concrete began to crack and delaminate.
To remedy this issue, steel post-tensioned strands were installed for additional structural support. The strands work to essentially hold the walls against the additional stress put on them by the asymmetric flow.
Due to the interstice prohibiting the post-tensioning of the reinforcing steel hoops to be placed uninterrupted around the silo, holes had to be drilled through it. This allowed for the reinforcing steel hoops to maintain constant contact with the silo walls to minimize future cracking and delamination of the walls.
Product flow was not commonly studied until the 1980's. Asymmetric flow is when the material, instead of flowing straight down through the middle, actually flows against the silo wall. Experts now know that this flow type creates stress against the concrete wall. The silos pictured were not designed to handle this added stress and therefore needed the additional structural support.
Initial design in any new construction is imperative, and with continuing education of how the silos function, material flow is considered during new silos in the design phase. Today this issue could have been prevented prior to construction with the advances in technology.