# Concrete Silos: Failures, Design Issues and Repair/Strengthening Methods
## 2 Failure of Reinforced Concrete Silos
### Material Variability and Loading/Unloading Factors  
According to research, there are diverse wall loading scenarios for reinforced concrete silo structures, depending on whether the materials are used in the construction of the hopper walls or barrel walls [23]. Based on the wall loading assessment, materials with advanced mechanical properties were necessary for the cylinder and hopper walls. An experiment conducted on a silo handling bulk maize noted that the loading and unloading impacted the structure of the silo shows that the optimal accumulation of vertical pressure in the silo bottom was recorded from day 1 to day 40. The decline in silo pressure due to bulk grain unloading was noted from day 40 to day 58, a trend that was also correlated with the decline in the column strength. A significant decline in column strength was noted from day 40 to day 58 [24].  
The load and unloading scenarios listed in Table 1 are influenced by thermal ratcheting--a condition that has been observed in both metallic and reinforced concrete silo; it is characterized by the expansion and contraction of the hopper walls in line with the fluctuations in the daytime and nighttime temperatures. Lab experiments have shown that the temperatures may vary by up to \(\pm\)35-40 \({}^{\circ}\)C, leading to variations in the internal frequency angles [11]. Each temperature cycle contributes to the buildup of pressure and stress on the wall structures leading to failure [1; 11]. Even though the phenomenon is highly common in metallic silos (metals have a higher coefficient of thermal expansion), thermal ratcheting has also been observed in reinforced concrete silos. The coefficient of thermal expansion for concrete varies between \(6\times 10^{-6}\) and \(8\times 10^{-6}\)\(1/^{\circ}\)C in the transverse and longitudinal directions, respectively [25]. Even though thermal ratcheting is a significant problem for reinforced concrete structures, it has been under-appreciated in silo design codes [1]. The exclusion or disregard of thermal ratcheting in the design of modern silos increases the risk of structural failure, as evidenced by the collapse of a 9000-ton silo in South West US [11]. The materials used in reinforced concrete structures are reviewed in Section 2.2.  
The four globally accepted silo design codes are British Standard BS EN 1991-4:2006, Australian Standard AS 3774-1996, American Society of Agricultural Engineers ANSI/ASAE EP433 DEC1988 (R2011), and American Concrete Institute ACI 313-97 [1]. However, the Australian Standard AS 3774-1996, and American Concrete Institute ACI 313-97 do not provide estimates for thermal ratcheting on silo--a factor that elevates the risk errors in construction. Additionally, the estimates for thermal ratcheting effects outlined in the American Society of Agricultural Engineers ANSI/ASAE EP433 DEC1988 (R2011) are based on lab experiments and models of steel silos rather than in-situ data derived from large physical silos. Additional constraints associated with the design codes was the inclusion of the Janssen's Cd parameters to cover for the inaccuracy in the estimation of the bulk grain discharge pressure [26]. Even though the parameter helped to ease the pressure during the solid bulk discharge, it is inadequate because the design codes largely disregard the asymmetric flow patterns.