# Concrete Silos: Failures, Design Issues and Repair/Strengthening Methods
## 2 Failure of Reinforced Concrete Silos
### Material Design Factors and the Failure of Reinforced Concrete Structures  
The case studies of failed concrete silos have demonstrated the need for careful selection of construction materials. [PERSON] noted that the use of bolts with unsatisfactory tensile strength was one of the key factors that contributed to the failure of RC silos [7]. Similar to the observations made by [PERSON] [7], [PERSON] [47] observed failure in silos due to improper material design factors that led to concrete surface wear. Impact of Bulk Grain Abrasion on the Hopper Wall Materials  
The wear and tear of the silo walls caused by bulk grain abrasion is an issue of concern considering that it reduces the cross-sectional area of the walls; it may also result in the exposure of the reinforcement bars, which might, in turn, trigger corrosion caused by changes in relative humidity or moisture accumulation on the bulk grains [47]. The risk of corrosion attributed to the bulk grain materials is most pronounced in concrete silo structures reinforced with special aging steel and carbon steels [48]. The accumulation of moisture in the bulk grain is influenced by the type of bulk grain stored--previous research evidence demonstrated dampness in rapeseed meal stored in bulk silo structures [36]. In other circumstances, moisture accumulation resulted in caking and arching. On the downside, the properties of bulk solids and their effect on abrasion have been under-investigated. The observations are consistent with [PERSON] and [PERSON] [49], who noted that \"bulk solid materials were of the least understood areas associated with solid processing plants.\" According to the Janssen formula in Equation (1), the lateral and vertical pressures caused by bulk grains is dependent on silo diameter (\(D\)), acceleration of gravity (\(g\)), coefficient of wall friction (\(\mu\)), and the Johansen's constant (\(k\)), depth of grain in the silo (\(z\)), \(\sigma_{\mathrm{x}}\) and \(\sigma_{\mathrm{z}}\) lateral and vertical pressure, respectively. \[\sigma_{z}\ =\ \frac{\rho gD}{4k\mu}\bigg{(}1-e^{-\frac{4k\mu}{D}z}\bigg{)} \tag{1}\]  
\[\sigma_{x}\ =\ k\sigma_{z}\]  
The limited scholarly data relevant to the subject could be attributed to the impact of other interrelated factors such as the moisture content, flow type, gravity, density, compactness, and the model materials [49; 50]. The bar graph in Figure 1 shows that there was a positive correlation between the moisture content of the bulk solid and the coefficient of friction for wheat. The moisture content of 20% translated to the highest coefficient of internal friction. The patterns observed in Figure 1 are consistent in other cereals, namely corn, oats, and barley, as shown in Table 5. However, in the case of corn, a saturation point was achieved at 27%, after which any further increase in the moisture content did not translate to a higher friction coefficient. The pressure ratios derived using Jaky and Euro-code designs had an undefined relationship with the angle of internal friction and the moisture content. Based on these data, the wearing of the internal walls of silo hopper structures can be mitigated by controlling the moisture content of the bulk solids [49]. Alternatively, the liners and sacrificial plates [47] could help to mitigate friction in silo structures holding barley, corn, and other grains.