Silo Repair and Protection

Reinforced concrete silos can fail over their useful lives because of issues related to foundation and soil factors, material, inadequate design, construction factors, silo design codes, seismic activity, and other natural phenomena and durability. The flow and frequency pattern of a silo loading and unloading contributes to the creation of stress on the structure, with high loading and unloading frequencies increasing risks of failure over time. Other variables to consider are the physical properties of the material being stored, pressure applied when filling and discharging silos, wind action, and the expansion and contraction of the walls, in line with the fluctuations in the daytime and nighttime temperatures. Although thermal action is more of an issue in metallic structures, it has also been observed in concrete-reinforced silos.

Durability problems are often due to poor or inappropriate construction materials, inadequate design or construction, and aggressive environments not properly understood during the design stages. They also are often the result of increased structure life-span requirements made on aging infrastructure. Cement abrasion, or the wearing of concrete due to contact with abrasive materials, is another common problem that can affect concrete silos. To prevent or repair this type of damage, it is important to use high-quality concrete and to maintain silos properly.

The choice of materials to be used in the construction of concrete structures is influenced by their mechanical properties, variability, thermal behavior, and loading scenarios. Replacing steel reinforcements in the concrete with carbon fiber-reinforced polymers (CFRP) reduces the coefficient of thermal expansion and thermal ratcheting as the coefficient of thermal expansion for steel is significantly higher relative to CFRP. Thus, CFRPs are a popular choice for repairing and strengthening concrete silos because they are lightweight, corrosion-resistant, and have high tensile strength. One common repair method that uses CFRPs with high modulus of elasticity is external prestressing, where tendons made of CFRP are tensioned and anchored to the outside of the concrete silo. The pre-stressing can be applied to both new and existing structures.

Carbon Fiber Reinforced Polymer (CFRP) physical and mechanical properties are perfectly suited for strengthening the flexural, shear, compressive, fatigue, seismic, wind, crack, and deflection control of any given structure, making them ideal for wrapping silos. In turn, this means that the foundations and buildings will not experience punishment from these conditions either, as they will be heavily protected by such a strong and impervious substance. The Carbon fibers do not absorb water and are resistant to many chemical solutions. They withstand fatigue excellently and neither corrode nor show any creep or relaxation. They are easy to apply, economical, and durable and upgrades are possible even with limited access. They are supplied in rods, strips, pre-saturated and uni and bi-directional fabric. Carbon Fiber reinforcements have an upper limit beyond which they do not translate into an improvement in tensile strength. Replacing steel with fiber-reinforced polymers is also wise economically as it avoids steel corrosion implications on the concrete structure. From a long-term view, the benefits outweigh the limitations considering that the tendons can be easily repaired; friction losses and dead loads are diminished, and the anchorages are customizable.

Globally accepted silo design codes include 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.

Insertion or removal of steel inserts is another repair strategy that can be used to strengthen concrete silos. This involves adding or removing concrete or other materials from the silo to adjust its structural properties. Shear columns, which are vertical elements that help resist shear forces, can also be added to reinforce a silo.

Overall, the main challenges in maintaining concrete silos are to prevent and repair damage and to ensure that the silo is structurally sound. Using materials and repair methods that are resistant to corrosion, such as FRPs, and properly maintaining the silo can help to address these challenges. Inadequate or absent protective coatings on concrete surfaces expose them to significant risks over time. When concrete is not properly sealed, cracks can develop, serving as entry points for water and corrosive agents like sea-salt. This infiltration initiates a destructive process known as carbonation, wherein carbon dioxide reacts with the calcium hydroxide in concrete to form calcium carbonate. This weakens the concrete’s alkalinity, leaving it vulnerable to corrosion of embedded steel reinforcement.

As this corrosion progresses, it compromises the structural integrity of the concrete, posing significant risks to the stability and safety of the affected structure. Moreover, the presence of sea-salt exacerbates the corrosion process, accelerating the deterioration of the concrete.

The consequences of neglected protective coatings are manifold. Not only does it jeopardize the structural soundness of the concrete, but it also escalates repair costs substantially. Addressing corrosion and structural damage requires extensive remedial measures, often involving extensive repair work or even complete replacement of affected components.

Furthermore, the implications extend beyond immediate financial burdens. Structural deterioration can lead to disruptions in operations, potential safety hazards, and even legal liabilities in some cases.

Effective protective coatings are indispensable for safeguarding concrete structures against the detrimental effects of environmental factors. They create a barrier that shields the concrete from moisture ingress and corrosive agents, thereby prolonging its lifespan and minimizing the need for costly repairs. Investing in high-quality protective coatings not only preserves the structural integrity of concrete but also ensures long-term durability and resilience against the harsh elements.”

A typical Silo Repair Application

In a typical concrete silo repair application, there is a need for de-rusting the old rebars or replacing them. The corrosion protection can be done through the application of StoCrete TK, a cement-bound and polymer-modified corrosion protection, which offers very good adhesion to concrete reinforcing bars. It is applied in three layers with the average consumption rate per layer of 0.15 kg/linear meter. It is applied in two different colors, light color alternating with dark color, to allow the applicator to monitor the number of layers applied.

Thereafter, StoCrete TS 136, a low rebound polymer-modified cementitious dry shotcrete (SRC) with strength and deformation properties, is used as a concrete repair product. It is applied in two layers. The first layer’s thickness, should leave the surface with a rough spray texture and the second layer should be planned as a levelling layer. When spraying the levelling layer, the first layer should still be slightly damp and free from substances that have a separating effect. With a total thickness of 50 mm and an average consumption rate per mm thickness of 2.1 kg/m², the consumption of both layers hovers around 110 kg/m². This concrete repair application is adapted to both old concrete and substrates with lower strength.

To provide a concrete dynamic crack bridging protective coating, three layers of StoCrete FB are thereafter applied. Each of the three layers applied has a different thickness, with the first-layer averaging a consumption of 1 kg/m², the second layer, a consumption of 1.5 kg/m², and the third layer, an average consumption of 2 kg/m². Thereafter, two final coats of a concrete crack bridging product coating, StoCrete RB, are applied, with average consumption rates of 0.35 liters per m².

Finally, the application of StoCryl EF, with an average consumption of 0.15 liters per m², is optional as protection against dirt pick-up.