Concrete producers are constantly faced with changing technology. Computer control systems, reclamation equipment, mixing techniques, chemical admix mechanics, and material handling advancements are but a few of the current technologies in the ever-evolving concrete production industry. At least some things never change: take storage silos, for example. These vertical “tanks” are about as simple as it gets, right? Dead wrong — and ‘dead’ may prove the critical term if a concrete producer or his plant engineer is careless in the design or use of a silo.

It may surprise some that storage silos are not just tanks but, in fact, structures that have function criteria including how the silo is to fill, empty, and store a given bulk product; flow pattern, i.e., mass or funnel flow; structural geometry comprising the shape and materials incorporated in silo design; and how these are affected in full or empty operations and under varied ambient conditions.

Understanding the material to be stored

A common mistake on the part of concrete plant owners arises from the assumption that a “cement silo” design can be used for storing any bulk product around the concrete plant. The fact, however, is that cement storage design criteria are distinct; it is not safe to assume that slag, fly ash, silica fume or silica sand, lime, or other bulk fines can be contained in a silo originally designed for cement storage. Producers need to understand the characteristics, properties and flow patterns of any material to be stored in a silo.

To determine a material property and flow pattern, contacting the material supplier and asking for complete product data worksheets is recommended. Such measures have become increasingly important as replacement ingredients in bulk are being added with far greater frequency to concrete and concrete products. Self Compacting Concrete (SCC) and High Performance Concrete (HPC), often calling for various bulk powder or fines additives, are frequently purchased without consideration of their respective silo storage requirements. For instance, cement is aerated to make it flowable or fluid-like. Although weighing in some cases as little as 60 percent of certain cements (by volume), fly ash can be handled much like cement, as the latter has higher design criteria — but not always (Figure 1).

Design criteria

Consider, for example, a conical or round silo constructed of steel containing a product that has a lower friction coefficient (wall friction angle) than a referenced designer's “cement on steel.” Assume a producer selects a product, such as fly ash (60-74 lb. per cu. ft.), and places the product in a storage silo without consulting the silo designer. Because the material is lighter than the cement product (85-94 lb. per cu. ft.), the producer presumably feels safe in doing so. A problem, however, arises in that the cement silo was designed to accommodate the properties of normal weight cement. The silo designer, assuming a mass flow discharge of a fluidized product that may typify fly ash as well, was also taking into account a wall friction angle creating a structural vertical transfer of weight to the cylinder wall of the silo. With its round surface area compared to the coarse surface area of cement, the lighter fly ash has a lower friction angle; consequently, a greater amount of the silo product weight is now creating pressure on the silo hopper or bottom cone section — far greater than specified in the original design. A complete hopper or cone section failure could result. Should the friction angle be greater, higher compressive loads will be transferred to the cylinder wall, which might cause buckling or the silo's collapse (Figure 2).

Thus, informed producers recognizes that avoiding errors in silo use contributes to a safer environment and workplace. Silo failure can be catastrophic to workers and their businesses. Contacting your silo designer before placing any product in the vessel is always advisable.