As Published in MN Grain and Feed, Spring 2010. 
Written by Doug R. Nagel, PE

MN Concrete silos develop cracks for a variety of reasons.  The number of cracks, and the severity of cracking, is contingent upon several different factors, such as silo age, frequency of product cycling, and design intent vs. actual silo use (was a given silo designed to be emptied with a side draw?).  Regardless of the reason for cracking, it can eventually lead to performance or safety concerns for existing concrete storage bins.  If an existing silo has extensive cracking, one economical solution to extend the service life of the silo may be the construction of a shotcrete liner. 

The American Concrete Institute (ACI) defines shotcrete as "concrete placed by a high velocity pneumatic projection from a nozzle".  In other words, shotcrete is concrete sprayed onto a surface through a hose rather than being poured into forms.  Gunite is another term that has historically been used to describe this process. Gunite is a proprietary term for shotcrete with a specific mixing process.  Shotcrete has been used to reinforce existing structures for about 100 years.

Before the construction of a shotcrete liner, a qualified engineer should examine the existing silo to determine what structural integrity, if any, remains in the walls.  The design of a shotcrete liner should also be performed by a qualified engineer.

A shotcrete liner can be used to reinforce or replace a portion of a deteriorated silo wall, or the wall in its entirety.  A liner is essentially a new wall constructed within an existing silo.  This is achieved by anchoring short rebar dowels into the existing wall and tying new vertical and horizontal liner reinforcement to the dowels.  Shotcrete is then applied to the wall at the required thickness.  Since shotcrete is still concrete, it is available in the same strengths as poured concrete (including strengths typically used for slipformed concrete).  The finished product is as strong as any slipformed or jumpformed wall of the same thickness with the same reinforcement and concrete compressive strength.

Shotcrete construction can be advantageous because no formwork is required.  Because shotcrete is sprayed onto the existing wall, the existing wall acts as the form for the new liner.  The existing wall should be free of any dirt or foreign material prior to the application of shotcrete.  One way to achieve this is by power washing the surface that will receive the liner.  Spalling or loose concrete should also be removed. 

There are a number of finishing options available with shotcrete.  The cheapest option is the natural gun finish.  The finish is very rough because there is no exterior form or vibrating of concrete after placement.  Depending on the product stored in the bin, this may not be a desirable interior surface.  There are other options available, such as a broom finish or a trowel finish, but because these types of finishes require additional labor they will result in additional costs.

In summary, a shotcrete liner may be an economical way to extend the service life of aging or worn concrete silos.  Shotcrete can be used to reinforce or replace part or all of a deteriorated silo wall.  Readers wishing to learn more about shotcrete can visit ACI (www.concrete.org), the American Shotcrete Association (www.shotcrete.org), or contact VAA, LLC (Van Sickle, Allen) by visiting www.vaaeng.com.  VAA has designed several shotcrete liners for silos ranging in diameter from 14 to 60 feet.