As many of you know there have been a number of projects in the last two years to increase the export capacity in the Pacific Northwest (PNW) region (Oregon and Washington). I am not a world grain-trading expert nor do I have data about supply and demand. What I do know is the total storage, cleaning capacity, rail capacity and ship-loading capacity has and is increasing significantly. I believe this will have an impact on grain movement from the Midwest and the construction industry in the following ways.

First, there will be a need to increase other infrastructure. In order to utilize this capacity the rest of the system needs to catch up. This means additions to rail lines in terms of cars, track, and power. I believe Warren Buffet saw this coming long before I did. Inland terminals will also have to increase to meet this demand. The number of shuttle loading facilities is increasing as I write this article. Other modifications will have to happen to meet railroad requirements for loading times. Inland terminal (cleaning, drying, and scalping) will have to increase to meet customer demands.

Secondly, we have seen a new export facility added in the PNW along with many additions and upgrades to existing facilities. To me, this looks similar to the changes in the late 70’s and early 80’s at some of the same locations. Now the export infrastructure will be greater than the feeder infrastructure. It will take some time for these two to come back to equilibrium. There may be a period after this boom ends of less construction in the PNW until the infrastructure catches up, and more construction inland to improve the feeder facilities.

What does all this mean to the people and facilities in the middle? More demand for shuttle loading facilities, services, and more demand for the crops. This sounds like positive opportunities for the agriculture industry over the next few years.

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.