Many older concrete tower silos are in use in Ontario today. The trend toward the construction of this type of silo has slowed. Farmers continue to fill their older structures, often without considering the deterioration that may have taken place due to age, weathering and the effects of silage acid attack. In recent years, there has been an increased incidence of collapse of these structures.

Figure 1. A silo collapse can destroy surrounding buildings

Figure 2. The effect of placing material in the silo that is too wet

The loss of a tower silo through collapse usually involves serious consequences. In addition to the loss of the structure we commonly see the complete or partial loss of the contained silage or grain. As well, the silo may fall onto an adjacent barn, resulting in an additional loss, and the possibility of injuring or killing any animal or person unfortunate to be in that part of the barn at the time. In Ontario, farm owners or workers have died as a result of the collapse of tower silos. Figure 1 shows the impact that a silo collapse can have on surrounding buildings.

Most of the problems with deterioration of conventional concrete tower silos are due to the attack of silage acids. When moist plant material is put into a silo it goes through the ensiling process which produces silage acids, principally lactic and acetic acids. These acids, when they come into contact with concrete silo walls, react with the Portland cement matrix that binds the aggregates together. This results in a gradual decline in strength as the structure ages. In some cases, the strength will decrease to the point where the concrete can no longer perform its required function. In addition these same acids will corrode silo hoops, reinforcing steel or hardware associated with the silo. Without proper maintenance and repair, this sort of action can ultimately lead to a silo failure. Figure 2 shows the effect of placing material in the silo that is too wet.

Silage acids cause deterioration to all types of concrete silos, cast-in-place (poured), as well as pre-cast, stave. The rate and severity of this deterioration depends on a number of factors such as the size of the silo, the moisture content of the ensiled material, and finally the amount of protection given to the concrete on a continuous basis.

Silage pressure has a large part to play in determining the rate and extent of acid deterioration. In any silo, the highest pressure is at the bottom of the wall. Larger silos produce higher pressures. This results in an increased squeezing effect on the ensiled mass, thereby creating even more free liquid and seepage. As well, the silage juices containing the acids are forced into the tiny pores in the concrete. Because of this, larger silos often suffer more acid deterioration than smaller silos. Ensiling higher moisture material leads to more fermentation and with it a higher level of acid production. This results in accelerated concrete deterioration.

Material placed into a tower silo creates vertical and horizontal loads or pressures. Acid attack is of concern and will eventually reduce the ability of the structure to carry these loads.

Pre-cast Stave Silos

The corrosive action of silage acids reduces the ability of stave walls to carry the vertical friction load imposed by the contained silage. As the strength of the inner surface of the silo wall decreases, the effective thickness of concrete resisting the vertical load is also reduced. Since stave silo walls have relatively thin sections to start with, any corrosion will cause a significant reduction in the wall strength.

Figure 3. Mechanism of failure of stave silos

The effect of acid deterioration is compounded in that it attacks the bottom of the silo wall, which is also the part of the wall that carries the greatest amount of the vertical load. In a normal, top-unloading tower silo, approximately 50 percent of the contained weight of silage is transmitted to the footings through the walls due to the friction effect. Thus, at some point in time, the cross-section of effective concrete can be decreased to the point where it will not longer be able to carry the imposed compressive load and the silo wall will begin to crush. Figure 3 shows this mechanism of failure.

Stave silo builders usually use the same thickness of staves for all sizes of structures. Thus, although there is some safety factor built into a wall to accommodate the vertical friction load, the larger the silo the lower will be the safety factor, and the sooner this can be reduced to the point of failure by acid deterioration.

Cast-in-Place (CIP) Silos

Although less apparent, acid deterioration is also a serious matter for CIP silos. Although there is more mass in a CIP wall with a typical thickness of 6 inches, the concrete in this type of wall is often not as strong or dense as in pre-cast staves, and therefore not as resistant to acid attack.

The horizontal strength of a CIP silo wall is due to the reinforcing bars which are located close to the centre of the wall. If the silage acids are allowed to penetrate the concrete cover, the reinforcing bars will become corroded, reducing the effective cross-sectional area of steel.

Equally important is the fact that the reinforcing bars that encircle the silo are made up of sections which are joined together only by the concrete that surrounds the lapped ends, and thus the continuity of the strength of the steel ring is entirely dependent on the bond strength of the surrounding concrete. Once silage acids penetrate the concrete around the steel much of the bond strength will be lost. This means that at some point in time the silo wall will not be able to carry the imposed horizontal load and the silo will collapse.

Prevention of Silo Deterioration

1. Construction

There are several things that an owner can do to prevent, or at least reduce the severity of silage acid action. The first step is to build or buy a quality silo: one with walls made of high quality concrete. Strong, dense concrete provides a good level of acid resistance. The next step is to protect the silo walls by preventing silage acids from coming into contact with the concrete. This should be done by applying a suitable acid resistant coating to the bottom 1/4 to 1/3 of the inside wall surface of the new silo prior to use, and renewing it as required to maintain a barrier.

2. Management

1. Moisture control: One of the things you can do to reduce deterioration caused by silage acid is to harvest your crops at a moisture content low enough that seepage will not occur. Ideally, whole-plant silages should contain enough moisture for good fermentation, yet it should be dry enough to avoid free liquid being squeezed out. In order to know when material is at the proper moisture level for storage, the use of a moisture tester is highly recommended. Table 1 shows the recommended maximum moisture content in whole-plant silages at time of harvest to avoid seepage problems with various sizes of silos.
   
   
2. Wall Exposure: All silage should be removed from the silo on a yearly basis, if at all possible. This will reduce the length of time the bottom of the silo wall will be in contact with wet silage. As well, there is a structural benefit from allowing the inner silo wall surface to dry out between fillings.

What to Look for

If your silo is showing signs of distress, before you empty it, you should contact a professional engineer who can follow up with further investigation.

Concrete silos are commonly converted as dry grain storage. Silos previously designed for whole plant silage or haylage have a limited capacity for dry grain unless additional reinforcing is provided. This is usually in the form of steel hoops at vertical intervals on the outside of the silo. You should contact an engineer to design this increased reinforcing.

Signs of distress: cracks in the concrete are one of the only signs to warn you of an impending failure. You should scan the entire outside of the silo to determine if new cracks have developed. You can sometimes use binoculars to do a cursory inspection.

The consequences of a structural failure are very critical and can be life-threatening. Emptying a silo can cause a significant increase in the loads applied to the structure. If a failure is about to happen, unloading the silo can cause an instantaneous structural failure. If you suspect that your silo has structural problems, do not fill or empty it before having a professional engineer on-site to evaluate the situation.