Concrete Tower Silo Maintenance and Repair

Agdex#:	732
Publication Date:	1990
Order#:	90-236
Last Reviewed:	1996
History:	Original Factsheet
Written by:	H.E. Bellman - Argue & Associates/ Guelph

Table of Contents

1. Introduction
2. Repair and Maintenance Procedure
3. Silo Coatings
4. Points Regarding the Use of Silo Coatings

Introduction

Just as farm tractors require regular maintenance, so do farm structures - concrete tower silos included. Unfortunately, most farmers consider a concrete silo to be indestructible, and thus give no thought to regular care and maintenance. With time this can lead to serious problems - even structural failure.

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Repair and Maintenance Procedure

All owners of concrete tower silos should follow a proper repair and/or maintenance schedule to get the greatest possible service-life out of their structures. This involves:

Inspection

The bottom portion of the silo wall (lower 10-15') should be carefully inspected, both inside and out, on an annual basis (or whenever the silo is empty). It is very important to use a good portable light when making this inspection in order to be able to see clearly the actual condition of the wall. A screwdriver and/or a hammer and cold chisel are simple but effective instruments for evaluating the wall condition.

Any softening of the inside wall surface and/or signs of roughness due to exposed concrete aggregate indicates that protected initially with a barrier coating, deterioration of the coating will take place with time, and replacement will be periodically required.

Figure 1. It is important to inspect the condition of the interior wall surface on a regular basis. Testing with a cold chisel is one way or estimating the extent of silage acid attack.

With stave silos a check should be made of the condition of the bottom of the staves where they rest on the foundation wall. Often they become badly eroded - even to the point of complete loss of the bottom 6" or so.

Figure 2. The greatest erosion of stave silo walls due to silage acids is often at the bottom where the staves meet the foundation.

Door jambs should also be inspected. In situation where considerable seepage has occurred, acid deterioration can be severe, particularly at the bottom of the silo.

Cleaning

If any work is to be done on the silo wall, the inside surface must first be thoroughly cleaned. A clean wall surface is essential to ensure that any repair or coating silage and/or corroded concrete - right down to the clean, solid, unaffected concrete underneath. Either a sandblaster or a high-pressure water blaster (3000 psi or more) can be used; the latter is more effective in some difficult situations.

Repairs

Any necessary structural repair work should be done before putting on an interior coating.

Stave silos

All severely deteriorated bottom stave should either be replaced with new staves, or the silo wall supported by installing a new, reinforced, cast-in-place concrete ring around the outside of the silo, and resting on the foundation. In the latter case some of the poorer bottom staves should be removed to allow the concrete to be placed under the upper staves and thus provide the necessary vertical support. Restoration work of this nature should only be done by a skilled, knowledgeable contractor (not a suitable "do-it- yourself" project!).

Figure 3. Door jambs, particularly at the bottom of the silo are also subjected to severe acid attack, and often need repair or replacing to prevent air leakage.

At the same time the silo hoops should be re-tightened to provide a strong, stable structure. Loss of hoop tension makes a stave silo more vulnerable to damage from high winds. Acid etching of stave joints also contributes to the problem of loss of hoop tension.

Stave silos should be constructed on foundation walls that extend above grade level all around the silo. When stave wall go below grade any seepage through the wall will be held by the surrounding soil that the staves walls go below grade any seepage through the wall will be held by the surrounding soil so that the staves are continuously immersed in the acid-containing liquid. This will greatly increase the rate of wall deterioration. In those cases where the wall does go below grade, a line of drainage tile, taken to an appropriate liquid runoff storage, should be place around the silo at the bottom of the wall and the trench back-filled with crushed stone.

Cast-in-place silos (CIP)

CIP silos that have been seriously affected by silage acid should be reinforced by adding extra hoops around the bottom portion.

Figure 4. Silo walls must be thoroughly cleaned, right down to solid unaffected concrete, before any type of coating is applied.

Figure 5. Where stave silo repairs include a concrete collar, it must replace some of the deteriorated staves, and extend inward underneath better quality upper staves in order to carry the vertical wall load down to the foundation.

All Concrete Silos

Deteriorated door jambs should be repaired (or replaced) to ensure the doors fit tightly to prevent air leakage. Often the interior drainage system needs upgrading. One suggestion is to use a ring of 4" drainage tile or tubing on the inside of the silo wall, covered by 6"-8" crushed stone and with an outlet to a suitable liquid runoff storage on the outside. NOTE: - Drainage from a silo should not be allowed to go into any type of surface water (ditches, watercourses, field drainage tile, etc). Also to lessen the volume of liquid runoff storage needed ensure that there is adequate surface drainage away from the base of the silo.

Barrier Coating

There are many different types of coatings available, only some of which are suitable for farm silo application. The two basic types of coating include (a) ones which are applied to the surface of concrete and act as an acid-resistant barrier; and (b) ones which penetrate the concrete surface, filling the voids with acid-resistant material which prevents penetration of the silage juices.

Regardless of type, all coatings for farm silo application should be:

• highly resistant to silage acids (mainly lactic and acetic)
• suitable for use in the food chain (not toxic to animals or humans in any way)
• suitable for farm silo application, considering such qualities as degree of tolerance to a dam wall, required curing temperature, length of curing time, method of application and the possible production of toxic fumes.

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Silo Coatings

Silo coatings can be loosely categorized as either:

Cementitious Coatings

These are coatings that are basically a mixture of sand and a binding agent. These are usually mortar-like in consistency and have considerable thickness or body. The simplest cementitious coating is a mixture of sand and Portland cement. Because the basic binding agent is Portland cement, this type of coating is susceptible to silage acid attack. Thus, if this is applied to a silo wall, it should either be considered as a sacrificial coating, and replaced at fairly frequent intervals, or it should be protected by the use of an additional coating that is acid resistant (usually some type of liquid coating).

Figure 6. Stave silo walls that go below grade should be protected with a ring of drainage tile which is taken to a proper outlet.

In order to prevent silage acid deterioration, a number of coatings manufacturers have produced various "proprietary" cementitious coatings. In these cases acid resistance is achieved by either the use of admixtures (often some type of acrylic), or the use of binding agents other than Portland cement (epoxy, polyester resin, etc.).

Cementitious coating may be applied to a silo wall either by hand with a trowel, or spray-applied using some type of gun. The term "gunite" is often used to denote a sand/Portland cement mixture that is applied with a gun.

Figure 7. The walls of a badly deteriorated cast-in-place silo can be reinforced by the addition of external hoops.

Figure 8. One method of draining excess silage liquid is to install an internal ring of 4" drain tile on the silo floor next to the wall and cover with a layer of crushed stone.

Liquid Coatings

These are coatings that are water-like or syrup-like in consistency, with little or no thickness or body. One type of liquid coating, when cured, forms a layer or film on the surface of the concrete and acts as an acid resistant barrier. Another type penetrates the concrete surface and either fills the voids with acid-resistant material (thereby preventing acid penetration), or chemically reacts with the free lime in the Portland cement concrete to form an acid-resistant matrix.

Liquid coatings may be applied in a number of ways - hand applied using a roller, brush, etc., or spray applied (equipment required depends on the nature of the material).

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Points Regarding the Use of Silo Coatings

Both field experience and controlled testing have suggested the following with regard to the application of silo coatings.

1. Regardless of the type of coating used, a clean wall is essential. Tests have shown that whereas coating failures were initially blamed on the coatings themselves, subsequent investigation proved the failures to be due to poor bonding to the silo walls because of inadequate cleaning.
2. Care should be taken to use the right type of coating for the particular wall situation. The condition of the wall after cleaning should dictate the type of coating to be used. Research ha shown that it is almost impossible to apply liquid-type of coating (one that forms a surface file) to a rough wall surface and get a pinhole-free film. Any holes will allow silage acids to get behind the film and attack the concrete, thus nullifying the effectiveness of the coating.

   If the wall is smooth (eg. New or only slightly used silo) a liquid-coating may be used. However, if the wall is rough (i.e. - has been etched by silage acids), consideration should be given to the use of a cementitious or mortar-like coating to bring the wall back to its original, smooth condition. If a proprietary coating is used (one that has acid-resistance built into the mixture), nothing more is required. If, however, a plain sand/cement mortar is applied, it in turn should be protected by the addition of a suitable liquid-type coating.
   
   
   
   

   Figure 9. Cementitious coating may be applied to a silo wall using a gun (sometimes referred to as "gunite").
   
   
   
   Figure 10. The condition of a cleaned wall surface dictates the type of coating that should be used.
   
   

3. Cementitious coatings that are gun-applied tend to have a fairly rough surface finish. Thus, since a relatively smooth finish is highly desirable (reduced friction, better self cleaning, etc.) The coating should have a troweled finish. NOTE: care must be taken to trowel the surface of gunit at the right stage of setting up, or pulling will occur, resulting in highly undesirable cracks and voids.
4. Proper curing conditions are very important. Each coating type has specific curing requirements. For example, cementitious coatings require the maintenance of a moist condition to prevent cracking during the curing period.

   Where cracking occurs, during subsequent use silage acid will penetrate the coating through these cracks, attack the concrete underneath, and reduce the strength of the bond to the wall.

5. If an attempt is made to apply a liquid-type coating, particularly to a rough wall, spray application is preferred. If it is put on with a roller, a stiff bristled brush should be used immediately afterwards to work the material into all surface crack and hollows (to try to get as continuous a film as possible).
   
   
   

   Figure 11. Gun-applied cementitious coatings should usually be given a troweled finish to ensure a smooth surface.
   
   
   
   

   Figure 12. Proper curing conditions are important for most coatings. Cracks will allow silage acids to penetrate to the concrete wall underneath, thereby nullifying most of the benefit of using a coating.
   
   

6. Regardless of the type of coating used, it is extremely important to follow the recommendations of the manufacturer or supplier.

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