Anchor Guard: Understanding and Preventing

Corrosion on Existing Structures

To this point we have discussed what corrosion is, how it works in the example of tower anchors, and we have given several methods for its control. The question now becomes what about existing facilities that may or may not have already sustained structural damage due to corrosion. This section will outline methods for investigating this potentially serious problem and give methods of remedial action that can be taken.

Anchor Inspection

It is becoming increasingly clear that many guyed towers have already sustained anchor corrosion. This has placed the structural capacity of the tower in question. Typical inspection procedures in the past have been to dig around the anchor to an approximate depth of twelve inches below grade. In many cases, corrosion is found during such an inspection, but is often determined to be acceptable. Based upon the principals of corrosion previously discussed, this method of detection is likely insufficient. The most extensive corrosion will usually occur closest to the concrete anchorage, which is often several feet below the surface of the soil. If any amount of corrosion exists at the upper levels of the anchor shaft, further investigation is usually warranted.

The most proven method of anchor investigation is to fully expose the steel shaft portion of the tower anchorage. This involves carefully removing the soil around the shaft to the depth of the concrete footing. Care should be taken so that the concrete footing is not overly exposed, which could result in the failure of the anchorage and the tower. If structural capacity of the anchor is in question, temporary anchorages may be advisable. Once the soil has been removed from around the anchor shaft, the shaft should then be cleaned so as to allow measurement of the steel member. Typically, sand blasting or some other acceptable method of cleaning the shaft to a bare metal condition will be required. It is important that the shaft be free of soil and corrosion scale so that accurate measurements can be taken. It would not be uncommon for the shaft to appear to be satisfactory based on appearance prior to cleaning. This is true in part because of the thick corrosion scale build-up that clings to the face of the steel, giving the false impression that the shaft size or dimensions have not been altered substantially by corrosion.

After cleaning the shaft, it can then be measured by means of micrometer or ultra-sonic thickness tester and reference given to the portion of the shaft above grade that has likely not sustained corrosion damage. Repair or replacement may be required, if corrosion has left question as to structural capacity of the shaft.

Applying Protective Measures

Epoxy Coating. If the shaft is determined to be structurally sound, then the bare metal anchor shaft provides an excellent surface for application of epoxy coating. Bituminous epoxy is the most commonly used and best alternative for coating of buried steel. Care should be taken to ensure complete saturation of the entire surface of the shaft. If anomalies or "holidays" are left in the coating, a corrosion cell could be even more severe than if the shaft were left alone.

Cathodic Protection. Once the shaft has been coated property, additional measures such as cathodic protection should be applied. A properly coated member will allow a substantial reduction in the amount of cathodic protection needed. Cathodic protection has been shown to be the most economical method of anchor shaft corrosion control. Its monitoring capabilities also add to the benefits of this form of corrosion control. Research in the field of cathodic protection for tower anchors has produced a viable product for the protection of anchors that is now available. For more information about this product, contact the author.