Anchor Guard: Understanding and Preventing

5. Corrosion Control Measures

5.1 All anchorage steel below grade shall be hot-dip galvanized.

5.2 Tower anchors determined to be at risk from corrosion shall be protected by a galvanic anode cathodic protection system designed for tower anchors (AnchorGuard® or equivalent), with magnesium sacrificial anodes and monitoring system. The monitoring system should be capable of providing anchor to reference cell corrosion potential readings and system current output readings. Zinc anodes may be substituted when soil resistivity is low. Galvanic anode cathodic protection uses the known variables of the corrosion cell to direct corrosion away from where it is not wanted. Other corrosion control measures listed herein are secondary options.

5.2.1 AnchorGuard model selection is based primarily on the amount of metal underground to protect, approximated by tower face width. Model selection is as follows: Less than 19" face tower = AG1, 19" to 60" face tower = AG2, Over 60" face tower = AG3. Based on site conditions, an upgrade may be necessary.

5.2.2 Installation and testing of galvanic anode cathodic protection system shall be in accordance with manufacturer's instructions.

5.3 Impressed current cathodic protection employs a DC rectifier to supply current to anodes. This method is effective but generally involves higher design and maintenance costs than with galvanic anode cathodic protection.

5.4 Anchor coatings can assist in limiting corrosion but cannot be considered adequate corrosion control in themselves. Virtually all coatings have or develop flaws. A small anomaly in the coating can cause accelerated corrosion in one area and have a detrimental effect on the anchor. Coatings can be beneficial, however, when used in combination with cathodic protection.

5.4.1 Anchor coatings should be suitable for hot-dip galvanized steel. Surface preparation and application are critical to coating performance and should be done in accordance with coating manufacturer's instructions.

5.5 Concrete encasement of the entire anchor shaft may be employed to control anchor corrosion. Total concrete encasement passivates the anchor and impedes an anode/cathode relationship on the anchor shaft itself. One disadvantage of this approach is if the concrete becomes seriously cracked or broken, water or soil could fill the cracks and lead to accelerated corrosion at the exposed area.

5.5.1 Concrete encasement should extend at least 6 inches above grade. Care should be taken to ensure that the entire anchor shaft is solidly encased in a large concrete block, such as with the caisson type design. The concrete mix should be sulfate resistant (Type V) when soluble sulfates exist in the soil or groundwater.

5.6 Electrical isolation using guy wire strand insulators eliminates the electrical path between the tower and the anchor support. If properly installed, this method protects the anchor from corrosion associated with copper grounding systems and from stray current corrosion. Electrical isolation does not, however, prevent an anode/cathode relationship on the anchor shaft itself.