STEEL
:
TOWERS
What You Can’t See, Can Hurt You
Why Below-Grade Inspection, Mitigation & Repair is Critical for Steel Structures
Although once thought to be maintenance-free, steel has proven that it does indeed require inspection and maintenance. Steel
structures degrade over time due to corrosion activity and, to a lesser degree, from mechanical damage. According to a recent article
in Electricity Today¹, the associated cost of corrosion for electric power generation and delivery is well in excess of 6.9 billion dollars
annually.
A review of 25,000 steel T&D structures (35-45 years of age) inspected by Osmose Utilities Services, Inc. over the last nine years
revealed the following:
• 12% of structures had 10% - 24.9% cross section loss and required mitigation or repair
• 8% of structures were in need of significant repair with an average cross section loss of 25% or greater. Of these structures,
2% were priority structures in need of immediate attention (with an average cross section loss of 50% or greater)
Failure: How & Why
Original protection systems, including factory-applied galvanizing and coatings, help to reduce or eliminate the effects of corrosion
by creating a barrier between the steel and the soil in its environment. As structures age, the initial protection systems begin to
deteriorate. Deterioration of the original protection allows environmental influences to directly impact the steel itself. In many cases
failed factory-applied coatings have actually become detrimental to structures because they not only allow moisture to come in
contact with the structure, they encapsulate the moisture, holding
it against the structure and accelerating the corrosion process.
Once the steel is exposed the natural process of corrosion activity sets in and begins
to thin and weaken the steel. This typically begins at the ground line and proceeds
downward along the surface of the structure below grade. The speed at which this
process occurs can be accelerated by several contributing factors including:
• Soil type
• Moisture
• Agricultural activity
Other factors that aid in the corrosion process are typically
not associated with the environment but are directly related
to the structure. These include:
• Design features
• Construction materials
• Dissipation of static current
At Risk Structures
Structures most at risk for corrosion activity are those structures whose initial protective coatings have begun to deteriorate, as well as
those structures placed in environments that can contribute to accelerated corrosion. Other structures at risk include:
SELF-WEATHERING STEEL LATTICE TOWERS
These towers are susceptible to a phenomenon referred to as “pack rust” or “pack out.” Pack rust occurs when water enters into a bolted
joint and does not dry out. As the water permeates the original corrosion layer the un-activated steel beneath it reacts to the water and
creates another layer of corrosion. This process sacrifices a small layer of good steel in order to create the layer of corrosion. As this
occurs, the steel in the area of the pack rust activity thins, eventually weakening the steel. Pack rust will continue to create more and
more layers as it remains wet and will result in a wedge of rust or “pack out” in the joints causing strain within the bolt group.
SELF-WEATHERING STEEL POLES
These poles are subject to pack rust primarily at the ground line, especially in areas where the factory-applied coating has failed. In
these instances, the pack rust continues to build layer upon layer until it sloughs off, thinning out the pole in a manner similar to lattice
towers which can create perforations in the pole wall.
CONCRETE ENCASED STRUCTURES WITH OVERBURDEN SOIL
Oberburden soil occurs when migrating soil from water, wind, or agricultural activity builds up on top of the concrete foundations,
directly contacting the steel. This is especially destructive on structures where either the galvanizing or coatings have deteriorated. It
typically results in a concentrated band of corrosion extending from the top of the concrete foundation to the top of the soil.
GUY ANCHORS
Anchors on both wood and steel structures are at risk for corrosion, especially those found to conduct current to ground. In areas where
current is dissipating from the anchor, as much as one pound of steel can be lost for every one amp of direct current (DC) annually.
Typically, this current is measured in milliamps of current so the loss of steel occurs more slowly, but is significant nonetheless.
STRUCTURES WITHIN A SHARED RIGHT-OF-WAY
Structures that share a right-of-way with other utilities can be subject to additional influences that contribute to corrosion activity. A
gas pipe line is a prime example. In some instances, cathodically protected (CP) gas pipe lines can indirectly impact the corrosion
activity on electric utility structures including steel towers, poles and anchors. In such cases, current from the CP system (mostly from
rectifiers) finds its way onto the steel structure through the soil and then discharges back into the ground. This process is typically
referred to as “DC interference” or “DC uptake.” In these situations, damage does not occur where the current is drawn onto the
structure, but rather where the current discharges back into the ground.
In many cases, one or more of these issues can exist at the same site. Through thorough inspection and by taking environmental
condition measurements at the site, technicians can help determine the extent of corrosion activity currently taking place and help
identify contributing influences.
Challenges for Utilities
Many utilities currently don’t have a sustainable program to address corrosion and corrosion-related issues. Without identifying
the level of need most cannot create a business case to acquire the funding necessary to support a full-fledged program including
inspection, repairs and mitigation.
The few utilities that do have programs in place typically do not have the necessary resources in personnel and expertise to manage and
support it appropriately.
Developing an Inspection Program
Program drivers are similar, but vary by level of importance to the specific utility. These include, but are not limited to:
•
•
•
•
•
Age
Structure Type (including foundation)
Material Type
Geographic Location
Line Importance
•
•
•
•
Failure and maintenance history
Previously installed corrosion control system
Previous inspection history
Grounding system
Most often the first step in determining whether an inspection program is necessary is to determine the most critical items from the list
of program drivers and then weight them accordingly. By utilizing this approach, a prioritized list of lines can be developed to focus on
those structures most important to the utility. Once developed, a sampling of structures from several of the most critical lines can be
selected to initiate a pilot project.
Components of a pilot project usually include:
• Line Structure Selection – This is a critical aspect of the project as it will define its results. Selecting a sampling of lines and
structures that are representative of the utility’s system as a whole will provide more representative results of the entire system. This
will allow for a more clear assessment of the system condition and help to determine guidelines for the development of a larger
system-wide corrosion program.
• Inspections – The inspection process involves two primary types of evaluations: 1) structural assessment of each structure to
help determine existing corrosion and its effect on the integrity of the structure 2) determination of key predictive environmental
indicators present at each site which influence the rate of corrosion activity. Structural members encased in concrete receive
rudimentary concrete evaluation only. In addition, all structures receive a visual overhead inspection primarily for safety purposes.
• Excavation – Steel structures are excavated to a depth of approximately eighteen inches. The below-grade surfaces are
cleaned to allow for accurate thickness measurements of the steel to calculate section loss. If thinning is noted, excavation
continues to a maximum depth of two feet in an attempt to determine the extent of corrosion damage.
• Structural – Section loss is measured and the condition of the concrete footings (where applicable) is evaluated. Mechanical
damage is also evaluated to determine its effect on the structure. The structural assessment results are used to determine the
following:
• Mitigation application
• Current structural condition
• Repair recommendations
• Environmental – The rate at which steel corrodes below-grade varies significantly based on the physical and chemical
properties of the surrounding site conditions. Key predictive corrosion indicators are measured at each structure during the
inspection process. These include:
•
•
•
•
•
•
Soil resistivity
pH
Oxidation Reduction (REDOX)
Half-cell/Structure-to-soil potential measurements
Soil type
Moisture level
The values of these indicators are assessed to determine the following:
• Potential risk of corrosion to the individual structures (the potential risk of corrosion over the entire footprint of the
line, in some cases)
• Recommended corrosion inspection cycle intervals
• Future mitigation options
• Additional inspections needed outside of the regular inspection cycle interval.
• Coatings – Coatings represent the primary form of mitigation in most anti-corrosion programs. Coatings are available in
several different types and for a variety of applications. It is very important that the correct material is selected based on the
construction materials and structure type to achieve the desired level of protection.
• Cathodic Protection- When coatings alone are not sufficient, cathodic protection (in the form of sacrificial anodes) can
be applied as an additional measure of protection. Sacrificial anodes sacrifice themselves to protect steel structures from
corrosion. These systems are sized specific to the size and type of structure. They are typically engineered to protect
structures within their specific environment.
• Data Analysis – The data from the inspection process is reviewed and evaluated by Corrosion Engineers and the structures are
categorized.
• Summary Report – A summary report of the pilot project findings is written and reviewed by engineering staff to convey the
comprehensive findings in a consolidated report.
• Repair Recommendations – Structures that have deteriorated beyond the protective capabilities of coatings and anodes are
usually significantly weakened by section loss of their supporting structural members or foundations. In most cases, in-place
repairs can be individually engineered for these structures in order to avoid the high cost of replacing the entire structure.
Based on the results of the pilot inspection repair and mitigation, options can be evaluated for inclusion into a more comprehensive
program approach.
The Impending Maintenance “Wave”
It is important to understand the extent of the coming “wave” of maintenance the industry is likely to encounter relative to steel
transmission structures (illustrated below). This chart represents one large utility’s steel transmission structure population and gives us
an idea of what the maintenance commitment for the industry as a whole may face in the coming years.
Maintenance on most steel transmission structures is currently focused on those structures built prior to the 1970’s and on some
critical lines. However, these structures only represent 35.4% of the total structure population¹ which means that a majority of the
remaining structures are now 25 to 50 years old and beginning to require significant maintenance.
The approaching age-population where
maintenance will be required
{
The age-population where most maintenance
is currently being performed
{
1900
1910
1920
1930
1940
1950
1960
1970
1980
1990
2000
¹ “Protecting Transmission Structures - Research Focuses on Corrosion and its Impact on System Components.” (Murray, Electricity Today, March 2013, p. 66-67).
Corrosion Photos
Pack Rust - layer upon layer
of corrosion has resulted in
a “wedge” of rust
Corrosion on anchor caused
by use of dissimilar metals
(copper and galvanized steel)
Corrosion damage due to failed coating on galvanized steel poles
Overburden soil - migrating soil
built up and came into direct
contact with the steel causing
severe corrosion
Severe corrosion damage on selfweathering steel lattice tower leg
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