STEEL TOWERSWhy Below-Grade Inspection, Mitigation & Repair is Critical for Steel Structures

What You Can’t See, Can Hurt You

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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)

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.

Failure: How & Why

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

SELF-WEATHERING STEEL POLESThese 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 SOILOberburden 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.

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.

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:

STRUCTURES WITHIN A SHARED RIGHT-OF-WAYStructures 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.

GUY ANCHORSAnchors 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.

Developing an Inspection Program

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

• 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.

• 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:

The values of these indicators are assessed to determine the following:

• Soil resistivity• pH• Oxidation Reduction (REDOX)• Half-cell/Structure-to-soil potential measurements • Soil type• Moisture level

• 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

Program drivers are similar, but vary by level of importance to the specific utility. These include, but are not limited to:

• 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.

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.

¹ “Protecting Transmission Structures - Research Focuses on Corrosion and its Impact on System Components.” (Murray, Electricity Today, March 2013, p. 66-67).

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

The Impending Maintenance “Wave”

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The age-population where most maintenance is currently being performed

The approaching age-population where maintenance will be required

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 self-weathering steel lattice tower leg

© 2013 Osmose Utilities Services, Inc.

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