Close Interval Survey on a 96-Inch MLCP Water Transmission Line
Case Study: Identifying Mortar Coating Damage and Active Corrosion Through Above-Ground Survey and Excavation Verification​
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Project Summary
ICG conducted a close interval survey on a 96-inch Mortar Coated and Lined Concrete Cylinder Pipe (MLCP) water transmission line to locate areas of mortar coating damage and potential corrosion activity. Using a temporary impressed current source and pipe-to-soil potential measurements, ICG identified specific locations along the alignment where coating damage was likely. Subsequent targeted excavations confirmed the survey findings: at every recommended site, the mortar coating was found to be deteriorating and active corrosion had initiated on the steel beneath.
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Background: How MLCP Protects Itself
Concrete pipelines, when installed correctly, take advantage of an inherent property of the material. The high pH of properly cured concrete and mortar coatings creates a passivating environment for the steel pipe beneath, mitigating the steel's tendency to corrode electrochemically when exposed to soil. As long as the mortar coating remains intact, the steel surface sits in an alkaline environment and is largely protected.
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When the mortar coating breaks down (through cracking, delamination, soil chemistry shifts, or mechanical damage), the alkalinity at the steel surface drops, passivation is lost, and corrosion begins. Because the steel is hidden beneath the coating and the concrete cylinder, the damage is invisible from the surface and progresses for years before any external sign appears. By the time a leak develops, the corrosion has typically been active for a long time.
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The Diagnostic Approach
Steel embedded in passivating concrete has a characteristic electrochemical signature. Without any externally impressed voltage applied, a properly passivated steel surface inside concrete has a potential of 0 to -200 mV referenced to a copper-copper sulfate electrode. A more negative potential indicates one of three things: damage to the mortar coating, stray current activity, or active corrosion on the steel surface.
This means a pipe-to-soil potential survey, run correctly along an MLCP alignment, can identify the segments where the mortar coating is breaking down or where corrosion has already started. The challenge is that the signal is small compared to typical pipeline survey conditions, and the interpretation requires understanding the specific behavior of steel in concrete rather than steel in soil.
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The Project​
Asset
A 96-inch Mortar Coated and Lined Concrete Cylinder Pipe (MLCP) water transmission line.
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Objective
Identify segments of the pipeline where the mortar coating was likely damaged and recommend excavation locations to directly verify coating condition and any corrosion activity beneath.
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Method
ICG set up a temporary ground bed adjacent to the pipeline and applied 710 mA of impressed current. Pipe-to-soil potential measurements were then collected at close intervals along the full alignment of the pipeline. The induced current amplified the electrochemical signal where coating damage was present, making areas of compromised mortar coating visible as large peaks in the survey data.
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Findings
The resulting survey plot showed clear peaks at specific locations along the alignment, each indicating a likely zone of mortar coating damage. ICG identified the highest-magnitude peaks as recommended excavation sites and provided station-by-station guidance to the project team.
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Verification
The recommended sites were excavated. At each location, ICG inspectors measured remaining mortar thickness using a depth gauge and tested the pH of the exposed coating with a pHydrion Insta-Chek 0-13 system to verify whether the passivating environment was still intact. At every site, the survey findings were confirmed: the mortar coating was actively deteriorating, the passivating pH had dropped, and corrosion had initiated on the steel surface beneath.
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Why This Project Matters
For owners of MLCP and other concrete cylinder pipe systems, this kind of survey delivers something that is otherwise very difficult to obtain: a way to locate hidden coating damage before it becomes a failure. The alternative is to either excavate randomly (expensive and unlikely to find the worst spots) or wait for a leak (much more expensive and disruptive).
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The approach used on this project applies to any MLCP, PCCP, or concrete cylinder pipe system where:
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The pipeline is buried and not accessible for direct external inspection
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The owner needs to prioritize rehabilitation, repair, or replacement decisions across a long alignment
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Random excavation is not a practical or affordable inspection strategy
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The owner needs defensible technical justification for capital spending
A well-executed survey can dramatically reduce the number of excavations required to characterize the condition of a pipeline, and it produces a data record that supports future capital planning, asset management, and regulatory documentation.
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Related Services
This project drew on several of ICG's core service capabilities: