COOLING TOWERS
Assess your cooling tower's condition
The aggressive operating environment that cooling towers are subjected to can accelerate their deterioration in many unexpected ways. This article explores how the following environmental effects must be considered when developing measures to extend the service life of a cooling tower:
- Freeze-thaw deterioration. Cooling towers in northern environments are exposed to cyclical freezing and thawing of wet concrete. When ice forms within the concrete, expansive forces can cause cracking and deterioration of the concrete matrix.
- Corrosion of embedded steel reinforcement. Concrete normally has a pH of about 12.5, providing a highly alkaline environment for embedded steel reinforcement. A thin passivating film normally forms on the steel as a protective coating against corrosion. If this film is compromised, corrosion can result.
- Chloride ion levels. Some cement constituents in concrete can chemically bind with chloride ions, averting corrosion until a threshold concentration is reached at the surface of the embedded steel. The generally accepted threshold is 0.20% acid-soluble chloride ion by weight of cement. In the presence of moisture and oxygen, chloride concentrations at and above this level can penetrate the passive film on the steel and initiate corrosion.
- Carbonation. Carbonation occurs when CO2 in the air reacts with calcium hydroxide and other hydration products present in concrete to form predominantly calcium carbonate and water. Calcium hydroxide has a pH of 13, while the pH of calcium carbonate and water is 7. When the pH of the concrete at the steel surface drops to about 10, the alkalinity is insufficient to maintain the passivating film on the steel and, in the presence of moisture and oxygen, the reinforcing steel will corrode.
- Sulfate attack. Sulfates can react with aluminates and water to form expansive compounds capable of breaking down the integrity of cement paste in concrete. Sulfate attack is a particular problem in areas with high-sulfate soils and groundwater. Elevated sulfates in cooling tower water can lead to gradual degradation of surface concrete.
- Demineralized or acidic water. Where cooling water is demineralized, it can slowly break down surface concrete by leaching cement paste from the concrete matrix. Similarly, cooling water that is mildly acidic (as from some natural sources) can erode surface concrete over time.
- Alkali-silica reaction (ASR). ASR is a reaction between alkali constituents within concrete and certain siliceous aggregates. A primary source of alkalis is portland cement, but other concrete constituents can also contribute alkalis. In addition, because ASR is fed by moisture, the cooling tower environment can produce ASR-related distress while it may not occur in nearby structures built with similar aggregates.
Many NDT options available
Owners need to routinely evaluate the condition of their cooling tower structures, as many of the failure mechanisms outlined above are hidden or occur without warning. Simple steps—such as visual inspections of all accessible areas by an experienced engineer—should be taken annually.
Discovery of excessive cracking, rust staining, concrete spalling, or surface softening are clues requiring a more thorough investigation. If needed, a variety of nondestructive testing (NDT) techniques are available, such as hammer sounding, impulse radar testing, and covermeter to evaluate the presence and depth of the steel; stress-wave methods to find deteriorated concrete; and half-cell potential tests to identify the presence of active corrosion in a structure. ACI 228.2R, Nondestructive Test Methods for Evaluation of Concrete in Structures, provides a detailed description of these and more concrete test methods.
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