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:

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.

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.

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 occurs when CO2 in the air reacts with calcium hydroxide and other hydration products present in concrete to form predominantlycalcium 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.

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.

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.

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.

Two hyperbolic cooling towers were the subject of a
detailed condition assessment by CTLGroup consultants

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 halfcell 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.

Two case studies
The first case study involves a Midwestern two-unit nuclear plant with two reinforced concrete hyperbolic natural-draft cooling towers. Our interest was with Unit 2’s tower, which had been placed into service 21 years earlier. CTLGroup was engaged by the owner to perform a condition assessment during an April 2007 scheduled outage. A similar inspection five years earlier had found soft concrete areas near the top and evidence of poor concrete consolidation at horizontal concrete placement lift joints. Also, several canopy beams had impact damage caused by falling ice. In addition to a follow-up inspection of the soft concrete,CTLGroup was asked to evaluate the condition of various structural elements, record any observed deterioration, identify any long-term durability concerns, inventory those areas that needed near-term corrective action, and recommend corrective measures.

The investigation began with hammer sounding at representative and accessible locations at the bottom of the shell exterior and at thrust blocks, columns, basin walls, and slabs. In addition, covermeter testing at representative areas at the bottom of the shell exterior and at an abandoned cooling structure column were completed. The investigators identified cracking
and localized concrete deterioration to the tower’s structural components. The observed cracking and deterioration were not judged to be an immediate threat to structural integrity. CTLGroup also recommended a series of repairs be made immediately, including removal and repair of all loose cracked and spalled concrete. A number of potential future problem areas were identified for the owner to make routine inspections of to ensure that the deteriorated areas don't expand.

The second case study involves a two unit coal project in the eastern U.S. that was placed into service about 35 years ago. By 1985 the two natural-draft concrete cooling towers began exhibiting signs of concrete deterioration, embedded steel corrosion, and water leakage through the thin shells.An investigating team composed of company staff engineers and CTLGroup
consultants was assembled and charged with preparing a comprehensive condition analysis report on the towers.

The size and geometric configuration of the towers made access for thorough inspection and repair challenging. A platform was erected on the outside of the shell just below the top of the tower to provide access for inspection and repair.

The inspection program on Unit 2 included the floor of the cold water basin of cross-struts, thrust blocks, portions of the abandoned hot water basin,and the underside of canopy beams. Courtesy

Before rehabilitation, 370-foot-tall cooling towers exhibited significant inspection of every square foot of the tower. Courtesy

Areas where an NDT examination revealed soft
concrete or rebar spalling were opened up and repaired.

Below the platform, a trolley rail was mounted to provide horizontal movement for suspended cable-climber scaffolds. Plant staff removed core samples and sent them to a laboratory for compressive strength and accelerated freeze/thaw testing. Copper-copper sulfate half-cell measurements were used to assess active corrosion An NDT examination team hammer-sounded every square foot of concrete in both towers, and questionable areas were further examined using pulse-echo micro-seismic tests. Inspectors used paint to mark deteriorated concrete for subsequent removal by the contractor. Grid markers also were attached to the shell. These were used in mapping the deteriorated and repaired areas, for planning the work, and in quality control documentation.

The team determined that the main cause of the shell deterioration was freeze/thaw action on non-air-entrained, poorly consolidated concrete. The inside shell deterioration was the result of leaching of cement paste by demineralized water from the tower operation. Cracks in the support columns and ringbeam that paralleled the reinforcing steel were attributed to drying shrinkage of the concrete.

The final study report recommended the deteriorated concrete be replaced, the cracks sealed, and a coating applied to the inner surface of the shells. This would seal and dry out the existing non-air-entrained, saturated concrete to increase its long-term resistance to the freezing/thawing environment.

CTLGroup’s rehabilitation team used then-state-of-the-art computer modeling to help ensure that structural integrity was maintained throughout the duration of the project.The structural analysis program also allowed for continuous updating of input data, taking into account the progress of the repair work. A comprehensive quality control program incorporated materials testing, pre-qualification of shotcrete crew members, and oversight and inspection of repair installation. Another inspection 15 years after these repairs were completed found no sign of concrete distress. In fact, the entire rehabilitation project earned a longevity award from the International Concrete Repair Institute in 2005.

0