Cooling Tower Concrete Maintenance
All things deteriorate over time. Even structures constructed of reinforced concrete require maintenance in order to continue to function as a critical foundation support asset. Cooling Tower structural elements are no different except that the concrete can provide a foundation, containment for cooling process fluids or housing critical mechanical equipment. Regardless of whether the Cooling Tower is based on Natural Draft or Mechanical Draft Technology, deterioration in aggressive environments are generally time dependent as long as regularly scheduled maintenance is performed and changes to the process are minimal. However, the only constant in life is “change” and these changes can provide wide swings in available service life as well as the assumption that the structure was constructed in accordance with industry “Best Practices”.
Unfortunately original construction practices, poor quality materials of construction, harsh environmental service exposure conditions as well as changes to process streams can culminate in premature failure in structural performance. Understanding these deterioration trends, well in advance of deficient structural behavior, is the goal of every Owner. Tools, in line with “Risk Based Inspection” (RBI) Programs developed for mechanical and electrical systems, are also available for civil assets. These tools provide indications as to the present condition of the structure and assist in prioritizing Maintenance/Repair Programs, allowing Owners to capitalize maintenance budgets over extended periods of time.
Forecasting the future is rarely successful in business without adequate preparation and research. Similarly, understanding the life-cycle of a civil asset can be a gamble without adequate preplanning, investigation and enlightened deliberation regarding data collection of existing structural conditions. Employing strategic planning processes such as the LQQ Method, an Owner can begin to understand and determine priorities for repair and maintenance of these critical assets. The LQQ Method involves Locating, Qualifying and Quantifying deterioration and associated root-cause mechanisms. Once root-cause deterioration mechanisms are identified, a Maintenance/Repair Program can be formulated and prioritized to meet extended service life criteria as well as an Owner’s Maintenance Budget.
Building to Specifications
Regardless of how stringent construction documents are formatted, unless appropriate Quality Assurance/Quality Control (QA/QC) practices are in effect, the specter of poor quality construction will be present in all aspects of the original construction. All too often however, Owners depend heavily on “strong” specifications and “fall-down” miserably with QA/QC during the execution of the Work due to the additional costs associated with monitoring and inspection. Construction defects originating during original construction can come in many forms such as:
- Poor construction document detailing
- Inappropriate specified materials of construction
- Inferior material product substitution
- Lack of consistent adherence to original Construction Documents
- Poor quality construction craftsmanship
- Disregard for manufacturer recommendations concerning application and resulting installation of specified construction materials
- Abuse/Damage to the structure during the original construction process
Unless a methodical review (i.e., Punch List Walk-Through) is scheduled and performed by qualified construction professionals, construction defects can remain latent until significant deterioration is detected at a much later date. Unless blatantly apparent, “discovery” generally occurs beyond most stipulated warranty periods agreed upon during the original contract negotiations. Whereupon this leaves the Owner with interpreted “maintenance” items that in reality were attributed to poor original design, contracting practices or a combination of both.
Regardless of the historical aspects associated with the initiation of detected deterioration, determining the “root-cause” of the deterioration is a critical feature in developing an enduring repair strategy. Frequently, Owners get caught in a repetitive cycle of “repairing-the-repair” instead of providing a quality restoration that provides a significant service life extension to the asset.
Scheduling Planned Cooling Tower Maintenance
Assuming structures are “built-to-specs”, as all things built by man degrade over time, regularly planned inspections and maintenance should be incorporated into the operating budget for an asset, including Cooling Towers and their associated concrete elements. As much of the access to view the internal status of Cooling Towers are predicated on drained basin conditions, many times an inspection will identify “discovery” deterioration with repair opportunities noted but not acted upon due to time limitations. In cases such as these, planned maintenance during a future scheduled outage can be prudent and provide an opportunity for the Owner to secure funding for the necessary maintenance. The kinds of potential reinforced concrete maintenance issues associated with an operating Cooling Tower can include:
- Leaking cracks in the reinforced concrete Walls and Base Slab
- Adhesion/cohesion Joint Sealant failures
- Embedded Waterstop leakage
- Expansion Joint deterioration and leakage
- Mechanical penetration leakage
- Anchor Bolt corrosion
- Lack of foundation support for the Cooling Tower superstructure
- Embedded reinforcing steel bar corrosion and resultant structural member spalling
- Erosion of concrete surface paste
- Differential settlement of the Cooling Tower Basin Wall/Base Slab
- Algal growth and vegetation obstruction
History and experience have shown that each Cooling Tower Structure poses unique challenges to a Repair Contractor. Regardless of whether the required repair involves partial or full-depth structural member section repairs, foundation stabilization, containment liner, crack repair or simply stopping cooling water egress, it’s imperative to utilize an engineered solution. A proper repair strategy should consist of:
- Identifying and determining the root-cause of failed concrete/structural elements
- Employing proper materials in construction and repair techniques
- Using a qualified experienced Contractor who can provide a solution, as well as a well-planned Quality Assurance/Quality Control (QA/QC) program for the repair
Location, Qualification and Quantification (LQQ) of Deterioration
Should significant distress conditions be exposed during a regularly scheduled maintenance outage, an evaluative approach, as discussed earlier, should be initially employed:
- Locate the deterioration
- Qualify the distress mechanisms and determine the “root-cause”
- Quantify the amount of repair to assess repair methodology – determine whether to Repair or Replace-in-Kind
Concrete deterioration comprises both obvious and latent characteristics that are not easily understood without gathering further information through investigation. Employing a combination of Non-Destructive (NDT) and Semi-Destructive Testing (SDT) techniques, characterizations as to the physical and chemical properties of the reinforced concrete structures can be determined quickly. Using cutting-edge analytical and diagnostic tools, the evaluator establishes these repair parameters:
- An evaluation that investigates further, and qualifies causes & effects;
- A quantification of the problem that expresses its extent in concrete terms (e.g. square feet, cubic feet, linear feet, etc.); and
- Documentation describing where the distressed conditions are located – and what it will cost to repair them – arranged from highest to lowest priority.
Once adequately characterized, a thoughtful and detailed repair approach can be developed addressing thermodynamic, chemical and construction material properties of the structure operating within the Cooling Tower process service environment – optimally resulting in a long-term repair program.
Project: Mechanical Draft Wood Construction Fill Cooling Tower Basin
Owner: Petrochemical Plant
Location: Midwest USA
Deterioration: Leaking Construction/Expansion Joints & Waterstops
Background: An Owner of a Mechanical Draft Cooling Tower was experiencing extensive leakage requiring vast amounts of “make-up” cooling water for losses associated with actively leaking joints in the Cooling Tower Basin. Attempts at grouting observed leakage conditions while the Cooling Tower was on-line were largely unsuccessful. Recognizing that a more thoughtful water mitigation program would be necessary and coinciding with a scheduled maintenance outage, a comprehensive plan was developed that addressed root-cause leakage conditions that included the restoration of existing expansion and construction joints in Walls and Base Slab areas as well as sub-surface stabilization of voided conditions below the Base Slab created by the transport of fines caused by water flow conditions.
Once Basin areas were water-blasted clean, interior regions of the Cooling Tower Basin could be assessed. Sealant failures (Figure No. 1) due to the lack of maintenance at construction/expansion joints and poorly installed “fixes” (Figure No. 2) were prevalent as well as original construction defects in the form of misplaced waterstops (Figure No. 3). Once identified, the areas were marked and then scheduled for repair. Repairs included restoration of vertical Basin Wall surfaces (Figure No. 4), subsurface grouting of voids below existing Base Slab expansion joints (Figure No. 5), and the installation of a new flexible hypalon gland overtop of expansion joint locations (Figure Nos. 6 & 7).
Project: Mechanical Draft Precast Concrete Construction Cooling Tower
Owner: Fossil-Fuel Power Generation Plant
Location: Southwest USA
Deterioration: Spalling concrete as a result of embedded metal corrosion
Background: The Head of Maintenance at a large Natural Gas-fueled Power Plant became concerned when the 25 year-old Mechanical Draft Cooling Tower (Figure No. 8) was experiencing advancing deterioration in the form of spalling concrete created by embedded metal corrosion activity within the Cooling Tower’s Fan Shrouds (Figure No. 9). Reviewing construction detailing, the Fan Shrouds were segmental components cast at a precast concrete fabricator and then transported to the site for assembly. Individual Fan Shroud components incorporated a framework of structural “ribs” integrally cast-inplace with arching pan forms that were reinforced with embedded steel bars in low protective concrete coverage conditions at the precasting plant. Concrete material chemical testing of the Fan Shrouds, via powder sampling, revealed the concrete was extensively carbonated and had elevated chloride ion content levels. Understanding that the service environment was very aggressive including atmospheric “swings” in temperature and relative humidity, a cathodic protection system was recommended including both aspects of sacrificial and impressed current. Due to economic budget restraints, the Owner did not have funding available to address the corrosion potential of a conventionally repaired concrete element adjacent to a concrete substrate of poor chemical characteristics. As such, a repair hybrid was developed that incorporated highquality concrete repair techniques and materials-of-construction. The repair hybrid developed provided enhanced durability as well as a relatively simple connection to a future sacrificial/impressed current cathodic protection system installation, should future maintenance budgets provide funding for the additional corrosion prevention.
After identifying regions of repair on the precast concrete Fan Shroud components, the deterioration tended to focus on the vertical members of the structural framework within the Fan Shroud. Detecting deterioration on the Fan Shroud not only included visual examination of the concrete members but also employed hammer sounding techniques. Using a hammer, concrete surfaces were impacted in an attempt to detect tonal differences and determining “sound” concrete versus “unsound” concrete for repair. After marking Fan Shroud members for repair, a 4” diamond grinder then sawcut the perimeter of the marked area, providing a defined edge for repair confinement. Using electric chipping hammers to minimize collateral concrete damage to adjacent yet sound concrete, unsound concrete was removed within the sawcut perimeters. Exposed reinforcing steel bars observed at the base of the excavation revealed significant corrosion losses requiring rebar augmentation to restore reinforcing continuity in the Fan Shroud. Unfortunately, the original as-built dimensions provided only ¾” of concrete cover in a service environment requiring at least 2” of protective concrete. To provide adequate protection, the Fan Shroud rib width was expanded in size for additional reinforcing coverage as well as incorporating an extremely dense concrete material within formed repair cavities. Once cast, the repairs cured within the formwork until achieving adequate strength. The formwork was then removed and the resultant cast surfaces ground to match surrounding surface contours (Figure No. 10).
The repair included an opportunity, at a later date, to apply thermally sprayed aluminum alloy (TSA) and/or install an impressed current cathodic protection (ICCP) system. This ability was accomplished by the complete electric connectivity of the embedded reinforcing steel bars within and adjacent to the repair area. Additionally, by incorporating a repair product that’s not only durable but also compatible with cathodic protection systems, regardless of the type, gave the Owner options on how to best spend their maintenance budget funding.
Decisions regarding whether to “repair-or-replace” a civil asset can be extremely frustrating to an Owner possessing incomplete information. Maintenance budgets seem to be shrinking yearly due to economic restraints placed upon Maintenance Departments by Management in efforts to improve process efficiency and product quality versus the need to maintain these assets at a minimum level of functionality. Although civil assets don’t necessarily make an Owner money in product sales, their failure in function can certainly cost money in lost production. A thoughtful phased approach in Cooling Tower evaluation, repair prioritization, cost estimation and repair contracting can provide Owners with significant service life extensions for these critical civil assets.