INTRODUCTION

Mechanical Draft Cooling Towers are critical fixed Assets that need to be maintained and remain in service in order to cool various plant operations and systems. Essentially, the Cooling Tower Basin in mechanical draft technology serves a two-fold purpose, one as containment for cooling water and two as the foundation for supporting overlying "fill" structures. Almost all of these Basin Structures are constructed of conventionally reinforced concrete, either partially or totally placed below grade. Their service environment subjects them to various aggressive deterioration mechanisms including embedded metal corrosion, original construction defects, environmental degradation (i.e., freeze-thaw, algal growth, etc.) as well as chemical attack resulting in erosion and structural section loss. Since these structures contain process water and function as support foundations, the existing condition of these structures should be evaluated regularly and repairs performed, should conditions warrant, on a timely basis.

CONDITION SURVEY/FORENSIC INVESTIGATION

Concrete deterioration comprises both obvious and latent characteristics that are not easily understood without gathering further information through investigation. Like forensic efforts in other process units, the Cooling Tower environment can be hostile for an in-process evaluation. However, techniques have been developed to assess concrete exterior surfaces On-Line and interior surfaces quickly during short duration outages in attempts to determine the causes and effects of concrete deterioration. Employing a combination of Non-Destructive and Semi-Destructive Testing (SDT) techniques, characterizations as to the physical and chemical characteristics 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.

Condition Survey Flowchart identifying the inter-relationship between Field Investigation, Laboratory Tests and Documentation.

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.

 STRUCTURE DETERIORATION TRENDS

Depending on process requirements, reinforced concrete is the construction material of choice for subsurface structures involved with partial or direct burial. However, as in all site-built construction projects, construction defects can be significant depending on the effort exercised with on-site Quality Control/Quality Assurance (QC/QA). Unfortunately, due to the aggressive operating service environment, containment structures with even small construction defects (e.g. honeycomb concrete, misplaced embedded reinforcing steel, waterstops, etc.) can greatly diminish the anticipated service-life of the subject structure.  

It's not unusual when reviewing prematurely deteriorated containment structures that the original designer omitted provisions in the building code, specific to environmental structures. These code provisions take into account corner cracking and require an increase in embedded reinforcing steel to address fluid containment¹.

While in operation, Cooling Tower Basin Structures are continuously exposed to elements that are detrimental to the integrity of reinforced concrete which leads to shortened life-expectancy. Numerous types of aggressive deterioration mechanisms exist within Cooling Tower Basin Structures and these mechanisms need to be accurately identified and mitigated effectively. Initially, all reinforced concrete is exposed to long-term material shrinkage which is actually desiccation of the "moisture-of-convenience" which makes the concrete placeable, but is not part of the hydration/strength-gain of the concrete. This mechanism subjects the concrete mass to volumetric shrinkage stresses that can result in concrete cracks. Concrete as a construction material is strong in compression but weak in tension. Therefore steel reinforcing systems, generally embedded into the concrete mass, resist applied tensile forces.  Structure designers generally understand this phenomenon and specify low Water/Cement ratio (W/C) concrete (i.e., concrete mixtures with an extremely low "moisture-of-convenience" amount) with sufficient embedded reinforcing steel to accommodate these "tensile" stresses. However, many times designers apply the wrong provisions of the Building Code and the result is insufficient steel reinforcement.

Figure 2 - Corrosion-Induced Deterioration caused by embedded metal corrosion.

Cracks, acting as conduits, allow deleterious substances to deeply penetrate the concrete mass as shown in Figure No. 2, above. Treated water, chemically altered for process considerations, can be detrimental to concrete. Chemicals in the water can serve as electrolytes (e.g. chlorides added to control algal growth) initiating premature deterioration of the concrete via corrosion processes associated with the embedded steel reinforcing systems or attack the concrete matrix (e.g. sulfuric acid added to modulate pH) and essentially erode the concrete mass by Sulfate Attack.

Sulfate Attack can be insidious within Cooling Towers as concentrated Sulfuric Acid is used to modulate the pH of some cooling water systems. The chemical injection point can concentrate the acid adjacent to unprotected concrete surfaces. Expansion forces within concrete, as shown below, are detrimental to the durability of concrete in service. The reaction causes significant surface erosion on concrete Cooling Tower Basin surfaces, sometimes only observable during a Unit outage.

Sulfate Attack of Concrete

• Sulfate & Calcium Ions form Gypsum
  (CaSO4•32 H2O) - expands 124% in volume
• Sulfate & Calcium Aluminate form Calcium Sulfoaluminate (ettringite) (3CaO•Al2O3•3CaSO4•3H2O) - expands 227% in volume

View of Cooling Tower Basin Wall Deterioration - Embedded metal corrosion and chemical attack of the concrete matrix affected the concrete wall section approximately 5" in depth.

COOLING TOWER REPAIR PROCESS

History and experience have shown that each Cooling Tower Basin Structure poses unique challenges to a Repair Contractor. Regardless of whether the required repair involves partial or full-depth wall/floor 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 the following elements:

• Identifying and determining the root-cause of the failed concrete;
• Employing proper materials in construction and repair techniques; and
• Using a qualified, experienced contractor who can provide a solution, as well as a well-planned quality control and assurance program (QC/QA), for the repair.

These three steps will assure the Owner that the repair-failure-repair cycle is eliminated and a sound structure put back into operation. A more comprehensive view of how these steps translate into the Repair Process is shown in Figure 3.

Figure No. 3 - Concrete Repair is a Team Process involving the Owner, Engineer and Repair Contractor working toward a common goal of an enduring repair.

REPAIR SCENARIOS

As each Cooling Tower Basin Structure is unique in construction and service, so to, many repair opportunities exist for structural restoration. Repair, based on the results of the Condition Survey/Forensic Investigation discussed above, can take many forms including, but not limited to, repair of leaking cracks (expansive grout injection, polymer crack stabilization, battened membranes, etc.), repair of structural components (e.g. walls, floor slabs, fill support columns, etc.), foundation stabilization (e.g. subsoil modification, compaction grouting, pier support, etc.), corrosion mitigation systems (e.g. passive cp with embedded anodes, active cp with impressed current, corrosion inhibitors, etc.), and establishment of new protective interior surface liners (e.g. fluid applied coating, sheet applied membrane, spray-applied lining system, etc.). Expansion joint and embedded waterstop repairs reestablish integrity to the details of construction, critical in process fluid containment.

When repairing concrete elements "In-Kind" in order for it to function adequately, it's important that the repair materials selected be compatible with the existing concrete substrate, matching as closely as possible:

• Modulus of Elasticity (Y = σ/ε)
• Thermal Expansion (?l/l = α?T)
• Low Material Drying Shrinkage (crack-free)
• Repairing like-with-like!

View of a Cooling Tower Basin Wall Repair, being prepared On-Line - note splash wall placement above repair. Deteriorated concrete removed, mechanical anchors installed, embedded reinforcing steel cleaned and augmented, just prior to formwork installation.

Obviously, besides matching some of the engineering properties for repair construction, cementitious repairs should incorporate corrosion inhibitors as the concrete will be subjected to ample oxygen and moisture in service. Being chemically resistant to sulfates is also important, especially in those Cooling Towers that depend on Sulfuric Acid for cooling water pH control. Generally, the sulfate resistance of a Portland Cement Concrete stems from low levels of Tricalcium Aluminate, C₃A, so as to not react with sulfate ions which can initiate expansive reactions within the concrete mass⁴. Chemical resistance can also be improved by the reduction of the Portland Cement fraction within the repair concrete and replacing that portion of the cement with mineral or pozzolan admixtures (e.g. flyash, microsilica, etc.) that also have cementitious properties⁵. Typically, a durable Cooling Tower Basin repair design involves one or more of the following details:

• Non-Corrosive Fiber-Reinforced Plastic embedment products (FRP)
• Dense, durable conventionally-reinforced cast-in-place concrete
• Mechanical anchorages for composite bonding of repairs to existing concrete substrates

Implementation of corrosion mitigation systems (i.e., sacrificial or impressed)

Crack Repair: Crack repair requires a basic knowledge as to why reinforced concrete cracks. Modern concrete is the end-product of an 80-year trend toward faster hydrating cements and ever-higher cement contents. This trend has produced very strong but also very crack-prone concrete. Major reinforced concrete structures exhibit significant distress because they are more restrained against volume change, undergo greater moisture and temperature changes, the concrete is stronger, has a high modulus, and little creep capacity to relieve the self-stress from thermal contraction, autogenous shrinkage, and drying shrinkage¹. Understanding the root-cause mechanisms associated with observed cracking will assure that the repair, when implemented, will be long lasting with two repair types shown in Figure No. 4.

Figure No. 4 - Two types of common crack repair employed at Cooling Tower Basins. Rout & Seal works best on "positive-side" (i.e., water-side) applications and can be performed only during Off-Line, drained Basin conditions. Pressure-Injecting Chemical Grouts into crack fissures consists of either hydrophilic or hydrophobic urethane grouts which expand in the presence of moisture and can be performed while the Cooling Tower Basin is On-Line.

Standard crack repair technology typically "glues" disjointed concrete members together, stabilizes individual segments of a once unified concrete mass or stops the ingress of groundwater/egress of process fluids. Occasionally cracks form in Cooling Tower Basin structures that result from movement during service. Often, designers place joints within structures that are designed to move, such as in the case of expansion joints. Both moving cracks and expansion joints require special attention. Durable yet flexible construction materials are required to address in-service movement as "rigid" repair efforts will fail from forces developed via restraint. Typically, chemically-resistant, high-temperature tolerant membranes (e.g. Hypalon, etc.) are specified for repair of moving cracks, glued using high-strength paste adhesives or mechanically fastened to crack shoulders. Expansion joints will generally incorporate neoprene gland assemblies or pneumatically inflated diaphragm glands installed into joint cavities, allowing the structure to move, yet maintaining containment integrity².

Structural Member Repair: Aggressive deterioration mechanisms associated with embedded metal corrosion and/or sulfate-related chemical attack of the concrete mass can significantly affect the structural integrity of reinforced concrete members within a structural system (e.g. walls, base slab, column support pedestals, etc.). Reduction in both concrete and embedded reinforcing steel bar cross-sections can create conditions of impending Structural Risk, in some cases requiring immediate action in the form of temporary support shoring or process bypass. At-Risk structural behavior can range from slow, barely noticeable, structural member deflections to "failure-without-notice" of structural systems supporting Cooling Tower fill components.

View of Failed Cooling Tower Basin Wall Water-Stop - an original constructin defect as the water-stop was misplaced and failed in service.

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

Once a repair methodology has been selected, follow the Concrete Repair Industry Best-Practices²:

• Demolish and remove unsound/deteriorated concrete materials
• Prepare resultant sound/competent concrete substrate surfaces
• Assess and augment, if necessary, deteriorated embedded steel reinforcing systems³
• Implement corrosion control measures, if evidence indicates significant embedded metal corrosion activity - embed sacrificial zinc anodes, metalize exterior repaired concrete surfaces or install an impressed current system
• Select appropriate concrete repair materials that have consistent plastic and hardened characteristics and properties to ensure composite behavior between the existing concrete substrate and new "cured" repair materials

Concrete Placement of a Cooling Tower Basin Wall Repair performed On-Line - repair materials were mixed on-site and then hydraulically pumped into formwork cavities. Note FRP formwork ties and temporary splash wall.

Install the selected repair materials using placement/application techniques consistent with the desired end repair product that achieves adequate bond and results in low shrinkage cracking

In conclusion, reinforced concrete Cooling Tower Basin Structures can be successfully repaired On-Line & Off-Line, providing a significant extension to their service-life. These types of repairs however, can only be implemented once we understand:

• Owners Requirements;
• Process items specific to the Facility;
• Deterioration mechanisms in-place within the structure and;
• Securing of Repair Professionals who offer an "engineered approach" and have the background and experience to implement the repair successfully.

 

References

1. Burrows, R. W., The Visible and Invisible Cracking of Concrete, American Concrete Institute Monograph No. 11, 1998, pg. 1.

2. Concrete Repair Manual, 1999 Edition, Published jointly by the International Concrete Repair Institute, Sterling , VA and the American Concrete Institute, Farmington Hills, MI, 1999, 861 pgs.

3. Manual of Standard Practice, 27^th Edition (MSP-2-01), Concrete Reinforcing Steel Institute, Schaumburg, IL, 2001, pgs. 4-4 & 4-5.

4. "Guide to Durable Concrete," ACI Manual of Practice, Part 1, ACI 201.2R-92, American Concrete Institute, Detroit, MI, 1998.

5. Kosmatka, Steven H., Panarese, William C., Design and Control of Concrete Mixtures, Portland Cement Association, 13^th Edition, Portland Cement Association, Skokie, IL, 1988, pgs. 68-70.