By Leandro Etcheverry, Walter P. Moore & Assoc.; Jorge Costa, CRT; and Mike Szoke, Structural Preservation Systems Industrial Division
As the supply source for operations in power generating plants, water intake structures serve an important function. These structures usually are elevated deck structures spanning the water source or closed, compartmentalized cells or units separated by walls, which serve as a house for pumps, traveling screen motors and ancillary equipment.
Because of the constant exposure to water – whether fresh, brackish or marine – these intake structures are extremely susceptible to corrosion-induced deterioration. However, with an understanding of the root cause for corrosion-induced concrete deterioration and the technologies available to address it, plant and maintenance professionals can adopt the proper mechanisms and rehabilitation strategies.
Figure 1 Cell water intake structure Click here to enlarge image
The water source plays a major role on the deterioration of the intake structure’s reinforced concrete. Water sources with higher chloride levels increase the propensity for corrosion-induced deterioration. Concrete is a beneficial environment for steel because of its high alkalinity or pH value. This high alkalinity causes the steel to react and form a passive oxide film on its surface, protecting it against corrosion. Should the passive film be removed, the steel is subject to corrosion damage. Few conditions cause the passive film to be disrupted; the most common condition is chloride contamination of the concrete itself.
During embedded steel metal corrosion, corrosion products require and occupy more space than the original volume of steel. As such, significant tensile stresses develop on the concrete in the immediate proximity of the corroding steel. Although inherently strong in compression, concrete is relatively weak in tension. The result is that unrestrained portions of the concrete mass crack at the corroding bar interface. Cracking exposes steel to the elements, which fuel the corrosion process. As corrosion continues, delaminations occur, most typically characterized by partially attached concrete sections due to the volume expansion of the corroding reinforcing steel. Spalling represents the corrosion process’s final stage, where the delaminated areas become completely detached and the corroded reinforcement is left exposed.
Getting to the Root
Many repair programs simply mask the effects yet do nothing to alleviate the cause. For example, in marine environments, surface repairs alone may not effectively address the deterioration. Typically, upon removing all deteriorated concrete, high chloride concentrations remain in native concrete areas. Without a means to address the root cause of the corrosion deterioration, extensive surface repairs alone could exacerbate the problem by introducing corrosion cells between steel embedded in chloride-free patch material and steel embedded in chloride-contaminated native concrete. This phenomenon is referred to as the “halo” or “anode ring” effect.
Cathodic protection, achieved by supplying a source of electrical current to counteract the internal current existing in the corrosion cell, has been shown to help address the root cause of corrosion-induced deterioration. During cathodic protection, current flows from an auxiliary anode material through the concrete electrolyte to the surface of the reinforcing steel, either mitigating or minimizing the corrosion process.
In sacrificial cathodic protection systems, the anode corrodes preferentially-producing electrons. The sacrificial anode needs to be more active than iron to protect the reinforcing steel. The most widely used sacrificial anode for reinforced concrete structures is zinc. The resistance of the concrete electrolyte is a crucial element to determine whether sacrificial cathodic protection is viable. In cases where the electrolyte resistance is too high, the effective potential difference between the steel and the anode may not be sufficient to adequately protect the structure.
In impressed current systems, a small direct current is passed from a permanent anode to the reinforcing steel to supply the electrons. An external power supply must be connected between the anode and the steel with appropriate polarity and voltage to prevent the reinforcing steel from giving up electrons. Commonly used where the current requirements for corrosion protection are high and where the driving voltage is greater than what can be obtained with sacrificial systems, impressed current cathodic protection systems can be controlled to deliver just enough protective current to the structure. Their disadvantage lies in that they require more monitoring.
Structural elements within the tidal zone or in closest proximity to the body of water are most affected by corrosion-induced deterioration. Tidal fluctuations result in wetting and drying cycles that fuel corrosion. The wetting and drying that result from traveling screens and pumps also generates splash (Figure 2).
Figure 2 Corrosion-induced deterioration on a water intake structure girder Click here to enlarge image
Surface repairs of corrosion-induced deterioration involve removing deteriorated concrete, undercutting around the reinforcing steel, cleaning the reinforcing steel and reestablishing the original concrete section. Where corrosion-induced deterioration is extensive, phasing the repairs or shoring over time might be required to avoid undermining structural capacity in the process of conducting repairs. The concrete removal process often extends concrete extraction beyond the limits of what is deteriorated. Here too, repairs may need to be phased. Completing concrete repairs involves allowing time for the repair material to develop sufficient strength. Shoring involves temporarily installing an extra skeletal support frame to bypass loads from sections compromised by extensive corrosion-induced deterioration. In the case of an elevated deck supported by piles, the shoring design often involves installing friction collars around the piles or columns to transfer loads to sound sections of these elements.
Certain sacrificial cathodic protection systems have shown to be effective in mitigating corrosion-induced activity in substructure elements. The anode can take different forms, depending on exposure conditions, corrosion protection needs and the geometry of the elements. For example, columns, piers, piles and other elements within the tidal zones are especially suited for jackets that incorporate zinc cathodic protection (Figure 3). Such a proprietary jacket system consists of two C-shaped fiberglass sections that clamp around the element. The system (a stay-in-place form filled with cementitious grout) incorporates a sacrificial zinc mesh anode, which is secured to the inside face of the fiberglass sections. In the process of filling the annulus between the original concrete surface and the interior of the jacket, any opened repair cavities are filled with the grout, which eliminates surface repairs.
Figure 3 Installed cathodic protection jacket on substructure piles Click here to enlarge image
A variation of the jacket system for elements other than piles or columns with exposure to a wet environment is zinc mesh secured to the concrete by compression fiberglass panels. This system can be used for substructure elements where the contact resistance for the anode is low, such as elements within the tidal zone or areas that remain wet most of the time.
For areas above the tidal zone, thermally-sprayed zinc can provide an effective long-term protection strategy. The main advantage of these systems is that they can conform to any surface geometry or orientation while adding minimal deadload to the structure. Metallized cathodic protection systems have seen increased acceptance for corrosion protection of steel reinforcing in concrete since their introduction in the early 1980s. More than two million square feet of metallized anodes for cathodic protection systems are in operation in North America. The bulk of these installations use zinc as the anode material. In a sacrificial cathodic protection system, zinc is sprayed on to the concrete surface (Figure 4). Contact is made to the rebar with specially designed attachments. These systems do not require outside sources of power and are “self-regulating.” This means that the current output from the anode is primarily controlled by the concrete’s resistance. If the resistance is lower (a condition generally conducive to higher corrosivity) the anode will deliver higher current outputs. If the resistance is higher, the opposite will be true. On top of the thermally-sprayed zinc system, most docks receive a coating to protect the zinc anode from undue atmospheric oxidation.
Figure 4 Thermally sprayed zinc on repaired water intake structure girder Click here to enlarge image
Regardless of the type of sacrificial zinc system used, bulk zinc anodes can be used to compensate for the consumption of the zinc anode (metallized zinc or zinc mesh) at the tidal area or splash zone. Bulk zinc anodes take the form of solid zinc pieces, which are secured to submerged portions of the elements to be protected via clamps or brackets.
Among the challenges associated with this type of work is the fact that a significant portion of the work is often performed in the presence of tidal fluctuations. As a result, access often requires floating platforms that provide a safe and stable stage for construction operations. The floating platforms should have proper railings; personnel working on them are often required to have flotation devices.
Traveling screens also represent a safety hazard for workers. Plant owners often require special clearances for workers before each workday to ensure that nearby traveling screens and pumps will be shut down. To maintain operations, concrete repair and protection should be phased to shut down only a certain section of the water intake structure at a time. Proper containment is also required around traveling screens to avoid damage from concrete debris or abrasives resulting from concrete removal and surface preparation operations.
Ponding Water Challenges
Decks are also often affected by corrosion-induced deterioration from ponding water. Since the deck serves as a traffic surface, corrosion-induced deterioration in the form of spalling can cause a tripping hazard. Where exposure conditions are aggressive and deterioration is significant, a long-term solution for repair and protection at the top of the reinforced concrete decks includes low-permeability overlays modified with fly ash or silica fume with proper grading. Unfortunately, the demand for a fast turn-around for overlays often results in improper material selection, which can lead to shrinkage and premature failure (Figure 5).
Figure 5 Failed overlay on water intake structure deck Click here to enlarge image
Another option is an impressed current cathodic protection system. Such systems involve installing an anode on the deck’s surface. The anode, which usually consists of mixed metal oxide titanium mesh ribbon, is embedded within a grid of slots cut in sound areas of the deck or inserted within repair cavities.
Corrosion-induced deterioration also affects water intake structure handrails, which (as shown in Figure 6) are often steel (bare or galvanized) or concrete, precast or cast-in-place. For steel handrails, deterioration often originates where the steel posts are embedded into the concrete. In cases of severe and widespread deterioration, total replacement with surface-mounted handrails may offer a long-term strategy.
Figure 6 Corrosion-induced deterioration on precast concrete handrails Click here to enlarge image
Concrete Repair Is a Process
Rehabilitating water intake structures presents unique challenges and opportunities. Of utmost importance is close coordination between the owner, engineer and repair contractor for the safe execution and successful implementation of the repairs. Understanding the root cause of the deterioration is also essential. A condition evaluation conducted by a team of forensic engineers and technicians is useful in locating, qualifying and quantifying corrosion-induced concrete deterioration. Engaging specialty engineering and contracting firms that are familiar with all the highlighted critical aspects will ensure the most cost-effective and long-lasting repairs.
Leandro Etcheverry, Ph.D., P.E is a Senior Associate with the Structural Diagnostics Services Group of Walter P. Moore and Associates, Inc., specializing in the diagnostic evaluation and rehabilitation of corrosion-induced deterioration in concrete structures. He is a member of the American Concrete Institute, NACE International and the International Concrete Repair Institute.
Jorge Costa, P.E. is president and founder of Corrosion Restoration Technologies, Inc. (CRT). He has worked in cathodic protection and corrosion control for 26 years. He is a licensed Corrosion Engineer in California and holds a P.E. license in Metallurgical Engineering in the State of Florida.
Mike Szoke is Division Manager in the Los Angeles office for the Industrial Division of Structural Preservation Systems. His 25 years of professional experience covers all phases of concrete repair.