Pointing to a pervasive problem within bridges and roadways, U.S. Federal Highway Administration (FHWA) bridge statistics and sufficiency ratings indicate
Pointing to a pervasive problem within bridges and roadways, U.S. Federal Highway Administration (FHWA) bridge statistics and sufficiency ratings indicate that more than 30 percent of the nation’s bridges were deemed deficient or substandard. Since corrosion significantly affects concrete’s functional characteristics by inducing cracks and spalls, which compromise structural integrity, that situation represents a growing corrosion liability, estimated to require tens of billions of dollars to correct.
Continuously rising construction costs are driving corrosion-resistance methodologies to facilitate conservation of structures rather than replacement. Corrosion Restoration Technologies’ LifeJacket concrete-repair and corrosion-control system offers what product engineers contend is a cost-effective solution. Key to the system is the use of sacrificial anode technology, whereby a specially designed anode is encapsulated within an annular cavity created between the existing structure and a two-piece fiberglass form, providing both concrete restoration and corrosion protection. In most cases, contractors can install the system while the structure remains in service.
DEFINING THE DILEMMA
Corrosion may be defined as the destruction or deterioration of a metal or alloy substrate by direct chemical or electrochemical attack. The corrosion resulting from interaction between the metal and its environment is driven by an electrochemical potential; and, the magnitude of the potential determines the severity of the corrosive reaction.
Steel reinforcement in concrete is initially protected from corrosion by the development of a stable oxide film on its surface. Formed by a chemical reaction between the highly alkaline concrete, pore water, and the steel surface, the film offers protection against corrosion until that layer becomes saturated with chloride ions or sufficient carbonation occurs to reduce the concrete’s pH. Once that occurs, the steel begins to corrode rapidly.
In regions of low resistance, chlorides attack the oxide film and develop anodic and cathodic sites on the steel. The most oxygen-rich regions function as a cathode that fuels the corrosive reaction. As active corrosion proceeds, the lower pH in and around the anodic sites increasingly reduces the passive protective layer. Expansive iron oxide developing as a by-product of corrosion imparts considerable internal tensile forces that lead to cracking and spalling of the covering concrete.
PRESCRIBING THE ANTIDOTE
In 1982, the FHWA issued a statement describing cathodic protection as the only rehabilitation technique proven to stop corrosion in salt-contaminated structures, regardless of concrete chloride content. Both forms of cathodic protection Û impressed current and sacrificial (galvanic) cathodic protection Û control corrosion by supplying electrical current to the affected region. A rectifier or permanent external power source is required by the impressed current system to provide a current flow from the externally placed anode to the corroding reinforcement. By contrast, sacrificial cathodic protection systems, such as LifeJacket, use galvanic cells (dissimilar metals) as the energy source to supply current to the corroding steel.
In both cases, a current flow is established from an externally placed anode that provides energy to negatively polarize the steel reinforcement. The steel in relation to the newly placed anode is thus established in an electrically homogeneous cathodic condition, as the polarization removes isolated anodic and cathodic sites from the reinforcement. Since a galvanic system provides its own power and regulates its current output according to changing environmental conditions, minimal post-installation maintenance and monitoring are required, as compared to impressed current technologies.
Galvanic cathodic protection is achieved by creating a current flow from the sacrificial zinc anode (electrical potential of minus-1.10V with respect to a Cu/CuSO4 half-cell) to the embedded reinforcing steel. When coupled together in a common electrolyte (chloride saturated concrete), the sacrificial anode and the reinforcing steel form a circuit wherein current flows from the zinc anode to the steel cathode until it is sufficiently polarized or electrical equilibrium occurs.
The loss of electrical energy at the anode deteriorates the metal over a period of time. Consequently, the life span of the anode is determined by the system’s current flow and the anode’s consumption rate. The corrosion rate of the zinc anode, therefore, is directly proportional to the potential difference between the anode and the reinforcing steel cathode for a given circuit resistance of the electrochemical cell.
Studies conducted by the Florida Department of Transportation (FDOT) have shown that zinc anodes can provide long-term sacrificial cathodic protection. In the early 1990s, various configurations of galvanic zinc anodes were successfully applied to provide cathodic protection for tidal zone marine structures. Further, in 1994, FDOT tested a system encapsulating internally placed zinc mesh anodes within a stay-in-place fiberglass form filled with a sand-cement mortar. Comprising the mesh was a high-purity, expanded zinc alloyed with trace elements to improve formability and generate better anodic performance. The optimum configuration was identified as a 0.5-in. hex pattern of A-190 zinc mesh balancing critical anode mass with available surface area.
Additionally, on the basis of proven anode performance from earlier trials utilizing zinc penny web (Cathodic Protection Using Scrap and Recycled Materials, paper No. 555, NACE Corrosion91 in Cincinnati), the LifeJacket system consists of a durable, stay-in-place fiberglass form that secures the anode relative to the embedded steel and creates the essential annular space to be filled with an approved material. Moreover, a supplemental bulk anode was introduced to protect the submerged portion of the structure against corrosion and to minimize the current demand on the lower portion of the anode mesh, typically subjected to more frequent wetting by tidal action.
As concrete is highly permeable, chloride ions from saltwater tend to saturate its pore structure over time in those areas most subjected to wet/dry tidal cycling. Thus, the rate and depth of chloride penetration in different areas depends on exposure. The zone of deepest penetration will ultimately establish the highest corrosion rate of the steel reinforcement. A self-regulating system is therefore advantageous for providing cathodic protection.
In a galvanic system like LifeJacket, current output will self-adjust according to environmental conditions to achieve electrical equilibrium with embedded reinforcement. Once equilibrium has been established, the system will continue to regulate its current output, preventing overprotection of the steel.
APPLYING THE REMEDY
The LifeJacket system is a two-piece assembly with interlocking seams joining the jacket sections. Once positioned, the joint is held together with nonconductive fasteners; and, an approved adhesive provides a moisture-tight seal. A wooden bottom-form, used to retain grout during pouring and curing, is later removed to allow a normal influx of saltwater to wet the anode interface and cleanse self-corrosion by-products from the anode surface within the jacket. Once the final wire connection is effected, the system becomes operational, providing uninterrupted current to the corroded structure and, thereby, supplying cathodic protection.
Ready to install with all wiring, nonconductive standoffs, sealed connections, fiberglass jacket form, bulk anode, pumping ports and anode mesh material, the LifeJacket system is implemented by qualified contractors using simple construction procedures. Deteriorated concrete is removed with chipping hammers and pneumatic demolition equipment. After the corroded steel is exposed and all loose concrete has been removed, the piling is prepared by sand- or hydro-blasting to produce a clean interface for good conduction essential to system activation once filler material is added.
If lacking, continuity of steel reinforcement must be established throughout the structure prior to making final connections in order for the system to function properly. The connection can be made through a single excavation to a sound reinforcement bar or strand in the region requiring protection. That anode-cathode connection must be done in compliance with design specifications. A standard junction box houses all functional wired connections and may serve as a site for shunting the circuit when current and voltage measurements are required.
SIGNS OF SUCCESS
With its many bridges in saltwater environments, Florida has a significant stake in the battle against corrosion; thus, FDOT played a key role in the development and testing of the LifeJacket system. Amid constant repair and protection of pilings, Florida in the mid-1990s began using LifeJacket sacrificial anode technology.
In 1994, the first LifeJackets were installed at the Broward River Bridge on an experimental basis, observes Ivan Lasa, FDOT Corrosion Mitigation and Rehabilitation Technologist. Following the successful Broward River implementation, FDOT installed additional LifeJacket systems in 1995 at Orange Avenue and Main Street Bridges in Volusia County. Ten years later, these bridges are still functioning in a satisfactory manner with no additional corrosion damage observed, Lasa affirms.
Since the Broward River Bridge installation, FDOT has specified approximately 3,000 LifeJackets for 70 bridges throughout the state. Today, LifeJacket is one of department’s standard repair methods for pilings in marine environments where corrosion damage is present; the additional service life required does not exceed 25 years; and/or, the limited number of piles fails to justify additional costs associated with impressed-current, cathodic protection.
Lasa explains that FDOT’s experience with LifeJacket proves it is well-suited for the splash area of marine structures. LifeJacket combines concrete restoration and the corrosion control, reducing the time needed to execute repairs.
In certain situations, however, LifeJacket is not the best solution. Due to the zinc mesh anode’s limited voltage output, LifeJacket is not the best approach for high electrical-resistance concretes, Lasa asserts. It is also not recommended for piles beyond those in direct contact with water in marine environments. Our evaluations suggest that under dry conditions, the zinc mesh tends to passivate. The fiberglass stay-in-place forms retain moisture to keep the anode active.
Û Corrosion Restoration Technologies, Tequesta, Florida, 561-744-2258; www.crt-norust.com
Article adapted from a report by LifeJacket system co-inventor Û Douglas Leng, Business Development, Corrosion Restoration Technologies
KEYS TO SUCCESS
LifeJacket’s successful history in Florida supports its use as a technology for repair and protection against corrosion in marine environments. Addressing both repair and prevention issues, LifeJacket holds promise for corrosion challenges when several conditions are met to ensure success:
- LifeJacket is used for marine applications, not to exceed 10 feet above the highest water level (unless ambient conditions allow).
- Electrical continuity exists between rebar and the jacket’s electrical connection.
- All cracked or delaminated concrete has been removed from the structure, and adequate surface preparation has been provided.
- Filler materials are approved by the manufacturer.
- Accepted construction practices are used, and quality control is maintained during installation by personnel trained in cathodic protection.