Encapsulating pier rehab

Concrete Construction, July, 2003 by Tom Klemens

Reinforced concrete structures installed in saltwater are at 'high risk of damage from rebar corrosion. Cathodic protection is one solution, but its high cost and the perceived complexity of its design and installation have limited its application.

A systems approach developed by Zinc Products Co., Greeneville, Tenn., offers a relatively simple, cost-effective long-term solution to corrosion. Its components are easy to install and provide a self-powered, self-regulating system requiring no further maintenance or monitoring.

Why metals corrode

The same molecular structure that makes a metal a good conductor of electricity and heat also makes it prone to corrosion. Corrosion is the process where a metal loses electrons. Electrochemical corrosion begins when a metal with a high electrical potential, such as steel, comes in contact with an area of lower electrical potential through an electrolyte solution. Even the mix water in concrete can set the scene for a galvanic current, from the high-potential steel, through the electrolytic moisture in the concrete, to the lower potential soil, water, or other materials.

Controlling this electron migration requires reversing the current flow, turning the steel rebar's function from that of anode'--giving up electrons--to cathode. And that is where a sacrificial supply of zinc can be put to good use. Connecting the steel to a higher potential metal, such as zinc, by a wire reverses the current and halts the rebar corrosion.

Zinc Products Co. offers prefabricated panels with expanded zinc sheets inside to protect concrete piers in saltwater. With locking seams to hold the two sides together, the panels are easy to place around existing round or square piers. The fiberglass jacket is then filled with sand-cement grout. The lead wire from the zinc is connected to a lead attached to the steel rebar, and protection begins. This system has been used by the Florida Department of Transportation on a number of bridges. Because it is a complete system, rather than individual components from a variety of sources, a typical bridge maintenance crew can install the protection system and get a neat appearance.

Is it really a problem?

The Portland Cement Association (PCA), in an article in Concrete Technology Today, July 2001, suggests that the potential for problems does exist, mostly above the waterline.

"The results of the 39-year study revealed that seawater had no damaging effect on submerged concrete specimens, regardless of their cementitious composition," the article says. "On reinforced concrete, the program produced some predictable results--namely, that a low water-cement ratio and high concrete cover reduce rebar corrosion. But the program also produced a more interesting result: Concrete positioned above high tide suffered more corrosion damage than concrete placed at mean tide levels."

The PCA study included a variety of cement types, ordinary reinforcing bars (some coated and some not), and stressed prestressing strands. "The only major source of distress in reinforced beams stored in and above seawater was longitudinal cracking attributed to corrosion of embedded reinforcing steel ... [and that] only in certain concrete beams stored above high tide." The PCA report suggests that this resulted from the low solubility of oxygen in seawater, which therefore limits oxygen available to affect the steel in submerged concrete. For beams stored above high tide, ample oxygen in the atmosphere, chloride from sea-driven fog and mist, and minimal concrete cover combined to produce conditions conducive to rebar corrosion and longitudinal cracking in the concrete.

The full text of the PCA article and the report can be found at www.portcement.org. For more information about Zinc Products Co.'s system visit www.allzinc.com/apps/jacket .htm.