Power plants have long experienced maintenance problems related to reinforced concrete structures such as cooling towers and flue stacks, typically manifested in age-related physical degradation and/or material corrosion via exposure to salts, chlorides or acids. Although various arresting schemes and repair processes are available, none have proven widely successful.
For the last 150 years, the use of reinforced concrete construction has broadly been viewed as a way to construct maintenance-free structures. Where properly constructed, reinforced concrete should last 100 years or more with little or no repairs. The older reinforced concrete structures in service today have survived because they were constructed with proper consideration given to their service environment.
“This is not the case in a majority of the reinforced concrete structures built in the last 30 years, however,” said Ed Young, B.S.M.E., vice president of business development with the Shaffer Group. “Inadequate design criteria, competitive construction costs and inattention during construction have resulted in structures that are not able to provide the service life we had come to expect. In some instances, previously unanticipated design criteria such as blast mitigation and impact resistance have suddenly become critical.
Power plants have attempted various solutions, including a number of repair materials and cathodic protection systems. In most cases, however, little consideration is given to the underlying problem of the anodic/cathodic cycle generating the corrosion current. Simply patching the damage is treating the symptom, not the cause.
“The two prototype cathodic protection systems we have seen apparently generated sufficient currents to overcome the corrosion current in the immediate area, but were considered too difficult to operate and maintain and were later abandoned,” said Fredrick Shaffer, P.E., CEO of the Shaffer Group (www.theshaffergroup.com). “The additional problem with retrofitting cathodic protection on a concrete structure is that the corrosion current is not constant throughout the structure. Therefore, it is not practical to induce a current sufficient to overcome the highest corrosion current without causing damage at the point where there is little or no current.”
The Shaffer Group has developed an integrated system of design and repair that combines existing and new restoration technologies. This system, called VAPIS, includes a project-specific formulation of chemical treatments to arrest the corrosion and improve the physical properties of the concrete; an innovative repair mortar; and comprehensive engineering for system implementation. By delivering the active ingredients under pressure directly into the concrete and into direct contact with the embedded steel, its effectiveness in limiting corrosion is greatly improved.
Preliminary testing at a large southeast fossil fuel power plant has been promising. The plant uses two precast concrete swamp cooling towers to treat river water when the intake water temperature reaches 80 F. The cooling towers have been severely affected by the salinity of the water and the wet/dry cycling operation, exhibiting extensive corrosion of the reinforcing steel and attendant spalling of the concrete.
The Shaffer Group conducted tests on one of the concrete columns that had been undergoing significant corrosion. Previous testing on this tower had shown chloride contents of 4,000-5,000 ppm. The pre-test and post-test (one hour after inoculation) data show a significant reduction in both corrosion and concrete porosity (higher resistivity). Corrosion current dropped from 350-400 µM/yr to 20-40 µM/yr and resistivity increased from +0.2 kOhms to +4-6 kOhms.
“The results are comparable to those obtained after 60 to 120 days of cure time for a topically applied, migratory corrosion inhibitor,” said Young. “Additionally, where we would expect to get a reduction in corrosion current with a topical application, we have achieved a total cessation of corrosion current using the VAPIS system. Considering that the chemistry used in our treatment chemicals is inorganic in nature, the result is permanent protection against corrosion.”
The repair mortar the Shaffer Group has developed can be applied without the extensive preparations necessary for conventional repairs. The material has zero shrinkage; therefore, it won’t crack, and will feather down to a 2-mm thickness. It also has a bond strength greater than the tensile strength of the host concrete and requires no surface preparation or binding agent. As a result, the demolition costs associated with a conventional repair are greatly reduced.