Repair and Cathodic Protection of Natural Draft Cooling Towers

A 400-foot-tall concrete hyperbolic shell, natural draft cooling tower at an electrical plant in the Mid-Atlantic region of the United States had been operating with brackish water since it was put into service in 1975. Over time, concrete delamination and spalling occurred in the shell and on the supporting x-columns.

An assessment was performed to identify the root causes of deterioration, determine their structural integrity and to develop a repair/rehabilitation strategy that would give the structures an increased service life of 25 years. Chloride-induced corrosion was rapidly crippling the aging structure and it was determined that concrete repair alone would not accomplish the service life extension. So, as part of the cooling tower rehabilitation, Vector Construction, designed and installed a custom cathodic protection system to provide a long-term solutions to the structure’s corrosion problem.

Repair Development: Design & Construction

With consideration to safety, cost and durability, the owner established an objective to achieve a service life extension of at least 25 years while repairing the tower without modifying its design. To achieve this, the cooling tower remained on “stand-by” to allow as much of the repairs as possible and minimize operational downtime.

Due to the extent of deterioration and delamination in the upper section of the tower shell, it was decided to perform an engineered demolition and rebuild of the shell above the throat. Below the throat, where concrete deterioration was less severe, localized partial-depth concrete repairs at both the interior and exterior of the shell were selected. The X-columns were found to have sufficient capacity in their current deteriorated state and did not need to be restored to their original design strength. As such, the X-columns were repaired to maintain their current structural capacity by way of a mitigation strategy.

Cathodic Protection Design and Installation: Hybrid Cathodic Protection Scheme

Cathodic Protection (CP) was selected as a reliable and adaptable repair strategy to protect the tower shell and X-columns from continued deterioration. Impressed current cathodic protection (ICCP) was selected for the shell based on the ability to control voltage levels at different locations of the large shell over the course of the system service life.

Three types of ICCP systems were used on different sections of the shell. Ebonex® Discrete titanium suboxide ceramic tube anodes were used in the heavily reinforced, thickened concrete base of the shell. Mixed metal oxide coated titanium ribbon mesh anodes were grouted into slots cut into the cover concrete to protect the repaired height of the existing shell; and mixed metal oxide coated titanium ribbon mesh anodes were cast between the reinforcing layers in the reconstructed portion of the shell above the throat for corrosion (cathodic) prevention.

Each vertical stack is controlled by a rectifier and monitored by a Vector Corrosion Services’ (VCS) StructureView® remote monitoring unit (RMU) located on the canopy walkway. The RMU can be controlled by web-based software to measure voltage, current, and potentials and to perform polarization decay testing.

Galvanic cathodic protection (GCP) through galvanic jackets was selected for the X-columns based on the durability of its components within the harsh basin environment. The galvanic jackets (Galvashield® DAS Jackets) containing alkali-activated distributed zinc anodes were installed with cement mortar inside stay-in-place fiber reinforced polymer (FRP) forms.

These galvanic anodes essentially operate as low-voltage batteries supplying protective current to the column reinforcing steel. These jackets do not require any maintenance or monitoring to provide long-term corrosion protection. They are suitable for use in wet environments and can be installed without external junction boxes or conduit, which can fill with water and lead to wiring failures. Four of the thirty-two X-columns were also instrumented and connected into the VCS StructureView monitoring system.

Summary

The client was provided with a complete design-build solution to address the corrosion of the reinforcing steel inside the natural draft cooling tower, and piece-of-mind that their cooling tower will now be able to provide reliable service for the next 25 years.