Located at St. John's River Power Plant in Jacksonville, Fla., this project featured work on the No. 2 hyperbolic cooling tower. Used in large power generation plants, hyperbolic natural draft cooling towers are known for their distinct shape. Although eye-catching in form, the distinct shape serves a functional purpose in that it cools the water received from the steam condensers at a much lower operating cost than mechanical draft cooling towers.
This particular cast-in-place tower, in operation since 1987, is 450-feet tall and 360-feet in diameter. Constructed using traditional formwork for the columns and lintel beam and slip-form construction for the veil (shell), the structure's purpose is to cool hot water from the generator's steam confdensers through a pipeline entering at ground level. Water is pumped upward to the tower interior, through four flumes and a series of pipes and is eventually discharged in spray form at about the forty-foot level. During the downward movement, the water is cooled by the air being drawn upward (by the natural draft) and undergoes evaporative cooling.
During the operation, practically all surfaces are subject to "immersion" conditions. The columns and lintels are subject to intermittent wet and dry conditions. As a result, these towers are extremely susceptible to corrosion-induced deterioration. Maintaining existing towers is crucial because construction of new hyperbolic natural draft cooling towers, represents a very large capital investment.
The owners, a joint venture between the Jacksonville Electric Authority (JEA) and Florida Power & Light Company (FPL), first noticed deterioration on the veil, perimeter columns and lintel beam in the form of severe corrosion of the reinforcing steel. Visual inspections noted concrete cracking, spalling, rust staining and delamination. Several factors were contributing to the deterioration. To begin, the cooling tower uses brackish water from the nearby St. John's River. This water contains a high volume of chlorides - a substance that is highly corrosive to steel embedded within the concrete. Additionally, airflow from the nearby Atlantic Ocean and St. John's River traveling though and around the cooling tower produces high oxygen and chloride levels at the columns and lintel beam. Once the brackish water and salty air start the corrosion process, the reinforcing steel begins to rust and expand - causing cracks to form in the concrete which become greater conduits for more chloride and oxygen intrusion. The result is delamination and spalling.
Repairing corrosion-induced deterioration typically involves removal of deteriorated concrete, undercutting around the reinforcing steel, cleaning and protection of the reinforcing steel, and re-establishing the original concrete section. However, one thing this project clearly recognized was the importance of installing a protection system.
The scope of our repair included installation of 120 lintel beam jackets and 240 column jackets for a total of 34,000 sq ft of jacketing. Procedures included removing delaminated concrete with pneumatic chipping guns, profiling concrete surfaces to a minimum concrete surface and cleaning the corroded reinforcing bars utilizing 35,000 PSI ultra-high pressure water blasting equipment and pneumatically rotated handguns prior to placing and grouting the fiberglass jackets.
Gaining access to the repair areas was a significant challenge. Nearly all of the work on this project was done off of aerial lifts. A total of 16 articulating aerial lifts and two 4x4 scissor lifts were required to provide access. This type of activity was new to most of the crew, so the contractor arranged for instructors to come to the jobsite and perform onsite field and classroom training. In addition to giving the crews the opportunity to practice operating the lift, these sessions taught the crews the dangers involved in using this equipment and what needed to be checked daily before using the lift. The crews also engaged in safety courses highlighting communication and the safe use of such large equipment in tight quarters.