In 2007, a condition survey and evaluation was performed on the exterior shell of two natural draft cooling towers owned by Progress Energy. Both towers, located in Crystal River, Florida, measured approximately 450 feet in height with basin diameters of 381 feet. The cooling tower veil of each tower is supported at the bottom by 40 pairs of precast, diagonally oriented rectangular concrete column pedestals. The inner diameter is 349 feet at the bottom of the veil, 209 feet at the throat (location of the minimal veil diameter), and 215 feet at the top of the veil. The thickness of the reinforced concrete veil wall ranges from 8 inches to 39 inches at the top of the veil and is reinforced with interior and exterior mats of Grade 60 reinforcement in both the vertical and circumferential directions with a clear concrete cover of approximately 1.5 inches.
Based on the data obtained through the survey, it was determined that corrosion of the reinforcing steel was actively present within the structure. The areas of deterioration appeared to have suffered from the cycles of wetting and drying prevalent in natural draft cooling towers. In this kind of environment, small hairline cracks would have originally formed - leading to carbonation down the cracks, a loss of the protective film and the acceleration of corrosion by the ingress of chlorides.
The design-build project included over 75,000 square feet of veil, top-ring and column leg concrete repairs; 680,000 square feet of impressed current cathodic protection installation; design, fabrication and installation of circumferential cantilever access platforms; tower rigging and high-rise access; basin wall strengthening and waterproofing; and sacrificial cathodic protection of 160 each columns and 80 each pier caps. STRUCTURAL performed 100 percent of all concrete restoration and cathodic protection installation.
The cathodic protection system for this project was designed using three anode types:
- Catalyzed Titanium Ribbon installed within slots in the concrete surface interconnected with titanium conductor bar. Installed when veil thickness is less than 10 in. (Zones 17 to 80)
- Discrete mixed metal oxide (MMO) probe anodes, installed in small ½ in holes interconnected with conductive titanium wire. Installed when veil thickness is greater than 10 in. (Zones 1 to 16 and 81 to 88)
- Galvanic LifeJacket® system (Columns) comprised of a fiberglass shell with a zinc anode mesh installed within a pre-assembled shell custom made to fit the dimensions of the legs and caps.
In addition to developing an effective design for the ICCP system, an external current in excess of the existing corrosion current at the reinforcing steel was needed. This project utilized a state of the art system involving 32 distributed outstations (DO) mounted at the bottom and the top of each veil. Each DO controls 5 to 6 zones depending on location. All 32 DOs are connected to a common power supply and a Main Control Unit (MCU). The MCU was installed in a pre-manufactured air conditioned structure located between the towers and will be connected to a web based network via a fiber-optic cable which will allow the unit to be accessed and monitored from anywhere in the world.
Successful operation and completion of this project involved five project managers and project engineers, eight jobsite superintendents/foremen; coordination of a 100-plus field crew; training, safety, subcontractor management; quality control; inspections; rigging and high angle access plan; scheduling; budgeting; planning; monitoring progress; and extensive client communication and coordination.