Hyperbolic cooling towers remove heat from power plant condensers using natural draft. Air rises through the center of the tower, and hot water is discharged as a spray in the interior. The evaporative cooling is very effective and costs relatively little to operate.
But there's a downside to the design. Nearly all surfaces in the tower's basin are subject to "immersion" conditions, and the columns and lintels are in a "splash zone" environment that alternates between wet and dry. The soaking and cycling make hyperbolic towers very susceptible to corrosive deterioration. That's hardly a good characteristic for a structure that must use as little material as possible to reduce its weight but still be strong enough to resist high winds. Loss of structural material due to corrosion is simply unacceptable.
The hyperbolic tower that cools Unit 2 of the St. John's River Power Plant in Jacksonville, Fla., is a case in point (Figure 1). Erected in 1987, the cast-in-place tower is 450 ft tall and 360 ft in diameter. Its columns and lintel beam were constructed with traditional formwork, but its veil (shell) was built using slip forms. The plant's owners—Jacksonville Electric Authority (JEA) and Florida Power & Light Co. (FPL)—first noticed severe corrosion of the reinforcing steel bars (rebar) of the veil, perimeter columns, and lintel beam. Visual inspections noted concrete cracking, spalling, rust staining, and delamination (Figure 2).
Chloride, from two sources, was identified as the main cause of deterioration. The tower's makeup, taken from the St. John's River, is brackish water that is high in chloride content. The tower's air supply only exacerbates the problem. Coming from the nearby Atlantic Ocean, the salty air increases the 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. The cracks that form in the concrete admit more chloride and oxygen, creating a vicious circle.
No time to lose
Because corrosion-induced deterioration is a progressive condition, it was essential for the plant's owners to get a handle on the causes, consequences, and scope of the problem as quickly as possible. The first step was to hire a specialist to perform a detailed visual and hands-on inspection of the lower 50 ft of the tower (limited access precluded going higher). The contractor did the following:
- Measured and documented structural geometry, deflections, displacements, cracks and other damage.
- Extracted samples at various depths and measured their chloride content.
- Mapped the electrical potential and continuity of the entire lower section of the tower.
- Used a pachometer to determine the location, depth, and size of the structure's reinforcing bars.
The nondestructive testing (which was conducted in accordance with concrete industry standards) confirmed that the lintel beam and columns were in poor condition, with heavy cracking and spalling. Separately, testing of the samples revealed chloride levels above 2.2 pounds per cubic yard at all measured depths. Together, the two tests verified that ongoing corrosion of rebar was the cause of the structural deterioration.
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