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 at a much lower operating cost than mechanical draft cooling towers. The hot water is cooled from the generator's steam condensers through a pipeline entering at ground level. Water is pumped upward to the tower interior, through flumes and a series of pipes and is eventually discharged in spray form. During the downward movement, the water is cooled by the air being drawn upward (by the natural draft) and undergoes evaporative cooling.
While this process is extremely effective, practically all surfaces in the basin are subject to "immersion" conditions. The columns and lintels are in a "splash zone" environment subject to intermittent wet and dry conditions. As a result, these towers are extremely susceptible to corrosion-induced deterioration. Construction of new hyperbolic natural draft cooling towers, however, represents a large capital investment so maintaining existing towers is crucial.
Such was the case for the Unit No. 2 Hyperbolic cooling tower located at St. John's River Power Plant in Jacksonville, Fla. 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 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 was delamination and spalling.
Inspection Method and Cause of Deterioration Because of the progressive nature of the corrosion-induced deterioration, understanding the root cause, the consequences and associated costs was essential. As such, a condition evaluation was conducted. A visual and hands-on inspection by trained professionals formed the investigation's focus. Given the logistical challenges of gaining access, the inspection addressed the lower 50-feet of the massive structure. The data gathered through visual inspection was augmented with:
Review of existing plans, specifications and records.
Measurement and documentation of geometry, deflections, displacements, cracks and other damage.
Extraction of samples and testing for chloride concentration at various depths.
Electrical Potential Mapping.
Continuity testing.
Pachometer testing.
The non-destructive testing, conducted in accordance with International Concrete Repair Institute (ICRI) and the American Concrete Institute (ACI) standards, showed the lintel beam and columns to be in poor condition as they exhibited heavy cracking and spalling. Chloride testing results, which exceeded the chloride threshold value of 2.2 lb/cu yd at all measured depths, indicated that active corrosion of the reinforcement was the cause of the deterioration.
Repair Recommendation
Repairing corrosion-induced deterioration typically involves removal of deteriorated concrete, undercutting around the reinforcing steel, cleaning and protecting the reinforcing steel, and re-establishing the original concrete section. However, the investigation team also recognized the importance of installing a protection method and recommended a cathodic protection system consisting of encapsulating zinc mesh anodes within a stay-in-place fiberglass form filled with cementitious grout. This system utilizes proven zinc anode technology to provide long-term solutions to corrosion issues for both steel and steel-reinforced concrete structures. The zinc mesh is attached to the existing reinforcing steel by welding a wire to the zinc mesh and then welding it onto the reinforcing steel of the structure. This process causes the zinc mesh on the fiberglass jacket to corrode before the reinforcing steel - giving the structure a more durable repair. Because the system is self-regulating, easy to install, maintenance-free and cost-effective, it was ideal for this application.
Structural Preservation Systems (SPS), one of the leading specialty concrete repair contractors in the country, was selected to install the system based on their proven track-record in assembling large, highly-trained, concrete repair crews capable of handling tight repair schedules, as well as experience with large concrete demolition and cathodic protection installation projects. The project started with pre-project planning activities involving a site visit by all of the team's leadership. The team gathered for more than a week to determine schedules, logistics, jacket-lifting system design, temporary formwork design, delegation of responsibilities and more. The next step involved a detailed submittal process. Satellite images of the site were utilized to identify the lay-down and staging areas, the location of temporary facilities and the flow of work - an important factor considering the tight working conditions with other contractors.
The scope of the repair project included installation of 120 lintel beam jackets and 240 column jackets for a total of 34,000 square feet of jacketing. Procedures included removing delaminated concrete with pneumatic chipping guns, profiling concrete surfaces to a minimum concrete surface profile number three 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.
Design Challenges Although the tower was originally built to withstand a 110 mph wind load, an evaluation of the tower stability was necessary to ensure that the tower, when subjected to the design lateral forces of a 110 mph wind load as well as 72 mph wind load, could meet the design criteria for the non-hurricane seasonal repair. The total weight of the tower and the static pressure on each column also was determined. Utilizing the collected data, the tower was recreated using a structural engineering computer modeling program called STAAD Pro 2004. This software includes model generation, static, dynamic, p-delta and non-linear analyses. First, the tower was modeled under its original design criteria of a 110 mph wind load. Next, the structure was modeled under its demolished state with a 72 mph wind load. Based on hand calculations and computer models, it was determined that concrete could be safely removed from all lintel sections and 40 of the 80 columns of the tower at one time. Additionally, the lintel and columns could be stripped of 3-inches of concrete on all faces.
Since the lintel jackets were an odd shape and size, SPS had to determine how to lift them into place and support them while grouting. Several methods were considered, and all but one were determined to be too cumbersome and time-consuming for the project timeline. The method chosen involved building a grillage in which the jacket would be placed prior to mounting, keeping it in place and fastened to the structure until the grout dried. For this method, structural steel brackets were first mounted to the interior and exterior of the tower and were utilized, along with steel rods, to suspend the grillage formwork and jacket. First, however, the grillage had to accommodate the locations for installing the permanent stainless steel fasteners that hold the jackets in place. Because the final placement of the fastener was important for aesthetic reasons, the grillage had to allow space for them to be installed at precise locations. Using an aerial lift, the jackets were attached to the lintel beam and 4,000 psi cementitious grout was pumped through ports on the back face of the jackets.
Jacket installation on the columns was challenging because of the compound angle of the columns. To address this concern, SPS designed and fabricated a lifting bracket that, once lifted off the ground, was at the correct angle to slide the jacket into place. For each column, the jacket was divided into six pieces - two pieces each for the bottom, middle and top. Placement started at the bottom with the two pieces resting on the foundation and subsequent pieces supported by the ones below. Each section of the jacket was lap-spliced and held in place with stainless steel fasteners. Brackets held up the jackets through the grout port-holes and the jackets were held open with straps attached to the lifting bracket. Once the jacket was around the column, the strap was released and the jacket closed around the column. Next, ratchet straps were wrapped around each section of jacket to increase hoop strength and to keep the jackets from warping during grouting. Grouting was performed through ports built into the jacket at 2.5-foot vertical intervals, alternately placed on either face of the column.
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. Since this type of activity was new to most of the crew, SPS 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.
"Outage" Schedule
Because the cooling tower had to be shut-down for this project, work was scheduled for a five week period during an outage. An unforeseen delay, however, occurred to accommodate a chemical cleaning of the internal tower "fill" or "packing" media. The result was an eight day loss in an already tight schedule. In response and with plant management's approval, SPS proceeded with a 24/7 work week to accommodate the client's aggressive deadline.
Adding to the scheduling challenges were three other contractors working on the tower during the same period. One was working inside the structure cleaning and replacing some of the fill, using Bobcats and other machinery below the SPS work area. Two other contractors were working on the veil of the tower -- one performing hydro-demolition and the other applying zinc mesh and shotcreting an overlay above the SPS work. Both had multiple 120-to-150-foot aerial lifts that required constant vigilance to avoid mishap.
With this incredibly busy work environment, coordination and communication were essential. Daily meetings with the owner and other contractors were invaluable. Coupled with the skill of the crew of 35 putting in more than 16,000 hours with no injuries, these challenges were successfully addressed and the client's aggressive schedule met.
Project Success
SPS' years of experience working in a shut-down environment clearly made a difference to the successful outcome of what turned out to be the largest cathodic protection system installation. The entire project team is very proud of meeting the tremendously challenging schedule despite many interruptions, complications, cold weather and constant changes. Completing this project successfully opens the door for the industry for applications of this technique on other cooling tower projects.