Materials Performance

NOV 2018

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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Page 18 of 72

16 NOVEMBER 2018 W W W.MATERIALSPERFORMANCE.COM MATERIAL MATTERS Protecting Concrete Cooling Towers from Corrosion W hen exposed to harsh operating environments, reinforced concrete cooling towers can develop severe corro- sion of embedded steel reinforcement that leads to concrete delamination and spalling. Because of their size, geometry, and operational constraints, determining the condition of cooling towers can be challenging; however, careful assessment provides critical knowledge for effectively ascertaining their structural health so maintenance and repairs can be planned to maximize service life. At an electric power plant in the Mid- Atlantic region of the United States, two identically designed reinforced concrete hy perbolic shell natural draft cooling towers serve two generating units (Unit #3 and Unit #4) that run intermittently to meet peak energ y grid demands. Each tower is the main component of a closed- cycle water cooling system that draws brackish process water from the nearby river. The older of the two towers, which serves Unit #3, was constructed in 1974 and placed into service in 1975. The sec- ond tower, serving Unit #4, was con- structed in 1975 and placed into service in 1981. Both cooling towers are approxi- mately 400-ft (122-m) tall with horizontal cast-in-place construction joints located at every ±5-ft (1.5-m) height increment to form 70 identif iable " lifts." Each tower's shell is ty pically 8-in (203-mm) thick along its height, with the thickness incre- mentally increasing in the f ive lifts at the bottom to form a 36-in (914-mm) thick ring beam and in the three lifts at the top to form a 26-in (660-mm) thick cornice. The interior and exterior faces of the concrete shells are each reinforced with a mat of steel reinforcing bars running verti- cally and circumferentially. From the tow- er's throat upwards, the reinforcing bars are typically #5 bars (~16 mm in diameter) spaced at 12 to 16 in (305 to 406 mm) on center in each direction. The volume of reinforcing steel increases toward the tow- er's base, where #7 bars (~22 mm in diam- eter) are spaced vertically at 5 in (127 mm) on center and #8 bars (~25 mm in diame- Overall view of the cooling towers in 2010. ter) are spaced circumferentially at 6 in (152 mm). The nominal concrete cover over the reinforcement is 2 in (51 mm). The shell is supported by 32 precast reinforced concrete X-columns that are approxi- mately 60-ft (18-m) tall with a 20- by 33-in (508- by 838-mm) cross section containing 16 vertical #11 bars (~36 mm in diameter) and #4 ties (~12 mm in diameter). In the mid-1980s, concrete deteriora- tion was initially obser ved on the exte- rior face of the Unit #3 cooling tower shell. Over time, concrete deterioration progressed in the cooling towers for both Unit #3 and Unit #4. This included large areas of concrete delamination and spalling at the shell exteriors, areas of concrete delamination at the shell interi- ors, and crack ing and corrosion staining in the supporting X-columns. To limit the risk of deteriorated concrete falling from the shells, the ow ner employed a contractor on several occasions to remove loose delaminated concrete from the towers. Multiple condition assessments were performed in 2006 to characterize dete- rioration mechanisms and determine the structural integrit y of the t wo towers, as well as develop repair plans to meet the ow ner's ser v ice life extension require- ment of 25 years. The towers' assessments included visual inspections performed from grade, catwalks, tower ladders, the cornice, and the basin. Hands-on inspections also were performed from swing stages and aerial lifts. During these inspections, nondestructive testing (NDT) was per- formed, such as hammer sounding, cover readings, and half-cell potential readings. Inspection windows were made at areas of interest in the concrete, and core sam- ples were extracted for petrographic analysis and chloride content testing. The results of the evaluation and sub- sequent repair strateg y for the Unit #3 tower are presented in a CORROSION 2018 paper by Matthew B. Gries, Kevin A. Michols, and John S. Lawler with Wiss, Janney, Elstner Associates, Inc. (North- brook, Illinois, USA); David W. Whitmore with Vector Corrosion Technologies, Ltd. (Winnipeg, Manitoba, Canada); and Matt Miltenberger with Vector Corrosion Ser- vices, Inc. (Tampa, Florida, USA). According to the authors, the 2006 assessment found large areas of spalled

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