ABSTRACT

Reinforced concrete cooling towers exposed to harsh operating environments can develop severe corrosion of embedded steel reinforcing, concrete delamination and spalling. Condition assessment of cooling towers is challenging due to their size, geometry and operational constraints but careful investigation can provide critical knowledge to effectively characterize structural health, plan maintenance, and prioritize repairs to maximize service life. A case history is presented to illustrate how assessment data was collected, interpreted, and used to develop and implement repair strategies for two 400 foot (121 m) tall hyperbolic shell cooling towers exhibiting widespread corrosion-induced deterioration. Evaluation of concrete properties, corrosion rate and desired service-life objectives were considered in the decision to utilize impressed current cathodic protection (ICCP) and galvanic cathodic protection (GCP) as part of the repair program. Review of the design and installation of the ICCP and GCP systems highlight the unique challenges associated with repair and protection of reinforced concrete cooling towers.

INTRODUCTION

At an electric power plant in the Mid-Atlantic region of the United States, two identically designed reinforced concrete hyperbolic shell natural draft cooling towers serve two generating units. Each tower is the main component of a closed-cycle water cooling system, in which the process water is sourced from the nearby brackish river.

The older of the two towers, serving Unit #3, was constructed in 1974 and placed into service in 1975; the second tower, serving Unit #4, was constructed in 1975 and placed into service in 1981. Concrete deterioration was initially observed on the exterior face of the Unit #3 tower shell in the mid-1980s. Over time, concrete deterioration progressed in both the Unit #3 and Unit #4 towers, including large areas of concrete delamination and spalling at the shell exteriors, areas of concrete delamination at the shell interiors, and cracking and corrosion staining in the supporting X-columns.

Beginning in 2006, the authors of this paper have performed multiple condition assessments of the two towers with the objectives of characterizing their deterioration mechanisms, assessing their structural integrity, and developing repairs to meet owner service life requirements. Of particular interest, and the focus of this paper, is the rapid increase in observed concrete deterioration at the Unit #3 shell due to chloride-induced corrosion since 2006 and continuing during repairs executed starting in 2014. Concurrent inspections of the relatively younger Unit #4 tower further solidified the understanding of observed deterioration at the Unit #3 tower and support the repair approach selection, which included impressed current cathodic protection.

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