By Bruce A. Collins, Vice President Restruction Corporation Sedalia Colorado
Four separate concrete cooling towers at a coal fired electrical generation plant exhibited
reinforcing steel corrosion causing concrete deterioration. The presentation will cover investigation of the corrosion sources, determination of the corrosion control solutions, implementation of the repair construction, and review of the corrosion measurements taken to prove effectiveness of the solution.
As early as the 1960’s, corrosion of reinforcing steel in structural concrete was recognized as a threat to concrete structure durability. Steel embedded in newly cast concrete is protected by the high pH of the concrete. The passive film on the surface of the reinforcing is broken down over time by the infiltration of ionic materials, typically salts. Carbonation of the concrete, caused by diffusion of atmospheric carbon dioxide can also break down the reinforcing steel passivation, allowing the initiation of corrosion. Many variables effect the time it takes for corrosion to initiate, including the concrete cover depth, temperature, concrete moisture content, pH level of the concrete, presence of oxygen and others. Upon corrosion initiation, one region of reinforcing will become anodic and other regions will act as a cathode. The ensuing chemical reactions cause oxidation of the metal at the anode. The oxidation product occupies a larger volume than passivated steel. As the bar oxidizes, an expansive tensile force is applied to the cover concrete creating cracks and ultimately a concrete spall. At this point, typically, the owner of the facility becomes concerned about long term durability and repair project planning is launched.
Repair Project Planning
During June 2000, the engineering team of a 1660 MW coal-fired electrical generation station located in the southwest United States began planning of a concrete repair project. The team had been observing concrete spalling and corrosion of reinforcing steel on specific reinforced concrete assemblies integral to the 4 concrete mechanical draft cooling towers on property. The four towers were separated into Unit 1 and Unit 2, consisting of two towers each. The cooling towers are constructed primarily of pre-cast concrete elements, with some conventionally reinforced elements. In plan, the cooling tower is a 12-sided shape with an approximate 212-foot diameter. Each tower has 12 motorized fans designed to draft air from the lower level up through the tower. The moving air is heat exchanged with hot process water, ejecting steam out the top of concrete fan stack assemblies. (See figure 1). The fan stack assemblies are constructed of a lower conventionally reinforced concrete ring measuring 41 feet 2 inches in diameter by 4 feet tall. The upper assembly is constructed of pre-cast concrete panels bolted to the lower assembly. Corrosion of the reinforcing steel in the lower assembly was causing concrete spalling. The plant engineering team wanted to understand the source of deterioration and institute a repair plan for the problem.