POWER July 2011 - 48

COOLING TOWERS
Defeating Concrete Reinforcing
Steel Corrosion
Four concrete cooling towers at a coal-fired electrical generation plant exhibited
reinforcing steel corrosion that was causing concrete deterioration.
This case study follows the repairs to those towers-how the corrosion
control solution was selected, how repairs were made, and how follow-up
tests found the repairs to be effective three years later.
By Bruce A. Collins, Restruction Corp.
A
s early as the 1960s, 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. However, the passive film on
the surface of the reinforcing steel 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 affect the time it takes for
corrosion to begin, including the concrete
cover depth, temperature, concrete moisture
content, pH level of the concrete, the presence
of oxygen, and others. When corrosion
begins, 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 (flaking or crumbling).
At this point, typically, the facility's owner becomes
concerned about long-term durability,
and repair project planning is launched.
Repair Project Planning
During June 2000, the engineering team of a
1,660-MW coal-fired electrical generation station
located in the southwest U.S. began planning
a concrete repair project. The team had
been observing concrete spalling and corrosion
of reinforcing steel on specific reinforced
concrete assemblies integral to the plant's four
concrete mechanical draft cooling towers. Two
towers served Unit 1 and two served Unit 2.
The cooling towers are constructed primarily
of precast concrete elements, with some
conventionally reinforced elements. In the
plan, the cooling tower is a 12-sided shape
with an approximate 212-foot diameter.
48
1. Four of a kind. The configuration of one of four cooling towers repaired as part of the
project. Courtesy: Restruction Corp.
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 (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 precast 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.
Investigation Program and Results
Investigation of the corrosion source consisted
of the following tasks:
■ Visual assessment of all 48 lower fan stack
assemblies and field measurement of concrete
cover.
■ Review of original structural drawings and
details for the lower fan stack assembly.
■ Chloride content analysis at depths of 1,
4, and 7 inches through the depth of the
lower assembly concrete at six separate
locations. A total of 18 chloride samples
were tested.
■ Delamination mapping of the assemblies.
■ Petrographic analysis of two cores removed
from different fan stack assemblies.
www.powermag.com
2. Much spalling found. The upper
and lower fan stack assembly and the areas
requiring repair on one of the four towers is
shown. Courtesy: Restruction Corp.
Visual assessment of the lower assemblies
determined several items of concern, including
the high-temperature environment, minimum
concrete cover of ¾ inch, and water
seeping down the outer face of the assembly at
the joint between the upper precast and lower
cast-in-place concrete. Constant contact with
steam was causing elevated concrete temperatures
and high moisture content. Each of
these variables was contributing to the rebar
corrosion (Figure 2).
Review of the original structural drawings
detailed the location, size, and placement of
cement grout in the joint between the upper
precast and lower cast-in-place concrete.
Reinforcing details for the lower assemblies
POWER | July 2011
http://www.powermag.com

POWER July 2011

Table of Contents for the Digital Edition of POWER July 2011

Contents
POWER July 2011 - Cover1
POWER July 2011 - Cover2
POWER July 2011 - Contents
POWER July 2011 - 2
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