POWER July 2011 - 50

COOLING TOWERS
the primary defense against future corrosion.
Cutting off the moisture ingress would increase
the concrete resistance, lowering the
reinforcing steel corrosion rate. Reducing
the concrete temperature was not a realistic
expectation.
A coating system designed to lower concrete
permeability was selected. The coating
system selected started with a 100% solids
moisture-tolerant epoxy primer applied at the
rate of 100 square feet per gallon. The body
of the coating was a 100% solids polyamide
based, flake-filled epoxy (Figure 3).
Access, surface preparation using 20,000psi
water lances, and coating installation
would be accomplished during a 30-day
plant shutdown.
Access was built using prefabricated
wood joist and planks bearing on steel angle
assemblies attached to existing structure.
Partial removal of the grout joint between the
upper and lower assembly to facilitate waterproofing
was also completed during the shutdown
and would minimize water penetrating
through the outer concrete face. The grout
was removed to a depth of approximately 2
inches. Open cell backer rod was saturated
with urethane multi-grout, activated, and installed
in the joint. A polysulfide caulking
was installed over the top to provide a finished
appearance.
The coating system was installed in two
topcoats to minimize pin-holing. A moisture
vapor test was completed prior to installation
to minimize the chance of debonding. The
project was completed in two phases and entailed
applying 14,400 square feet of coating
inside 24 separate assemblies in each phase.
The repair program included other rebar
repassivation techniques that would be constructed
during plant operation. With the
vapor drive cut off, we believed the concrete
would " dry out " rather quickly. Our attention
was now focused on the outside face of the
assemblies.
The high concrete temperature would
continue to drive corrosion at a faster than
4. Repairs completed. The spall repairs
are completed at the base of the cooling tower.
Courtesy: Restruction Corp.
normal rate. To counter this effect, the repair
program included installation of passive cathodic
anodes inside the concrete repair areas.
The density of the reinforcing steel was
evaluated and anodes were specified for installation
at approximately 4 feet on center at
the partial depth repair perimeters. Standard
concrete repairs, per ICRI Guideline 310.12008,
formerly No. 03730 (the International
Concrete Repair Institute's Guide for Surface
Preparation for the Repair of Deteriorated
Concrete Resulting from Reinforcing
Steel Corrosion), were completed. Dry mix
shotcrete was measured for resistivity compatibility
with the anodes and utilized as the
repair material. Dry-mix shotcrete was chosen
for ease of application and low shrinkage
characteristics.
A larger than normal amount of shrinkage
cracking was anticipated, and occurred, due
to the difficult curing conditions and elevated
substrate temperature. A 40% solids silane
sealer was applied to the outside face of the
assembly upon curing to assist in maintaining
low concrete moisture content from the
high-humidity environment outside the assembly.
Spall repair totals were 5,900 square
feet, and those repairs were completed in two
phases (Figure 4).
Reducing the concrete moisture levels,
installing passive anodes, and completing
concrete repairs to the outside face of the assemblies
provided a complete rebar repassivation
project.
Corrosion Measurements
As the first repair construction phase of the
project was nearing completion in 2005, the
plant engineering team raised the question of
how to measure the repair program's success.
Quickly, a plan was implemented to measure
corrosion. Luckily, the second phase, Unit 2
project was still in the active corrosion mode.
Unit 1, consisting of 24 assemblies in the first
phase, was repaired and the reinforcing steel
repassivated.
5. Pre- and post-repair data. Data labeled " 2B " are pre-repair values. Data labeled " 1A "
and " 1B " are immediate post-repair values. No data signifies that readings weren't taken at that
location. Source: Restruction Corp.
300
250
200
150
100
50
2B04
2B04
-50
-100
-150
-200
-250
-300
Sample location
300
250
200
150
100
50
2B04
50
2B08 1A09 1A02 1A06 1A04 1A07 1B05 1B09
Sample location
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POWER | July 2011
2B08 1A09 1A02 1A06 1A04 1A07 1B05 1B09
Sample location
2B08 1A09 1A02 1A06 1A04 1A07 1B05 1B09
Concrete resistance (kOhms)
Corrosion potential (mV)
Corrosion rate (micrometers/yr)

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POWER July 2011

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