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CONDITION ASSESSMENT AND CATHODIC PROTECTION OF REINFORCED
CONCRETE COOLING TOWER
Zia Chaudhary and Fahad M. Al-Mutlaq
SABIC Technology Center Jubail
P.O. Box 11669, Al-Jubail Industrial City 31961
Saudi Basic Industries Corporation (SABIC)
Kingdom of Saudi Arabia
E-mail: [email protected],
&Ali A. Al-Beed
Saudi Petrochemical Company (SADAF)
P.O.Box 10025, Al-Jubail Industrial City 31961
Kingdom of Saudi Arabia
ABSTRACT
Investigations were conducted to assess condition and determine root cause of the ongoing concrete
deterioration of the cooling tower. The beams, columns, wall panels of end walls, roof slab, bund wall,and louvers, were visually exhibiting severe concrete deterioration in many areas across the entire
structure. In some areas, the concrete deterioration was very advanced and posing serious threat to
integrity of the structure. Chloride had penetrated to full depth of the concrete cover in concentrations
significantly higher than threshold level. Electrochemical measurements showed that the reinforcing
steel was actively corroding under the sound concrete in >50% areas of the entire structure.
The visual condition of the exposed steel and the survey results concluded that the deterioration of
concrete resulted due to chloride-induced corrosion of the reinforcing steel. There was no risk of
carbonation-induced corrosion of steel and sulfate attack on concrete. Patch repair and cathodic
protection (CP) repair method was recommended to arrest the ongoing corrosion of the steelreinforcement. The CP system design, installation, and initial commissioning and monitoring results are
also described and discussed.
Key Words: concrete deterioration, delamination, chloride, sulfate, half-cell potential measurements,
probability of corrosion, patch repairs, cathodic protection, titanium mesh anode.
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Paper No.
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INTRODUCTION
The cooling tower provides potable water supply (cooling system) for plants in a petrochemical
company located in Jubail industrial city, Saudi Arabia. The cooling tower structure comprises pre-cast
concrete units mounted on a reinforced structural frame. The cooling tower structure is 27.45 m long, 24
m wide and 18.22 m in height. The cooling tower was built in 1981.
Major parts of the structure are as follows:
End Walls (north and south elevation), are comprised of reinforced concrete in-situ castcolumns and pre-cast beams and wall panels.
Roof Slab, it contains pre-cast slab panels, fan plinths, and parapet walls. Bund wall, built all around the tower to contain the water (2.97 m high) Reinforced concrete Louvers units, (east and west elevation)
A schematic illustration of the cooling tower with major components identified is given in Figure 1 and
a view of south elevation is shown in Figure 2.
The cooling tower was commissioned in 1981 and was showing signs of concrete distress in the form
of cracking and spalling of concrete for the last few years, believed to be caused by corrosion of the
reinforcement. In some areas, the extent of deterioration was very severe and posing a safety hazard to
personnel and plant below. A condition survey was conducted to determine the cause and extent of
deterioration and recommend appropriate repair methods for the rehabilitation of the structure. This
paper describes and discusses the condition survey results, available repair options, and design,
installation, and monitoring of the recommended patch repair and cathodic protection (CP) repair
method.
CONDITION SURVEY
Standard condition survey techniques were employed throughout this investigation, which includes
the following:
Visual Inspection & Hammer tapping survey of concrete surface Chemical Analysis for chloride and / or Sulfate content determination Cement content & compressive strength analysis Depth of carbonation & reinforcement Half-cell potential & Corrosion rate measurements
Visual Inspection
South & North End Walls (External):-The extent of concrete deterioration on south elevation was
relatively more significant than on other parts of the structure. The cracking, delamination, and spalling
of concrete was visible and noted at many locations. At some locations, the condition of concrete
elements was posing safety hazard threat to personnel working in that area. Several water leaks were
also visible.
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Large and wide cracks were visible on many reinforced concrete beams. At some locations,
propagation of cracks had already converted into spalling of concrete and exposing the steel bars, which
were severely corroded, hence confirming the cause of concrete cracking & spalling. Whitish salt
deposits were found on beams. Several columns were also exhibiting signs of cracking of concrete. At
some locations, the cracks were very long and extremely wide as shown in photo 3. The crack pattern
appears to be in line with reinforcing steel. Water leaks were noted on column / beam joints at many
locations, particularly on the central row of columns. Rust staining and longitudinal as well as horizontal
cracks in concrete were found in some areas of columns close by the leakage points. Scale deposits werenoted at many locations.
Wall panels were showing by far the highest degree of concrete deterioration. Large sections ofconcrete repairs were also visible on many panels. Extensive cracking and delaminated concrete was
visible on about 12 panels out of the total of 20.
On the north elevation, visible signs of concrete deterioration were relatively less and few in
numbers. However, where visible the extent of deterioration was quite advanced and had already led to
spalling of concrete. Beams and columns on top east side of the elevation were exhibiting extensive
cracking and at some points spalling of concrete. Water leakage was also underway at many locations. It
was occurring either at the beam/wall panel joint or beam/column joint. The exposed steel bars were
severely corroded hence confirming that deterioration of concrete is associated with corrosion of the
reinforcing steel.
East & West End Walls (External):- The top beams on both east and west elevations were showing
signs of concrete distress. At some locations, the cracking was highly advanced and had already resulted
in concrete spalling. The exposed rebars were severely corroded. Cracking and delamination was also
visible on both columns and wall panels, but relatively at less no. of locations.
Louvers:- Several louvers from both east and west elevations are showing advanced concretedeterioration. At many locations, the cracks were very long and extremely wide (>5 mm). The
delaminated concrete was also visible. At some points, spalling of concrete had already occurred and
rebars were exposed. The exposed rebars were extensively corroded and had significant section loss.
End Walls Internal :- Internally, the visual condition of columns, beams, and wall panels was
generally good except the top and upper wall portion, which was subject to frequent wet and dry cycles.
Cracks were visible on beams and wall panels located in the uppermost chamber of the tower, just under
the fans. The beams were exhibiting advanced stage of concrete cracking. Long and very wide
longitudinal cracks were visible on many beams, which were in line with the reinforcing steel andappear to resulting due to corrosion of steel. The jointing mortar between the beams and top roof slab
was also broken out.
Roof Slab (Top & Soffit):- The roof slab top and fan plinth foundations were generally in good
condition. The edges of the roof slab panels were repaired throughout the entire roof slab top, whichwere also visible on the soffit side as well. Some cracks and delaminated concrete sections were visible
on parapet walls. Old repairs were also noted at several locations. The slab panel soffit was severely
cracked and delaminated around the opening points and also in areas close by these openings. Exposed
bars were severely corroded. The jointing mortar between the slab panels and between the slab and
beams was either eroded or broken out at several locations. Most of the beams, which are supporting theroof slab panels, were severely cracked. The cracks are very long and generally wider than 2 mm. The
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exposed steel showed that cracks were in line with the reinforcing steel. The exposed steel was severely
corroded hence confirming that cracking has resulted due to corrosion of the reinforcing steel.
Bund Wall:- The coating on external side of the bund wall was severely deteriorated and peeled off
throughout the wall length. However, the concrete underneath was visually in good condition except the
top of the wall, which was cracked at many locations.
In summarizing the visual inspection records, the reinforced concrete elements of the cooling tower
are exhibiting advanced stage of concrete deterioration on the external side of the structure. At manylocations, it has already resulted in spalling of concrete and posing safety hazard to personnel at number
of other locations. The exposed rebars are severely corroded and confirm that deterioration of concrete
is associated with corrosion of the reinforcing steel.
Chloride Contents
Concrete powder samples were taken from each concrete element for chloride content analysis. The
acid soluble chloride content of concrete powder samples was determined in the laboratory usingconventional titration method by BS 1881: Part 124. The results are given in Table 1 below.
South & North Elevations (Beams, Columns, & Wall Panels):-The average chloride content andprofile is shown in Figure 6 below. The chloride contents showed a decreasing profile with depth, which
is indicative of ingress from an external source. The results show that chloride had penetrated into each
concrete element beyond the external steel reinforcement depth and its chloride concentration at the
external rebar level was well in excess of the threshold limit (0.03%) for chloride-induced corrosion in
OPC concrete1-3
.
Bund Wall:- The chloride content profile is shown in Figure 6 below. The results show that chloride
content at the external steel depth is 0.21%, which is significantly (7 times) higher than the threshold
limit (0.03%). The profile also indicates that the chloride penetration was mainly from the external side
of the bund wall, however, some penetration from the internal side could also be possible and the
chloride content could also be higher at the internal steel depth.
Roof Slab:- The chloride profile is shown in Figure 7 below. The chloride content at the top and
bottom reinforcing steel depths (0.17% and 0.06% respectively) are higher than threshold level. The
profile shows that chloride penetration was mainly from the external top of the slab, however, it appears
that chloride penetration might also be from the soffit side as well though at relatively much less rate
and in quantity.
Roof Slab Beam:-The chloride profile is shown in Figure 7 below. The results show that chloride
penetration had been from both sides of the beam and their concentration at the steel depth on both sides
is significantly higher than threshold level.
Louvers:- The average chloride content profile is shown in Figure 7 below. The results show that
chloride content at the external steel depth is 0.25%, which is significantly (8 times) higher than the
threshold limit (0.03%). The profile also indicates that the chloride content was higher on the external
side of the both louvers.
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Sulfate Content
The acid soluble sulfate content of concrete powder samples was determined in the laboratory using
conventional titration method by BS 1881: Part 124. The results are given in Table 3 below. Most of the
results from all different elements showed that sulfate content was generally less than the threshold limit
of 0.6% sulfate by weight of concrete1. Only one result value from south elevation column (2% at 0-20
mm depth) was significantly higher than the threshold limit and some 6 results from 5 different locations
were slightly higher than the threshold limit and ranged between 0.65% and 0.77%. Each profile has
shown that sulfate penetration into the concrete was from the external side of each element.
Cement content & Compressive Strength
The cement content and compressive strength of concrete was determined from three cores CM1,
CM3, and CM4, extracted from south elevation beam, column and roof slab respectively. The testing
was carried out using CaO method in accordance with BS 1881: Part 124. The cement content in each
core was >16.4% and the compressive strength was >35 N/mm2. These results suggest that both the
cement content and the compressive strength of the concrete were sufficient.
Carbonation
The carbonation depth was determined using phenolphthalein solution spray on three cores CM1,
CM3, and CM4, extracted from south elevation beam, column and roof slab respectively. No colorless
zone was found in all three samples, which suggests there was no carbonated concrete at all.
Half cell potentials
The free corrosion potential of the reinforcing steel was measured in selected areas on the concrete
surfaces of two beams and wall panels, one column and roof slab using hand held Ag/AgCl reference
electrode. The half-cell survey results are summarized in Table 4 below. The interpretation of half-cell
survey results was carried using the Van Daveer criteria and in accordance with ASTM C876-91standard.
The results indicated 90% risk of corrosion of the reinforcing steel in 19%, 50% and 100% areas of
the north & south elevations and top of the roof slab respectively. Whereas, 50% risk of corrosion was
also evident in 61% and 50% areas of north and south elevations. This implies that risk of corrosion ofthe reinforcing steel was very high throughout the entire cooling tower structure and widespreadcorrosion activity is occurring underneath the sound concrete areas though this has not yet transpired as
visible damage. .
Corrosion Rate
The corrosion rate of the reinforcing steel was measured in selected areas using Gecor 6 linear
polarization device. The interpretation of the results was carried out in accordance with the following
criteria4:
Low to moderate corrosion: Icorr 0.1 to 0.5 uA/cm2Moderate to high corrosion Icorr 0.5 to 1.0 uA/cm
2
High corrosion rate Icorr >1.0 uA/cm2
Using the above criteria, the summary of corrosion rate results is given in Table 5 below. The results
showed that >50% of the reinforcing steel in south elevation concrete elements and in roof slab was
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actively corroding at moderate to high corrosion rates. At north elevation, the corrosion rate in most of
the areas (~66%) is in low range.
DIAGNOSIS
Cause of Concrete Deterioration
There are five classes of concrete deterioration that are recognized by ACI Committee 2014
. In theArabian Gulf environment, most commonly reported
5causes of concrete deterioration are; a) Corrosion
of the steel reinforcement and b) sulfate attack. Corrosion of steel in concrete occurs due to chlorideattack and/or carbonation of concrete6. Carbonation of concrete is not very common in the Gulf
environment.
It is evident from the visual condition of the exposed steel bars (resulted due cracking and spalling of
concrete at many locations throughout the structure) that deterioration of concrete is associated with
corrosion of the reinforcing steel. Corrosion of steel in concrete occurs due to either chloride attack or
carbonation of concrete.
The chemical analysis of concrete samples has shown that the carbonation depth of concrete wasnegligible, and the sulfate content was mostly lower than threshold level of 0.6% by weight of concrete.
Extensive chloride penetration was, however, found in all test samples from all different concrete
elements. This confirms that the cause of concrete deterioration is chloride-induced corrosion of thereinforcing steel.
The chlorides profile in all different elements shows that chloride penetrated into the concrete
externally. Since the deterioration has occurred in areas that are well above the grade level the source of
chloride ions appear to be the cooling tower water, i.e. being sprayed onto the concrete from top of thecooling tower. The chloride concentration in the cooling tower water varies between 300 and 500ppm.
The continuous water spray and subsequent evaporation due to high temperatures can result in gradual
build-up of chloride concentration in the concrete.
Extent of Damage
It is evident from the chloride profiles that chloride penetration was very deep and its concentration
at all depths was significantly higher than the threshold limit. This shows that not only external steel
layer but also the internal steel layer would also be subject to chloride attack. According to the half-cell
potential data, the risk of corrosion appears to be greater than 95% in about 50% area of the entire
structure, whereas in remaining areas, likelihood of corrosion is between 5% and 50%. The corrosion
rate data has confirmed that steel is actively corroding at moderate to high corrosion rates in about 50%
areas of the structure. This implies that the extent of concrete deterioration is well spread and deep
across the whole structure and therefore there is an urgent need for remedial works to arrest this ongoingconcrete deterioration.
Based on the above-mentioned observations, it was diagnosed that deterioration of the concrete has
resulted due to chloride-induced corrosion of the reinforcing steel. The reinforcing steel is also activelycorroding in several other areas of the structure where concrete apparently is in good condition.
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SELECTION OF APPROPRIATE REPAIR METHOD
The pros and cons and feasibility of the following three options were compared in selecting the most
appropriate and durable repair method for the cooling tower structure.
Option 1 Local Patch Repairs
Option 2 Re-Skinning or Traditional Repairs
Option 3 Patch Repairs and Cathodic Protection
The option 1 was very economical, as this method would involve breakout and removal of only
delaminated concrete and reinstatement. However, past experience has shown that in chloride-
contaminated structures this method provides only a temporary or short-term solution since it deals withonly the damaged areas and not the cause6. It could result in enhancing the corrosion activity and/or
developing incipient anodes in the adjacent un-repaired areas. By comparison, the re-skinning of the
structure (option 2) would provide durable and well-extended service life, as it involves removing all
delaminated and chloride-contaminated concrete. But it would require extensive concrete breakout (20
mm beyond the main external reinforcement), which was not desirable considering the operations
constraints and the time required. Due to extensive concrete breakout/removal and also concrete coating
for durable repairs, this method was not likely to be an economical option when compared with option 3,
which would involve patch repairs of the delaminated areas and installation of cathodic protection (CP)
system to the concrete surfaces. Since, only delaminated concrete would be broken out and removed,
risk of operational constraints would also be minimum or negligible. The ongoing chloride-induced
corrosion of the reinforcing steel would be arrested or controlled when the steel is sufficiently
cathodically polarized. Hence, it was concluded that option 3 method, would provide a durable and long-term solution for rehabilitation of the cooling tower structure.
Therefore, for overall repairs of the structure, patch repair and impressed current cathodic protectionrepair method was recommended. The major and minor defects of different elements and recommended
repairs are summarized in Tables 6-7 below.
CATHODIC PROTECTION SYSTEM
Design
Based on literature guidance8-10and exposed condition of the reinforcing steel in different elements
of the structure, a design current density of 20 mA/m2of steel surface area was used in calculating the
current requirement for the protected steel in the cooling tower structure. The selected anode system
comprised expanded titanium mesh anode with cementitious overlay. This selection was based on
extensive and good track record of mesh overlay anode system in Middle East region. The overlaythickness was limited to 25-30 mm in order to keep the dead load onto the structure within acceptable
levels.
For effective performance, assessment, and control, the whole protected area of the structure was
split into 16 CP anode zones, which both current and voltage outputs can be controlled independently.Multiple anode feeder and current return (steel) connections were allowed for each anode zone to
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acquire good and uniform current distribution and also ensuring 100% redundancy. The precise number
and positioning of these connections was based on design calculations that ensured minimum and
acceptable level of voltage drops across the anode current distributors and mesh. About 2 to 5
embeddable reference electrodes (Ag/AgCl / 0.5M KCl) were allowed for different anode zones to
monitor the system performance. More design details are given in Table 8 below.
Installation
All concrete surfaces of all different elements of the structure that were to be protected, were
hammer tapped in order to identify the delaminated areas. Subsequently, all delaminated concrete was
removed. In some areas, particularly upper elements of the south elevation (extending down to 3-5
meters from top of the structure, extent of damage was very significant and deep. The reinforcing steel
was severely corroded with a section loss of >10-20%. Therefore, a lot of steel replacement was made in
such areas.
As the structure was made up of pre-cast elements, a comprehensive electrical continuity testing was
conducted. All exposed steel was utilized for this testing and additional concrete breakouts were also
made where needed. The reinforcing steel bar was considered electrically discontinuous when any
individual resistance reading; was greater than 1 ohm or it changed more than 1 ohm in 15 seconds orwhen the instrument leads were reversed. In general, the reinforcing steel in all different elements, i.e.
wall panels, columns and beams, was not electrically continuous. Similarly, the steel in top slab panelsand parapet walls was also electrically discontinuous. The testing also identified about 150-200
discontinuous steel rods in the bund wall that were probably used to hold the scaffolding platform
during the construction. The electrical continuity of all different elements within each anode zone was
established using welded rebar links as shown in Figure 9.
Double junction Ag/AgCl /0.5 M KCl reference electrodes (REs) were installed preferably at sites
where risk of corrosion was high or steel was already actively corroding. In general, REs were located ina manner that whole area within each zone would be appropriately represented. Some REs were also
embedded close to the rear layer of the steel reinforcement. All steel connections were made and then allexposed areas were repaired using cementitious material to the original concrete profile. All cables were
secured and repaired areas were cured using wet hessian for a minimum period of 7 days. After curing,
pull-off tests were conducted using 50 mm diameter dollies to determine bond strength between the
repair material and the parent concrete. The pull-off tests were carried out at a rate of 1 set (3 no. per
set) for the first 50 m2 and then 1 set per 100 m
2. The repair approval criteria was that the point of
fracture should occur within the substrate or if it occurred at the repair / substrate interface it should
exceed the mean value of 1.5 N/mm2with no individual value below 1 N/mm
2.
The reinstatement of expansion and mortared joints was also completed at this stage. Two part poly-
sulfide flexible sealant was applied to both type of joints using gun. Following this concrete surfaces
were cleaned and abraded by mechanical scrabbling (using hatching tool) to acquire roughened surfacefor good bond between the substrate and CP overlay. Prior to overlay placement, electrical continuity
testing was conducted to ensure all components of the anode system are electrically continuous within
each zone and also there are no short circuits between the anode and the steel. All connection cables
were appropriately tied to the mesh anode and carried to terminate in the nearest junction box. The
overlay was installed in small sections to keep the shrinkage cracking under control and minimum.
Special attention was given to the joint locations between the columns and walls and beams. The
overlay was cured using wet hessian for a minimum period of 14 days. After curing pull-off tests were
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carried out (as described above) to determine bond strength between the substrate and overlay.
Commissioning & Monitoring
Four transformer rectifiers (T/Rs) enclosures were installed, each containing multiple independentoutputs (T/Rs) and placed at four different locations around the structure. All dc output and monitoring
cables were run between the associated T/Rs and junction boxes. Prior to energizing of the CP system,
steel natural potentials were established at the location of all embedded reference electrodes and pre-commissioning checks were conducted to verify the circuit wiring and steel polarization in negative
direction. The CP system was powered after the completion of 28 day curing of the overlay. All zones
were initially energized at a steel current density of 5mA/m2and then current was gradually increased to
achieve a current-on potential shift of 100-200 mV. After 7 days of system operation, instant-off steel
potentials were measured and the applied current was increased in each zone to a level of 15 mA/m2of
steel surface area.
The natural and instant off steel potentials, and potential decay measurement results are summarized
in Table 9 below. In general, the polarization growth in the negative direction increased steadily with
increase in applied current and time and was noted quite uniform in all areas within each anode zone
and also overall in all zones.
After 9 months operation of the CP system, the monitoring results have shown that 100 mV decay
criterion was met at all the 50 monitoring locations except at two, i.e. RE-8-4 and RE-14-2. These two
reference electrodes appear to have gone faulty, as the potential readings were not stable. This implies
that the CP system is affording the required protection and reinforcing steel is adequately protected.
Based on these results it can be inferred that the CP system has been successfully commissioned and
operating satisfactorily, and meeting its design objectives in controlling the chloride-induced corrosion
of the reinforcing steel of cooling tower structure.
CONCLUSIONS
1. The deterioration of the cooling tower concrete occurred due to chloride-induced corrosion of thesteel reinforcement. The chlorides are present in the concrete cover in excess of the threshold limit
of 0.03% by wt. of concrete, which poses a very high risk of corrosion of the reinforcing steel. The
chloride penetration was from the external side and resulted due to continuous cooling tower water
spray.
2. There was no risk of carbonation-induced corrosion of steel and sulfate attack on concrete.3. The electrochemical measurements showed that the reinforcing steel was actively corroding under
the sound concrete in >50% areas of the entire structure. Therefore, repair works should not be
limited to the damaged areas only.
4. Patch repairs and CP repair method offer durable, long-term & economical solution with no ormuch less installation constraints for rehabilitation of the structure. Therefore, this method wasrecommended.
5. The CP system has been successfully installed and commissioned. The 9 month system operation& monitoring results have shown that the 100 mV day criterion was met at 48 monitoring locationsout of the total of 50. This shows that CP system is affording required protection to all protected
areas and meeting its design objective in controlling the ongoing corrosion of the reinforcing steel.
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ACKNOWLEDGEMENT
The author would like to thank the management of SABIC Technology Center and SABIC R & T forthe encouragement and approval for the preparation and presentation of this paper.
REFERENCES
1. M.S., Eglinton, Concrete and its Chemical Behaviour Pub. Thomas Telford Ltd.1987
2. ACI 224, Causes, Evaluation, and Repair of Cracks in Concrete American ConcreteInstitute, Detroit, USA, 1987.
3. BS 8110: Part 1, The Structural Use of Concrete British Standards Institution, 1985.4. ACI 201, Guide to Durable Concrete American Concrete Institute, Detroit, USA,
1987.
5. O.S.B. Al-Moudi, Durability of Reinforced Concrete in Aggressive SabkhaEnvironments ACI Materials Journal, May-June 1995.
6. J.P. Broomfield, Corrosion of Steel in Concrete Pub. E & FN Spon, 1997.7. Rasheeduzzafar, S.E. Hussain, and S.S. Al-Saadoun, Effect of tricalcium aluminate
content of cement on chloride binding and corrosion of reinforcing steel in concrete
ACI Materials Journal, January-February, 1992.
8. Concrete Society/CEA Technical Reports No. 36. Corrosion Engineering Association,1989. U.K.
9. NACE Standard RP0390-90, Item No. 53072, Cathodic Protection of ReinforcedSteel in Atmospherically Exposed Concrete Structures.
10. European Standard BS EN 12696, Cathodic Protection of Steel in Concrete Atmospherically exposed concrete 2000.
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TABLE 1:- Chloride content analysis results.
Dust sample increment depths (mm)
170-220120-17080-12060-8040-6020-400-20
Chloride concentration % by Wt. of concrete sample
Element/Face
0.060.070.060.060.060.070.09Beam/South Elev.
0.060.060.070.1Beam/Nort Elev.
0.070.110.240.69Column/South Elev.
0.070.060.070.09Column/North Elev
0.080.120.21Wall/South Elev.
0.160.160.21Wall/South Elev.
0.070.090.10Wall/North Elev.
0.130.120.150.180.210.230.28Bund Wall/East Elev.
100-15080-10060-8040-6020-400-20
0.060.050.070.110.170.18Roof Slab
0.130.110.090.120.150.18Roof Slab Beam
0.190.210.25Louver/East Elev.
0.260.290.38Louver/East Elev.
TABLE 2:-Sulphate content analysis results.
Dust Sample increment depths (mm)
170-220120-17080-12060-8040-6020-400-20
Sulfate content % by wt. of concrete sample
Element/Face
0.470.410.380.400.430.450.47Beam/South Elev.
0.360.390.500.56Beam/Nort Elev.
0.440.530.772.00Column/South Elev.
0.400.390.400.44Column/North Elev
0.540.530.58Wall/South Elev.
0.420.470.58Wall/South Elev.
0.330.360.49Wall/North Elev.
0.360.320.340.370.470.550.64Bund Wall/East Elev.
100-15080-10060-8040-6020-400-20
0.410.380.320.430.510.68Roof Slab Beam
0.660.550.400.490.600.73Roof Slab
0.510.530.57Louver/East Elev.
0.450.550.65Louver/East Elev.
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Table 3:- Cement content and compressive strength test results.
CM4CM3CM1CORE IDENTIFICATION
16.416.816.6Cement Content (%)
38.547.246.6Corrected Compressive Strength N/mm
TABLE 4: Summary of half cell potentials
North Elevation South Elevation Roof Slab
Half-cell
potentials
(mV)
-351 -351 -351
%of values within each potential range
Beam 0 60 40 0 30 70 - - -
Column 0 0 100 - - -
Wall 36 63 1 0 65 35 - - -
Roof Slab - - - - - - 0 0 100
Total* 20 61 19 0 50 50 0 0 100
* Totals are based on all data from each elevation
TABLE 5:- Summary of corrosion rate results.
North Elevetion South Elevetion Roof Slab
Corrosion
RateLow Moderate High Low Moderate High Low Moderate High
% of values within each corrosion rate range
Beam 100 0 0 0 33 67
Column 100 0 0
Wall 34 33 33 17 17 66
Roof Slab 33 67
Total* 66 17 17 27 18 55 33 67
* Totals are based on all data from each elevation.
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TABLE 6:- Repair methods for End wall concrete elements.
Defect Concrete repairsSupplementary
protection
A.
External South & North Elevations and Top Suspended Walls on East & WestElevations (Beams, Columns & Wall panels).
B. Internal South & North Elevations, (Only upper portion of beams, columns, &wall panels) and Internal West & East Elevations.
Spalls and delaminated
concrete.
Cracks >0.3mm.
Leaks.
Old repairs.
Weathering
Cavities, and Cracked and
broken mortar sealant in
joints.
Electrical discontinuity ofreinforcing steel in
different concreteelements.
Break out only delaminated
concrete and patch repair (repair
material to be suitable for CP
repairs). Evaluate loss of steelsection, replace as required.
Local breakout and inspect steel,inject or reinstate mortar.
Replace mortar fillets externally
and sealant internally.
Breakout to remove existing
repair material and reinstate
using material suitable for CP.
Apply a coating to reduce rate of
weathering.
Breakout old mortar at corner
joints and reinstate mortar.
Test and establish electrical
continuity of reinforcing steel in
all different concrete elements.
Impressed current CPsystem (ICCP),
comprising mixed metal
oxide (MMO) coatedexpanded titanium mesh
and cementitious overlay
anode system.
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TABLE 7: Repair methods for roof slab, louvers, and bund wall.
Defect Concrete repairsSupplementary
protection
Roof Slab (Top & Soffit), Parapet walls & Bund wall.
Spalls and delaminated
concrete, Cracks >0.3mm,Old repairs.
Cavities, and Cracked andbroken mortar sealant in
joints.
Cracked and peeled off
coating
Electrical discontinuity of
reinforcing steel indifferent concrete
elements.
As recommended in table 6
above.
Breakout old mortar at corner
joints and reinstate mortar.
Remove coating from all
concrete surfaces.
Test and establish electrical
continuity of reinforcing steel in
all different concrete elements
Impressed current CP
system (ICCP),
comprising mixed metaloxide (MMO) coated
expanded titanium mesh
and cementitious overlayanode system.
Louvers units of East & West Elevations.
Spalls and delaminated
concrete, Cracks >0.3mm,Old repairs.
Weathering
Replace all damaged units with
new units. New units shall have
epoxy-coated steel.
Apply a coating to reduce rate of
weathering..
All new units shall have
polyurethane coating.
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TABLE 8:- Summary of CP system design details
Design
Current
Anode
CapacityCooling Tower Area
Anode
Zone
RE
(Nos.)Amps
TR Output
Ratings
Bund Wall North 1 2 2.04 2.47 3A, & 10V
Bund Wall East 2 2 3.54 5.10 5A, & 10V
Bund Wall South 3 2 3.40 5.10 5A, & 10V
Bund Wall West 4 2 0.88 1.26 2A, & 10V
Bund Roof Slab
South5 2 5.15 6.56 7A, & 10V
Bund Roof Slab
North6 2 1.87 2.26 3A, & 10V
South End Wall,
Lower7 4 2.41 2.91 3A, & 10V
South End Wall,
Upper8 4 2.41 2.91 3A, & 10V
North End Wall,
Lower9 4 2.04 2.47 3A, & 10V
North End Wall,
Upper10 4 3.54 5.10 5A, & 10V
West End Wall,
Upper11 2 3.40 5.10 5A, & 10V
East End Wall, Upper 12 2 5.15 6.56 7A, & 10V
Upper- Inner Beams,Columns & Walls-
North
13 5 5.10 6.27 7A, & 10V
Upper- Inner Beams,Columns & Walls-
South
14 5 1.87 2.26 3A, & 10V
Roof Slab West 15 4 0.87 1.26 2A, & 10V
Roof Slab East 16 4 5.10 6.27 7A, & 10V
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TABLE 9:- Summary of CP system monitoring results.
After
3M
After
6M
After
9M
After
3M
After
6M
After
9MNatural
Instant-off 24 Hour decay
TR
Encl.
#
Anode
ZoneRE No.
Steel Potential mV Ag/AgCl mVRE 1-1 -326 -578 -655 -496 232 298 178
3 1RE 1-2 -259 -562 -617 -562 260 302 218
RE 2-1 -318 -474 -286 -452 161 103 129
4 2 RE 2-2 -364 -572 -574 -587 160 175 130
RE 3-1 -351 -619 -543 -613 226 159 2111 3
RE 3-2 -474 -734 -746 -790 183 217 206
RE 4-1 -343 -567 -572 -524 188 187 1464
RE 4-2 -307 -588 -609 -503 266 316 188
RE 5-1 -147 -474 -539 -401 356 430 2635
RE 5-2 -252 -289 -212 -290 120 91 109
RE 6-1 -105 -407 -416 -328 335 351 236
2
6RE 6-2 -162 -233 -164 -406 186 97 296
RE 7-1 -230 -396 -405 -372 216 243 220
RE 7-2 -233 -513 -563 -516 184 204 155
RE 7-3 -192 -485 -437 -441 352 299 3157
RE 7-4 -187 -637 -692 -643 376 419 335
RE 8-1 -248 -447 -429 -435 229 193 221RE 8-2 -188 -346 -322 -410 180 145 178
RE 8-3 -292 -424 -593 -379 226 336 208
1
8
RE 8-4 -131 -339 -179 -23 244 99 9
RE 9-1 -259 -716 -783 -723 296 312 286
RE 9-2 -351 -526 -513 -540 171 168 189
RE 9-3 -352 -616 -657 -648 238 237 2539
RE 9-4 -190 -538 -599 -604 353 396 406
RE 10-1 -355 -467 -487 -557 272 320 126
RE 10-2 -199 -454 -486 -466 196 189 193
RE 10-3 -313 -658 -651 -557 219 394 166
3
10
RE 10-4 -187 -414 -437 -403 294 299 265
RE 11-1 -179 -315 -537 -505 190 302 325
1 11 RE 11-2 -230 -443 -372 -347 374 119 216RE 12-1 -203 -361 -345 -326 250 247 215
4 12RE 12-2 -174 -357 -331 -386 252 214 234
RE 13-1 -185 -309 -338 -399 191 204 254
RE 13-2 -238 -308 -304 -400 168 165 124
RE 13-3 -151 -267 -267 -412 149 133 267
RE 13-4 -166 -298 -269 -396 227 217 249
3 13
RE 13-5 -246 -375 -404 -396 174 178 145
RE 14-1 -145 -536 -405 -447 408 302 224
RE 14-2 -138 -409 - -86 260 - 32
RE 14-3 -198 -351 -428 -510 246 248 254
RE 14-4 -187 -314 -565 -476 182 330 251
1 14
RE 14-5 -359 -317 -457 -441 202 294 216
RE 15-1 -269 -457 -463 -585 257 293 202RE 15-2 -262 -443 -577 -546 286 378 321
RE 15-3 -445 -484 -578 -528 114 156 1213 15
RE 15-4 -172 -402 -409 -473 302 325 274
RE 16-1 -282 -392 -548 -523 231 329 320
RE 16-2 -250 -338 -548 -550 170 343 296
RE 16-3 -236 -323 -493 -527 122 268 2644 16
RE 16-4 -289 -375 -578 -572 175 302 286
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FIGURE 1:- A schematic illustration of cooling tower shows major components of end walls.
FIGURE 2:- View of south elevation and east side of the cooling tower.
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FIGURE 3: Large & wide cracks on beam, water leakage and corroded bar.
FIGURE 4: Extensive cracking of wall panel and edge column (A safety hazard).
FIGURE 5: Severe cracking of internal beam and roof slab soffit.
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0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0-20 20-40 40-60 60-80 80-120 120-170 170-220
Concrete Powder Samples Depth (mm)
ChlorideConc.(%byWt.ofCon
crete)
Beam (Aver.) Column (Aver.) Wall Panel (Aver.) Bund Wall Threshold Value
FIGURE 6: Chloride profile in the End Wall elements and Bund Wall.
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0-20 20-40 40-60 60-80 80-100 100-150
Concrete Powder Samples Depth (mm)
ChlorideConc.(%b
yWt.ofConcrete)
Roof Slab Roof beam Louver (Aver.) Threshold Value
FIGURE 7:- Chloride profile in the Roof Slab, Beam and Louvers.
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