Condition Assessment and Cathodic Protection of Reinforced Concrete Cooling Tower

8/13/2019 Condition Assessment and Cathodic Protection of Reinforced Concrete Cooling Tower

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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.

08301


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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.


0.070.090.10Wall/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.


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



0.330.360.49Wall/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



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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.


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.


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.

19


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Documents

Condition Assessment and Cathodic Protection of Reinforced Concrete Cooling Tower