dr p bamforth - durability study of a diaphragm wall - concrete cracking jl

Al Sowwah Island Durability of the diaphragm wall

Dr P B Bamforth 1 V.1 20 July 2009

AL SOWWAH ISLAND Durability study for the diaphragm wall and facia Dr P B Bamforth



Executive Summary A durability assessment has been carried out for the diaphragm wall and in situ facia for Al Sowwah Island. Concrete mixes are proposed based on the requirements of CS163 and the exposure conditions unique to the site. With the use of blended cements (PC/ggbs/ms) which exhibit both sulfate resistance and a high resistance to chloride penetration it has been estimated that a 70-year life will be achieved using C50 concrete with 70% ggbs, 2-5% microsilica and 80mm cover in the retaining wall, and C60 concrete with 60% ggbs, 5-10% microsilica and 70mm cover in the facia. Specific mix proportions must be derived through mix trials. The higher ggbs content in the diaphragm wall is required to minimise temperature rise in the thicker section. Options for reducing cover included;

a) An integral corrosion inhibitor to increase the threshold level of chloride at which corrosion commences. If used this may lead to a reduction in cover to about 50mm in both the diaphragm wall and the facia, depending on the dosage.

b) Stainless steel or stainless steel clad reinforcement. In this case the cover will be determined primarily by the structural requirements.

The principal risk to durability is from cracking. It is recommended that in the extreme splash zone exposure condition, crack widths are no greater than 0.2mm, the level at which it is generally accepted that some degree of self healing will occur in a humid environment. In the aggressive and moderately aggressive zones cracks widths up to 0.3mm may be permitted. To avoid the risk of corrosion in the facia an option is to use stainless steel. As corrosion of reinforcement presents the more serious risk to achieving a 70-year life, testing should focus on the chloride levels in the constituent to minimise the background level and on the chloride resistance of the concrete. Nordtest NT 443 is recommended as this provides a value of effective diffusion coefficient than may be used to validate the estimates used in the predictive model. As it has been estimated that a service life of 70-years will be achieved, extensive maintenance should not be necessary. However, visual inspections are recommended at 5 year intervals for the first 20 years and at 10 year interval thereafter. A warning of potential problems may be obtained through the use of embedded corrosion monitors, or by periodically taking drilled samples to determine the extent of chloride ingress. The latter would be included as part of the visual surveys



Contents 1. Introduction................................................................................................................. 4 2. The diaphragm wall .................................................................................................... 4 3. Exposure conditions.................................................................................................... 5

3.1 General conditions .............................................................................................. 5 3.2 Exposure to sulfate.............................................................................................. 5 3.3 Exposure to chlorides.......................................................................................... 5 3.4 Exposure to combined sulfate and chloride ........................................................ 8

4 Estimating service life using CSTR61........................................................................ 9 4.1 The model ....................................................................................................... 9 4.2 Assumptions.................................................................................................... 9 4.3 Estimating service life................................................................................... 11 4.3.1 The facia - splash zone.............................................................................. 11 4.3.2 Back of the wall in dredged granular fill .................................................. 12

5. Delayed ettringite formation ..................................................................................... 12 6 Recommendations for concrete and cover for 70-year life....................................... 13 7 Cracking.................................................................................................................... 14

7.1 The effect of cracking on corrosion of reinforcement ...................................... 14 7.2 Other effects of cracking................................................................................... 15

8 Measures to minimise corrosion at cracks ................................................................ 15 8.2 Use of stainless steel reinforcement or stainless steel cad reinforcement. ....... 15 8.2 The use of a corrosion inhibitor ........................................................................ 16

9 Construction of the facia........................................................................................... 17 9.1 Early thermal effects in pours 1 and 3 .............................................................. 17 9.2 Shrinkage of pour 2........................................................................................... 19

10 Testing and quality control ................................................................................... 21 10.1 Background chloride content ............................................................................ 21 10.2 Testing for resistance to chloride penetration................................................... 22 10.3 Batching plant trials .......................................................................................... 23 10.4 Full scale trial.................................................................................................... 23

11. Monitoring, Maintenance and inspection in service ............................................. 23 12 Conclusions........................................................................................................... 24 References......................................................................................................................... 25



1. Introduction A project specific durability study has been undertaken for a typical diaphragm wall cross-section and given design conditions. According to Concrete Society Report CS163 [1], the diaphragm wall falls into the category of a special structure because of the requirement for a design life is significantly greater than 30 years. As stated in CS163, the durability study must therefore lead to a project specific durability plan which includes;

consideration of the micro exposure conditions which apply to the major and/or critical elements

recommendations of measures in the design to provide durability, including recommendations for the concrete specification, cover requirements and any additional corrosion protection measures required to achieve the specified design life of 70 years

specify testing and quality control during construction specify monitoring, inspection and maintenance in service, developed as a

project maintenance plan To ensure a long design life, attention must be given to the design, the construction process and subsequent maintenance. In particular it is important that the concrete is properly compacted and cured. For the purposes of this durability assessment it will be assumed that current best practice will be used in the construction process as described in CIRIA C577 [2].

2. The diaphragm wall The diaphragm wall has a uniform thickness of 1200mm. A reinforced concrete facia is to be cast in situ to a level from 3m above Abu Dhabi Datum 1m to 2m belowAbu Dhabi Datum. This may include polypropylene fibre as additional protection against scaling. The profile of the facia has been designed to match other sections of the wall constructed using blocks. The durability of reinforced concrete is determined by the inherent durability of the concrete and the ability of the concrete, through the combination of quality and cover depth, to prevent corrosion of the reinforcement. In Abu Dhabi, deterioration of the concrete may occur as a result of either aggressive agents in the environment or as a result of the internal degradation as shown below.

External attack Internal attack

Sulfate attack Alkali silica reaction

Salt weathering Delayed ettringite formation Reinforcement is protected in newly cast concrete by virtue of the high pH (>12.5). However, the protection may be lost under one of two conditions;

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Mechanism Effect on steel passivation

Carbonation Reaction of CO2 in the air the calcium hydroxide in the hydrated cement leads to the production of calcium carbonate with a much lower pH and the passivation of reinforcement is lost

Chloride penetration to the steel

Chlorides, at a sufficient (threshold) level at the steel surface, interrupt the continuing formation of the passivating layer and again protection is lost

As the concrete is either immersed or regularly wetted, carbonation is not considered to be a major issue, with the predominant risk of reinforcement corrosion resulting from chlorides.

3. Exposure conditions

3.1 General conditions Abu Dhabi is a hot, arid climate (i.e. one in which evaporation exceeds precipitation). Summer shade temperatures exceeding 40oC with very high humidity near the coast. Other particular features of the climate include;

High (day-to-night) temperature changes Strong drying winds High change in relative humidity with condensation at night High solar radiation

The diaphragm wall will be exposed at various locations to seawater and to sulfate and chloride bearing groundwater. CS163 provides guidance on exposure classes for concrete in the Arabian Peninsula, recognising that the use of other National codes may not be sufficiently robust to deal with significantly more aggressive exposure.

3.2 Exposure to sulfate Test results from the soil [3] have shown that sulfate levels are in the range from 300 to 2400 mg/l (as SO4 by 2:1 water extraction) and the pH is 8.0-8.7. Under these conditions with mobile groundwater, Table 6 of CS163 classifies the sulfate exposure as S-3. However, CS163 also states that all concrete below ground level must have effective and durable tanking. Where this is not practical CS163 recommends that the next higher aggressive ground class should be used, in this case the highest class S-4. Mix requirements to meet both S-3 and S-4 conditions are given in Table 1.

3.3 Exposure to chlorides The exposure class for chloride varies from the permanently submerged part of the retaining wall to the exposed area in the splash zone. CS193 exposure conditions are described in Table 2.

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Table 1 Requirements of CS163 for S-3 and S-4 exposure to sulfate

Sulfate exposure

class Minimum strength

class

Maximum free w/c or

combination content

Minimum cement or

combination content for

20mm aggregate

Recommended cement and combination group

S-3 C50 0.40 380 E 55-70% PC 35-25% fly ash 10-5% ms

30-45% PC 60-60 % ggbs1 10-5% ms

B Sulfate resisting

C 65-75% PC

35-25% fly ash 40-50% PC

60-50% ggbs1 C50 0.40 400

D 60-74% PC

40-36% fly ash 25-34% PC

75-66% ggbs1

S-4

C60 0.35 400 E As above As above 1 When the cement or combination is specified for sulfate resistance there is an additional requirement that if the alumina content of the slag exceeds 14%, the tricalcium aluminate content of the Portland cement component shall not exceed 10%. Table 2 Chloride exposure classes according to CS163

Exposure class

Examples

Moderately aggressive

(Permanently submerged). Surface of under-sea structures which are permanently at least 5m below low tide level

Aggressive (Wet, rarely dry)

(i) Surfaces underground in areas with saline groundwater including the capillary rise zone (but see also sulfate exposure classes)

(ii) Surfaces of structures containing saline water which are in permanent contact with the water

(iii) Surfaces between high tide level and 5m below sea level Severe (Moderate humidity)

External surfaces which are not affected by condensation, condensation run-off, irrigation or leakage and which are more than 3m above ground level of structures which are

a) in geographical locations with high saline water table, salinas or sabkhas, or b) between 50m and 1km from the sea

Extreme (Cyclic wet and dry)

(i) External surfaces in geographical locations which are between 50m and 1km from the sea or with high saline groundwater, salinas or sabkhas, which are a) less than 3m above ground level or within the capillary rise zone, or b) affected by condensation, condensation run-off, irrigation or leakage

(ii) Surface of water retaining structures where the contained water is saline and which are

a) affected by fluctuating water levels, spray or splash b) on the opposite side of the member from the contained water and could be

affected by leakage (iii) Internal surfaces of water excluding structures such as tunnels and basements

where the excluded water is saline (iv) Splash zone external surfaces of structures from high tide level to 50m inland



On the seaward face the retaining wall is part exposed in the splash zone, part permanently immersed in sea water and part buried in the seabed. On the landward face the wall is part exposed (permanently) to saline water in the dredged granular fill and part buried in the seabed. The range of exposure classes is illustrated in Figure 1.

Diaphragm

wall

Dredged granular fill MHHW

(+0.76mNADD)

MLLW (-1.075mNADD)

Rap layer (-3.6mNADD)

EXTR

EM

EA

GG

RE

SSIVE

Bed level (-8.0mNADD)A

GG

RE

SS

IVE

Capping/facing beam

MO

DE

RA

TELY A

GG

RE

SS

IVE

MO

DE

RA

TELY A

GG

RE

SS

IVE

EXTR

EM

E

Transition depends on water table

Figure 1 Exposure classes according to CS163 In the worst case the exposure is defined as extreme for the upper part of the wall. On the seaward face the exposure conforms to Extreme (iv) splash zone and on the landward face the exposure conforms to Extreme (ii) - Surface of water retaining structures where the contained water is saline and which are a) affected by fluctuating water levels, spray or splash. From high tide level down to 5m below sea level, where the concrete is wet, rarely dry the exposure class is defined as aggressive on both faces. The transition on the landward face will depend on the level of the water table. From 5m below sea level the exposure is defined as moderately aggressive. The requirements for concrete to meet the relevant CS163 chloride exposure classes are given in Table 3 for an intended life of 30 years. The diaphragm wall has an intended life of 70 years and the requirements provided in Table 3 have been checked using the chloride migration model of Concrete Society 61 to establish the requirements for the much longer life.



Table 3 Concrete requirements (strength class, maximum w/c ratio, minimum cement content) and cover for an intended 30 year life

Strength class, maximum w/c and minimum cement content (kg/m3) for cover (mm) of;

Exposure class

35+ c 50 + c 55 + c 60 + c 65 + c 70 + c 75 + c

Cement type (see Table 1)

Moderately aggressive

C50 0.40 380

A, B

C40 0.45 360

C,D,E

Aggressive C60 0.35 400

C50 0.40 380

C40 0.45 360

C

C50 0.40 380

C40 0.45 360

D

C50 0.40 380

C40 0.45 360

E

Extreme C60 0.35 400

C50 0.40 380

C40 0.45 360

C

C60 0.35 400

C50 0.4 380

C40 0.45 360

D

C60 0.35 400

C50 0.40 380

C40 0.45 360

E

3.4 Exposure to combined sulfate and chloride Structures on or close to the coast will be exposed to both sulfate and chloride. In such circumstances the aluminates in the cement tend to react preferentially with the chlorides (hence the improved chloride resistance of high C3A cements which have enhanced chloride binding capacity [4]). While not dealing specifically with sea water exposure, BRE SD1, Concrete in aggressive ground [5], acknowledges that indeed, they [chlorides] may be beneficial since there is considerable evidence, from seawater studies, that the presence of chloride generally reduces sulfate attack. This is taken into account for brackish water in brownfield sites. Furthermore, BS8500-1:2006 [6] states that Where concrete is to be in contact with seawater, it needs to be of sufficient quality to resist seawater attack. The recommendations to resist reinforcement corrosion induced by seawater provide concretes with adequate resistance to the chemical attack on concrete by the seawater. Taking into account the high levels of chloride reported in the groundwater (0.98 4.00%) the requirement to increase the sulfate class from S-3 to S-4 may therefore be unnecessarily onerous.



4 Estimating service life using CSTR61

4.1 The model The rate of chloride ingress is determined by the level of chloride in the environment, the depth of cover and the ability of the concrete to resist chloride migration (defined as the apparent chloride diffusion coefficient. The level of background chloride in the concrete (derived for the mix constituents) must also be taken into account. For this durability assessment the model of CSTR 61 [7] is used. The CSTR61 model estimates the time to the onset of reinforcement corrosion and the time to cracking. As there is greater uncertainty attached to the prediction of the time between onset of corrosion and cracking, the service life is conservatively estimated on the basis of the time to onset of corrosion, i.e. when the chloride content at rebar depth exceeds the threshold level. The error function equation is used to predict the rate of chloride ingress using equation 1

+=t

ttD2

xerf1)C(CCCn

mca(tm)

bsnbx (1)

where, Cx is the chloride content at depth x after time t Csn is the notional surface chloride level Cb is the level of background chloride (e.g. from the aggregate and sand) Dca(tm) is the apparent diffusion coefficient, Dca derived from a measurement at time tm n is the age factor

4.2 Assumptions In making the service life predictions the following assumptions have been made. (Notional) surface chloride level, Csn Based on a review of measurements from structures [7, 8] a characteristic (95 percentile) value of 0.9% by weight of concrete (about 5.4% by weight of cement) is used assuming splash zone conditions. Background chloride level, Cb - The chloride content of the fresh concrete must be limited by either the selection of non-contaminated aggregate or by washing. CIRIA C577 [2] recommends chloride limits, following acid extraction to BS12: Part 117, as follows;

Coarse aggregate 0.03%

Fine aggregate/sand 0.06%



Assuming a typical 2:1 ratio of coarse to fine aggregate this is equivalent to 0.04% by weight of combined aggregate, this being the limit in the Employers specification. With these limits the contribution of chloride to the concrete from the aggregate would be expected not to exceed about 0.7 kg/m3. This is about 0.03% weight of the concrete. With a cement content exceeding 400kg/m3 the maximum chloride expressed as percent of cement is therefore limited to less than 0.18 % wt of cement. Chloride threshold level, Ct This is the level at the surface of the steel at which it is assumed that corrosion may commence. In temperate climates, the threshold level of chloride for corrosion activation is generally assumed to be 0.4% wt of cement. However, the threshold level is lower at elevated temperature [7, 9]. The threshold level may also be reduced by the use of microsilica and ggbs. At an assumed mean temperature of 30oC and with 60%ggbs the threshold level is assumed to be 0.21%. It should be noted that this threshold level is only marginally higher than the level of background chloride permitted and it is important therefore that if the background chloride level is close to the maximum permissible, no further chloride should be able to penetrate to the depth of the reinforcement Apparent chloride diffusion coefficient Dca - The diffusion coefficient is dependent on the mix constituents (in particular the cement type) and the mix proportions. The model of CSTR 61 uses algorithms to estimate Dca based on the cement content, the cement type and the w/c ratio. The age factor, acknowledging the increasing resistance to chloride ingress with age, is also estimated from the mix parameters. Estimated values for C40, C50 and C60 mixes using 60% ggbs are given in Table 4. [NB The model does not generate diffusion coefficients for triple blend mixes and in this analysis the additional benefit from microsilica has been ignored providing a margin of safety]. Dca is assumed to vary with temperature and the assumed mean temperature is 30oC. Table 4 Estimated values of Dca used in the CSTR61 chloride diffusion model [Assumes 60% ggbs by weight of cementitious material]

Apparent diffusion coefficient, Dca (m2/s) Strength

class w/c 28 days 20 years

C40 0.45 9.39 x 10-12 0.150 x 10-12

C50 0.40 11.1 x 10-12 0.176 x 10-12

C60 0.35 12.8 x 10-12 0.207 x 10-12



4.3 Estimating service life

4.3.1 The facia - splash zone Using the above assumptions the relationship between cover and time to corrosion has been estimated and the results are shown in Figure 2 for the three strength classes. From these curves the cover requirements for 30-year and 70-year have been estimated and the results are given in Table 5.

0

20

40

60

80

100

40 50 60 70 80 90 100

Cover (mm)

TIm

e to

cor

rosi

on a

ctiv

atio

n (y

rs)

C60 C40C50

Figure 2 The relationship between the estimated time to corrosion and cover

Table 5 Estimated cover requirements compared with the requirements of CS163 using type E cement combination with 60% ggbs.

Cover requirements (mm) Strength class CSTR61 30yrs CS163 30yrs CSRT61 70yrs

C40 75 65 89

C50 66 60 77

C60 58 55 68 Cover requirements based on the CSRT61 model are marginally more conservative than the requirements of CS163 (possibly because additional benefits from the microsilica have been ignored) but are in the same order, indicating that the assumptions behind the mode are consistent with those of CS163. With C60 the estimated cover for 30 years life is 58mm and this increases by only 10mm, to 68mm, to give a 70-year life. The relatively small increase in cover to achieve 40 years additional life is due to the ageing effect associated with ggbs concrete and with microsilica concrete at low w/c.



Hence the use of C60 concrete with type E cement combination with cover of 70 mm on seaward faces would be expected to provide protection from corrosion of reinforcement for 70 years. Using the C50 concrete the cover with the same cement combination would require 77mm cover to achieve a 70-year life. The proposed minimum cover is 100mm and hence this would provide a significant margin of safety with regard to protection of reinforcement using either C50 or C60 concrete. Note that tolerances, c, should be added to the estimated cover requirements (see Table 3) to account for variations that can occur during placement and concreting

4.3.2 Back of the wall in granular fill While the back of the wall at the upper level is in conditions that define it as extreme i.e. Surface of water retaining structures where the contained water is saline and which are, a) affected by fluctuating water levels, spray or splash, the build up of chloride on the surface is likely to be less severe. However, the effect of reducing the surface chloride level to 0.5% has been estimated to permit a reduction in cover of only about 5mm. It would be appropriate therefore to use the same cover on each face, 70mm for C60 and 80mm for C50, rather than 100mm on the seaward face and 50mm on the landward face. This would achieve about the same distance between inner and outer reinforcement.

5. Delayed ettringite formation Delayed ettringite formation is a form of internal sulfate attack. Ettringite is normally formed during cement hydration, resulting in a volume increase in the fresh, plastic concrete. However, due to the concretes plastic condition, the expansion is harmless and unnoticed. However in some concretes which have been exposed to temperatures over about 70C, the high temperature decomposes any initial ettringite formed and holds the sulfate and alumina tightly in the calcium silicate hydrate (C-S-H) gel of the cement paste. The normal formation of ettringite is thus impeded but in the presence of moisture, ettringite may form after months or years in confined locations within the hardened concrete. Since the concrete is rigid the ettringite volume increase leads to expansion and cracking. BRE IP11/01 [10] states that initial research has shown that fly ash at levels > 20% or ggbs at levels > 40% will prevent DEF-induced expansion in concrete subject to peak temperatures of up to 100oC but states that further work is needed to ensure that DEF is not merely delayed. For this reason BRE IP11/01 categorises concretes using ggbs as low risk (rather than no risk). Furthermore, Concrete Society Report No.30 [11] states that the alkali contribution from ggbs may be ignored when the ggbs content exceeds 40%; hence the effective Na20equ (for the CEMI/fly ash combination) is likely to be well

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below 0.85%, permitting the 75oC limit to be used. This is the limit recommended by CS163 for concrete using blended cement. As the wall is 1200mm thick there is a risk of very high temperature rise during early hydration and to minimise the risk of delayed ettringite formation, a mix using cement with low heat of hydration and the lowest practical cement content should be used. This will be achieved with the triple blend mix including ggbs and microsilica, the former slowing the rate of heat output during hydration and the latter, which has a cementing efficiency of at least 2 times that of Portland cement, enabling the total cement content to be reduced. Estimates of the peak temperature have been made using the model of CIRIA C660 [12] for C50 and C60 concrete with varying levels of ggbs. The placing temperature of the concrete is assumed to be 32oC and the ground temperature 30oC. The results are given in Table 6. Table 6 Estimated peak temperature in the 1200mm diaphragm wall

Peak temperature (oC) with ggbs proportions of: Strength

class

Cement content (kg/m3) 60% ggbs 65% ggbs 70% ggbs

C50 380 77 75 72

C60 400 79 77 75 Using a higher proportion of ggbs enable the peak temperature to be reduced and will also increase resistance to sulfate attack and chloride ingress.

6 Recommendations for concrete and cover for 70-year life

Mix requirement

Diaphragm wall

Facia

Strength class C50 C60

Min cement content (kg/m3)

380 400

PC 25-28% 35-30%%

Ggbs 70% 60%

microsilica 5-2% 5-10%

Cover (mm) 80 + c 70 + c Based on the assessment using CSTR61 no additional protective measures are required to protect the reinforcement for 70 years.

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7 Cracking 7.1 The effect of cracking on corrosion of reinforcement Reinforced concrete is designed to crack and an inherent part of the design is the control of crack widths. However, the impact of cracking and the specific influence of crack width is still not easily quantifiable in relation to corrosion of reinforcement despite much research in this area. In 1972, a study by the University of Texas at Austin [13] led to the following conclusion ... Although flexural cracking of concrete was found to promote corrosion of reinforcement at the crack location, the severity of the long term corrosion damage to the bars was primarily dependent on the depth of concrete cover. Large cracks, usually found in conjunction with large cover, promoted early corrosion at the crack locations but further development of the corrosion, as well as longitudinal cracking of the cover over the bars, were inhibited for the larger covers. In a more recent study, Arya and Ofori-Darko [14] reached a similar conclusion and suggested that ...an effective measure against corrosion may be to limit the frequency of intersecting cracking by increasing the depth of cover to the reinforcement, rather than by controlling surface crack widths. Ohno et al [15] also reported reduced corrosion with increasing cover, but were surprised by these findings, suggesting that, for a given load, increased cover should lead to wider cracks. Effects of crack widths and w/c ratio were also investigated and it was concluded that the most significant factor was w/c ratio. Francois and Arliguie [16] reported that in cracked beams with low cover (10mm), the secondary cracks resulting from steel corrosion seemed to be randomly located and without any correlation with the presence of service cracking (for cracks up to 0.25mm). In beams with greater cover (40mm), the extent of cracking was generally less than in the low cover beams, despite the crack width being greater (up to 0.5mm) supporting an argument that the quality of the cover is more influential than the crack width in determining the extent of steel corrosion. The research evidence suggests, therefore, that the use of high cover with low water/cement ratio concrete (as proposed) will minimise the influence of cracking, by stifling any initial corrosion that may be initiated at the core of the crack, and that it is more appropriate to limit the frequency of cracks than to limit crack widths Nevertheless codes of practice still aim to limit the crack width. For example EN1992-1-1 [17] permits surface crack widths up to 0.3mm for splash zone conditions in more moderate climates. Where there is a requirement for water retention (as described in EN1992-3 [18]), the allowable crack width is reduced to 0.2mm for a hydraulic gradient up to 5 and reducing linearly as the hydraulic gradient increases to 35. This is because there is evidence that at crack widths of 0.2mm or less some degree of self-healing can occur. Swedish and Danish Standards (SS13701 [19] and DS411 [20] also limit crack

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widths to 0.2mm in extra aggressive environments. Furthermore, recognizing the more aggressive nature of the Arabian Peninsular some authorities demand more stringent limits, e.g. 0.15mm in the Dubai Municipality for concrete in the ground. In view of the very aggressive environment in the extreme splash zone condition it would be appropriate to design for a crack that is likely to have some self-healing capacity and on this basis an allowable value of 0.2mm would be appropriate. In the less severe exposure conditions wider cracks may be tolerated. This is especially so in those part of the diaphragm wall that are permanently immersed. In these conditions, even if chlorides penetrate to the steel, corrosion will be inhibited by the lack of oxygen. In these conditions a crack width of 0.3mm will be acceptable, this being the magnitude of crack width generally accepted as being adequate in marine structures [21] In selecting an appropriate crack width it has to be recognized that increasing cover to enhance durability will also increase the surface crack width and therefore increase the amount of reinforcement required. One way to overcome this is to design for a crack width at the design cover, rather than that determined by durability requirements [1]. The argument for this is that the limiting crack widths developed over the years have generally been associated with more typical cover depths up to about 50 mm. As the concern is the potential to protect the reinforcement and hence the potential for self-healing, this could be achieved by ensuring that the crack width, within the cover required for structural purposes, is 0.2mm or less.

7.2 Other effects of cracking In addition to acting as potential corrosion sites, cracks may also be unacceptable visually if they are too wide or if they lead to rust staining. As discussed in Section 7.1, corrosion may initiate at the root of cracks but may then be stifled if the concrete quality and cover is adequate. However, even a small amount of corrosion may lead to rust staining. It may be prudent therefore to adopt additional measures to prevent corrosion at cracks. If the cracks are sufficiently narrow to be self-healing then the measures to prevent corrosion may only need to be effective for a limited period.

8 Measures to minimise corrosion at cracks Additional measures to minimize or prevent corrosion, particularly at the location of cracks, are as follow

8.1 Use of stainless steel reinforcement or stainless steel clad reinforcement. One obvious way to avoid corrosion of the reinforcement is to use steel that is resistant to corrosion. Stainless steel (SS) is such a material and may provide a simple solution in areas that are particularly vulnerable. There are various grades of SS but the two most commonly used for reinforcement are designations 1.4301 (304 S31) and 1.4436 (316 S33): both are austenitic SS. 316 S33 offers the highest corrosion resistance and is generally recommended when used in coastal or marine applications.

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A particular benefit of SS is that the cover may be reduced, permitting less steel to achieve the same surface crack width. In addition, the design crack width could itself be relaxed to level that is visually acceptable, further reducing the amount of the SS required. Hence some of the additional cost associated with SS may be offset by a reduction in the amount of reinforcement required. Consider the difference between the steel required to control crack widths to 0.2mm at 70mm cover with that required to achieve, say, 0.3mm cracks with 50mm cover. Stainless steel clad carbon steel is also available (e.g. Nouvinox). This comprises carbon steel with a skin of 316 S33 of 1-2mm (depending on diameter) and provides the same corrosion resistance as solid SS of the same grade, but is about half the price of solid stainless steel reinforcement. If SS is used it would only be necessary in the facia which is exposed to splash zone conditions, i.e. above high tide level (see Table 2). Based on a review of extensive research, Concrete Society Report 51[21] recommends that no particular precautions are necessary when coupling SS and carbon steel, but that it may be prudent to use the higher quality molybdenum-bearing alloy (316 S33 or better) to keep the risk to a minimum

8.2 The use of a corrosion inhibitor Calcium nitrite, the most commonly used corrosion inhibitor, is widely available commercially as a 30% solution. It is normally added to the concrete as a separate material at the mixer at a rate between 10 and 30 litres per cubic meter of concrete, depending on the amount of chlorides predicted to accumulate in the vicinity of the reinforcing steel during the intended design life of the structure. Extensive laboratory research and numerous field studies have established the effectiveness of calcium nitrite in reducing the risk of corrosion in chloride contaminated concrete. The inclusion of calcium nitrite in concrete appears (from limited research studies) to have no significant effect on the rate of ingress of chloride towards the reinforcing steel, it has, when added in sufficient quantities, been found to have a considerable effect in two respects: i) it delays the onset of corrosion (by raising the chloride threshold level), and ii) after corrosion starts, it reduces the corrosion rate. Evidence emerging from existing structures, which have been in exposure for up to 20 years, confirms satisfactory performance. Within the model of CSTR61, calcium nitrate is assumed to increase the chloride threshold level according to the equation

tttI f

I0.06CC += (2) where CtI is the inhibitor modified chloride threshold level Ct is the chloride threshold level with no inhibitor I is the inhibitor dose in l/m3 ft is a safety factor (applied by the user)



With a dose of 20 l/m3 and a safety factor of 3 (i.e. assuming the inhibitor is only 1/3 as effective as claimed) the threshold level would increase by 0.4 (from 0.32 to 0.72) at 20oC, reducing to 0.48 at 30oC. Estimates of the cover requirements using increasing levels of corrosion inhibitor in the C50 and C60 concretes are as follows:

0 10 l/m3 20 l/m3 30 l/m3

C50 80mm 60mm 55mm 50mm

C60 70mm 55mm 50mm 45mm As demonstrated this would also provide the opportunity to reduce cover and achieve better control of crack widths with less steel.

8.3 Epoxy-coated reinforcement The contractor has allowed for the use of fusion-bonded epoxy coated reinforcement (FBECR) as an additional protective measure. Over the years, and particularly in the US, there have been many instances of failures using epoxy coated steel and its reputation has understandably suffered. The risk in using FBECR is that its performance requires

a) that it is manufactured well, with no inherent defects, and b) that no defects are introduced during the process of cutting, bending and fixing or

during the concreting process. This is practically very difficult to achieve and to guarantee, particularly if the facia is to be cast as a single deep lift.

Where defects occur, corrosion may be accelerated as it is concentrated locally, leading to significant loss of section. An added consideration is the reduced bond achieved using FBECR, which nay be assumed to be about 80% of that of uncoated steel [1]. This may lead to the requirement for an increase in the area of reinforcement to achieve the limiting crack widths and for increased lap lengths In view of the track record of FBECR and the uncertainties associated with its use, it would be difficult to guarantee a 70-year life if it is used and its use is not recommended.

9 Construction of the facia The facia is to be cast in situ. It is has a complex profile (Figure 3) and trials are being undertaken to demonstrate that it may be completed as a single pour.

9.1 Early thermal effects The lower and upper sections of the facia (1 and 3) are relatively thick, up to about 1000mm, while the middle section 2 may be as thin as 400mm. It was originally proposed that the wall be cast against a scabbled surface of the wall to provide good bond. In this case there would be a significant a risk of early-age cracking due to early



thermal effects and/or shrinkage with the diaphragm wall offering considerable restraint to shrinkage.

1000mm

1200mm

400mm

1805mm

125mm

1350mm

500mm

1000mm1

2

3

Figure 3 Section through the diaphragm wall and facia including the location of

construction joints. Estimates of the early age temperature rise have been made using the model of CIRIA C660 for walls. In each case a notional section thickness has been estimated from the expression; 2 x cross sectional area/perimeter. Potential crack widths arising from restraint to early-age thermal strains have been estimated and the results are given in Table 7. The calculation assumes that the crack width wk = Sr,max cr where Sr,max is the characteristic crack spacing and cr is the crack inducing strain. According to EN1992-1-1 [21] for under reinforced concrete the natural crack spacing for a wall is 1.3 x height. In this case the height is replaced with the thickness of the respective elements of the facia. The crack inducing strain is the restrained strain (thermal and shrinkage) less the mean residual strain in the concrete after cracking, assumed to be 50% of the tensile strain capacity of the concrete (see CIRIA C660 [12] for further details of the method of calculation of crack width). Estimated crack widths are 0.11mm, 0.06 and 0.15mm for pours 1, 2 and 3 respectively. Hence, with no reinforcement and provide a good bond is achieved between the wall and the facia, crack widths will be acceptably small. Any longitudinal reinforcement will act to reduce crack widths further.



Table 7 Estimate of the natural crack spacing and crack width due to early-age thermal contraction

Pour 1 Pour 2 Pour 3 Thickness (mm) 650 400 1000 Notional thickness (mm) 520 400 600 Estimated peak temperature (oC) 66 62 68 Temperature drop to mean ambient of 30oC - T1 (oC) 36 32 38 Coefficient of thermal expansion - c (microstrain/oC) 10 Free thermal contraction - T1 c (microstrain) 360 320 380 Autogenous shrinkage - ca 29 Estimated restraint - R 0.75 0.8 0.65 Creep coefficient - K 0.65 Restrained strain r = R K ( T1 c + ca) (microstrain) 190 181 173 Tensile strain capacity of C50/60 concrete ctu 122 Crack-inducing strain cr = r 0.5ctu 129 120 117 Natural crack spacing Sr,max (=1.3 x thickness) 845 520 1300 Estimated crack width (ignoring effect of reinforcement) wk = Sr,max cr (mm)

0.11 0.06 0.15

To minimise the risk of thermal cracking, it has been proposed that the facia be separated from the diaphragm wall using polythene sheet and there will be no continuity of reinforcement. Full scale trials are proceeding on this basis. In this case the facia will simply be supported by the wall but the latter will offer very little restraint to movement of the facia. In this case the only risk of thermal cracking will be as a result of the temperature differentials between the different elements of the facia due to the differing thickness. During the early thermal cycle the thicker sections 1 and 3 will expand and contract to a greater extent than section 2. During cool-down section 1 and 3 will contract to a greater extent than section 2 and may therefore lead to modest compression in section 2 in the horizontal direction.

9.2 Shrinkage of pour 2 Pour 2 may be as narrow as 400mm and the thicker sections 1 and 3, will offer restraint to the more rapid drying shrinkage in the thinner section. This could potentially lead to cracking in the longer term. An estimate of the relative drying shrinkage has been made using the method of EN1992-1-1 (also described in CS163). The following conservative assumptions have been made; Sections 1 & 3 have a thickness of 800 mm Section 2 has a thickness of 400mm Drying is from one face only



Relative humidity = 60% Restraint = 0.9 Creep factor = 0.65 (ref CIRIA C660) The results are shown in Figure 4. In the worst case, after about 3 years, the tensile strain capacity is more than double the differential restraint strain (even without taking account the beneficial effect of creep).

0

50

100

150

200

250

10 100 1000 10000 100000

Age (days)

Mic

rost

rain

Sections 1 & 3Section 2

Differential shrinkage

Tensile strain capacity

Figure 4 Drying shrinkage estimated using the method of EN1992-1-1 and the resulting

differential strains between section 1 & 3 and section 2 of the facia. Provided the facia is prevented from rapid drying during its early life, significant cracking arising from drying shrinkage is not expected. Based on the above assessment there is no reason why the facia cannot be cast to full depth provided it can be demonstrated that the concrete can be properly placed and fully compacted without disturbing the reinforcement and jeopardising the cover and without damaging the polythene sheet used to ensure that the facia does not bond to the wall.

9.3 Appropriate selection of reinforcement Reinforcement is required primarily to carry tensile stress after cracking. Hence, whatever form of reinforcement is used, it must be sufficiently strong to prevent yielding when a crack occurs and transfers the load previous carried by the concrete into the reinforcement. It has been suggested that polypropylene fibres may be used for the facia concrete. In general, polypropylene fibres offer the greatest benefit when the concrete is in its plastic and hardening state by minimising the extent of plastic cracking and are most useful in slabs. In this case they have been proposed to provide resistance to scaling as a result of salt crystallization. In concrete with an already very low w/c = 0.35 the additional benefit from the fibres is questionable. Furthermore, when the concrete has achieved its



structural capacity, polypropylene fibres would not be expected to be effective in controlling cracking. If polypropylene fibres are to be used they would therefore be as an addition to, rather than a replacement for, conventional steel reinforcement.

10 Testing and quality control As the principal deterioration mechanism is likely to be corrosion of the reinforcement the testing and quality control should focus on ensuring that the level of chloride at the depth of the reinforcement remains below the threshold level, estimated to be 0.21% at a mean temperature of 30oC. This involves ensuring that,

a) the background level of chloride in the fresh concrete is controlled to an acceptably low level

b) that the concrete has sufficient resistance to chloride penetration to prevent the chloride level increasing at reinforcement depth from the background level to the threshold level

10.1 Background chloride content Background chlorides derive mainly for the aggregate. For the durability assessment it has been assumed that the maximum levels recommended by CIRIA C577 [8], i.e. coarse aggregate 0.03%, sand 0.06%. This will result in chlorides of about 0.03% wt of concrete and about 0.18% wt of cement. As the threshold level may be as low as 0.21% an upward deviation of 15% of the total chloride content may lead to intermittent premature corrosion and a reduction in service life. Conversely, if the background chloride can be maintained at a level below that permissible, then either the life may be extended (providing an additional factor of safety) or the cover requirements may be reduced as shown in Figure 4 for C50 and C60 concrete.

0

20

40

60

80

100

0.010 0.015 0.020 0.025 0.030

Background chloride content (% wt concrete)

TIm

e to

cor

rosi

on a

ctiv

atio

n (y

rs)

60mm

75mm70mm65mm 80mm

55mm

70mm65mm60mm

C50C60

Figure 3 The estimated time to corrosion as affected by the background chloride content



Testing of the chloride content of the aggregates should be a normal part of the material selection and the quality control process.

10.2 Testing for resistance to chloride penetration Appendix E of CS163 describes several tests that are used to measure permeability of chloride diffusion but states that, None of the permeability test methods which are commonly used in the region has sufficient precision or reliability for use as a routine quality control test during concrete production. The best approach, if they are to be used, is to use them at mix development stage. Once a mix has been developed which meets the necessary requirements, its performance can be monitored during production by strength tests and by checking the ingredients and their quantities from the batching plant records. The ASTM rapid chloride permeability test could be used as a coarse screen during production, for example as a check that silica fume has been included in a mix, but not as part of the overall compliance requirement Of the available tests, the Nordtest NT Build 443 accelerated chloride penetration test is the most useful for mix development as it provides both a chloride transport parameter and a surface chloride level which can be used in durability models. The principle of this test is that a saturated concrete specimen is exposed to a chloride solution for at least 35 days. After exposure, samples are taken from the exposed face at successive depths by grinding and the chloride results are analysed by curve fitting to give an effective diffusion coefficient or a penetration parameter. NT Build 443 recommends a test temperature of 23 2oC. For application in Abu Dhabi, the test temperature should be increased to 30 2oC to provide a representative value for this exposure condition. In addition, if time is available prior to start of construction, samples should be exposed for increasing periods to establish the effect of ageing, e.g. 28, 90 and 180 days with further specimens maintained for longer term testing. IMPORTANT NOTE; To determine the effect of ageing, all specimens should be exposed to chloride at 28 days, with the period of exposure being extended to the age at test. This is because the exposure to chloride itself affects the ageing process. A different result may be achieves if concrete is cured conventionally for a longer period and then tested after the same relatively short period of exposure. This is one of the limitations of the other rapid tests in which more mature specimens may be tested at late age, but which have not had long term exposure to chloride prior to testing If time is limited a more rapid test may be used in accordance with Nordtest NT 492 - chloride migration from non-steady-state migration test, APPENDIX E of CS163 describes this as follows, The concrete sample is exposed to sodium chloride solution on one side and to calcium hydroxide solution on the other. An initial voltage of 30V is applied across the specimen. This voltage is adjusted according to the initial current passed. The test duration varies between 6 and 96 hours depending on the initial current. At the completion of this part of



the test, the specimen is split open and the depth of chloride penetration is determined by spraying the fractured face with silver nitrate. The non-steady-state migration coefficient is calculated from the applied voltage and the average chloride penetration depth. This again gives an indication of the effective diffusion coefficient but with less reliability than the BT443 test.

10.3 Batching plant trials Having selected the materials and mix proportions for the C50 (diaphragm wall) and C60 (facia) concretes, batching plant trials should be undertaken. This will involve casting three separate batches of each concrete with no two batches of the same strength class produced within the same shift. Sampling and testing for fresh and hardened concrete properties shall follow the general recommendations of CIRIA C577. In addition, 100mm cylinders shall be cast for the provision of specimens for testing to NT443 and NT492. Three 100mm cylindrical samples shall be obtained from each batch for each strength class. From each of these sets of 3 specimens, testing shall be as follows;

Cylinder Test Age

1 NT443 & NT492 28-days

2 NT443 & NT492 28-days

3 NT443 Later age to be agreed with the Engineer to assess the age factor

10.4 Full scale trial The facia has a complex profile and the Contractor has undertaken a full scale trial to demonstrate the practicality of a full height pour and to demonstrate the surface finish that can be achieved. Ideally this would employ the same materials and plant (and ideally the same operatives) as planned for the construction but at this stage the mix design has not yet been finalized. As the facia is to be separated from the wall by polythene sheet to prevent bonding, the wall onto which the facia is cast need not be the same strength class as the diaphragm wall. When the mix has been finalised, it would be appropriate to repeat the full scale trial to ensure that any differences between the mix used in the first trial (details not known) and the selected mix do not impact on either the procedure or the surface finish. Furthermore, having constructed the trial facia using the agreed mix, 3 x 100mm cores should be extracted from each level (1, 2 and 3), spread over its length, and tested in accordance with NT443 exposure and at a later age to be agreed with the Engineer (but the later the better to highlight the effect of ageing).

11. Monitoring, Maintenance and inspection in service Based on the durability assessment and application of current best practice during construction (according to the recommendations of CIRIA C577), the selection of appropriate concrete mixes using blends of Portland cement with ggbs (for C50 for the



diaphragm wall) and Portland cement with ggbs and microsilica (for the C60 facia concrete) together with adequate cover would be expected to ensure that the reinforcement is protected for 70-years. Furthermore, the cement combinations will provide resistance to the reported levels of sulphate (S-3) and recognising that sulfate resistance is improved in the presence of chlorides would also be expected to achieve resistance for 70 year. Furthermore, only the facia concrete will be visible and easily accessible for inspection. However, this is in the most severe exposure condition and would be expected therefore to be the first location to exhibit any sign of problems. To be able to address problems of reinforcement corrosion before they are exhibited as cracking and spalling over corroded steel, corrosion monitors may be embedded in the cover zone to detect high levels of chloride before they reach the reinforcement. Alternatively, incremental dust drillings may be obtained periodically to measure chloride profiles directly, to determine the rate of chloride ingress and hence to forecast potential onset of corrosion. This would enable early intervention, for example by the application of hydrophobic surface treatment to reduce the moisture content of the concrete and the associated rate of chloride penetration. Recognising that there may be areas where a combination of construction events and exposure levels might lead to local problems a regular visual inspection should be undertaken. These should be initially at 5 year intervals, for the first 20 years and then at 10 year intervals.

12 Conclusions Concrete mixes are proposed for the diaphragm wall and the facia based on the requirements of CS163 and the exposure conditions unique to the site. With the use of blended cements (PC/ggbs/ms) which exhibit both sulfate resistance and a high resistance to chloride penetration it has been estimated that a 70-year life will be achieved using C50 concrete with 80mm cover in the retaining wall, and C60 concrete with 70mm cover in the facia. Details are as follows;

Mix requirement Diaphragm wall

Facia

Strength class C50 C60

Min cement content (kg/m3) 380 400

PC 25-28% 35-30%%

Ggbs 70% 60%

microsilica 5-2% 5-10%

Cover (mm) 80 + c 70 + c



The higher ggbs content in concrete for the diaphragm wall is required to minimise temperature rise and the risk of delayed ettringite formation. Specific mix proportions must be derived through mix trials. Options for reducing cover included;

c) An integral corrosion inhibitor to increase the threshold level of chloride at which corrosion commences. If used this may lead to a reduction in cover to about 50mm in both the diaphragm wall and the facia, depending on the dosage.

d) Stainless steel or stainless steel clad reinforcement. In this case the cover will be determined primarily by the structural requirements.

The use of epoxy coated steel is not recommended because of its variable track record and the practical difficulties in ensuring that the epoxy coating is not damage during handling, fixing and concreting. The principal risk to durability is from cracking. It is recommended that in the extreme splash zone exposure condition, crack widths are no greater than 0.2mm, the level at which it is generally accepted that some degree of self healing will occur in a humid environment. In the aggressive and moderately aggressive zones cracks widths up to 0.3mm may be permitted. To avoid the risk of corrosion in the facia an option is to use stainless steel. As corrosion of reinforcement presents the more serious risk to achieving a 70-year life, testing should focus on the chloride levels in the constituent to minimise the background level and on the chloride resistance of the concrete. Nordtest NT 443 is recommended as this provides a value of effective diffusion coefficient than may be used to validate the estimates used in the predictive model. When a mix has been approved, normal quality control measures and monitoring of the batching plant should be used to ensure that the materials and mix proportions remain within specification and tolerances. As it has been estimated that a service life of 70-years will be achieved, extensive maintenance should not be necessary. However, visual inspections are recommended at 5 year intervals for the first 20 years and at 10 year interval thereafter. A warning of potential problems may be obtained through the use of embedded corrosion monitors, or by periodically taking drilled samples to determine the extent of chloride ingress. The latter would be included as part of the visual surveys

References

1 CONCRETE SOCIETY, Guide to the design of concrete structures in the Arabian Peninsular, Report of a Concrete Society Working Party, CS163, Camberley, Surrey, October 2008, ISBN978-1-904482-47-5

2 CIRIA, Guide to the construction of reinforced concrete in the Arabian Peninsular, CIRIA report C577, ed M Walker, London, 2002, ISBN 0 966691 93 4



3 Soil/groundwater analysis 4 BAMFORTH, P B, PRICE, W F and EMERSON, M, AN international

review of chloride ingress into structural concrete, Transport Research Laboratory, Scotland, Contractor Report 359, 1997, ISSN0266-7045

5 BUILDING RESEARCH ESTABLISHMENT, Concrete in aggressive ground, BRE Bookshop, Garston, Watford WD25 9XX Third edition 2005, ISBN 1 86081 754 8

6 BS 8500-1:2006, Concrete Complementary British Standard to BS EN206-1, Part 1: Method of specifying and guidance for the specifier, ISBN 0 580 48251 0

7 BAMFORTH, P B, Enhancing reinforced concrete durability, Guidance on selecting measures for minimising the risk of corrosion of reinforcement in concrete, Concrete Society Technical Report No 61, Camberley, Surrey, 2004, ISBN 1 904482 11 2.

8 BAMFORTH, P B, Definition of exposure classes and concrete mix requirements for chloride contaminated environments, in Corrosion of Reinforcement in Concrete Construction (ed Page, Bamforth & Figg), SCI Special Publication 183, 1996, pp 176-190, ISBN 0-85404-731-X

9 BENJAMIN, S E and SYKES, J M, Chloride induced pitting corrosion of Swedish iron in ordinary Portland cement mortars and alkaline solutions: the effect of temperature, 3rd International Symposium on Corrosion of Reinforcement in Concrete, (ed Page, Treadaway & Bamforth), Published for SCI by Elsevier Science Publishers Ltd, Essex, 1990, pp 59-64, ISBN 1-85166-487-4

10 BUILDING RESEARCH ESTABLISHMENT, Delayed Ettringite Formation: In situ concrete, Information Paper IP 11/01, June 2001

11 CONCRETE SOCIETY, Alkali-silica reaction: minimising the risk of damage to concrete, Report of a Concrete Society Working Party, Technical Report No 30, 1999

12 BAMFORTH, P B, Early age thermal crack control in concrete, CIRIA Report C660, London, 2007, ISBN 978-8-86107-660-2

13 HOUSTON, J T, ATIMTAY, E AND FERGUSON, P M. Corrosion of reinforcing steel embedded in structural concrete. Research Report 112-1F, Project 3-5-68-112, Centre for Highway Research, The University of Texas of Austin, March 1972.

14 ARYA, C AND OFORI-DARKO, F K. Influence of crack frequency on reinforcement corrosion in concrete. Cement and Concrete Research, Vol. 26, No. 3, 1996, pp345-353.

15 OHNO, Y, PRAPARNTANATORN, S AND SUZUKI, K. Influence of cracking and water cement ratio on macrocell corrosion of steel in concrete, Corrosion of Reinforcement in Concrete Construction, Ed Page, Bamforth, Page, SCI, 1996 pp24-36

16 FRANCOIS, R and ARLIGUIE G, The influence of service cracking on the corrosion of reinforcement, Journal of Materials in Civil Engineering, February1998, pp 14-19.



17 EN1992-1-1 Eurocode 2, Design of concrete structures Part 1-1, General rules and rules for buildings.

18 EN1992-3:2006, Eurocode 2, Design of concrete structures Part 3: Liquid retaining and containment structures.

19 Swedish Standard, SS 137010 Concrete structures Concrete cover 20 Danish Standard, DS 411 Code of practice for the Structural use of

Concrete 21 CONCRETE SOCIETY, Guidance on the use of stainless steel

reinforcement, Report of a Concrete Society Working Party, Report No 51, Camberley, Surrey, UK, 1998

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