Sustainable Bridges – 300 Year Design Life for Second ...Sustainable Bridges – 300 Year Design Life for the Second Gateway Bridge Connal & Berndt - 1 - ... were able to build sustainable

Sustainable Bridges – 300 Year Design Life for the Second Gateway BridgeConnal & Berndt - 1 -

Sustainable Bridges – 300 Year Design Life for SecondGateway Bridge

John Connal, Industry Director, and Marita Berndt, Principal Engineer, MaunsellAustralia Pty Ltd, Melbourne, Australia

SynopsisThe Second Gateway Bridge is being built next to the original bridge to duplicate theGateway Arterial crossing of the Brisbane River. It is 1627m long with a main span of260m. The three river spans are constructed by the cast in-situ balanced cantilevermethod with segments varying from 15m to 5m deep and the approach spans are typically71m long and constructed by match cast segmental construction with epoxy joints.

The new bridge is designed and is being built with a design life target of 300 years.Designing for a very long service life has considerable economic and community benefitsand is a means of maximising the return on community investment in infrastructure.Delaying replacement and minimising maintenance costs and the disruption caused bymaintenance activities is an aim of asset owners. Designing for such a long service lifecan represent sustainable structural engineering without significant cost premium.

The durability design process for extended life requires the specific analysis of theenvironmental conditions in which the structure is placed, the strategic use of a range ofmaterials and an understanding of the means by which they deteriorate and the rate of thatdeterioration. The process involves:

Definition of the characteristics of the environment Identification of the potential deterioration mechanisms in that environment Determination of the likely rate of deterioration Assessment of the material life Definition of the required material performance Consideration of a probabilistic approach to the variability of the relevant parameters Assessment and definition of the need for further protection

This process goes beyond the deemed-to-comply approach of the codes, where materialsperformance is assumed, and the strength of the materials and the concrete cover isdefined by an adopted exposure classification. The process takes a first-principles anddeterministic approach to modelling the environmental influences and materialperformance, but rather than just adopting mean values of the governing parameters, itoverlays an understanding or assumption of their variation using known or assumedcoefficients of variation to account for the stochastic nature of the deterioration process.

This paper describes the background to the selection of a 300 year design life target, thenprovides an explanation of the design process for addressing durability, and describes theoutcomes.

1.0 IntroductionThe term “sustainable” can be defined as “avoiding depletion of natural resources”. Thevery act of building a bridge uses natural resources but is done so with the objective ofproviding a transport solution that gives enduring benefits to society that also help the


cause of conserving the planet’s natural resources. Assuming the right planning decisionhas been made to build the bridge, how do we maximise the sustainability of the bridge?

Collings (2006) has performed an environmental comparison of bridge forms that providessome indication of embodied energy and CO2 emissions in bridges, assuming these are areasonable measure of the environmental burden of the structure. The major findings fromthat comparison suggest:

The environmental burden during the construction of a bridge is approximatelyproportional to its cost, which in turn generally equates to the volume of materialsconsumed by the bridge. Larger spans use more material and have a higher burden.The use of materials with lower embodied energy and CO2 are beneficial. For mostbridge forms the concrete bridge has the lower embodied energy and CO2 emissionvalues. Whilst concrete bridges use cement that has a high embodied energy, this canbe mitigated by the use of supplementary cementitious materials as partial cementreplacement.

There is an ongoing environmental burden during the maintenance of paintwork,bearings, joints etc, and a particular spike in that burden when resurfacing isundertaken. This is increased when traffic diversions are accounted for.

The CO2 emissions during the life of the bridge from repair and maintenance can besimilar to those of the construction phase, for bridges of moderate spans.

These findings suggest sustainable bridge construction should aim to conserve naturalresources and minimise waste (be an efficient, minimalist design, avoiding extravagantarchitectural statements), minimise the embodied energy in the structure (appropriateselection of materials and material sources for the functional demands of the bridge), andhave a long life with minimal maintenance input (long service life and durable materials).Sustainability objectives for bridges are best accomplished by ensuring durable bridgeswith long service life and low maintenance inputs, that, on a whole-of-life basis, minimisematerial consumption over the long term. It is likely that such a bridge also has the lowestwhole-of-life economic cost.

Such objectives were perhaps an innate characteristic of the ancient engineers whosestructures still stand today as a testament to their sustainable design and constructionpractices.

Figure 1 The Pantheon, Rome


The Pantheon in Rome is a marvellous example of engineering sustainability. Thisstructure is almost 2000 years old. Compression is a wonderful thing! Our engineeringpredecessors knew that the materials available to them work well under compression andproportioned their structures accordingly. The dome of the Pantheon is an unreinforcedconcrete structure, maintained in compression and with no reinforcing steel to corrode.The selection of materials and structural form has produced a very long life structure.

Figure 2 Ponte Vecchio, Florence

The Ponte Vecchio in Florence was built around 1345 and is more than 600 years old.Bridge engineers have, for centuries, been capable of designing and building bridges ofvery long life. The early bridge engineers, working with stone and again keeping thestructure substantially in compression, were able to build sustainable bridges. Moremodern bridge practice seeks to be more frugal with materials, and has at its disposal anarray of more sophisticated materials, capable of resisting tension. The sustainabilitychallenge now involves a large range of complex issues.

2.0 Economics of Service LifeFenwick and Rotolone (2003) explored the economics of service life for civil infrastructurefrom a long term asset owner’s perspective. They adopted the AASHTO (2006) definitionof service life as the period of time that the structure is expected to be in operation, andconsidered a simple example of a culvert, exploring service lives between 30 and 300years. For almost all cases over a range of feasible interest rates, a service life of 300years before replacement proved to be the most economical outcome on a net presentvalue basis. Whilst this is a simple example, the results of course hold provided the ratioof initial to replacement cost is similar. For many cases, particularly infrastructure in busyurban areas, the replacement cost can be many times the initial cost when all the costs ofdiversion and traffic congestion are taken into account. From a sustainability perspective,these costs are also likely to be representative of embodied energy and CO2 emissionlevels.

The Fenwick and Rotolone (2003) study did not consider the maintenance costsassociated with the example. It could be expected that a structure designed for a verylong service life might be more durable than a structure designed for a very short servicelife. On the other hand perhaps a structure with a short life would be neglected in the


expectation that it will be replaced quicker. Either way it is likely that the structure designedfor a very long service life will have a lower annual maintenance cost.

In many circumstances, designing for 300 year service compared to say 100 years servicedoes not represent an order of magnitude increase in material consumption, embodiedenergy, CO2 emission or cost. The Fenwick and Rotolone (2003) study assumed astructure with a 300 year service life would have an initial cost 10 percent higher than onedesigned for a 30 year design life. This is a broad assumption and one hard to verify forvarious scenarios.

3.0 Service Life and Design LifeIn targeting a service life performance for a structure, the asset owner needs to be awarenot only of the initial cost of creating the structure, but its service life and the long term costof maintaining and repairing it over that service life, and finally its replacement cost. Thiscan be done on a net present value basis, but requires knowledge of the impact of thedurability performance on initial and long term costs. Those owners concerned withsustainability can take this analysis to a new level and also evaluate the options withrespect to embodied energy and CO2 emission levels. These evaluations are not usuallydone to establish a service life performance target that can be embedded in a design orproject specification.

A service life performance must be formulated into specific design criteria to provide thedesigner with objectives for the design which are then encapsulated in specificationrequirements that must be met by the builder during the construction. The codes usuallydefine a design life that implies a particular service life regarded by society as acceptable.AS5100 (2004) adopts a design life of 100 years. BS5400.1 (1988) assumes a design lifeof 120 years and AASHTO (2006) is based on a design life of 75 years. It is usual tosimply apply these codes and standards that imply a particular assumption of service lifeperformance. These are based on current materials and technologies and perhaps inhibitachieving very long service lives through the use of new materials or combinations ofmaterials and new technologies.

For the Second Gateway Bridge the service life performance was “defined” by a design lifeof 300 years. This selection was made in recognition of the size and importance of thebridge in the transport network and with the background indicated above that suggests forsuch a large asset, designing for a very long life provides the owner with the best whole-of-life value. In simplistic terms this philosophy can be described as “aim higher, it doesn’tcost much more initially, but can be significantly cheaper in the long term”.

A 300 year design life target is a new and interesting challenge. It requires the designer toexplore outside the current codes, evaluate environmental loading and establish materialperformance over a long period, requiring extrapolation of current knowledge of climateand material properties as well as the extrapolation of material deterioration models.Importantly, it also focuses the design and construction team to go beyond the standarddurability response, to explore new means of achieving an extended service life andachieve a higher level of performance.

4.0 Second Gateway Bridge DurabilityThe Second Gateway Bridge is a prestressed and reinforced concrete bridge that sits in arange of environmental exposure conditions. The durability of the concrete comprising thebridge elements is the major factor in achieving a long service life.


The project scope and technical requirements (PSTR) for the bridge specified that thedurability be applied diligently and continuously throughout the process of design,construction and throughout the maintenance period, and that the Second Gateway Bridgehave a design life of 300 years, with some replaceable sub-items having design livesranging from 20 years (wearing course) to 100 years (bearings). Design life was definedas the period assumed in design for which the structure or structural element is required toperform its intended purpose without replacement or major structural repairs.

In addition to a 300 year design life dictated for the bridge structure, several otherrequirements were mandated from the PSTR. These were aimed at ensuring a minimumlevel of durability:

Minimum B2 exposure classification (Notwithstanding B2 exposure classification isdescribed in AS5100.5 as within 1km of the coastline and the Gateway Bridge isapproximately 7 km from the coast of Moreton Bay)

Minimum 40 MPa concrete strength Minimum 20% fly ash Electrical connectivity of reinforcement in concrete piles, pile caps and piers for

possible future installation of cathodic protection, and Mix requirements for concrete in potential acid sulphate soil.

For the Second Gateway Bridge, the durability challenge is significant and varied. Thebridge comprises a concrete structure with various elements in a range of exposureconditions:

In the tidal/splash zone, where the main span piers are located in the Brisbane Riverwhich is essentially sea water

Permanently submerged, where the foundations close to the Brisbane River are belowthe tidal zone

In contact with the ground where the soil has, in places, high chloride content and acidsulphate potential, and

Air-exposed.

The philosophy adopted in meeting the extended design life is based on “building in” therequired durability at the outset, where feasible, and minimising the need to take measureslater in the life of the bridge to achieve a 300 year service life. Integral with this philosophyis the appropriate selection of high quality materials chosen to address the particulardurability issues that are posed by the range of exposure conditions.

The design process for addressing durability is summarised as follows:

Define the characteristics of the environment Identify the potential deterioration mechanisms in that environment Determine the likely rate of deterioration Assess the material life Define the required material performance Take a probabilistic approach to the variability of the relevant parameters Assess and define the need for further protection

This process goes beyond the deemed-to-comply approach of the codes, where materialsperformance is assumed, and the strength of the materials and the concrete cover is


defined by an adopted exposure classification. The process takes a first-principles anddeterministic approach to modelling the environmental influences and materialperformance, but rather than just adopting mean values of the governing parameters, itoverlays an understanding or assumption of their variation using known or assumedcoefficients of variation.

This approach is similar to the ACI Life 365 design methodology which is a deterministicmethodology, but incorporates the probabilistic components of “DuraCrete” (1999); aEuropean research project on the probabilistic performance based durability design ofconcrete structures. Both of these methodologies are summarised by Rostam (2005).

5.0 Concrete Durability DesignIn order to assess the durability risks and required preventative measures for reinforcedand prestressed concrete, the exposure environments were classified as follows:

Tidal/Splash Submerged Buried Atmospheric

The exposure environments were evaluated for aggressivity towards concrete due todeterioration mechanisms such as reinforcement corrosion caused by chloride ingress orcarbonation, sulphate attack, microbiological attack and degradation resulting from acidsulphate soil exposure. In addition, other forms of deterioration and durability risks wereconsidered such as alkali-aggregate reaction and thermal cracking. This paper focuses onthe methods used to predict chloride ingress and carbonation.

5.1 Chloride IngressIn terms of chloride ingress, the greatest concern was for pile caps in the tidal/splash zoneof the Brisbane River. The following describes the approach taken to predict chloridepenetration. The same principal was used for other chloride-bearing exposureenvironments. Modelling of chloride ingress versus depth of cover was performed. It wasassumed that all transport of chloride ions in concrete would occur by ionic diffusion. Suchdiffusion can be modelled using a solution to Fick’s Second Law of Diffusion as shown inEquation 1 below:

tDxerfcc stx .2

1., (1)

where D = diffusion coefficient of chloride through concrete (m2/s)cx,t = chloride concentration at depth x and time t (%)cs = surface chloride concentration (%)erf = numerical error functiont = time (s)

Variations on Equation 1 have been proposed to account for phenomena such as chloridebinding, convection, intermittent exposure, moisture content, and temperature. However,Equation 1 generally describes the diffusion process reasonably well for mostcircumstances in uncracked concrete.


The changes in diffusion coefficient with time can be modelled according to Equation 2 asgiven in ACI Life 365:

m

tt ttDD 0

0 (2)

where Dt = diffusion coefficient at time t (m2/s)Dt0 = diffusion coefficient at time of testing t0 (m2/s)t0 = time at test (s)t = time (s)m = age factor depending on mix proportions

Equations 1 and 2 can be combined to give Equation 3 which predicts chloride ingressversus time for a changing diffusion coefficient (Luping and Gulikers, 2007).

s

m

s

mstx

tttt

tt

Dm

xerfcc

....)1(

12

1.0

, (3)

where ts = age at start of exposure (s)

The limitation of Equation 3 is that it effectively allows the diffusion coefficient tocontinually decrease over the selected time period. However, it is generally recognisedthat after approximately 30 years the diffusion coefficient remains constant becausehydration is virtually complete. Therefore, use of Equation 3 to predict chloride diffusionover a prolonged period could potentially result in underestimation of chlorideconcentration at a given depth of cover. For a life of 100 years, Equation 3 still gives anacceptable prediction. The 300 year design life for the Second Gateway Bridge requiredconsideration of how underestimation of chloride concentration due to assumed ongoingreduction of diffusion coefficient could be avoided.A time weighted average diffusion coefficient (DTWA) was calculated assuming reduction ofthe diffusion coefficient over the first 30 years according to Equation 2, followed by aconstant value thereafter. The time weighted diffusion coefficient was calculatedaccording to Equation 4:

n

ii

n

iiti

TWA

t

tDD

1

1.

(4)

where DTWA = time weighted average diffusion coefficient (m2/s) Dti = diffusion coefficient at time ti (m2/s)


The concrete mix proposed for the pile caps was a 50 MPa grade ternary blend consistingof 30% fly ash and 21% blast furnace slag. The total cementitious content was 560 kg/m3

and the maximum water/cementitious material ratio was 0.32. In addition to improvingdurability properties such as chloride and sulphate resistance, the use of fly ash and slaghad the added benefits of reducing heat of hydration and greenhouse gas emissions of theconcrete compared with a 100% cement mix and thereby improving sustainability in termsof materials usage. Testing in accordance with NordTest NTBuild 443 was performed ontrial mixes and it was determined that the initial chloride diffusion coefficient was 1.2 - 1.4 x10-12 m2/s at 56 days. A conservative initial diffusion coefficient of 2 x 10-12 m2/s wasselected for the modelling.The Brisbane River was determined to have a chloride concentration up to 18,000 ppmwhich is similar to that of seawater. Testing of core samples from pile caps on the existingGateway Bridge indicated surface chloride concentrations of 0.4 to 0.5% by weight ofconcrete. A “worst case” surface chloride concentration of 0.65% by weight of concretewas used in the analysis and is similar to published data for marine splash environments(e.g., Lee and Chisholm, 2005; Bertolini et al., 2004; Lay et al., 2003). Furthermore, thevalue of 0.65% also corresponds with the tendency for fly ash- and slag-modifiedconcretes to have higher surface chloride concentrations.There is experimental and field data available for the age factor (e.g., Mangat and Molloy,1994; Bamforth, 1999; Thomas and Bamforth, 1999; Stanish and Thomas, 2003; Lee andChisholm, 2005; Nokken et al., 2006; Thomas et al, 2008). However, data for ternaryblends is limited. Hence, it was decided to use the ACI Life 365 guidelines and therebyuse conservative values in order to ensure long-term durability. Table 1 summarises theinput parameters used for modelling chloride ingress in the pile caps.

Table 1 Input Parameters for Chloride Diffusion Modelling of Pile Caps in Tidal/Splash Zone

SupplementaryCementitious

Material

Age FactorCalculation

AgeFactor(“m”)

Cs (% bywt.

concrete)

Dt1(m2/s)

DTWA(m2/s)

30% Fly Ash (FA),21% Blast Furnace

Slag (BFS)

m = 0.2 +0.4[(%FA/50) +

(%BFS/70)]

0.56 0.65 2 x 10-12 1.1 x 10-13

The chloride threshold concentration above which corrosion is initiated depends on factorssuch as cement chemistry, presence of supplementary cementitious materials, pH, oxygenconcentration and exposure conditions. Typically, a value of 0.06% by weight of concreteis assumed as an initiation threshold for black steel (Bertolini et al., 2004; Lay et al., 2003).For Type 316L/LN stainless steel the threshold chloride concentration was taken as 0.5%by weight of concrete based on recommendations by the ACI Life 365 model. The valueof 0.5% by weight of concrete was regarded as conservative since other data suggest thatthe value may be higher, particularly if the bars are pickled (Bertolini et al, 2004; Clemeñaand Virmani, 2004; Hurley and Scully, 2006). The threshold chloride concentration forpickled LDX 2101duplex stainless steel was assumed to be 0.35% by weight of concretein the pickled condition (Hurley and Scully, 2006) and 0.15% by weight of concrete ifunpickled (Clemeña and Virmani, 2004) .


Depth of Cover (mm)

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150Chl

orid

e C

once

ntra

tion

(% b

y w

t. co

ncre

te)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7DTWA = 1.1 x 10-13 m2/s; 30% FA; 21% BFS; t = 280 years

316 LN

LDX 2101 Pickled

LDX 2101 Unpickled

Black Steel

The proposed pile cap design was to have 150 mm minimum cover to black steelreinforcement and 75 mm minimum cover to stainless steel reinforcement. The outer layerof concrete and stainless steel was regarded as sacrificial in the structural design.Formwork to pile caps was to be kept in place for a minimum of 14 days to prevent earlyingress of chloride ions to the concrete. The predicted chloride ingress versus depth ofcover after 280 years is presented in Figure 3. A period of 280 years was selected as therequired time to corrosion initiation, following which corrosion and spalling may take placeover a subsequent 20 year period, resulting in a 300 year service life before major repairs.The corrosion threshold concentrations for the different types of reinforcement consideredare also indicated on Figure 3.

Figure 3 Predicted Chloride Profile at Year 280 (Pile Caps, Grade S50 Ternary Blend Concrete).Corrosion threshold for black and stainless steels indicated.

The predicted chloride ingress profile in Figure 3 indicated that 150 mm cover to blacksteel and 75 mm cover to LDX 2101 or 316LN stainless were likely to provide adequateprotection and prevent initiation of corrosion within 280 years. The LDX 2101 had costsavings compared with 316LN and was therefore the favoured stainless steel.

The above deterministic model has limitations because the inherent variability of concreteproperties and depth of cover in practice are not taken into account. Hence, it wasappropriate to also use a probabilistic approach to modelling of chloride ingress ratherthan the deterministic approach above. Accounting for inherent variabilities anduncertainties in parameters such as diffusion coefficient, corrosion threshold, age factor,depth of cover and surface chloride concentration enables a more realistic assessment ofthe adequacy of proposed protective measures. Statistical variations of the input variablefor chloride modelling have been documented (e.g., Bentz, 2003; Enright and Frangopol,1998). The coefficients of variation used in the Second Gateway Bridge analysis aresummarised in Table 2.

The reliability index for concrete in the tidal/splash zone with different supplementarycementitious materials was calculated. This was based on a Monte Carlo samplingtechnique using 1000 iterations. The target reliability index, , was > 1.28, equating to a


Time (years)

0 50 100 150 200 250 300

Rel

iabi

lity

Inde

x

-2

-1

0

1

2

3

4

5

6

Ternary Blend; 150 mm Cover, 5% COVTernary Blend; 150 mm Cover; 10% COVTernary Blend; 150 mm Cover; 20% COV

= 1.28

failure probability of 10%, where failure refers to initiation of corrosion within 280 yearsrather than any loss of structural capacity. The reliability index was selected based onrecommendations from Fluge (2001). Plots of the reliability index versus time for theternary mix and 150 mm depth of cover to black steel in the pile caps are given in Figure 4.The effect of changing the coefficient of variation for the depth of cover is shown.

Table 2: Input Data for Probabilistic Time to Corrosion Initiation Predictions

Variable Mean Coefficient ofVariation

(COV)Depth of cover (mm) 75

(stainless),150 (black)

0.05, 0.1, 0.2

Initial 56 day Chloride Diffusion Coefficient (m2/s) 2 x 10-12 0.25Age Factor “m” 0.56 0.25Surface Chloride Concentration (% by weight ofconcrete) (Cs)

0.65 0.2

Threshold Chloride Concentration for black (carbon)steel (% by weight of concrete) (Ct)

0.06 0.25

Threshold Chloride Concentration for LDX 2101pickled stainless steel (% by weight of concrete) (Ct)

0.35 0.25

Figure 4 Reliability Index versus Time for Grade S50 Ternary Blend Pile Cap Concrete with 150 mmCover to Black Steel. Effect of COV for cover indicated.

Figure 4 predicts that the target reliability index ( = 1.28) is met over 300 years for 150mm cover to black steel reinforcement steel with S50 ternary blend concrete even if thecoefficient of variation for depth of cover is 20%. However, it was recommended that tightconstruction quality control be implemented to reduce the coefficient of variation.


Time (years)

0 50 100 150 200 250 300

Rel

iabi

lity

Inde

x

-2

-1

0

1

2

3

4

5

6

Ternary Blend; 2101; 75 mm Cover; 5% COVTernary Blend; 2101; 75 mm Cover; 10% COVTernary Blend; 2101; 75 mm Cover; 20% COV

= 1.28

The reliability index calculations for the concrete with an outer layer of pickled LDX 2101stainless steel reinforcement 75 mm cover are presented in Figure 5.

Figure 5 Reliability Index versus Time for Grade S50 Ternary Blend Pile Cap Concrete with 75 mmCover to LDX 2101 Pickled Stainless Steel. Effect of COV for cover indicated.

Figure 5 predicts that the S50 ternary blend concrete with 75 mm cover to pickled LDX2101 stainless steel would achieve the required life. It is also noted that the predictedcorrosion rate of stainless steel will be significantly lower than that for black steel and thisfurther enhances design for durability

5.2 CarbonationAnother potential risk, particularly for concrete exposed to atmospheric conditions, iscarbonation. This was primarily of concern for the superstructure elements of the SecondGateway Bridge. The process of carbonation results in reduction of concrete pH. Thepassive iron oxide layer, which protects reinforcement from corrosion in concretestructures, is only maintained at higher pH levels. If the concrete becomes carbonated tothe depth of reinforcement, the passive iron oxide layer is no longer stable and corrosioncan occur in the presence of sufficient water and oxygen.

The rate of carbonation of concrete is related to a combination of factors. These includeCO2 concentration, moisture content of the concrete and diffusivity of hardened cementpaste. The diffusivity in turn depends on mix design (cementitious content, presence andproportion of supplementary cementing materials, water/cementitious material ratio),extent of curing, pore size and distribution within the concrete, and connectivity of pores.The presence of cracks permits local ingress of CO2 and can result in carbonation andsubsequent corrosion ahead of the main carbonation front in sound concrete.

The risk of reinforcement corrosion initiation by the process of carbonation can be reducedby using high quality concrete and sufficient depth of cover. The rate of carbonation inatmospheric environments is expressed typically expressed by Equation 5.


Time (years)

0 50 100 150 200 250 300

Dep

th o

f Car

bona

tion

(mm

)

0

20

40

60

80

100

120Carbonation Rate = 3.0 mm/yr0.5, S50 25% FACarbonation Rate = 4.5 mm/yr0.5, S40 25% FA

55 mm

45 mm

Depth of Carbonation (mm) = C.t0.5 (5)

where C = carbonation rate (mm/year0.5) t = time (years)

The carbonation rate can be expressed as a function of the controlling factors and theseare described by Lay et al. (2003) and Maage and Smeplass (2001). Other moresophisticated models of carbonation have also been developed (e.g., CEB, 1997; Saettaand Vitaliani, 2004; Steffens et al., 2002). However, the simpler Equation 5 was used inthis instance.

The concrete mix designs considered for the bridge superstructure were 40 and 50 MPawith 25% fly ash. The maximum water/cementitious material ratios of the S40 and S50concretes were 0.46 and 0.4, respectively and the minimum total cementitious contentswere 450 and 390 kg/m3, respectively. The fly ash was required to provide protectionagainst alkali-aggregate reaction. However, a disadvantage of fly ash in this case wasincreased carbonation rate for concrete with equivalent water/cementitious material ratioand total cementitious content. Published data (Ho and Lewis, 1987; Burden, 2006;Collepardi et al, 2004; McPolin, 2007; Sisomphon and Franke; 2007) for similar mixdesigns to those proposed was used to estimate the carbonation rates for predictivepurposes. The rates selected were 4.5 and 3.0 mm/yr0.5 for S40 and S50 concrete with25% fly ash, respectively, and an atmospheric CO2 concentration of 0.04% has beenassumed. The carbonation predictions are presented in Figure 6.

Figure 6 Predicted Depth of Carbonation versus Time for S40 and S50 25% Fly Ash Concretes.

The depth of carbonation predictions in Figure 6 suggested that the proposed 55 mmcover to exterior surfaces of superstructure elements would be acceptable in preventingpremature carbonation and corrosion initiation provided an S50 concrete or other mix witha maximum carbonation rate of 3.0 mm/yr0.5 was used. Reliability analysis was alsoperformed using the procedure outlined by Maage and Smelplass (2001) as part of theDuracrete work on service life prediction. Therefore, the philosophy adopted was to usehigh quality concrete with predicted carbonation rate 3.0 mm/yr0.5 and cover of 55 mm.To provide more cover was not regarded as practical or beneficial because the


superstructure comprises relatively slender elements and additional cover would onlyresult in wider crack widths under load and have a detrimental effect on durability.

5.3 Other Durability Issues

In addition to reinforcement corrosion due to chloride ingress or carbonation, durabilityassessment for the Second Gateway Bridge considered other mechanisms of deteriorationincluding sulphate attack, alkali-aggregate reaction and acid sulphate soils. The approachtaken to ensure durability was to use high quality concrete with supplementarycementitious materials, appropriate cover, and, where necessary additional protectionsuch as sacrificial steel casing. Guidelines produced by Carse (2004) for protectionagainst acid sulphate soils and the research findings by Carse and Dux (1988, 1989) onthe use of fly ash for control alkali-aggregate reaction were implemented.

6.0 Construction ControlThe long life sought by the design will not be achieved without the necessary controlapplied by the construction team to realise the design assumptions of the durability plan.The first step is to supply a concrete mix with properties equal to or better than thoseassumed in the design. In this case trial concrete mixes were tested to ensure the chloridediffusion coefficients were achievable, and the concrete cast was tested during productionto verify that the properties were achieved in the structure.

However apart from using high performance concrete, the single most important elementin achieving durable structures is to ensure the correct thickness of high quality coverconcrete. The design must ensure the detailing is practicalenough to prevent reinforcement congestion, but theconstruction must ensure the requisite cover is achieved, thecompaction of the concrete achieves a good layer of densecover concrete, and the concrete is sufficiently cured toprevent any early age cracking or incomplete hydration. Forthe Second Gateway Bridge the contractor undertook anextensive education program with a theme of 3 C’s toencourage commitment of all workers to achieve a 300 yearlife. The slogan adopted is shown in Figure 7.

Figure 7 The 3C’s Logo

7.0 ConclusionsVery long life concrete structures have been achievable through the ages and areachievable now. Structures that meet the challenge for very long life go a long waytowards sustainability and provide asset owners and the community in general with abetter return on their investment in major infrastructure.

This paper describes a design methodology that can be adopted to achieve very longservice life. The process goes beyond the deemed-to-comply assumptions of the codes,and uses a first-principles, materials science approach to durability planning.

The cost to achieve very long life is not great. In the case of the Second Gateway Bridge,the price paid to achieve a design life of 300 years rather than notionally 100 yearsinvolved:


Selection of good quality concrete. This is not a cost impost but rather business asusual and can often be based on current concrete specifications of the State roadauthorities. The extra requirement is the need to define target chloride diffusion andcarbonation coefficients, and achieve them.

Selection of greater cover to reinforcement. In this case 55mm cover was adoptedfor 50MPa superstructure concrete when the AS5100.5 code would have permitted45mm.

Use of a ternary concrete mix and selective use of stainless steel as surfacereinforcement in the splash zones.

Provision of electrical continuity for reinforcement in substructure elements in moreaggressive environments, to enable future cathodic protection to be installed ifnecessary.

Good detailing to enable compaction of concrete, along with good vibration andsubsequent curing during construction, to ensure a dense layer of cover concrete.

It takes a commitment from owners, designers and builders to achieve the goal of verylong life. This commitment was demonstrated by all involved on the Gateway Bridgeproject down to the men placing, finishing and curing the concrete. This commitmentachieves results when supported by an adequate level of supervision and guidance, plusactive and independent surveillance.

8.0 AcknowledgementThe authors wish to thank Queensland Motorways Limited, Leighton Abigroup JointVenture, and the Maunsell-SMEC Joint Venture for permission to publish this paper. Theviews expressed in this paper are those of the authors

9.0 ReferencesAASHTO, LRFD Bridge Design Specifications, Edition 3, American Association of State Highway andTransportation Officials, Washington DC, (2006).

Bamforth, P.B., Definition of Exposure Classes and Concrete mix Requirements for Chloride ContaminatedEnvironments, SCI Conference on Corrosion of Reinforcement in Concrete Construction, pp. 176-190, 1996.

Bentz, E.C., Probabilistic Modelling of Service Life for Structures Subjected to Chlorides, ACI MaterialsJournal, Vol. 100, No. 5, pp. 391-397, 2003.

Bertolini, L., Elsener, B., Pedeferri, P. and Polder, R.P., Corrosion of Steel in Concrete, Wiley-VCH,Weinheim, 2004.

British Standards Institute, BS5400, Steel, Concrete and Composite Bridges, 1988

Burden, D., The Durability of Concrete Containing High Levels of Fly Ash, PCA R&D Serial No. 2989, 2006.

Carse, A., The Design of Durable Concrete Structures for Aggressive Environments, Road SystemEngineering and Technology Forum, 2004.

Carse, A. and Dux, P., Alkali-Silica Reaction in Concrete Structures, University of Queensland, Departmentof Civil Engineering Research Report No. CE-88, May, 1988.

Carse, A. and Dux, P., Alkali-Silica Reaction in Australian Concrete Structures, Proceedings EighthInternational Conference on Alkali-Aggregate Reaction, Kyoto, pp. 25-30, 1989.


CEB, New Approach to Durability Design - An Example for Carbonation Induced Corrosion, Bulletin No. 238,Comité Euro-International du Béton, Lausanne, 1997.

Clemeña, G.G. and Virmani, Y.P., Comparing the Chloride Resistances of Reinforcing Bars, ConcreteInternational, Vol. 26, No. 11, pp. 39-49, 2004.

Collepardi, M., Collepardi, J.J., Olagot, J.J. O. and Simonelli, F., The Influence of Slag and Fly Ash on theCarbonation of Concretes, ACI SP-221: Eighth CANMET/ACI International Conference on Fly Ash, SilicaFume, Slag, and Natural Pozzolans in Concrete, Malhotra, V. (ed), 2004.

Collings, D., An Environmental Comparison of Bridge Forms, Proc. Institution of Civil Engineers, BridgeEngineering, Vol 159, Issue BE4, Dec. 2006, pp. 163-168

Enright, M.P. and Frangopol, D.M., Probabilistic Analysis of Resistance Degradation of Reinforced ConcreteBridge Beams under Corrosion, Engineering Structures, Vol. 20, No. 11, pp. 960-971, 1998.

Fenwick, J.M., and Rotolone, P., Risk Management to Ensure Long Term Performance in Civil Infrastructure,21st Biennial Conference of the Concrete Institute of Australia, Brisbane, Australia, pp. 647-656, 2003.

Fluge, F., Marine Chlorides: A Probabilistic Approach to Derive Provisions for EN 206-1, DuraNet, ThirdWorkshop, Service Life Design of Structures-From Theory to Standardisation, Tromsø, June 2001.

Fredericksen, J.M., Method for Determination of Chloride Threshold Values for Steel in Concrete, NordtestReport TR 500, 2003.

Ho, D.W.S. and Lewis, R.K., Carbonation of Concrete and Its Prediction, Cement and Concrete Research,Vol. 17, pp. 489-504, 1987.

Hurley, M.F. and Scully, J.R., Threshold Chloride Concentrations of Selected Corrosion Resistant RebarMaterials Compared to Carbon Steel, Corrosion, Vol. 62, No. 10, pp. 892-904, 2006.

Lay, S., Schießl, P. and Cairns, J., LIFECON Deliverable 3.2, Service Life Models, 2003.

Lee, N.P. and Chisholm, D.M., Durability of Concrete Structures Under Marine Exposure in New Zealand,BRANZ Study No. 145, 2005.

Luping, T. and Gulikers, J., On the Mathematics of Time-Dependent Apparent Chloride Diffusion Coefficientin Concrete, Cement and Concrete Research, Vol. 37, pp. 589-595, 2007

Maage, M. and Smeplass, S., Carbonation: A Probabilistic Approach to Derive Provisions for EN 206-1,Duranet, 2001.

Mangat, P.S. and Molloy, B.T., Prediction of Long Term Chloride Concentration in Concrete, Materials andStructures, Vol 27, pp. 338-346, 1994.

Mc Polin, D.O., Basheer, P.A.M., Long, A.E., Grattan, K.T.V. and Sun, T, New Test Method to Obtain pHProfiles due to Carbonation of Concretes Containing Supplementary Cementitious Materials, Journal ofMaterials in Civil Engineering, Vol. 19, No. 11, pp. 936-946, 2007.

Nokken, M., Boddy, A., Hooton, R.D. and Thomas, M.D.A., Time Dependent Diffusion in Concrete-ThreeLaboratory Studies, Cement and Concrete Research, Vol. 36, No. 1, pp. 200-207, 2006.

Rostam, S,. Design and Construction of Segmental Concrete Bridges fro Service Life of 100 to 150 Years,American Segmental Bridge Institute Convention, Washington DC, USA, 2005.

Saetta, A.V., and Vitaliani, R.V., Experimental Investigation and Numerical Modelling of Carbonation Processin Reinforced Concrete Structures: Part I: Theoretical formulation, Cement and Concrete Research, Vol. 34,No. 4, pp. 571-579, 2004.

Sisomphon, K. and Franke, L., Carbonation Rates of Concretes Containing High Volume of PozzolanicMaterials, Cement and Concrete Research, Vol. 37, No. 12, pp. 1647-1653, 2007.


Steffens, A., Dinkler, D. and Ahrens, H., Modelling Carbonation for Corrosion Risk Prediction of ConcreteStructures, Cement and Concrete Research, Vol. 32, No. 6, pp. 935-941, 2002.

Thomas, M.D.A. and Bamforth, P.B., Modelling Chloride Diffusion in Concrete Effect of Fly Ash and Slag,Cement and Concrete Research, Vol. 29, No. 4, pp. 487-495, 1999.

Thomas, M.D.A., Scott, A., Bremner, T., Bilodeau, A, and Day, D., Performance of Slag Concrete in MarineEnvironment, ACI Materials Journal, Vol. 105, No. 6, pp. 628-634, 2008.

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