Influence of active crack width control on the chloride penetration resistance and global warming...

Construction and Building Materials xxx (2013) xxx–xxx

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Construction and Building Materials

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Influence of active crack width control on the chloride penetrationresistance and global warming potential of slabs made with flyash + silica fume concrete

0950-0618/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.conbuildmat.2013.10.032

⇑ Corresponding author. Tel.: +32 9 264 55 22; fax: +32 9 264 58 45.E-mail address: [email protected] (N. De Belie).

Please cite this article in press as: Van den Heede P et al. Influence of active crack width control on the chloride penetration resistance and global wpotential of slabs made with fly ash + silica fume concrete. Constr Build Mater (2013), http://dx.doi.org/10.1016/j.conbuildmat.2013.10.032

Philip Van den Heede, Mathias Maes, Nele De Belie ⇑Magnel Laboratory for Concrete Research, Ghent University, Technologiepark Zwijnaarde 904, B-9052 Ghent, Belgium

h i g h l i g h t s

� Uncracked fly ash + silica fume concrete is very resistant to chloride penetration.� Uncracked fly ash + silica fume concrete has a long service life (>100 years).� The seemingly uncracked condition only exists for crack widths below 0.1 mm.� Limiting the maximum crack width allowed requires more reinforcing steel in concrete slabs.� More reinforcing steel results in a substantial increase of the slab’s global warming potential.

a r t i c l e i n f o

Article history:Available online xxxx

Keywords:Concrete crackingFly ashSilica fumeChloride penetrationService lifeLife cycle assessment (LCA)

a b s t r a c t

Service life predictions for concrete exposed to chloride-induced corrosion usually result from durabilitytests performed on uncracked concrete. Chloride migration coefficients for uncracked concrete shouldonly be used if the structure can be considered as uncracked. The seemingly uncracked condition requirescrack widths below 0.1 mm. The extra reinforcing steel to achieve this in concrete slabs, results in a30–43% increase of the global warming potential. Fly ash + silica fume concrete may be preferred becauseof its low 28 day migration coefficient (3.4 � 10�12 m2/s), its long service life (>100 years) and itsautogenous healing ability.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Recent sustainability studies show that concrete’s global warm-ing potential (GWP) is mainly governed by its binder composition,strength and service life [1,2]. With respect to the latter, research-ers are advised to implement data from durability tests into mod-els that simulate the main deterioration mechanism of theenvironment to estimate the concrete’s life span. When lookingat chloride-induced corrosion, the end of service life is oftenequaled with steel depassivation. For this failure event the modelof Fib Bulletin 34 [3] based on Fick’s second law, looks straightfor-ward. Experimental chloride migration coefficients can be used toestimate when the critical chloride concentration will reach the re-bars and end service life. However, this approach does not take intoaccount the unavoidable presence of cracks in concrete due to themechanical loads applied. True, a structure should always be de-signed as such that the maximum allowed crack width (0.3 mm

for a submerged marine environment according to Eurocode 2[6]) is not exceeded. Nevertheless, even 0.3 mm wide cracks inthe tensile zone of a concrete slab – the case study of this paper– can easily extend beyond the location of the rebars and thereforeoffer direct pathways for chlorides. As a consequence, it makessense to limit the maximum crack width allowed even more. Ofcourse, this design approach will have its implications on theamount of reinforcing steel needed and therefore on the environ-mental impact of the slab.

In this research, we conducted chloride migration tests on con-crete representative mortar samples containing a crack of 0.3, 0.2and 0.1 mm in width. This was done to see whether crack widthreduction could decrease the chloride penetration around the cracksignificantly. If not, it may be necessary to aim for very fine crackwidths that can heal autogenously. Jaroenratanapirom and Saha-mitmongkol reported a fast and complete natural crack closingfor crack widths 60.05 mm in Ordinary Portland cement (OPC)mortars, OPC + 10% silica fume (SF) mortars and OPC + 30% flyash (FA) mortars [7]. Since the binder of the mortar compositionsstudied in this paper consisted of a combination of the same

arming

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2 P. Van den Heede et al. / Construction and Building Materials xxx (2013) xxx–xxx

materials (50% OPC, 40% FA and 10% SF), it is certainly relevant toconsider the 0.05 mm crack width criterion here as well. In a nextresearch phase, concrete slabs made with traditional concrete andfly ash + silica fume concrete were designed according to the mostsuitable crack width criterion. Then, a full probabilistic service lifeprediction cf. Fib Bulletin 34 [3] was performed in the Comrel soft-ware [4], followed by a life cycle assessment (LCA) in the SimaProsoftware [5]. These calculations were done to see the effect of crackwidth limitation on the GWP of the studied concrete slabs.

2. Materials and methods

2.1. Concrete representative mortar mixes

Two concrete compositions were studied (Table 1). Mix T(0.45) has a cementcontent and water-to-cement (W/C) ratio of 340 kg/m3 and 0.45, respectively. Itis seen as the appropriate OPC reference concrete for exposure class XS2 [8]. Theexposure class corresponds with an environment where steel reinforced concreteis permanently submerged in sea water. As a consequence, the concrete is exposedto chlorides and this can induce steel corrosion. The other concrete mix is charac-terized by the same total binder content (340 kg/m3) as the OPC reference. Onlynow it consisted of three different cementitious materials: 50% Portland cement,40% FA and 10% SF. The water-to-binder ratio (W/B) equaled 0.35 to ensure astrength class at least equal to the strength class of the OPC reference (C50/60).By doing so, composition SF(0.35) was characterized by a strength just one strengthclass higher (C55/67). The environmental consequences associated with differencein strength between the two mixtures under investigation were taken into accountby choosing a strength related functional unit for LCA (see Section 2.7.1). Because ofits high cement replacement level, cement related greenhouse gas emissions couldbe reduced significantly with the latter concrete mix. Therefore, it is seen as apotentially ‘green’ concrete type.

An equivalent mortar mix was designed for the two concrete compositions inaccordance with the Concrete Equivalent Mortar (MBE) method [9]. Within aMBE mortar mix, the gravel mass fractions of the corresponding concretemix – in this case fgravel 2/8 and fgravel 8/16 – are replaced with the amount of sandDfsand 0/4 that has the same specific surface. This sand fraction can be calculatedby means of Eq. (1) in which Sgravel 2/8, Sgravel 8/16 and Ssand 0/4 represent the specificsurface areas of the applied coarse aggregates and sand used in the studied concretemixes. The ratios 2/8, 8/16 and 0/4 refer to the minimum and maximum aggregatesizes in mm. The first figure of the ratio represents the lower sieve size, while thesecond figure is the upper sieve size.

Dfsand 0=4 ¼fgravel 2=8 � Sgravel 2=8 þ fgravel 8=16 � Sgravel 8=16

Ssand 0=4ð1Þ

Table 1Concrete compositions, specific surface areas and water absorption coefficients of thesand and aggregates and MBE mortar mix proportions.

Concrete composition T(0.45) SF(0.35)

Sand 0/4 (kg/m3) 778 791Gravel 2/8 (kg/m3) 676 687Gravel 8/16 (kg/m3) 447 454CEM I 52.5 N (kg/m3) 340 170Fly ash (kg/m3) 0 136Silica fume (kg/m3) 0 34Water (kg/m3) 153 119W/B 0.45 0.35FA/B (%) 0 40SF/B (%) 0 10

Sand/aggregate properties Sand 0/4 Gravel 2/8 Gravel 8/16

Specific surface area (m2/kg) 4.889 0.398 0.194Absorption coefficient 0.008 0.018 0.011

MBE composition MBE T(0.45) MBE SF(0.35)

Sand 0/4 (kg/m3) 850.8 864.9CEM I 52.5 N (kg/m3) 340 170Fly ash (kg/m3) 0 136Silica fume (kg/m3) 0 34Water (kg/m3) 136.5 102.2Superplasticizer (ml/kg B) 3.0 14.0W/B 0.40 0.30FA/B (%) 0 40SF/B (%) 0 10Strength class C50/60 C55/67

Please cite this article in press as: Van den Heede P et al. Influence of active cracpotential of slabs made with fly ash + silica fume concrete. Constr Build Mater

Replacing the gravel 2/8 and gravel 8/16 by the much finer sand 0/4 obviouslyaffects the water demand of the mortar. As a result, its required water amountneeds to be adjusted in accordance with the difference in water absorption betweenthe gravels and the sand. This can be done with Eq. (2):

Dfwater ¼ �fgravel 2=8 � Agravel 2=8 � fgravel 8=16 � Agravel 8=16 þ Dfsand 0=4 � Asand 0=4 ð2Þ

with Agravel 2/8, Agravel 8/16 and Asand 0/4 the water absorption coefficients of the coarseaggregates and the sand. The measured water absorption coefficient and the specificsurface areas are shown in Table 1. The resulting two MBE mortar compositions canbe found there as well. By following this method the workability of the MBE mortarsshould be identical to the workability of the corresponding concrete mixtures. Theuse of MBE mortar instead of concrete normally reduces the material cost and effort[9].

2.2. Manufacture of MBE mortar with an artificial crack

15 Cylindrical specimens (diameter: 110 mm, height: 53 mm) were made foreach of the two MBE mortar mixes in PVC tube moulds: 3 samples without crackplus 3 � 4 samples containing an artificial crack as a result of putting thin metalplates with a nominal thickness of 0.1, 0.2 and 0.3 mm at a depth of 15 mm inthe cylindrical moulds just before casting. Fig. 1a shows a schematic of the mouldsetup with the metal plate fixed at the desired crack depth cf. Mu [10].

Note that artificial cracks created by means of thin metal plates are differentfrom more naturally induced cracks induced by mechanical loading. Both tech-niques are in use and have their advantages and disadvantages. By means of thinmetal plates it is indeed not possible to reproduce a concrete crack which is realisticin all its properies. However, it is seen as a very convenient way to study the effectof one crack property in particular, being the crack width (which is the main crackproperty considered in crack controlled slab design). The reproducibility of crackscreated as such is high. On the other hand, displacement steered mechanical load-ing to create more natural cracks does not always guarantee the same predefinedcrack width. Moreover, with the latter method it is difficult to ensure that the crackdoes not go all the way through the specimen. This condition is required to be ableto conduct the chloride migration test. With thin metal plates fixed at a certainheight in the sample mould the crack depth is rather easy to control. For this casestudy we therefore adopted the thin metal plate technique. Though, one should re-main aware of the differences between these artificial cracks and naturally inducedcracks. The walls of the voids created with thin metal plates should be considered ascast surfaces. These surfaces are subject to the so-called wall effect which meansthat the more fine (usually cementitious) materials will be present in the vicinityof the crack walls. Its unhydrated fraction can still react later on and initiate autog-enous healing. However, this may be the only effect that favors this mechanism forthe artificial cracks. It is also unknown if the fraction of unhydrated materials nearthe crack walls is sufficient to induce full closure of the crack. For naturally inducedcracks on the other hand, there can be several beneficial effects. There, the crackwidth can seriously vary length of a crack. The crack tortuosity and crack wallroughness will evidently be higher [11]. Moreover, cracks will contain more con-crete particles broken from the surface due to cracking [12]. All these conditionscontribute to a partial blocking of the crack followed by the autogenous healingphenomenon. These favorable conditions are not present in the artifical cracks. Gi-ven these differences, a more detailed comparison between artifically and naturallycracked specimens in relation to their autogenous healing capacity would certainlybe relevant. This investigation is for the moment still ongoing.

After casting, the specimens were kept at a constant temperature and relativehumidity (RH) of 20 �C and 95%, respectively. The metal plates were carefully re-moved from the samples after approximately 12 h whereupon the cylinders weredemoulded. From then on, they were stored again under the same conditions untilthe age of 28 days.

2.3. Microscopic crack width measurements

After 28 days, the obtained crack widths were measured after mechanical flat-tening of the cylinders’ troweled surfaces and on the saw cut perpendicular to thecrack of the fourth cylinder of each cracked series. The latter samples were onlyused for the evaluation of the cross-sectional crack width and not exposed to chlo-rides. All crack width measurements were done on micrographs taken with a LeicaS8 APO stereo microscope (SM) (magnification: 20�) while using the LAS 3.7software.

After the cross-sectional crack width evaluation with the stereo microscope, thecracked area of the non-exposed MBE mortar SF(0.35) with the 0.1 mm wide crackwas also subjected to scanning electron microscope (SEM) analysis to study the par-tial closing of the crack more in detail (see Section 3.1). By then, the sample was196 days old. Three 20 � 20 � 10 mm3 prisms containing a cross-section of thecrack, were cut from one cylinder halve and then put in an ultrasonic bath with iso-propanol to remove all loose particles inside the crack. Afterwards, the sampleswere vacuum dried for one week and gold coated by means of a Baltec SCD030Sputter Coater before being examined in a FEI QUANTA 200F SEM at an acceleratingvoltage of 20 kV. Secondary electron imaging was used for electron micrography.

k width control on the chloride penetration resistance and global warming(2013), http://dx.doi.org/10.1016/j.conbuildmat.2013.10.032


Fig. 1. Mould setup for creating artificial cracks in the mortar samples (a), expected AgNO3 colour change boundary in a cracked (b) and a seemingly uncracked (c) samplesection.

P. Van den Heede et al. / Construction and Building Materials xxx (2013) xxx–xxx 3

2.4. Chloride migration tests

A rapid chloride migration test was done on the uncracked MBE mortar samplesin compliance with NT Build 492 [13]. First, the cylindrical specimens were vacuumsaturated in a 4 g/l Ca(OH)2 solution. After 18 ± 2 h of immersion in this solution,the specimens were fixed inside silicon rubber sleeves with a 0.3 N NaOH (anolyte)solution on top. The bottom surface of the samples in the sleeves was brought incontact with a 10% NaCl solution (catholyte). Then, an external electrical potentialwas applied axially across each cylinder, which forces the chloride ions to migrateinto the specimens. After 24 h the specimens were removed from the sleeves andsplit axially, whereupon a 0.1 M silver nitrate solution was sprayed onto the freshlysplit sections. When the white silver chloride precipitation had become clearly vis-ible, the penetration depth was measured from the center to both edges at intervalsof 10 mm. From the chloride ingress obtained, a non-steady state migration coeffi-cient can be calculated with Eq. (3):

Dnssm ¼0:0239ð273þ TÞL

ðU � 2Þt xd � 0:0238

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffið273þ TÞLxd

U � 2

r !ð3Þ

where Dnssm, U, T, L, xd and t represent the non-steady state migration coefficient(�10�12 m2/s), the absolute value of the applied voltage (V), the average value ofthe initial and final temperatures in the anolyte solution (�C), the thickness of thespecimen (mm), the average value of the penetration depths (mm) and the test dura-tion (h), respectively.

The cracked MBE mortar specimens went through the same test procedure ex-cept for the fact that the external electrical potential was imposed for only 4 h. Thisis much less than the normal 24 h test duration. The short test period was chosen tomake sure that the overall chloride ingress would not be more than 13 mm, theaverage depth of the artificially induced cracks. Obviously, in case the crack widthis too wide, the chloride penetration close to the crack would be higher than 13 mm(Fig. 1b). The 10% NaCl solution would almost immediately reach the deepest pointof the crack and chloride migration would start from there on. For each of the spec-imens containing a 0.1, 0.2 or 0.3 mm crack, it was evaluated whether the chloridepenetration around the crack extended much beyond its deepest point. If not, thespecimen could be considered as uncracked (Fig. 1c) and the Dnssm value measuredfor the uncracked specimen would also be valid. As such, service life predictioncould be done using Dnssm values measured on uncracked concrete. However, it alsomeans that a concrete structure needs to be designed according to the stricter max-imum crack width criterion.

2.5. Crack width reduction in concrete slabs

When designing a concrete slab for a given mechanical load, an evaluation ofthe expected crack width in the service limit state is imperative. Eurocode 2 [6] pro-vides an Eq. (4) to calculate a characteristic crack width wk (mm) for the structuralelement under investigation.

wk ¼ 3:4 � c þ 0:425 � k1 � k2 �UAs

Ac;eff

0@

1A � rs

Es� kt �

fct;effAs

Ac;eff� Es� 1þ a � As

Ac;eff

� �0@

1A ð4Þ

with c: concrete cover d + 10 mm, factor k1: coefficient accounting for the bond prop-erties of the reinforcing steel (=0.8 in case of high bond), factor k2: coefficientaccounting for the strain distribution (=0.5 in bending mode), U: diameter of the re-bar, As: cross-sectional area of the steel (mm2), Ac,eff: effective cross-sectional area ofthe concrete in the tensile zone (mm2), rs: steel stress (N/mm2), Es: design value forthe steel’s modulus of elasticity, kt: factor accounting for the load duration (=0.4 for along-term mechanical load), fct,eff: concrete’s effective tensile strength (N/mm2), a:effective ratio of the moduli of elasticity for the steel (Es) and the concrete (Ecm).


Eurocode 2 [6] imposes a characteristic crack width of 0.3 mm for exposureclass XS2. For a given diameter of the rebars, the characteristic crack width canbe reduced by increasing the overall steel cross-sectional area. Thus, more rebarswould be needed to meet stricter crack width criteria.

2.6. Service life prediction

Fib Bulletin 34 [3] is a design code providing the necessary models for a fullprobabilistic service life prediction. The design approach consists of defining a suit-able limit state Eq. (5) containing the necessary load and resistance variables for thedeterioration mechanism under investigation, in this case chloride-inducedcorrosion:

Ccr ¼ C0 þ CS;Dx � C0ð Þ � 1� erfd� Dx

2 �ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiDapp;C � t

p" #

ð5Þ

with Ccr: the critical chloride content (wt.%/binder), C0: the initial chloride content(wt.%/binder), CS,Dx: chloride content at depth Dx (wt.%/binder), d: concrete cover,Dx: depth of the convection zone (mm), t: time (years), erf(.): error function andDapp,C: apparent coefficient of chloride diffusion through concrete (mm2/years).The latter coefficient can be obtained from the experimental non-steady state migra-tion coefficient using Eq. (6):

Dapp;C ¼ exp be1

Tref� 1

Treal

� �� DRCM;0 � kt �

t0

t

� �a

ð6Þ

with be: a regression variable (K), Tref: the standard test temperature (K), Treal: thetemperature of the structural element or the ambient air (K), DRCM,0: the non-steadystate chloride migration coefficient (mm2/years), kt: a transfer parameter, t0: a refer-ence point of time (years), t: time (years) and a: the ageing exponent. A combinationof (5) and (6) enables an estimation of the time to steel depassivation. Table 2 gives aquantification of all the input parameters which are normally used in the model.

Note that the critical chloride content (1.9 wt.%/binder, cf. [14]) differs from the0.6 wt.%/binder prescribed by Fib Bulletin 34 [3]. This value should be valid for sub-merged OPC concrete with W/C ratios ranging between 0.5 and 0.4. A parameterstudy conducted by Van den Heede et al. [2] showed that 1.9 wt.%/binder is proba-bly a more realistic than 0.6 wt.%/binder. Since it is still uncertain whether the crit-ical chloride content increases or decreases in the presence of FA or othercementitious materials [15], the same Ccr value was adopted for both T(0.45) andSF(0.35). Nevertheless, further investigation on the actual Ccr value for each con-crete type is imperative. Therefore, specimens with embedded steel reinforcementbars would need to be manufactured for corrosion potential monitoring of the rein-forcing steel as a function of the Cl� concentration near the steel surface. However,within the early stages of optimizing a potentially ‘green’ concrete type such as thesuggested SF(0.35) composition, these long-term experiments were not yet per-formed. An experimentally determined and mix specific critical chloride thresholdvalue for service life prediction would certainly be of value once the focus can shiftfrom studying a representative mortar of the concrete to the characterization of thefully optimized concrete mix design.

The applied ageing exponents are the values suggested by Fib Bulletin 34 [3] forOPC (0.3) and FA (0.6) concrete in general. These values are not mix specific. It is notsure whether the latter value is also valid for FA + SF concrete. In Duracrete [14], acharacteristic ageing exponent of 0.62 is given for concrete that contains SF. Fornow, we decided to use the lowest of these two values as the ageing exponent ofFA + SF concrete. To estimate the actual ageing exponents per concrete mix, itwould be necessary to perform chloride diffusion tests on specimens that have beenimmersed in realistic sea water solutions. Only if one has obtained the correspond-ing chloride diffusion coefficients at minimum three different ages, it is possible toquantify the ageing exponent by means of non-linear regression analysis. The latterrather time consuming methodology was already applied in Van den Heede et al.



Table 2Quantification of the input parameters for the probabilistic limit state function as defined by (5) and (6).

Parameter Distribution Mean Stdv Lower bound Upper bound

Ccr (wt.%/binder) Beta 1.9 0.15 0.2 2.0C0 (wt.%/binder) Constant 0 – – –CS,Dx (wt.%/binder) Normal 3.0 0.8 – –d (mm) Lognormal 40 8 – –Dx (mm) Constant 0 – – –be (K) Normal 4800 700 – –Tref (K) Constant 293 – – –Treal (K) Normal 283 5 – –DRCM,0 (mm2/yrs) Normal 336.2 (MBE T(0.45)) 32.2 – –

107.5 (MBE SF(0.35)) 20.2 – –kt Constant 1 – – –t0 (yrs) Constant 0.0767 (28d) – – –a Beta 0.30 (OPC) 0.12 0.0 1.0

0.60 (FA) 0.15 0.0 1.0


[16] to estimate the ageing exponent of high-volume fly ash concrete (a = 0.4). Itsexperimental determination for FA + SF concrete is for the moment still ongoingat our laboratory.

The probabilities of failure (Pf) and reliability indices (b) that result from thelimit state equation defined by (5) and (6) were calculated using the First OrderReliability Method (FORM) available in the probabilistic Comrel software [4].According to Fib Bulletin 34 [3], these parameters need to meet the requirementsfor the depassivation limit state (Pf 6 0.10 and b P 1.3) to qualify for use in a XS2environment.

2.7. Life cycle assessment

In compliance with ISO 14,040 [17], the LCA consisted of four major steps: thedefinition of goal and scope, the inventory analysis, the impact analysis and theinterpretation.

2.7.1. Definition of goal and scopeThis LCA was conducted to quantify the reduction in greenhouse gas emissions

that result from replacing 50% of concrete’s OPC with 40% FA and 10% SF fume whiletaking into account the differences in strength and durability with OPC concrete.Therefore, a concrete slab located in a submerged marine environment carrying avariable load of 5 kN/m2 was chosen as functional unit. The strength classes ofthe two studied MBE mortars (Table 1) were assumed to be similar to the strengthclasses of their corresponding concrete compositions. The same strength classeswere considered in the design of the concrete slab (span: 5 m, width: 1 m). Ribbedsteel bars with a diameter of 16 mm and steel quality 500 were used as reinforce-ments. All design calculations were done in accordance with Eurocode 2 [6]. An-other reason for choosing a concrete slab as functional unit is because this typeof structural element allows for a crack controlled design. As a result, it can be cal-culated how much extra reinforcing steel would be needed to obtain a slab wherethe concrete in the tensile zone would behave as uncracked cf. the drawing inFig. 1c. Only for a slab designed as such, a service life prediction based on the chlo-ride migration tests performed on uncracked concrete samples would be valid.Thus, a quantification of the additional environmental impact attributed to thecrack-controlling efforts done to enable the use of chloride migration coefficientsfor uncracked concrete as input to the service life prediction models would be veryuseful. With our functional unit choice this aspect can be included as well.

2.7.2. Inventory analysisPer concrete constituent, the necessary inventory data were collected from the

Ecoinvent database [18]. Their proper short descriptions as mentioned in the data-base together with the amounts used to manufacture 1 m3 of each concrete mix, areshown in Table 3.

Mean values and standard deviations for the sand and aggregates were calcu-lated from the amounts of each material needed according to Fuller’s optimal par-ticle size distribution curve for three deliveries of sand and aggregates to ourlaboratory. The probabilistic distribution of these amounts was assumed to be nor-mal. The required amounts of cement, FA, SF, water and superplasticizer (SP) forconcrete manufacture were assumed to be accurately weighed and therefore con-sidered as constants. For the allocation of impacts attributed to the industrial by-products FA and SF, the economic allocation coefficients as proposed by Chenet al. [19] and Chen [20] were applied (Table 4). For the former by-product, thisis 1.0% of the impact of the coal fired electricity production corresponding withthe production of 1 kg FA. For the latter by-product this is 4.8% of the impact of sil-icon metal production corresponding with the production of 1 kg SF. SP inventorydata were obtained from an environmental declaration published by the EuropeanFederation of Concrete Admixture Associations [21]. The transport of eachconstituent to the concrete plant was not incorporated in the LCA since its environ-


mental impact is always very case specific. With respect to the steel reinforce-ments the following Ecoinvent inventory data were used: ‘Reinforcing steel, atplant/RER U’.

For all Ecoinvent data, unit processes (U) were used in the modelling of eachconcrete mix. This was done to enable a full probabilistic uncertainty analysis ofthe calculated environmental scores using Monte Carlo simulation.

2.7.3. Impact analysis and interpretationThe IPCC 2007 GWP 100a impact method was used to calculate the Global

Warming Potential (GWP) expressed in CO2 equivalents for a timeframe of100 years. All calculations were done in the LCA software SimaPro 7.3.3 [5].

3. Results and discussion

3.1. Microscopically measured crack widths

Table 5 shows that the observed crack widths at the surface didnot differ much from the thicknesses of the thin metal plates (0.1,0.2 and 0.3 mm). The cross-sectional crack widths measured onone cylinder of each cracked series further confirm this good matchfor both mortar compositions. Note that the cross-sectional crackwidth observed on the MBE SF(0.35) sample with a 0.1 mm crackhad a rather high standard deviation on the individual values(stdv.: 0.08 mm). This is mainly due to the fact that the measuredcrack widths – recorded over the entire crack depth at regular dis-tances of 1 mm – quite often equaled zero. At several places along-side its cross-sectional area, the SM micrographs (age: 28 days)showed crack closure which could not be attributed to a mere fill-ing by loose particles originating from the mechanical surface flat-tening (Fig. 2a). The additional SEM micrographs that were takenafterwards (age: 196 days) further confirm this observation(Fig. 2b). The (partial) crack closure practically always consistedof a bridging with solid material. In the second SEM micrograph,a very fine crack (<5 lm in width) seems to be going through thesolid crack filling material. Re-cracking was quite often observedduring the SEM analysis. This phenomenon was probably inducedby the sawing operations done to obtain the 20 � 20 � 10 mm3

prismatic SEM samples from the MBE SF(0.35)_0.1 mm cylinderhalve. The fact that this very fine crack is going through the wholelocal bridging of the crack demonstrates that it was once a solidcrack filling material.

The partial crack closure observed both with SM and SEM mayindicate that for a crack width of ±0.1 mm, partial autogenouscrack healing can occur. Since this phenomenon was not observedon the MBE T(0.45) sample with a 0.1 mm crack, the partial autog-enous healing that was detected is most likely induced by furtherhydration of the unreacted alternative binders (FA and SF) presentin MBE SF(0.35). Jaroenratanapirom and Sahamitmongkol reportedfast natural healing of cracks P0.05 mm in OPC mortars with 10%silica fume [7]. Fig. 2 shows that this may also be the case for theMBE SF(0.35) mortar containing 50% OPC, 40% FA and 10% SF.



Table 3Overview of the life cycle inventory data used per concrete mix.

Material data (kg) Distribution T(0.45) SF(0.35)

Sand, at mine/CH U Normal 766 ± 44 778 ± 45Gravel, round, at mine/CH U Normal 1135 ± 109 1154 ± 111Portland cement, strength class Z 52.5, at plant/CH U Constant 340 170Fly asha Constant 0 136Silica fumeb Constant 0 34Tap water, at user/CH U Constant 153 119Superplasticizer (EFCA 2006) Constant 1.1 5.2

Processing data (kWh) Distribution T(0.45) SF(0.35)

Electricity, low voltage, production BE, at grid/BE U Constant 3.83 3.83

a Patially contains Ecoinvent data: ‘Electricity, hard coal, at power plant/BE U’, through allocation.b Partially contains Ecoinvent data: ‘MG-silicon, at plant/NO U’, through allocation.

Table 4Economic allocation coefficients for FA and SF cf. Chen et al. [19] and Chen [20].

Product Mass produced Market price Allocation by economic value

Electricity 1 kWh* 0.1 €/kWh 99.0%FA 0.052 kg 20 €/t 1.0%a

Si metal 1 kg 1200 €/t 95.2%SF 0.15 kg 400 €/t 4.8%b

* Equivalent to 0.367 kg of hard coal used to produce electricity.a Allocation percentages applied to the following Ecoinvent data: ‘Electricity,

hard coal, at power plant/BE U’.b Allocation percentages applied to the following Ecoinvent data: ‘MG-silicon, at

plant/NO U’.


3.2. Measured chloride migration coefficients

MBE mortar SF(0.35) is characterized by a 28 day non-steadystate chloride migration coefficient Dnssm of 3.4 ± 0.6 � 10�12

m2/s. This is only around one third of the 28 day Dnssm value(10.7 ± 1.0 � 10�12 m2/s) obtained for OPC reference MBE T(0.45).Thus, when uncracked, the former mortar composition is muchmore resistant to chloride penetration than the latter. In terms ofservice life, this finding suggests that less rehabilitation actions(repair/replacement) will be needed for an uncracked FA + SF fumeconcrete slab exposed to seawater within a 100 years timeframe.To evaluate this quantitatively, the Dnssm values were expressedin mm2/years and used as DRCM,0 input (Table 2) to the service lifeprediction model defined by Eqs. (5) and (6).

3.3. Determination of the maximum crack width allowed

Fig. 3 shows one representative photo of the chloride penetra-tion for each mortar-crack combination. Per mortar mix and crack

Table 5Microscopic crack width measurements of MBE T(0.45) and MBE SF(0.35).

MBE T(0.45)

0.1 mm Flattened surface Cross-section

n 33 12Mean 0.08 mm 0.09 mmStdv. 0.01 mm 0.01 mm






width all broken cylinder surfaces (n = 6) looked similar. The chlo-ride penetration always extended beyond the deepest point of thecrack. This was the case for all three nominal crack widths (0.1, 0.2,0.3 mm) that were experimentally assessed in this research, alsowhen already some partial autogenous healing had occurred forthe 0.1 mm crack of MBE SF(0.35). In other words, none of thestudied cracked samples could be considered as uncracked cf.Fig. 1c. This means that within the slab design, the maximum crackwidth allowed should be reduced even more, preferably to a valuethat ensures complete autogenous healing of the crack, i.e. the0.05 mm crack width suggested by Jaroenratanapirom and Saha-mitmongkol [7]. For the latter crack width, complete crack closurewas observed within 12 days, which is much earlier than the age ofour cracked samples at the time of inspection with SM (at 28 days)and SEM (at 196 days).

Obviously, it would be interesting to manufacture another ser-ies of cracked samples containing a 0.05 mm wide crack and sub-ject them to chloride migration tests as well. However, it isdifficult to create these very fine artifical cracks in mortar withthe current mould setup (Fig. 1a). Metal plates with a thicknessof only 0.05 mm have almost no stiffness. As a result, it is practi-cally impossible to maintain them at a fixed position in the freshmortar. Moreover, their intact removal from the hardened mortaris also not evident. More research is needed on how this could beachieved in the future.

3.4. Concrete slab dimensioning

When following Eurocode 2 [6], the slab made with T(0.45) con-crete should have a thickness of 150 mm and contain 6 rebars witha 16 mm diameter. For the slab made with SF(0.35), the slab thick-ness can be reduced to 140 mm due to the higher strength class ofthe concrete. The required number of rebars remained the same.Under these conditions, the estimated crack widths in the tensile

MBE SF(0.35)









Fig. 2. Partial crack healing of the 0.1 mm wide crack in MBE SF(0.35) as observed on SM and SEM micrographs.

Fig. 3. Observed AgNO3 colour change boundary on the cracked samples of MBE T(0.45) and MBE SF(0.35). (For interpretation of the references to colour in this figure legend,the reader is referred to the web version of this article.)

Table 6Influence of active crack width control on the slab dimensioning, service life andglobal warming potential of T(0.45) and SF(0.35) concrete.

Without crack width control T(0.45) slab SF(0.35) slab

Slab thickness 150 mm 140 mm


zone equaled 0.18 and 0.21 mm, respectively. To reduce the theo-retical crack width of the two slabs to 0.05 mm – the value pro-posed by Jaroenratanapirom and Sahamitmongkol [7] – thenecessary number of rebars would need to be increased to 14and 15, respectively (Table 6).

Number of rebars 6 � 16 mm 6 � 16 mmGWP slab 279 ± 30 kg CO2eq 227 ± 14 kg CO2eq

Service life 25 years* >100 years*

Number of slab replacements 3* 0*

GWP slab + replacements 1120 ± 116 kg CO2eq* 227 ± 14 kg CO2eq

*

With crack width control T(0.45) slab SF(0.35) slab

Slab thickness 150 mm 140 mmNumber of rebars 14 � 16 mm 15 � 16 mmGWP slab 367 ± 29 kg CO2eq 325 ± 18 kg CO2eq

Service life 25 years >100 yearsNumber of slab replacements 3 0GWP slab + replacements 1460 ± 119 kg CO2eq 325 ± 18 kg CO2eq

* in case concrete could be considered as uncracked (cf. laboratory specimens),which is not the case in reality.

3.5. Service life prediction

Comparing the reliability indices b and probabilities of failure Pf

with the prescribed criteria (b P 1.3 and Pf 6 0.1) leads to the fol-lowing findings. It shows that the estimated service life of MBET(0.45) is much lower than 100 years (25 years). While for MBESF(0.35) the predefined service life of 100 years (b = 2.7 andPf = 0.003) is exceeded by far (Table 6). This means that a concreteslab made of T(0.45) concrete will need a certain number of reha-bilitation actions within the 100 years time span. For this casestudy it was assumed that rehabilitation comprised completereplacement of the slab, and not a labor intensive local repair.The concrete and steel needed for three T(0.45) slab replacementsneed to be included in the LCA study to have a correct, durabilityrelated environmental impact calculation. Since a 100 years servicelife seems easily achievable for composition SF(0.35), only the con-


crete volume for the initial manufacture of the slab needs to beconsidered in the LCA.




3.6. Life cycle assessment

When simply looking at the initial production stage – thus,without consideration of the service life aspect – it is clear thatthe SF(0.35) concrete slab (227 ± 14 kg CO2eq) has a lower carbonfootprint (�19%) than the T(0.45) concrete slab (279 ± 30 kg CO2eq)(Table 6). This can mainly be attributed to the high cement replace-ment level (50%) applied in the former composition. Replacing 50%of the cement with supplementary cementitious materials (40%FA + 10% SF) does not result in 50% reduction of the GWP becausestill considerable environmental impacts were assigned to FA andSF through economic allocation. The environmental benefit forthe SF(0.35) concrete in the production stage adds onto the benefitthat can be achieved with its good service life performance(replacement-free for more than 100 years). This is in contrast withfor instance ultra-high performance concrete (UHPC) for which theproduction related CO2 emissions are usually higher when com-pared with traditional concrete. Only when considering the longservice life of UHPC, there is a substantial decrease in GWP [22,23].

Without active crack width limitation, the GWPs of the T(0.45)and SF(0.35) concrete slabs (inclusive the required slab replace-ments within a 100 years life span) amounts to 1120 ± 116 and227 ± 14 kg CO2eq, respectively. Increasing the amount of reinforc-ing steel to 14 and 15 rebars to achieve a characteristic crack widthof only 0.05 mm, will increase the environmental score substan-tially to 1460 ± 119 kg CO2eq (+30%) and 325 ± 18 kg CO2eq

(+43%), respectively (Table 6).

4. Conclusions

In uncracked condition, a concrete representative FA + SF mor-tar is characterized by a very low chloride migration coefficientafter 28 days (3.4 � 10�12 m2/s) which highly contributes to theestimated long service life (>100 years) of the material. As a result,a slab made with the corresponding FA + SF concrete has a muchlower GWP (�80%) when compared with a OPC reference concreteslab.

This very significant decrease in GWP was obtained in a LCAbased on a preliminary service prediction in which not all inputparameters to the probabilistic prediction model for chloride-in-duced corrosion were mix specific. Within the further develop-ment of the FA + SF mix design, it would certainly berecommended to determine every relevant model input parameter(ageing exponent, critical chloride content, etc.) experimentally.This way, the applicability of the now available default valuesmentioned in the Fib Bulletin 34 model code could be evaluatedand further optimized. It would also enable a more precise GWPquantification for the concrete slabs in the uncracked condition.

To correct for the unavoidable presence of cracks in these slabs,the additional amount of reinforcing steel needed to reduce thecharacteristic crack width to 0.05 mm, a value that should nor-mally ensure fast and complete autogenous crack healing, needsto be included in the life cycle assessment as well. This extraimpact should be taken into account when service life predictionis based on chloride migration coefficients measured on uncracked


concrete/mortar. The extra reinforcing steel can increase the GWPof the FA + SF concrete slab with another 43%.

Acknowledgements

The authors would like to thank Ghent University for the re-search funding. The PhD grant of Mathias Maes is funded by theAgency for Innovation by Science and Technology (IWT).

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