Construction and Building Materials 39 (2013) 71–76

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Construction and Building Materials

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Assessment of high volume replacement fly ash concrete – Concept ofperformance index

Obada Kayali a,⇑, M. Sharfuddin Ahmed b

a School of Engineering and Information Technology, University of New South Wales, Australian Defence Force Academy, Canberra, Australiab Roads ACT, Territory and Municipal Directorate, Canberra, Australia

h i g h l i g h t s

" 50% fly ash replacement may significantly reduce strength and E-modulus." Total chloride and RCPT values in 50% fly ash concrete are larger than their values in OPC concrete." 38 MPa 50% fly ash concrete may be obtained with industry practices using 225 kg Portland cement." 50% fly ash concrete was superior to 450 kg OPC concrete in resistance to chloride caused corrosion." Performance index concept is suggested assigning numerical values for strength and durability.

a r t i c l e i n f o

Article history:Available online 31 May 2012

Keywords:Fly ashConcreteCementChlorideCorrosionPerformance indexStrengthSustainability

0950-0618/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.conbuildmat.2012.05.009

⇑ Corresponding author. Tel.: +61 2 62688329; fax:E-mail address: o.kayali@adfa.edu.au (O. Kayali).

a b s t r a c t

This paper examines the practicality and suitability of high proportion replacement of cement by class Ffly ash. Binary and ternary blends of fly ash/Portland cement and fly ash/silica fume/Portland cement,were tested. The investigation focussed on the realistic conditions of concrete making on site and theeffects on the mechanical aspects as well as the consequences on corrosion of reinforcement.

It has been found that class F fly ash may replace 50% of the Portland cement and at the same timeresult in improving resistance to chloride initiated corrosion. Such replacement however, may signifi-cantly reduce the values of the mechanical properties. Nevertheless, such concrete is considered a highperformance concrete. The authors therefore suggest that the mechanical and durability characteristicsof concretes may be assigned numerical Performance Index values. These values may provide the meansfor making informed decisions on the extent of cement replacement by other cementitious materials.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Several researchers have advocated the use of high volume flyash in high-performance high-strength concrete [1–4]. Thoseresearchers have generally used large cement quantities togetherwith 28 day curing in laboratory conditions to achieve the requiredhigh strength [5,6]. Papayianni and Anastasiou [3] reported highvolume replacement by high calcium fly ash up to 50% of the ce-ment. They obtained strength value similar to that of plain OPCconcrete. However, the curing was also for 28 days [3]. Durán-Her-rera et al. studied the replacement effect but also with normal cur-ing all the time [7]. Yazici examined ultrastrength concrete withlarge replacement up to 60%, and with very high cement content[8]. He achieved strength above 120 MPa, but the total cementi-tious content was 850 kg/m3 and the curing was either standard,

ll rights reserved.

+61 2 62688337.

where strength exceeded 120 MPa or autoclaved, where strengthexceeded 170 MPa.

It is of paramount importance when advocating high fly ash con-crete that conditions of making such concrete are similar to thoselikely to be encountered in practice. Structural concrete design isoften based on the strength of 28 day laboratory cured samples.However, in actual structures, operators tend to avoid prolongedcuring, mainly for cost reasons [9]. Anecdotal evidence suggeststhat proper curing is hardly applied even for a minimum of 7 days.In a report by the Cement Concrete and Aggregates of Australia [10],concrete cured for 7 days could in general achieve about 70% of itspotential compared to continuously cured concrete. Fly ash con-crete needs rigorous curing much more than plain OPC concrete[11–13]. This means that translating results of 28 day laboratorycured fly ash concrete into actual practice is less credible and maybe more problematic than in the case of plain OPC concrete.

This paper examines the properties of high volume replacementof fly ash concrete which has been cured for only 7 days. This is theminimum that can be expected from good concrete practice

72 O. Kayali, M. Sharfuddin Ahmed / Construction and Building Materials 39 (2013) 71–76

[14,15]. The concrete samples are tested after 1 year in relativelydry conditions. The properties include the mechanical characteris-tics as well as chloride permeability and corrosion behaviour. Theauthors further assess these properties using a performance indexconcept. This method allows assigning numerical values to theinvestigated properties.

2. Experimental

2.1. Materials and testing

Crushed Dacite coarse aggregates of 9.5 mm maximum size, complying withASTM C 33, were used. The aggregates were washed and dried before casting.Washed river bed sand was used as fine aggregates.

Table 1Chemical composition of ordinary Portland cement, silica fume and fly ash.

Chemical composition OPC% Silica fume% Fly ash%

SiO2 21.1 >90 67.5Al2O3 5.2 <0.9 23Fe2O3 4.3 <1.5 4.5CaO 64.2 <0.4 <1MgO 1.2 <0.1 <1Na2O, K2O 0.05, 0.47 <0.4, <0.9 0.5, 1.5SO3 2.6% <0.03 0.1Loss on ignition 0.8% – 1.0Specific gravity 3.13 2.24 2.13Fineness index (m2/kg) 350 23, 500 310

Table 2Fresh and mature properties of the control, silica fume and fly ash mixes; w/b: 0.38a.

Materials and properties OPC S10 F25

Cement (kg/m3) 450 405 337.5Silica fume (kg/m3) – 45 –Fly ash (kg/m3) – – 112.5Total cementitious content (kg/m3) 450 450 450Coarse aggregate (kg/m3) 1110 1101 1083Fine aggregate (kg/m3) 680 675 664Superplasticizer L/100 kg binder 1.14 1.3 1Water-effective (kg) – (Free) 171 171 171Slump (mm) 65 85 130Air content (%) 1.65 0.9 1.2Hardened concrete (kg/m3) – 365 days 2360 2344 2298

a Aggregate quantities are based on oven dry condition, while the water quantity rec

Fig. 1. Details of the rein

Concrete mixes were cast with total cementitious materials content of 450 kg/m3. The OPC was replaced with low, medium, and high percentage of fly ash at thereplacement levels of 25%, 50%, and 70%. Silica fume was used at 10% replacementof the total cementitious materials content in all the ternary mixes. The chemicalanalysis of OPC, silica fume, and fly ash used in this series is shown in Table 1.

A total of eight types of mixes were cast using a constant water to binder (w/b)ratio of 0.38 and varying dosage of superplasticizer. The mixes cast and tested areshown in Table 2. The mixes were named in Table 2 as follows: OPC is the controlplain ordinary Portland cement concrete. Mix S10 stands for the mix with 10% silicafume as a weight for weight replacement of Portland cement. Mix F25 stands for themix with 25% fly ash replacing Portland cement. Mix F25S10 stands for the concretewith 25% fly ash and 10% silica fume replacing Portland cement, and so on. A poly-carboxylic ether hyperplasticizer usually used in producing high performance con-crete was used in this series.

2.2. Concrete specimens

A reinforced concrete slab panel of size 500 � 500 � 60 mm was cast for eachmix. The concrete cover at the top and bottom were 30 and 15 mm, respectively.The slabs were air dried in the laboratory for a period of 28 days after 1 week offog curing. On the 29th day, the slabs were ponded with 3% sodium chloride solu-tion (chloride ion concentration of 18,198 ppm) placed on the top of the slab withaverage depth of 10 mm. The slabs were placed in an internal enclosure where theambient temperature and RH were approximately 23 �C and 40% respectively. Mar-ine grade aluminium was used to enclose the sodium chloride solution on top of theslabs. The sodium chloride solution was completely removed on weekly basis andwas replenished with freshly prepared solution. The solution was continually stir-red to avoid stratification. The ponding period reported in this paper lasted for2 years after which concrete powder specimens were extracted from the range of5–25 mm which represents a case of shallow concrete cover, and the range of25–45 mm depth, which represents the reinforcement vicinity in a medium to thickcover. The samples were analysed for acid soluble chlorides following the AASHTO-T260 method [16]. Details of slab and reinforcement are shown in Fig. 1.

F25S10 F50 F50S10 F70 F70S10

292.5 225 180 135 9045 – 45 – 45112.5 225 225 315 315450 450 450 450 4501074 1056 1046 1034 1025658 646 642 633 6281 0.6 1 0.8 1171 171 171 171 17160 50 50 25 351.5 1.2 1.75 1.35 1.952276 2232 2226 2164 2135

orded is the free water.

forced concrete slab.

O. Kayali, M. Sharfuddin Ahmed / Construction and Building Materials 39 (2013) 71–76 73

For each mix, cylindrical specimens of size 100 mm diameter by 200 mm lengthwere cast for the tests of compressive strength, modulus of elasticity, and tensilestrength. The plain and blended cylindrical concrete specimens were fog curedfor a period of 7 days then exposed to environmental room conditions maintainedat 23 �C and 50% R.H. Mechanical properties of the concrete specimens were testedafter 365 days.

2.3. Rapid chloride permeability test (RCPT) specimens

For each mix, 8 disc specimens (with the exception of the control mix whichhad 7 specimens) of 100 mm diameter and 50 mm thickness were cast. The speci-mens were demoulded 24 h after casting and were fog cured for a period of 7 days.They were then exposed in an environmental room maintained at 23 �C and 50% RH.The test was performed after 350 days following the procedures outlined in theAASHTO Standards [17].

Fig. 3. Tensile strength reduction as a result of fly ash and/or silica fumereplacement in all mixes.

2.4. Corrosion measurement

The corrosion evaluation was performed using ‘‘GECOR6’’ corrosion rate metredeveloped by GEOCISA in collaboration with two leading Spanish research centres.The apparatus works on the principle of linear polarisation [18]. The apparatus al-lows the measurement of corrosion potentials values using copper/copper sulphatehalf-cell electrode. The corrosion rate is measured in terms of the corrosion currentdensity, Icorr and is expressed as micro-ampere per square centimetre (lA/cm2).Values between 0.1 and 1 lA/cm2 are the most frequently observed. A corrosioncurrent density less than 0.1 lA/cm2 is associated with passivity or negligible cor-rosion activity. Values between 0.5 and 1.0 lA/cm2 are considered in the range ofmoderate corrosion activity. Values above 1.0 lA/cm2 are in the high range of cor-rosion activity [19]. The corrosion rate for all slab reinforcement was monitored forthe total reported period of 2 years. The results presented here are those recorded atthe conclusion of the 2 year testing period. Each result represents the average of 12measured values.

Fig. 4. The reduction in modulus of elasticity with replacement of OPC by fly ashand/or silica fume.

3. Results and discussion

3.1. Mechanical properties

Replacing OPC with fly ash has resulted in lower compressivestrength (Fig. 2), lower tensile strength (Fig. 3) and lower modulusof elasticity (Fig. 4). It is noticed that the presence of silica fume asa further 10% replacement of cement, did not significantly improvethe mechanical properties.

Compressive strength of concrete where a portion of the cementhas been replaced by only fly ash is shown to decrease as the ratioof replacement increases, as apparent in the trend shown in Fig. 5.The results for tensile strength show a similar trend (Fig. 6). BothFigs. 5 and 6 may be useful for predicting strength values.

The effect of only fly ash replacement on the modulus of elastic-ity could also be predicted from the trend shown in Fig. 7. Theauthors however, emphasise that at this stage these trends canonly represent the case where the initial OPC content is 450 kg.These trends might be significantly different when the initial ce-ment content changes.

Fig. 2. Effect on 1 year compressive strength of fly ash and silica fume replacementof Portland cement.

Fig. 5. Trend of the compressive strength when only fly ash replaces OPC.

3.2. Chloride resistance

The effect of fly ash and silica fume replacement on concrete’sresistance to chloride ion penetration as tested according to thestandard RCPT is shown in Fig. 8. This figure shows that silica fumereduced the permeability of chloride ions as indicated by the RCPT.Silica fume concrete has been found to decrease chloride diffusion[20]. This has been largely attributed to the filling of the pores withhydration products, better packing of the fine particles and adsorp-tion of chloride ions by silica fume [21]. Nevertheless, such appar-ently beneficial effect seems to disappear when the fly ashproportion was increased and even it has reversed when the flyash proportion exceeded 50%. This behaviour may be explained

Fig. 6. Trend of reduction in tensile strength with only fly ash replacement.

Fig. 7. Trend of the elastic modulus variation as a result of replacing OPC with onlyfly ash.

Fig. 8. RCPT results for 1 year old fly ash and/or silica fume concretes.

Fig. 9. Chloride penetration in the layer between 5 and 25 mm after 2 yearsponding.

Fig. 10. Chloride penetration in the layer between 25 and 45 mm after 2 yearsponding.

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by the fact that silica fume’s content, being limited to 10% of thebinding material, can only adsorb a certain quantity of chlorideions. Thus its presence would be effective and apparent only whenthe fly ash proportion is low. As for the fly ash, it has been shownthat fly ash inclusion significantly changes the electrical conductiv-ity of concrete [22]. Some class F fly ashes were demonstrated tohave caused an increase in electrical conductivity [22]. Hence,when the fly ash proportion is high, the effect of silica fume is pos-sibly counter-balanced by an increase in electrical conductivitycaused by fly ash. It is appropriate at this stage to draw attentionto the fact that the RCPT is mainly a measure of the ability to passa direct current charge [22,23]. For the determination of the chlo-ride ion content as a result of exposure to chloride solution, theAASHTO ponding test should be used [24].

The effect of fly ash on the chloride ion permeability as revealedby determination of the total chloride content, is shown in Figs. 9and 10. These two figures show the total chloride contents in thelayer from 5 mm till 25 mm and from 25 mm till 45 mm respec-tively. These layers represent the proximity to steel reinforcementsin various values of depth of cover. It can be seen that at the shal-low depth of 5–25 mm the chloride ion content was far more thancan be tolerated as a limit for chloride initiated corrosion [25].Fig. 10 shows that, with the exception of the 70% fly ash substitu-tion level, the concentration of chloride ions within the 25–45 mmdepth is quite similar in all the types of concretes. Furthermore, itis seen that the level of chloride concentration in concretes whenthe fly ash proportion was 50% or less, was well below the criticalvalue needed for initiating corrosion [22,25].

3.3. Corrosion of reinforcement

The results of corrosion potentials and corrosion current in rein-forcing steel placed with 30 mm cover, are presented in Figs. 11and 12, respectively. These results are obtained after 2 years ofponding under chloride solution. Fig. 11 shows that only the 70%replacement level resulted in potentials that are considered condu-cive for the possibility of corrosion occurring. As it is well known, ahighly negative corrosion potential by itself is not sufficient to con-clude that corrosion is active [18,26]. Thus corrosion current valuesneed to be known so as to determine whether there is active cor-rosion. The Fig. 12 shows that even the concretes with 70% flyash replacement showed negligible corrosion activity where corro-sion rate was less than 0.1 lA/cm2 [19]. Interestingly enough, thehighest value of corrosion activity was recorded for the concretewith straight OPC. Concrete where the proportion of replacementby fly ash was 50% of cementitious materials had a current rate va-lue less than 0.06 lA/cm2. This value is considered to be in therange of minimal corrosion activity [19].

Fig. 11. Corrosion potentials after 2 years ponding.

Fig. 12. Corrosion current after 2 years ponding.

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3.4. The concept of performance index

Rather than using qualitative terms to describe the performanceof high fly ash replacement in concrete, a quantitative value can beassigned instead. Considering the performance of OPC concrete as1.0 for any mechanical and durability property of interest, the per-formance of fly ash concrete may be expressed as either a valueless than 1.0, which means its performance is relatively inferiorto OPC concrete, or as a value greater than 1.0, which means thatthe concrete in question is performing better than OPC concretein relation to the particular property or functionality in question.

Applying this concept of performance index, the performance ofthree concretes used in this research was determined and shown inTable 3. The fly ash concrete shown in this table is that of the 50%replacement level. This concrete used only 225 kg of cement whenthere was no silica fume addition. It used 180 kg of cement whensilica fume was used. The results show that using silica fumemay not be justified especially when considering that silica fumeis sometimes several folds more expensive than OPC [27,28].Replacing OPC by 50% fly ash in a typical concrete whose OPC con-tent is 225 kg in the cubic metre has resulted in a practical and sat-isfactory compressive strength of 38 MPa. This value is adequatefor normal strength concrete. But perhaps the performance index

Table 3Performance index values.

Concrete type Compressivestrength

Tensilestrength

Modulusofelasticity

RCPT Chloridepenetration at5–25 mm

OPC 1 1 1 1 1OPC + 50% fly ash 0.49 0.65 0.7 0.54 0.92OPC + 50% fly

ash + 10%silica fume

0.45 0.52 0.6 0.54 1.3

becomes of more interest when it is found that such a concretepossesses the values 1.14 and 1.9 when it comes to corrosion ofreinforcement as measured by corrosion potentials and currentvalues respectively. That is; it performs either slightly better(based on potential values) or nearly twice as good (based on cor-rosion current values) as straight OPC concrete that contains twicethe amount of cement.

However, it should always be acknowledged that there is notone factor that determines the performance. For example, Table 3shows that the tensile strength of such concrete is only 0.65 asgood as the totally OPC concrete. It also shows that the modulusof elasticity of this concrete is 0.7 of its value when the OPC istwice as much as the quantity of the fly ash. This only means thatthe requirement of the concrete structural element is what deter-mines whether a replacement on large magnitude is the wise deci-sion. Indeed, consideration of the performance using this index orsimilar yardstick may allow using large volume of fly ash providedthat engineering measures are taken to compensate for losses suchas those experienced in compressive strength, tensile strength ormodulus of elasticity. One of such methods is to include fibre rein-forcement [29].

4. Conclusions

1. The majority of reports on high strength high volume fly ashconcretes were based on either very large quantity of bindersor 28 day curing or both. It is believed that such concretes donot represent the reality of practice. The industry, understand-ably, is keen on reducing expensive ingredients and costly prac-tices. Curing for more than 7 days is rarely applied. The strengthand performance of high volume fly ash concrete should reflectthis reality.

2. Replacing Portland cement with fly ash while applying theexpected curing practices may result in steady reduction in cer-tain mechanical values. For example, a 50% fly ash replacementmay reduce compressive strength, tensile strength and the E-modulus by approximately 50%, 35% and 30% respectively,when compared with 100% Portland cement concrete.

3. The values of the total chloride content and the conducted elec-trical charge in high volume fly ash concrete are larger thantheir values in plain Portland cement concrete. However, thevalues of corrosion potentials and corrosion currents are lessin high volume fly ash concrete up to 50% replacement. Theseresults indicate better performance from the corrosion resis-tance aspect.

4. A medium and practical strength concrete with 38 MPa com-pressive strength may be obtained with normal industry prac-tices using 225 kg Portland cement and an equal quantity ofclass F fly ash. Such a concrete is superior to a 450 kg Portlandcement concrete in as far as resistance to chloride initiated cor-rosion is concerned.

Chloridepenetration at25–45 mm

Corrosion potentials at2 year aggressiveenvironment

Corrosion current at2 year aggressiveenvironment

1 1 10.94 1.14 1.91.16 0.88 2.1

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5. A concept of performance index is suggested. This conceptallows assigning a numerical value for certain mechanical anddurability characteristics. Such means of evaluation may helppractitioners making an informed decision based upon priori-ties of the job.

6. Although silica fume inclusion may yield desirable results formechanical as well as durability properties, these results dimin-ish greatly with the increase of fly ash content. In the ternaryblends reported here, there was no appreciable benefit of silicafume addition. In view of the current excessively high price ofsilica fume, such addition is not justified.

Acknowledgement

The authors acknowledge the financial support that this re-search has received from the University of New South Wales atthe Australian Defence Force Academy, Canberra, Australia.

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