Influence of silica fume on mechanical and physical properties of recycled aggregate concrete

HBRC Journal (2014) xxx, xxx–xxx

Housing and Building National Research Center

HBRC Journal

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Influence of silica fume on mechanical and physical

properties of recycled aggregate concrete

* Corresponding author. Tel.: +90 2123835242; fax: +90

2123835133.

E-mail addresses: [email protected] (O. Cakır), omerozkansofyanli

@windowslive.com (O.O. Sofyanlı).1 Tel.: +90 2123835242; fax: +902123835133.

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http://dx.doi.org/10.1016/j.hbrcj.2014.06.002

Please cite this article in press as: O. Cakır, O.O. Sofyanlı, Influence of silica fume on mechanical and physical properties of recycled aggregate concrete,Journal (2014), http://dx.doi.org/10.1016/j.hbrcj.2014.06.002

Ozgur Cakır *, Omer Ozkan Sofyanlı 1

Yıldız Technical University, Civil Engineering Faculty, Construction Materials Department, 34210 Istanbul, Turkey

Received 29 January 2014; revised 7 May 2014; accepted 2 June 2014

KEYWORDS

Recycled aggregate concrete;

Silica fume;

Mechanical properties;

Physical properties

Abstract Several studies related to sustainable concrete construction have encouraged develop-

ment of composite binders, involving Portland cement, industrial by-products, and concrete mixes

with partial replacement of natural aggregate with recycled aggregate. In this paper, the effects of

incorporating silica fume (SF) in the concrete mix design to improve the quality of recycled aggre-

gates in concrete are presented. Portland cement was replaced with SF at 0%, 5% and 10%. Spec-

imens were manufactured by replacing natural aggregates with recycled aggregates. Two size

fractions (4/12 mm and 8/22 mm) as recycled aggregates were used and four series of concrete mix-

tures were produced. In all concrete mixtures, a constant water/binder ratio at 0.50 was used and

concrete mixtures with a target initial slump of S4 class (16–21 cm) were prepared. Concrete prop-

erties were evaluated by means of compressive strength, tensile splitting strength, water absorption

and ultrasonic pulse velocity and it was found that, using 10% SF as a cement replacement for recy-

cled aggregate concretes enhanced the mechanical and physical properties of concrete. At all the test

ages the tensile splitting strength gain of the natural aggregate concrete mixture (NA) with and

without SF was higher than that of the recycled concrete mixtures. Continuous and significant

improvement in the tensile splitting strength of recycled aggregate concretes incorporating SF

was observed. Similar to compressive strength test results, concrete incorporating 10% SF and con-

taining 4/12 mm fraction recycled aggregates showed better performance among recycled aggregate

concretes.ª 2014 Production and hosting by Elsevier B.V. on behalf of Housing and Building National Research

Center.

Introduction

Many structures in Turkey are now either reaching the end oftheir design life time or were not constructed according to the

specifications. The Law ‘on Transformation of the Areasunder Disaster Risk’ dated 16 May 2012, and numbered6306, is called Urban Renewal Law among society, regulates

the circumstance of recent structures. Due to the UrbanRenewal Law, demolition and maintenance of these structuresresult in a large amount of concrete rubbles. The improper dis-

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Table 1 Properties of cement.

Contents Composition (%)

SiO2 20.1

Al2O3 4.9

Fe2O3 3.6

CaO 63.5

MgO 1.2

SO3 2.9

Loss on ignition 1.7

Specific gravity (g/cm3) 3.14

Specific surface area (cm2/g) 3942

2 O. Cakır, O.O. Sofyanlı

posal of construction and demolition (C and D) waste is aproblem faced by municipalities, not only in Turkey, but alsoin other countries of the World. In the European Union

roughly 75% of the waste material is disposed to landfills,despite its major recycling potential [1]. One of the construc-tion sector’s major contributions to the preservation of the

environment and sustainable development is the reuse andrecycling of the waste materials it generates. One way ofachieving this is to introduce recycled aggregates from C and

D debris and rubble into the production processes. Thisincreases the life cycle of these materials, thereby reducingthe amount of waste dumping and natural resource extraction[2]. On the other hand, construction activities demand a signif-

icant amount of natural materials, such as sand and gravel,and extraction of these natural materials modifies the courseof rivers and its beds, creating environmental problems [3].

Although construction industry world-wide is promotingthe use of recycled aggregates for concrete production, mainlyto respond to the problem of the depletion of natural aggre-

gates, the current specifications in many parts of the worldare not able to support and encourage the recycling of C andD waste. Since recycling of demolition waste was first carried

out after the Second World War in Germany, research workcarried out in several countries has demonstrated sufficientpromise for developing use of construction waste as a constit-uent in new concrete [4].

Recycled concrete aggregates mainly differ from naturalaggregates in that they are composed of two different materi-als: natural aggregate and cement mortar attached. Cement

mortar is the origin of the worse properties of recycled aggre-gates: lower density, higher absorption, and Los Angeles abra-sion [5]. Recycled aggregates are also highly heterogeneous and

porous, as well as contain a high content of impurities. Theheterogeneity influences the characteristics of recycled aggre-gates and these aggregate properties have a negative influence

on recycled aggregate concrete quality such as reduction of thecompressive strength, tensile strength due to the increased con-crete porosity and a weak aggregate–matrix interfacial bond[6,7].

Chandra [1] reported that the construction and demolitionwaste coarse aggregate can produce a range of concretes withacceptable compressive strength, exceeding 30% water reduc-

ing admixtures or larger amount of cement is to be used. Recy-cled aggregate concrete mixes require more water than naturalaggregate concrete to maintain the same workability without

the use of admixtures and this affects the quality and strengthof the concrete, resulting in lower concrete strength. It isknown that when using the same water to cement ratio, asrecycled aggregate percentage increase, the mechanical and

physical properties of the recycled aggregate concrete deterio-rate. Although concrete production is one of the high-gradeapplications where recycled aggregate can be used, concrete

strength decreases when recycled aggregates were used andthe strength reduction could be as low as 40% [8–10]. Concretemade with 100% of recycled coarse aggregates has 20–25%

less compression strength than conventional concrete at28 days, with the same effective w/c ratio (w/c = 0.50) andcement quantity (325 kg/m3) [11]. On the other hand, the gen-

eral trend in the tensile strength as measured by the splittingtensile test, depends mainly on binder rather than aggregatetype [12]. The use of coarse aggregate made from recycledconcrete with strength equal to 50 MPa results in compressive

Please cite this article in press as: O. Cakır, O.O. Sofyanlı, Influence of silica fumeJournal (2014), http://dx.doi.org/10.1016/j.hbrcj.2014.06.002

and tensile strengths comparable with that achieved whenusing natural coarse aggregate [13].

Recycled aggregate is more absorptive than natural aggre-

gate and one of the marked physical differences between recy-cled and natural aggregate is higher water absorption rate. Thehigher water absorption of the recycled aggregate results from

the higher absorption rate of cement mortar attached to theaggregate particles. This characterizes concrete made withhigher water absorption recycled aggregate reduced compres-

sive strength than with natural aggregate [14–18]. Under thesame mixture proportions, the mechanical properties of recy-cled aggregate concrete were worse than those for naturalaggregate concrete. When recycled aggregate was washed,

these negative effects were greatly improved [10]. Moreover,adding SF as a supplementary binder material can alsoimprove the mechanical and physical properties of concrete

prepared with recycled concrete aggregate [4,19–22].This paper presents the experimental results of the use of

silica fume as a cement replacement in proportion to the recy-

cled aggregate concrete. The effects of silica fume on themechanical and physical properties such as compressivestrength, tensile splitting strength, water absorption, capillarity

coefficient and ultrasonic pulse velocity, of recycled aggregateconcrete that was cured in water up to 90 days wereinvestigated.

Experimental details

Materials

Binders

The cementitious materials used in this experimental studywere Portland Cement CEM I 42.5R cement type accordingto TS EN 196-1:09 [23] and SF according to ASTM C

1240:12 [24] with a specific surface area of about 15,000 m2/kg and relative density of 2.3. The chemical composition, phys-ical and mechanical properties of the cement and SF are listed

in Tables 1 and 2, respectively.

Aggregates

Natural and washed recycled aggregates were used in the con-

crete mixes. In this study, two size fractions of crushed lime-stone coarse aggregates were used, one with nominal sizes of4/12 mm (NA1) and the other with nominal size of 8/22 mm

(NA2), as the natural coarse aggregate. Size fraction of silicabased natural aggregate (river sand) with nominal sizes of 0/4 mm was used as the natural fine aggregate (sand) in the con-

on mechanical and physical properties of recycled aggregate concrete, HBRC


Table 2 Properties of SF.

Contents Composition (%)

SiO2 >85

CaO <1

SO3 <2

Structure of material Condensed microsilica

Color Amber

Density (kg/liter) 0.55–0.70

Chlorine ratio <1

Specific surface area (m2/kg) 15,000

Activity index (%) >95

Particle ratio (<0.045 mm) <40

Table 4 Properties of superplasticizer.

Content Composition (%)

Structure of material Polycarboxylic ether

Color Amber

Density (kg/liter) 1.082–1.142

Chlorine ratio (%) <0.1

Alkaline ratio (%) <3

Influence of silica fume on recycled aggregate concrete 3

crete mixes. The recycled aggregate contained almost entirelyof crushed concrete rubbles obtained from building demolition

projects. By using laboratory jaw crusher, demolition wasteaggregates were crushed to obtain a cumulative grain size dis-tribution curve similar to that prepared with the natural coarse

aggregates. Two size fractions (4/12 mm-RA1; 8/22 mm-RA2;and both of two-RA1/2) as recycled aggregates were used.Density test and water absorption test were carried accordingto TS EN 1097-6:02 [25], Los Angeles abrasion resistance tests

were carried according to TS EN 1097-2:2000 [26]. The physi-cal properties of natural and recycled aggregates are shown inTable 3.

Concrete mixtures

Four series of concrete mixtures were prepared in the labora-

tory using a Pan mixer. SF was used as a cement replacementon a weight basis. In all concrete mixtures, a constant water/binder ratio of 0.50 was used and concrete mixtures with a tar-get initial slump of S4 class (16–21 cm) were prepared. In order

to obtain constant workability, water-reducing admixture(Glenium Sky) was used. Approximately the same amount ofwater-reducing admixture (0.9–1.2%) was used in the concrete

mixtures. Properties of superplasticizer are given in Table 4.The abbreviation of the mixtures is presented in Table 5 andthe mixture proportions of the concrete are presented in

Table 6.

Specimen casting and curing

100/200 mm cylinders, 70 mm and 150 mm cubes were cast foreach concrete mixture. The cylinders were used to evaluate thetensile splitting strength. 150 mm cubes were used to determinethe compressive strength and ultrasonic pulse velocity, 70 mm

cubes were used to determine the capillary and water absorp-tion. All the specimens were cast in steel molds and compactedusing vibrating table. After demoulding, the specimens were

Table 3 Physical properties of river sand, natural aggregates, and r

Type Bulk density (kg/m3) Water absorptio

Natural sand 2550 1.7

4/12 mm natural aggregate 2670 2.2

8/22 mm natural aggregate 2670 2.0

4/12 mm recycled aggregate 2370 7.4

8/22 mm recycled aggregate 2380 5.1


cured in water-curing tank at 20 ± 2 �C till test age at 7, 28,

and 90 days.

Tests

Compressive and tensile splitting strengths

The compressive strength test was carried out according to TSEN 12390-3:10 [27], while the tensile splitting strength was

conducted following TS EN 12390-6:10 [28]. Compressiveand tensile splitting strengths of concrete were determinedusing a compression machine with a loading capacity of

3000 kN. The loading rates applied in the compressive and ten-sile splitting strength tests were 10.6 kN/s and 1.6 kN/s, respec-tively. The compressive and tensile splitting strengths were

measured at the ages of 7, 28 and 90 days.

Water absorption

Capillary water absorption test and volumetric water absorp-

tion test were conducted following TS EN 480-5:01 [29] andTS 12390-7:10 [30], respectively. Water absorption tests wereperformed on 70 mm cubic specimens. Three specimens for

the capillary water absorption and another three for the volu-metric water absorption tests were used for each of the con-crete mixes. For the capillary water absorption test, lower

face (parallel to the trowelled upper face) was brought in con-tact with water in a tray. Environmental temperature was20 ± 2 �C during the test. Absorbed water was measured atdifferent intervals. Initial slope of the curve of absorbed

water-square root of time was calculated representing the cap-illary water absorption coefficient. For the volumetric waterabsorption test, specimens were immersed in water and the

mass was measured until a constant value was achieved.Absorbed water was calculated as the difference between satu-rated surface dry and dry masses and the values were given as

percent by the volume of specimen.

Ultrasonic pulse velocity

Ultrasonic pulse velocity test was carried out according to

ASTM C597:09 [31]. The pulse velocity measurements weremade with commercially available portable equipment (PUN-DIT); paraffin wax was used as a couplant between the trans-

ecycled aggregates.

n (%) Initial moisture content (%) Abrasion resistance (LA)

1.2 –

0.7 LA20

0.8 LA20

3.4 LA35

2.9 LA30



Table 5 Notation of abbreviations.

Abbreviation Notation

NA Conventional concrete containing NS, NA1 and NA2

RA1 Concrete containing NS, RA1 and NA2

RA2 Concrete containing NS, NA1 and RA2

RA1/2 Concrete containing NS, RA1 and RA2

NASF5 Conventional concrete containing NS, NA, NA2 and SF content 5%

RA1SF5 Concrete containing NS, RA1, NA2 and SF content 5%

RA2SF5 Concrete containing NS, NA1, RA2 and SF content 5%

RA1/2SF5 Concrete containing NS, RA1, RA2 and SF content 5%

NASF10 Conventional concrete containing NS, NA, NA2 and SF content 10%

RA1SF10 Concrete containing NS, RA1, NA2 and SF content 10%

RA2SF10 Concrete containing NS, NA1, RA2 and SF content 10%

RA1/2SF10 Concrete containing NS, RA1, RA2 and SF content 10%

(NA: Natural Aggregate, NS: Natural Sand, SF: Silica Fume, RA1: Recycled Aggregate (Size 1), RA2: Recycled Aggregate (Size 2), RA1/2:

Recycled Aggregates (Size 1 + Size 2), SF5: Silica Fume with replacement percentage of cement 5%, SF10: Silica Fume with replacement

percentage of cement 10%).

Table 6 Concrete mix proportions.

Abbreviation Constituents (kg/m3) (Liter/m3) (cm)

Composite of binder Natural aggregate Recycled aggregate Water reducing admixture Slump

Water Cement SF Sand NA1 NA2 RA1 RA2

NA 190 380 0 523 640 640 0 0 0.342 18

RA1 190 380 0 523 0 640 640 0 0.456 20

RA2 190 380 0 523 640 0 0 640 0.456 18

RA1/2 190 380 0 523 0 0 640 640 0.456 20

NASF5 190 361 19 523 640 640 0 0 0.342 18

RA1SF5 190 361 19 523 0 640 640 0 0.456 19

RA2SF5 190 361 19 523 640 0 0 640 0.456 20

RA1/2SF5 190 361 19 523 0 0 640 640 0.456 20

NASF10 190 342 38 523 640 640 0 0 0.342 18

RA1SF10 190 342 38 523 0 640 640 0 0.456 18

RA2SF10 190 342 38 523 640 0 0 640 0.456 18

RA1/2SF10 190 342 38 523 0 0 640 640 0.456 20


ducers and the concrete surface. The pulse velocity readings,reported for a given age, are the average of six measurements

through a path length of 150 mm in three cubes, taken in adirection perpendicular to the casting direction.

Results and discussion

Compressive strength

The results of the compressive strength of the concretes withand without recycled coarse aggregate at the ages up to 90 days

are shown in Table 7. Each presented value is the average ofthree measurements. Although the use of recycled aggregatesdiminishes the compressive strength of the concrete, dependingon the percentage of recycled aggregates used [3,7,9], for con-

crete mixtures with similar mixture proportions and workabil-ity, there was no significant difference at the 5% confidencelevel in the 7, 28 and 90 days compressive strengths of concrete

made with recycled aggregate (RA1, RA2 and RA1/2) andcompressive strengths of natural aggregate concrete (NA) inthis study. Crentsil and Taylor [12] reported that compressive


strengths of concrete made with commercial recycled aggregateand normal weight natural basalt aggregate concrete are simi-

lar at 28 days. Moreover, Fonteboa and Abella [2] reportedthat, with the same workability, it is possible to produce recy-cled aggregate concrete with 50% recycled aggregates with

almost the same compressive strength as natural aggregateconcrete. By washing recycled aggregates prior to using in con-crete production, the negative effects on concrete strength were

greatly improved [10].The compressive strength of both the natural and recycled

aggregate concretes with and without silica fume with standardwater curing significantly increased with curing age. In general,

the concrete incorporating silica fume underwent a reductionin early age compressive strength of natural and recycledaggregate concretes. The compressive strength decreased with

an increase in the silica fume content. At 7 days, the compres-sive strength of the concrete mixture NA with 5% and 10% ofsilica fume was reduced by 6% and 12%, respectively, in com-

parison to the strength of the control concrete without silicafume. The compressive strength of concrete mixture RA1 with5% and 10% of silica fume was reduced by 3%, in comparisonto the strength of the control concrete without silica fume.



Table

7Testresultsoftheconcretemixturesattheages

of7,28and90days.

Abbreviation

Compressivestrength

(MPa)

Tensile

splittingstrength

(MPa)

Volumetricwaterabsorption(%

)Capillarity

coeffi

cient(10�6cm

2/s)

Ultrasonic

pulsevelocity

(km/s)

728

90

728

90

728

90

728

90

728

90

NA

40.4

46.7

49.2

2.9

3.3

3.2

11.5

9.3

8.4

4.4

3.4

3.1

4.54

4.53

4.83

RA1

39.3

46.9

48.9

2.8

3.8

3.8

13.6

11.2

9.6

7.1

6.2

5.3

4.62

4.64

4.87

RA2

41.6

46.4

52.0

3.6

3.7

3.8

13.4

10.6

9.8

6.5

5.3

4.6

4.44

4.51

5.10

RA1/2

39.5

48.6

51.7

3.4

3.4

3.5

14.1

11.4

10.2

7.3

6.5

5.4

4.45

4.48

5.14

NASF5

38.1

46.2

51.4

3.6

3.9

3.7

11.4

8.7

8.1

4.2

3.3

3.0

4.31

4.34

4.72

RA1SF5

38.2

50.5

58.2

3.1

3.4

4.3

12.9

10.9

9.3

6.0

5.5

4.9

4.44

4.47

4.91

RA2SF5

40.3

42.5

55.3

3.6

3.8

4.0

11.9

9.7

9.1

5.8

5.0

3.2

4.40

4.40

4.67

RA1/2SF5

38.8

46.4

54.8

3.3

3.4

3.8

13.1

11.0

9.9

6.2

5.7

5.1

4.41

4.51

4.67

NASF10

35.5

47.5

51.8

3.3

3.9

4.2

11.2

8.2

7.7

3.9

3.2

2.9

4.30

4.29

4.67

RA1SF10

38.0

57.7

59.1

3.1

3.9

4.4

11.9

9.9

9.0

4.7

4.5

3.9

4.38

4.34

4.91

RA2SF10

37.8

47.4

58.6

3.3

3.5

4.1

10.4

9.1

8.2

5.4

4.2

3.0

4.23

4.35

4.88

RA1/2CSF10

38.0

48.6

58.0

3.1

3.2

3.7

12.0

10.5

9.1

5.6

4.8

4.0

4.34

4.48

4.92



Moreover, concrete mixture RA2 with 5% and 10% of silicafume was reduced by 3% and 9%, and concrete mixtureRA1/2 with 5% and 10% of silica fume was reduced by 2%

and 4%, in comparison to the strength of the control concretewithout silica fume. Fig. 2 shows the compressive strength ofthe concretes. Compressive strength loss of concretes contain-

ing recycled aggregate was less than the concretes containingnatural aggregate at 7 days due to SF usage. At 28 days, thestrengths of the concrete mixtures NA, RA2 and RA1/2 with

5% and 10% of silica fume were reduced but concrete mixtureRA1 with 5% and 10% of silica fume was increased by 8%and 23%, in comparison to the strength of the control concretewithout silica fume. The highest percentage gain in compres-

sive strength was recorded for 10% silica fume RA1 followedby 5% silica fume RA1. The concrete mixture RA1SF10 hadthe highest compressive strength gain (23%). At 90 days, the

strength of all the concrete mixtures with 5% and 10% of silicafume was increased, in comparison to the strength of the con-trol concrete without silica fume. The strength of the concrete

mixture NA with 5% and 10% of silica fume was increased by4% and 5%, respectively, in comparison to the strength of thecontrol concrete without silica fume. Moreover, the compres-

sive strength of concrete mixture RA1 with 5% and 10% of sil-ica fume was increased by 19% and 21%, respectively, incomparison to the strength of the control concrete without sil-ica fume. Concrete mixture RA2 with 5% and 10% of silica

fume was increased by 6% and 13%, respectively, in compar-ison to the strength of the control concrete without silica fume.Concrete mixture RA1/2 with 5% and 10% of silica fume was

increased by 6% and 12%, respectively, in comparison to thestrength of the control concrete without silica fume.

Fig. 1 shows the compressive strength of the concretes. The

highest percentage gain in compressive strength was recordedfor 10% silica fume RA1 followed by 5% silica fume RA1.The concrete mixture RA1SF10 had the highest compressive

strength gain (21%) and this might be attributed to incorpo-rating silica fume into the recycled aggregate concrete. SF’seffects (the pozzolanic effect and filler effect) improve all themechanical properties of the concrete but, particularly, its

compressive strength [2]. Due to the recycled aggregates beingmore porous, some part of the cement and silica fume wouldbe able to penetrate into the aggregate, which subsequently

would increase the bond strength between the aggregates andhydrated cementitious matrix. With the presence of silicafume, the cracks in the recycled aggregates were reduced due

to the healing effect after longer curing of silica fume blendedcement pastes. Therefore the concrete made with recycled con-crete aggregate, and the quality of the interfacial transitionzone, was better than that of the old paste and natural aggre-

gate concrete. The bond between the new cement paste andrecycled concrete aggregate was enhanced [21,32].

Tensile splitting strength

The tensile splitting strengths of the concretes at the ages up to90 days are shown in Table 7. The tensile splitting strengths of

recycled aggregates concretes are higher than the tensile split-ting strengths of natural aggregate concrete up to 90 days,except the strength of concrete mixture R1 at 7 days. This

increase might be due to the high quality of washed recycledaggregates and constant workability of concrete mixtures.



Fig. 1 Compressive strength of concrete mixture.


Chen et al. [10] concluded that when the recycled aggregate

was washed, negative effects were greatly improved and thisis especially true for the flexural strength of the recycled aggre-gate concrete. Tabsh and Abdelfateh [13] found that, the use of

coarse aggregate made from recycled concrete with strengthequal to 50 MPa results in compressive and tensile strengthscomparable with that obtained when using natural coarse

aggregate. At all the test ages the tensile splitting strength gainof the concrete mixture NA with and without SF was higherthan that of the recycled concrete mixtures. At 7 days, the ten-sile splitting strengths of the concrete mixture NA with 5%

and 10% of silica fume were increased by 23% and 12%,respectively, in comparison to the tensile splitting strengths

Fig. 2 Tensile splitting stre


of the control concrete without silica fume. Moreover, the ten-

sile splitting strengths of concrete mixture RA1 with 5% and10% of silica fume were increased by 10% and 11%, respec-tively, in comparison to the strengths of the control concrete

without silica fume. Compressive strength loss was observedin concrete mixtures RA2 and RA1/2. Concrete mixtureRA2 with 5% and 10% of silica fume was reduced by 2%

and 9%, respectively, in comparison to the strength of the con-trol concrete without silica fume and concrete mixture RA12with 5% and 10% of silica fume was reduced by 3% and9%, respectively, in comparison to the strength of the control

concrete without silica fume. Fig. 2 shows the tensile splittingstrengths of the concretes. Tensile splitting strengths of con-

ngth of concrete mixture.



Table 8 Ratio of tensile splitting strength to compressive

strength for different percentages of SF and RA.

Abbreviation RA (%) SF (%) Ratio

7 28 90

NA 0 0 0.072 0.070 0.065

NASF5 5 0.094 0.083 0.072

NASF10 10 0.093 0.082 0.082

RA1 35 0 0.071 0.081 0.078

RA1SF5 5 0.080 0.068 0.071

RA1SF10 10 0.082 0.068 0.074

RA2 35 0 0.087 0.078 0.072

RA2SF5 5 0.088 0.088 0.073

RA2SF10 10 0.087 0.074 0.070

RA1/2 70 0 0.085 0.070 0.068

RA1/2SF5 5 0.084 0.073 0.069

RA1/2SF10 10 0.081 0.067 0.064


crete series except NA and RA1 decreased with use of SF. Thehighest percentage gain in the tensile splitting strengths wasrecorded for 5% silica fume NA followed by 10% silica fume

NA at 7 days. At 28 days, the tensile splitting strengths of theconcrete mixture NA with 5% and 10% of silica fume wereincreased by 17% and 19%, respectively, in comparison to

the strengths of the control concrete without silica fume butconcrete mixtures RA1, RA2, and RA1/2 with 5% and 10%of silica fume were decreased in comparison to the strengths

of the control concrete without silica fume. The highest per-centage gain in tensile splitting strength was recorded for10% silica fume NA followed by 5% silica fume NA. At90 days, the tensile splitting strengths of the concretes contain-

ing both natural and recycled aggregates with and without SFwere increased significantly. The concrete mixture NA with5% and 10% of silica fume was increased by 16% and 33%,

respectively, in comparison to the tensile splitting strength ofthe control concrete without silica fume. Moreover, the com-pressive strength of concrete mixture RA1 with 5% and 10%

of silica fume was increased by 13% and 15%, respectively,in comparison to the strength of the control concrete withoutsilica fume. Concrete mixture RA2 with 5% and 10% of silica

fume was increased by 8% and 9%, respectively, in compari-son to the strength of the control concrete without silica fume.Concrete mixture RA1/2 with 5% and 10% of silica fume wasincreased by 8% and 6%, respectively, in comparison to the

strength of the control concrete without silica fume. The high-est percentage gain in the tensile splitting strengths amongrecycled aggregate concretes (RA1, RA2 and RA1/2) was

recorded for 10% silica fume RA1 followed by 5% silica fumeRA1, similar to compressive strength test results. However,comparing the results at 90 days shows that there was a contin-

uous and significant improvement in the tensile splittingstrength of recycled aggregate concretes. Adding ultra fineSF particles decreases the porosity and enhances the matrix

and transition zone between aggregates and cement mortar.The volume of micro pores is reduced causing benefits in termsof mechanical performances [21,32].

As reported by many researchers [7,11], the workability of

recycled aggregate concretes for the same water content inthe concrete is lower, especially when the replacement levelsexceed 50% [33]. In this study, the recycled aggregate concretes

showed a better mechanical performance rather than the natu-ral aggregate concrete up to 90 days due to the SF usage andthe constant workability of the concrete mixes.

Table 8 also shows the ratio of tensile splitting strength tocompressive strength for different percentages of SF and RA.

Water absorption

The volumetric water absorption values of concrete containingthe recycled aggregates are higher than those of the normalconcrete up to 90 days and it shows an increasing trend toward

higher recycled coarse aggregate content as shown in Table 7and Fig. 3. At 7 days, the highest absorption value is attainedfor RA1/2 specimens followed by RA1 and RA2, which are

22%, 18% and 16% times higher, respectively, than NA. At28 days, the highest absorption value is attained for RA1/2specimens followed by RA1 and RA2, which are 23%, 20%

and 14% times higher, respectively than NA. At 90 days, thehighest absorption value is attained for RA1/2 specimens fol-


lowed by RA2 and RA1, which are 21%, 17% and 14% timeshigher, respectively than NA. It was due to the high absorption

capacity of recycled coarse aggregate itself, which has createdhigher osmosis pressure within the concrete. When the dryspecimens are immersed into water during the testing, the spec-

imens with high osmosis pressure tend to absorb more waterfrom the surrounding of the specimens. Recycled aggregatesurface coated with mortar phase has a higher porosity than

aggregate phase [2,34]. The absorption values of concrete con-taining the recycled coarse aggregates are higher than those ofthe normal concrete and results show an increasing trendtoward higher recycled concrete aggregate content [35,36].

However, the volumetric water absorption values of concretecontaining the recycled aggregate with SF were decreased sig-nificantly especially at later ages. This effect is more significant

in recycled aggregate concretes incorporating 10% SF ratherthan recycled aggregate concretes incorporating 5% SF.

Capillarity coefficient

According to the experimental results of capillary pore investi-gation, it can be seen from Table 7 that the capillarity coeffi-

cient of the specimens with recycled aggregate is highercompared to the specimens with natural aggregate. As seenfrom Fig. 4, the capillarity coefficient decreases in all concretemixes with time for both NA and RA. At 7 days, the highest

capillarity coefficient value is attained for RA1/2 specimensfollowed by RA1 and RA2, which are 92%, 82% and 57%times higher, respectively, than NA. At 28 days, the highest

absorption value is attained for RA1/2 specimens followedby RA1 and RA2, which are 73%, 71% and 49% times higher,respectively than NA. At 90 days, the highest absorption value

is attained for RA1/2 specimens followed by RA1 and RA2,which are 74%, 71% and 48% times higher, respectively thanNA. A similar trend can be seen up to 90 days among recycledaggregate concretes. The capillarity coefficient test results

show an increasing trend toward higher recycled aggregatecontent as shown in Table 6. The capillarity coefficient valuesof concrete containing the recycled aggregate with SF were

decreased significantly especially at later ages. This effect ismore significant in recycled aggregate concretes incorporating



Fig. 3 Volumetric water absorption of concrete mixture.


10% SF rather than recycled aggregate concretes incorporat-ing 5% SF, similar to the volumetric water absorption testresults. This situation can be explained as the SF finer than

the cement particles fill the capillary pores physically and alsopozzolanic property of SF is affective on gel structure [21].

Ultrasonic pulse velocity

The ultrasonic pulse velocity values of all specimens were inthe range of 4.23 km/s to 5.14 km/s and the values tend toincrease with the ages as shown in Table 7 and Fig. 5. The

results presented in Fig. 5 clearly show the relation betweenthe average UPV values of specimens with the recycled aggre-gate content and the age of testing. At 7 days, the UPV value

of the concrete mixtures NA and RA1 with 5% and 10% ofsilica fume was reduced by 5% in comparison to the UPV of

Fig. 4 Capillarity coefficie


the control concrete without silica fume. The UPV value ofconcrete mixture RA2 with 5% and 10% of SF was reducedby 1% and 5%, in comparison to the UPV of the control con-

crete without silica fume. Concrete mixture RA1/2 with 5%and 10% of silica fume was reduced by 1% and 3%, in com-parison to the UPV of the control concrete without silica fume.However, the UPV values of concrete containing the recycled

aggregate with SF were slightly increased especially at laterages due to the hydration and pozzolanic properties of SF.According to the rating suggested by Malhotra, the specimens

are classified as in ‘‘good’’ condition as their UPV values fall inthe range of 3.66 km/s–4.58 km/s [37]. At 7, 28 and 90 days, allspecimens are evidently being in ‘‘good’’ condition. The UPV

test is used to predict the characteristic of the internal particlesof concrete and the quality of the concrete. The pore structuresin the concrete may have an impact on the UPV values and

nt of concrete mixture.



Fig. 5 Ultrasonic pulse velocity of concrete mixture.


their strength. Even though adhered mortars are porous mate-rials and it would reduce the UPV at early tages as proven in

the experiment, the UPV value of the all replacement levels ofthe recycled coarse aggregate content is still acceptable as theyfall in the ‘‘good’’ category according to Malhotra. As long as

the UPV values lie within the ‘‘good’’ category, it implies that aparticular concrete does not contain any large voids or crackswhich would affect the structural integrity. Therefore, the recy-

cled aggregate is still considered suitable to replace finer andcoarse aggregates in large portions as long as it achieves thetarget strength.

Conclusion

Based on this experimental study, the following conclusions

can be drawn:

1. Concretes produced with natural and recycled aggregatesincorporating silica fume underwent a reduction in early

age compressive strength. The compressive strengthdecreased with increase in the silica fume content. However,compressive strength loss of concretes containing recycled

aggregate was less than the concretes containing naturalaggregate at early age due to the SF usage. At 28 and90 days, the strength of all the concrete mixtures with 5%

and 10% of silica fume was increased, in comparison tothe strength of the control concrete without silica fume.The pozzolanic effect of the SF that usually occurs after7 days tends to increase the compressive strength of the

concretes with this mineral admixture. This effect is moresignificant in recycled aggregate concrete incorporating10% SF and containing 4/12 mm fraction recycled aggre-

gates rather than the other concrete series.2. Usage of SF is more effective on the compressive strength

of concrete having 4/12 mm fraction recycled aggregate

than 8/22 mm fraction aggregate incorporating the sameSF content.


3. Continuous and significant improvement in the tensile split-ting strength of recycled aggregate concretes incorporatingSF was observed with time. The concrete with 10% SF and

having 4/12 mm fraction recycled aggregates showed a bet-ter performance than the other concrete series in terms ofthe physical and the mechanical properties.

4. Water absorption values of concretes containing the recy-

cled aggregates with SF were decreased significantly espe-cially at later ages. This effect is more significant inrecycled aggregate concretes incorporating 10% SF rather

than recycled aggregate concretes incorporating 5% SF.

Conflict of interests

The authors have no conflicts of interest to declare.

Acknowledgements

The authors would like to express their gratitude to the _ISTAC(The _Istanbul Environmental Protection and Waste ProcessingCorporation), AKCANSA Cement Industry and Trading Co.,

and BASF Chemical Company for providing recycled aggre-gates, cement and mineral/chemical admixtures.

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Influence of silica fume on mechanical and physical properties of recycled aggregate concrete

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