Construction and Building Materials 61 (2014) 50–59

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Construction and Building Materials

journal homepage: www.elsevier .com/locate /conbui ldmat

An investigation on mechanical and physical properties of recycledaggregate concrete (RAC) with and without silica fume

http://dx.doi.org/10.1016/j.conbuildmat.2014.02.0570950-0618/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +90 2123835242; fax: +90 2123835133.E-mail addresses: hdilbas@yildiz.edu.tr, hasandilbas@gmail.com (H. Dilbas),

msimsek@yildiz.edu.tr, mesutsimsek@gmail.com (M. S�ims�ek), cozgur@yildiz.edu.tr(Ö. Çakır).

H. Dilbas, M. S�ims�ek, Ö. Çakır ⇑Yıldız Technical University, Department of Civil Engineering, 34220 Istanbul, Turkey

h i g h l i g h t s

� RAC with 5% SF increases the ratio of tensile splitting strength to compressive strength.� The ratio of tensile splitting strength to compressive strength of RAC decreases with 10% SF.� Suitable proportion of the replacement of NA with RA is 30%.

a r t i c l e i n f o

Article history:Received 13 November 2013Received in revised form 13 February 2014Accepted 17 February 2014Available online 21 March 2014

Keywords:Recycled aggregateSilica fumeMechanical propertiesPhysical propertiesRegression analysis

a b s t r a c t

Experimental studies for determining the mechanical and the physical properties of the recycledaggregate concrete (RAC) with and without silica fume (SF) is inspired by the Urban Renewal Law whichregulates circumstance of existing structures in Turkey. According to this law, the structures which havebeen built lacking quality engineering, built without considering urban planning, and are risk prone (i.e.susceptible to earthquakes), will be demolished and rebuilt using recent Turkish Standards. Implement-ing this law is expected to increase the quantity of waste concrete. Minimizing waste disposal throughstructural and non-structural areas without a harmful effect on nature has a vital importance in Turkey.In this study, demolished-building-rubble is used as recycled aggregate (RA) with and without SF in con-crete mixtures. Twelve concrete mixtures in three groups are produced, and the mechanical properties ofthe concrete specimens such as compressive strength, tensile splitting strength and elasticity modulus,and physical properties of the concrete specimens such as density and water absorption ratio are deter-mined. The proportion 30% of RA in concrete mixtures is proposed as the optimum ratio. Low regressioncoefficient of RAC with SF is observed in the short-term. It is found that 5% SF content in the RAC is moreconvenient to improve the low properties of RAC (i.e. compressive strength).

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

In the construction industry, concrete is the most common anduseful material. Concrete has contributed to the advancement ofcivilizations throughout history. In recent years, the acceleratingurbanization causes excessive works of destruction and construc-tion activities. Constructional waste storage, management, andtransformation into recycled aggregate (RA) for construction usagerequires large amounts of land and is costly. The most commonapproaches to minimizing waste are through landfills androad-bed applications [1–4].

In many countries, regulations and procedures on reusing wastematerials in construction applications have been established, andmany countries have also begun transforming constructionalwastes into RA [2,4–8]. In Turkey, The Law on Transformation ofthe Areas under Disaster Risk, (Turkish Law 6306, May 16, 2012),regulates the durability of existing structures. The structureswhich were built lacking quality engineering, without utilizingurban planning, and are risk prone (i.e. susceptible to earthquakes)will be demolished and rebuilt using recent Turkish Standards andUrban Renewal Plans of local administrations [9]. It is estimatedthat after a few decades all risk prone structures in Turkey willbe demolished and rebuilt according to the new standards. InMarch 2006, the Istanbul Metropolitan Municipality and theIstanbul Environmental Management in Industry and Trade Inc.(_ISTAÇ) prepared a plan, called Construction and Demolition WasteManagement Plan. According to the plan, it was decided thatimport centers would be established at each municipality to collect

Table 1Properties of cement and SF.

Contents Cement SF

SiO2 (%) 22.0 >85CaO (%) 64.9 <1SO3 (%) 2.7 <2Al2O3 (%) 5.9 –Fe2O3 (%) 3.5 –MgO (%) 0.9 –Structure of material – Condensed microsilicaDensity (g/cm3) 3.16 0.55–0.70Chlorine ratio (%) – <1Specific surface area (m2/kg) 3540 15000Activity index (%) – >95Particle ratio (<0.045 mm) – <40%Loss on ignition (%) 1.2 –

H. Dilbas et al. / Construction and Building Materials 61 (2014) 50–59 51

local wastes. The import centers will send the wastes to tworecycling construction waste facilities which are located on theAnatolian and European side of Istanbul [10].

The production of RA has only recently been studied in Turkey.Therefore, the studies on recycling concrete are still very limited[10–15]. The studies that have been conducted indicate concernswith RA. In other countries, in order to evaluate the demolitionwaste in concrete, researchers studied the mechanical behaviorand other properties of RAC [16–33]. It was revealed that RAC upto C32/40 strength class was able to be produced by using 70% nat-ural aggregate (NA) and 30% RA [16], and similarly the proportionof RA up to 30% in concrete was suitable replacing fine NA with fineRA [27]. It was reported that the Poisson’s ratio was independent ofthe RA ratio in concrete and Poisson’s ratio was found ranging from0.14 to 0.20 for all replacement [17]. Belen et al. [22] determinedstress–strain curves of RAC and compared the curves with theproposed model equation. The experimental results of their studyincluded the code curve and fitted curves for a specimen. It wasdetected that the use of recycled aggregate affected the values ofmodulus of elasticity. Pereira et al. [25] used two types of superp-lasticizers in RAC with fine recycled aggregate. They found that theperformance of RAC with incorporation of recycled aggregate onlywas poorer than the performance of NAC. However, the mechanicalperformance of RAC was generally increased when superplasticizerwas utilized in the mixture. Sheen et al. [26] produced RAC usingconcrete wastes from the earthquake of Chi-Chi in Taiwan. Theyobserved that the compressive strength of RAC was affected byRA; because fine ingredients decreased the compressive strength.Also, it was observed that high water absorption had a negative ef-fect on the strength of RAC. Sagoe-Crenstil et al. [29] examined themechanical and workability properties of RAC. They found that RA,produced in a plant, had smoother and spherical particles, whichmade the workability of RAC easy.

Moreover, mineral additions usage at various ratios and typesreplacing cement in RAC was found suitable to enhance theproperties [18–20,30–32]. Kou et al. [32] prepared some mixturescontaining NA, RA, and mineral additions such as fly ash, SF,metakaolin, and ground granulated blast slag. The study concludedthat mineral additions increased the performance of RAC. Forexample, SF and metakaolin improved both the mechanical andthe durability properties. Fly ash and ground granulated blast slagimproved essentially durability performance.

On the other hand, in recent researches [28,34], RA in concretehas been utilized in reinforced concrete elements. The reinforcedRAC elements were produced in a scale of various ratios withrespect to current size [34], the behavior reinforced RAC elementswere observed in laboratory conditions [28,34] and Gonzalez andMoriconi [34] concluded that the use of 30% RA content in rein-forced RAC under cycling loading was convenient for the structuresin seismic areas.

The objective of this study is to investigate the mechanical andthe physical properties of concretes containing SF at various ratios(0–5–10%) and replacing fine and/or coarse NA with RA. In thisstudy, the rubble of a demolished building in the Sütlüce neighbor-hood in Istanbul is used. The RA (with and without SF) is utilized toexamine the usability of RA and SF with content of 0%, 5%, and 10%instead of the conventional concrete. The crushed basalt aggregates(natural fine aggregate (NA1) and natural coarse aggregate (NA2))and siliceous sand are used in the concrete mixtures. Also recycledfine aggregate (RA1) and recycled coarse aggregate (RA2) are usedas RA in the concrete mixtures. For this purpose, twelve concretemixtures in three groups are produced, and the mechanical proper-ties such as the compressive strength, the tensile splitting strength,elasticity modulus, physical properties such as density and waterabsorption of RAC are investigated. Each group has four concretemixtures. The conventional concrete mixture with natural

aggregate (NA), also named as natural aggregate concrete (NAC),is included in the first group. The groups, mixture names and nota-tions are listed in the tables. The regression analysis between thetensile splitting strength, the compressive strength, the ratios ofthe tensile splitting strength to the compressive strength, and theevaluation of the tensile splitting strength only are examined. Somesuggestions about RA and RAC properties are indicated to research-ers and designers who will use the RA in future.

2. Experimental studies

2.1. Materials

2.1.1. Cement and SFType I general use Portland cement (PC) compatible with Turkish Standard

‘‘Cement-Part 1: Composition, specifications and conformity criteria for commoncements’’ (TS EN 197-1 (2012)), and SF suitable with American Society for Testingand Materials ‘‘Standard Specification for Silica Fume Used in CementitiousMixtures’’ (ASTM C 1240-12) are used in the concrete mixtures. The chemical andphysical properties of cement and SF are given in Table 1.

2.1.2. AggregatesNatural aggregate (NA) and recycled aggregate (RA) are used as the fine and the

coarse aggregate in the concrete mixtures. In this analysis, crushed basalt aggregateis employed as NA. The particle size distribution of NA and RA is performed to therequirements of Turkish Standard ‘‘Aggregates for concrete’’ (TS 706 EN 12620(April 2003)). The RA sources are supplied from rubble of a demolished buildingfrom the Sütlüce neighborhood in Istanbul. The demolished material undergoeson-site crushing and on-laboratory crushing in two steps. In the first step, the rub-ble is collected on site area without classification of the rubble components such asconcrete, brick, marble and etc., and then the rubble is crushed into small piecesapproximate diameter <30 mm by using hammer manually in the laboratory. Inthe second step, a laboratory jaw crusher is employed in order to obtain RA havingsize fractions <30 mm. The crusher has two jaws, and one of the jaws is replaceableand another jaw is fixed. Also it is possible to adjust the distance between the jawsreplacing the jaw with another one.

After production of RA, at first crushed rubble is classified as RA1 and RA2 withparticle sizes 4–8 mm and 8–32 mm, respectively, as similar to NA1, and NA2 usingsieves with sieve apertures 32, 16, 8, 4, 2, and 1 mm. Then, the physical propertiesof RA1 and RA2 are determined, and are classified using sieves with sieve apertures32, 16, 8, 4, 2, and 1 mm. Afterwards, any quantity of retained RA on any sieve isstocked separately in a container which have same number of the sieve aperture.RA1 and RA2 contain not only crushed concrete but also various impurities asshown in Table 2 and Fig. 1. Siliceous sand, which has particles whose diameteris smaller than 4 mm, is used in all concrete mixtures. The physical and mechanicalproperties of aggregates are determined using Turkish Standards. Density andwater absorption tests are performed in accordance with ‘‘Tests for mechanicaland physical properties of aggregates-Part 6: Determination of particle densityand water absorption’’ (TS EN 1097-6/AC (2006)), Los Angeles abrasion loss isdetermined in accordance with ‘‘Tests for mechanical and physical properties ofaggregates-Part 2: Methods for the determination of resistance to fragmentation’’(TS EN 1097-2 (2010)). Chemical properties of RA are also determined accordingto ‘‘Tests for chemical properties of aggregates – Part 1: Chemical analysis’’ (TSEN 1744-1 (2011)) and results are listed in Table 3.

2.1.3. SuperplasticizerPolycarboxylic ether based superplasticizer is utilized in order to enhance low

workability of the mixtures. Therefore, the slump class of all mixtures is set toslump class S4 so that the workability of all mixtures is constant. Note that the

Table 2The component of RA1, and RA2.

Content RA1 (%) RA2 (%)

Concrete 84.64 72.32Ceramic, and tile 4.96 10.68Brick 5.48 10.4Marble 4.92 5.01Styropor – 0.11Other (insulation materials, wallpaper, gypsum) – 1.58

Total 100 100

Fig. 1. A scene after tensile splitting strength test is applied on a specimencontaining recycled aggregate. Colorful aggregates demonstrates the variety inrecycled aggregate. (For interpretation of the references to color in this figurelegend, the reader is referred to the web version of this article.)

52 H. Dilbas et al. / Construction and Building Materials 61 (2014) 50–59

slump class is chosen according to Turkish Standard ‘‘Concrete-Part 1: Specification,performance, production and conformity’’ (TS EN 206-1 (2002)). The density ofsuperplasticizer ranges 1.082–1.142 kg/l with color of amber. Other properties ofthe superplasticizer are presented in Table 4.

2.2. Concrete mixtures

Twelve groups of concrete mixtures that contain the previously mentionedaggregates and have the target initial slump class S4 are produced in the laboratory.For the sake of the convenience, the notations of the concrete mixtures are specifiedin Table 5. The absolute volume method is employed to design the mix proportionsof the concrete mixtures shown in Table 6. In all the mixtures, the water/binderratio (w/b) is a constant value of w/b = 0.5, and the quantity of the cement is350 kg/m3.

The concrete is mixed in a laboratory in a pan mixer. The coarse/fine aggregatesand sand are first dry blended for one minute. PC and SF are then added and dryblended for a further minutes. Two thirds of water is added and mixing is continuedfor another minute. The remaining water and superplasticizer are then added andthe total mixing time is five minutes. Concrete is cast in accordance with ‘‘StandardPractice for Making and Curing Concrete Test Specimens in the Laboratory (ASTMC192/C192M–13a (2013))’’ and vibrated till large air bubbles occurs and blows atthe top surface. In addition the slump of mixes with NA and RA with/without SFis similar (18 cm ± 2 cm). Due to the rougher surface textures of crushed particlesand greater angularity compared with the smooth, rounded natural aggregates, itis expected that the RAC mixtures are less workable. Owing to its high water

Table 3The physical properties of sand, NA and RA.

Type Density(kg/dm3)

Waterabsorption (%)

Initial moisturecontent (%)

Chloridescontent, (%)

Watersulfate

Sand 2.55 1.2 1.05 – –NA1 2.75 0.8 0.67 – –NA2 2.72 0.6 0.53 – –RA1 2.33 3.8 2.10 0.57 0.65RA2 2.23 4.3 2.32 0.53 0.71

absorption rate, the RA is pre-soaked in water for 24 h before casting. Hence asapproximately the same amount of superplasticizer is used with increasing RA con-tent in the mixtures as demonstrated in Table 5 and S4 slump class is obtained.Poon et al. [35] mentioned that a small change in the initial slump of concretes in-cluded RA in surface-dried condition at any replacement percentages was observed.Also there is no observation of any bleeding or segregation for any of the concretemixtures tested.

Moreover, in order to determine how SF has an effect on the mechanical and thephysical properties of the specimens, 0%, 5% and 10% SF content is used in themixtures. For instance, the specimen RA12CSF10 with 10% SF contains 315 kg/m3

cement and 35 kg/m3 SF, as demonstrated in Table 6.Sand content is constant in all the concrete mixtures. The amounts of RA in RAC

mixtures are presented in Table 6.

2.3. Specimens and curing

In the production stage of this study, 100£ � 200 mm cylinder specimens areproduced to evaluate the tensile splitting strength, density and water absorptionvalue. 150£ � 300 mm cylinder specimens are also produced to determine thecompressive strength and the static modulus of elasticity. The specimens are castin plastic moulds, and compacted using a vibrating table. After remolding, all spec-imens are cured in a water-curing tank at 20 ± 2 �C, until 28 days. It should be notedthat these curing conditions are compatible with Turkish Standard ‘‘Testing hard-ened concrete-Part 2: Making and curing specimens for strength tests’’ (TS EN12390-2 (2010)).

2.4. Tests

2.4.1. Compressive and tensile splitting strengthsIn order to assess the compressive and the tensile splitting strengths of speci-

mens, experimental studies are performed in accordance with Turkish Standard‘‘Testing hardened concrete – Part 3: Compressive strength of test specimens’’(TS EN 12390-3 (2010)), and ‘‘Testing hardened concrete-Part 6: Tensile splittingstrength of test specimens‘‘(TS EN 12390-6 (2010)). The tests are conducted atthe age of 28 days. A compression machine with a loading capacity 3000 kN is usedin the experiment. The loading rates are applied first to the compressive strengthtest with value 10.6 kN/s, and secondly to the tensile splitting strength test withvalue 1.6 kN/s. The results for the compressive and the tensile splitting strengthsof specimens are displayed in Table 7.

2.4.2. Static modulus of elasticityThe tests of the static modulus of elasticity are conducted according to the

American Society for Testing and Materials ‘‘Standard Test Method for StaticModulus of Elasticity and Poisson’s Ratio of Concrete in Compression’’ (ASTM C469 (2002)). The compressometer equipment (Fig. 2) have been specially desig-nated considering ASTM C 469 (2002). The tests are applied on the 150£ � 300 mmcylindrical specimens at the age of 28 days. As a result of the tests, the stress–straincurves are obtained. The results of the static modulus of elasticity are demonstratedin Table 7.

2.4.3. Density and water absorption ratioThe density and water absorption ratio tests of specimens are applied on the

specimens 100£ � 200 mm cylindrical dimensions in accordance with TurkishStandard ‘‘Testing hardened concrete-Part 7: Density of hardened concrete’’ (TS12390-7 (2010)) at the age of 28 days, and the results are shown in Table 7.

3. Results and discussions

3.1. Compressive strength

The compressive strength results of the concrete at the age of28 days are shown in Table 7 and Fig. 3. Each presented value isthe average of test results of the three specimens. The results

-solubles (%)

Totalsulfates (%)

Total sulfurcontent (%)

Finenessmodulus

Los Angelesabrasion (%)

– – 2.12 –– – 5.57 –– – 6.41 24.320.76 0.81 5.50 –0.80 0.86 6.44 41.40

Table 4The properties of superplasticizer.

Content Superplasticizer

Structure of material Polycarboxylic etherColor AmberDensity (kg/l) 1.082–1.142Chlorine ratio (%) <0.1Alkaline ratio (%) <3

Table 5The notation of mixtures, expansion of notations, and groups.

Notation Expansion of notation

1st GroupNAC Conventional concrete containing NA1 and NA2RA1C Concrete containing RA1 and NA2RA2C Concrete containing NA1 and RA2RA12C Concrete containing both RA1 and RA2

2nd GroupNACSF5 Conventional concrete containing NA1, NA2 and SF content 5%RA1CSF5 Concrete containing RA1, NA2 and SF content 5%RA2CSF5 Concrete containing NA1, RA2 and SF content 5%RA12CSF5 Concrete containing both RA1 and RA2, and SF content 5%

3rd GroupNACSF10 Conventional concrete containing NA1, NA2 and SF content 10%RA1CSF10 Concrete containing RA1, NA2 and SF content 10%RA2CSF10 Concrete containing NA1, RA2 and SF content 10%RA12CSF10 Concrete containing both RA1 and RA2 and SF content 10%

H. Dilbas et al. / Construction and Building Materials 61 (2014) 50–59 53

demonstrate that the compressive strength decreases with thereplacement of the NA with RA.

The compressive strengths of RA1C, RA2C and RA12C are re-duced by 7.8%, 4.7%, and 18.7% in comparison to the strength ofNAC. The strength of concrete depends on the strength of theaggregates, the cement matrix and the interfacial transitionzone (ITZ) between the matrix and the aggregates is well-known.The failure in the concrete occurs at the weakest point. It is dem-onstrated in macroscopic-scale in Fig. 1 that RA in the concrete isfound broken after the tests are applied to the specimen. Hence,the weakest point, being in these RAC, is the RA itself. In otherwords, the discontinuities in the structure of RA, are cracks dueto crushing processes and higher porosity due to the propertiesof adhered old mortar, decrease the strength of RAC.

Table 6Concrete mix proportions.

Mixes Constitution (kg/m3)

Water Composite of binder Chemical admixture

Cement Mineral admixture

1st GroupNAC 175 350.0 0 0.096RA1C 175 350.0 0 0.289RA2C 175 350.0 0 0.289RA12C 175 350.0 0 0.289

2nd GroupNACSF5 175 332.5 17.5 0.289RA1CSF5 175 332.5 17.5 0.385RA2CSF5 175 332.5 17.5 0.385RA12CSF5 175 332.5 17.5 0.385

3rd GroupNACSF10 175 315.0 35.0 0.501RA1CSF10 175 315.0 35.0 0.462RA2CSF10 175 315.0 35.0 0.385RA12CSF10 175 315.0 35.0 0.385

The data from Table 7 shows that using SF in RAC improvesgenerally the strength of RAC. On the other hand, despite usingSF content, a sharp decrease is observed in the strength ofRA12CSF10 with value 28.9 MPa. It is known that SF’s effects (thepozzolonic effect and filler effect) improve all the mechanical prop-erties of the concrete but, particularly, its compressive strength[36]. Due to the recycled aggregates being more porous, some partof the cement and silica fume would be able to penetrate into theaggregate, which subsequently would increase the bond strengthbetween the aggregates and hydrated cementitious matrix. Withthe presence of silica fume, the cracks in the recycled aggregateswere reduced due to the healing effect after longer curing of silicafume blended cement pastes. Therefore, the concrete made withrecycled concrete aggregate, and the quality of the interfacial tran-sition zone, 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 [37,38].

In this study, RA12CSF10 contains great amount of RA approxi-mately 70%. The RA contains old aggregates with adhered old mor-tars which have linked and open cracks. However the cracks arefilled and RA is enhanced by SF and the positive effect on thestrengths may take a long time. In short-term (at the age of28 days), the result of those a sharp decrease in compressivestrength in comparison to NAC is observed. In long-term thechange in compressive strength of RAC included SF and impuritiescan be investigated in further researches. On the other hand,Wagih et al. [39] used recycled construction and demolition wasteconcrete as aggregate in the mixtures at various ratios (0–25–50–75–100%) in their experimental study. It was found that 10% SFcontent in the mixtures included RA with no impurities improvedthe compressive strength at the age of 28 days. It is concluded byauthors that the impurities of RA may reduce the effect of SF onthe compressive strength in the present study.

It is found that the compressive strength of RA12CSF5 is higherthan the results of RA12C and RA12CSF10. The compressivestrengths of RA1CSF5, RA2CSF5 and RA12CSF5 are reduced by12.8%, 11.8% and 16.8% in comparison to the compressive strengthof NACSF5. Also, the compressive strength of RA1CSF10, RA2CSF10and RA12CSF10 are reduced by 18.2%, 15.4% and 35.5% in compar-ison to the compressive strength of NACSF10.

Another interesting result is due to the fact that the compres-sive strengths of RA1CSF10 and RA2CSF10 are higher than NACwith compressive strength values 37.2 and 38.5 MPa, respectively.In addition, the compressive strength of RA2CSF5 is closer to NAC

Natural aggregate Recycled aggregate (over sieve)

Sand Fine Coarse 16 mm 8 mm 4 mm 2 mm 1 mm

538 777 576 0 0 0 0 0538 0 576 29 384 205 26 4538 777 0 202 259 2 0 0538 0 0 231 642 208 26 4

538 777 576 0 0 0 0 0538 0 576 29 384 205 26 4538 777 0 202 259 2 0 0538 0 0 231 642 208 26 4

538 777 576 0 0 0 0 0538 0 576 29 384 205 26 4538 777 0 202 259 2 0 0538 0 0 231 642 208 26 4

Table 7The results of tests of the concrete mixtures at the age of 28 days.

Notation SF (%) RA (%) Slump (cm) Slump class Compressivestrength (MPa)

Static elasticitymodulus (MPa)

Splitting tensilestrength (MPa)

Density (kg/m3) Waterabsorption (%)

1st GroupNAC 0 0 17 S4 35.8 28095 2.25 2478 4.8RA1C 0 40 19 S4 33.0 23437 2.24 2202 7.0RA2C 0 30 18 S4 34.1 25167 2.41 2234 6.4RA12C 0 70 18 S4 29.1 22896 1.58 2038 9.1

2nd GroupNACSF5 5 0 18 S4 39.9 25619 2.62 2347 5.4RA1CSF5 5 40 19 S4 34.8 25541 2.52 2175 7.7RA2CSF5 5 30 17 S4 35.2 25571 2.97 2211 7.2RA12CSF5 5 70 18 S4 33.2 22026 1.92 2031 10.1

3rd GroupNACSF10 10 0 19 S4 45.5 27721 3.40 2375 3.6RA1CSF10 10 40 16 S4 37.2 24968 2.46 2200 5.5RA2CSF10 10 30 20 S4 38.5 21162 2.63 2252 4.7RA12CSF10 10 70 16 S4 28.9 22098 1.62 2061 7.2

Compressometer

Fig. 2. A scene while compressive strength test is being done. Also picture showscompressometers which is located on the specimen.

54 H. Dilbas et al. / Construction and Building Materials 61 (2014) 50–59

and that is 35.2 MPa. It can be concluded from the above resultsthat RA1CSF10, RA2CSF10 and RA2CSF5 are more suitable for useinstead of NAC, if compressive strength is considered only. As dem-onstrated in Table 7, the proportion of RA content in RAC is 40%,

30% and 30% for RA1CSF10, RA2CSF10 and RA2CSF5. This indicatesthat when RA content is approximately 30–40% in RAC, SF contenthas a positive effect on the compressive strength, as reported in thestudy of Corinaldesi and Moriconi [18].

3.2. Tensile splitting strength

The tensile splitting strength of the specimens is determined atthe age of 28 days. The results of the tensile splitting strength andthe ratios of the tensile splitting strength to the compressivestrength with the change of SF and RA content are presented inTables 7 and 8.

In the literature, there are two approaches to evaluate the ten-sile splitting strength [29–31]. In the first method, the ratio of thetensile splitting strength to the compressive strength is considered.The second procedure takes into account the tensile splittingstrength only. The evaluation is able to be done using both meth-ods, also using the regression analysis between the compressivestrength and/or the tensile splitting strength.

It is seen that the tensile splitting strengths of the specimensRA1C with 0% and 5% SF content are approximately equal to thoseof NAC specimens with same SF content. This means that the usageof the recycled fine aggregate, which is called RA1, has no signifi-cant impact on the tensile splitting strength. Besides, it is interest-ing to note that for 0% and 5% SF contents, the tensile splittingstrengths of the specimens contains the recycled coarse aggregatecalled RA2 are greater than the strengths of the conventional con-crete specimen (NAC). Another interesting result from Table 7 isthat the usage of SF content in RAC has a marginal increasing effecton the tensile splitting strength, i.e., the tensile splitting strength isobtained as 2.24, 2.52 and 2.46 for the specimens RA1C, RA1CSF5and RA1CSF10. Further, a drastic decrease is observed in the tensilesplitting strength if the aggregates RA1 and RA2 used in the samemixture.

Table 8 shows the effect of SF contents on the ratios of the ten-sile splitting strength to the compressive strength. It is observedfrom Table 8 that the ratios of the specimens RA1CSF5, RA2CSF5and RA12CSF5 (i.e., 0.072, 0.084, 0.058) are greater than those ofRA1C, RA2C and RA12C (i.e., 0.068, 0.071, 0.054). This incrementin the ratios stems from the usage of 5% SF content. On the otherhand, when the percentage of SF content is increased from 5% to10%, as seen from Table 8, the ratios of RAC are affected, inversely.At the same time, it is clear that for 5% and 10% percentages, SFcontent has an increasing effect on the ratios of all NAC specimens.Hence the ratio of splitting tensile strength to compressive

0

10

20

30

40

50

NA

C

RA

1C

RA

2C

RA

12C

NA

CS

F5

RA

1CS

F5

RA

2CS

F5

RA

12C

SF

5

NA

CS

F10

RA

1CS

F10

RA

2CS

F10

RA

12C

SF

10

CO

MPR

ESS

IVE

ST

RE

NG

TH

, FC

, MPA

CONCRETE SERIES

Fig. 3. Compressive strength of specimens.

Table 8The ratio of tensile splitting strength to compressive strength, RA content and SFcontent.

Notation SF (%) RA (%) Ratio

1st GroupNAC 0 0 0.063RA1C 40 0.068RA2C 30 0.071RA12C 70 0.054

2nd GroupNACSF5 5 0 0.066RA1CSF5 40 0.072RA2CSF5 30 0.084RA12CSF5 70 0.058

3rd GroupNACSF10 10 0 0.075RA1CSF10 40 0.066RA2CSF10 30 0.068RA12CSF10 70 0.056

H. Dilbas et al. / Construction and Building Materials 61 (2014) 50–59 55

strength of RAC is 5.4–8.4% and the ratio of splitting tensilestrength to compressive strength of NAC is 6.3–7.5%. It is clear thatthe gap between the upper and lower limits of the ratios of RAC isgreater than that of NAC. The similar results is found by Jau et al.[40] that the ratio for RAC is 7.44–12.72% and that of NAC is8.25–11.13%.

In Table 8, the variation of the ratios with RA content is pre-sented for the three groups tested. The results show that RA pro-portion in concrete mixture up to 40% increases the ratios of thespecimens in the first and the second groups, and the further in-crease in the RA content (i.e., 70%) yields a decrease in the ratiosof both groups. A careful inspection of Table 8 indicates that themost appropriate proportion of RA content is 30% for RAC with/without SF. A similar recommendation for RA content was madeby the study of Evangelista and Brito [27]. However, it is explicitthat the ratios of the third group are reduced with all RA propor-tions when SF content is 10%.

3.3. Correlation between compressive strength and tensile splittingstrength

The correlation between compressive strength and tensilestrength was tested. In Figs. 4 and 5, the relationships between

the compressive strength and the tensile splitting strength of theconcrete are displayed. From Fig. 4, the correlation coefficient forall specimens is found as 0.46. It can be realized from Fig. 5 thatthere is a high correlation between the results of NAC, NACSF5and NACSF10 (i.e. 0.70), as represented by the red trend line. How-ever, the low correlation coefficients, i.e., 0.28, 0.27 and 0.38 forspecimens included RA1, RA2 and RA12 are obtained from theregression analyses. Similarly in an experimental study by Kouand Poon [30], low correlation coefficient (0.355) between thecompressive strength and the tensile splitting strength of RACwas mentioned. The analyses points out that the correlation be-tween the compressive strength and the tensile splitting strengthof RAC is poor. This might be due to the fact that RA content signif-icantly improves the tensile splitting strength of the concreteaccording to the compressive strength, as can be seen from Table 7.

On the other hand, as presented in Table 9 the strength resultsof specimens included NA have the greatest standard deviations(i.e. 6.1115 MPa for compressive strength and 0.6723 MPa for ten-sile splitting strength). Also it is presented in Fig. 5 that the speci-mens included NA, have the highest correlation coefficient (i.e.0.70). Evaluating Table 9 and Fig. 5, the specimens included RAhave the low standard deviations and low correlation coefficients.Hence it can be commented that low correlation coefficients be-tween compressive and tensile splitting strengths is not originat-ing from the high scatter in the strength results of specimens,however it is originating, as it is stated above, that RA content sig-nificantly improves the tensile splitting strength of the concreteaccording to the compressive strength, as can be seen from Table 7.

In [41], concrete cube specimens used for compressive strengthtesting and a precast reinforced concrete column were crushed toproduce RA. The compressive strength class of the cubes specimensand the precast column were C30/37 and C40/50, respectively [41].The standard deviations of compressive strengths of the specimensincluded 0–50–100% RA contents were calculated such as1.5769 MPa, 1.2089 MPa and 3.5016 MPa, respectively [41] andhigh standard deviation was found due to 100% RA content. Itshould be noted that the highest standard deviation is calculatedfor specimens included NA in this study.

In addition new prediction approaches of splitting tensilestrength can be developed in further researches. In the past someresearchers occupied with the relation of between the compressivestrength and tensile splitting strength to predict the close results

y = 0.07x - 0.19R² = 0.46

0.00

1.00

2.00

3.00

4.00

5.00

15 20 25 30 35 40 45 50 55 60

TE

NSI

LE

SPL

ITT

ING

ST

RE

NG

TH

, FC

TS,

MPA

COMPRESSIVE STRENGTH, FC, MPA

Fig. 4. Regression between tensile splitting strength, and compressive strength of all specimens.

y = 0.09x - 1.00R² = 0.70

y = 0.07x + 0.10R² = 0.28

y = 0.06x + 0.64R² = 0.27

y = 0.04x + 0.47R² = 0.38

0.00

1.00

2.00

3.00

4.00

5.00

10 15 20 25 30 35 40 45 50 55 60 65

TE

NSI

LE

SPL

ITT

ING

ST

RE

NG

TH

, FC

TS,

MPA

COMPRESSIVE STRENGTH, FC, MPA

NA RA1 RA2 RA12

Fig. 5. Regressions between tensile splitting strength, and compressive strength of specimens (Correlation coefficients are 0.70, 0.28, 0.27, and 0.38 for specimens includingNA, RA1, RA2, and RA12).

56 H. Dilbas et al. / Construction and Building Materials 61 (2014) 50–59

[i.e. 25,40,42–45]. In [25,40,43,44], formulas were developed forpredicting tensile strength of RAC with high regression coefficients.In [42], the formulas predicted tensile strength given in literatureand by ACI were compared. The formula given by ACI predictedthe tensile strength higher for RAC and NAC. In [45], the relationbetween the relative splitting tensile strength and the relativecompressive strength was examined and high regression coeffi-cient between the relative splitting tensile strength and the rela-tive compressive strength was calculated. The relative splittingtensile strength is the ratio between the 90-day splitting tensilestrength of RAC to that of NAC and the relative compressivestrength is the ratio between the 7-day compressive strength ofRAC to that of NAC.

3.4. Static elasticity modulus

The test results of the modulus of elasticity of the specimens atthe age of 28 days are presented in Table 7. In order to present the

relative elasticity modulus in Fig. 6, the elasticity modulus of eachspecimen is normalized by the elasticity modulus of NAC speci-men. As seen from Table 7, the elasticity modulus of all RAC spec-imens is lower than the elasticity modulus of the controlspecimens NAC for all groups, and the value of the elasticity mod-ulus of all RAC specimens decrease with an increase of RA content[30]. Generally concrete’s elasticity modulus on large scale de-pends on the stiffness phases (interfacial transition zones, cementpaste and aggregates) [31]. It is well-known that elasticity modu-lus is influenced considerably by modulus of aggregates [46]. Inthis context, the elasticity modulus of RAC is affected by RA. There-fore the low properties of RA such as cracks and higher porosity ofadhered old mortar with comparatively low modulus of elasticitydecreases the elasticity modulus of RAC. On the other hand, itwas reported that brick and tile content in the RA did not haveremarkable effect on the value of elasticity modulus [47].

It is found that relative elasticity modulus of NACSF5, RA1CSF5,and RA2CSF5 are close to each other with value 0.91 in the same

Table 9Standard deviations and mean values of compressive strength and tensile splittingstrength of specimens.

Test Statisticalvalue

Specimens included:

NA RA1 RA2 RA12

Compressivestrength (MPa)

Standarddeviation

6.1115 3.5862 3.4158 4.7702

Mean value 40.83 35.00 35.93 30.41

Tensile splittingstrength (MPa)

Standarddeviation

0.6723 0.4440 0.3688 0.3132

Mean value 2.76 2.41 2.67 1.71

H. Dilbas et al. / Construction and Building Materials 61 (2014) 50–59 57

group, although their compressive strengths are different. Thecompressive strength of NAC increases steadily while SF contentin NAC increases from 0% to 10%. However, the elasticity modulusof NAC decreases while SF content is up to 5% in NAC, and then theelasticity modulus of NAC increases when SF content exceeds 5% inNAC.

3.5. Density and water absorption ratio

The experimental studies, the effects of aggregate type and SFcontent on the density and the water absorption are evaluated atthe age of 28 days. The test results are displayed in Table 7 andFig. 7. It is well-known that the absorption capacity of recycled

1.00

0.830.90

0.810.91 0.9

0.00

0.20

0.40

0.60

0.80

1.00

1.20

NA

C

RA

1C

RA

2C

RA

12C

NA

CS

F5

RE

LA

TIV

E E

LA

STIC

ITY

MO

DU

LU

S

CONCR

Fig. 6. Relative elasticity modulus of specimens

1500

1750

2000

2250

2500

2750

3000

0 0.02 0.04 0.06

Den

sity

, kg/

m3

Water Ab

NA RA1

Fig. 7. Correlation between water absor

aggregate is higher than that of natural aggregate. The higherabsorption rate of the cement mortar attached to the aggregateparticles causes the higher water absorption of the RAC. Underthe light of this information, it can be inferred that the inclusionof RA content in the specimens makes the specimens more porous,and this leads to a decrease in the density and an increase in thewater absorption [22,26].

In this context, the greatest density, and the lowest waterabsorption is obtained for NAC specimens. It is well-known thatwater absorption requires linked and open cracks in the structureof aggregate and RA contains cracks due to the crushing process. Asseen from Table 7, the increase of RA content in RAC increases thewater absorption of RAC as expected for all mixtures. Also theimpurities in RA increases the water absorption of RAC. In addition,the water absorption of all NAC and RAC specimens is raised until5% SF content, after this value, all water absorption values aredecreased.

The relation between the water absorption and the density isplotted in Fig. 7. It is estimated that there is an inverse relationshipbetween the density and the water absorption ratio.

4. Conclusions

In this study, the effects of RA and SF on the mechanical and thephysical properties of concrete are presented. Based on the aboveresults, the following conclusions can be drawn:

1 0.910.78

0.990.89

0.75 0.79

RA

1CS

F5

RA

2CS

F5

RA

12C

SF

5

NA

CS

F10

RA

1CS

F10

RA

2CS

F10

RA

12C

SF

10

ETE SERIES

in comparison to elasticity modulus of NAC.

y = -6068.69x + 2614.24R² = 0.72

0.08 0.1 0.12 0.14sorption, %

RA2 RA12

ption ratio and density of concrete.

58 H. Dilbas et al. / Construction and Building Materials 61 (2014) 50–59

1. The compressive strength of the RAC decreases with thereplacement of the NA with RA. However, SF addition isan alternative way to increase the compressive strengthof RAC to use concrete in structural industry. RA1CSF10,RA2CSF10 and RA2CSF5 can be suitable to use instead ofNAC, if compressive strength is considered only.

2. The tensile splitting strength of the specimens containing0% and 5% SF contents increases by replacing the NA withRA.

3. The addition of the 5% SF content in RAC increases the ratioof the tensile splitting strength to the compressivestrength. However, the percentage of SF content isincreased from 5% to 10%, the ratio of the tensile splittingstrength to the compressive strength of RAC decreases.

4. The proportion of RA in concrete mixture up to 30% affectsthe ratios of the tensile splitting strength to the compres-sive strength increasingly. After 30% proportion of RA inthe mixture, the ratio decreases.

5. The recycled aggregate (RA) affects more on the splittingtensile strength of RAC rather than on the compressivestrength of RAC. Hence the low correlation coefficientbetween the compressive strength and the tensile splittingstrength is obtained.

6. For all groups the value of the elasticity modulus of all RACspecimens decreases with the increase of RA content.

7. The inclusion of RA content in the specimen decreases thedensity and increases the water absorption.

In summary, it can be demonstrated to researchers and design-ers that suitable proportion of the replacement of NA with RA is30%. Also, SF is a mineral addition that improves the performanceof RAC. The properties of recycled aggregates (RA) can differdepending on their source. Therefore, before the utilization of recy-cled aggregate concrete (RAC) in structural elements, the trial spec-imens should be produced by using available recycled aggregateand they should be tested to determine the physical and themechanical properties of current RAC.

Acknowledgements

This work forms a part of the MSc thesis which will be submit-ted by the first author to Institute of Science and Technology ofYıldız Technical University, _Istanbul.

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