Scholarly article on topic 'Influence of silica fume on mechanical and physical properties of recycled aggregate concrete'

Influence of silica fume on mechanical and physical properties of recycled aggregate concrete Academic research paper on "Civil engineering"

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Abstract of research paper on Civil engineering, author of scientific article — Özgür Çakır, Ömer Özkan Sofyanlı

Abstract Several studies related to sustainable concrete construction have encouraged development 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 aggregates in concrete are presented. Portland cement was replaced with SF at 0%, 5% and 10%. Specimens were manufactured by replacing natural aggregates with recycled aggregates. Two size fractions (4/12mm and 8/22mm) as recycled aggregates were used and four series of concrete mixtures 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 (16–21cm) were prepared. Concrete properties 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 recycled 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 containing 4/12mm fraction recycled aggregates showed better performance among recycled aggregate concretes.

Academic research paper on topic "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

http://ees.elsevier.com/hbrcj

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

OzgUr CCakir *, Omer Ozkan Sofyanli 1

Yildiz 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 development 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 aggregates in concrete are presented. Portland cement was replaced with SF at 0%, 5% and 10%. Specimens 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 mixtures 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 properties 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 recycled 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 containing 4/12 mm fraction recycled aggregates showed better performance among recycled aggregate concretes.

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

Center.

* Corresponding author. Tel.: +90 2123835242; fax: +90 2123835133.

E-mail addresses: cozgur@yildiz.edu.tr (O. Cakir), omerozkansofyanli @windowslive.com (O.O. Sofyanli).

1 Tel.: +90 2123835242; fax: +902123835133. Peer review under responsibility of Housing and Building National Research Center.

Introduction

Many structures in Turkey are now either reaching the end of their design life time or were not constructed according to the specifications. The Law 'on Transformation of the Areas under Disaster Risk' dated 16 May 2012, and numbered 6306, is called Urban Renewal Law among society, regulates the circumstance of recent structures. Due to the Urban Renewal Law, demolition and maintenance of these structures result in a large amount of concrete rubbles. The improper dis-

1687-4048 © 2014 Production and hosting by Elsevier B.V. on behalf of Housing and Building National Research Center. http://dx.doi.Org/10.1016/j.hbrcj.2014.06.002

posal of construction and demolition (C and D) waste is a problem faced by municipalities, not only in Turkey, but also in 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 construction sector's major contributions to the preservation of the environment and sustainable development is the reuse and recycling of the waste materials it generates. One way of achieving this is to introduce recycled aggregates from C and D debris and rubble into the production processes. This increases the life cycle of these materials, thereby reducing the amount of waste dumping and natural resource extraction [2]. On the other hand, construction activities demand a significant amount of natural materials, such as sand and gravel, and extraction of these natural materials modifies the course of rivers and its beds, creating environmental problems [3].

Although construction industry world-wide is promoting the use of recycled aggregates for concrete production, mainly to respond to the problem of the depletion of natural aggregates, the current specifications in many parts of the world are not able to support and encourage the recycling of C and D waste. Since recycling of demolition waste was first carried out after the Second World War in Germany, research work carried out in several countries has demonstrated sufficient promise for developing use of construction waste as a constituent in new concrete [4].

Recycled concrete aggregates mainly differ from natural aggregates in that they are composed of two different materials: natural aggregate and cement mortar attached. Cement mortar is the origin of the worse properties of recycled aggregates: lower density, higher absorption, and Los Angeles abrasion [5]. Recycled aggregates are also highly heterogeneous and porous, as well as contain a high content of impurities. The heterogeneity influences the characteristics of recycled aggregates and these aggregate properties have a negative influence on recycled aggregate concrete quality such as reduction of the compressive strength, tensile strength due to the increased concrete porosity and a weak aggregate-matrix interfacial bond [6,7].

Chandra [1] reported that the construction and demolition waste coarse aggregate can produce a range of concretes with acceptable compressive strength, exceeding 30% water reducing admixtures or larger amount of cement is to be used. Recycled aggregate concrete mixes require more water than natural aggregate concrete to maintain the same workability without the use of admixtures and this affects the quality and strength of the concrete, resulting in lower concrete strength. It is known that when using the same water to cement ratio, as recycled aggregate percentage increase, the mechanical and physical properties of the recycled aggregate concrete deteriorate. Although concrete production is one of the high-grade applications where recycled aggregate can be used, concrete strength decreases when recycled aggregates were used and the strength reduction could be as low as 40% [8-10]. Concrete made with 100% of recycled coarse aggregates has 20-25% less compression strength than conventional concrete at 28 days, with the same effective w/c ratio (w/c = 0.50) and cement quantity (325 kg/m3) [11]. On the other hand, the general trend in the tensile strength as measured by the splitting tensile test, depends mainly on binder rather than aggregate type [12]. The use of coarse aggregate made from recycled concrete with strength equal to 50 MPa results in compressive

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

Recycled aggregate is more absorptive than natural aggregate and one of the marked physical differences between recycled and natural aggregate is higher water absorption rate. The higher water absorption of the recycled aggregate results from the higher absorption rate of cement mortar attached to the aggregate particles. This characterizes concrete made with higher water absorption recycled aggregate reduced compres-sive strength than with natural aggregate [14-18]. Under the same mixture proportions, the mechanical properties of recycled aggregate concrete were worse than those for natural aggregate concrete. When recycled aggregate was washed, these negative effects were greatly improved [10]. Moreover, adding SF as a supplementary binder material can also improve 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 recycled aggregate concrete. The effects of silica fume on the mechanical and physical properties such as compressive strength, tensile splitting strength, water absorption, capillarity coefficient and ultrasonic pulse velocity, of recycled aggregate concrete that was cured in water up to 90 days were investigated.

Experimental details

Materials Binders

The cementitious materials used in this experimental study were Portland Cement CEM I 42.5R cement type according to 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, physical 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 concrete mixes. In this study, two size fractions of crushed limestone coarse aggregates were used, one with nominal sizes of 4/12 mm (NA1) and the other with nominal size of 8/22 mm (NA2), as the natural coarse aggregate. Size fraction of silica based natural aggregate (river sand) with nominal sizes of 0/ 4 mm was used as the natural fine aggregate (sand) in the con-

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

Table 2 Properties of SF.

Contents Composition (%)

SiO2 CaO SO3 Structure of material Color Density (kg/liter) Chlorine ratio Specific surface area (m2/kg) Activity index (%) Particle ratio (<0.045 mm) >85 <1 <2 Condensed microsilica Amber 0.55-0.70 <1 15,000 >95 <40

Table 4 Properties of superplasticizer.

Content Composition (%)

Structure of material Color Density (kg/liter) Chlorine ratio (%) Alkaline ratio (%) Polycarboxylic ether Amber 1.082-1.142 <0.1 <3

crete mixes. The recycled aggregate contained almost entirely of crushed concrete rubbles obtained from building demolition projects. By using laboratory jaw crusher, demolition waste aggregates were crushed to obtain a cumulative grain size distribution curve similar to that prepared with the natural coarse aggregates. Two size fractions (4/12mm-RA1; 8/22mm-RA2; and both of two-RA1/2) as recycled aggregates were used. Density test and water absorption test were carried according to TS EN 1097-6:02 [25], Los Angeles abrasion resistance tests were carried according to TS EN 1097-2:2000 [26]. The physical properties of natural and recycled aggregates are shown in Table 3.

Concrete mixtures

Four series of concrete mixtures were prepared in the laboratory using a Pan mixer. SF was used as a cement replacement on a weight basis. In all concrete mixtures, a constant water/ binder ratio of 0.50 was used and concrete mixtures with a target 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 of water-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 and the 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 for each concrete mixture. The cylinders were used to evaluate the tensile splitting strength. 150 mm cubes were used to determine the compressive strength and ultrasonic pulse velocity, 70 mm cubes were used to determine the capillary and water absorption. All the specimens were cast in steel molds and compacted using vibrating table. After demoulding, the specimens were

cured in water-curing tank at 20 ± 2 0C till test age at 7, 28, and 90 days.

Compressive and tensile splitting strengths The compressive strength test was carried out according to TS EN 12390-3:10 [27], while the tensile splitting strength was conducted following TS EN 12390-6:10 [28]. Compressive and tensile splitting strengths of concrete were determined using a compression machine with a loading capacity of 3000 kN. The loading rates applied in the compressive and tensile splitting strength tests were 10.6 kN/s and 1.6 kN/s, respectively. 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 absorption test were conducted following TS EN 480-5:01 [29] and TS 12390-7:10 [30], respectively. Water absorption tests were performed on 70 mm cubic specimens. Three specimens for the capillary water absorption and another three for the volumetric water absorption tests were used for each of the concrete mixes. For the capillary water absorption test, lower face (parallel to the trowelled upper face) was brought in contact with water in a tray. Environmental temperature was 20 ± 2 0C during the test. Absorbed water was measured at different intervals. Initial slope of the curve of absorbed water-square root of time was calculated representing the capillary water absorption coefficient. For the volumetric water absorption 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 saturated 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 were made with commercially available portable equipment (PUNDIT); paraffin wax was used as a couplant between the trans-

Table 3 Physical properties of river sand, natural aggregates, and recycled aggregates.

Type Bulk density (kg/m3) Water absorption (%) Initial moisture content (%) Abrasion resistance (LA)

Natural sand 2550 1.7 1.2 -

4/12 mm natural aggregate 2670 2.2 0.7 LA20

8/22 mm natural aggregate 2670 2.0 0.8 LA20

4/12 mm recycled aggregate 2370 7.4 3.4 LA35

8/22 mm recycled aggregate 2380 5.1 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)

Çomposite 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 a direction perpendicular to the casting direction.

Results and discussion

Compressive strength

The results of the compressive strength of the concretes with and without recycled coarse aggregate at the ages up to 90 days are shown in Table 7. Each presented value is the average of three measurements. Although the use of recycled aggregates diminishes the compressive strength of the concrete, depending on the percentage of recycled aggregates used [3,7,9], for concrete mixtures with similar mixture proportions and workability, there was no significant difference at the 5% confidence level in the 7, 28 and 90 days compressive strengths of concrete made with recycled aggregate (RA1, RA2 and RA1/2) and compressive strengths of natural aggregate concrete (NA) in this study. Crentsil and Taylor [12] reported that compressive

strengths of concrete made with commercial recycled aggregate and normal weight natural basalt aggregate concrete are similar at 28 days. Moreover, Fonteboa and Abella [2] reported that, with the same workability, it is possible to produce recycled aggregate concrete with 50% recycled aggregates with almost the same compressive strength as natural aggregate concrete. By washing recycled aggregates prior to using in concrete 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 standard water curing significantly increased with curing age. In general, the concrete incorporating silica fume underwent a reduction in early age compressive strength of natural and recycled aggregate 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% of silica fume was reduced by 6% and 12%, respectively, in comparison to the strength of the control concrete without silica fume. The compressive strength of concrete mixture RA1 with 5% and 10% of silica fume was reduced by 3%, in comparison to the strength of the control concrete without silica fume.

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Moreover, concrete mixture RA2 with 5% and 10% of silica fume was reduced by 3% and 9%, and concrete mixture RA1/2 with 5% and 10% of silica fume was reduced by 2% and 4%, in comparison to the strength of the control concrete without silica fume Fig 2 shows the compressive strength of the concretes Compressive strength loss of concretes containing recycled aggregate was less than the concretes containing natural aggregate at 7 days due to SF usage At 28 days, the strengths of the concrete mixtures NA, RA2 and RA1/2 with 5% and 10% of silica fume were reduced but concrete mixture RA1 with 5% and 10% of silica fume was increased by 8% and 23%, in comparison to the strength of the control concrete without silica fume The highest percentage gain in compres-sive strength was recorded for 10% silica fume RA1 followed by 5% silica fume RA1. The concrete mixture RA1SF10 had the highest compressive strength gain (23%) . At 90 days, the strength of all the concrete mixtures with 5% and 10% of silica fume was increased, in comparison to the strength of the control concrete without silica fume The strength of the concrete mixture NA with 5% and 10% of silica fume was increased by 4% and 5%, respectively, in comparison to the strength of the control concrete without silica fume Moreover, the compres-sive strength of concrete mixture RA1 with 5% and 10% of silica fume was increased by 19% and 21%, respectively, in comparison to the strength of the control concrete without silica fume Concrete mixture RA2 with 5% and 10% of silica fume was increased by 6% and 13%, respectively, in comparison 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 the strength of the control concrete without silica fume

Fig 1 shows the compressive strength of the concretes The highest percentage gain in compressive strength was recorded for 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 incorporating silica fume into the recycled aggregate concrete SF's effects (the pozzolanic effect and filler effect) improve all the mechanical properties of the concrete but, particularly, its compressive strength [2] Due to the recycled aggregates being more porous, some part of the cement and silica fume would be able to penetrate into the aggregate, which subsequently would increase the bond strength between the aggregates and hydrated cementitious matrix With the presence of silica fume, the cracks in the recycled aggregates were reduced due to the healing effect after longer curing of silica fume blended cement pastes Therefore the concrete made with recycled concrete aggregate, and the quality of the interfacial transition zone, was better than that of the old paste and natural aggregate concrete The bond between the new cement paste and recycled concrete aggregate was enhanced [21,32]

Tensile splitting strength

The tensile splitting strengths of the concretes at the ages up to 90 days are shown in Table 7 The tensile splitting strengths of recycled aggregates concretes are higher than the tensile splitting 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 recycled aggregates 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 this is especially true for the flexural strength of the recycled aggregate concrete. Tabsh and Abdelfateh [13] found that, the use of coarse aggregate made from recycled concrete with strength equal to 50 MPa results in compressive and tensile strengths comparable with that obtained when using natural coarse aggregate. At all the test ages the tensile splitting strength gain of the concrete mixture NA with and without SF was higher than that of the recycled concrete mixtures. At 7 days, the tensile 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

of the control concrete without silica fume. Moreover, the tensile splitting strengths of concrete mixture RA1 with 5% and 10% of silica fume were increased by 10% and 11%, respectively, in comparison to the strengths of the control concrete without silica fume. Compressive strength loss was observed in concrete mixtures RA2 and RA1/2. Concrete mixture RA2 with 5% and 10% of silica fume was reduced by 2% and 9%, respectively, in comparison to the strength of the control concrete without silica fume and concrete mixture RA12 with 5% and 10% of silica fume was reduced by 3% and 9%, respectively, in comparison to the strength of the control concrete without silica fume. Fig. 2 shows the tensile splitting strengths of the concretes. Tensile splitting strengths of con-

10 20 30 40 50 60 Days 70 80 90 100

♦ NA ■ NASF5 A NASF10 RA1 X RA1S5

• RA1S10 + RA2 - RA2S5 RA2S10 ♦ RA1/2

■ RA1/2S5 A RA1/2S10 -NA NASF5 ...........NASF10

RA1 RA1S5 -Log. (RAI S5) RA1S10 RA2

RA2S5 RA2S10 RA1/2S5 RA1/2S10

Fig. 2 Tensile splitting strength of concrete mixture.

crete series except NA and RA1 decreased with use of SF. The highest percentage gain in the tensile splitting strengths was recorded for 5% silica fume NA followed by 10% silica fume NA at 7 days. At 28 days, the tensile splitting strengths of the concrete mixture NA with 5% and 10% of silica fume were increased by 17% and 19%, respectively, in comparison to the strengths of the control concrete without silica fume but concrete 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 percentage gain in tensile splitting strength was recorded for 10% silica fume NA followed by 5% silica fume NA. At 90 days, the tensile splitting strengths of the concretes containing both natural and recycled aggregates with and without SF were increased significantly. The concrete mixture NA with 5% and 10% of silica fume was increased by 16% and 33%, respectively, in comparison to the tensile splitting strength of the 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 without silica fume. Concrete mixture RA2 with 5% and 10% of silica fume was increased by 8% and 9%, respectively, in comparison to the strength of the control concrete without silica fume. Concrete mixture RA1/2 with 5% and 10% of silica fume was increased by 8% and 6%, respectively, in comparison to the strength of the control concrete without silica fume. The highest percentage gain in the tensile splitting strengths among recycled aggregate concretes (RA1, RA2 and RA1/2) was recorded for 10% silica fume RA1 followed by 5% silica fume RA1, similar to compressive strength test results. However, comparing the results at 90 days shows that there was a continuous and significant improvement in the tensile splitting strength of recycled aggregate concretes. Adding ultra fine SF 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 terms of mechanical performances [21,32].

As reported by many researchers [7,11], the workability of recycled aggregate concretes for the same water content in the concrete is lower, especially when the replacement levels exceed 50% [33]. In this study, the recycled aggregate concretes showed a better mechanical performance rather than the natural aggregate concrete up to 90 days due to the SF usage and the constant workability of the concrete mixes.

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

Water absorption

The volumetric water absorption values of concrete containing the recycled aggregates are higher than those of the normal concrete up to 90 days and it shows an increasing trend toward higher recycled coarse aggregate content as shown in Table 7 and Fig. 3. At 7 days, the highest absorption value is attained for RA1/2 specimens followed by RA1 and RA2, which are 22%, 18% and 16% times higher, respectively, than NA. At 28 days, the highest absorption value is attained for RA1/2 specimens followed by RA1 and RA2, which are 23%, 20% and 14% times higher, respectively than NA. At 90 days, the highest absorption value is attained for RA1/2 specimens fol-

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

lowed by RA2 and RA1, which are 21%, 17% and 14% times higher, respectively than NA. It was due to the high absorption capacity of recycled coarse aggregate itself, which has created higher osmosis pressure within the concrete. When the dry specimens are immersed into water during the testing, the specimens with high osmosis pressure tend to absorb more water from the surrounding of the specimens. Recycled aggregate surface coated with mortar phase has a higher porosity than aggregate phase [2,34]. The absorption values of concrete containing the recycled coarse aggregates are higher than those of the normal concrete and results show an increasing trend toward higher recycled concrete aggregate content [35,36]. However, the volumetric water absorption values of concrete containing the recycled aggregate with SF were decreased significantly especially at later ages. This effect is more significant in recycled aggregate concretes incorporating 10% SF rather than recycled aggregate concretes incorporating 5% SF.

Capillarity coefficient

According to the experimental results of capillary pore investigation, it can be seen from Table 7 that the capillarity coefficient of the specimens with recycled aggregate is higher compared to the specimens with natural aggregate. As seen from Fig. 4, the capillarity coefficient decreases in all concrete mixes with time for both NA and RA. At 7 days, the highest capillarity coefficient value is attained for RA1/2 specimens followed 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 followed by 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 than NA. A similar trend can be seen up to 90 days among recycled aggregate concretes. The capillarity coefficient test results show an increasing trend toward higher recycled aggregate content as shown in Table 6. The capillarity coefficient values of concrete containing the recycled aggregate with SF were decreased significantly especially at later ages. This effect is more significant in recycled aggregate concretes incorporating

0 10 20 30 40 50 Days 60 70 80 90 100

♦ NA ■ NASF5 A NASF10 RA1 X RA1SF5 • RA1SF10

+ EA2 - RA2SF5 RA2SF10 ♦ RA1/2 ■ RA1/2SF5 RA1/2SFK

-NA NASF5 ...........NASF10 RA1 RA1SF5 RA1SF10

RA2 RA2S5 RA2S10 RA1/2 RA1/2SF5 RA1/2SF1C

Fig. 3 Volumetric water absorption of concrete mixture .

10% SF rather than recycled aggregate concretes incorporating 5% SF, similar to the volumetric water absorption test results . This situation can be explained as the SF finer than the cement particles fill the capillary pores physically and also pozzolanic property of SF is affective on gel structure [21] .

Ultrasonic pulse velocity

The ultrasonic pulse velocity values of all specimens were in the range of 4 23 km/s to 5 14 km/s and the values tend to increase with the ages as shown in Table 7 and Fig 5 The results presented in Fig 5 clearly show the relation between the average UPV values of specimens with the recycled aggregate content and the age of testing . At 7 days, the UPV value of the concrete mixtures NA and RA1 with 5% and 10% of silica fume was reduced by 5% in comparison to the UPV of

the control concrete without silica fume The UPV value of concrete mixture RA2 with 5% and 10% of SF was reduced by 1% and 5%, in comparison to the UPV of the control concrete without silica fume . Concrete mixture RA1/2 with 5% and 10% of silica fume was reduced by 1% and 3%, in comparison 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 later ages 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 in the range of 3 . 66 km/s-4 . 58 km/s [37] . At 7, 28 and 90 days, all specimens are evidently being in "good" condition . The UPV test is used to predict the characteristic of the internal particles of concrete and the quality of the concrete . The pore structures in the concrete may have an impact on the UPV values and

Fig. 4 Capillarity coefficient of concrete mixture .

Fig. 5 Ultrasonic pulse velocity of concrete mixture.

their strength. Even though adhered mortars are porous materials and it would reduce the UPV at early tages as proven in the experiment, the UPV value of the all replacement levels of the recycled coarse aggregate content is still acceptable as they fall in the "good" category according to Malhotra. As long as the UPV values lie within the "good" category, it implies that a particular concrete does not contain any large voids or cracks which would affect the structural integrity. Therefore, the recycled aggregate is still considered suitable to replace finer and coarse aggregates in large portions as long as it achieves the target strength.

Conclusion

Based on this experimental study, the following conclusions can be drawn:

1. Concretes produced with natural and recycled aggregates incorporating silica fume underwent a reduction in early age compressive strength. The compressive strength decreased with increase in the silica fume content. However, compressive strength loss of concretes containing recycled aggregate was less than the concretes containing natural aggregate at early age due to the SF usage. At 28 and 90 days, the strength of all the concrete mixtures with 5% and 10% of silica fume was increased, in comparison to the strength of the control concrete without silica fume. The pozzolanic effect of the SF that usually occurs after 7 days tends to increase the compressive strength of the concretes with this mineral admixture. This effect is more significant in recycled aggregate concrete incorporating 10% SF and containing 4/12 mm fraction recycled aggregates 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 same SF content.

3. Continuous and significant improvement in the tensile splitting strength of recycled aggregate concretes incorporating SF was observed with time. The concrete with 10% SF and having 4/12 mm fraction recycled aggregates showed a better performance than the other concrete series in terms of the physical and the mechanical properties.

4. Water absorption values of concretes containing the recycled aggregates with SF were decreased significantly especially at later ages. This effect is more significant in recycled 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 Processing Corporation), AKCCANSA Cement Industry and Trading Co., and BASF Chemical Company for providing recycled aggregates, cement and mineral/chemical admixtures.

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