Properties of freshly mixed carbon fibre reinforced self-consolidating concrete

Properties of freshly mixed carbon fibre reinforced self-consolidating concrete

Construction and Building Materials 46 (2013) 224–231 Contents lists available at SciVerse ScienceDirect Construction and Building Materials journal...

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Construction and Building Materials 46 (2013) 224–231

Contents lists available at SciVerse ScienceDirect

Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Properties of freshly mixed carbon fibre reinforced self-consolidating concrete M. Yakhlaf, Md. Safiuddin, K.A. Soudki ⇑ Department of Civil and Environment Engineering, Faculty of Engineering, University of Waterloo, Waterloo, ON, Canada N2L 3G1

h i g h l i g h t s  Carbon fibres greatly affected the filling ability, passing ability, and segregation resistance of SCC mixes.  The T50 slump flow time was increased with the increase in carbon fibres content.  All CFRSCC mixes clearly passed the segregation resistance requirement.  The CFRSCC mixes with carbon fibres content up to 0.75% passed the requirements of SCC.  The carbon fibres were well distributed in all concrete mixtures, as observed from the SEM test.

a r t i c l e

i n f o

Article history: Received 3 January 2013 Received in revised form 1 April 2013 Accepted 5 April 2013 Available online 30 May 2013 Keywords: Carbon fibres Filling ability Passing ability Segregation resistance Self-consolidating concrete Visual stability index

a b s t r a c t This study examined the effects of discrete pitch-based carbon fibres on the fresh properties of self-consolidating concrete. Different carbon fibre reinforced self-consolidating concrete mixtures were produced incorporating 0%, 0.25%, 0.5%, 0.75% and 1% carbon fibres by concrete volume with two water-to-binder ratios (0.35 and 0.40). The flowing ability (filling ability and passing ability) of the concrete mixtures was determined with respect to slump flow, J-ring slump flow, and T50 slump flow time. The segregation resistance of the concrete mixtures was evaluated by using the sieve stability test. Visual stability index (VSI) was also used to assess the segregation resistance of concrete. Moreover, the freshly mixed concrete mixtures were tested for air content and unit weight. The hardened concretes were tested by a Scanning Electron Microscope to observe the distribution of fibres. Test results revealed that the increased amount of carbon fibres decreased the filling ability and passing ability of concrete. However, carbon fibres had no adverse effects on the segregation resistance of concrete. Also, no significant air entrapment occurred in the presence of carbon fibres. Carbon fibres were well-distributed and they slightly decreased the unit weight of concrete. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Self-consolidating concrete (SCC) is a special kind of concrete, which was developed in Japan. In construction industry, Japan faced the shortage of skilled concrete workers to produce highquality durable concrete structures; hence Japanese engineers and researchers started to develop a different kind of concrete that can be placed and finished with less skilled workers. As a result, the concept of SCC was first proposed in Japan in 1986 [1]. Ozawa and Maekawa carried out many studies to develop SCC at the University of Tokyo in Japan [2,3]. Japan was the first country that worked intensively to pragmatically use SCC in civil engineering structures [4].

⇑ Corresponding author. Tel.: +1 519 888 4494; fax: +1 519 888 4349. E-mail address: [email protected] (K.A. Soudki). 0950-0618/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2013.04.017

SCC has many advantages over conventional concrete. SCC flows under own weight and does not require any vibrating equipment; it is an ideal material for smooth finishing and for heavily reinforced structural members. The key fresh properties of SCC are flowing ability (filling ability and passing ability) and segregation resistance. The performance requirements for filling ability, passing ability, and segregation resistance must be met to produce SCC successfully. Therefore, these properties should be determined carefully using proper test methods. The three key fresh properties of SCC can be evaluated by using various methods. These methods are either available as standard tests or proposed by some researchers. Currently American Society for Testing and Materials (ASTM) has the standard test methods to evaluate the aforementioned three key fresh properties of SCC [1,5,6]. Many studies have been carried out on the fresh and hardened properties of SCC [7]. Recently, several types of fibre such as steel, glass, and polypropylene fibres have been used to produce fibre

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reinforced SCC [8–13]. In comparison, limited studies were carried out to use carbon fibres in SCC. Adding carbon fibres to SCC significantly decreases its flowing ability (filling ability and passing ability). Carbon fibres may restrict and prevent coarse aggregates from moving uniformly, thus causing flowing ability problem [14]. However, the incorporation of carbon fibres improves many mechanical and electrical properties of SCC such as compressive and flexural strengths, toughness, and electrical conductivity [14–16]. Carbon fibres are attractive to engineers due to their low density and high thermal conductivity. Carbon fibres can be used to eliminate or reduce drying shrinkage and cracking problems. Many studies have been conducted since 1970s to investigate the effectiveness of carbon fibres on the various properties of concrete [17]. Carbon fibre reinforced concrete has been used in many projects because of its good thermal conductivity, lightweight, and high modulus of elasticity [18,19]. It has also been used significantly to produce curtain walls, partition panel, and formwork for walls [18]. However, none of the aforementioned studies used carbon fibres in SCC for applications in civil engineering structures. Two types of carbon fibres are commercially available for use in concrete; polyacrylonitrile (PAN)-based and pitch-based carbon fibres. PAN-based carbon fibres have a very high modulus of elasticity and high tensile strength; they have been mostly used to produce aerospace and sport equipment [17,18]. Although PAN-based fibres were the first type of discrete short carbon fibres used in reinforced concrete, presently they are rarely used in civil engineering applications due to their high cost. Pitch-based carbon fibres are usually used in civil engineering applications because of their lower cost even though pitch-based carbon fibres have lower modulus of elasticity than PAN-based fibres. Pitch-based carbon fibres are used in many industrial fields due to their lightweight, good chemical stability, and high heat and excellent abrasion resistance [18]. The use of pitch-based carbon fibres in the reinforced concrete leads to increases in flexural strength about 85%, flexural toughness about 205%, and compressive strength about 22%; on the other hand, the drying shrinkage can be decreased by up to 90% and the electrical resistivity up to 83% [20,21]. These advantages make pitch-based carbon fibres more attractive for use in SCC. Although many studies were conducted on the use of pitch-based carbon fibres in concrete, limited research has been carried out to produce SCC using this kind of fibres. In the present study, pitch-based discrete carbon fibres have been used to produce SCC. The freshly mixed carbon fibre reinforced SCC mixtures were tested for filling ability, passing ability, segregation resistance, air content, and unit weight. In addition, the distribution of carbon fibres in hardened SCC mixtures was examined using a Scanning Electron Microscope. From the test results, the effect of pitch-based carbon fibres on the tested properties was observed.

2. Research significance Carbon fibre reinforced concrete without SCC properties has been well researched. Also, SCC incorporating steel and polymer fibres has been significantly studied. However, limited study has been conducted on the use of carbon fibres in SCC. Incorporating carbon fibres in SCC can produce a high quality special concrete known as carbon fibre reinforced self-consolidating concrete (CFRSCC). CFRSCC would offer the benefits of both carbon fibres and SCC. The main purpose of this study was to examine the effects of discrete pitch-based carbon fibres on the major fresh properties of SCC. The research outcome shall be useful to produce and

commercialize CFRSCC as a new or repair material for use in concrete structures. 3. Experimental investigation 3.1. Constituent materials Normal (Type I) Portland cement, crushed limestone (coarse aggregate, CA), manufactured sand (fine aggregate, FA), silica fume (SF), high-range water reducer (HRWR), and tap water (W) were used in this study. The manufactured sand conformed with the specification OPSS 1002 [22]. Fig. 1 shows the pitch-based carbon fibres (CFs) that was used in this study. Table 1 shows the physical properties of the concrete constituent materials. 3.2. Concrete mixture proportions A total of ten non-air-entrained SCC mixtures incorporating different contents of pitch-based carbon fibres were produced in this study. Two water-to-binder (W/B) ratios of 0.35 and 0.40 were used in these concrete mixtures. Silica fume was kept constant at 10% by weight of binder. Two mixtures were dealt as control with no fibres and eight mixtures had different percentages of carbon fibres. Table 2 presents the details of the CFRSCC mixture proportions. The mixtures were divided into two groups based on the W/B ratio. The first group had a W/B ratio of 0.35, and the second group had a W/B ratio of 0.4. HRWR was added to the CFRSCC mixtures to enhance their filling ability and passing ability; the HRWR dosages for the first and second groups were 1.5–8.0% and 1.0–7.0%, respectively. 3.3. Preparation of concrete The concrete mixtures were prepared using a pan-type revolving mixer of 50 l maximum capacity. The coarse and fine aggregates were first charged and mixed together with a 1=4 of the total mixing water for 60 s. Then, the binder (cement

Fig. 1. Pitch-based carbon fibres.

Table 1 Physical properties of constituent materials. Material

Properties

Normal Portland cement (C) Crushed limestone coarse aggregate (CA)

Relative density: 3.15 Maximum aggregate size: 10 mm Saturated surface-dry based relative density: 2.74 Absorption: 1.13% Moisture content: 0.393% Relative density : 2.68 Absorption: 1.15% Moisture content: 0.144% Relative density: 1.85 Tensile strength: 1770 MPa Tensile modulus: 180 GPa Length: 10 mm Diameter: 17 lm Relative density: 1.064 Solid content: 33% Relative density: 2.2 Total solids: 430 mg/L Density at 24 °C: 997.28 kg/m3

Manufactured concrete sand (FA)

Pitch-based carbon fibres (CFs)

High-range water reducer (HRWR) Silica fume (SF) Normal tap water (W)

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Table 2 Details of concrete mixture proportions. Concrete mix

W/B ratio

CA (kg)

FA (kg)

M1 M2 M3 M4 M5 M6 M7 M8 M9 M10

0.35 0.35 0.35 0.35 0.35 0.40 0.40 0.40 0.40 0.40

784.2 778.4 767.1 746.4 736.2 811.9 806.7 802.2 783.5 770.8

958.5 951.4 937.6 912.3 899.8 992.3 985.9 980.5 957.6 942.1

Binder (B)

CFs

W (kg)

C (kg)

SF (kg)

(vol.%)

(kg)

432.7 432.7 432.7 432.7 432.7 378.6 378.6 378.6 378.6 378.6

48.1 48.1 48.1 48.1 48.1 42.1 42.1 42.1 42.1 42.1

0.00 0.25 0.50 0.75 1.00 0.00 0.25 0.50 0.75 1.00

0.0 4.7 9.5 14.2 18.9 0.0 4.7 9.5 14.2 18.9

HRWR (% B)

189.8 189.8 189.8 189.8 189.8 189.8 189.8 189.8 189.8 189.8

1.50 2.00 3.50 6.70 8.00 1.00 1.45 1.75 5.00 7.00

Notation: CA = coarse aggregate, FA = fine aggregate, C = cement, CFs = carbon fibres, SF = silica fume, W = water, HRWR = high range water reducer.

and silica fume) was added with a 1=4 of the total mixing water and mixed for 120 s. After that, the mixing operation was stopped for 180 s and the mixture was covered with wet burlap to prevent loss of water due to evaporation. After the resting time of 180 s, for the control mixture with no fibres, the remainder of the mixing water (½ of the total mixing water) including the HRWR dosage was added into the mixer and the mixture was mixed further for 180 s. The same sequence was followed for the mixtures with fibres, except that the fibres were added before adding the rest of the mixing water including HRWR dosage. The concrete mixtures were mixed for a total time of 6 min excluding the resting time. Various trial concrete batches were prepared to obtain the optimum dosage of HRWR for use in the CFRSCC mixtures. The mixture quantity was 25L for all concrete batches.

bucket was filled with SCC and the concrete was poured onto the sieve and rested for 5 min to allow some mortar pass through the sieve, the sieve and pan with the concrete were weighed together, the sieve with the concrete retained was separated, the pan including the mortar was weighed, the concrete retained on the sieve was washed to obtain coarse aggregates in saturated surface-dry (SSD) condition; then the SSD coarse aggregates were weighed, finally the segregation index was determined using the following equation [25,26]:

SI ¼ M p =M c  100%

ð1Þ

where SI is the segregation index (%), Mp is the mass of the mortar that passed the sieve (kg), Mc is the mass of the mortar contained in concrete = concrete mass  aggregate mass in SSD condition (kg).

3.4. Test procedures Immediately after the completion of mixing operation, the concrete mixtures were sampled and tested for filling ability, passing ability, segregation resistance, air content, and unit weight. The slump flow, J-ring slump flow, and segregation index were measured simultaneously for each mixture to determine its filling ability, passing ability, and segregation resistance. Then the fresh concrete mixtures were tested for air content and unit weight. Moreover, the hardened concretes were examined to observe distribution of fibres. The test procedures are discussed below. 3.4.1. Filling ability tests Filling ability is the ability of SCC to flow horizontally and vertically under its self-weight. The slump flow and T50 slump flow time tests were followed in accordance with ASTM C1611/C1611M-09b [23] to measure the filling ability of SCC. The slump flow was determined by measuring the diameter of the concrete spread in two perpendicular directions (D1 and D2), where D1 is the largest diameter of the flow patty. T50 slump flow time was also measured for all concretes from the same test; it is the time that concrete took to reach a diameter of 50 cm. A stopwatch was used to measure T50. The stopwatch was started as the mould was lifted upward and the concrete was allowed to flow. Once the concrete touched the 50cm diameter circle marked on the base plate, the stopwatch was stopped and the T50 slump flow time was recorded. 3.4.2. Passing ability test Passing ability is the ability of SCC to flow through the limited spaces between re-bars with no blocking. It mainly depends on the maximum size and volume of the aggregates. The J-ring slump flow test was used in this study according to ASTM C1621/C1621M-09b [24] to measure the passing ability. This test is similar to the slump flow test, except it is carried out in the presence of a J-ring around the slump cone. The two perpendicular diameters (D1, D2; D1 > D2) of the concrete spread in the presence of a J-ring were measured to determine the J-ring slump flow. 3.4.3. Segregation tests Segregation resistance or stability is a vital property of SCC. It is defined as the ability of the concrete to remain consistent and uniform during mixing, transport, and placing. Segregation resistance affects the hardened properties such as strength and durability. Therefore, it should be measured properly. There are several techniques such as column segregation, penetration, and sieve stability tests to measure the segregation resistance of SCC. The segregation resistance can also be qualitatively assessed based on visual observation. In this study, the sieve stability test [25,26] was followed to quantify the segregation resistance of SCC. In addition, the segregation resistance of SCC was visually assessed with respect to visual stability index (VSI). 3.4.3.1. Sieve stability test. The sieve stability apparatus consisted of a bucket (2 l volume), a sieve (4.75 mm opening size) attached to a pan, and a stopwatch. The test procedure was as follows: the sieve and pan were weighed separately, the

3.4.3.2. Visual stability index. Visual stability index (VSI) indicates the bleeding condition and stability (segregation level) of freshly mixed SCC. ASTM C1611/C1611M09b [23] categorizes the visual stability index for SCC into four groups; these groups are described below: (a) (b) (c) (d)

VSI = 0 VSI = 1 VSI = 2 VSI = 3

refers to highly stable SCC. indicates stable SCC. implies unstable SCC. infers highly unstable SCC.

In this study, the VSI of SCC was determined by observing the visual quality of concrete mixture in the slump flow test. Each concrete mixture was given a value, which indicated the stability of the CFRSCC. The index value was varied from 0 to 3 to describe the degree of concrete stability. 3.4.4. Air content and unit weight tests The procedure given in ASTM C231/C231M-09b standard was followed with an exception for compaction to determine the air content of fresh concretes by using a Type B air meter [27]. The concrete was poured in one layer to fill the air meter bowl with no compaction. To measure the unit weight of the concrete, the procedure depicted in ASTM C138/C138M-10b [28] was followed. A bowl was weighed with no concrete and then filled with the concrete without any compaction, and weighed again to obtain the unit weight of the concrete. 3.4.5. Scanning electron microscopy The distribution of carbon fibres in hardened concretes was examined through scanning electron microscopy. This test was carried out using the prism specimens tested for flexural strength and toughness. The specimens for this test were prepared from the fractured prism specimens by cutting them into 20  20 mm size (without affecting the fracture surface). The test specimens were cleaned and dried; then the fractured surface was coated with a thin gold-coating (conductive material) in order to be properly scanned. A scanning electron microscope was used to produce the magnified images of the fracture surface. These images are known as scanning electron micrographs (SEMs).

4. Test results and discussion 4.1. Filling ability The filling ability was measured with respect to slump flow and T50 slump flow time. The effect of carbon fibres on the slump flow and T50 slump flow time are plotted in Figs. 2 and 3, respectively. Carbon fibres greatly affected the slump flow of SCC. This is because, as the fibre volume increases, the interaction between car-

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900 Upper limit

800

Lower limit

Slump flow (mm)

700 600 500

W/B=0.35 W/B=0.40

400

4.2. Passing ability

300 200 100 0 0

0.25

0.5

0.75

1

Carbon fibres (%) Fig. 2. Effect of carbon fibre content on the slump flow of CFRSCC.

35 30

T50 slump flow (s)

whereas carbon fibres decreased concrete filling ability. Therefore, finding the optimum balance between the amount of carbon fibres and the dosage of HRWR is important for concrete mixtures to be successful in meeting the requirements for SCC fresh properties. This study shows that carbon fibres with a content up to 0.75% can be used to produce SCC with acceptable filling ability.

25 20

W/B=0.35 W/B=0.40

15 10

Upper limit

Lower limit

5 0 0

0.25

0.5

0.75

The J-ring slump flow test was used to measure the passing ability of the concrete mixtures. As shown in Fig. 4, the J-ring slump flow of different CFRSCC mixtures varied from 477.5 to 730 mm. It was generally lower than the slump flow. The J-ring slump flow was used to calculate blocking index. Subtracting the J-ring slump flow from the slump flow provided the blocking index. The blocking index for different CFRSCC mixtures produced in the present study was in the range of 0–50 mm as shown in Fig. 5; this is within the range specified in ASTM C1621/C1621M09b [24]. The incorporation of carbon fibres increased the blocking index of SCC. This is because the presence of fibres restricted the concrete mixture from moving through spacing between obstacles (re-bars). However, it greatly depended on HRWR dosage. An adequate HRWR dosage significantly decreased the blocking index of CFRSCC. For example, mixture M4 had 20 mm blocking index, whereas mixture M5 had 5 mm blocking index although it had the highest amount of carbon fibres. But a substantially high HRWR dosage was used in M5 mixture. At a greater HRWR dosage, the J-ring slump flow was substantially increased; therefore, the blocking index was decreased. This made the M5 concrete mixture to move easily around the re-bars during flow. Furthermore, mix-

1

Carbon fibres (%) J-ring slump flow (mm)

bon fibres can restrict the filling ability in SCC [29]. As shown in Fig. 2, the slump flow value for the mixtures varied from 550 mm to 745 mm. The slump flow for SCC typically ranges from 550 mm to 850 mm [30–32]. Mixtures M5 and M10 had 1% carbon fibres by volume which is the highest amount of fibres used in the present study; these two mixtures successfully fulfilled the slump flow requirements of SCC. However, they required a higher HRWR dosage than the rest of the concrete mixtures to achieve the target slump flow of SCC. This is because the filling ability of concrete was significantly reduced in these two mixtures due to the highest volume of carbon fibres. A higher HRWR dosage was needed to improve the filling ability of CFRSCC mixtures. Increased HRWR dosage increases the deformability of concrete to achieve the target filling ability [33]. Mixtures M1 and M6 (0% carbon fibres) showed higher deformability than the other CFRSCC mixtures with respect to T50 slump flow time. Fig. 3 shows the effect of carbon fibres on the T50 slump flow time. T50 slump flow time increased with higher volume of carbon fibres because the inclusion of fibres makes SCC mixture more viscous, and thus slows the flow of concrete. T50 slump flow time for the concrete mixtures without and with carbon fibres varied from 2.4 to 30 s. The T50 slump flow time of SCC typically varies in the range of 2–7 s [33,34]. Hence, the T50 slump flow time results for the concrete mixtures (M4, M5, M9 and M10) including 0.75% and 1% carbon fibres did not strictly meet the requirements for SCC. However, it should be mentioned that the aforementioned criteria are for SCC without any fibres. Despite higher T50 slump flow time, the concrete mixtures M4, M5, M9 and M10 passed the minimum criterion of slump flow for SCC. In fact, HRWR played a very significant role in improving the filling ability of SCC mixture

800 700 600 500 400

W/B=0.35

300

W/B=0.40

200 100 0 0

0.25

0.5

0.75

1

Carbon fibres (%) Fig. 4. Effect of carbon fibre content on the J-ring slump flow of CFRSCC.

100 90 80

Blocking index (mm)

Fig. 3. Effect of carbon fibre content on the T50 slump flow time of CFRSCC.

70 60

Maximum limit W/B=0.35

50

W/B=0.40

40 30 20 10 0 0

0.25

0.5

0.75

1

Carbon fibres (%) Fig. 5. Effect of carbon fibre content on the blocking index of CFRSCC.

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ture M1 (control mixture without fibre) had a blocking index of 15 mm whereas mixture M5 had a blocking index of 5 mm. This indicates that HRWR dosage had a greater influence on the blocking index of concrete mixture. Thus, it is clear that the HRWR played a vital role to improve the passing ability of CFRSCC. 4.3. Segregation index (SI) The sieve stability test was adopted to investigate the segregation resistance with respect to segregation index. The segregation index varied from 9% to 12% for concrete mixtures with the W/B ratio of 0.35. On the other hand, the segregation index was between 3.4% and 7.8% for concrete mixtures with the W/B ratio of 0.4. The effect of carbon fibres on segregation index is presented in Fig. 6. All CFRSCC mixtures had their segregation index below the maximum limit of 18% reported by Perez et al. [35]. Mixture M1 had the highest segregation index (12%) whereas mixture M3 had the lowest segregation index (9%) among the mixtures produced with the W/B ratio of 0.35. Moreover, mixture M7 had the highest segregation index (7.8%) and mixture M9 had the lowest segregation index (3.4%) among the concrete mixtures produced with the W/B ratio of 0.40. In general, the segregation index decreased when the carbon fibres content increased. This is because the increased volume of carbon fibres decreased the fluidity of concrete mixture. SCC is more prone to segregation due to higher fluidity [33]. This segregation tendency is reduced in the presence of fibres [33]. Thus, the increased volume of carbon fibres greatly decreased the segregation in CFRSCC mixture. Nevertheless, it should be mentioned that HRWR dosage also affected the segregation index of concretes produced in the present study. HRWR dosage significantly improves the filling ability and passing ability of CFRSCC by enhancing its fluidity, which affects segregation index. For example, mixtures M4 and M5 required a very high dosage of HRWR, which is 6.7% and 8%, respectively. The very high dosages of HRWR increased the fluidity of these two concrete mixtures. Therefore, the effect of carbon fibres in reducing segregation index was counter balanced in these two cases in the presence of a higher HRWR dosage. As a result, the segregation indices of the M4 and M5 concrete mixtures were still significant although they remained below the maximum limit.

and showed no evidence of segregation or bleeding; hence, they were highly stable (VSI = 0). Similarly, mixture M5 was highly stable (VSI = 0), as shown in Fig. 11. Also, it can be seen from Fig. 8 that mixture M2 showed very slight bleeding and mixture M7 showed negligible coarse aggregate concentration at the middle of the flow; therefore, they were designated as the moderately stable concrete mixtures (VSI = 0.5). Mixtures M8 and M4 were moderately stable with no evidence of bleeding but with negligible coarse aggregate concentration or a very small mortar halo (VSI = 0.5) whereas mixtures M3, M9, and M10 were stable with slight bleeding as sheen on the surface and a small mortar halo

M1 W/B: 0.35 CF = 0%

(a) Mix M1: VSI = 0 (highly stable); no evidence of bleeding, mortar halo, and aggregate piling.

M6 W/B: 0.4 CF = 0%

(b) Mix M6: VSI = 0 (highly stable); negligible coarse the flow patty but no evidence of bleeding and mortar halo.

Fig. 7. Visual stability of M1 and M6 concrete mixtures.

4.4. Visual stability index Visual stability index (VSI) of CFRSCC mixtures was obtained from the slump flow test. The photos of slump flow patty for all concrete mixtures are shown in Figs. 7–11. The concrete mixtures were in the highly stable state (VSI = 0) to stable state (VSI = 1). As it can be seen from Fig. 7, mixtures M1 and M6 were homogenous

M2 W/B: 0.35 CF = 0.25%

20 (a) Mix M2: VSI = 0.5 (moderately stable); very slight bleeding and a very small mortar halo < 5 mm.

Segregation index (%)

18 Maximum limit

16 14 12

W/B=0.35

10

W/B=0.40

8 6 4

M7 W/B: 0.4 CF = 0.25%

2 0 0

0.25

0.5

0.75

1

Carbon fibres (%) Fig. 6. Effect of carbon fibre content on the segregation index of CFRSCC.

(b) Mix M7: VSI = 0.5 (moderately stable); negligible coarse aggregate concentration at the middle of the flow patty and a very small mortar halo < 5 mm.

Fig. 8. Visual stability of M2 and M7 concrete mixtures.

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M3 W/B: 0.35 CF = 0.50%

M5 W/B: 0.35 CF = 1%

(a) Mix M3: VSI = 1 (stable); concrete shows a small mortar halo < 10 mm and slight bleeding as a sheen on the surface.

(a) Mix M5: VSI = 0 (highly stable); no evidence of bleeding, mortar halo, and aggregate piling.

M8 W/B: 0.40 CF = 0.50%

M10 W/B: 0.40 CF = 1%

(b) Mix M8: VSI = 0.5 (moderately stable); no evidence of bleeding and mortar halo but negligible coarse aggregate concentration at the middle of the flow patty.

(b) Mix M10: VSI = 1 (stable); concrete shows as a sheen on the surface.

Fig. 9. Visual stability of M3 and M8 concrete mixtures.

Fig. 11. Visual stability of M5 and M10 concrete mixtures.

10

M4 W/B: 0.35 CF = 0.75%

(a) Mix M4: VSI = 0.5 (moderately stable); no evidence of bleeding but negligible coarse aggregate concentration at the middle of the flow patty and a very small mortar halo < 5 mm.

Entrapped air content (%)

9 8 7 6 W/B=0.35

5

W/B=0.40

4 3 2 1 0 0

0.25

0.5

0.75

1

Carbon fibres (%) Fig. 12. Effect of carbon fibre content on the entrapped air content of CFRSCC.

2600 (b) Mix M9: VSI = 1 (stable); concrete shows a small a sheen on the surface.

(VSI = 1), as shown in Figs. 9–11. In summary, the photos of slump flow patty revealed that all mixtures passed the VSI requirements of SCC.

Unit weight ( kg/m 3 )

Fig. 10. Visual stability of M4 and M9 concrete mixtures.

2550 2500 W/B=0.35

2450

W/B=0.40

2400

4.5. Air content and unit weight

2350

The air content of the CFRSCC mixtures was 1.4–3.5%, as shown in Fig. 12. The concrete mixtures were designed to be non-air entrained with an entrapped air content of 2% [28]. The air content results indicate that carbon fibres did not cause any significant

2300 0

0.25

0.5

0.75

1

Carbon fibres (%) Fig. 13. Effect of carbon fibre content on the unit weight of CFRSCC.

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air entrapment. This is attributed to the high filling ability of concrete. However, the overall entrapped air content of the concretes produced with the W/B ratio of 0.35 was relatively low, as compared to the concretes fabricated with the W/B ratio of 0.40. This is because the concretes with the W/B ratio of 0.35 possessed a higher filling ability, as can be seen from Fig. 2. The increased filling ability facilitated to release the entrapped air-voids from concrete. The unit weights of the freshly mixed CFRSCC mixtures are shown Fig. 13. The unit weight varied in the range of 2360– 2460 kg/m3. The unit weight decreased as the carbon fibres increased; the reason is that carbon fibres were the lightest solid component in the concrete mixtures.

M2 W/B: 0.35 CF = 0.25%

4.6. Scanning electron micrograph The SEMs of the fracture surface for different hardened CFRSCC mixtures revealed that the carbon fibres were well distributed in the mortar matrix of all concrete mixtures. As evidence, the SEMs of the CFRSCC mixtures including 0.25% and 1% carbon fibre contents are provided in Figs. 14 and 15, respectively. These figures show that no fibre clumping or balling occurred in concrete. This suggests that the fibres were well dispersed during the mixing of concrete. Indeed, the use of SF along with HRWR facilitated the dispersion of carbon fibres in concrete [18,36]. Thus, it was understood that the fresh properties of concrete was not influenced by fibre clumping or balling.

5. Conclusions Based on the test results of this study for the fresh properties of CFRSCC, the following conclusions can be drawn:

(a) Mix M2: SEM of the fracture surface, well distribution of 0.25% fibres. M7 W/B: 0.40 CF = 0.25%

(b) Mix M7: SEM of the fracture surface, well distribution of 0.25% fibres.

Fig. 14. Scanning electron micrographs of M2 and M7 concrete mixtures including 0.25% carbon fibres.

M5 W/B: 0.35 CF = 1%

(a) Carbon fibres greatly affected the filling ability, passing ability, and segregation resistance of SCC mixtures. HRWR facilitated to achieve the target filling ability and passing ability properties of the SCC mixtures including carbon fibres. (b) The SCC mixtures with 1% carbon fibres (mixtures M5 and M10) required a very high amount of HRWR to improve the filling ability of concrete. The CFRSCC mixtures with carbon fibres content up to 0.75% satisfactorily passed the requirements of SCC. (c) The T50 slump flow time was increased with the increase in carbon fibres content because the inclusion of fibres slowed the flow of CFRSCC mixture by making it more viscous. (d) The blocking index for all SCC mixtures was below the maximum limit of 50 mm because of their relatively high filling ability and high segregation resistance. (e) All CFRSCC mixtures clearly passed the segregation resistance requirement as their segregation index was significantly below the maximum limit (18%). (f) The visual stability index (VSI) of the freshly mixed CFRSCC mixtures revealed that the concrete mixtures were highly stable (VSI = 0) to stable (VSI = 1), thus indicating excellent and good segregation resistance, as observed from the sieve stability test. (g) The carbon fibres were well distributed in all concrete mixtures, as observed from the scanning electron micrographs. This is because the fibres were well dispersed without any fibre clumping or balling in the presence of SF and HRWR.

Acknowledgements (a) Mix M5: SEM of the fracture surface, well distribution of 1% fibres. M10 W/B: 0.40 CF = 1%

The authors acknowledge the financial support from the Higher Education Ministry of Libya. The authors are also grateful to BASF Construction Chemicals Canada Ltd. for supplying chemical admixtures and to Mitsubishi Company in USA for supplying the carbon fibres. References

(b) Mix M10: SEM of the fracture surface, well distribution of 1% fibres.

Fig. 15. Scanning electron micrographs of M5 and M10 concrete mixtures including 1% carbon fibres.

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