Properties of self-compacting lightweight concrete reinforced with steel and polypropylene fibers

Construction and Building Materials 226 (2019) 388–398

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Properties of self-compacting lightweight concrete reinforced with steel and polypropylene fibers Xi Liu, Tao Wu ⇑, Xue Yang, Hui Wei School of Civil Engineering, Chang’an University, Xi’an 710061, China

h i g h l i g h t s
SCLC was produced using steel and polypropylene fibers.
The workability of SCLC was slightly influenced by the fibers.
The steel and polypropylene fibers in hybrid form resulted in performance synergy.
Microstructure in aggregate/paste interface improved with increasing curing time.

a r t i c l e

i n f o

Article history: Received 18 January 2019 Received in revised form 1 June 2019 Accepted 26 July 2019

Keywords: Self-compacting lightweight concrete Hybrid fiber Workability Mechanical properties Microstructure

a b s t r a c t The aim of this study is to investigate the effect of incorporating steel (ST) and polypropylene (PP) fibers and silica fume on the rheological properties, mechanical properties and microstructure of selfcompacting lightweight concrete (SCLC). ST fiber combined with PP fiber in four volume fractions (ST/ PP = 0.5%/0, 0.5%/0.5%, 0.5%/0.75%, 0.5%/1.0%) is introduced in this study. The self-compacting characteristic of concrete is evaluated by means of slump flow, V-funnel, L-box and U-box tests. Compression, splitting tensile and flexural tests are performed to characterize the mechanical properties of SCLC. In addition, a microscopic study on the aggregates/paste and fibers/paste interfaces were conducted using Scanning Electron Microscopy (SEM). The results of the workability test show that all of the mixes can be defined as SCLC with good flowability, viscosity and passing ability. An improvement in the mechanical properties of SCLC is observed following the incorporation of silica fume and fibers. In particular, ST and PP fibers in hybrid form effectively enhance the mechanical behavior of SCLC, and result in a positive synergy. Moreover, good bond behavior at aggregates/paste and fibers/paste interfaces is observed. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Self-compacting concrete (SCC), also known as selfconsolidating concrete, is an innovative concrete that can be placed and consolidated under its self-weight alone, requiring no external vibration. It is able to fill formworks as well as flow through the constricted areas and highly congested reinforcements without showing bleeding or segregation [1–3]. SCC was originally introduced by Okamura in the late 19800 s to promote the quality of construction work [4]. Meanwhile, a mix design method for SCC production was further developed with fixed coarse and fine aggregate contents [5]. Artificial lightweight aggregates such as sintered fly ash, expanded clay and expanded shale are mostly used in the production of lightweight concrete (LWC), showing some characteristics ⇑ Corresponding author. E-mail address: [email protected] (T. Wu). https://doi.org/10.1016/j.conbuildmat.2019.07.306 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

such as lower density, better durability properties and higher specific strength. Self-compacting lightweight concrete (SCLC) is developed from SCC, the normal coarse aggregates in SCC are replaced by lightweight coarse aggregates to produce SCLC. SCLC can be defined as a new type of high-performance material that combines the well-known advantages of SCC with that of LWC. The key advantages of SCLC are its self-consolidating capability and the reduction in self-weight of the structure. SCLC was first used in the main girder of a cable-stayed bridge in 1992 [6]. Since then, the application of SCLC in actual structure has gradually increased, especially in the area of poor working conditions, long construction circle and rehabilitation and reconstruction of structure [7,8]. Besides that, SCLC might be used in underwater concreting, heavily congested structural elements, and precast structural components where there is a high degree of quality control [9,10], which makes SCLC potentially more competitive. Application of SCLC in construction practice yields economic benefits and technical potentials such as reducing on-site noise emissions,

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and increasing the rate of construction [11,12], which ensure a better working environment during the construction process. In addition, compared to normal vibrated concrete, a comparatively higher powder content is required in the production of SCLC in order to improve its rheological characteristics. It was reported that a powder content in the range of 450–600 kg/m3 is recommended for SCC [13]. Furthermore, mineral admixtures such as fly ash, blast-furnace slag and silica fume are commonly used as a partial replacement for cement to avoid segregation and achieve high deformability [14]. In the application of lightweight aggregates, it should be considered that the brittleness of SCLC is higher than that of SCC at the same strength level. Incorporating fibers into concrete mixes is recognized as an efficient way to resolve the brittle texture and dramatically improve the material properties of concrete [15]. However, there is a reduction in the workability of concrete when fibers are added. The fresh and mechanical properties of SCC reinforced with fibers have been conducted by many researchers. In most cases, steel fiber is incorporated, although non-metallic fiber can also be utilized. Khaloo et al. [16] reported that acceptable workability can be achieved for SCC with steel fiber. Alberti et al. [17] studied the effect of polypropylene fibers on SCC and normal vibrated concrete and showed that an increase of fiber content increased the tensile strength but reduced the compressive strength. Sahmaran et al. [18] demonstrated that it is possible to produce a hybrid fiber reinforced SCC with superior properties in both hardened and fresh states. However, only a limited number of studies regarding fiber reinforced SCLC have been conducted. Shahid et al. [19] demonstrated that for SCLC reinforced with different amounts of steel fiber content, an increase in fiber content resulted in a strong reduction of workability, but significantly increased the tensile strength. Klein et al. [20] analysed SCLC reinforced with steel or polyester fibers and obtained an optimal polye-

ster fiber reinforced SCLC having a density of 1665 kg/m3, a slump flow of 605 mm and a 28-day compressive strength of 22.3 MPa. Since the investigation on SCLC containing fibers and mineral admixtures is of great importance, additional work is required. This paper describes the effect of silica fume and two types of fibers (steel and polypropylene fibers) on the workability, mechanical properties and microstructure of SCLC. Slump flow, V-funnel, U-box and L-box tests were performed to assess the selfconsolidating capability of fresh concrete. Moreover, the mechanical performance of SCLC in a hardened state were also determined by means of compressive strength with elapsed age, splitting tensile strength and flexural strength. Furthermore, two types of interfacial transition zones (ITZs) at aggregate/paste and fiber/paste were observed to investigate the bond properties in SCLC.

2. Experimental program 2.1. Materials 2.1.1. Coarse aggregate Crushed expanded shale ceramist from Guangda Co. Ltd. in Hubei, China was used as a coarse aggregate. Physical properties of the selected lightweight aggregates determined by Chinese standard GB/T 17431.2 [21] are listed in Table 1.

2.1.2. Fibers Two types of fiber were selected: Dramix straight-shape steel (ST) fiber and polypropylene (PP) fiber. The ST and PP fibers were provided by Bekaert Co. Ltd., Belgium and Hansen Co. Ltd., Wuhan, China, respectively. The properties of the selected fibers, used for SCLC mixtures, are shown in Table 2.

Table 1 Physical properties and grading of expanded shale aggregate. View

Bulk density (kg/m3)

Apparent density (kg/m3)

Total porosity (%)

1 h /24 h water absorption (%)

Cylinder compressive strength (MPa)

Aggregate volume fraction (%) 2.36–5 mm

5–10 mm

10–16 mm

855

1507

43.7

4.7/6.4

7.5

23

64

13

Table 2 Overview of investigated fibers. Fibers

View

Diameter (mm)

Length (mm)

Shape

Density (kg/ m3)

Elongation at break (%)

Modulus of elasticity (GPa)

Tensile strength (MPa)

Steel

200

13

Straight round

7800

3.2

200

2500

Polypropylene

80

20–25

Straight round

910

17

3.75

680

390

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Table 3 The physical properties of cement. Blaine fineness (m2/ kg)

Loss in ignition (%)

Initial setting time (min)

Final setting time (min)

3-day compressive strength (MPa)

28-day compressive strength (MPa)

350

2.15

100

330

32.5

56.4

Table 4 Mix proportions of SCLC (kg/m3). Mixture ID

SCLC-1 SCLC-2 ST0.5SP00 ST0.5SP0.5 ST0.5SP0.75 ST0.5SP1.0

Mixture No.

1 2 3 4 5 6

Binder Cement

Fly ash

Silica fume

395 374 374 374 374 374

170 170 170 170 170 170

– 22.7 22.7 22.7 22.7 22.7

2.1.3. Other materials The following additional materials were used: - Cement used in the test was No. 42.5 Ordinary Portland cement in accordance with Chinese standard GB175 [22]. The physical properties of the cement are listed in Table 3. - The fine aggregate was natural river sand of maximum size 4 mm with a fineness modulus of 2.83. - A polycarboxylate-based superplasticizer (SP) was used as an admixture to obtain the desired workability as well as to improve the fiber dispersion. - Two mineral admixtures: fly ash, following Chinese standard GB/T 1596 [23] with a specific gravity of 2.30 g/cm3; and silica fume with a specific gravity of 2.79 g/cm3. 2.2. Mix proportions and procedures The concrete mix design was based on the over calculation method with fixed coarse and fine aggregate content [5,24,25]. In the experiments, two mixtures with different target strength are presented in Table 4. The first concrete class (denoted as SCLC-1) was designed with cement (70% in mass) and fly ash (30% in mass) as cementitious materials. For the second concrete class (denoted as SCLC-2), cement (66% in mass), fly ash (30% in mass) and silica fume (4% in mass) were used. The effective water to binder ratios (w/b) of mixtures SCLC-1 and SCLC-2 were 0.33 and 0.30, respectively. In addition, four additional mixtures of SCLC containing hybrid ST/PP fibers are listed in Table 4. Mix 2 can be defined as the control mix for the fiber reinforced SCLC. The ST fiber combined with PP fiber in different volume fractions (ST/PP = 0.5%/0, 0.5%/0.5%, 0.5%/0.75%, 0.5%/1.0%) are used. The concretes were mixed using a double-axis mixer under labconditions. Before mixing, a sufficient amount of lightweight aggregate (LWA) was immersed in water for 1 h to obtain a saturated surface dry (SSD) condition. Firstly, cementitious materials (cement, fly ash and silica fume) and fine aggregates were drymixed for 1 min, followed by the addition of ST and PP fibers. Mixing continued for 1–2 min to ensure sufficient dispersion of fibers. Next, an approximately 3/4 mix of water with superplasticizer was added and mixed for 3–5 min. Next, LWAs were poured into the mixture and mixed for another 3 min and the remaining water was gradually added. 2.3. Workability test After the mixing procedure, the concrete’s deformability and flowability was determined immediately based on the usual slump

LWA

Sand

Water

SP

ST

PP

450 450 450 450 450 450

750 760 760 760 760 760

186 170 170 170 170 170

3.3 5.0 5.1 5.3 5.7 6.2

– – 39 39 39 39

– – – 4.55 6.825 9.1

flow, V-funnel, L-box and U-box tests, as recommended by EFNARC committee [26] together with other possible codes. As suggested by EFNARC, the workability of SCC can be evaluated by the three categories of flowability, viscosity and passing ability. 2.3.1. Slump flow test A normal slump cone and a plate with dimensions of 1000  1000 mm2 were adopted to evaluate the slump flow of SCLC, as shown in Fig. 1(a). In the test, the final slump flow diameter (Dm, the median of two orthogonal diameters) and time required to reach 500 mm of slump flow (T500) were measured. The flowability of fresh SCC can be described with the slump flow value and, based on that, the SCC can be divided into three slump flow (SF) classes: SF1, SF2 and SF3 corresponding to slump flow diameters of 550–650, 660–750 and 760–850 mm, respectively [26]. 2.3.2. V-funnel Test The time required for SCLC to flow through a V-funnel was tested to characterize its resistance to flow as well as the filling ability. The test apparatus for the V-funnel test is shown in Fig. 1 (b). The time required to flow out of the funnel is recorded as the V-funnel time. Viscosity of the SCC can be assessed by T500 during the slump flow test (VS) and time required to flow through the Vfunnel (VF). SCC can be classified in two viscosity classes: VS1/VF1 and VS2/VF2 measuring T500  2 s/V funnel time  8 s and T500 greater than 2 s/V-funnel time range from 9 s to 25 s, respectively [26]. 2.3.3. L-box test The L-box test was conducted to evaluate the flowing ability through tight openings such as spaces between reinforcing bars. The L-box consists of three parts – the chimney section, a horizontal section, and a gate with three reinforcing bars. The vertical distance of concrete in the chimney section, h1, the vertical distance in horizontal section, h2, and time required to reach 400 mm from the gate, T400, as shown in Fig. 1(c), can be measured. An L-box passing ratio h1/h2 larger than 0.8 is recommended to confirm good passing ability and this field is called the self-flow zone [26]. However, there is no specification for T400 in EFNARC or other codes. 2.3.4. U-box test The U-box test can be used in combination with the L-box test to evaluate the passing ability as well as the filling ability of SCLC. The filling height difference of concrete between two boxes is measured as shown in Fig. 1(d). The value of height difference (Dh)

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391

Fig. 1. Workability tests and apparatus for (a) Slump flow test, (b) V-funnel test, (c) L-box test, (d) U-box test.

should not exceed 30 mm to achieve sufficient filing and passing ability [26]. 2.4. Test of mechanical properties Mechanical properties, including compressive strength, splitting tensile strength and four-point flexural strength were tested in accordance with Chinese standards GB/T 50,081 [27] and CSCE13 [28]. In total, fifteen cubic specimens with dimensions of 100  100  100 mm3 and three prismatic specimens with dimensions of 100  100  400 mm3 were casted for each concrete mixture without compacting. De-moulding was performed after 24 h of casting and specimens were then exposed in a standard curing room maintained at 23 ± 2 °C and 100% relative humidity until testing. The compressive strength was measured with cubic specimens at ages of 3, 7, 28 and 90 days, respectively. The test was carried out using a 2000 kN capacity universal testing machine (WAW-3100, Changchun, China) at a loading rate of 6 kN/s. The splitting tensile strength was determined at 28 days for cubic specimens under a constant loading rate of 1 kN/s. Prismatic specimens were used to determine the flexural strength of SCLC at 28 days under a loading rate of 0.3 kN/s. A universal testing machine (YA300, Changchun, China) of 300 kN capacity was applied for both splitting tensile and flexural strength. The specimens were simply supported between two rollers and two loading knifes with spans of 300 mm and 100 mm, respectively. The test setups for mechanical properties are shown in Fig. 2. The following equations were used to compute the splitting tensile and flexural strengths of all mixes:

rt ¼

2P pA

ð1Þ

where P is the maximum tensile load; and A is the cross section of a cubic specimen. Next,

rf ¼

Fl bh

2

ð2Þ

Fig. 2. Mechanical properties test details: (a) compressive strength, (b) splitting tensile strength, (c) flexural strength.

where F is the total flexural load; l is the span length (300 mm); and b and h are the width (100 mm) and height (100 mm) of the prismatic specimens, respectively.

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Fig. 3. Test details of microstructure investigation: (a) sub samples, (b) spray conducting material treatments, (c) S-4800 SEM.

Table 5 Details of Micro-examination specimens. Sub samples

Age

Aggregates/paste interface of SCLC

M1

Effect of silica fume on aggregate/paste interface ST fiber/paste interface PP fiber/paste interface

M1, M2

3 day, 28 day 28 day

M3 M4

28 day 28 day

2.5. Microstructure investigation The aggregate/paste and fiber/paste interfaces of SCLC were evaluated using scanning electron microscopy (SEM). After stirring, three prismatic specimens having dimensions of 40  40  160 mm3, were casted for mix Nos. 1–4 without compacting. The specimens were broken under a bending load and afterwards sampled, the sub-samples M1–M4 of 1 cm2 were immersed in acetone solution to the stop hydration process. The microstructure investigation was performed by S-4800 SEM with detectors of backscattering electrons, and applying 3.0 kV to the image generation with a working distance of 9.9 mm. Before SEM analysis, the sub samples were subjected to high vacuum and were then coated with a layer of gold-palladium to enhance their electrical conductivity [29]. Fig. 3 depicts the test details of the microstructure investigation. The main research content is shown in Table 5.

3. Results and discussion 3.1. Fresh state properties The variation of slump flow diameter in slump flow tests of all six mixes are depicted in Fig. 4. Slump flow diameters fall into the range of 660–750 mm and can be classified as SF2, except for the result of SCLC with 0.5SF/0.1PP hybrid fibers (i.e. mix No. 6). SF2 is suitable for many common applications [26]. SCLC with 0.5SF/1.0PP hybrid fibers satisfies the highest level of requirements for SF1. Therefore, slump flow results indicate an adequate rheology performance for all six mixes of SCLC. It can be seen that the slump flow values show a very slight change (from 690 to 640 mm) with the presence of fibers, and also an increasing total fiber content. Incorporating 1.5% volume fractions of hybrid fibers (0.5% ST/1.0% PP) to SCLC resulted in a merely 5% reduction in slump flow diameter (from 680 mm to 645 mm). A study conducted for high strength SCC did not significantly alter the slump flow values of concrete (from 800 to 740 mm) with the addition of SF fiber, while an obvious reduction in slump flow values can

800

SF3

750 700

690

680 665

670

SF2 660 645

650 600 550

Slump flow classes

Research content

Slump flow diameter / mm

850

SF1

1

2

3

4

5

6

Mix No. Fig. 4. Variation of slump flow diameter for SCLC.

be observed with ST fiber content above 0.75% [16]. Another study indicated a negative effect on the flowability of fresh SCC, especially with the addition of ST fiber compared with PP fiber because of its higher stiffness and configuration [30]. Moreover, during the tests, there is a visual decrease in the flowability rate of concrete with the addition of fibers though the slump flow values decreased slightly. The time taken to flow through the V-funnel and the time taken to reach a 500 mm slump flow for SCLC is shown in Fig. 5. These parameters can be used to assess the viscosity as well as the segregation resistance ability of SCLC [31]. The V-funnel flow time of all mixes exhibited results in the range of 11.2–19.3 s, SCLC1 mixture (i.e. mix No. 1) had the lowest V-funnel time of 11.2 s, while the highest V-funnel time was 19.3 s as measured for the ST0.5SP1.0 mixture (i.e. mix No.6). The T500 slump flow time was in the range of 1.9–4.1 s. As recommended by EFNARC [26], all mixtures except the SCLC1 mixture were in the category of VS2VF2. The SCLC1 mixture was less than the minimum limitation but still filled the accepted ranges. The viscosity class of VS2VF2 is helpful in some special cases, such as in limiting the formwork pressure or improving segregation resistance. However, considering the increasing flow time, the concrete having the viscosity class of VS2VF2 was more likely to exhibit thixotropic effects. The V-funnel and T500 slump flow times were 16.5 s and 2.3 s, respectively, for the control mix (i.e. mix No. 2). For mix Nos. 2– 6, both the T500 and V-funnel flow times increased, accompanied by an increase in total fiber content. For instance, applying 0.5% ST fiber to the control mix, the T500 slump flow times increased to 2.8 s. The T500 flow time for ST0.5SP0.5, ST0.5SP0.75, and ST0.5SP1.0 mixtures were 3.2 s, 3.4 s, and 4.1 s, respectively. The

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6

40

VS2VF2

Slump flow diameter / mm

6

4 5 3

4

3 2

2

1

VS1VF1

30

21 20

5

10

15

20

4

5

10

10

5 0

0

17

15

1 0

18

1

25

2

3

6

Mix No.

V-funnel time / s

Fig. 7. Variation of height difference Dh in U-box test for SCLC.

Fig. 5. Relationship of time taken to flow through V-funnel and to reach 500 mm slump flow for SCLC.

results for the increase in flow time can be explained by the fiber orientation and distribution, which is beneficial in restraining the segregation of lightweight aggregates [32]. Good fiber orientation and distribution in fresh mixtures, which entails variation of fiber number, angle and embedded length, are necessary to obtain optimum benefits of the fibers without fiber balling and fiber clumping [33]. Figs. 6 and 7 illustrate the L-box passing ratio h1/h2 and U-box height difference Dh change for various mixes, respectively, to determine the passing ability of SCLC. The L-box passing ratio and U-box height difference Dh in the ranges of 0.85–0.98 mm and 5–21 mm, respectively, met the standards recommended by EFNARC [26], which confirmed the good passing ability of SCLC. The lowest passing ratio h1/h2 of 0.85 was measured for the SCLC1 mixture, while the ST0.5PP1.0 mixture had the highest h1/h2 of 0.98. Similarly, the SCLC1 mixture had the lowest height difference, Dh, while the ST0.5PP1.0 mixture had the highest Dh. These results emphasize that applying fibers to concrete weakens the passing ability of SCLC. Referring to Figs. 10–13, it can be seen that incorporation of fibers in concrete indeed decreases the workability of

1.1

28-day compressive strength/MPa

65

36 28-day compressive strength Specific strength 34

60

32

55 30

50

Specific strength/[MPa/(t/m3)]

T500 slump flow time/ s

5

28 1

2

3

4

5

6

Mix No. Fig. 8. Compressive strength of SCLC and its relationship with specific strength.

SCLC, while the results of these tests met the limitations recommended by EFNARC committee [26] and the all six mixes in this study can be recognized as SCLC with sufficient workability.

7 6.2 5.9

1.0

6

3.2. Mechanical properties

5

3.2.1. Specific strength and compressive strength The specific strength (strength-to-density ratio) can be used to evaluate the light weight and high strength characteristics of LWC. As can be seen from Fig. 8, the specific strength almost keeps in line with the changing trend of the 28-day compressive strength of SCLC. It indicates that the compressive strength shows a corresponding relationship with the density of SCLCs. The 28-day compressive strength of SCLC was within the range of 52.8–61.7 MPa for an oven dry density of 1817–1894 kg/m3, which satisfies the requirements for structural LWC, with a 28-day compressive strength of more than 17 MPa and an equilibrium density of 1120–1920 kg/m3 as well as the requirements for high strength LWC (>40 MPa) [34]. Compared with SCLC without silica fume (i.e. mix No. 1), incorporation of silica fume to the concrete (i.e. mix No. 2) exhibited higher compressive strength and specific

5.1

0.9 0.85

0.96 0.92

0.98 0.94

4

4.1

3.2

3

0.8

2 0.7 1 0.6

1

2

3

4

5

6

Mix No. Fig. 6. Variation of passing ratio h1/h2 in L-box test for SCLC.

0

T400 /s

0.91

h1/h2

5.7

X. Liu et al. / Construction and Building Materials 226 (2019) 388–398

100

Compressive strength / MPa

90-day 7-day

28-day 3-day

80

60

40

20

0

1

2

3

4

5

6

Mix No.

0.09 0.06 0.03 0.00

6

Flexural strength

5 4 3 2 1 0

Fig. 9. Variation of compressive strength with concrete mixes incorporation various amount of fibers.

Flexural-compressive strength ratio

0.12

Flexural strength /MPa

Flexural-compressive strength ratio

394

1

2

3

4

5

6

Mix No.

Splitting tensile-compressive strength ratio Splitting tensile strength /MPa

Fig. 11. Flexural strength and flexural-compressive strength ratio of SCLC.

Splitting tensile-compressive strength ratio

0.12 0.09 0.06 0.03 0.00

Splitting tensile strength

8 6 4 2 0

1

2

3

4

5

6

Mix No. Fig. 10. Splitting tensile strength and splitting tensile-compressive strength ratio of SCLC.

strength. Silica fume in concrete participated a pozzolanic reaction, and thus efficiently reduced the overall capillary porosity and densified the microstructure, which in turn improved the compressive strength of SCLC [35,36]. Mix No. 2, with a 28-day compressive strength of 57.3 MPa is considered as the control concrete for the fiber reinforced SCLC. The effect of ST fiber on the compressive strength of SCLC can be neglected. However, there was a reduction in specific strength, which was attributed to the high density of ST fiber, and following increased voids in the concrete. The maximum compressive strength and specific strength were both obtained at 0.5% ST /0.5% PP hybrid fibers with values of 61.7 MPa and 33.7 MPa/[t/ m3], respectively. With fixed ST fiber content, the increase in PP fibers content decreased the compressive strength as well as the specific strength, but the results are still higher compared with SCLC containing only ST fiber. The compressive strength of SCLC for 3 days, 7 days, 28 days and 90 days for all mixes is presented in Fig. 9 and Table 6. The

compressive strength of SCLC increased obviously with increasing curing time. The range of compressive strength varies from 23.2 MPa to 26.6 MPa for 3 days, from 36.2 MPa to 42 MPa for 7 days, from 52.8 MPa to 61.7 MPa for 28 days, and from 66.8 MPa to 80.6 MPa for 90 days, depending on the fiber content. The best performing SCLC containing both silica fume and 0.5 ST/0.5 PP hybrid fibers has a compressive strength of 42 MPa for 7 days, 61.7 MPa for 28 days, and 80.6 MPa for 90 days, which is 15%, 15% and 14% higher than the control concrete at the same test ages, respectively. This is due to the positive synergetic effect between ST and PP fibers [37]. Hoverer, there is a reduction in the strength at 3 days with the addition of fiber in any volume fraction compared with the control concrete. It can also be observed that the enhancement of strength from 0 to 3 days is much higher than that from 3 to 7 days for all mixes. In addition, the strength enhancement of concrete from 28 days to 90 days is within the range of 24%–31% in comparison with compressive strength at an age of 28 days. 3.2.2. Splitting tensile and flexural strengths The splitting tensile and flexural strength of SCLC, as well as the splitting tensile and flexural-compressive strength ratios, are shown in Figs. 10 and 11, respectively. Test results indicate that a combination of ST and PP fibers have a significant positive effect on the splitting tensile and flexural strength. The maximum increments in splitting tensile and flexural strength were obtained for SCLC containing 0.5 ST/0.75 PP hybrid fibers and 0.5% ST/1% PP hybrid fibers, increasing by 80% and 61.1%, respectively, with presence of silica fume. This improvement can be explained by the utilization of fiber and the bridging action of these fibers in preventing the initiation and propagation of cracks in the concrete. Besides that, the results show that fibers used in hybrid form could lead to superior performance in tensile strength compared with mono SF fiber composites. Accordingly, PP fiber in concrete was highly more effective in improving the efficiency than ST fiber. The high elastic modulus and stiffness of ST fiber resulted in an apparent increase of tensile strength, while low modulus PP fiber performed better for improving the ductility of concrete [38]. Regardless of the fiber reinforcement in concrete, incorporation of silica fume played an important role in strength enhancement.

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Compared with SCLC without silica fume (i.e. mix No. 1), adding silica fume to concrete (i.e. mix No. 2) resulted in a 2.6% and a 9.1% increase of splitting tensile and flexural strengths of SCLC, respectively. This can be attributed to the improvement of bond properties between aggregates and cement paste. Obviously, incorporation of fiber appeared to be more effective than silica fume in increasing splitting tensile and flexural strength of SCLC. Figs. 10 and 11 show the splitting tensile and flexural strengths to the cubic compressive strength ratio, respectively. It is known that this ratio for LWC is usually lower than that of normal weight concrete (NWC) at the same compressive strength level. The LWC exhibited higher brittleness compared to NWC of same strength grade [39]. However, it can be seen that the tension to compression ratio and flexural to compression ratio are in the range of 7–11% and 10–13%, respectively, which shows that this ratio for fiberreinforced SCLC falls within the range of NWC, which is 7–14.3% [40]. 3.3. Microstructure At the micro- level, concrete composite can be divided into three phases – coarse aggregate, cement paste, and an ITZ between aggregate and cement paste – and this characteristic of multiphase is considered to be the main factor affecting the physical and mechanical properties of concrete. Among the constituents, ITZs are considered to influence the properties of concrete significantly as they contain more pores and soluble calcium hydroxide [41]. In this section, the microstructure and morphology of two types of ITZs, including aggregate-paste and fiber-paste interfaces, are investigated by using SEM as a surface analysis tool. 3.3.1. Microstructure of the aggregate-paste interface Figs. 12 and 13 present the SEM images of the interfacial zone between lightweight aggregates and cement paste at 3 days and 28 days, respectively. The lightweight aggregates and cement paste can be clearly identified at low magnification due to their distinct morphological differences. Aggregates on the underside are characterized by an interior porous structure, while the cement paste exhibits a much denser structure without many visible voids at

395

low magnification. The porous structure of aggregates serves to explain their lightweight characteristic. As can be also observed, the boundary between aggregates and cement paste curing for 3 days and 28 days were hardly distinct at low magnification. However, when the ITZ was magnified up to 1000 and 5000 times, the microstructure of the ITZ between aggregates and cement paste at 3 days and 28 days showed a noticeable difference. Empty spaces and loose microstructure between aggregates and cement paste can be clearly seen in sample M1, which cured for three days. Hydration products existed in the interface, including amorphous calcium silicate hydrates (C-S-H) gel, ettringite (AFt) with an elongated shape, and plate-like calcium hydroxide (CH) crystals, overlapped with each other and infiltrated into the pores on the lightweight aggregate surfaces, resulting from the insufficient hydration reaction at early hydration. With the process of hydration, further hydration reaction occurs and the spaces between aggregates and cement paste are filled with poorly crystalline C-S-H and a second generation of AFt [42], which serves to increase the cohesiveness and hence the strength of the ITZ. When the curing time increased to 28 days, there were few visible spaces and flaws between aggregates and cement paste at high magnification, and a very dense microstructure can be identified from Fig. 16(d). The decrease in the w/b at the surface of the aggregates can be explained by the non-existent ‘‘wall effect” around the lightweight aggregates, which shows the favorable effect of aggregates on the abundant hydration reaction in the interfacial zone. Fig. 14 shows the microstructure of the aggregate/paste interface at 28 days for sample M2, which has a silica fume addition. Compared to the microstructure at the ITZ between aggregates and cement paste of sample M1 (without silica fume), shown in Fig. 13, the microstructure at the aggregate/paste interface of sample M2 is much denser under identical magnification. This result is mainly attributed to the improved C-S-H gel, which is the main binding phase in Portland cement-based composites. The pozzolanic C-S-H gel formed from the reaction between silica fume and CH crystals presents a much denser structure than that of CS-H gel produced from conventional hydration of cement [43]. As can be seen from Fig. 14(c) and (d), spherical particles filled the voids of aggregate surface and hydration products, thereby making

Fig. 12. SEM of ITZ between aggregates and cement paste at 3 days of sample M1, magnification: (a) 100, (b) 500, (c) 1000, (d) 5000.

Fig. 13. SEM of ITZ between aggregates and cement paste at 28 days of sample M1, magnification: (a) 100, (b) 500, (c) 1000, (d) 5000.

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Table 6 Mechanical properties test results and strength development of SCLC. Mixture ID

Mix No.

SCLC-1 SCLC-2 ST0.5SP00 ST0.5SP0.5 ST0.5SP0.75 ST0.5SP1.0

1 2 (control concrete) 3 4 5 6

Compressive strength (MPa) 3-day

7-day

28-day

90-day

26.2 26.6 26.3 24.7 23.2 25.3

38.4 36.5 36.7 42.0 36.2 40.2

52.8 57.3 56.9 61.7 58.8 56.7

66.8 70.9 70.5 80.6 72.6 71.8

(-) (-1.1%) (-7.1%) (-12.8%) (-4.9%)

(-) (0.5%) (15.1%) (-0.8%) (10.1%)

(-) (0.5%) (15.1%) (-0.8%) (-1.0%)

(-) (-0.6%) (13.7%) (2.4%) (1.3%)

Splitting tensile strength(MPa)

Flexural strength (MPa)

3.9 4.0 5.9 6.9 7.2 6.9

3.3 3.6 3.9 4.7 5.3 5.8

(-) (47.5%) (72.5%) (80%) (72.5%)

(-) (8.3%) (30.5%) (47.2%) (61.1%)

Note: The data in parentheses show the percentage of strength increase over that of control concrete.

Fig. 14. SEM of ITZ between aggregates and cement paste at 28 days of sample M2, magnification: (a) 500, (b) 1000, (c) 3000, (d) 5000.

the microstructure denser. In addition, owing to its fine particle size, silica fume as partial replacement for cement particles packs more efficiently in the aggregate/paste interface among other larger particles [44]. Consequently, the incorporation of silica fume can effectively improve the interfacial bond characteristic between aggregates and cement paste. 3.3.2. Microstructure of fiber-paste interface SEM images of morphology of the virgin ST and PP fiber state are depicted in Fig. 15. It can be observed that the diameter of SF

Fig. 15. SEM micrographs for surface texture of virgin fibers: (a) ST fiber, (b) PP fiber.

and PP fibers is approximately 200 mm and 80 mm, respectively, and both feature a relatively smooth surface. The SEM micrographs of ST fiber/paste and PP fiber/paste interfaces, shown in Figs. 16 and 17, came from the fractured surface of samples M3 and M4, respectively. In contrast to fibers in the virgin state, presented in Fig. 15, fibers in the mixture were abundant in densely hydrated products, which implies a good bond characteristic between fibers and cement paste. Moreover, the fiber surface roughening caused by the collision with aggregates during mixing was observed. It is believed that the friction and bond behavior between fibers and cement paste were expected to be enhanced by the fiber surface damage [45]. A water film formed around the fiber surface due to the inner bleeding in the fresh mixture, indicated by a higher w/b in the fibers/paste interface. In addition, autogenous shrinkage of ordinary cement paste served to increase the spaces between fibers and cement paste [46]. As a result, a relatively loose structure can be observed in the ITZ between fibers and cement paste. Distinct spaces existing between fibers and cement paste can be found at high magnification, which indicates that no chemical reaction has occurred. Compared with the spaces in the aggregates/paste interface depicted in Fig. 17(c), the width of spaces in the ITZ between fibers and cement paste was much larger at the same magnification. Therefore, it can be speculated that the weak

Fig. 16. SEM of ITZ between ST fiber and cement paste at 28 days of sample M3, magnification: (a) 50, (b) 150, (c) 300, (d) 5000.

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Fig. 17. SEM of ITZ between PP fiber and cement paste at 28 days of sample M4, magnification: (a) 100, (b) 500, (c) 1000, (d) 5000.

zone between fibers and cement paste was introduced by adding fibers.

4. Conclusions LWC having adequate self-compacting properties were reinforced with steel and polypropylene fibers and exhibited a 28day compressive strength between 52.8 MPa and 61.7 MPa for a density range of 1817–1894 kg/m3. Base on the experimental results of this study, the following main conclusions can be summarized: (1) All six mixes satisfied the limitations of SCC recommended by EFNARC and can be recognized as SCLC with good flowability, viscosity, and passing ability. The addition of fibers to mixtures decreased the workability of the SCLC; however, this decrease was less sensitive to the fiber content. (2) The incorporation of silica fume had a positive effect on the mechanical properties as well as the interfacial bond characteristic of SCLC. This enhancement can be mainly attributed to the pozzolanic reaction between silica fume and CH crystals. (3) The effect of ST fibers on the compressive strength of SCLC can be neglected, while the combination of ST and PP fibers in hybrid form resulted in performance synergy and improved the compressive strength of SCC. With a fixed ST fiber content, compressive strength decreased with an increase in PP fiber content. Moreover, the changing trend of specific strength was in agreement with that of the 28day compressive strength. (4) Splitting tensile and flexural strength were significantly improved because of the addition of fibers. The effectiveness of hybrid fibers on the mechanical properties was superior to that of mono ST fibers. (5) With an increase in curing time, the microstructure of the aggregates/paste interface became denser and fewer visible spaces and flaws could be found. (6) A good bond between the fiber surface and cement paste can be indicated by the surrounding densely hydrated products. However, the interfaces between fibers and cement paste were relatively weak compared with those between lightweight aggregates and cement paste.

Declaration of Competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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