Effect of basalt fibers on mechanical properties of calcium carbonate whisker-steel fiber reinforced concrete

Construction and Building Materials 192 (2018) 742–753

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Effect of basalt fibers on mechanical properties of calcium carbonate whisker-steel fiber reinforced concrete Mehran Khan a,⇑, Mingli Cao a, Majid Ali b a b

Department of Civil Engineering, Dalian University of Technology, Dalian, China Department of Civil Engineering, Capital University of Science and Technology, Islamabad, Pakistan

h i g h l i g h t s  CaCO3 whiskers, steel and basalt fibers in concrete are considered.

 Respective fibers lengths are 20–30 lm, 35 mm and 12 mm.

 Various basalt fiber contents, i.e. 0.34%, 0.68%%, 1.02% and 1.36% are studied.  Concrete with 0.68% basalt fiber content is an optimized one.  SEM analysis shows that the hybrid fibers arrest cracking at different levels.

a r t i c l e

i n f o

Article history: Received 12 April 2018 Received in revised form 15 October 2018 Accepted 18 October 2018

Keywords: Hybrid fiber reinforced concrete CaCO3 whiskers Steel fibers Basalt fibers Mechanical properties Scanning electron microscopy

a b s t r a c t Nowadays, hybrid fiber reinforced concrete is being considered for structural applications due to its enhanced mechanical properties compared to concrete without fibers/plain concrete. In this work, the mechanical properties of new kind of hybrid fiber reinforced concrete, i.e. CaCO3 whisker-steel fiberbasalt fiber reinforced concrete (CSBFRC) with various basalt fibers percentages are studied. All the fiber-reinforced concretes are compared with that of plain concrete (PC). The CaCO3 whisker, steel and basalt fiber lengths are 20–30 lm, 35 mm and 12 mm, respectively. Steel fibers and CaCO3 whiskers contents are 0.32% and 0.9%, by volume, respectively. Various basalt fiber contents of 0.34%, 0.68%, 1.02% and 1.36%, by volume, are added. For each batch, cylinders and beam-lets are cast and tested under respective compressive, splitting tensile and flexural load as per ASTM standards. Stress-strain curves and loaddeflection curves are obtained. Strengths, energy absorptions and toughness indices are determined against for each type of loading. The scanning electron microscopy (SEM) analysis is performed to reveal the behavior (interfacial bonding) of CaCO3 whiskers, basalt fibers and steel fibers. It is concluded that, with increasing content of basalt fibers up to 0.68%, there is an increase in the mechanical properties of hybrid fiber reinforced concrete and CSBFRC4 is found to be an optimum. However, beyond 0.68%, the mechanical properties of CSBFRC decrease with an increase in the basalt fiber content. The resistance against cracking provided by hybrid fibers is observed by SEM images. Ó 2018 Elsevier Ltd. All rights reserved.

1. Introduction Hybrid fiber reinforced concrete (HFRC) is being considered nowadays due to its several structural benefits as compared to that of plain concrete (PC). HFRC can control the multi-level cracking of concrete which can result in improved toughness. By controlling cracks at multi-level, the performance of concrete structures can be increased. At the initial stage of micro-cracking, micro-fibers arrest micro-cracks by bridging them. When the stress increases, ⇑ Corresponding author. E-mail address: [email protected] (M. Khan). https://doi.org/10.1016/j.conbuildmat.2018.10.159 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.

these micro-cracks propagate and become meso-cracks where meso-fibers are more effective to resist cracks. With further increase in stresses, the meso-cracks propagate which results in macro-cracks. Macro-fibers arrest macro-cracks and this ultimately results in improved toughness. In addition to crack control phenomena, HFRC has the advantage of improving durability by controlling cracking. For the same type of loading, the corrosion rate of reinforced concrete specimens made with HFRC is less than that of plain concrete specimens [1]. The fibers in concrete provide the confinement effect by limiting cracks [2,3]. The engineering properties of concrete (i.e. load carrying capacity after cracking, toughness, abrasion, fatigue, splitting tensile and flexural

M. Khan et al. / Construction and Building Materials 192 (2018) 742–753

strengths) can be improved by the addition of fibers [4–5]. Fiber reinforced concrete (FRC) is widely used due to sufficient durability, high ductility and high resistance against corrosion [6]. The addition of steel fibers in concrete had the enhanced impact resistance and flexural toughness [7–9]. The composites reinforced with CaCO3 whiskers showed excellent mechanical properties [10,11]. The structural durability can be greatly increased by producing a more crack-resistant concrete [12]. Therefore, the use of multi-scale fibers in concrete may result in the improvement in toughness and durability. CaCO3 whiskers have the low production cost of about $236 per ton which may be helpful in producing low cost cement based composites [11]. CaCO3 whiskers are a new type of inorganic micro-fibers which are used in cement mortar to improve its mechanical properties [11]. Recently, basalt fibers have gained the popularity due to its environment friendly manufacturing process and excellent mechanical properties in concrete [13]. The tensile strength of basalt fibers is greater than that of E-glass and steel fibers [13]. Basalt fibers are extruded from melted basalt rock and are a new kind of commercially available inorganic fiber [6]. The manufacturing process of basalt fibers is the same as that of glass fibers, but with the consumption of less energy and without additives, which make it more economical than carbon and glass fibers [14–16]. The current commercial availability, excellent interfacial shear strength, good resistance to chemical attack, heat resistance and high modulus are the other benefits which make it a good alternative in a concrete matrix as compared to carbon and glass fibers [17]. The mechanical properties of fiber reinforced concrete made with both industrial and recycled steel fibers are studied by many researchers [18–22]. Caggiano et al. [18] clearly demonstrated that industrial steel fibers can be replaced by an equal (or slightly higher) amount of recycled ones without a significant decay in the relevant mechanical properties, provided that the recycled steel fibers are characterized by adequate geometrical characteristics. Kizilkanat et al. [23] performed an experimental study on the mechanical properties of basalt fiber reinforced concrete (BFRC). The mix design was 360 kg/m3, 40 kg/m3, 931 kg/m3, 509 kg/m3 and 436 kg/m3 for cement, fly ash, crushed sand, river sand and coarse aggregate, respectively. The w-c ratio was 0.5, with various contents of superplasticizer ranging from 3.2–8 kg/m3. The basalt fibers in the amount of 6.75 kg/m3, 13.5 kg/m3, 20.25 kg/m3 and 27 kg/m3 were added. The compressive strength (r), flexural strength (fs) and splitting tensile strength (SS) were increased by 5.1%, 13% and 24%, respectively; with the addition of 0.5% basalt fibers content. Jiang et al. [6] studied the mechanical properties of BFRC. The mix design for cement, fly-ash, sand and aggregate was 448 kg/m3, 126 kg/m3, 624 kg/m3, 1024 kg/m3, respectively, with a water-cement (w-c) ratio of 0.60. The length of basalt fibers was 12 mm with the contents of 0.05%0.3% by volume friction. The compressive strength (r), flexural strength (f s) and splitting tensile strength (SS) were improved by 0.2–4.7%, 6.3–9.6% and 8–24.3%, respectively. Ayub et al. [24] studied the effect of basalt fibers on the mechanical properties of fiber reinforced concrete. The various mixes were prepared in which basalt fibers were added in the range of 0–3%, by volume friction. The average increase in the splitting tensile strength and flexural strength was found to be 24% and 27%, respectively, when basalt fibers were added as 3%, by volume, in the concrete. This shows that 3% basalt fiber volume is the maximum fiber volume. Therefore, the mechanical properties and durability of concrete can be improved by hybridization of CaCO3 whiskers, basalt fibers and steel fibers. This can also be helpful in constraining the cracking at multi-level. To the best of authors’ knowledge, no research has been done on multi-scale hybrid fibers (CaCO3 whiskers, basalt fibers and

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steel fibers) in concrete. The CaCO3 whisker is micro-fiber; basalt fiber is meso-fiber and steel fiber is macro-fiber. Therefore, a new kind of hybrid fiber reinforced concrete is considered to increase the resistance against micro-cracks, meso-cracks and macro-cracks. The steel fibers, CaCO3 whiskers and basalt fibers are combined together to make a new composite for controlling cracking at multi-level. In addition to this, the mechanical properties of plain concrete (PC), steel fiber reinforced concrete (SFRC), CaCO3 whisker fiber reinforced concrete (CWRC), steel fiberCaCO3 whisker reinforced concrete (CSFRC) and steel fiber-CaCO3 whisker-basalt fiber reinforced concrete (CSBFRC) with various basalt fibers percentages i.e. 0.34%, 0.68%, 1.02% and 1.36%, by volume, are investigated. The contents of steel fibers and CaCO3 whisker are 0.32% and 0.9%, respectively, by volume. The lengths of CaCO3 whisker, steel and basalt fiber are 20–30 lm, 35 mm and 12 mm, respectively. The stress-strain curves and load-deflection curves are recorded. Also, the strengths, pre/post and total energy absorbed and toughness under compression, splitting tensile and flexural loads are determined. The scanning electron microscopy is also performed for micro-structure study of composite. 2. Experimental process 2.1. Raw ingredients The CaCO3 whisker, basalt fibers, steel fibers, coarse aggregate, sand, cement, water and superplasticizer were the raw ingredients used in this research. The chemical composition of cement and CaCO3 whisker are shown in Table 1. The maximum size of aggregates was 18 mm. The properties of CaCO3 whisker, basalt fibers and steel fibers provided by manufacturer are shown in Table 2. The CaCO3 whiskers, basalt fibers and steel fibers are shown in Fig. 1. The micromorphology and XRD pattern of CaCO3 whisker is shown in Fig. 2(a) and Fig. 2(b), respectively. In Fig. 2(b), the peak in XRD pattern shows the concentration of CaCO3 whisker at 2h. The gradation curve of fine aggregates is shown in Fig. 3. 2.2. Mixing, casting and curing All concrete mixes include PC, SFRC, CWRC, CSFRC and CSBFRC (with various basalt fibers percentages, i.e. 0.34%, 0.68%, 1.02% and 1.36%, by volume fraction) are cast. This recipe is selected based on trial mixes of CaCO3 whisker steel fiber reinforced concrete. After obtained the proper mix of CSFRC, the different contents of basalt fibers are added. The maximum content of basalt fiber was 30 kg/m3 to obtain a uniform, homogenous and workable mix. The CSBFRC batches were prepared with combination of CaCO3 whiskers, basalt fibers and steel fibers. The lengths of CaCO3 whisker, steel fibers and basalt fibers are 20–30 lm, 35 mm and 12 mm, respectively. Steel fibers and CaCO3 whiskers contents are 0.32% and 0.9%, respectively, for each fiber reinforced concrete. The superplasticizer of 4.7 kg/m3 is added to all CSBFRC mixes. For PC, the mix design was of 470 kg/m3: 940 kg/m3: 705 kg/m3: 200 kg/m3 (cement: sand: aggregate: water). The same contents of materials are used for preparation of FRC along with the addition of fibers as mentioned in Table 3. The addition of fibers would produce more additional cylinders depending upon quantities of added fibers. The whole material was added into the mixer for PC and mixer was rotated for four minutes. For production of SFRC, CWRC, CSFRC and all CSBFRC mix, the whole material was added into the mixer in layers as adopted by Ali et al. [25] to avoid balling effect. A layer of one-third aggregate was added first, followed by layers of one-third sand, steel fibers, cement, CaCO3 whisker and basalt fibers. The other layers were

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M. Khan et al. / Construction and Building Materials 192 (2018) 742–753

Table 1 Chemical composition of cement and CaCO3 whiskers (wt.%). Composition

CaO

SiO2

Al2O3

Fe2O3

CO2

MgO

K2O

SO3

Na2O

P2O5

MnO

Cement CaCO3 Whisker

61.13 54.93

21.45 0.29

5.24 0.11

2.89 0.07

2.37 42.07

2.08 2.14

0.81 –

2.50 0.31

0.77 –

0.07 –

0.06 –

Table 2 Properties of raw ingredients (provided by manufacturer). Properties

Materials CaCO3 whiskers

Density

2.86 g
cm

3

Basalt fibers 2.63–2.65 g
cm

Steel fibers 3

7.8 g
cm

3

Size

Length Diameter

20–30 lm 0.5–2 lm

12 mm 7–15 lm

35 ± 3.5 mm 0.55 ± 0.055 mm

Mechanical property

Tensile strength Elastic modulus

3000–6000 MPa 410–710 GPa

3000–4800 MPa 91–110 GPa

1345 ± 200 MPa 210 GPa

Fig. 1. Raw materials: (a) CaCO3 whiskers (b) basalt fibers and (c) steel fibers.

Fig. 2. CaCO3 whiskers: (a) micromorphology and (b) XRD pattern.

Fig. 3. Gradation curve of fine aggregates.

also added using the same sequence until all materials were finished. The mixer was rotated for 30 s with dry material for uniform distribution of fibers. After that, the half of water was poured into the mixer and was rotated for two minutes. More water was then added and mixer was again rotated for four minutes. The superplasticizer was mixed with water before pouring into the mixer. For preparing concrete samples, the moulds were filled with the mix and were put on mechanical vibrator for compaction. After 24 h, the concrete specimens were de-moulded and placed into the curing room for 28 days. The fiber reinforced concrete prepared with basalt fibers has smooth finish. The ASTM C192 [26] is followed for making and curing concrete specimens. The size of cylinders was 100 mm diameter and 200 mm height for compression and splitting-tensile tests. The prisms of size 100 mm  100 mm  400 mm were cast for flexural tests. A total of nine specimens were cast for each batch of PC, SFRC, CWRC, CSFRC, CSBFRC2, CSBFRC4, CSBFRC6 and CSBFRC8. Three specimens were cast from each batch for each property. The CSBFRC denotes

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M. Khan et al. / Construction and Building Materials 192 (2018) 742–753 Table 3 Configuration of all concrete types.

hybrid fiber reinforced concrete containing steel fiber, CaCO3 whisker and basalt fiber and the numbers 2, 4, 6 and 8 denote the basalt fiber content of 0.34%, 0.68%, 1.02% and 1.36%, by volume, respectively. Table 4 shows the total number of specimens per number of mixtures and per number of tests for all concrete types. 2.3. Testing methods 2.3.1. Slump test The slump test was performed for all mixes before pouring into moulds following ASTM C143/143M-15a [27] for workability of PC. To the best of authors’ knowledge, no ASTM standard is available to determine workability of SFRC, CWRC, CSFRC and all CSBFRC. Thus, the same procedure was used to determine the workability of all concrete mixes. 2.3.2. Compressive, splitting tensile and flexural strengths tests ASTM C39/C39M-17b [28] was followed for testing compressive strength of cylinders. All the cylinders were capped with gypsum before testing for uniform distribution of load. Universal testing machine (UTM) was used for testing all cylinders to study compressive behaviour, to calculate compressive strength (r) and to determine pre-crack energy absorbed in compression (EACPr), post-crack energy absorbed in compression (EACPo), total energy absorbed in compression (EACT) and compression toughness index (CTI). The stress-strain curves were recorded for all concrete types. The strain was taken as the ratio of change in length by original length. The stress-strain curve was determined by load displacement curve.

ASTM C496/C496M-11 [29] was followed for splitting tensile strength tests of cylinders. Compression testing machine was used for splitting-tensile strength tests to study splitting tensile behaviour, to calculate splitting tensile strength (SS) and to determine pre-crack energy absorbed in splitting tensile (EASPr), post-crack energy absorbed in splitting tensile (EASPo), total energy absorbed in splitting tensile (EAST) and splitting tensile toughness index (STI). ASTM C1609/C1609M-12 [30] was followed for flexural strength tests, to study flexural behaviour, to calculate flexural strength (f s), and to determine pre-crack energy absorbed in flexion (EAFPr), post-crack energy absorbed in flexion (EAFPo), total energy absorbed in flexion (EAFT) and flexion toughness index (FTI). The load-deflection curves were recorded for all concrete types. 2.3.3. Scanning electron microscopy (SEM) analysis SEM analysis was performed to study the micro structure of CSBFRC. The purpose of SEM analysis was to observe the interface between matrix and fibers, cracking, fibers de-bonding and fibers bonding. The Zeiss Gemini 2 Merlin FESEM machine with accelerating voltage range of 5–15 kV was used for SEM imaging. The samples were coated with iridium (Ir). 3. Experimental outcomes 3.1. Slump of fresh concrete The slump values for PC, CWRC, SFRC, CSFRC and all CSBFRCs are shown in Table 5. The slump values for CSBFRC are reduced

Table 4 Number of specimens for all concrete types. Concrete type

PC CWRC SFRC CSFRC CSBFRC2 CSBFRC4 CSBFRC6 CSBFRC8 Total

Number of specimens Compressive strength test

Flexure strength test

Splitting tensile strength test

Total

3 3 3 3 3 3 3 3 24

3 3 3 3 3 3 3 3 24

3 3 3 3 3 3 3 3 24

9 9 9 9 9 9 9 9 72

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Table 5 Slump value of all concrete mixes.

PC CWRC SFRC CSFRC CSBFRC2 CSBFRC4 CSBFRC6 CSBFRC8

Slump Values (mm)

% Reduction in slump with respect to PC

115 100 105 90 50 40 30 30

(–) 13 9 22 57 65 74 74

CWRC CSBFRC4

SFRC CSBFRC6

CSFRC CSBFRC8

30

Stress (MPa)

Mixes

PC CSBFRC2

20

10

0

in a decreasing trend. The maximum slump value is decreased up to 74% in CSCWRC6 and CSBFRC8 as compared to that of PC. The value of slump is smaller (30 mm) due to the addition of CaCO3 whiskers, steel fibers and basalt fibers. Even with the addition of super plasticizer in CSBFRC, the slump was further decreased due to the addition of higher basalt fiber content as compared to that of PC. There were more shorter fibers per unit volume which resulted in large fiber distribution density [6]. So, the quantity of short fibers is larger (1.02% and 1.36%) in concrete than other mixes (0.34% and 0.68%); and it is difficult to distributed fibers uniformly which may affects the workability of concrete. The large surface area and high content of fibers results in slump loss because the more cement-paste is utilized by fibers to wrap around which leads to higher viscosity of the mix [31]. Moreover, the distributed fibers form a network structure which restrains the flow and segregation of concrete mix. The reduction in slump with addition of increasing basalt content is also reported by Jiang et al. [6]. 3.2. Compressive behaviour, compressive strength (r), energy absorbed in compression and toughness index Stress-strain curves for PC, SFRC, CWRC, CSFRC and all CSBFRC are shown in Fig. 4. As expected, CSBFRC shows greater strength than PC due to the addition of CaCO3 whisker which has filler effect. Also, CSBFRC shows improved post-cracking behavior of stress-strain curve as compared to that of PC. The stress-strain curves of fiber reinforced concrete with improved post-cracking behavior are also reported by other researchers [34,35]. The CSBFRC6 has high strain after peak stress which shows the synergy of different fibers. At the maximum load, pieces from PC chipped off; while in CSBFRC, pieces of concrete were held together as shown in Fig. 5. The reason is the presence of hybrid fibers which provided the bridging effect across the cracks in CSBFRC. Furthermore, it was visually observed that crack width, length and number of cracks were greater in PC than that of CSBFRC at the maximum stage. However, the CSBFRC specimen looks in a good appearance after fracture with less vertical cracks. The fiber can resist lateral propagation of cracks; ultimately delaying fracture during compression testing. The expanding way of cracks is changed with the addition of fibers which results in good appearance of CSBFRC. As the basalt fiber content increases, the resistance against crack propagation increases and ultimately results in further good appearance of CSBFRC specimens as compared to that of PC. Compressive strength (r) is taken as the maximum stress from the stress-strain curve. Pre-crack energy absorbed in compression (EACPr) is calculated as the area under the stress-strain curve up to the maximum stress. The area under the stress-strain curve from maximum stress to the ultimate stress is taken as the post-crack energy absorbed in compression (EACPo). Total energy absorbed in compression (EACT) is calculated as the area under the stressstrain curve from zero to ultimate stress. Compression toughness index (CTI) is the ratio of total energy absorbed in compression to the pre-crack energy absorbed in compression (i.e. EACT/EACPr).

0

0.01

0.02 Strain (-)

0.03

0.04

Fig. 4. Typical stress-strain curves of all concrete types.

The mean values and standard deviation for r, EACPr, EACPo, EACT and CTI of all concrete types are shown are Table 6. The r of CSBFRC shows increasing trend up to addition of 1.02% basalt fiber content and then reduces from value of PC at increased basalt fiber content of 1.36%. For CSBFRC, the higher r is achieved with addition of 0.68% basalt fiber content and then r reduces with increasing in basalt fiber content up to 1.36%. The compressive strength is increased due to the filler effect of CaCO3 whisker and also the crack arresting mechanism of multi-scale fibers. The decrease in r may be due to the creation of air voids with the addition of fibers at higher content; and may due to decreased workability at higher fiber content. However, the r of all CSBFRC is still higher than that of PC except CSBFRC8. Another possible reason for reduced r may be the reduced amount of cement in concrete due to the addition of higher content of fibers. The increase in compressive strength by 4.68% and 5.1% with addition of 12 mm basalt fiber content at 28 days is also reported by Jiang et al. [6] and Kizilkanat et al. [23], respectively. Similar increasing trend with addition of basalt fiber in concrete is also reported by High et al [36]. Cao et al. [32] observed the increment in compressive strength with addition of CaCO3 whisker in cement based composite. Song and Hwang [33] reported that addition of steel fibers in concrete results in 7.1% improved compressive strength. The EACPr of all mixes is higher than that of PC. The PC SFRC, CWRC and all CSFRC shows increasing trend first and after addition of 0.68% basalt fiber content, the decreasing trend is observed for CSBFRC. The increment in EACPr may be due to the addition of fibers up to optimum dosage which effect the shear resistance ultimately enhance the bond strength. The reduction in EACPr may be due to the higher dosage of different fibers which results decreased bond strength. The EACPo of SFRC, CWRC and all CSFRC is improved than that of PC. In CSBFRC, all the value of EACPo is higher than PC. The improved EACPo is due to the addition of multi-scale hybrid fiber which results in increased strain capacity after peak stress. In contrast to PC, the EACT of SFRC, CWRC and CSFRC are enhanced showing the increasing trend with moderate slope. The increased magnitude range of EACT for CSBFRC demonstrates the increasing trend up to CSBFRC4 and after that shows decreasing trend. The EACT of SFRC, CWRC, CSFRC and all CSBFRC is enhanced than that of PC due to the incorporation of hybrid fiber which provides resistance against stresses. In comparison to PC, the CSBFRC4 showed highest CTI followed by CSBFRC6, CSBFRC8, CSBFRC2, CWRC, CSFRC and SFRC. A growing trend is observed for CTI up to CSFRC4 of all concretes. As expected, the CTI of CSBFRC is higher than that of PC because the addition of fibers provides sustainable resistance especially after the peak stress by bridging the cracks at multilevel. The improved CTI also showed the positive synergy effect caused by hybrid fibers.

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Fig. 5. Behaviour of PC and CSBFRC4 under compressive load.

Table 6 Compressive properties of all concretes. Mixes

Parameters

r PC CWRC SFRC CSFRC CSBFRC2 CSBFRC4 CSBFRC6 CSBFRC8

(MPa)

EACPr (MPa)

EACPo (MPa)

EACT (MPa)

CTI (–)

23.7 ± 2.3 24.8 ± 1.8 23.3 ± 3.1 23.6 ± 3.0 27.0 ± 1.5 24.8 ± 1.9 24.1 ± 1.1 19.3 ± 2.1

0.128 ± 0.01 0.141 ± 0.01 0.164 ± 0.02 0.151 ± 0.01 0.172 ± 0.01 0.226 ± 0.00 0.204 ± 0.03 0.161 ± 0.02

0.048 ± 0.00 0.100 ± 0.01 0.107 ± 0.01 0.102 ± 0.01 0.155 ± 0.01 0.260 ± 0.03 0.225 ± 0.02 0.165 ± 0.02

0.176 ± 0.02 0.241 ± 0.01 0.271 ± 0.03 0.253 ± 0.03 0.328 ± 0.03 0.486 ± 0.02 0.428 ± 0.04 0.326 ± 0.04

1.37 ± 0.17 1.71 ± 0.22 1.65 ± 0.13 1.68 ± 0.18 1.90 ± 0.20 2.15 ± 0.25 2.11 ± 0.30 2.03 ± 0.27

Note: An average of three readings is taken.

and then reduces at CSBFRC8. The EACPr shows the increasing trend up to optimum content of basalt fiber and then demonstrates the declining trend for CSBFRC with increase in basalt fiber content. There is growing trend in EACT of all mixes up to CSBFRC4 and then

The comparisons of compressive properties of all concrete types are shown in Fig. 6. It may be noted than the comparison of all compressive properties is with respect to that of PC. The r of CSFRC, CSBFRC2, CSBFRC4 and CSBFRC6 is significantly increased

Percentage

300

PC CSBFRC2

CWRC CSBFRC4

SFRC CSBFRC6

CSFRC CSBFRC8

200

100

0 σ

EACpr

EACT Parameters

Fig. 6. Comparison of compressive properties of all concrete types.

CTI

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CSBFRC shows slightly decrease in EACT with increased basalt fiber content. The CTI is improved from a minimum of 20% to a maximum of 56% of CSBFRC and presents a good increasing trend up to CSBFRC4. The decrease compressive properties are due to the heterogeneity of mix caused by higher content of hybrid fibers and which also results in reduced cement quantity. The reason for enhanced compressive properties is due to the filler effect of CaCO3 whisker and positive synergy effect caused by hybridization of fibers up to optimum content. Furthermore, the strong bridging effect and pull-out resistance of fibers results in improved properties. The maximum percentage of standard deviation errors for r, EACPr, EACPo, EACT and CTI is up to 13%, 13%, 14%, 12% and 14%, respectively.

3.3. Splitting tensile behaviour, splitting tensile strength (SS), energy absorbed in splitting tensile and toughness index Splitting tensile strengths of all concrete types are shown in Fig. 7. The SFRC, CSFRC and all CSBFRC show more strengths than PC. The SFRC, CSFRC and all CSBFRC shows higher load capabilities at the peak load as compared to that of PC. The CSBFRC4 shows higher load absorption capacity at peak which shows the positive synergy of different fibers. At first crack load, the PC and CWRC specimens were broken into two pieces; while in SFRC, CSFRC and all CSBFRC, the specimens were not broken in to two pieces as shown in Fig. 8. Similar trend is observed in all specimens except PC and CWRC with different maximum load. The first crack and peak load of PC and CWRC is same because the specimens is broken into two pieces at first crack load. The hybrid fibers provided the resistance across the cracks in SFRC, CSFRC and all CSBFRC. The crack width, length and number of cracks at maximum load are greater than that of first crack load in SFRC, CSFRC and all CSBFRC. Similar behaviour is also observed at ultimate load as compared to that of first crack and maximum load. The bridging effect caused by hybrid fibers does not allow the specimen to break into two pieces after first crack load. Splitting tensile strength (SS) is calculated from the maximum load of the load-time curve. Pre-crack energy absorbed in splitting tensile (EASPr) is calculated as the area under the load-time curve up to the first crack load. The area under the load-time curve from first crack load to the peak load is taken as the post-crack energy absorbed in splitting tensile (EASPo). Total energy absorbed in splitting tensile (EAST) is calculated as the area under the load-time curve from zero to peak load. Splitting tensile toughness index (STI) is the ratio of total energy absorbed in splitting tensile to the pre-crack energy absorbed in splitting tensile (i.e. EAST/EASPr).

Splitting tensile strength (MPa)

5 4

The mean value and standard deviation for SS, EASPr, EASPo, EAST and STI of all concrete types are shown are Table 7. The SS of shows the growing trend up to CSBFRC4 and then displays the declining trend. For CSBFRC, the SS is enhanced with addition of 0.68% basalt fiber content and then reduced with increase in basalt fiber content up to 1.36%. However, the SS of all CSBFRC is still higher than that of PC. Jiang et al. [6] and Kizilkanat et al. [23] reported the improvement in SS with addition of 12 mm basalt fiber in concrete up to 9.5% and 34%, respectively. The hybrid fibers provide resistance against cracking at multi-level which ultimately results in improved SS. The SS may be decreased due to the low bond strength caused by addition of higher content of basalt fiber. The EASPr of CSBFRC is increased up to 1.02% basalt fiber addition and then decreased with increase in basalt fiber content at 1.36%, than that of PC. The EASPr of CSBFRC8 is reduced due to the heterogeneity of mix caused by higher content of basalt fiber which results in filled voids. The increment in EASPr up to optimum content may be due to the filler effect and also the crack arresting mechanism of CaCO3 whisker at early stage. The EASPo of all the concrete types containing steel and basalt fibers is higher than PC and CaCO3. The improved EASPo is due to positive synergy of multi-scale hybrid fiber which results in increased load capacity after first crack load. In contrast to PC, the increment in EAST of SFRC, CWRC, CSFRC and all CSBFRC mixes is observed and shows the growing trend up to CSBFRC4. The EAST of SFRC, CWRC and CSFRC and all CSBFRC is enhanced than that of PC due to the incorporation of hybrid fiber which offers resistance to cracking at different levels. The STI of PC and CWRC is 1 because there is no EASPo. The maximum enhancement in STI of CSBFRC is observed with addition of 0.68% basalt fiber content. As expected, the STI of CSBFRC is higher than that of PC because of improved postcracking behavior and energy absorption. The addition of multiscale hybrid fiber may result in improved interface bonding between aggregate and cementitious composites up to optimum content of basalt fiber. Furthermore, the crack arresting power of CaCO3 whiskers, basalt fibers and steel fibers results in improved EACT and STI. The comparisons of splitting tensile properties of all concrete types are shown in Fig. 9. It may be noted than the comparison of all splitting tensile properties is with respect to that of PC. The SS of is enhanced showing a growing trend initially and then result in decline with increase in basalt fiber content. The EASPr is increased up to 14%. There is an increment in EAST of all mixes. The STI is improved from a minimum of 14% to a maximum of 73% of CSBFRC. The crack propagation is effectively slow down by addition of fibers which results in enhanced properties. Also, the random distribution of hybrid fiber in the matrix results in improved splitting tensile properties. The percentage of standard deviation errors for SS, EASPr, EASPo, EAST and STI is up to 10%, 10%, 14%, 9.8% and 7%, respectively. 3.4. Flexural behaviour, flexural strength (f s), energy absorbed in flexion and toughness index

3 2 1 0

Concrete Type Fig. 7. Splitting tensile strengths of all concretes.

Load-deflection curves for PC, SFRC, CWRC, CSFRC and all CSBFRC are shown in Fig. 10(a). As expected, SFRC, CSFRC and all CSBFRC curves show more deflection than PC. The enlarge view of load-deflection curve up to 2 mm deflection is shown in Fig. 10(b). It can be clearly seen that the CSBFRC4 and CSBFRC6 still have higher load than other FRCs after 2 mm deflection. On the other hand, initially the SFRC, CSFRC, CSBFRC2 and CSBFRC8 has peak load at small deflection as compared to CSBFRC4 and CSBFRC6. The CSBFRC shows better deflection capacity at ultimate load and shows higher load capacity in load-deflection curve at and after the peak load as compared to that of PC, SFRC, CWRC and CSFRC. The deflection capacity at ultimate load of CSBFRC is up

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Fig. 8. Behaviour of PC and CSBFRC4 under splitting tensile load.

Table 7 Splitting tensile properties of all concretes. Mixes

Parameters

PC CWRC SFRC CSFRC CSBFRC2 CSBFRC4 CSBFRC6 CSBFRC8

SS (MPa)

EASPr (kN.s)

EASPo (kN.s)

EAST (kN.s)

STI (–)

2.79 ± 0.23 3.02 ± 0.31 3.23 ± 0.29 3.31 ± 0.14 3.49 ± 0.35 3.90 ± 0.32 3.47 ± 0.27 2.96 ± 0.18

1954 ± 151 2229 ± 201 1995 ± 79 2100 ± 101 1969 ± 123 2181 ± 212 2016 ± 203 1882 ± 104

0 ± 0.00 0 ± 0.00 807 ± 85 554 ± 57 1097 ± 101 1598 ± 51 954 ± 89 273 ± 41

1954 ± 151 2229 ± 201 2802 ± 176 2654 ± 228 3066 ± 241 3779 ± 312 2970 ± 179 2156 ± 211

1.00 ± 0.00 1.00 ± 0.00 1.40 ± 0.03 1.26 ± 0.03 1.56 ± 0.05 1.73 ± 0.09 1.47 ± 0.06 1.15 ± 0.08

Note: An average of three readings is taken.

Percentage

300

PC CSBFRC2

CWRC CSBFRC4

SFRC CSBFRC6

CSFRC CSBFRC8

200

100

0 SS

EASpr

EAST

STI

Parameters Fig. 9. Comparison of splitting tensile properties of all concrete types.

to 10.9 mm which is approximately 10 times of PC. The CSBFRC shows higher load absorption capacity after peak deflection to ultimate deflection which shows the synergy of different fibers. At first crack load, the PC and CWRC specimens were broken into two pieces; while in SFRC, CSFRC and all CSBFRC, the specimens were not broken into two pieces (refer Fig. 11). Similar trend is observed in all specimens except PC and CWRC with different maximum and ultimate load. The reason is the presence of hybrid fibers which provided the bridging effect across the cracks in SFRC, CSFRC and all CSBFRC. Also it was visually observed that crack width and

length at maximum load is greater than that of first crack load in SFRC, CSFRC and all CSBFRC. Similar behaviour is also observed at ultimate load as compared to that of first crack and maximum load. The resistance against crack propagation increases with increase in basalt fiber content and shows flatter load-deflection curve after peak load of CSBFRC specimens. The bridging effect offer by hybrid fibers results in improved the ductility and toughness and thus eliminating the sudden brittle failure after maximum load. In SFRC, CSFRC and all CSBFRC the load after peak drops slowly and display excellent load absorption capacity than PC and CWRC.

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PC

CWRC

SFRC

CSFRC

CSBFRC2

CSBFRC4

CSBFRC6

CSBFRC8

25

Load (kN)

20 15 10 5

(a)

0 0

2

4 6 Deflection (mm)

8

10

PC

CWRC

SFRC

CSFRC

CSBFRC2

CSBFRC4

CSBFRC6

CSBFRC8

25

Load (kN)

20 15 10 5

(b)

0 0

0.5

1 Deflection (mm)

1.5

2

Fig. 10. (a) Typical load-deflection curves of all concrete types; (b) Enlarge view of load-deflection curve up to 2 mm.

Flexural strength (f s) is calculated from the maximum load of the load-deflection curve. Pre-crack energy absorbed in flexion (EAFPr) is calculated as the area under the load-deflection curve up to the maximum load. The area under the load-deflection curve from maximum load to the ultimate load is taken as the post-crack energy absorbed in flexion (EAFPo). Total energy absorbed in flexion (EAFT) is calculated as the area under the load-deflection curve from zero to ultimate load. Flexion toughness index (FTI) is the ratio of total energy absorbed in flexion to the pre-crack energy absorbed in flexion (i.e. EAFT/EAFPr). Table 8 shows the mean and standard deviation values of f s, EAFPr, EAFPo, EAFT and FTI for all concrete types. The f s of SFRC, CWRC and CSFRC and all SCBFRC shows a growing trend. For CSBFRC, the f s is enhanced with addition of 1.02% basalt fiber content and then reduced with increase in

basalt fiber content up to 1.36%. The f s is increased due to the crack arresting mechanism of multi-scale hybrid fibers. The hybrid fibers provide resistance against cracking at micro-, meso- and macrolevel. The addition of steel fibers results in increased flexural strength [33]. Jiang et al. [6] and Kizilkanat et al. [23] observed the increment in flexural strength with addition of 12 mm basalt fiber in concrete up to 9.5% and 34%, respectively. The possible reason for reduced f s may be the low bond strength caused by addition of higher content of basalt fiber due to lesser amount of cement. Another reason in decreased f s may be the heterogeneity of the concrete mix with the addition of fibers at higher content. However, the f s of all CSBFRC is still higher than that of PC. The EAFPr of SFRC, CWRC, CSFRC and CSBFRC shows the increasing trend. The improved EAFPr of CWRC may be due to the addition of micro fiber (CaCO3 whisker) which offers resistance at early stage. The increment in EACPr of SFRC and CSFRC may be due to the addition of fibers which affect the shear resistance and increase the bond strength. Compared to SFRC and CSFRC, the reduction in EACPr of CSBFRC may be due to the higher dosage of different hybrid fibers which results in filled voids ultimately cause early initiation of micro cracks. In contrast to PC, the maximum enhancement in EAFPo is noted up to CSBFRC4. The declining trend is observed after CSBFRC6 to CSBFRC8. The improved EAFPo is due to the addition of multi-scale hybrid fiber which results in increased deflection capacity after peak load. For EAFT of SFRC, CWRC, CSFRC and CSBFRC an increasing trend is observed up to CSBFRC4; beyond that the decreasing pattern is observed. The EAFT of SFRC, CWRC, CSFRC and all CSBFRC is enhanced than that of PC due to the incorporation of hybrid fibers which provide the resistance against stresses. The FTI of PC and CWRC is 1 because there is no EAFPo. In contrast to PC, the FTI of all CSBFRC is marginally higher than for SFRC and CSFRC. As anticipated, the FTI of CSBFRC is higher than that of PC because of improved post-cracking behavior and higher energy absorption. The constrainment effect and crack arresting mechanism of CaCO3 whisker, basalt fiber and steel fiber results in improved EAFT and FTI due to the positive hybridization of fibers. The enhanced toughness index for the addition of 12 mm basalt fiber is also reported by Jiang et al. [6]. Fig. 12 demonstrates the comparisons of flexural properties of all concrete types. The comparison of all flexural properties is with respect to that of PC. The f s shows an upward trend with a moderate slope. The increment in EAFPr is up to 116%. The EAFT of demonstrate growing trend up to CSBFRC4; and then display declining trend with increase in basalt fiber content. The improvement in FTI is observed up to 565%. The reason for enhanced flexural properties is the strong bridging effect and pull-out resistance offered by hybrid fibers. The percentage of standard deviation errors for f s, EAFPr, EAFPo, EAFT and FTI is up to 10%, 11%, 14%, 11% and 11%, respectively.

Fig. 11. Behaviour of PC and CSBFRC4 under flexural load.

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M. Khan et al. / Construction and Building Materials 192 (2018) 742–753 Table 8 Flexural properties of all concretes. Mixes

Parameters

PC CWRC SFRC CSFRC CSBFRC2 CSBFRC4 CSBFRC6 CSBFRC8

Ƒs (MPa)

EAFPr (kN.mm)

EAFPo (kN.mm)

EAFT (kN.mm)

FTI (–)

4.59 ± 0.43 5.00 ± 0.39 5.08 ± 0.52 5.70 ± 0.47 5.12 ± 0.46 5.79 ± 0.32 6.70 ± 0.63 5.63 ± 0.5

5.6 ± 0.55 6.6 ± 0.64 12.2 ± 1.32 12.1 ± 1.41 8.3 ± 0.89 8.7 ± 0.54 10.3 ± 0.31 9.7 ± 1.02

0.0 ± 0.00 0.0 ± 0.00 43.8 ± 6.30 39.8 ± 3.70 44.8 ± 2.33 49.3 ± 5.20 45.5 ± 4.90 29.7 ± 3.70

5.6 ± 0.55 6.6 ± 0.64 56.0 ± 7.84 51.9 ± 5.67 53.9 ± 6.01 58.1 ± 4.10 55.8 ± 2.30 39.4 ± 3.57

1.00 ± 0.00 1.00 ± 0.00 4.60 ± 0.34 4.28 ± 0.41 6.51 ± 0.63 6.66 ± 0.40 5.43 ± 0.59 4.07 ± 0.47

Note: An average of three readings is taken.

for the structural applications. The comparison of tested concrete types is shown in Table 9. The highest strength, pre-crack energy absorbed, total energy absorbed and toughness index for compressive, splitting tensile and flexural properties are presented. The suggested optimum concrete type is CSBFRC4, i.e. 0.32% steel fiber, 0.9% CaCO3 whisker and 0.68% basalt fiber content, by volume. The addition of hybrid fibers shows enhanced mechanical properties of concrete. The mechanical properties of CSBFRC are improved as compared to that of PC. Wright et al. [37] stated that improving r of concrete with material other than cement results less early age micro cracking (EAMC) which ultimately enhancing the durability of structural members. The improved tensile strength controls the early age micro cracking [38,39]. Therefore, the improved r and f s will lead towards less EAMC ultimately improving durability. A more crack-resistant concrete can greatly increase the lateral load carrying capacity and structural durability [12]. The mechanical properties of CSBFRC are increased due to multi-level crack-arresting mechanism by CaCO3 whiskers, steel fibers and basalt fibers at multi-level. The crack arresting mechanism and better bond strength of steel fibers, basalt fibers and CaCO3 whisker are also observed by SEM analysis as discussed above in Section 3.5. Burgueno et al. [2] reported that the performance of bridge structures is greatly dependent on the energy dissipating capacity and tough behaviour of piers. The better energy absorption capacity of CSBFRC will result in more energy dissipation in concrete, first improving the capacity/tough behavior and then ultimately enhancing the performance of bridge structure. The deterioration reduction through micro- and macro-crack control is a performance based design methodology which consists of hybridization [40]. Thus, the crack arresting power of hybrid fibers at micro-, meso- and macro-levels does not allow the moisture to penetrate in to the concrete which ultimately reduces the deterioration of concrete and improves the durability of bridge structure.

3.5. Microstructural analysis The scanning electron microscopy (SEM) images of macrocrack, meso-crack, fibers de-bonding and basalt fibers bonding are shown in Fig. 13. The steel fiber bridged across macro-crack and did not allow the crack to propagate on the other side of the matrix (refer Fig. 13(a)). Furthermore, the embedded steel fiber showed the proper bonding between the matrix and fiber. The bundle of basalt fiber provided the resistance against cracking (Fig. 13(b)) ultimately enhanced mechanical properties. The meso-crack in the matrix propagated after the pull-out of basalt fiber which converted into the macro-crack. The basalt fiber had smooth surface, ultimately resulting de-bonding of fibers from the matrix. The micro-crack converted into the meso-crack after the de-bonding of CaCO3 whisker; this meso-crack then converted into the macro-crack after the de-bonding of basalt fiber as shown in Fig. 13(c). Also, the fiber pull-out is due to the smaller development length on one side of the crack creating lesser bond strength caused by heterogeneity of the mix due to the higher content of basalt fiber. At certain locations, the fiber-matrix bond shows the proper bonding which can be observed by cement matrix/paste attached on the surface of basalt fiber (refer Fig. 13(d)). On the other hand, the basalt fiber is covered with dense hydrated cement matrix which shows good bond between basalt fiber and cement matrix. The dense interfacial transition zone of basalt fiber and cement matrix showing proper bonding can also be observed from Fig. 13(d). This phenomenon can result in improved mechanical properties of CSBFRC as compared to PC. 4. Discussion In this research, hybrid fiber reinforced concrete with addition of CaCO3 whiskers, steel fibers and basalt fibers is investigated

Percentage

1200

PC CSBFRC2

CWRC CSBFRC4

SFRC CSBFRC6

CSFRC CSBFRC8

900 600 300 0 ƒs

EAFpr

EAFT Parameters

Fig. 12. Comparison of flexural properties of all concrete types.

FTI

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M. Khan et al. / Construction and Building Materials 192 (2018) 742–753

Macro-Crack

Bundle of Basalt Fibers

Steel Fiber Basalt Fiber Pull-out

Fiber-Matrix Interface

Basalt Fiber De-bonding Meso-Crack

(a)

Basalt fiber

(b)

Fiber-Matrix Interface

CaCO3 Whisker De-bonding

CaCO3 Whisker Basalt Fiber

Crack Propagation Basalt Fiber De-bonding (c)

(d) Fig. 13. SEM analysis (a) macro-crack (b) meso-crack (c) fibers de-bonding (d) basalt fibers bonding.

Table 9 Comparison of tested concrete types. Parameters Highest strength Highest pre crack energy Highest total energy Highest toughness index

Compressive

Splitting tensile

Flexural

CSBFRC2 (14%) CSBFRC4 (76%)

CSBFRC4 (39%) CWRC (14%)

CSBFRC6 (46%) SFRC (116%)

CSBFRC4 (176%) CSBFRC4 (56%)

CSBFRC4 (93%)

CSBFRC4 (931%) CSBFRC4 (565%)

CSBFRC4 (73%)

Note: Percentage in the bracket shows percentage increase with respect to PC.

Zhang and Cao [9] reported that the macro-fibers arrest macrocracks, ultimately resulting in improved toughness. However, the CSBFRC favors its utility to be used for the structural applications like bridge structures due to its improved energy dissipation capacity and toughness.

 The compressive, splitting tensile and flexural total energy absorbed are improved up to 176%, 93% and 931%, respectively, than that of PC.  In contrast to PC, the increments in compressive, splitting tensile and flexion toughness index are up to 56%, 73% and 565%, respectively.  The proper bonding and bridging of hybrid fibers in the matrix is observed by scanning electron microscopy.  The CSBFRC4 is suggested to be an optimized mix, i.e. the concrete with steel fibers (0.32%), CaCO3 whisker (0.9%) and 0.68% basalt fibers, by volume fraction. The CSBFRC with combination of CaCO3 whiskers, steel fibers and basalt fibers showed satisfactory results with 12 mm basalt fibers length. Therefore, the next step is to optimize various basalt fiber lengths (12 mm, 25 mm, 37 mm and 50 mm) and contents in hybrid fiber reinforced concrete to get overall best properties. Conflict of interest

5. Conclusions None. In this study, the CaCO3 whisker steel fiber and basalt fiber reinforced concrete (CSBFRC) is investigated. The lengths of steel fibers, basalt fibers and CaCO3 whiskers are 35 mm, 12 mm and 20– 30 lm, respectively. Properties under compressive, splitting tensile and flexural loading are experimentally determined. Following conclusions are drawn:  The workability of different CSBFRC mixes is reduced up to 74% as compared to that of plain concrete (PC).  There is an increase in compressive, splitting tensile and flexural strength up to 14%, 39% and 46%, respectively, as compared to that of PC.

Acknowledgements The authors would like to acknowledge the support of this work by the Natural Science Foundation of China under Grant No. 51678111 and No. 51478082. The financial support from China Scholarship Council (CSC) for PhD studies of first author at Dalian University of Technology, China is gratefully acknowledged. The authors are also thankful to Dr. Mingli Cao research group and Engr. Shakeel Ahmad for their help during the lab work. The careful review and constructive suggestions by the anonymous reviewers are gratefully acknowledged.

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