a r c h i v e s o f c i v i l a n d m e c h an i c a l e n g i n e e r i n g 1 9 ( 2 0 1 9 ) 3 4 8 – 3 5 9

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Original Research Article

On the development and studies of nano- andmicro-fiber hybridized strain hardenedcementitious composite

B.S. Sindu a,b,*, Saptarshi Sasmal a,b

aAcademy of Scientific and Innovative Research (AcSIR), INDIAbCSIR-Structural Engineering Research Centre, CSIR Campus, Taramani, Chennai, 600113, INDIA

a r t i c l e i n f o

Article history:

Received 6 March 2018

Accepted 23 November 2018

Available online 29 December 2018

Keywords:

Cementitious composite

Carbon nanotubes

PVA fibers

Digital image correlation technique

Strain-hardening

a b s t r a c t

In this study, strain hardened cementitious composite is developed by systematically

incorporating fibers of two different length scales, viz., PVA fibers (micro-fibers) and CNTs

(nano-fibers) to improve the load transfer and crack formation mechanism at their corre-

sponding [17_TD$DIFF]scales. At first, the influence of individual fibers on the tension associated (axial

tension-, fracture- and flexure-) properties is investigated. Then, the composite is developed

using hybrid fibers with appropriate dosage to cater the desired performance. The tensile

strength, stiffness, strain carrying capacity and fracture energy of the developed composite

is found to be improved by almost 2 times, 3 times, 220 times and 130 times respectively to

that of the original cement composite. The outstanding performance of the developed

composite is resulted from the effective crack bridging and preferred load transfer in

micro-scale due to incorporation of (a meagre amount of) hetero fibers of distinctly different

length [17_TD$DIFF]scales. In order to investigate the fracture and crack propagation phenomenon of the

developed cementitious composite, Digital Image Correlation (DIC) technique is also

employed. The findings of this study will lead towards development of multi-performance

cementitious composite (MPCC) by tailoring the material to attain the desired level of

strength, stiffness and ductility.

© 2018 Politechnika Wrocławska. Published by Elsevier B.V. All rights reserved.

1. Introduction

With the increased demands from the infrastructure sector,there is a pressing need to engineer the properties of thecementitious composite with multiple performance targets.However, themajor challenge in engineering the cementitious

* Corresponding author at: Nano Mechanics and Engineering Group, SEngineering Research Centre (CSIR-SERC), CSIR Campus, Taramani, C

E-mail address: sindu@serc.res.in (B.S. Sindu).https://doi.org/10.1016/j.acme.2018.11.0081644-9665/© 2018 Politechnika Wrocławska. Published by Elsevier B.V

composites is that they are heterogeneous and porous innaturewith components and defects at different length scales.Hence, in order to effectively enhance the properties ofcementitious composite, intervention has to be done carefullyto overcome the absence of mechanism to delay the crackinitiation phenomenon. Incorporation of fibers into cementi-tiousmatrix has been proven to be advantageous as it helps in

pecial and Multifunctional Structures Laboratory CSIR-Structuralhennai 600113, India.

. All rights reserved.

[(Fig._1)TD$FIG]

Fig. 1 – Crack bridging of fibers at different length scales.

a r c h i v e s o f c i v i l a n d m e c h an i c a l e n g i n e e r i n g 1 9 ( 2 0 1 9 ) 3 4 8 – 3 5 9 349

bridging the cracks developed in it, thereby increasing thetensile strength and ductility of the cementitious composite.Several attempts have been made to incorporate fibers of fewmillimetres in length into cementitious composite to improveits tensile strength and ductility. Incorporation of chemicallyactive polymeric fibers has been found to be highly potential asit creates both the chemical- and frictional-bond with thecementitious matrix; thereby able to enhance its straincapacity by many folds. The cementitious composite rein-forced with poly vinyl alcohol (PVA) fibers exhibit pseudo-strain hardening and multiple cracking behaviour after firstcrack [1].

Li et al. [2] developed PVA fibers (few millimetres in length)incorporated engineered cementitious composite (ECC) thatexhibited moderate tensile strength of 4–12 MPa and is highlyductile with strain capacity of 3–7%. Ranade et al. [3] tailoredthe constituents of ECC and developed the composite whichexhibited high tensile strength of 16 MPa, high ductility in theorder of 5% and high energy dissipation capacity of 20%. Yuet al. [4] developed PVA fibers incorporated ECC with thetensile strength of 20 MPa and elongation of 8.7%. Apart fromexhibiting outstanding mechanical properties, ECC alsoexhibited reduced permeability [5], good thermal conductivity[6] and high impact resistance [7]. However, the properties likestrength, ductility and strain hardening behaviour of ECCdepend largely on the method of mixing of ingredients, curingof specimens, distribution of fibers in the matrix [8] and fiber-matrix interactions. PVA incorporated cementitious compos-ite also served as a promising repair [9,10] and retrofit material[11] for reinforced concrete beams.

Similar to incorporation of micro-fibers of few millimetresin length, attempts have also been made, in recent years, toincorporate nano-fibers like carbon nanofibers (CNFs) andcarbon nanotubes (CNTs) of few micrometres in length intocementitious composite. CNTs have extraordinarymechanicalproperties with Young's modulus in the range of 1–6 TPa,tensile strength in the range of 20–60 GPa, and an ultimatestrain of 12% and are extremely light in weight [12].Incorporation of very small amount of CNT (0.04–0.5% byweight of cement) into cementitious composite improved themechanical properties like elastic modulus [13], strength[14,15], fracture toughness [16,17], durability [18] and piezo-resistive [19] properties. Hence, it has been identified thatCNTs are promising candidates to improve the properties ofthe cementitious composite.

1.1. Usage of hybrid fibers for multi-scale crack bridging

Incorporation of both micro- and nano-fibers into cementi-tious compositewill be highly beneficial as it will provide crackbridging mechanism in the composite at different lengthscales (as shown in Fig. 1). Different types of hybridization hasbeen explored by researchers. Zhang and Cao [20] demon-strated that incorporation of calcium carbonate whisker inaddition to PVA and steel fibers into cementitious compositelead to improvement in flexural strength, toughness, straincapacity and also changed the mode of failure to the ductileone. It was also demonstrated that the mechanical properties,microstructure, the uniaxial and triaxial compression ofcementitious composite improved tremendously due to

multiscale cracking resistance offered by the presence of bothnano- and micro-fibers [21]. Sbia et al. [22] demonstrated thatCNFs (nanofibers) can act synergistically with PVA fibers(micro-fibers) by providing reinforcing action at[13_TD$DIFF] differentlength scales. Meng and Khayat [23] incorporated CNFs inconcrete containing steel micro-fibers and found that incor-poration of 0.3% of CNFs was able to improve the tensilestrength, flexural strength and toughness significantly whencompared to singly reinforced or plain cement paste. Similarobservations were made by Metaxa et al. [24], Alshaghel et al.[25], Alrekabi et al. [26] and Jiang et al. [27] by incorporation ofhybrid nano- and micro-fibers. Attempts were even made toelectro-deposit graphene oxide on carbon fibers beforeincorporating it into cementitious composite to improve itsmechanical properties [28]. From the above studies, it isevident that the incorporation of both the nano- and micro-fibers into cementitious composite is highly beneficial.However, the research in this area is very limited and gainingthe interest in recent years towards developing innovativecomposites.

This crack bridging phenomenon depends on the fiber–matrix interface properties which is governed by the chemicalbond strength and frictional bond strength. The PVA fibers(micro-fibers) considered in this study has a chemical bondand it also displays strain-hardening effect. On the other hand,CNTs are capable of delaying the nucleation and growth ofcracks at the micro-scale; thereby, further delay the propaga-tion of crack to macro scale. Also, due to higher modulus, theyhelp in improving the micro-stiffness of the composite. HenceCNTs can act as complements to traditional fibers and help indeveloping high strength, high ductile and high modulusmulti-performance cementitious composite (MPCC) by suit-ably tailoring at nano- and micro-scale.

In view of this, an attempt has been made in this study todevelop cementitious composites by incorporating fibers oftwo different length scales, viz., PVA fibers (micro-fibers) andCNTs (nano-fibers) to get significantly improved mechanicalproperties, such as tensile strength, ductility, stiffness(modulus), delayed crack formation, fracture energy, etc.Since cementitious composites are complex in nature andare deficient in tensile properties, the primary focus of thepresent work is to understand the influence of hybridizationon the key aspects of tension related properties, i.e., tensile,fracture and flexural properties. A thorough investigation has

a r ch i v e s o f c i v i l a n d m e ch an i c a l e n g i n e e r i n g 1 9 ( 2 0 1 9 ) 3 4 8 – 3 5 9350

been carried out to understand the influence of incorporationof each type of fiber into the composite and the optimumdosage for each case has been arrived at based on whichsuitable hybridization has been carried out. In order to get anin-depth understanding of the cracking behaviour of thedeveloped composite (at considerable lower scale than that forthe usual usage of conventional sensors), non-contact opticaltechnique, viz., digital image correlation (DIC), has also beenused. Various key features like strain localization, crackdevelopment and crack propagation which cannot beextracted through conventional strain gauges has beeninvestigated using DIC technique. The findings of this studywill lead towards development of the high performance strainhardened cementitious composites.

2. Experimental programme

2.1. Development of strain hardened cementitiouscomposite using hybridization

The basic ingredients of the mix include cement, PVA fibers,CNTs, water and superplasticizer (SP). Cement used for thestudy was Ordinary Portland cement (OPC) of grade 53.Polycarboxylate based high range water reducing admixture(HRWRA)was used as SP to increase theworkability of themix.The specifications of PVA fibers and CNTs used in the study aregiven in Table 1. CNTs were dispersed in Gum Arabic solutionusing ultrasonication for 1 h. The choice of surfactant andultrasonication parameters for optimum dispersion of CNT isfrom the work of Sindu et al. [29]. The proportion of variousmaterials used in the composite is given in Table 2. Measuredquantity of cement was poured into a rotary [18_TD$DIFF]mixer with a flatbeater. To this, water mixed with superplasticizer was addedand mixed for 2 min. Then, PVA fibers were added and mixedfor 3 min. Finally dispersed CNT solution was added andmixed for 2 min. The process of specimen preparation is

Table 1 – Specifications of nano- and micro-fibers used in the

Material Diameter Length Bul

PVA fibers 40 mm 12 mm 1MWNT Outer: 50–80 nm

Inner: 5–15 nm10–20 mm 0

Table 2 – Mix proportion.

Nomenclature Cement Water(in weight % of cement) (in weig

PF0.5 1 0.4PF1 1 0.4PF1.5 1 0.4PF2 1 0.4PGA06 1 0.4PGA08 1 0.4PGA1 1 0.4PGA12 1 0.4PGA1F2 1 0.4PGA12F2 1 0.4

shown in Fig. 2. After proper mixing of all the ingredients, themix is poured into moulds. Specimens were demoulded after24 h and cured in water for 28 days.

2.2. Studies on strain hardened cementitious composite

2.2.1. Tension studiesObtaining the direct tensile strength of cementitious compos-ite (with very thin sectional area) is cumbersome due todifficulties involved in fabricating the test samples anddesigning the test set up for gripping the specimens forcarrying out experiments with perfect concentric alignment.Keeping these issues in mind, grips were designed andfabricated in-house. The grips are capable of holding thespecimen of size 330 mm � 60 mm � 12 mm. In order torestrict the failure in the gauge section, the width of thespecimen is reduced to 30 mm in that region (shown in Fig. 3).The specimens were then subjected to tensile loading with aloading rate of 0.1 mm/min using high precision servocontrolled UTM of 25 kN capacity. Strain gauges were pastedon one side of the specimen to measure the strain data of thespecimen. On the other side, colour spraywas applied to createa speckle pattern for carrying out DIC studies.

2.2.2. Flexural studiesFlexural tests were conducted on prism specimens of dimen-sion 40 � 40 � 160 mm as per ASTM C348-14. Three pointbending tests were conducted on specimens with a loadingspan of 150 mm. Load was applied at a rate of 0.1 mm/min.Deflection at the mid-span of the specimen was monitoredthrough two laser LVDTs (with a resolution of 80 mm). Perplexsheet affixed to the specimen at mid height served as thetarget for these laser LVDTs.

2.2.3. Fracture studiesThe fracture tests were carried out on prism specimens ofdimension 40 � 40 � 160 mm using Japanese Standard, JCI-S-

study.

k density Manufacturer

.33 g/cc Kuraray Co. Ltd.

.18 g/cc Nanostructured and Amorphous Materials, Inc.

CNTht % of cement)

PVA(in volume % of cementitious composite)

– 0.5– 1.0– 1.5– 2.00.06 –

0.08 –

0.10 –

0.12 –

0.10 2.00.12 2.0

[(Fig._2)TD$FIG]

Fig. 2 – Fabrication of cementitious composite incorporated with hybrid fibers.

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001-2003. The specimens were notched at the centre and weresubjected to three-point bending. The depth of the notch (a [14_TD$DIFF])was kept as 12 mm (=0.3D) and the width was kept as 2 mm.The loading span was taken as 150 mm. Clip-on gauge with agauge length of 10 mm and a travel of 4 mm was used tomeasure the crack mouth opening displacement (CMOD). Therate of loading (CMOD rate) was maintained at 0.2 mm/min.The load-CMOD curve was obtained for each specimen. Fromthis curve, the fracture energy (N/mm2

[16_TD$DIFF]) is obtained using theformula,

GF ¼ 0:75W0 þW1

Alig(1)

W1 ¼ 0:75SLm1 þm2

� ��g�CMOD (2)

where, W0 is the area below load-CMOD curve, W1 is the workdone by the deadweight of specimen and loading jig,Alig is the

[(Fig._3)TD$FIG]

Fig. 3 – Test set up for conducting direct tension test.

area of broken ligament, m1 and m2 are the mass of specimenand loading fixture, respectively, S and L are the loading spanand the length of the specimen.

2.3. Digital image correlation

Digital image correlation (DIC) technique is a non-contact,optical technique which gives contour of deformation,displacement, strain, stress, vibration, etc. of the specimen.This technique can also be used to capture the strainconcentration, crack tip formation and the crack propagationin the component. This method employs pattern recognitionalgorithm to detect the changes in the grey scale distributionof surface pattern of the component when subjected toloading. For a three dimensional DIC technique, two synchro-nized high speed cameras mounted on the either side of thetripod stand (shown in Fig. 4) capture a series of photographsof the component containing patterns. A rectangular gridknown as 'facet' is mapped onto the image which defines thecorrelation area. The movement of these facets in thesuccessive images of the component under test is trackedby image correlation algorithm. In this study, DIC technique

[(Fig._4)TD$FIG]

Fig. 4 – Digital image correlation (DIC) test set up.

[(Fig._5)TD$FIG]

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was used to investigate the crack behaviour of all specimensunder all loading cases.

Fig. 5 – Stress–strain curves of cementitious compositeincorporated with (a) CNTs (revealing improvement inmodulus and strength), (b) PVA fibers (revealingimprovement in strain) and (c) hybrid fibers (revealingimprovement in strength, modulus and strain).

3. Results and discussion

3.1. Influence of hybridization on the improvement intensile properties

3.1.1. Effect of CNTsIt can be observed from Fig. 5a and Table 3 that both thestrength and stiffness of the composite consistently increaseas the dosage of CNTs increase. After a certain percentage(�0.08%), though there is an increase in stiffness, strengthincrease is found to be not considerable (compared to lowerdosage level). The increase in strength of the composite with0.08% of CNTs accounts to 58% and the increase in stiffnessaccounts to 69%. When composite is reinforced with 0.12% ofCNTs, stiffness increases to as high as 143% whereas thestrength increases only by 10%.

3.1.2. Effect of PVA fibersIt can be observed from Fig. 5b and Table 3 that as againstCNTs, incorporation of PVA fibers into cementitious compositedoes not increase the stiffness of the composite. However, thestrength and ductility (strain capacity) increase by manifolds.Incorporation of 2% of PVA fibers into cement paste increasesthe strength by 80% and ductility by 200 times. The mainadvantage of using PVA fibers into cementitious composite isto increase the strain carrying capacity and make the brittlematerial highly ductile. It is due to the dual action of chemicalbonding and mechanical crack bridging offered by PVA incement composite. When a small quantity of fibers (0.5%) isintroduced, brittle cement paste gets converted into ductilematerial with strain softening behaviour. As the incorporatedpercentage of fibers is increased, the ductility of the compositeincreases furthermore and the strain softening characteristicsgot transformed into strain hardening behaviour. The firstdrop in the stress-strain curve is when the matrix reaches itstensile strength. After the matrix cracks, fibers in thecomposite bridges the crack and tends to transfer the loadduring which strain softening/hardening behaviour is exhib-ited. DIC technique was employed to investigate the crackformation and propagation behaviour of PVA incorporatedcementitious composite. As a typical case, the crack behaviourof composite incorporated with 2% PVA fibers is demonstratedin Fig. 6. The strain distribution in the gauge section showsregions of strain localization, which indicates the onset ofmultiple cracking (Fig. 6a). The local strain distribution alongthe gauge section (marked in Fig. 6a) at the mid-width of thespecimen is depicted in Fig. 6b. This strain captured andcalculated by DIC corresponds to actual strain of the specimenand a part of rigid bodymotion with the crack opening. Virtualstrain gauges were placed across the cracks to measure theevolution of crackwidth during different loading stages. Fig. 6cshows the crackwidth at certain key loading stages (marked inFig. 6d) of the specimen.

Table 3 – Properties of cementitious composite subjected to direct tension test.

Specimen [1_TD$DIFF]Elastic modulus (stiffness)(GPa)

Strength(MPa)

Strain at failure (%)

W0.4 [2_TD$DIFF]9.68 1.47 0.014PGA06 [3_TD$DIFF]15.10 1.98 0.014PGA08 [4_TD$DIFF]16.40 2.32 0.014PGA1 [5_TD$DIFF]18.41 1.68 0.008PGA12 [6_TD$DIFF]23.59 1.62 0.006PF [7_TD$DIFF]0.5 9.68 1.42 0.824PF1 [8_TD$DIFF]9.68 1.89 2.348PF [9_TD$DIFF]1.5 9.68 1.94 1.702PF2 [10_TD$DIFF]9.68 2.62 2.849PGA1F2 [11_TD$DIFF]21.41

(121%)4.44(202%)

3.136(22,300%)

PGA12F2 [12_TD$DIFF]9.97(3%)

3.22(119%)

1.411(9979%)[(Fig._6)TD$FIG]

Fig. 6 – Features extracted from a dog bone specimensubjected to tensile loading using DIC technique (a) contour ofstrain distribution along the gauge section showing regions ofstrain concentration which will lead towards multiplecracking, (b) distribution of strain (strain corresponds to actualstrain + crack width with rigid body motion) in the gaugesection along the section X–X (shown in (a)), (c) crack width atdifferent loading stages (shown in (d)) of all cracks (shown in(a)) and (d) load versus strain behaviour of the specimen.

[(Fig._7)TD$FIG]

Fig. 7 – Load–displacement curves of cementitiouscomposite incorporated with (a) CNTs (revealingimprovement in strength and modulus), (b) PVA fibers(revealing multiple crack formation and improvement inenergy dissipation capacity) and (c) hybrid fibers.

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Table 4 – Flexural properties of cementitious composite.

Specimen [1_TD$DIFF]Energy dissipated(�10�3 J)

% increase in thedissipated energy wrtthe reference (W0.4)

W0.4 0.0563 –

PGA06 [3_TD$DIFF]0.0530 0PGA08 [4_TD$DIFF]0.1351 140PGA1 [5_TD$DIFF]0.0917 62PGA12 [6_TD$DIFF]0.0254 �54PF [7_TD$DIFF]0.5 0.3478 517PF1 [8_TD$DIFF]1.0284 1726PF [9_TD$DIFF]1.5 1.6409 2814PF2 [10_TD$DIFF]1.9848 3425PGA1F2 [11_TD$DIFF]3.9208 6862PGA12F2 [12_TD$DIFF]3.6718 6400

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3.1.3. Effect of hybrid fibersIt can be observed from Fig. 5c that incorporation of both PVAfibers and CNTs into cementitious composite takes theadvantages of both the fibers (stiffness increment due toCNTs and ductility improvement and strain hardening due toPVAfibers). It is observed that there is a tremendous incrementin strength, ductility and stiffness of the composite due toincorporation of a meagre amount of both the fibers. Thecementitious composite with hybrid fibers had a straincarrying capacity of 3%.

[(Fig._8)TD$FIG]

Fig. 8 – (a) Distribution of strain in a specimen subjected to flexumicroscopic images showing (b) single crack and (c) multi-crack

3.2. Influence of hybridization on the improvement inflexural properties

3.2.1. Effect of CNTsIt can be observed from Fig. 7a and Table 4 that incorporationof CNT into cementitious composite has altered its loadcarrying capacity, stiffness and strain capacity considerably. Itis evident from Fig. 7a that as the dosage of CNT increases, thestiffness of the composite increases significantly. The straincapacity and load carrying capacity increases up to incorpo-ration of 0.08% of CNT and then decreases as dosage isincreased. This is similar to the trend observed in direct tensiletests. The load carrying capacity of the composite increased by187% with the incorporation of 0.08% of CNTs.

3.2.2. Effect of PVA fibersIt can be observed from Fig. 7b that there is a tremendousincrease in both the load carrying capacity and strain capacitywith incorporation of a meagre amount of PVA fibers.Incorporation of PVA fibers has converted the failure patternof the cementitious composite from brittle to that of strainhardening one. Multiple peaks in the load–displacement curveindicate multiple cracking phenomenon observed (shown inFig. 8) in the PVA fibers incorporated specimen. Thisphenomenon is very important since it gives warning aboutthe failure of the specimen at a much earlier stage itself. Theincrease in load carrying capacity of PVA fibers (2%) incorpo-rated cementitious composite is around 400% and the increase

ral loading obtained through DIC technique, Opticalbridging phenomenon of PVA fibers.

[(Fig._9)TD$FIG]

Fig. 9 – Load vs CMOD curves of cementitious compositeincorporatedwith (a) CNTs, (b) PVA fibers and (c) hybrid fibers(revealing improvement in energy dissipation capacity).

Table 5 – Fracture energy of cementitious composite.

Specimen [1_TD$DIFF]Fractureenergy(J/m2

[2_TD$DIFF])

% increase in thefracture energy wrtthe reference (W0.4)

W0.4 [2_TD$DIFF]17.43 –

PGA06 [3_TD$DIFF]18.99 9PGA08 [4_TD$DIFF]24.38 40PGA1 [5_TD$DIFF]19.64 13PGA12 [6_TD$DIFF]20.24 16PF [7_TD$DIFF]0.5 441.48 2433PF1 [8_TD$DIFF]908.87 5114PF [9_TD$DIFF]1.5 1459.13 8271PF2 [10_TD$DIFF]1561.33 8858PGA1F2 [11_TD$DIFF]2374.58 13,524PGA12F2 [12_TD$DIFF]3193.04 18,219

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in strain carrying capacity is more than five times. This hasamounted to around 3400% increase in the energy dissipationcapacity.

3.2.3. Effect of hybrid fibersIncorporation of both CNTs and PVA fibers has takenadvantage of benefits from both the fibers that there is anincrease in the strength, ductility and stiffness of thecementitious composite. Hybrid fibers (Fig. 7c) improvedthe strain carrying capacity around ten times when com-pared to that of compositewithout any fibers and double thatof the improvement that is observed due to incorporation ofonly PVAfibers. The improvement in load carrying capacity ishowever equivalent to that of only PVA fibers. This has led to

the enhancement in energy dissipation capacity around 65timeswhen compared to that of compositewithout anyfibersand double that of the improvement that is observed due toincorporation of only PVA fibers.

3.3. Influence of hybridization on the improvement infracture properties

3.3.1. Effect of CNTsIt can be observed from Fig. 9a and Table 5 that there is aconsiderable increase in fracture energy of the composite dueto incorporation of CNTs. The increase in fracture energy isabout 40%.

3.3.2. Effect of PVA fibersIt is evident from Fig. 9b that incorporation of PVA fibers intocementitious composite has improved its fracture behaviourtremendously. The load carrying capacity and the straincapacity has improved manifolds. The increase in fractureenergy is about 9000% due to incorporation of 2% of PVA fibers.It can be observed from Table 5 that the increase in fractureenergy with respect to the percentage of fibers is almost linearup to 1.5%, whereas beyond that it gets almost saturated.

Fig. 10 shows the strain distribution of the specimensincorporated with different percentages (0.5% and 1.5%) ofPVA fibers. For the sake of demonstration, the straindistributions of both the specimens are displayed at aparticular CMOD level (=1 mm). The figure also shows thecrack width across the depth of the specimen. It can be notedthat as the percentage of PVA fibers increases, the crackdevelopment in the specimen also varies. When thepercentage of fibers is less, there is a single prominent crackwhereas as the percentage of fibers increase, there aremultiple crack paths. This is the reason for the delayedfailure of specimens with higher percentage of fibers. Thecrackwidth of the specimen also decreases as the percentageof fibers increases. It is evident fromFig. 11 that the specimenwith 0.5% of fiber is almost brittle as the onset of crack is onlyat the 100% of peak load. However, in specimen with 1.5% offibers, the crack begins to develop when the load is only 70%of the peak load. The length of the crack is also lesser as thepercentage of fibers increases (25% decrease in length of thecrack in this case).

[(Fig._10)TD$FIG]

Fig. 10 – Strain contour and crack width along the depth of the cement mortar specimen incorporated with differentpercentage of PVA fibers when subjected to fracture test.

[(Fig._11)TD$FIG]

Fig. 11 – Crack length of PVA incorporated cementitiouscomposite subjected to fracture test.

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3.3.3. Effect of hybrid fibersThough there is notmuch change in the load carrying capacityof hybrid fibers when compared to that of only PVA fibers, thestrain capacity and the fracture energy of the composite hasconsiderable improvement. The improvement in fractureenergy is 180 times when compared to that of compositewithout any fibers and twice that of composite only with PVAfibers. Fig. 12 shows the consolidated picture of fracture energyof all the specimens. Though composite only with CNTs alsoshow a considerable increase in fracture energy, the effect isovershadowed by the effect of PVA fibers. It is also interestingto observe that addition of a very small quantity of CNT intothe composite with only PVA fibers has improved the fractureproperties enormously.

It can be observed from Fig. 12a and c that the strength ofthe composite decreases if the CNTs incorporated into it isbeyond 0.1% by weight of cement. However, the modulus(Fig. 12b) and fracture energy (Fig. 12e) increases proportion-ately with the CNT content. From these observations it isevident that if our objective is to enhance the strength of thecomposite, then the amount of CNTs incorporated into itshould be limited to 0.1%.

However, if the objective is to enhance the modulus of thecomposite, thenmore quantity of CNTs can be incorporated. Itcan also be observed that, as the percentage of PVA fibers isincreased, the strength and ductility of the compositeincreases. However, modulus of the composite does not alterdue to incorporation of PVA fibers. After judiciously incorpo-rating both the fibers into the composite, the strength, ductilityand modulus of the composite got increased. The developedcomposite has a strength of 4.5 MPa, strain carrying capacity of3% and modulus of 20 GPa. The enhancement in propertiesobtained due to judicious incorporation of fibers in the presentstudy is compared with the experimental investigationsreported in earlier works and presented in Table 6. The studyclearly indicates that the judicious tailoring of cementcomposite at both nano- andmicro-scale can help in attainingthe multi-performance (high strength, high stiffness and highductility) cement composite (MPCC).

4. Conclusions

An attempt has been made in this study to identify a suitablehybridization technique to develop [15_TD$DIFF] high strength, high ductileand high modulus cementitious composite by incorporatingfibers of two different length scales (micro- and nano-fibers).At first, the influence of different quantities of individual fiberson the mechanical properties of cementitious composite isidentified to arrive at the optimum dosage level. It has beenobserved that incorporation of 0.08% of CNTs is capable ofimproving the tensile properties of cementitious composite.With the further increase in the dosage of CNT, the improvedmechanical properties (expect stiffness) got deteriorated.There is a tremendous increase in ductility due to incorpo-ration of PVA fibers. However, PVA fibers did not contributetowards the stiffness improvement of the composite. Hence, inorder to develop high strength, high ductile and high stiffnesscementitious composite, hybrid fibers consisting of CNT of 0.1–0.12% and PVA fibers of 2% were incorporated. It has beenobserved that the tensile strength of the composite improvedby 200%, stiffness improved by 121%, strain carrying capacityimproved by 220 times, energy dissipating capacity improvedby 68 times and fracture energy improved by 130 times due to

[(Fig._12)TD$FIG]

Fig. 12 – (a) Tensile strength, (b) elastic modulus, (c) flexural strength, (d) energy dissipated and (e) fracture energy of hybridfibers incorporated cementitious composite.

a r c h i v e s o f c i v i l a n d m e c h an i c a l e n g i n e e r i n g 1 9 ( 2 0 1 9 ) 3 4 8 – 3 5 9 357

incorporation of a meagre amount of both the fibers (hybridfibers). The outstanding performance of the developedcomposite is resulted from the effective crack bridging andpreferred load transfer in micro-scale due to suitable hybrid-ization of fibers of distinctly different length scales. Thedeveloped composite has a strength of [21_TD$DIFF]4.5 MPa, strain carryingcapacity of 3% and modulus of 20 GPa. This study provides

vital information towards developing multi-performancecement composite (MPCC) where all three key performanceparameters, i.e., strength, stiffness and ductility can beimproved in desired amount. In order to understand the crackbehaviour in the developed composite, DIC techniquewas alsoemployed. It is found that the DIC information on crack lengthand crack width along the depth of the specimen during near-

Table 6 – Comparison of the results obtained in the present work with the literature.

Literature % nano-fiber % micro-fiber Improvement in property with respect toplain cementitious composite

Flexural strength Modulus Toughness Energy absorption

Zhang and Cao [20] 0.5(CaCO3)

1.5 + 0.4(Steel + PVA)

130 – – –

Sbia et al. [22] 0.12(CNF)

2.4(PVA)

146 – – 2500

Metaxa et al. [24] 0.048(CNF)

0.54(PVA)

– 65.9 7000 –

Alrekabi et al. [26] 0.025(CNT)

2(Steel)

106 – – –

Present work 0.1(CNT)

2(PVA)

400 200 18,000 6862

a r ch i v e s o f c i v i l a n d m e ch an i c a l e n g i n e e r i n g 1 9 ( 2 0 1 9 ) 3 4 8 – 3 5 9358

and post-peak period is able to provide vital information onbehaviour of the composite. Based on the extremely encour-aging results of the present study, authors are activelyworkingon development of various types of MPCCs (with differentcriteria and performance levels) for concrete structures.

Conflicts of interest

No conflicts of interest.

Ethical statement

Article was conducted according to the ethical standards.

Funding body

None.

Acknowledgements

The authors would like to acknowledge the support and helpreceived from the technical staffs and project students ofSpecial and Multifunctional Structures Laboratory (SMSL) ofCSIR-SERC during conducting the experiments. Kind help fromDr. V. Srinivas, Scientist of SMSL on carrying out the DICstudies is also thankfully acknowledged.

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