Construction and Building Materials 180 (2018) 405–411

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Construction and Building Materials

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Improvement in tensile and flexural ductility with the addition ofdifferent types of polypropylene fibers in cementitious composites

https://doi.org/10.1016/j.conbuildmat.2018.05.2800950-0618/� 2018 Elsevier Ltd. All rights reserved.

⇑ Corresponding author.E-mail address: nilanjan@civil.iitkgp.ernet.in (N. Mitra).

Sutapa Deb a, Nilanjan Mitra a,⇑, Subhasish Basu Majumder b, Swati Maitra c

aDepartment of Civil Engineering, Indian Institute of Technology Kharagpur, IndiabMaterial Science Centre, Indian Institute of Technology Kharagpur, IndiacRanbir and Chitra Gupta School of Infrastructure and Management, Indian Institute of Technology Kharagpur, India

g r a p h i c a l a b s t r a c t

Flexural test set –up along with sample Fig. 1. Flexural load-displacement diagram of different samples

Direct tensile test set –up along with dogbonesample

Fig. 2. Uniaxial direct tensile stress-strain diagram of different samples

a r t i c l e i n f o

Article history:Received 28 November 2017Received in revised form 3 May 2018Accepted 30 May 2018

Keywords:Cementitious compositesPolypropylene fiberTensile strengthFlexureDuctility

a b s t r a c t

The influence of addition of fibrillated and monofilament polypropylene fibers to a cementitious compos-ite mix on the tensile strength, flexural strength and ductility characteristics of the sample are beingprobed in this study. The study demonstrates that addition of fibrillated variety improves the strengthof the samples both in tension and flexure in comparison to that of monofilament variety, whereas theaddition of monofilament variety improves the tensile and flexural ductility characteristics of the sample.A combination of both these type of fibers improves the tensile strength and ductility characteristicsalong with improvement in flexural ductility with no significant improvement in flexural capacity.

� 2018 Elsevier Ltd. All rights reserved.

1. Introduction

Improved tensile and flexural ductility along with lightweightcharacteristics are typical needs of high-performance concreteand/or cementitious composite materials for use in infrastructure.

406 S. Deb et al. / Construction and Building Materials 180 (2018) 405–411

It is well known that certain mechanical properties of concreteand/or cementitious composites can be improved by the additionof reinforcing materials such as steel wires, glass and carbon fibers,synthetic polymeric fibers (polypropylene, polyethylene, poly-vinyl alcohol, acrylics, polyamide, polyester). Out of these differentsynthetic fibers, polypropylene (PP) fibers have been adjudged asthe most efficient [1–3] because of their low cost, ductility, easeof dispersal, good anchoring capability, no corrosion, thermal sta-bility (high melting point in comparison to other polymer fibres),being chemically inert and stable under alkaline environment ofconcrete/cementitious composite material as well as chemicallyinert and stable under strong acidic environments. In this manu-script, we have considered only cementitious composites withpolypropylene fiber as reinforcements.

There is numerous literature on improvement of mechanicalperformance with the addition of polypropylene fibers in cementconcrete mix [4–10] as compared to that in cementitious compos-ites [11–14]. It should be noted that the differences between con-crete and cementitious composite are in presence of coarseaggregates. Since the strength and stiffness of coarse aggregatesare significantly different from the surrounding matrix, observingthe influence of addition of fibers to the matrix becomes difficultin presence of coarse aggregates in matrix. Typically, coarse aggre-gates are like inclusions in a matrix. The entire matrix along withthe inclusions when subjected to load results in development ofinternal stresses due to the presence of aggregates which mighthelp in the initiation of cracks. On the other hand, fibers are addedto control cracks; thereby addition of the two materials: fiber –which helps in crack propagation mitigation and coarse aggregatewhich results in crack initiation may counter the effect of eachother. With this in perspective, the manuscript deals with cemen-titious composites with polypropylene fiber reinforcements. Gen-erally, polypropylene is available in commercial market asmonofilament and/or in fibrillated forms. The study makes anattempt to identify the comparative improvement in mechanicalperformance with addition of these different forms of polypropy-lene fibers in the cementitious composites.

In this regard, it should be mentioned that there are relatedstudy in literature in which the adherence of polypropylene tothe cementitious matrix has been explored through differentmethodologies such as alkaline and silane treatment [15,16],mechanical modifications such as fibrillation and indentation[17], altering surface polarity [18], altering surface chemistry andmorphology [19,20], plasma treatment [21], introducing functionalgroups through treatment with acids and other chemicals [22–24].It should be noted that the objective of this paper is not on modi-fication to the properties of commercially available polypropylenefibers so as to improve the mechanical performance. Moreover,most of the methodologies presented in the above references arequite complex to be practiced in the field. The objective of thispaper is to use commercially available polypropylene fibers in dif-ferent forms and observe the mechanical performance improve-ment if any.

2. Casting methodology

Two different types of polypropylene fibers have been consid-ered in this study – fibrillated and monofilament. Both the fibershave been tested in the laboratory for their mechanical properties.The fibrillated fibers (F) have tensile strength of 670 MPa, modulusof elasticity as 7.52 GPa and a contact angle of 71.310; whereasmonofilament fibers (M) have tensile strength of 550 MPa withmodulus of elasticity as 5.5 GPa and contact angle of 45.18�. Typi-cally, it is reported that polypropylene fibers are hydrophobic(with contact angle greater than 90�), however the supplied fibers

from the manufacturer were hydrophilic in nature. The fibers weresupplied by the companies typically used for infrastructural pur-poses, where spinning oil coating (trade name of Encimage) ofthe fiber is done. This is because when fibers are cut, due to highabrasion resistance the cutters are heated up which might damagethe fibers since the melting point of the fibers is around 165 �C.When the fibers are extruded, they are passed through spin-finish oil which along with water, dissipate the heat. This helpsthe individual fibers to form a bunch thereby helping in cuttingoperation. It was also mentioned that these fibers are used forinfrastructural applications and because of the hydrophobic naturethey tend to agglomerate and thereby disrupting dissipation [25].Both the fibers (fibrillated and monofilament) has a specific gravityof 0.91, with 12 mm cut length and 34 mm average diameter. Thepercentage of polypropylene fiber content in the cementitiousmix has been kept as 2% by volume.

Ordinary Portland cement (OPC 53 grade conforming to IS:12269–1987 [26]) has been used in preparing the cementitiousmix and the physical properties of cement have been determinedin the laboratory. The physical properties of OPC as estimatedare: specific gravity = 3.15, normal consistency = 30%, initial set-ting time = 147 min, final setting time = 273 min, soundness byLe-Chatelier apparatus = 0.20 mm, fineness by dry sieving = 4% alsoby Blaine apparatus = 399 m2/Kg. The oxide compositions of OPChave also been estimated in the laboratory using X-ray Fluores-cence spectrometer which are as follows: CaO = 64.60%, SiO2 =17.83%, Al2O3 = 5.04%, Fe2O3 = 3.34%, K20 = 0.40%, MgO = 1.44%,TiO2 = 0.33%, SO3 = 2.63%, P2O5 = 0.18%.

The fly ash (conforming to IS 3812-Part I [27]) used in the mixhas the following properties which are estimated in the laboratory:specific gravity = 2.20 and fineness by Blaine apparatus = 730 m2/Kg. The oxide compositions of the fly ash as determined in the lab-oratory are as follows: CaO = 1.84%, SiO2 = 53.71%, Al2O3 = 25.17%,Fe2O3 = 9.59%, K20 = 2.70%, MgO = 0.24%, TiO2 = 4.10, % SO3 = 0.8%,P2O5 = 1.07%. Another ultrafine pozzolanic material (apart fromfly ash) that has been added to the mix is silica fume (specificgravity = 2.20) having spherical particles less than 1 lm in diame-ter, with an average being about 0.15 lm. The oxide compositionsof silica fume as determined from X-ray Fluorescence spectrometerare as follows: CaO = 2.82%, SiO2 = 92.90%, Al2O3 = 0.70%, Fe2O3 =0.08%, K20 = 0.60%, MgO = 0.10%, TiO2 = 0.01%, SO3 = 1.80%, P2O5 =0.86%. As per IRC:SP:46-2013 [28] and ACI 226-1987 [29], the per-centage by volume of fly ash is kept around 35% of the OPC cementin this study. As per ACI 234R-2000 [30], the percentage by weightof Silica fume is kept around 15% of the OPC in this study.

Ultra-fine sand (specific gravity = 2.66) having particle size lessthan 150 mm has been used as a component of the mix. The propor-tion of sand to binder material (cement + pozzolonic materials likefly ash and silica fume) is kept as 0.1 by weight in this study. In thisregard, it should be noted that previous literature prescribes sandto cement ratio as 0.5 or lower [31] and sand to binder ratio as 0.3[32]. Ultra fine sand has been used as per specifications for themanufacture of strain-hardening cementitious mortars, which isknown to act as a good filler material. Polycarboxylate basedsuperplasticizer has been added to the mix to maintain the consis-tency level (slump value = 150–175 mm) with the specified water/cementitious material ratio of 0.3. Viscosity modifying agent(VMA) has been utilized to maintain the viscosity of the greencementitious mix such that proper dispersion of fibers is ensured.It should be noted that the amounts of VMA and superplasticizerutilized are negligible compared to the volume and/or the weightof the other components in the mix.

The methodology used for mixing of the cement compositematerial is as follows: at first cement, sand and fly ash are mixedfor a couple of minutes to get a uniform dry mix. Half of the totalamount of water, superplasticizer and VMA are then added to the

S. Deb et al. / Construction and Building Materials 180 (2018) 405–411 407

dry mixture and mixed for about two minutes. Silica fume is addednext and mixed for another two minutes. Remaining half portion ofwater, superplasticizer and VMA are then mixed with the previousmix for about three minutes. Lastly polypropylene fibers are addedand mixed until uniform mixture is obtained and then cast intomoulds. The specimens are demoulded after 24 h. After demould-ing, the specimens are cured in water chambers for 28 days. After28 days the specimens are considered to be ready for testingpurpose.

Cement mortar, conventional concrete and cementitious com-posite having the same composition as that of the proposed mixbut without the polypropylene fiber reinforcement have been con-sidered as the control samples in this work. The cement mortar (asper IS: 4031 part 6, 1988 [33]) prepared as part of the study doesnot contain coarse aggregates. The standard sand (Ennore Sand),has been taken as a combination of three different grades (gradeI, II and III) in equal proportions (IS: 650-1991 [34]). The ratio ofcement to standard sand is 1:3. The properties of the cement con-sidered for this preparation is similar to that used for the concrete.No fly ash or superplasticizer has been used for this sample prepa-ration. For the conventional concrete sample, the ratio of bindercontent: fine aggregates: coarse aggregates is 1:2:2 with a water-cement (w/c) ratio of 0.3. The above specified ratio is obtained

Fig. 1. Experimental setups for a) four-point

Table 1Different test results of M40 Cementitious Composites along with cementitious control sa

Sample ID Avg. Compressive Strength(MPa)

AS

7 Days’ 28 Days’ 2

F 38.39 51.80 5M 32.15 45.72 4M40 Concrete 30.46 45.52 5Mortar 40.52 53.17 4Cementitious Control Sample 49.94 66.28 2F7M3 32.34 51.30 4F5M5 32.31 49.62 4F3M7 32.27 47.85 4

based on the mix design as per ACI 318-1977 [35] to maintain acharacteristic compressive strength of 40 MPa. The binder materialincludes cement and fly ash (pozzolanic material) with a fly ash tocement ratio as 0.5. The amount of coarse aggregates taken for themix is around 2.8 times to that of the cement by weight. Anothertype of control sample (herein referred to as cementitious controlsample) has been prepared with all the ingredients at the sameproportion as that of the proposed cementitious mix but withoutthe fibers.

3. Experimental setup

Standardized tests have been carried out to determine the com-pressive, split tensile, direct tensile and flexure strength of themixes. Six specimens have been utilized for each of the differenttests for each mix proportion and the average of the values havebeen reported. After 7 and 28 days of curing, 70.6 mm cube speci-mens have been used for testing as per IS: 10080-1982 [36] for thedetermination of compressive strength. Standard splitting tensiletest has also been done on the coupons as per ASTM C 496-1991[37]. After 28 days of curing, rectangular slabs (500 mm � 100 mm � 30 mm) have been used for determination of flexuralcharacteristics (four-point bending test) in accordance with ASTM

bending test and b) direct tensile test.

mples, conventional concrete and mortar.

vg. Flexuraltrength (MPa)

Avg. Splitting TensileStrength (MPa)

Avg. Direct TensileStrength (MPa)

8 Days’ 28 Days’ 28 Days’

.14 3.14 2.15

.49 3.24 1.66

.80 3.28 3.12

.90 3.20 2.46

.75 1.51 1.74

.76 3.35 2.05

.61 3.34 1.97

.58 3.22 1.87

408 S. Deb et al. / Construction and Building Materials 180 (2018) 405–411

C1018-1998 [38]. The support span of the flexural test set-up iskept at 450 mm with the load span as 150 mm (shown inFig. 1a). The compression and flexural tests have been carried outon a 600 KN capacity Universal Testing Machine (TINIUS OLSENSuper ‘‘L” UTM) with a constant rate of displacement of 1 mm/min. Uniaxial direct tensile tests are carried out using a dog-boneshaped specimens (gauge length = 60 mm, with 30 mm at bridgeand thickness 50 mm as shown in Fig. 1b) in a servo-hydraulicUTM with 50 KN capacity with a constant rate of displacement of1 mm/min.

The variations in proportions of different types of polypropy-lene fibers used for the mix include different variations betweenthe fibrillated and the monofilament fibers. Samples have been

Fig. 2. Uniaxial direct tensile stress-str

Fig. 3. Uniaxial direct tensile stress-strain diagram of different cementitious c

prepared either with just monofilament fibers or with fibrillatedfibers or with a combination of these two types of fibers in differ-ent proportions. Sample abbreviations include F for 100% of fibril-lated fibers; M for 100% of monofilament fibers; F7M3 for 70% offibrillated and 30% of monofilament fibers; F5M5 for 50% each offibrillated and monofilament fibers; and F3M7 for 30% of fibrillatedand 70% of monofilament fibers.

4. Results and discussion

Table 1 presents results for the above-mentioned tests for thecementitious composites of different mix proportions along withthe results of the control concrete and cement mortar specimens.

ain diagram for different samples.

omposite samples with various proportions of different types of PP fibers.

(a)

(b)

F M Concrete Mortar ControlSample F7M3 F5M5 F3M7

Average Values 0.53 1.53 0.011 0.013 0.032 1.29 1.30 1.39

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Stra

in(%

) at m

ax. s

tres

s

F M Concrete Mortar ControlSample F7M3 F5M5 F3M7

Average Values 2.03 5.37 0.012 0.013 0.033 4.95 5.01 5.12

0

1

2

3

4

5

Post

-pea

k st

rain

(%) a

t 20

% m

ax. s

tres

s

Fig. 4. (a) Strain at maximum stress and (b) Post-peak strain at 20% of the maximum stress for different samples for direct tensile loading tests.

Fig. 5. Flexural load-displacement diagram of different samples.

S. Deb et al. / Construction and Building Materials 180 (2018) 405–411 409

Atleast six samples were considered to determine the averagemaximum strength of the samples subjected to different types ofloading conditions. It can be observed from the table that the com-pressive strength and splitting tensile strength is not compromisedfor different mixes used in this study, however, there is a slightdecrease in the flexural and tensile strength of the samples. Itshould be noted at this point that the strength values typicallyindicate just the peak value; for determination of the ductilityone should take a look into the curve (especially the strain at

maximum strength and the post-peak strain at 20% of the maxi-mum strength) rather than just a number indicating the value ofpeak strength.

Fig. 2, represents uniaxial stress-strain curve for different typesof polypropylene fiber additions compared to control samples. Thefigure demonstrates that if one is interested only in the maximumstrength then obviously conventional M40 concrete and the mortarare better choices compared to the cementitious composite (CC)samples prepared in this study. The cementitious control sample

F M Concrete Mortar

Cementitious

ControlSample

F7M3 F5M5 F3M7

Average values 6.75 12.96 0.87 0.9 1.512 9.12 9.89 10.96

0

2

4

6

8

10

12

14

Dis

plac

emen

t (m

m)

at M

ax. l

oad

Fig. 6. Displacement at maximum load from flexure test of different samples.

410 S. Deb et al. / Construction and Building Materials 180 (2018) 405–411

yields lower values of strength compared to the M40 concrete andthe conventional Mortar control samples, however demonstratinglarger ductility than the other two. If one is interested in ductilityof the samples, then the fiber reinforced cementitious compositesamples perform much better than the conventional concrete,mortar control samples (for which the failure strain is around0.01 and shown in a small figure as a zoomed in portion near zerostrain) as well as cementitious control sample. It is also interestingto note from the figure that amongst the fiber reinforced CC sam-ples, the one with (fibrillated fibers) F at 100% gives higherstrength but lower ductility in comparison to the one with(monofilament fibers) M at 100% which gives lower strength butlarge ductility. It can also be observed that initial stiffness is higherfor samples with F fibers in comparison to that of samples with Mfibers. The differences in behaviour between two types of fibers inhydrated cement paste may be attributed to the fibrillation of thefiber along with the differences in the strength and the modulus ofelasticity of the two types of fibers used. Fig. 2 also shows the min-imum and maximum scatter bounds of the samples with 100% Fand 100% M fibers. It should be noted that based on the boundsit can be ascertained that the behaviour observed from F fibers issignificantly different compared to that of M fibers.

Since significant variation could be observed between theresponse characteristics of cementitious composites made with Fand M type of polypropylene fibers, samples are also fabricatedwith different proportions of a combination of these two fiberswith a motive to improve the tensile strength characteristics alongwith ductility. As can be observed from Fig. 3, considering maxi-mum and minimum bounds of different combinations there ishardly any difference with regards to strength and as well as duc-tility (measured as a function of strain). Based on this observation,it may be stated that a combination of fibrillated and monofila-ment fibers is required to improve both the strength and ductilityof the samples in direct tensile tests.

In order to assess the ductility of the sample, it is necessary todetermine the strain at which the post-peak stress reaches 20%of the maximum stress (typically referred to as failure strain ofthe sample) [39,40], apart from the strain at maximum stress.Bar charts in Fig. 4 show that the addition of either kind ofpolypropylene fibers (F or M) leads to significant increase in thestrain at maximum stress as well as strain at failure stress (definedas 20% of the maximum stress). This increase in ductility can beattributed to the bridging action of the fibers in mitigating the evo-lution of cracks in the matrix. It should be noted that the increasein percentage of strain at maximum stress is around 950% on an

average compared to that of control samples for cement mortarand concrete. The monofilament fibers have been found to be moreeffective in increasing the strain at maximum stress compared tothat of the fibrillated fibers. Increase in percentage of failure strain(strain corresponding to 20% of the maximum stress) is observed tobe around 4000% in comparison to that of the control samples ofcement mortar and concrete. For this parameter also, the monofil-ament fibers perform better in comparison to that of the fibrillatedfibers.

Four-point flexure tests have also been carried out to determinebehavioural changes of the sample with addition of different typesof polypropylene fibers (F and M). Similar to that of Fig. 2, it can beobserved from the load-displacement curve in Fig. 5 that the flex-ure characteristics are significantly different for F type fibers incomparison to that of M type fibers. The F type fibers give higherload carrying capacity whereas M type fibers provide more ductil-ity. As expected, a combination of the two different types of fibersgives values in between the two bounds of 100% usage of eitherfiber. It can also be observed that there is a difference betweenthe initial stiffness of the samples containing either F or M typefibers with lower initial stiffness being provided by the M fibers.It should be noted that not much scatter is observed for the flexuretest of the samples and thereby the maximum and minimumbounds have not been provided. The scatter obtained in the sampletests for all different types has been estimated to be around 15%. Itcan also be mentioned from the study that addition of small per-centage of fibrillated fibers changes the initial stiffness of the curveas well as improves the strength characteristics compared to sam-ples with no addition of F fibers. As can be observed from Fig. 5, theaddition of any type of polypropylene fibers significantly improvesthe ductility characteristics as compared to that of the control sam-ples of concrete and mortar specimens.

The characterization of ductility has been done based on dis-placement at maximum load and the results are shown in Fig. 6.The figure demonstrates significant improvement in flexural duc-tility of the samples with addition of fibers. It should be noted thatirrespective of percentage variation of different types of fibers,there is a huge increase in the ductility characteristics of the spec-imen in flexure in comparison to the control samples of concrete,mortar and cementitious mix.

5. Conclusion

It is well established that addition of fibers to the cementitiouscomposites improves the ductility characteristics of the mix. This

S. Deb et al. / Construction and Building Materials 180 (2018) 405–411 411

study investigates improvement in ductility with addition of twodifferent types of commercially available polypropylene fibers: fib-rillated and monofilament fibers. The addition of fibrillated varietyimproves the tensile strength and flexural capacity of the mix incomparison to that of addition of the monofilament variety. Onthe other hand, the ductility improvement with addition of fibril-lated variety is smaller as compared to that with the addition ofmonofilament variety. Combination of both these two type offibers has also been investigated to reveal that addition of smallamount of fibrillated fibers to the mix containing monofilamentfibers improves the tensile strength and ductility characteristicsof the sample. For flexural loads, the addition of monofilamentvariety to the sample containing fibrillated fibers significantlyimproves the ductility characteristics at the cost of strengthcharacteristics.

6. Conflict of Interest

The authors declare that they have no conflict of interest in anypart of the manuscript.

Acknowledgements

The authors acknowledge the Future of Cities project (Projectcode: ECI) under Ministry of Human Research Development, Indiafor funding this research.

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