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

Construction and Building Materials 180 (2018) 405–411

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Improvement in tensile and flexural ductility with the addition of different types of polypropylene fibers in cementitious composites Sutapa Deb a, Nilanjan Mitra a,⇑, Subhasish Basu Majumder b, Swati Maitra c a

Department of Civil Engineering, Indian Institute of Technology Kharagpur, India Material Science Centre, Indian Institute of Technology Kharagpur, India c Ranbir and Chitra Gupta School of Infrastructure and Management, Indian Institute of Technology Kharagpur, India b

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 dogbone sample 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 2017 Received in revised form 3 May 2018 Accepted 30 May 2018

Keywords: Cementitious composites Polypropylene fiber Tensile strength Flexure Ductility

a b s t r a c t The influence of addition of fibrillated and monofilament polypropylene fibers to a cementitious composite mix on the tensile strength, flexural strength and ductility characteristics of the sample are being probed in this study. The study demonstrates that addition of fibrillated variety improves the strength of the samples both in tension and flexure in comparison to that of monofilament variety, whereas the addition 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 characteristics along with improvement in flexural ductility with no significant improvement in flexural capacity. Ó 2018 Elsevier Ltd. All rights reserved.

1. Introduction

⇑ Corresponding author. E-mail address: [email protected] (N. Mitra). https://doi.org/10.1016/j.conbuildmat.2018.05.280 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.

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

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It is well known that certain mechanical properties of concrete and/or cementitious composites can be improved by the addition of reinforcing materials such as steel wires, glass and carbon fibers, synthetic polymeric fibers (polypropylene, polyethylene, polyvinyl alcohol, acrylics, polyamide, polyester). Out of these different synthetic fibers, polypropylene (PP) fibers have been adjudged as the most efficient [1–3] because of their low cost, ductility, ease of dispersal, good anchoring capability, no corrosion, thermal stability (high melting point in comparison to other polymer fibres), being chemically inert and stable under alkaline environment of concrete/cementitious composite material as well as chemically inert and stable under strong acidic environments. In this manuscript, we have considered only cementitious composites with polypropylene fiber as reinforcements. There is numerous literature on improvement of mechanical performance with the addition of polypropylene fibers in cement concrete mix [4–10] as compared to that in cementitious composites [11–14]. It should be noted that the differences between concrete and cementitious composite are in presence of coarse aggregates. Since the strength and stiffness of coarse aggregates are significantly different from the surrounding matrix, observing the influence of addition of fibers to the matrix becomes difficult in presence of coarse aggregates in matrix. Typically, coarse aggregates are like inclusions in a matrix. The entire matrix along with the inclusions when subjected to load results in development of internal stresses due to the presence of aggregates which might help in the initiation of cracks. On the other hand, fibers are added to control cracks; thereby addition of the two materials: fiber – which helps in crack propagation mitigation and coarse aggregate which results in crack initiation may counter the effect of each other. With this in perspective, the manuscript deals with cementitious composites with polypropylene fiber reinforcements. Generally, polypropylene is available in commercial market as monofilament and/or in fibrillated forms. The study makes an attempt to identify the comparative improvement in mechanical performance with addition of these different forms of polypropylene fibers in the cementitious composites. In this regard, it should be mentioned that there are related study in literature in which the adherence of polypropylene to the cementitious matrix has been explored through different methodologies such as alkaline and silane treatment [15,16], mechanical modifications such as fibrillation and indentation [17], altering surface polarity [18], altering surface chemistry and morphology [19,20], plasma treatment [21], introducing functional groups through treatment with acids and other chemicals [22–24]. It should be noted that the objective of this paper is not on modification to the properties of commercially available polypropylene fibers so as to improve the mechanical performance. Moreover, most of the methodologies presented in the above references are quite complex to be practiced in the field. The objective of this paper is to use commercially available polypropylene fibers in different forms and observe the mechanical performance improvement if any.

2. Casting methodology Two different types of polypropylene fibers have been considered in this study – fibrillated and monofilament. Both the fibers have been tested in the laboratory for their mechanical properties. The fibrillated fibers (F) have tensile strength of 670 MPa, modulus of elasticity as 7.52 GPa and a contact angle of 71.310; whereas monofilament fibers (M) have tensile strength of 550 MPa with modulus of elasticity as 5.5 GPa and contact angle of 45.18°. Typically, 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 were supplied by the companies typically used for infrastructural purposes, where spinning oil coating (trade name of Encimage) of the fiber is done. This is because when fibers are cut, due to high abrasion resistance the cutters are heated up which might damage the fibers since the melting point of the fibers is around 165 °C. When the fibers are extruded, they are passed through spinfinish oil which along with water, dissipate the heat. This helps the individual fibers to form a bunch thereby helping in cutting operation. It was also mentioned that these fibers are used for infrastructural applications and because of the hydrophobic nature they tend to agglomerate and thereby disrupting dissipation [25]. Both the fibers (fibrillated and monofilament) has a specific gravity of 0.91, with 12 mm cut length and 34 mm average diameter. The percentage of polypropylene fiber content in the cementitious mix 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 cementitious mix and the physical properties of cement have been determined in the laboratory. The physical properties of OPC as estimated are: specific gravity = 3.15, normal consistency = 30%, initial setting time = 147 min, final setting time = 273 min, soundness by Le-Chatelier apparatus = 0.20 mm, fineness by dry sieving = 4% also by Blaine apparatus = 399 m2/Kg. The oxide compositions of OPC have also been estimated in the laboratory using X-ray Fluorescence 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 mix has 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 laboratory 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 from fly ash) that has been added to the mix is silica fume (specific gravity = 2.20) having spherical particles less than 1 lm in diameter, with an average being about 0.15 lm. The oxide compositions of silica fume as determined from X-ray Fluorescence spectrometer are 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 percentage by volume of fly ash is kept around 35% of the OPC cement in this study. As per ACI 234R-2000 [30], the percentage by weight of Silica fume is kept around 15% of the OPC in this study. Ultra-fine sand (specific gravity = 2.66) having particle size less than 150 mm has been used as a component of the mix. The proportion of sand to binder material (cement + pozzolonic materials like fly ash and silica fume) is kept as 0.1 by weight in this study. In this regard, it should be noted that previous literature prescribes sand to 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 the manufacture of strain-hardening cementitious mortars, which is known to act as a good filler material. Polycarboxylate based superplasticizer has been added to the mix to maintain the consistency 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 green cementitious mix such that proper dispersion of fibers is ensured. It should be noted that the amounts of VMA and superplasticizer utilized are negligible compared to the volume and/or the weight of the other components in the mix. The methodology used for mixing of the cement composite material is as follows: at first cement, sand and fly ash are mixed for a couple of minutes to get a uniform dry mix. Half of the total amount of water, superplasticizer and VMA are then added to the

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dry mixture and mixed for about two minutes. Silica fume is added next and mixed for another two minutes. Remaining half portion of water, superplasticizer and VMA are then mixed with the previous mix for about three minutes. Lastly polypropylene fibers are added and mixed until uniform mixture is obtained and then cast into moulds. The specimens are demoulded after 24 h. After demoulding, the specimens are cured in water chambers for 28 days. After 28 days the specimens are considered to be ready for testing purpose. Cement mortar, conventional concrete and cementitious composite having the same composition as that of the proposed mix but without the polypropylene fiber reinforcement have been considered as the control samples in this work. The cement mortar (as per IS: 4031 part 6, 1988 [33]) prepared as part of the study does not contain coarse aggregates. The standard sand (Ennore Sand), has been taken as a combination of three different grades (grade I, II and III) in equal proportions (IS: 650-1991 [34]). The ratio of cement to standard sand is 1:3. The properties of the cement considered for this preparation is similar to that used for the concrete. No fly ash or superplasticizer has been used for this sample preparation. For the conventional concrete sample, the ratio of binder content: fine aggregates: coarse aggregates is 1:2:2 with a watercement (w/c) ratio of 0.3. The above specified ratio is obtained

based on the mix design as per ACI 318-1977 [35] to maintain a characteristic compressive strength of 40 MPa. The binder material includes cement and fly ash (pozzolanic material) with a fly ash to cement ratio as 0.5. The amount of coarse aggregates taken for the mix is around 2.8 times to that of the cement by weight. Another type of control sample (herein referred to as cementitious control sample) has been prepared with all the ingredients at the same proportion as that of the proposed cementitious mix but without the fibers. 3. Experimental setup Standardized tests have been carried out to determine the compressive, split tensile, direct tensile and flexure strength of the mixes. Six specimens have been utilized for each of the different tests for each mix proportion and the average of the values have been reported. After 7 and 28 days of curing, 70.6 mm cube specimens have been used for testing as per IS: 10080-1982 [36] for the determination of compressive strength. Standard splitting tensile test has also been done on the coupons as per ASTM C 496-1991 [37]. After 28 days of curing, rectangular slabs (500 mm  100 m m  30 mm) have been used for determination of flexural characteristics (four-point bending test) in accordance with ASTM

Fig. 1. Experimental setups for a) four-point bending test and b) direct tensile test.

Table 1 Different test results of M40 Cementitious Composites along with cementitious control samples, conventional concrete and mortar. Sample ID

Avg. Compressive Strength (MPa)

Avg. Flexural Strength (MPa)

Avg. Splitting Tensile Strength (MPa)

Avg. Direct Tensile Strength (MPa)

7 Days’

28 Days’

28 Days’

28 Days’

28 Days’

F M M40 Concrete Mortar Cementitious Control Sample F7M3 F5M5 F3M7

38.39 32.15 30.46 40.52 49.94 32.34 32.31 32.27

51.80 45.72 45.52 53.17 66.28 51.30 49.62 47.85

5.14 4.49 5.80 4.90 2.75 4.76 4.61 4.58

3.14 3.24 3.28 3.20 1.51 3.35 3.34 3.22

2.15 1.66 3.12 2.46 1.74 2.05 1.97 1.87

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C1018-1998 [38]. The support span of the flexural test set-up is kept at 450 mm with the load span as 150 mm (shown in Fig. 1a). The compression and flexural tests have been carried out on a 600 KN capacity Universal Testing Machine (TINIUS OLSEN Super ‘‘L” UTM) with a constant rate of displacement of 1 mm/ min. Uniaxial direct tensile tests are carried out using a dog-bone shaped specimens (gauge length = 60 mm, with 30 mm at bridge and thickness 50 mm as shown in Fig. 1b) in a servo-hydraulic UTM with 50 KN capacity with a constant rate of displacement of 1 mm/min. The variations in proportions of different types of polypropylene fibers used for the mix include different variations between the fibrillated and the monofilament fibers. Samples have been

prepared either with just monofilament fibers or with fibrillated fibers or with a combination of these two types of fibers in different proportions. Sample abbreviations include F for 100% of fibrillated fibers; M for 100% of monofilament fibers; F7M3 for 70% of fibrillated and 30% of monofilament fibers; F5M5 for 50% each of fibrillated and monofilament fibers; and F3M7 for 30% of fibrillated and 70% of monofilament fibers. 4. Results and discussion Table 1 presents results for the above-mentioned tests for the cementitious composites of different mix proportions along with the results of the control concrete and cement mortar specimens.

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

Fig. 3. Uniaxial direct tensile stress-strain diagram of different cementitious composite samples with various proportions of different types of PP fibers.

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Strain(%) at max. stress

1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0

Average Values

F

M

0.53

1.53

Concrete Mortar 0.011

0.013

Control Sample 0.032

F7M3

F5M5

F3M7

1.29

1.30

1.39

F7M3

F5M5

F3M7

4.95

5.01

5.12

(a)

Post-peak strain(%) at 20% max. stress

5 4 3 2 1 0

Average Values

F

M

2.03

5.37

Concrete Mortar 0.012

0.013

Control Sample 0.033

(b) 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.

Atleast six samples were considered to determine the average maximum strength of the samples subjected to different types of loading conditions. It can be observed from the table that the compressive strength and splitting tensile strength is not compromised for different mixes used in this study, however, there is a slight decrease in the flexural and tensile strength of the samples. It should be noted at this point that the strength values typically indicate just the peak value; for determination of the ductility one should take a look into the curve (especially the strain at

maximum strength and the post-peak strain at 20% of the maximum strength) rather than just a number indicating the value of peak strength. Fig. 2, represents uniaxial stress-strain curve for different types of polypropylene fiber additions compared to control samples. The figure demonstrates that if one is interested only in the maximum strength then obviously conventional M40 concrete and the mortar are better choices compared to the cementitious composite (CC) samples prepared in this study. The cementitious control sample

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

Displacement (mm) at Max. load

410

14 12 10 8 6 4 2 0

Average values

F

M

Concrete

Mortar

6.75

12.96

0.87

0.9

Cementiti ous Control Sample 1.512

F7M3

F5M5

F3M7

9.12

9.89

10.96

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

yields lower values of strength compared to the M40 concrete and the conventional Mortar control samples, however demonstrating larger ductility than the other two. If one is interested in ductility of the samples, then the fiber reinforced cementitious composite samples perform much better than the conventional concrete, mortar control samples (for which the failure strain is around 0.01 and shown in a small figure as a zoomed in portion near zero strain) as well as cementitious control sample. It is also interesting to note from the figure that amongst the fiber reinforced CC samples, the one with (fibrillated fibers) F at 100% gives higher strength but lower ductility in comparison to the one with (monofilament fibers) M at 100% which gives lower strength but large ductility. It can also be observed that initial stiffness is higher for samples with F fibers in comparison to that of samples with M fibers. The differences in behaviour between two types of fibers in hydrated cement paste may be attributed to the fibrillation of the fiber along with the differences in the strength and the modulus of elasticity of the two types of fibers used. Fig. 2 also shows the minimum and maximum scatter bounds of the samples with 100% F and 100% M fibers. It should be noted that based on the bounds it can be ascertained that the behaviour observed from F fibers is significantly different compared to that of M fibers. Since significant variation could be observed between the response characteristics of cementitious composites made with F and M type of polypropylene fibers, samples are also fabricated with different proportions of a combination of these two fibers with a motive to improve the tensile strength characteristics along with ductility. As can be observed from Fig. 3, considering maximum and minimum bounds of different combinations there is hardly any difference with regards to strength and as well as ductility (measured as a function of strain). Based on this observation, it may be stated that a combination of fibrillated and monofilament fibers is required to improve both the strength and ductility of the samples in direct tensile tests. In order to assess the ductility of the sample, it is necessary to determine the strain at which the post-peak stress reaches 20% of the maximum stress (typically referred to as failure strain of the sample) [39,40], apart from the strain at maximum stress. Bar charts in Fig. 4 show that the addition of either kind of polypropylene fibers (F or M) leads to significant increase in the strain at maximum stress as well as strain at failure stress (defined as 20% of the maximum stress). This increase in ductility can be attributed to the bridging action of the fibers in mitigating the evolution of cracks in the matrix. It should be noted that the increase in percentage of strain at maximum stress is around 950% on an

average compared to that of control samples for cement mortar and concrete. The monofilament fibers have been found to be more effective in increasing the strain at maximum stress compared to that of the fibrillated fibers. Increase in percentage of failure strain (strain corresponding to 20% of the maximum stress) is observed to be around 4000% in comparison to that of the control samples of cement mortar and concrete. For this parameter also, the monofilament fibers perform better in comparison to that of the fibrillated fibers. Four-point flexure tests have also been carried out to determine behavioural changes of the sample with addition of different types of polypropylene fibers (F and M). Similar to that of Fig. 2, it can be observed from the load-displacement curve in Fig. 5 that the flexure characteristics are significantly different for F type fibers in comparison to that of M type fibers. The F type fibers give higher load carrying capacity whereas M type fibers provide more ductility. As expected, a combination of the two different types of fibers gives values in between the two bounds of 100% usage of either fiber. It can also be observed that there is a difference between the initial stiffness of the samples containing either F or M type fibers with lower initial stiffness being provided by the M fibers. It should be noted that not much scatter is observed for the flexure test of the samples and thereby the maximum and minimum bounds have not been provided. The scatter obtained in the sample tests for all different types has been estimated to be around 15%. It can also be mentioned from the study that addition of small percentage of fibrillated fibers changes the initial stiffness of the curve as well as improves the strength characteristics compared to samples with no addition of F fibers. As can be observed from Fig. 5, the addition of any type of polypropylene fibers significantly improves the ductility characteristics as compared to that of the control samples of concrete and mortar specimens. The characterization of ductility has been done based on displacement at maximum load and the results are shown in Fig. 6. The figure demonstrates significant improvement in flexural ductility of the samples with addition of fibers. It should be noted that irrespective of percentage variation of different types of fibers, there is a huge increase in the ductility characteristics of the specimen 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 cementitious composites improves the ductility characteristics of the mix. This

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study investigates improvement in ductility with addition of two different types of commercially available polypropylene fibers: fibrillated and monofilament fibers. The addition of fibrillated variety improves the tensile strength and flexural capacity of the mix in comparison to that of addition of the monofilament variety. On the other hand, the ductility improvement with addition of fibrillated variety is smaller as compared to that with the addition of monofilament variety. Combination of both these two type of fibers has also been investigated to reveal that addition of small amount of fibrillated fibers to the mix containing monofilament fibers improves the tensile strength and ductility characteristics of the sample. For flexural loads, the addition of monofilament variety to the sample containing fibrillated fibers significantly improves the ductility characteristics at the cost of strength characteristics. 6. Conflict of Interest The authors declare that they have no conflict of interest in any part of the manuscript. Acknowledgements The authors acknowledge the Future of Cities project (Project code: ECI) under Ministry of Human Research Development, India for funding this research. References [1] L. Sarvaranta, E. Mikkola, Fibre mortar composites under fire conditions: effects of ageing and moisture content of specimens, Mater. Struct. 27 (9) (1994) 532–538. [2] P. Kalifa, G. Chene, C. Galle, High-temperature behaviour of HPC with polypropylene fibres: From spalling to microstructure, Cem. Concr. Res. 31 (10) (2001) 1487–1499. [3] N. Banthia, R. Gupta, Influence of polypropylene fiber geometry on plastic shrinkage cracking in concrete, Cem. Concr. Res. 36 (7) (2006) 1263–1267. [4] P.S. Song, S. Hwang, B.C. Sheu, Strength properties of nylon-and polypropylene-fiber-reinforced concretes, Cem. Concr. Res. 35 (8) (2005) 1546–1550. [5] Z. Bayasi, J. Zeng, Properties of polypropylene fiber reinforced concrete, ACI Mater. J. 90 (6) (1993) 605–610. [6] Z. Sun, Q. Xu, Microscopic, physical and mechanical analysis of polypropylene fiber reinforced concrete, Mater. Sci. Eng. A 527 (1) (2009) 198–204. [7] M. Saidani, D. Saraireh, M. Gerges, Behaviour of different types of fibre reinforced concrete without admixture, Eng. Struct. 113 (2016) 328–334. [8] S. Kakooei, H.M. Akil, M. Jamshidi, J. Rouhi, The effects of polypropylene fibers on the properties of reinforced concrete structures, Constr. Build. Mater. 27 (1) (2012) 73–77. [9] J.H. Lee, B. Cho, E. Choi, Y.H. Kim, Experimental study of the reinforcement effect of macro-type high strength polypropylene on the flexural capacity of concrete, Constr. Build. Mater. 126 (2016) 967–975. [10] R.F. Zollo, Fiber-reinforced concrete: an overview after 30 years of development, Cem. Concr. Compos. 19 (2) (1997) 107–122. [11] D. Joo Kim, A.E. Naaman, S. El-Tawil, Comparative flexural behavior of four fiber reinforced cementitious composites, Cem. Concr. Compos. 30 (10) (2008) 917–928. [12] B. Felekoglu, K. Tosun-Felekoglu, R. Ranade, Q. Zhang, V.C. Li, Influence of matrix flowability, fiber mixing procedure, and curing conditions on the mechanical performance of HTPP-ECC, Compos. Part B: Eng. 60 (2014) 359– 370. [13] B.Y. Lee, V.C. Li, Y.Y. Kim, Polypropylene fiber-based strain-hardening cementitious composites, in: Proc. of the 2013 World Congress on Advances in Structural Engineering and Mechanics, 2013, pp. 444–457.

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