An investigation into influence of physical and chemical surface modification of macro-polypropylene fibers on properties of cementitious composites

Construction and Building Materials 244 (2020) 118340

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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

An investigation into influence of physical and chemical surface modification of macro-polypropylene fibers on properties of cementitious composites Rouhollah Rostami a, Mohammad Zarrebini a,⇑, Khaled Sanginabadi b, Davood Mostofinejad b, Sayyed Mahdi Abtahi b, Hossein Fashandi a a b

Department of Textile Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran Department of Civil Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran

h i g h l i g h t s  Influence of physical and chemical surface modification of fibers on properties of cementitious composites.  Fiber reinforced concrete as a cementitious composites were designed, produced and studied.  Hydrophobic and hydrophilic polypropylenes fibers were made.  The surface of fibers was physically modified by surface indentation.

a r t i c l e

i n f o

Article history: Received 23 June 2018 Received in revised form 31 August 2019 Accepted 2 February 2020

Keywords: Macro-fibers Concrete Indented fibers Hydrophilic polypropylene Mechanical properties Energy absorption capacity

a b s t r a c t During the past decades the use of polymeric fibers known as macro-fibers has been welcomed. The interface between these fibers and the cementitious matrix fundamentally influences the properties of the resultant composites. The induced indentation on the surface of macro-fibers enhances the adhesion of the fiber-matrix at the interface. Additionally hydrophilic fibers tend to adhere more positively with concrete matrix. In this research, hydrophobic and hydrophilic polypropylene fibers were made using ordinary and grafted anhydride maleic polypropylene granules. The surface of fibers was physically modified by surface indentation. Properties of the produced fibers, together with their moisture absorption tendency were determined. Various concrete mix designs were prepared with 0.55% fiber volume fraction. The concrete samples were tested for workability, compressive, splitting tensile and flexural strength. Results showed that the use of indented and hydrophilic fibers improves the mechanical properties of the reinforced concrete. The increase in flexural strength in case of indented and hydrophilic fibers was found to be 77%. The area under load-displacement curve as representation of the amount of energy absorption capacity were evaluated. The results pointed to 9 times increase in energy absorption capacity of the samples reinforced with indented and hydrophilic fibers in comparison with that of control sample. Additionally it was found that the effect of hydrophilicity of fibers on energy absorption capacity of the concrete is more pronounced than the surface indentation. Ó 2020 Elsevier Ltd. All rights reserved.

1. Introduction Concrete as a building material is composed of cement, aggregates and water. The hydration reaction between water and cement results in formation of a sticky paste that bonds the aggregates together. The resultant hard material is known as concrete. Low cost, ease of availability, high formability, adequate resistance ⇑ Corresponding author. E-mail address: [email protected] (M. Zarrebini). https://doi.org/10.1016/j.conbuildmat.2020.118340 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.

against fire and high resistance to compressive forces are the factors that have made concrete to be one of the highly favored building material worldwide. Mold-ability of fresh concrete and its strength and durability when hardened are the other advantages of cementitious composites. Despite of high compressive strength, the low tensile strength of concrete has hindered its applications in some end-uses. Brittleness of concrete during exertion of tensile forces is the major drawback associated with concrete. This drawback can be overcome by incorporation of steel rebars in the direction of the applied tensile forces. However, due to occurrence of

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cracks and penetration of aggressive and destructive ions, the steel rebars are corroded. This phenomenon leads to concrete spalling and its ultimate disintegration [1]. Additionally the steel rebars only form a fraction of the total cross-sectional area of the concrete section, thus the assumption of homogeneity of the concrete section cannot be held true. Therefore the use of other reinforcing materials that can be uniformly distributed in all direction is a promising alternatives [2]. Fiber reinforced concretes (FRC) are composites with almost uniform distribution of the reinforcing component in all directions. Natural and synthetic fibers can be used in cementitious composites such as concretes, mortars and shotcretes. These introduced composites are extensively studied [3,4]. Fiber strands due to flexibility, high aspect ratio, various crosssection area and molecular orientation can be used as reinforcement in cement-based composites [5]. The uniform distribution of fibers within the concrete matrix not only results in reduction of crack formation, but also improves concrete post-cracking performance due to control of crack propagation, which leads to increase in concrete energy absorption capacity [6]. So far both natural and synthetic fibers have been employed in concretes. Examples of the use of natural fibers are kenaf, jute, sisal, basalt, palm, coconut (coir), napier, akwara and bamboo [7]. As far as organic synthetic fibers are concerned, polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyvinyl alcohol (PVA), acrylic (PAN), nylon (PA) and inorganic synthetic fibers such as steel and glass are among the fibers that are used in manufacture of cement-based composites [3,4]. The use of recycled fibers such as PET and PP fibers in reinforced concretes is also prevalent [8]. Generally fibers as concrete reinforcement in civil engineering applications from dimensional point of view are classified into micro and macro-fibers. According to ASTM D 7508 [9] fibers with diameter smaller than 0.3 mm or fineness of less than 580 denier are known as micro and fibers with diameter larger or equal to 0.3 mm or fineness of more than 580 denier are classified as macro-fibers. Macro-fibers length is in the range of 30–60 mm and their cross-sectional area lies in the range of 0.6–1.0 mm2 [1,4]. The employment of polymeric fibers known as macro-fibers in concrete due to their beneficial properties has been of interest during the past decades. Macro-fibers due to their length, high tensile strength and elastic modulus, not only serve to prevent crack formation but may also improve tensile and flexural strength of the reinforced concrete [10]. These fibers can also control formation of cracks that can be created by plastic shrinkage of the concrete which is due to their high aspect ratio and specific surface area [11]. Macro-fibers are made from PP-homopolymer, PP-copolymer, co-polyethylene, combination of PE and PP, recycled PET and recycled PP [3,4]. Properties such as low weight, strength parity in wet or dry conditions and inertness in acid or especially in concrete alkaline environments are among the salient properties of PP fibers [12]. These have led to wide acceptance of PP fibers as concrete reinforcement. Additionally due to thermo-plasticity of PP, fibers can readily undergo operations that can change their geometry, both along length and cross-section [13]. These include surface indentation that is important as far as composite materials such as concrete is concerned. Macro-fibers used as concrete reinforcement, must enjoy high strength and modulus and be able to maintain their properties in long-term. In general, polymeric macro-fibers reinforcing concrete must have the following properties [2,5]:  High tensile strength and modulus. This is obtainable by manipulation of production parameters during fibers manufacturing.

 Minimum amount of variation in fiber strength.  Very low reduction in initial strength to the end of concrete service life. In this regard use of auxiliary materials during fiber manufacturing is of paramount importance. The use of auxiliaries such as anti-oxidants and UV stabilizers results in durability of the produced fibers during end-use.  Low dimensional variation of fibers. In this respect rheological properties of the polymer and compatibility between various stages of fibers production is of assistance. Researches show that the adhesion between fibers and concrete matrix at their interface is principally important [14–16]. Physical and chemical fiber surface modification technologies are very effective in enhancement of adhesion of fibers to the concrete. In general, physical surface modification of fibers can be achieved by fibrillation, indentation, crimping or any other means that can change fiber surface topology [15,17]. The use of chemical surface modification imparts hydrophilic property to the fibers. This can most positively assists the chemical adhesion of the fibers to the concrete at the interfaces. The use of hydrophilic fibers has resulted in enhancement of the concrete performance. This has been achieved due to formation of strong hydrogen bonds between the fibers and the water, which enhances the adhesion between fiber and concrete matrix [18]. Hydrophobic fibers are unable to form a strong bond at the interfaces of fiber-matrix. PP fibers can be made hydrophilic, if grafted anhydride maleic raw polymer is used. This has led to considerable increase in resistance of such concrete against cracks due to plastic shrinkage [19]. In this research, with the view to the requirements of the reinforced concrete technology, engineered textile macro-fibers were designed and manufactured, using ordinary and anhydride maleic grafted PP raw polymers. Surface of the fibers were physically modified by indentation. Physical-mechanical properties and hydrophilicity of the produced fibers were determined and then were added to the concrete in identical fiber volume fraction. Surface indentation of the macro-fibers enhances the physical bonds and hydrophilic surface of fibers can leads to formation of stronger chemical bonds between the fibers and the concrete matrix at their interfaces. The combined effect of the above phenomenon, seems to cause considerable increase in energy absorption capacity of the resultant concrete. The effects of physical and chemical fiber modifications on workability, compressive, splitting tensile, flexural strength and energy absorption capacity of the reinforced concretes were studied based on statistical analysis.

2. Experimental program 2.1. Reinforcing fibers 2.1.1. Production and characterization In this research, melt-spinning technology as the most widely manufacturing technique for production of the required PP fibers was used. Ordinary and grafted anhydride maleic PP granules were obtained from JAM Polypropylene Co., and Aria Polymer Pishgam Co., respectively. An industrial scale melt-spinning line made by PFE Engineering Co. was used. Melt-spinning technique using cold-air during quenching, is not suitable for production of macro-fibers, thus fibers are quenched using cold-water bath. Fibers after cooling are stretched in the first drawing zone and then are softened by hot oven prior to being stretched again by the second drawing zone. The selected draw ratio in this step is the key factor that controls the molecular orientation of the fibers structure, which determines the mechanical properties of the macro-fibers. By considering the type of used polymer and trial and error method, a draw ratio of 6:1 was judged

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to be the most suitable draw ratio. This was achieved by setting the linear speed of drawing zone rollers. Fibers after relaxation in the appropriate hot oven enter the third drawing zone and ultimately are wound on bobbins, ready for subsequent operations. Schematic representation of the macro-fiber spinning line is shown in Fig. 1.

In this research, variable production parameters during transformation of polymer granules to macro-fibers are as following: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

Fig. 1. Schematic representation of the macro-fibers spinning line.

Fig. 2. Schematic representation of surface indentation machine.

Fig. 3. Macro-fiber surface indent.

Extruding pressure, Temperature profile along the length of extruder, Gear pump revolution, Cooling rate of extruded fibers, Water height in the cold-water bath, Control of bath temperature and the amount of addition of auxiliaries, Threading path of the extruded fibers through the coldwater bath and drawing zone, Linear speed of various drawing zone, Hot oven temperature, Relaxation oven temperature, Fibers take-up speed.

Since the macro-fibers are more susceptible to variation in the above production parameter than micro-fibers, thus the above were carefully controlled during the production of fibers. Fig. 2 shows the schematic representation of surface indentation machine. As shown in Fig. 2, the spun macro-fibers were passed through the surface indentation operation. The characteristics of the indentation are influenced by factors such as linear speed of the fibers through the indenting roller, temperature of the rollers, surface geometry of the rollers and the applied nip pressure. These factors can alter depth and intensity of the created indentation on surface of the fibers. Indentation of the fiber surface was carried out at speed of 10 m/s, temperature of 135 °C and nip pressure of 2.7 bars. Fig. 3 shows the surface and transverse geometry of the produced macro-fibers. As shown in Fig. 3, the initial circular fiber cross-sectional area due to the indentation operation changes to rectangle or ellipse. After surface modification operation, the fibers were cut into staple length of 48 ± 2 mm long. Mechanical characteristics of the fibers were determined according to ASTM D 3822 [20] using Zwick (model 1446-60) tensile tester. The results are shown in Table 1. 2.1.2. Hydrophilicity of fibers Various methods are available to define the moisture absorption tendency of the materials. Static or dynamic measurement of contact angle or spectrometry as Fourier transform infrared (FTIR) spectroscopy are among these methods. In the former, the hydrophobic nature of the materials depends on the measured contact angle being smaller or larger than 90°. The material is defined as hydrophilic, if its measured contact angle is smaller than 90° otherwise the material is considered as being hydrophobic. In principle, hydrophobic materials due to low surface energy have more contact angle [17,19]. In FTIR method, the polymer functional groups can be investigated by examining the peaks appeared in the spectrum at specific wavenumbers. The existence of hydrophilic functional groups in the polymer chains can be demonstrated by the FTIR spectrum of the hydrophilic material. In this research, dynamic measurement of contact angle was established to determine the hydrophilicity of the ordinary and

Table 1 Fiber characteristics. Length (mm)

Tensile strength (MPa)

Specific gravity (g/cm3)

Young modulus (GPa)

Melting point (°)

48

200–300

0.91 [25]

3–5

140–170 [25]

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grafted PP fibers. DCAT-11 apparatus made by DATA PHYSICS Co., was used. Prepared 10  50 mm (diameter  length) samples were inserted in water at a very slow pace, the measured contact angle according to the Wilhelm method was reported in degrees. Sample preparation for FTIR spectroscopy was carried out by chopping of the produced fibers into very small particles and by using KBr in the weight ratio of 1:100 (fiber:KBr), the required tablets for use in HARTMAN & BRAUN apparatus (model MB-100) were prepared. FTIR spectrum in the wavenumber range of 400–4000 cm 1 with 8 scans and resolution of 8 cm 1 was obtained. 2.2. Cementitious composites 2.2.1. Concrete components The usual concrete components are cement, sand, gravels and water. In this research, Portland cement type I, manufactured by Sephahan cement Co., was used. Sand and gravels based on ASTM C 33 [21] were obtained from local mines. Characteristics of the aggregates are shown in Table 2. 2.2.2. Concrete mixing design In this research, the mixing design was finalized according to ACI 211 [22]. The selection is made based on the best performance, high durability and minimum cost of the resultant concrete. Table 3, shows the weight of the concrete components. The amount of fibers added to the concrete is defined as the bulk ratio of the volume of the fibers to the total volume of the concrete. In this research, the concrete mix designs were made with 0.55% fiber volume fraction. Mixing of the fibers with concrete is one of the most critical steps in production of FRCs and affects the performance of the resultant concrete. Inappropriate mixing, leads to undesirable orientation of fibers in the concrete matrix [23,24]. Undesirable orientation of fibers not only impairs the overall performance of the concrete, but also leads to localized variation in the mechanical properties of the concrete. In this research, based on trial and error method, it was decided to add the fibers in four stages of two minutes duration to the dry components. This was followed by addition of 75% of the water to the mixture and additional two minutes mixing. Eventually the remaining water was added and mixing was continued for a further one minute. In principal, the mixing operation must continue until a uniform dispersion of the fibers and other components is achieved. Ideal mixing is dependent on quality and appearance of the components together with mixer capacity and speed. In this research, the mixer used was large enough, therefore, one batch was adequate for each mixing design. In total, 5 mixes (5 batches) were considered as follows: 1- without any fibers, 2- with nonindent and hydrophobic fibers, 3- with indent and hydrophobic fibers, 4- with non-indent and hydrophilic fibers and 5- with indent and hydrophilic fibers. 2.2.3. Concrete preparing Internal structure of the concrete as much as possible must be free from voids. Compaction of the concrete, results in reduction of the internal air pockets and increases concrete density, as the result of which composite strength is increased. This increase can be achieved by vibration of the mould at desired frequencies. The

Table 3 Weight of components in terms of kg/m3. Sand

Gravel

Cement

Water

917

757

379

203

use of external vibrator for preparation of FRCs is obligatory due to the need for uniform distribution of fibers and also removal of the voids in cementitious composite [1,2]. Generally, fibers can be effective if are well mixed and attain suitable orientation within the concrete. Thus vibration of the mould is advantageous as far as uniform distribution of fibers within the matrix is concerned. In this research, the vibration of the mould was carried out by vibrating desk with rotation of 4000 RPM, when the mould was half full and full, each for two minutes duration. Four 150  300 mm cylindrical samples from each design were selected for determination of compressive and splitting tensile strength. Three prismatic 100  100  350 mm samples were used to measure the flexural strength. The samples were coded using a three letters code; letter (M) denotes macro-fiber, letter (N) denotes non-indented fibers, letter (I) denotes indented fibers and letters (N) and (H) point to hydrophobic or hydrophilic fibers, respectively. Samples were cured in standard conditions for a period of 28 days. 2.2.4. Concrete workability based on slump test The slump index determines the workability of the concrete. In this research, slump was measured according to ASTM C 143 [25]. 2.2.5. Compressive strength Compressive strength is the most important feature of concrete. In this research, the compressive strength is measured according to ASTM C 39 [26]. A 2000 kN single screw compressing jack made by ELE Co., was used, the loading rate was adjusted at 0.22 MPa/s. 2.2.6. Tensile strength Splitting test is used to determine the tensile strength of the concrete. In this research, splitting tensile strength of the samples was measured according to ASTM C 496 [27]. In this test, side way load is applied to a cylindrical sample with diameter of 150 mm and height of 300 mm. 2.2.7. Flexural strength and energy absorption capacity Bending properties of the fibrous concrete was determined according to ASTM C 1018 [28]. In this research, beams with dimension of 100  100  350 mm were used. Distance between the supports was set at 300 mm and the load was exerted in the middle one-third. The exerted loads momentarily were measured using a load-cell. Time and displacement of the mid-span together with the vertical displacement of the left and right hand supports, were measured simultaneously using three LVDT. The load-displacement curve was drawn and by using MATLAB software, area under load-displacement was calculated. The calculated area was studied as the ‘‘Energy Absorption Capacity”. Fourpoints flexural test is shown in Fig. 4.

Table 2 Characteristics of the aggregates. Aggregates

Bulk density (dry condition) (kg/m3)

Moisture SSD (%)

Maximum diameter (mm)

Fineness modulus

Fine Coarse (crushed angular)

1934 1683

0.70 0.50

4.75 9.50

3.10 –

R. Rostami et al. / Construction and Building Materials 244 (2020) 118340

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Fig. 4. Four-point flexural strength: (a) apparatus and (b) schematic.

3. Results and discussions 3.1. Hydrophilicity of fibers The hydrophilicity of the ordinary and grafted PP fibers was evaluated using, dynamic water contact angle measurement and FTIR spectroscopy methods. The advancing contact angles of produced fibers are shown in Table 4. The smaller than 90° of measured contact angle of grafted PP fibers, causes the increase in moisture absorption tendency of these fibers.

FTIR spectra of the ordinary and grafted PP fibers are shown in Fig. 5. The peaks appeared at wavenumbers lower than 3000 cm 1, 2850–3000 cm 1, are related to sp3 C–H stretching vibration [29]. The peaks at 1375 and 1455 cm 1 are attributed to the CH3 bending vibration [30]. The peaks centered at wavenumbers of 1750 and 1810 cm 1 are assigned to the stretching vibration of carbonyl (C=O) group of maleic anhydride grafted on PP fibers [29]. These hydrophilic functional group-related peaks are absent in the FTIR spectrum of the ordinary PP fibers. It can be stated that, FTIR spectra are well in line with results of water contact angle measurement. 3.2. Slump

Table 4 Advancing contact angle of ordinary and grafted PP fibers. Fibers

Advancing water contact angle (degrees)

PP (ordinary) PP (grafted with maleic anhydride)

98 77

The amount of slumps of control sample and FRCs are shown in Fig. 6. Results are indicative of the fact that, in general, addition of fibers leads to impairment of performance of the concrete. Addition of fibers both enhances the cohesiveness and homogeneity of the fresh concrete and can reduces its workability. This result

Fig. 5. FTIR spectrum of the grafted polypropylene.

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R. Rostami et al. / Construction and Building Materials 244 (2020) 118340 Table 5 Compressive strength Duncan test result. Specimen code

Subset for alpha = 0.05 1

Control M-N-N M-I-N M-N-H M-I-H Sig.

Fig. 6. Slumps of the concretes.

is consistent with other researches [31,32]. The lowest slump reduction of 16% was found, due to the use of non-indented and hydrophobic fibers. Indented and hydrophilic fibers showed the highest slump reduction of 48%. Additionally, indented fibers in comparison with non-indented fibers showed higher reduction in slump.

23.4 24.5

0.058

2

3

24.5 24.7 24.9 0.476

26.1 1.000

In order to verify the results, the Duncan test was performed using statistical SPSS software. As shown in Table 5, the analysis points to the existence of significant differences in compressive strength between FRCs and control sample at the 5% significance level. Statistical analysis points to the existence of significant differences in compressive strength between concrete reinforced with indented and hydrophilic fibers, i.e. M-I-H, and other concrete samples , i.e. Control, M-N-N, M-I-N and M-N-H, at the 5% significance level. Difference between FRC using non-indented and hydrophobic fibers, i.e. M-N-N and control sample is not significant at the 5% level.

3.3. Compressive strength

3.4. Splitting tensile strength

Compressive strength of the concrete samples is shown in Fig. 7. Addition of fibers has resulted in enhancement of compressive strength of the concrete. The lowest significant increase of 5% is obtained when adding indented and hydrophobic fibers, i.e. M-IN, to the concrete. FRC using indented and hydrophilic fibers i.e. M-I-H shows the highest significant increase of 11% in compressive strength to the control sample. In this research, fiber balling did not occurs due to the appropriate dispersion and employment of hydrophilic fibers, thus no reduction in compressive strength is observed in the FRCs. Balling or clumping of fibers, that occurs during mixing of fibers with the concrete component is more intense when using hydrophobic fibers such as PP fibers. This phenomenon occurs due to accumulation of static electricity. Hydrophilicity of PP fibers results in conduction of the accumulated static electricity, while hydrophobicity of fibers hinders the conduction of static electricity, that leads to balling or clumping the fibers used in concrete. Balling results in non-uniform distribution of fibers within the concrete and as the result of which, leads to reduction in compressive strength of the FRCs as reported by some researches [32,33]. Fig. 7, shows the positive effect of indentation and hydrophilization on compressive property of the FRCs.

Splitting tensile strength of the concretes is shown in Fig. 8. The high adhesion of fiber-matrix at the interfaces has caused fibers to withstand the tensile forces exerted to the concrete. The fibers prevent crack formation and crack propagation within the reinforced concrete, thus the observed increase in the resistance of concrete against the applied tension. As shown in Fig. 8, addition of fibers to the concrete has led to minimum increase of 25% in splitting tensile strength; the maximum increase in splitting tensile strength is 45%. Additionally it seems that, the uniform distribution of fibers in concrete, results in increase in exerted stress to the fibers. This in turn, leads to significant enhancement of tensile strength of the concrete. Significant increase in splitting tensile strength of reinforced concrete has shown in Table 6. Statistical analysis points to the existence of significant differences in splitting tensile strength between concrete reinforced with fibers, i.e. M-N-N, M-I-N, M-N-H and M-I-H, and control sample at the 5% significance level. Since water is one of the concrete components, then the wetting tendency of fibers is an influential factor in adhesion of the fibers to the concrete matrix. Enhancements of fiber-matrix adhesion directly affect the mechanical properties of the FRCs. The SEM micrographs shown in Fig. 9, illustrate the crosssectional fracture of the fiber reinforced concrete sample due to

Fig. 7. Compressive strength of the concretes.

Fig. 8. Splitting tensile strength of the concretes.

R. Rostami et al. / Construction and Building Materials 244 (2020) 118340 Table 6 Splitting tensile strength Duncan test result Specimen code

Subset for alpha = 0.05 1

Control M-N-N M-I-N M-N-H M-I-H Sig.

2

3

4

2.0 2.49 2.66

1.000

0.101

2.66 2.72 0.539

2.72 2.91 0.071

splitting tensile strength test. The cross-sectional fracture of the concrete reinforced with non-indented and hydrophobic fibers, i.e. M-N-N is depicted in Fig. 9.a, which shows the pulling out of the fibers during the conduct of the test. The fiber can be slipped and pulled out due to low adhesion between the fibers and the concrete matrix. The cross-sectional fracture of the concrete reinforced with non-indented and hydrophilic fibers, i.e. M-N-H is depicted in Fig. 9.b, which in contrast to Fig. 9.a, vividly shows the breakage of fibers during pulling out. Fiber breakage is due to high adhesion of fibers and the concrete matrix, which results in breakage of fibers. 3.5. Flexural strength and energy absorption capacity The load-displacement curve and the flexural strength of the concretes are shown in Fig. 10 and Fig. 11, respectively. Generally addition of fibers as is shown in Fig. 11, results to increase in flexural strength. Results show that, addition of

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non-indented hydrophobic fibers, i.e. M-N-N has led to 46% increase in flexural strength in comparison with the control sample. An increase of 77% in flexural strength is observed in case of sample reinforced by indented and hydrophilic fibers, i.e. M-I-H in comparison with the control sample. These differences are statistically significant according to analyses included in Table 7. The energy absorption capacity of the concretes is shown in Fig. 12. Addition of fibers also results in enhancement of reinforced concrete performance after crack occurrence. The modification carried out, can be responsible for enhancement of properties such as energy absorption capacity. Bridging of the cracks by the fibers is known as crack sewing. This phenomenon result in enhancement of load bearing capacity of the concrete structures which in turn leads to increase in energy absorption capacity. In case of FRCs due to the increase in ductility and the fact that fracture of concrete does not occur abruptly, the area under loaddisplacement curve is increased. This increase in some cases can be 9 times more in comparison with the control sample. Fig. 12, shows that the effect of fibers hydrophilicity on energy absorption capacity of the concrete is more than that of fiber surface indentation. Hydrophilicity increases the energy absorption capacity 8 times in comparison with the control sample, whereas surface indentation leads to increase the energy absorption capacity by 6 times. The result of the Duncan test for flexural strength of concretes is shown in Table 8. Statistical analysis points to the existence of significant differences in energy absorption capacity between concrete reinforced with fibers, i.e. M-N-N, M-I-N, M-N-H and M-I-H, and control sample, at the 5% significance level.

Fig. 9. SEM images captured from the cross-section of fractured area of cylindrical samples after splitting tensile test: (a) M-N-N concrete sample (b) M-N-H concrete sample.

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Fig. 10. Load-displacement curve of the concretes.

Fig. 11. Flexural strength of the concretes.

Fig. 12. Energy absorption capacity of the concretes.

Table 7 Flexural strength Duncan test result Specimen code

Subset for alpha = 0.05 1

Control M-N-N M-I-N M-N-H M-I-H Sig.

Table 8 Energy absorption capacity Duncan test result

2

Specimen code 3

2.56 3.83

1.000

1.000

4.38 4.48 4.60 0.235

4. Conclusions FRCs due to their interesting properties are employed by civil and structural engineers in some cases. Brittle performance of ordinary concrete when subjected to tensile or bending forces, forms the most predominant disadvantage of this widely used building materials. Overcoming this disadvantage has been evaluated by many researchers. The use of fibers is one of the key technologies that can reduce this intrinsic property of the ordinary concrete. Nowadays, efforts have been made to use appropriate fibers in

Subset for alpha = 0.05 1

Control M-N-N M-I-N M-N-H M-I-H Sig.

2

3

4

5

0.68 4.34 5.11 6.01 1.000

1.000

1.000

1.000

6.29 1.000

cementitious composites. In this research, the use of nonindented and indented, hydrophobic and hydrophilic PP macrofibers in concrete, was studied and the following conclusions were reached:  As expected macro-fibers using grafted PP polymer have higher moisture absorption tendency due to their hydrophilic nature.  Addition of fibers in general leads to reduction of the concrete workability. Fibers tend to enhance the cohesiveness and homogeneity of the concrete components. The lowest slump reduc-

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tion was obtained by samples containing non-indented and hydrophobic fibers. The highest reduction in slump was due to the use of indented and hydrophilic fibers. Hydrophilicity of fibers enhances the fiber-matrix adhesion. Adhesion between fibers and concrete as reinforcement and matrix, respectively is among the factors affecting mechanical properties of the concretes. Adhesion of non-indented and hydrophilic fibers seems to be higher than the adhesion of non-indented and hydrophobic fibers. Addition of fibers has resulted in enhancement of compressive strength of the concrete. The lowest significant increase of 5% is obtained when adding indented and hydrophobic fibers to the concrete. FRC using indented and hydrophilic fibers shows the highest significant increase of 11% in compressive strength to the control sample. Addition of non-indented and hydrophobic fibers resulted to 25% increase in splitting tensile strength of the FRCs. This was found to be 45% in case of indented and hydrophilic fibers. Addition of non-indented and hydrophobic fibers leads to 46% increase in the flexural strength of the samples. The increase in flexural strength of the concrete samples in case of indented and hydrophilic fibers in comparison with the control sample was found to be 77%. This increase is due to fibers bridging phenomenon that prevent crack formation and propagation. The difference between mechanical properties of the fiber reinforced concert in some cases, is not significant and very small. This negligible difference can be attributed to the level of hydrophilicity of the fibers, which can be very high to very low levels. In this paper, we are dealing with low level of hydrophilicity of the fibers which is supported by the results of dynamic contact angle. The difference between concrete reinforced by non-indented and hydrophobic fibers with other types of concretes is significant and quite high. This fact points to the importance of indention and hydrophilicity as physical and chemical modification, respectively.

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