Scholarly article on topic 'Mechanical performance and durability of treated palm fiber reinforced mortars'

Mechanical performance and durability of treated palm fiber reinforced mortars Academic research paper on "Materials engineering"

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Abstract of research paper on Materials engineering, author of scientific article — Nesibe Gozde Ozerkan, Bappy Ahsan, Said Mansour, Srinath R. Iyengar

Abstract The performance of cement mortar reinforced with varying percentages of treated bundled date palm fibers is investigated to appraise their feasibility for structural and non-structural applications. The study first entailed the evaluation of two different alkali pre-treatments at varying concentrations by subjecting treated and untreated bundled fibers to tensile testing. The suitable pre-treatment was then adopted while casting cement mortar mixes. The physical properties of fresh mortar was studied through setting times and, for mortar mixes cured up to 28days, through parameters such as drying shrinkage and water absorption. The unconfined compressive strengths, split tensile strengths as well as the flexural strengths of the cured mortar mixes at two different ages were undertaken to assess their mechanical properties; while the durability was gauged based on their sulfate resistance for up to a period of four months. Observed stress–strain behavior under tension led to the choice of 0.173% Ca(OH)2 as the preferred pre-treatment for the bundled fibers used in the mortar mixes. This was further supported by the microstructural examination on the hardened mortars which, revealed that the integrity of treated fibers remained intact within the cement matrix without hindering the hydration processes. Results also indicated that inclusion of fibers improves the flexural strengths as well as the sulfate resistance of the mortar mixes. However, the cylinder and cube compressive strengths decreased with the increase in treated fiber inclusion. Although, the work reported in this paper was carried out on cement mortars, conclusions are expected to be relevant to fiber reinforced concrete employing treated natural fibers.

Academic research paper on topic "Mechanical performance and durability of treated palm fiber reinforced mortars"

International Journal of Sustainable Built Environment (2014) xxx, xxx-xxx

Gulf Organisation for Research and Development International Journal of Sustainable Built Environment

ScienceDirect www.sciencedirect.com

Original Article/Research

Mechanical performance and durability of treated palm fiber

reinforced mortars

Nesibe Gozde Ozerkan a, Bappy Ahsan a, Said Mansour b, Srinath R. Iyengarc *

a Center for Advanced Materials, Qatar University, Qatar Qatar Energy & Environment Research Institute, Qatar c Department of Mechanical Engineering, Texas A&M University, Qatar

Received 10 November 2013; accepted 18 April 2014

11 Abstract

12 The performance of cement mortar reinforced with varying percentages of treated bundled date palm fibers is investigated to appraise

13 their feasibility for structural and non-structural applications. The study first entailed the evaluation of two different alkali pre-treat-

14 ments at varying concentrations by subjecting treated and untreated bundled fibers to tensile testing. The suitable pre-treatment was then

15 adopted while casting cement mortar mixes. The physical properties of fresh mortar was studied through setting times and, for mortar

16 mixes cured up to 28 days, through parameters such as drying shrinkage and water absorption. The unconfined compressive strengths,

17 split tensile strengths as well as the flexural strengths of the cured mortar mixes at two different ages were undertaken to assess their

18 mechanical properties; while the durability was gauged based on their sulfate resistance for up to a period of four months.

19 Observed stress-strain behavior under tension led to the choice of 0.173% Ca(OH)2 as the preferred pre-treatment for the bundled

20 fibers used in the mortar mixes. This was further supported by the microstructural examination on the hardened mortars which, revealed

21 that the integrity of treated fibers remained intact within the cement matrix without hindering the hydration processes. Results also indi-

22 cated that inclusion of fibers improves the flexural strengths as well as the sulfate resistance of the mortar mixes. However, the cylinder

23 and cube compressive strengths decreased with the increase in treated fiber inclusion.

24 Although, the work reported in this paper was carried out on cement mortars, conclusions are expected to be relevant to fiber

25 reinforced concrete employing treated natural fibers.

26 © 2014 The Gulf Organisation for Research and Development. Production and hosting by Elsevier B.V. All rights reserved.

27 Q4 Keywords: Fiber reinforced mortar; Durability; Flexural strength; Date palm fibers; Mortars (materials); Mechanical performance

1. Introduction

* Corresponding author. Tel.: +974 44230497; fax: +974 44230011. E-mail addresses: gozdeozerkan@qu.edu.qa (N.G. Ozerkan), ahsan@ qu.edu.qa (B. Ahsan), smansour@qf.org.qa (S. Mansour), srinath. iyengar@qatar.tamu.edu (S.R. Iyengar).

Peer review under responsibility of The Gulf Organization for Research and Development.

Concrete and mortars made with Portland cement are 30

known to be easy to form and relatively strong in compres- 31

sion but weak in tension, tend to be brittle and have poor 32

impact strength and toughness. The weakness in tension 33

could be overcome by the use of conventional rod 34

reinforcement and to some extent by the inclusion of a 35

sufficient volume of certain fibers (Mehta and Monterio, 36

1997; Neville and Brooks, 1990; Price, 1951). 37

2212-6090/$ - see front matter © 2014 The Gulf Organisation for Research and Development. Production and hosting by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ijsbe.2014.04.002

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2 N.G. Ozerkan et al. / International Journal of Sustainable Built Environment xxx (2014) xxx-xxx

38 In concrete, fibers can be introduced as primary or

39 secondary reinforcement. Fibers work as primary rein-

40 forcement in thin-sheet products in which conventional

41 reinforcing bars cannot be used and includes no coarse

42 aggregate and a matrix with markedly higher cement con-

43 tent than normal concrete. The fibers are used as primary

44 reinforcement to increase both the strength and toughness

45 of the composite. Fibers are also included in the matrix as

46 the secondary reinforcement to control cracking induced

47 by humidity or temperature variations or to provide

48 post-failure integrity in the event of accidental overload

49 or spalling (Filho et al., 1999; Bentur and Mindness, 2007).

50 Reinforcing cement matrices with various fibers have

51 been reported to resist rapid propagation of micro cracking

52 under applied stress as well as the ability to withstand loads

53 even after initial cracking, thereby improving toughness

54 (Yurtseven, 2004). Furthermore, increase in the flexural

55 strength of the fiber-cement composite up to 30% had been

56 observed; however, fiber inclusion reduces the workability

57 Q5 of the fresh concrete and mortars (ACI, 2010).

58 Several fiber types in a variety of sizes, both manmade

59 and natural, have been incorporated into cement based

60 matrices which composed of paste, mortar or concrete.

61 The choice of the most commercially significant types of

62 fibers varies from synthetic organic materials such as poly-

63 propylene or carbon, synthetic inorganics such as steel or

64 glass, natural organics such as cellulose or sisal to natural

65 inorganic asbestos. Although, most of the developments

66 involve the use of ordinary Portland cement; the use of

67 high alumina cement, cement with additives such as fly

68 ash, slag, silica fume, etc. to improve the durability of the

69 composite or to minimize chemical interactions between

70 the fibers and matrix have also been reported (Bentur

71 and Mindness, 2007).

72 Natural fibers; either unprocessed or processed, have

73 been used to reinforce cement based products in various

74 applications around the world. These include materials

75 Q6 obtained from different parts of plants. For example, fibers

76 of jute, ramie, flax, kenaf and hemp are obtained from the

77 stem whereas sisal, banana and pineapple are obtained

78 from the leaf and cotton and kapok from the seed. Natural

79 fibers are composites with a cellular structure including dif-

80 ferent proportions of cellulose, hemicellulose and lignin

81 which constitute different layers (Filho et al., 1999; John

82 et al., 2005).

83 Natural fibers have a high tensile strength and a low

84 modulus of elasticity; however, they have a high variation

85 on their properties which could lead to unpredictable

86 fiber-cement composite properties (Swamy, 1990; Li et al.,

87 2006). There are several studies on the evaluation of the

88 use of different types of natural fibers in concrete and mortar

89 applications. Aggarwal (1995) suggested that in countries

90 where bagasse is substantially available, it can be used for

91 the production of cement-bonded building materials, since

92 the results obtained from the study showed that the devel-

93 oped composites meet most of the requirements of various

94 standards on cement-bonded particle boards and have high

levels of performance even in moist conditions. Coconut 95

fibers can also be used as reinforcement and to substitute 96

sand in the development of composite reinforced coconut 97

fiber. Moreover, increasing the content of coconut fiber 98

increased the modulus of rupture and compressive strength 99

of the composites up to a certain optimum composition. The 100

composites manufactured with short coconut fibers and 101

ordinary Portland cement matrix presented a significant 102

reduction in toughness (Abdullah et al., 2011; Filho et al., 103

2000). On the other hand, bamboo fiber is a satisfactory 104

fiber for incorporation into the cement matrix, and does 105

not vary greatly in flexural strength and fracture toughness 106

values (Coutts, 1995). It is also proved that the composites 107

reinforced with sisal fibers are reliable materials to be used 108

in practice for the production of structural elements to be 109

used in rural and civil construction, and improvement in 110

flexural strength and splitting tensile strength was reported 111

(Filho et al., 1999; Al Rawi and Al Khafagy, 2009). Several 112

other studies have been carried out on evaluating the 113

behavior of cement composites with natural fibers from 114

bamboo (Ghawami, 2005), sisal (Filho et al., 2009), coir 115

(Aggarwal, 1992), vegetable origin (Agopyan et al., 2005; 116

Toledo Filho et al., 2005), etc. 117

Although natural fibers have some advantages like low 118

density, less abrasiveness, and lower cost when compared 119

to inorganic reinforcing fibers, they also have some disad- 120

vantages such as mechanical and thermal degradation dur- 121

ing processing, poor wettability and high moisture 122

absorption. Moreover, it is known that the natural fibers 123

include high content of hydroxyl groups (OH) which causes 124

the hydrophilic behavior. The hydrophilic behavior of fiber 125

produces poor adhesion between fiber and matrix when the 126

natural fiber is faced to develop composite material. This 127

problem is mainly improved by several chemical treatment 128 methods suggested by researchers. These treatment meth- Q7 129

ods range from being saline treatment, alkali treatment 130

and graft copolymerization of monomer. In order to mod- 131

ify and clean the surface of natural fiber, alkali treatment is 132

one of the most common methods employed. Alkali treat- 133

ment has been reported to decrease the surface tension and 134

to improve the interfacial adhesion between the fiber and 135

matrix. Furthermore, in the literature, several possible 136

explanations can be found discussing the positive effects 137

of alkali treatment on the properties and structure of nat- 138

ural fibers (Tan, 1997; Bledzki and Gassan, 1999; Rong 139

et al., 2001; Aziz and Ansell, 2004; Weyenberg et al., 140

2006; Bachtiar et al., 2008; Kriker et al., 2008; Bachtiar 141

et al., 2012; Nordin et al., 2013). 142

Date palm trees are native to the middle-east region; 143

their fibers can be easily and abundantly found in countries 144

like the State of Qatar. The idea of reinforcing concrete 145

with date palm fibers was studied by Kriker et al. (2005). 146

They had looked into the durability and mechanical prop- 147

erties of date palm surface fibers in hot-dry climate. They 148

concluded that Male date palm surface fibers (MDPSFs) 149

had the most tensile strength compared to other types of 150

date palm fibers. As the volume of fiber is increased in 151

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the concrete, more post-crack flexural strength and toughness coefficient were observed but at the same time a reduction in first crack and compressive strength was noted. Another research was launched by the same group to study the durability of male date palm surface fibers immersed in alkaline solutions which resulted in concluding that the durability of MDPSF reinforced concrete is poor (Rao and Rao, 2007). Also it was reported that the male date palm surface fiber had an average tensile strength and weak elastic modulus and the increase in percentage and length of the fiber in concrete has a beneficial effect on the ductile behavior (Kriker et al., 2005).

Hence, there is a need for further investigating properties of date palm fibers and understanding their contribution to the performance of cement-fiber composites.

2. Materials and methodology

The objective of this paper is to evaluate the performance of cement mortar reinforced with varying percentages of treated bundled date palm fibers so as to appraise their feasibility for structural and non-structural applications.

2.1. Materials

2.1.1. Fiber sampling and extraction

Date palms are classified as male and female tree. While the female trees produce flowers, the male trees produce pollen. In this study, the female date palm leaves were used according to the previous researches (AlMaadeed et al., 2013), it is discovered that female leaves have better tensile properties. The date palm leaves were obtained from the female trees which are planted at the women campus of the Qatar University. The female trees bear the date fruits. The bundled fibers (thickness 0.7-4 mm) were peeled out by means of scissors from date palm leaves and cutting them in lengths of 10 cm.

2.1.2. Fiber pretreatment

The treatment of date palm fibers was performed by NaOH and Ca(OH)2 solution immersion. The NaOH pellets used to prepare the solution were manufactured by the BDH Laboratory Supplies, U.K. and were of general purpose reagent category. Extra pure Ca(OH)2 was used to prepare the second solution and was supplied by Riedel-de Haen, Germany.

2.1.3. Mortar preparation

Sand (or fine aggregate) procured from the Qatar Sand Treatment Plant, having 2.73 specific gravity, unit weight (i.e. the weight per unit volume of a material) of 1.65 kg/l and water absorption of 2.15% and ordinary Portland Cement Type I supplied by Al Khalij Cement Company which conforms to BS EN 197-1 (BSI, 2000), were used in this study.

The mortar mixes used in this study have been detailed in Table 1.The volume fraction is defined as the volume of fibers divided by the volume of the composite (fibers and concrete), and typically ranges from 0.1 to 3%. In this study, the aim was to vary the volume fractions of the bundled fibers in the mix beyond 2.0%. However, while attempting to do so, during sample preparation, excessive separation of the mortar components was observed which pose difficulty in mix workability as well as in preparing and obtaining monolithic compacted samples. Hence, it was decided to limit the maximum fiber inclusion to 2.0%.

2.2. Experimental methods

2.2.1. Fiber pretreatment

The bundled fibers were treated by immersing individually either in 2.0% of NaOH solution (based on recommendations from study performed by AlMaadeed et al., 2013) or 0.173% Ca(OH)2 (as per room temperature solubility range). Additionally, for direct comparison, 0.173% of NaOH solution was also considered. The fibers were immersed in the solution for an hour and then placed in an oven at 60 °C for 3 h to dry. 10 cm length of treated fibers was then cut to prepare them for the tensile test.

2.2.2. Fiber properties

Tensile test was performed using a Lloyd 1KN tensile tester to examine the elongation and maximum tensile load that can be applied to bundle palm leaf fibers (treated and untreated) of 10 cm length whose thickness ranged between 0.7 mm and 4 mm. The procedure specified in ASTM-D3822 (ASTM, 2007) was followed. A minimum of at least 10 replicates of treated and untreated fibers were tested. The tensile tester was equipped with a linear variable differential transformer (LVDT) to measure the corresponding deformation/strains and Young's stiffness modulus (E).

2.2.3. Mortar and sample preparation

Sand, cement, water and treated palm fiber bundles were mixed to prepare the fresh mortar in a concrete mixer for four mixes with varying percentages (i.e. 0.0%, 0.5%, 1.0% and 2.0 wt.%) of treated date palm fiber bundles. The mortar was placed in molds to prepare four kinds of samples namely cylindrical, cubical, prism and shrinkage/ sulfate whose dimensions are provided in Table 2. After 24 h, the samples were demolded, marked, dimensions taken and immersed in large drums filled with tap water at room temperature to cure.

2.2.4. Physical properties of the fresh and hardened mortars

The setting time of the fresh mortar was recorded using

a Vicat's apparatus in accordance to ASTM C807 (2008). The ASTM C1403 (ASTM, 2012b) method was followed to measure the water absorption of the hardened mortar after 7 and 28 day curing time. The sorptivity test method was based on the ASTM C1585 and was used to determine

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Table 1

Details of the mortar mix designs.

Mix No. w/ca Fiber inclusion Sand Cement

(wt.%) content (kg/m3) content (kg/m3)

1 (control) 0.485 0.0 1197 400

2 0.515 0.5 1173 400

3 0.540 1.0 1157 400

4 0.550 2.0 1127 400

w/c denotes water-to-cement-ratio.

Table 2

Dimensions of mortar samples.

Sample type Dimensions (mm)

Cube 50 x 50 x 50

Cylinder 100 x 200 (diameter x length)

Shrinkage/Sulfate Bars 280 x 25 x 25

Prism 160 x 40 x 40

the rate of absorption of water by hydraulic cement concrete by measuring the mass increase of a disk specimen at 1, 5, 10, 20, 30, 60, 180, 240, 300 and 360 min time intervals when one surface of the specimen was submerged in 3-5 mm of water. The water absorption and the sorptivity tests were performed in triplicates. The change in length of the mortar bar samples was measured to identify the drying shrinkage according to ASTM C 596 (ASTM, 2009). Length comparator readings of the mortar samples were recorded on the 4, 11, 18, and 25 days of sample curing. Shrinkage results were based on observations made on six replicate specimens per mix.

2.2.5. Mechanical testing

Mechanical testing of hardened mortar samples after 7 and 28 days of curing tests was performed in triplicate. Cylindrical and cubical samples were subjected to compressive loading based on ASTM C 39 and C 109 (ASTM, 2012, 2012a), respectively; wherein the maximum load and stress at failure were recorded. The flexural strength of the prism samples was determined according to ASTM C293 (ASTM, 2010) by the three point bending test. The load at failure was noted and accordingly, the flexure strength of the sample was then calculated. The tensile splitting test of the cylindrical samples was performed as per ASTM C496/C496M (ASTM, 2011) by applying compressive force along the length of the specimen until failure.

2.2.6. Durability studies

The sulfate resistance of the mortar bars was determined according to ASTM C 1012 (ASTM, 2013a,b). The mortar bars were cured in lime water as per Section 9.2 of the aforesaid standard and then immersed in sodium sulfate solution. The length changes were measured using length comparators in the durations as provided in Section 9.4 of the standard. Six replicate specimens per mix were subjected to durability testing for a period of up to 4 months.

2.2.7. Microstructure analyses 288

The microstructure and crystallinity (viz. scanning elec- 289

tron microscopy and X-ray diffraction) of the various types 290

of cured mortars reinforced with treated date palm fibber 291

bundles were studied. FEI Quanta 400 Scanning electron 292

microscope was used to study the microstructure of fibers 293

and that of the hardened fiber reinforced mortars. SEM 294

of samples was employed by firstly being dried and then 295

mounted on an aluminum stub using a strong conductive 296

double-sided adhesive tape. 297

X-ray diffraction was used to identify the crystalline 298

phases and the corresponding orientation of various 299

compounds in the 28 day cured fiber reinforced mortars. 300

Rigaku Ultima IV 2-Theta-2-Theta type X-ray Diffractom- 301

eter fitted with a copper anode diffraction X-ray tube oper- 302

ating at 40 kV and 40 mA was used in this study. The Peak 303

Search and Qualitative Analysis software provided by 304

Rigaku using JCPDS-ICDD library (PDF-2 Release 305

2007) was employed to identify the peaks of the raw 306

XRD data. 307

3. Results and discussions 308

3.1. Fiber properties and choice of pretreatment 309

Tensile strength (TS) was chosen as one of the parame- 310

ters to assess the performance of the treated and untreated 311

fibers taking into account the maximum stress observed 312 along with Young's modulus (E); which is a measure of 313

the tensile stiffness of the fibers under service loads and is 314

experimentally determined from the slope of a stress-strain 315

curve. 316

In order to obtain meaningful comparison of the various 317

fibers, both the strength and stiffness parameters were con- 318

sidered together. Therefore, a straightforward scatter plot 319

in Fig. 1 indicates the relation between the TS and E. Results 320

fell between the curves: 40.00*TS < E (MPa) < 78.13*TS. 321 Moreover, it is seen from the figure that the majority of 322

the points fall close to a single center line, represented by 323 E«56*TS. Also, it can be observed that the data for Q8 324

0.173% Ca(OH)2 treated fibers display closer correlation 325

with less variation compared to the aforesaid trend. 326

It can be inferred that the tensile strengths and stiffness 327

properties of the fibers are generally better for those treated 328

with Ca(OH)2 than those treated with NaOH. Further- 329 more, Fig. 2 presents the stress-strain profiles for untreated Q9 330

and treated using 0.173% Ca(OH)2, 0.173% NaOH and 331

2.0% NaOH bundled fibers. It is evident that treated fibers 332

experienced an overall loss in stress resistance when com- 333

pared to the untreated fibers. 334

It is also observed that the stress-strain curves for 335

0.173% Ca(OH)2 treated fibers displayed lesser variation 336

and more consistency in comparison with the untreated 337

fibers and NaOH samples treated at different concentra- 338 tions. Furthermore, changing NaOH concentration from 339

2.0% to 0.173%; do make slight favorable difference in 340

the tensile behavior of the bundled fibers. 341

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4,000 -

2,000 -

1,000 -

A Untreated Fibers ^ A O J /

□ Treated with 2.0% NaOH o Treated with 0.173% NaOH A ^ /

0— * O A a ^ oaao ^ o n __<

O Treated with 0.173% Ca(OH)2

□ □ / — A o <x> □ 0 A O

/ / / /

/ / / /

30.0 45.0 60.0

Tensile Strength - TS (MPa)

Fig. 1. Tensile Strength (TS) versus Young's Modulus (E) relation for fibers with and without various treatments.

342 Hence, it was decided to pursue only 0.173% Ca(OH)2 Also, since cement hydration releases calcium hydroxide it 345

343 treatment for further investigations with mortar mixes, is unlikely that fibers already treated specimens will 346

344 because the fibers displayed more or less uniform behavior. undergo further structural degradation.

N.G. Ozerkan et al. /International Journal of Sustainable Built Environment xxx (2014) xxx-xxx

Table 3

Setting time test results of mortar samples.

Mix No. Fiber Inclusion (wt.%) Average setting times (h) Initial Final

1(Control) 0.0 3:00 4:00

2 0.5 3:15 3:55

3 1.0 4:25 5:00

4 2.0 2:20 2:45

348 3.2. Physical properties of the fresh and hardened mortars

349 The physical properties of fresh and hardened mortars

350 were evaluated by performing the setting time, drying

351 shrinkage, water absorption and sorptivity tests.

352 Table 3 shows the average setting times of the various

353 mortar mixes tested. The overall trend seems to suggest

354 that the setting times are prolonged with the increase in

355 the fiber content. Although, with 2.0% of fiber inclusion,

356 the setting time decreased; which could be due to water

357 being absorbed by the excess fibers per se thereby making

358 the mix set quickly.

359 Fig. 3 represents the results of drying shrinkage test for

360 each mortar mix prepared in the study. It can be seen that

361 all mixes including different palm fiber ratios show different

362 shrinkage behavior. The rate of increase in shrinkage for all

363 mortar, except the mixture including 2.0% palm fiber, was

364 high up to 18 days which is a result of curing process, and

365 after 18 days it can be noticed that palm fibers have effect in

366 reducing shrinkage. Moreover, it can be observed from the

367 figure that the mortars reinforced with 2.0% palm fiber,

368 which is the maximum ratio tested in this study, has the

369 least shrinkage. This finding is in agreement with findings

370 of Singh et al. (2010) which showed that a higher volume

371 fraction, 4% of oil palm trunk fiber reduces shrinkage for

372 different core diameters.

373 Water absorption and sorptivity test results for mortar

374 mixes are presented in Figs. 4 and 5, respectively. The level 375Q10 of accuracy is shown in the figures by the error bars and

376 was generally similar for all mixes with average and maxi-

377 mum values of 5.86% and 9.23% respectively. These figures

378 indicate, that the mortar reinforced with 0.5% palm fiber

Fig. 4. Effect of palm fiber ratio on water absorption capacity of mortar samples.

Fig. 3. Effect of palm fiber ratio on drying shrinkage.

i Fiber Ratio I

Fig. 5. Effect of palm fiber ratio on sorptivity of mortar samples.

has the highest water absorption capacity and water 379

absorption rate whereas the lowest water absorption capac- 380

ity and water absorption rate are given by the reference 381

sample. For the mortar mixes reinforced with 1.0% and 382

2.0% palm fiber (i.e. mixes 3 and 4 respectively), it can be 383

observed that the water absorption rate and capacity 384

decreased with increasing weight percent of palm fiber. 385

Although, these results differ from some published studies 386

(Aggarwal, 1995; Abdullah et al., 2011; Abdullah et al., 387

2013), they are consistent with those of other studies 388

(Ghavami, 1995; Filho et al., 2003; Bilba and Arsene, 389

2008; Savastano et al., 2001; Juarez et al., 2007; Parveen 390

et al., 2012) and suggest that alkali treatment applied on 391

natural fibers reduces the water absorption capacity by 392

removing hemicellulose and lignin or by imparting 393

hydrophobicity. 394

3.3. Mechanical testing 395

The mechanical test results including compressive 396

strength, flexural strength and split tensile strength of 397

mortars are presented in Figs. 6 and 7. For compression 398

test, two types of samples were tested viz. 5 x 5 x 5 cm 399

cubic samples and 10 x 20 cm cylindrical samples. 400

As seen in these figures, the compressive strength is 401

slightly increased in value with low fiber content as 402

compared with the control mix. High compaction between 403

the fibers and the cement matrix was likely achieved 404

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Fig. 6. The effect of palm fiber ratio on the compressive strength for cube mortar samples (the level of accuracy ranged between 3.53% and 15.22%).

Fig. 9. The effect of palm fiber ratio on split tensile strength of mortar samples after 7 and 28 days (the level of accuracy varied between 3.28% and 17.86%).

Palm Fiber Ratio (%) 3l

Fig. 7. The effect of palm fiber ratio on the compressive strength of

cylinder mortar samples (the level of accuracy ranged between 3.53% and 0.5

15.22%).

leading to good homogeneity in mix with 0.5% fiber inclusion, and this finding corroborates with the findings of Ismail (2005) who showed that the compressive strength and bulk density slightly improved with low fiber content in the range of 0.3-1.5%. However, if the fiber content exceeds the value of 0.5%, the compressive strength of mortar samples decreases which also seems to be consistent with other studies (Shimizu and Jorillo Jr., 1992; Sorovshian and Khan, 1992; Islam et al., 2011; Awwad et al., 2011).

Fig. 8 represents the results of flexural strength test performed on 4 x 4 x 16 cm prism mortar samples. The estimated error ranged between 1.80% and 11.72%. The results corroborate the findings of compressive strength test results, i.e. flexural strength increases with lower fiber content, 0.5% and 1% as compared with the control mix, and decreases with higher fiber content.

-♦-■Control Mix —■— 0.5% Palm Hber

—A— 1% Palm Hber —O— 2% Palm Hber

/' \ X--A

i L j

J ^ 'Vr--1 " \ "-Il

MJA^IJ

0 7 14 21 28 56 91 105 120 Immersion Period (days)

Fig. 10. Length change due to sulfate attack vs. immersion period.

The average splitting tensile strength at 7 and 28 days is shown in Fig. 9. Trend similar to compressive strength results was observed in this case. These results are consistent with those reported by other researchers which found that splitting tensile and flexural strength increases up to 1% of natural fiber volume (Ahmad and Ibrahim, 2010; Dawood and Ramli, 2011; Ahmad and Nurazuwa, 2008).

3.4. Durability studies

Fig. 10 presents the results of resistance against sulfate attack of conventional control mortar and palm fiber reinforced mortar samples in different ratios. The results are based on the average values obtained from six test specimens per mix. The highest length change at 120 days of immersion is observed in the control mix as well as mortar reinforced with low percentage of palm fiber (i.e. 0.5%); while, mixes with 1.0% and 2.0% appear to be more resilient to length changes. Hence, it can be inferred that higher inclusion of palm fibers within mortars offers better long term durability performance and advantage in resisting sulfate attack.

3.5. Microstructure examinations

Fig. 11 depicts some of the SEM micrographs obtained from this study which sheds some light on the tensile failure

Days 0 28 Days

0.0 0.5 1.0 2.0 Palm Fiber Ratio (°/o)

Fig. 8. The effect of palm fiber ratio on flexural strength of mortar samples.

8 N.G. Ozerkan et al. / International Journal of Sustainable Built Environment xxx (2014) xxx-xxx

Untested Failed sample undergone

Sample type tensile testing

_ H [4f w Kw

r- ' J*

. _ „1»^.: Ji B ÂMSiMi

ê'PÎÉmtr iÛ vil

Untreated bundled fiber MMfiftr iWJ jjK -4m^m m f ImfâjL iilMI 4/28/2013 mag WD HV del mode -200 pm-

0.173% Ca(OH)2 treated y ' M UmU jé / AwfnPÊ^

bundled fiber fil! -'M'iMïà

MwlldiTMIFl rT IiMIIILr.ki'iJy.....„!,-■ .......... Il Imi .•..■ ' ■ »vu.™

0.173% NaOH treated il • vw ■ !■ . M"

bundled fiber BB j^f ^ r1 J 4/28/2013 mai WD HV del mode - 200 pm —

mJIOIVM IT 1 IMII ,1. ,-„■. ',!•, ,■

__ 1 )

2.0% NaOH treated bundled 1

fiber H man WT1 HV tint mnrte -XIO 11m-

IL-LMinilllMNIIHII M

Fig. 11. Typical SEM micrographs for representative untreated and treated bundled fibers at x500 magnification.

445 behavior of the treated and untreated bundled fibers. It can

446 be observed that as the concentration of the treatment

447 increased (i.e. 2.0% NaOH), the fibers became more brittle

448 and this agrees with the tensile stress-strain profiles

449 reported in the previous sections. Also, while comparing

450 the failure morphologies of the 0.173% Ca(OH)2 vs. the

451 0.173% NaOH, the contribution of the individual bundle

452 fibers in resisting the tension is more conspicuous in the

former samples; a behavior that is considered favorable 453

for mortar and concrete applications. This supports our 454

choice of 0.173% Ca(OH)2 treatment for mortar studies. 455

Typical SEM micrographs from the mortar mix cured 456

up to 28 days have been presented in Fig. 12. The abun- 457

dance of fiber bundles is observed to increase progressively 458

with their inclusion in the mixes. Moreover, the treated 459

fibers appear to remain intact within the cement matrix. 460

N.G. Ozerkan et al. /International Journal of Sustainable Built Environment xxx (2014) xxx-xxx 9

Fig. 12. Scanning electron micrographs of the 28-day mortar mixes (with and without fiber inclusion) at two different magnifications.

461 Also, the dense morphology of the CSH gel which is the

462 product of complete hydration of cement is visible in the

463 images indicating that the hydration progressed normally

464 despite the inclusion of the fibers in the mixes.

465 X-ray diffraction of the mortar mixes cured up to

466 28 days has been compared in Fig. 13. Although some

467 peaks of early hydration products such as portlandite (p),

ettringite (e) and gypsum (g) were observed, their relative 468

intensities were significantly low again supporting the 469

above argument that the treated fibers remained within 470

the cement matrix without hindering the hydration 471

processes. Calcite (c) peaks were observed which can be 472

attributed to carbonation while the conspicuous peaks of 473

quartz (q) can be attributed to the sand (i.e. fine 474

N.G. Ozerkan et al. /International Journal of Sustainable Built Environment xxx (2014) xxx-xxx

Fig. 13. X-ray diffractograms for mortar mixes (with and without fiber inclusion) at 28 days.

Angular Range (

Fig. 14. X-ray diffractograms for mortar mixes (with and without fiber inclusion) subjected to sulfate attack at 120 days.

475 aggregates) present in the mortar mixes. Hence, although

476 the preferential orientation/crystallinity was slightly differ-

477 ent, no phase change was observed between the mixes.

478 In contrast, X-ray diffraction of the mortar mixes sub-

479 jected to sulfate attack was carried out and compared in

480 Fig. 14. The samples collected for this analysis were

481 obtained from mix specimens aged up to 120 days while 482Q11 being immersed in sodium sulfate solution. In addition to

483 some of the phases reported earlier, presence of ettringite

484 (e) and strong peaks of gypsum (g) especially at 11.6° 20

485 were conspicuous which, are typical products formed as a

486 result of sodium sulfate attack in cement mortars (Prasad

487 et al., 2006). Weak peaks of portlandite (p) were also pres-

488 ent; which could imply the possibility of conversion of

489 calcium hydroxide into the aforesaid sulfate products.

490 However, the overall trend seems to suggest that with

491 increased fiber inclusion in the mixes, the intensities of

492 products of sulfate attack became low thereby implying

493 that such mixes offer better resistance to sulfate attack.

494 This is in agreement with the previous observations that

495 mixes 1.0% and 2.0% fiber inclusion are more resilient to

496 length changes.

497 Nevertheless, it is recommended to investigate exposure

498 times much longer than 120 days as well as higher sodium

499 sulfate concentrations to better understand the deteriorating

effects on the mortar mixes as well as the influence of date 500

palm fibers on improving the durability. 501

4. Conclusions 502

The tensile performance of Portland cement mortars 503

and concrete had been reported to be enhanced via the 504

inclusion of certain fibers in sufficient quantities. Natural 505

fibers such as those obtained are known to exhibit high 506

tensile strength and a low modulus of elasticity despite high 507

variation. Date palm trees are native to the middle-east 508

region; their fibers can be easily and abundantly found in 509

countries like the State of Qatar. 510

The results in this paper are based on laboratory exper- 511

iments with four mortar mixes reinforced with varying 512

percentages of treated natural date palm fiber up to 513

2.0%. For fiber inclusions greater than 2.0%, poor work- 514

ability as well as difficulty in preparing and obtaining 515

monolithic compacted samples due to excessive separation 516 of mortar components was encountered. Hence, it is rec- Q12517

ommended that such date palm fiber inclusion in mortars 518

has to be limited to less than 2.0% by weight. 519

Fibers treated with 0.173% Ca(OH)2 displayed better 520

tensile strengths and stiffness properties than those treated 521

with NaOH and hence, the former was chosen as preferred 522

N.G. Ozerkan et al. /International Journal of Sustainable Built Environment xxx (2014) xxx-xxx 11

523 pre-treatment for the bundled fibers used in the mortar

524 mixes. This was further supported by the microstructural

525 examination on the hardened mortars which, revealed that

526 the integrity of treated fibers remained intact within the

527 cement matrix without hindering the hydration processes.

528 The setting time of the mortar pastes was observed to be

529 prolonged with the increase in the fiber content. Water

530 absorption rate and capacity decreased with increasing

531 weight percent of palm fiber in the mixes. Results also indi-

532 cated that inclusion of fibers improves the flexural

533 strengths. However, the cylinder and cube compressive

534 strengths decreased with the increase in treated fiber

535 inclusion.

536 Nevertheless, the results of the durability performance

537 of the studied mixes clearly suggest that incorporation of

538 1.0-2.0% of date palm fibers improved the resistance of

539 the mortar against sulfate attack. Also, the drying shrink-

540 age performance of the mixes improved with increased

541 fiber inclusion.

542 Based on this study, it can be concluded that inclusion

543 of treated palm fibers in cement mortars do offer flexural

544 strengths and durability performance improvements. How-

545 ever, these advantages come as a trade-off in the form of

546 initial loss in workability and subsequent poor compressive

547 strengths.

548 Although, the work reported in this paper was carried

549 out on cement mortars, conclusions are expected to be rel-

550 evant to fiber reinforced concrete employing treated natu-

551 ral fibers. Yet, for practical purposes, it is recommended

552 to conduct thorough feasibility studies on the use of such

553 natural fibers based on the actual application as well as

554 the desired final properties of the resulting cement

555 composites.

556 5. Uncited reference

557Q13 ACI Committee 544 (2002).

558 Acknowledgements

559 The joint initiative and financial backing of the Qatar

560 University (QU), Texas A&M University at Qatar (TAM-

561 UQ) and Qatar Energy & Environment Research Institute

562 (QEERI) for this collaborative study is duly acknowledged.

563 In particular, the authors are grateful to Dr. Eyad Masad

564 (TAMUQ), Dr. Mariam Ali (QU) and Dr. Fahhad Alharbi

565 (QEERI) for their strong support.

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