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Durability of chemically modified sisal fibre in cement-based composites
Construction and Building Materials 241 (2020) 117835
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Durability of chemically modified sisal fibre in cement-based composites M.D. de Klerk a, M. Kayondo a, G.M. Moelich a, W.I. de Villiers a, R. Combrinck a, W.P. Boshoff b,⇑ a b
Department of Civil Engineering, Stellenbosch University, South Africa Department of Civil Engineering, University of Pretoria, South Africa
h i g h l i g h t s Durability of sisal fibres in cement-based systems. Chemical treatment of sisal fibres. Single fibre pull-out tests on sisal fibres. Ageing tests and performance of fibres.
a r t i c l e
i n f o
Article history: Received 30 April 2019 Received in revised form 5 December 2019 Accepted 9 December 2019
Keywords: Sisal fibre Fibre reinforced concrete FRC Durability Treatment of fibres Mechanical behaviour Single fibre pull-out Mechanical properties Alkali treatment Acetylation
a b s t r a c t The degradation of sisal fibre in a cement-based matrix due to the highly alkaline environment reduces the strength of the composite. In this research, to avert this degradation, alkaline treatment and acetylation were respectively performed with sodium hydroxide (NaOH) and Acetic Acid or Acetic Anhydride to improve the resistance of the fibre to alkaline attack. In addition, single fibre pull-out (SFP) tests were performed to evaluate the influence of chemical treatment on fibre strength, fibre-matrix interaction and also determine the critical fibre length. Specimens were tested in indirect tension (flexure) at 28 days to determine the strength of the composite. Additional ageing tests by extended curing in water at 24 °C, lime saturated hot water at 70 °C, and alternate cycles of wetting and drying were done. Aged samples were further tested at 90 days to evaluate the durability of the fibre. It was found that a combination of alkali treatment and acetylation was the most effective treatment condition, followed by that of alkali treatment at low concentrations of sodium hydroxide. At higher concentrations of sodium hydroxide, a significant reduction in strength was observed. Chemical treatment improves the durability of sisal fibres in concrete, albeit slight degradation still occurs. Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction The global drive towards achieving sustainable concrete infrastructure has led to a concerted effort in the use of various natural fibres in cement-based composites [1]. This is evidenced by the growing literature in this direction [1–6]. Fibre reinforced cement-based composites (FRCC) are widely utilised in several applications in the concrete industry, especially due to their high strength, light weight, and ductile failure mechanisms [5]. Most FRCC are made using synthetic or steel fibres due to their exceptional properties within the cement matrix. The possibility of replacing synthetic and steel fibres in concrete with natural fibres such as sisal, hemp, jute or flax is one of the many areas of research into renewable building materials [7–9]. Natural fibres such as ⇑ Corresponding Author. E-mail address: [email protected] (W.P. Boshoff). https://doi.org/10.1016/j.conbuildmat.2019.117835 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.
sisal are largely available in tropical regions and fit into the sustainable materials category [10]. Some of these natural fibres have tensile strengths in excess of those of polypropylene (PP) fibres and are comparable to polyvinyl acetate (PVA) fibres, and exhibit performance similar to composites of synthetic or steel fibres [11]. Sisal fibre made from the Agave Sisalana plant, one of the most widely used natural fibres, has largely been studied for inclusion in cement-based composites [12–15]. Its abundance and excellent mechanical properties rank it highly among the available natural fibres for use in the construction industry [3,16]. Sisal fibre is low cost, has a high tensile strength, abrasion resistance and toughness, no health-related risk, as well as good thermal and acoustic properties [1,17,18]. This is in addition to being biodegradable and renewable. The mechanical properties of sisal fibre are generally dependent on factors such as the origin of the fibre, fibre orientation, fibre dispersion, type of matrix used, and the bonding between fibre and the matrix [17].
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The use of sisal fibres in cement-based composites enhances the overall mechanical properties of the composites [16]. This material property enhancement largely depends on the critical fibre length, orientation, and fibre cellular make-up. However, despite the several outstanding advantages of sisal fibre, a number of studies have shown that the durability of sisal fibre in cement-based composites is of concern, since the fibres tend to degrade in an alkaline environment [3,10,19–21]. Cement-based composites have high alkalinity resulting from the calcium hydroxide, Ca(OH)2, formed during the hydration of cement, and this presents a corrosive environment for the sisal fibre [22,23]. This degradation (due to alkaline hydrolysis which leads to loss of reinforcing capacity) is what compromises the performance of most natural fibres in cement-based composites. Additionally, at high concentrations of Ca(OH)2, delignification is known to take place and since lignin is what binds the fibres together, absence of lignin leads to defibrillation [2,24–26]. Other flaws of sisal fibre are the low combustion temperature, and poor fibre/matrix adhesion (attributed to the hydrophilic characteristics of natural fibre) [17,27,28]. Despite some of the challenges regarding natural fibre, sisal fibre utilisation in cement-based composites has attracted research with the sole aim of enhancing the material properties of the natural fibre to make it viable for use in cement-based composites [1– 6,12–15]. Techniques such as chemical pre-treatment (such as with sodium hydroxide, silane, acetic acid, and others) have been used to enhance the fibre properties such as bond strength through removal of impurities (fats, waxes, and mineral matter) from the fibre surface [12,29,30]. In this research, two different chemical treatment methods, alkali treatment and acetylation, were performed on sisal fibre. Alkali treatment was performed with sodium hydroxide (NaOH) at concentrations of 2, 6, 10, 20 and 30%, whereas Acetylation was performed with acetic acid (CH3COOH) and acetic anhydride ((CH3CO)2O), both at concentrations of 5 and 10%. A combination of alkali treatment and acetylation was performed by treating fibres pre-treated with 6% NaOH with 5 and 10% acetic anhydride respectively. The improvement of fibre mechanical properties is dependent on both the chemical concentration and exposure periods of the chemical treatment [14]. Longer exposure periods or larger than critical chemical concentration dosages lead to deterioration of the mechanical properties of the fibre. In this research, the viability of sisal fibre in cement-based composites, as a more sustainable building material, was further investigated by reinforcing a cement-based matrix with 1% sisal fibre (by volume) cut to a length of 20 mm. The challenges associated with the durability of natural sisal fibre in an alkaline environment were the main focus of the study thereafter. The experiments conducted and results achieved are discussed in the following sections. 2. Experimental procedure 2.1. Materials The mortar matrix consisted of Ordinary Portland cement (OPC) CEM I and a natural quarry sand, locally known as Malmesbury sand. In addition to fibres, a modified acrylic polymer was used as super-plasticiser and added to maintain workability of the mortar matrix. The sisal fibre was supplied by Rebtex RSA. The specific origin of the fibre is unknown. Sisal fibres were received in bundles, already washed and combed. Using SEM images of six fibres, the average diameter was found to be 260 mm.
then re-combed using a nail comb to remove any entanglement. Untreated fibres were stored in a conditioning chamber at a temperature of 23 ± 2 °C and relative humidity of 65 ± 2%. Two different fibre treatments, namely alkali treatment and acetylation were performed. In this study, 100% concentration is equivalent to one molar. 2.2.1. Alkali treatment Alkali treatment was performed using 2, 6, 10, 20 and 30% NaOH solutions. These concentrations were used since the highest crystallinity index reported by Mwaikambo and Ansell [31] was for fibres treated in a 6% NaOH solution. Higher crystallinity is said to result in stronger and stiffer fibres [31]. Fibres were soaked in the different NaOH concentrations for 48 h in a conditioning chamber, after which they were rinsed in 1% acetic acid to neutralise excessive NaOH and washed in water to remove excess acid from the fibre surfaces. They were then spread out in the conditioning chamber and left to dry over a period of 2–3 days until constant weight measurements were attained. 2.2.2. Acetylation Acetylation was performed using Acetic Acid and Acetic Anhydride, at concentrations of 5 and 10% for both. In addition, a combination of alkali treatment and acetylation was used. Fibres were soaked in Acetic Acid and Acetic Anhydride respectively for 1 h. Fibres, pre-treated with 6% NaOH, were also soaked in 5 and 10% Acetic Anhydride for 1 h. The fibres were removed from the solution and washed in water to remove excess acid from the fibre surface. The fibres were then spread out in the conditioning chamber and left to dry for a period of 2–3 days until constant weight was reached. This treatment procedure is similar to that by Reis [32], except that a hot hair oven was used to dry the treated sisal fibres. 2.3. Specimen production 2.3.1. Mix design and procedure A water/cement ratio of 0.6, with a cement/sand ratio of 0.28, was used for all the specimens. The sand grading is shown in Fig. 1. The workability and consistency were evaluated by the slump flow test performed in accordance to the flow test described in ASTM C1437-07 [33]. A 10-litre pan mixer was used to mix mortar for the single fibre pull-out (SFP) specimens. The mixing pan was initially saturated with water and towel dried before adding the materials. Dry ingredients were first mixed for 30 s, after which water was added and additional mixing of 2 min done. After the matrix was mixed in the 10-litre pan mixer, a 5 L mix, containing 1% fibres by volume, was re-mixed in a 25-litre pan mixer to determine the amount of super-plasticiser required to obtain the same workability as for a mix without fibres. It was found that 1% super-plasticiser (by mass of cementitious material) was sufficient, to avoid material segregation. The compressive, uni-axial tensile and flexural strengths of the matrix were determined to be 27.5 MPa, 2.6 MPa and 5.6 MPa respectively. The fibre volume ratio and length were optimised experimentally by De Klerk and Boshoff [34]. The critical fibre length was determined by analysing the pull-out lengths. For the various treatment conditions, the pull-out length was 10 mm on average. Therefore, a 20 mm fibre was selected to ensure sufficient anchorage. Fibre volume ratios of 0.5, 1, 1.5 and 2% were evaluated by comparing the flexural post-peak strength results. At a fibre volume of 1%, the flexural post-peak strength was the highest for a fibre length of 20 mm.
2.2. Fibre preparation Bundled fibres were separated into smaller bundles before being cut to required lengths using a paper guillotine. Fibres were
2.3.2. Single fibre pull-out (SFP) specimens Single fibre pull-out specimens were cast in a mould similar to the one used by Boshoff et al., [35]. The length, width and depth of
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Fig. 1. The sieve grading of the sand used.
the mould were 200, 50 and 10 mm respectively. Fibre strands were secured to a frame, spaced at a distance of 10 mm between strands. The mould was then filled halfway, after which the frame with the fibre strands were secured on top of the mortar and the rest of the mould was filled. The specimen was vibrated for 60 s to compact and spread the mortar evenly. Fig. 2 (a–d) shows the detail of the specimen preparation. After one day of curing at room temperature, the specimen was demoulded and cured in water at 24 ± 2 °C until 28 days from the date of casting. At 28 days, the specimen was dry-cut into smaller samples (50 10 10 mm), with a single cast-in fibre protruding on both sides (see Fig. 2 c and d), using a diamond blade cutter. As the fibre was embedded along the 50 mm length of the sample, the sample was simply cut shorter to obtain the desired embedment length.
interlock between the fibres can cause fibre balls which are not separated during the mixing process. Adding the fibres took approximately 10–15 min, after which super-plasticiser followed, and the mixing continued for another four minutes to allow the super-plasticiser to work effectively. The total mixing time was approximately 15–20 min. For flexure, specimens of 300x70x15 mm were obtained by cutting in half bigger specimens cast in 600x70x15 mm beam moulds. After mixing, the fresh mortar was placed in the moulds and vibrated on a vibrating table for two minutes. The surface of the specimen was smoothed using a trowel. After two days at room temperature, the specimens were demoulded and cured in water at 24 ± 2 °C until 28 days from the date of casting. The 600 mm beams were then cut in half to 300 mm beams that were used for flexure.
2.3.3. Flexural test specimens A 50-litre pan mixer was used for preparation of specimens for flexure testing. The addition of fibres was done slowly and by hand, while continuously separating them to ensure that no fibre balling took place. If the fibres are added too quickly, the friction and
2.4. Ageing methods In order to evaluate the durability of SFRCC, specimens for flexure were aged and tested at 90 days. The strength was compared to the reference SFRCC specimens tested at 28 days, both treated and
Fig. 2. Single fibre pull-out specimen preparation: a) Fibre strands secured to a frame at 10 mm spacing; b) Mould filled with mortar; c) Specimen cut into smaller samples; d) Single specimen ready for fibre pull-out test.
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untreated. After 28 days of water curing at 24 ± 2 °C and testing of the reference specimens, other specimens were subjected to three different ageing conditions. One set of specimens was placed in a hot water curing tank saturated with lime and kept at a constant temperature of 70 ± 2 °C. This was done according to ASTM C1560-03 [36]. The addition of lime simulates an alkali-based environment that accelerates the degradation of the sisal fibre in the matrix. Another set of specimens were subjected to alternate cycles of wetting and drying. The specimens were dried in a conditioning chamber for 4 days at 23 ± 2 °C and 65 ± 2% relative humidity (RH), and then saturated in water at 24 ± 2 °C for 3 days to make one cycle of 7 days. This cycle was repeated until 90 days. This ageing condition was used to simulate natural weathering. The third set of specimens was left in the curing tank at 24 ± 2 °C up to 90 days, to determine whether degradation takes place under normal curing conditions. Table 1 shows a summary of the curing and ageing conditions adopted in this investigation.
were continued for shorter embedment lengths. The average pull-out force of six specimens (of the fibre from the matrix) per millimetre of embedded fibre was calculated, as well as the fracture force based on the prevalent failure mechanism.
2.5. Test methods
ry ¼
2.5.1. Scanning electron microscope Scanning Electron Microscope (SEM) images of the fibre were taken using a LEO 1450 VP SEM. The images were used to study the effect of chemical treatment on the fibre appearance as well as the fibre structure and appearance after fibre fracture and pull-out.
2.5.3. Flexural tests Flexural tests were performed on 300 mm beams in a threepoint bending setup to evaluate the flexural strength of the matrix and to study the influence of the fibres on the post-peak (after cracking) behaviour of the specimens. The tests were also performed using the Zwick Z250. A span of 250 mm was used. The beams were loaded at a displacement rate of 5 mm/min. The tests were stopped after a displacement of 10 mm. For most of the series, six specimens were tested. The load and displacement was recorded and represented in graphs of flexural stress of the beam versus the crosshead displacement of the material testing machine. The ultimate flexural stress, ry, is calculated using the following equation:
3PL 2bd
2
ð1Þ
where, P is the applied load in Newton (N), L is the support separation, in this case 250 mm, b is the width of the specimen, in this case 70 mm, d is the thickness of the specimen in mm, in this case 15 mm. 2.6. ANOVA analysis
2.5.2. Single fibre pull-out Single fibre pull-out tests were performed to gain an understanding of the fibre-matrix interaction, the interfacial bond strength and the effect of chemical treatment on the bond strength as well as to determine the critical lengths for the fibres subjected to different treatments. Fibre pull-out tests were performed using a Zwick Z250 material testing machine. The bottom part of the specimen (mortar) was clamped, while the fibre was fixed with superglue to an aluminium surface attached to a Hottinger Baldwin Messtechnik (HBM) 10 N load cell as shown in Fig. 3. The clamp consisted of two plates which were adjustable by bolts. The two plates and the load cell were clamped in the grips of the machine. A tensile force was applied to the specimen at a constant displacement rate of 0.1 mm/s. A linear variable differential transformer (LVDT) was fixed across the clamps of the machine to record the displacement and thus the pull-out of the fibre. The load and displacement were logged using a HBM Spider8 data acquisition system. An initial maximum embedment depth of 50 mm, allowed by the size of the specimen, was used. The fibres were tested starting at embedment depths of 50, 40, 30, 20, 15 and 10 mm respectively. If fibre pull-out was the prevalent failure mechanism at a certain embedment depth, the tests were not continued for shorter embedment depths. Likewise, if fracture occurred, pull-out tests
Table 1 Curing and ageing conditions for test specimen series. Set/Time
Reference Set 1st Set 2nd Set 3rd Set
Curing/Ageing conditions 28 days
Up to 90 days
water curing at 24 ± 2 °C water curing at 24 ± 2 °C water curing at 24 ± 2 °C water curing at 24 ± 2 °C
– Hot lime water curing at 70 ± 2 °C 4 days drying at 23 ± 2 °C and RH 65 ± 2%; water at 24 ± 2 °C for 3 days; (7-day cycle) water curing at 24 ± 2 °C
To account for the large variability of natural fibres, a one-way Analysis of Variation (ANOVA) was conducted. The analysis determines if treating the fibre made a statistically significant difference, at a significance level of 0.05, in both the single fibre pullout and flexural test results. The results were used to compare the untreated to treated fibres as well as to compare the treated fibres to each other. Although the main conclusions were made on statistically significant results, trends were also considered when the results were evaluated. More on an ANOVA can be found elsewhere [37]. 3. Results 3.1. Single fibre pull-out In order to understand some of the results of the fibre pull-out tests, SEM images of the fibres under the different surface treatment approaches have been presented first. Figs. 4–9 show the surface observations for the different fibres before and after the treatment variations. Figs. 4 and 7 show an untreated fibre surface with some impurities, which are observed to be significantly reduced after the treatment process in Fig. 5. The effect of higher dosage of treatment leading to damage as reported by [1,24] are revealed in Figs. 6 and 8. Graphs of the average pull-out force (in N/mm) and fracture force (in N) for the different treatment conditions are presented in Figs. 10–13. The average pull-out force per unit length (in N/mm) was calculated (from six specimens) as the pull-out force (in N) divided by the original embedment depth. This computation assumes a uniform bond shear stress over the length of the embedded part of the fibre. The diameters of the individual fibres are not constant and are difficult to measure, therefore the pullout and fracture force are not expressed as an interfacial shear stress and tensile stress respectively. The error bars on the figures indicate the minimum and maximum force for the specific series. The ANOVA result of the pull-out force test is seen in Tables 2
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Fig. 3. Test set-up for single fibre pull-out tests. The protruding fibre is enhanced in red. Embedded lengths of 50, 40, 30, 20, 15 and 10 mm were tested. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. SEM image of untreated fibre showing surface impurities present (in red circles). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. SEM image of 20% NaOH treated fibre showing damage due to higher dosage of treatment.
Fig. 5. SEM image of 10% NaOH treated fibre, with significantly less impurities than the untreated fibre.
Fig. 7. SEM image of 5% Acetic Acid treated fibre revealing some surface impurities.
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Fig. 8. SEM image of 5% Acetic Anhydride treated fibre showing signs of delignification.
Fig. 9. SEM image of 6% NaOH-5% Acetic Anhydride treated fibre.
and 3. The ANOVA results of the fracture force test is seen in Table 4 and 5. Fig. 10 and Table 2 show that at a NaOH concentration of 2%, treated fibres have a statistically significant higher pull-out strength than the untreated fibre, while at higher concentrations of 20 and 30% NaOH, fibres had a lower pull-out strength. This differential performance is attributed to the surface differences arising from the different treatment approaches leading to improved
bond strength [12]. The low pull-out resistance observed in this research for fibres treated with high concentrations of NaOH is due to the damage to fibres at such alkaline concentrations [24–26]. The untreated and 20% NaOH fibres pulled out at an embedment depth of 15 mm, while the other alkali treated fibres pulled out at 10 mm and had a combination of pull-out and fracture at 15 mm. The increased resistance of the low concentrations NaOH fibres at a smaller embedment depth indicates an improved bond between the fibre and the matrix, due to the removal of surface impurities through alkali treatment. This is further confirmed through observation of the SEM images as in Figs. 4–6 after fibre treatment. Higher dosages of treatment with NaOH however showed declining resistance compared to the untreated, and this is due to delignification as confirmed by the damage visible in Fig. 6. For the untreated fibre, impurities are visible on the surface and encircled in red, whereas for the 10% and 20% NaOH, significantly less impurities are visible. The average pull-out force for the acetylated fibres is presented in Fig. 11 with the ANOVA results presented in Table 3. The resistance of the 5 and 10% Acetic Acid fibres to pull-out is much lower than the untreated and other acetylated fibres. The fibres treated with Acetic Acid pulled out at embedment depths of 15 mm and 20 mm and also fractured at 20 mm, while the fibres treated with Acetic Anhydride pulled out at 10 mm and fractured at 15 mm. A number of fibres pre-treated with 6% NaOH and then with 10% Acetic Anhydride pulled out at 15 mm, but at a higher force than the Acetic Acid specimens, indicating that the 6% NaOH-10% Acetic Anhydride had the highest resistance to pull-out. The higher average pull-out force and the smaller embedment depth at which it pulled out, indicates a stronger bond between the fibres and the matrix for the fibres treated with Acetic Anhydride than for the fibres treated with Acetic Acid. The fibres treated with 6% NaOH10% Acetic Anhydride pulled out at 15 mm while all the other fibres treated with Acetic Anhydride pulled out at 10 mm. In comparison, the fibres treated with only 2% NaOH had the highest resistance to pull-out of all the fibres, followed by fibres treated with 6% NaOH. Pre-treatment with NaOH was more effective in removing surface impurities. This is confirmed by the slightly higher average pull-out force of the 6% NaOH-5% Acetic Anhydride compared to the 5% Acetic Anhydride and also of the 6% NaOH-10% Acetic Anhydride compared to the 10% Acetic Anhydride. Acetic Anhydride improved the bond strength of the untreated fibre, but reduced the bond strength of the 6% NaOH treated fibre. Acetylation was not as effective as alkali treatment in removing surface impurities. This can be seen from the SEM images presented in Figs. 7 and 8, which show surface impurities on both the fibres treated with Acetic Acid and Acetic Anhydride. The fibre pre-treated with 6% NaOH, presented in Fig. 9, however, had a significantly cleaner surface.
Fig. 10. Average pull-out force for alkali treated fibres.
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Fig. 11. Average pull-out force for acetylated fibres.
Fig. 12. Average fracture force for alkali treated fibres.
Fig. 13. Average fracture force for acetylated fibres.
Table 2 Pull-out force ANOVA p-value results for alkali treated fibres. Sample
Mean
2% NaOH
6% NaOH
10%NaOH
20% NaOH
30% NaOH
Untreated 2% NaOH 6% NaOH 10%NaOH 20% NaOH 30% NaOH
0.420 0.697 0.596 0.565 0.287 0.363
0 –
0.052 0.986 –
0.262 0.916 1 –
0.024 0 0.015 0.044 –
0.259 0.014 0.245 0.445 0.997 –
sym.
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Table 3 Pull-out force ANOVA p-value results for acetylated fibres. Sample
Mean
5% A.Acid
10% A.Acid
5% A.Anh
10% A.Anh
6% NaOH, 5% A.Anh
6% NaOH, 10% A.Anh
Untreated 5% A.Acid 10% A.Acid 5% A.Anh 10% A.Anh 6% NaOH, 5% A.Anh 6% NaOH, 10% A.Anh
0.42 0.171 0.166 0.454 0.478 0.479 0.547
0 –
0 1 –
0.563 0.009 0.037 –
0.396 0.003 0.016 1 –
0.369 0.055 0.101 1 1 –
0.057 0.001 0.005 0.982 0.998 1 –
sym.
Table 4 Fracture force ANOVA p-value results for alkali treated fibres. Sample
Mean
2% NaOH
6% NaOH
10%NaOH
20% NaOH
30% NaOH
Untreated 2% NaOH 6% NaOH 10%NaOH 20% NaOH 30% NaOH
6.063 8.301 7.13 5.387 4.589 3.667
0 –
0.131 0.936 –
0.273 0.02 0.483 –
0.005 0.0148 0.264 0.999 –
0 0 0.001 0.466 0.997 –
sym.
Table 5 Fracture force ANOVA p-value results for acetylated fibres. Sample
Mean
5% A.Acid
10% A.Acid
5% A.Anh
10% A.Anh
6% NaOH, 5% A.Anh
6% NaOH, 10% A.Anh
Untreated 5% A.Acid 10% A.Acid 5% A.Anh 10% A.Anh 6% NaOH, 5% A.Anh 6% NaOH, 10% A.Anh
6.063 3.659 4.468 4.354 4.656 6.358 6.839
0 –
0 0.999 –
0.563 1 1 –
0.396 0.996 1 1 –
0.369 0.208 0.658 0.576 0.820 –
0.057 0.090 0.384 0.316 0.563 1 –
sym.
The fracture force for the alkali treated fibres increased for the 2% NaOH fibres compared to the untreated fibres as seen in Fig. 12 and Table 4. The fracture force decreases linearly as the NaOH concentration increased, with 20 and 30% NaOH resulting in a statistical significate decrease in fracture force. Only the 2% and 6% NaOH fibres had higher resistance to fracture than the untreated fibres. Although the strength of the fibres was higher, it fractured at a shorter embedment depth than the untreated fibre, indicating an improved bond through the removal of fats, waxes and mineral matter from the fibre surface. At higher concentrations of NaOH the fibre cells are damaged and the strength is reduced. This further confirmed the phenomenon of delignification which is known to take place at high alkalinity. The fibre structure and morphology is modified by the NaOH. At higher concentrations some of the hemi-cellulose and lignin is removed from the fibre, which results in a loss of strength and rigidity of the fibre. Fibres treated with Acetic Acid and Acetic Anhydride had a significantly lower fraction force than untreated fibres, with Acetic Anhydride fibres having a slightly higher strength than Acetic Acid fibres, as seen in Fig. 13 and Table 5. The fibres pre-treated with 5% NaOH before treatment with 10% Acetic Anhydride had significantly higher strength than the untreated fibres. In comparison, the fibres treated with only 2% NaOH had the highest strength followed by 6% NaOH, 6% NaOH-10% Acetic Anhydride and 6% NaOH-5% Acetic Anhydride in order of decreasing strength. Only treatments of 2% NaOH and 6% NaOH-10% Acetic Anhydride showed a statistically significant increase in strength. Fibres treated with only 6% NaOH had on average higher strengths than fibres treated with a combination of 6% NaOH-10% Acetic Anhydride. The results show that the fibre-matrix bond can be improved and the fibre strength can increase by chemical treatment. The type of treatment is important, as some treatments are more effective than others. The concentration is also important, since higher
concentrations may be detrimental to the fibre strength. Low concentrations of NaOH were more effective than high concentrations of NaOH. The combination of Acetic Anhydride and NaOH also proved to be effective with Acetic Acid being unfavourable. 3.2. Flexural test results The results of the specimens tested for three-point bending are presented in this section. All tests were stopped after a displacement of 10 mm was measured. The results of the unaged specimens tested at 28 days are presented first. Both the peak- and post-peak strength is presented for the unaged, but for the aged specimens tested at 90 days, only the post-peak strength is presented. The ageing was performed in order to evaluate the degradation of the fibres which can be studied best by evaluating the post-peak behaviour. The average peak stress was calculated from the individual peak stress for each specimen in a series. The average post-peak stress for an individual specimen was calculated as an average over a displacement of 2 mm starting at 0.5 mm away from the limit of proportionality (LOP), which is at the peak of the graph for the tested specimens as typically shown in Fig. 14. In flexural test results, the error bars indicate the maximum and minimum values recorded in that series. 3.2.1. Unaged specimens at 28 days Fig. 15 shows the observed trend for the specimens reinforced with alkali treated fibres. The trend does not reveal the expected decrease in strength with increasing NaOH concentration. The trend of a higher peak strength at lower concentrations can be observed. The mean strength of the 2, 6 and 10% NaOH specimens were higher when compared to the untreated specimen but only 6% NaOH was statistically significant as seen in Tables 6 and 7. The 20% NaOH treatment leads to a significant reduction in peak
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Fig. 14. Typical graph for a series of specimens’ test in three-point bending.
Fig. 15. Average peak and post-peak flexural strength for specimens with untreated and alkali treated fibres.
Table 6 Peak flexural strength ANOVA p-value results for alkali treated fibres. Sample
Mean
2% NaOH
6% NaOH
10%NaOH
20% NaOH
30% NaOH
Untreated 2% NaOH 6% NaOH 10%NaOH 20% NaOH 30% NaOH
5.432 5.917 5.931 5.678 4.803 5.044
0.085 –
0.021 1 –
0.229 0.972 0.958 –
0.020 0 0 0.001 –
0.097 0.001 0.001 0.052 0.970 –
sym.
Table 7 Post-peak flexural strength ANOVA p-value results for alkali treated fibres. Sample
Mean
2% NaOH
6% NaOH
10%NaOH
20% NaOH
30% NaOH
Untreated 2% NaOH 6% NaOH 10%NaOH 20% NaOH 30% NaOH
1.682 1.544 1.989 2.050 1.461 1.377
0.639 –
0.257 0.699 –
0.188 0.523 1 –
0.398 1 0.460 0.303 –
0.232 1 0.252 0.148 1 –
sym.
flexural strength. For the post-peak stress, the same trend can be observed if only the mean values are considered. However, the statistical analysis indicated that there is no significant difference in alkali treated post-peak stress. In Fig. 16, the peak and post-peak strengths can be observed for the acetylation treatment. The ANOVA results can be seen in Tables 8 and 9. A treatment of 5% and 10% of Acetic Acid lead to a signif-
icant reduction in peak flexural strength. On average, the Acetic Anhydride specimens had lower peak strength, but higher postpeak strength compared to the untreated specimens, with the 10% Acetic Anhydride also having a higher strength than the 5% Acetic Anhydride. Treatment with 5 or 10% Acetic Anhydride and 6% NaOH resulted in a statistically significant increase in both the peak and post-peak strengths.
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Fig. 16. Average peak and post-peak flexural strength for specimens with untreated and acetylated fibres.
Table 8 Peak flexural strength ANOVA p-value results for acetylated treated fibres. Sample
Mean
5% A.Acid
10% A.Acid
5% A.Anh
10% A.Anh
6% NaOH, 5% A.Anh
6% NaOH, 10% A.Anh
Untreated 5% A.Acid 10% A.Acid 5% A.Anh 10% A.Anh 6% NaOH, 5% A.Anh 6% NaOH, 10% A.Anh
5.432 4.514 5.046 5.232 5.180 5.927 6.146
0.001 –
0.092 0.185 –
0.379 0.016 0.996 –
0.259 0.034 1 1 –
0.052 0 0.001 0.022 0.010 –
0.007 0 0 0.001 0 0.985 –
sym.
Table 9 Post-peak flexural strength ANOVA p-value results for acetylated treated fibres. Sample
Mean
5% A.Acid
10% A.Acid
5% A.Anh
10% A.Anh
6% NaOH, 5% A.Anh
6% NaOH, 10% A.Anh
Untreated 5% A.Acid 10% A.Acid 5% A.Anh 10% A.Anh 6% NaOH, 5% A.Anh 6% NaOH, 10% A.Anh
1.682 1.606 1.623 1.771 1.760 2.075 2.468
0.767 –
0.835 1 –
0.773 1 1 –
0.826 1 1 1 –
0.175 0.632 0.682 0.963 0.953 –
0.061 0.018 0.023 0.117 0.105 0.830 –
sym.
Fig. 17. Average post-peak flexural strength for specimens with untreated and alkali treated fibres.
3.2.2. Aged specimens at 90 days The results of the specimens subjected to ageing and tested at 90 days are presented in Figs. 17 and 18. The 28d data in Figs. 17 and 18 respectively corresponds to the post-peak data shown in
Figs. 15 and 16. The ANOVA results can be seen in Tables 10-15. Only the post-peak strength is presented. Lime saturated hot water was the most severe ageing condition. Specimens, aged in hot lime water, with fibres treated with 10 and 30% NaOH had a statistically
11
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Fig. 18. Average post-peak flexural strength for specimens with untreated and acetylated fibres.
Table 10 Post-peak flexural strength ANOVA p-value results for alkali treated fibres aged in water. Sample
Mean
2% NaOH
6% NaOH
10%NaOH
20% NaOH
30% NaOH
Untreated 2% NaOH 6% NaOH 10%NaOH 20% NaOH 30% NaOH
1.414 1.815 1.699 1.721 1.261 1.435
0.025 –
0.264 1 –
0.171 1 1 –
0.362 0.439 0.757 0.698 –
0.907 0.845 0.984 0.972 1 –
sym.
Table 11 Post-peak flexural strength ANOVA p-value results for alkali treated fibres aged in wet-dry cycles. Sample
Mean
2% NaOH
6% NaOH
10%NaOH
20% NaOH
30% NaOH
Untreated 2% NaOH 6% NaOH 10%NaOH 20% NaOH 30% NaOH
1.174 1.407 1.188 1.379 1.072 1.181
0.332 –
0.957 0.999 –
0.329 1 0.999 –
0.612 0.980 1 0.977 –
0.971 0.999 1 0.999 1 –
sym.
Table 12 Post-peak flexural strength ANOVA p-value results for alkali treated fibres aged in hot lime water. Sample
Mean
2% NaOH
6% NaOH
10%NaOH
20% NaOH
30% NaOH
Untreated 2% NaOH 6% NaOH 10%NaOH 20% NaOH 30% NaOH
0.719 0.809 0.867 1.018 0.792 0.987
0.296 –
0.169 1 –
0.051 0.841 0.979 –
0.376 1 1 0.768 –
0.005 0.938 0.996 1 0.893 –
sym.
Table 13 Post-peak flexural strength ANOVA p-value results for acetylated treated fibres aged in water. Sample
Mean
5% A.Acid
10% A.Acid
5% A.Anh
10% A.Anh
6% NaOH, 5% A.Anh
6% NaOH, 10% A.Anh
Untreated 5% A.Acid 10% A.Acid 5% A.Anh 10% A.Anh 6% NaOH, 5% A.Anh 6% NaOH, 10% A.Anh
1.414 1.464 1.339 1.400 1.517 2.366 1.962
0.788 –
0.741 1 –
0.949 1 1 –
0.639 1.000 0.999 1 –
0.011 0.009 0.002 0.004 0.019 –
0.052 0.594 0.271 0.415 0.738 0.833 –
sym.
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M.D. de Klerk et al. / Construction and Building Materials 241 (2020) 117835
Table 14 Post-peak flexural strength ANOVA p-value results for acetylated treated fibres aged in wet-dry cycles. Sample
Mean
5% A.Acid
10% A.Acid
5% A.Anh
10% A.Anh
6% NaOH, 5% A.Anh
6% NaOH, 10% A.Anh
Untreated 5% A.Acid 10% A.Acid 5% A.Anh 10% A.Anh 6% NaOH, 5% A.Anh 6% NaOH, 10% A.Anh
1.174 1.563 1.767 1.579 1.781 1.835 1.466
0.08 –
0.02 0.999 –
0.227 1 0.999 –
0.092 0.998 1 0.999 –
0.016 0.987 1 0.992 1.000 –
0.388 1 0.973 1 0.963 0.902 –
sym.
Table 15 Post-peak flexural strength ANOVA p-value results for acetylated treated fibres aged in hot lime water. Sample
Mean
5% A.Acid
10% A.Acid
5% A.Anh
10% A.Anh
6% NaOH, 5% A.Anh
6% NaOH, 10% A.Anh
Untreated 5% A.Acid 10% A.Acid 5% A.Anh 10% A.Anh 6% NaOH, 5% A.Anh 6% NaOH, 10% A.Anh
0.719 0.718 0.741 0.814 1.191 1.033 1.103
0.993 –
0.751 1 –
0.235 0.999 1 –
0.01 0.015 0.026 0.116 –
0.004 0.318 0.425 0.805 0.971 –
0.01 0.101 0.152 0.445 1 1 –
sym.
significant higher post-peak strength than specimens with untreated fibres as seen in Table 12. At concentrations of 2, 6 and 10% NaOH, the average post-peak strength was higher than that of the untreated fibre specimens under all ageing conditions. Similar results were found in a study by Kabir et al. [2]. Mwaikambo & Ansell [31] also found that there are optimum concentrations of NaOH at which the best results are found. In this study, low concentrations of 2, 6 and 10% NaOH showed the best results in terms of post-peak strength with higher concentrations of 20 and 30% NaOH being detrimental to the 28-day strength. For the acetylated fibre specimens, the majority had a significant effect on the post-peak strength as shown in Fig. 18 and Tables 13–15. After ageing in lime saturated hot water, treatment with 10% Acetic Anhydride, 6%NaOH-5% Acetic Anhydride and 6% NaOH-10% Acetic Anhydride showed a higher 90 day post-peak strength. The combination of alkali treatment and acetylation proved to be the best treatment condition under all the tested conditions. At 28 days, the highest strength was recorded for specimens where a combination of the treatments was used. Mwaikambo & Ansell [31] stated that the dimensional stability of sisal fibre is improved through acetylation. Thus, under wet/dry cycle ageing, acetylated fibres were more stable than the alkali treated fibres, leading to the increased durability. At 90 days, the post-peak strengths were significantly higher than the untreated fibre specimens for all ageing conditions.
Higher concentrations of 20 and 30% NaOH caused damage to the fibre structure. Acetylation of sisal fibre with both Acetic Acid and Acetic Anhydride at concentrations of 5% and 10% reduced the fracture strength of the fibre considerably with Acetic Acid also reducing the fibre-matrix bond. However, pre-treatment with NaOH resulted in a statistically significant increase in fraction strength and pull-out force. Chemical treatment improved the flexural post-peak strength of the specimens in the majority of the specimens under all ageing conditions. At low concentrations of NaOH, the post-peak strength of the specimens was improved, but at higher concentrations the strength decreased. The use of Acetic Acid and Acetic Anhydride to treat fibres proved to be less effective in improving the durability of SFRCC. However, when Acetic Anhydride was combined with NaOH the flexural post-stress under all the ageing conditions significantly increased when compared to the untreated fibre specimen. Acetic Anhydride treatment with a NaOH pre-treatment showed an improved fibre-matrix bond strength, fibres fracture strength and post-peak strength (under all the ageing conditions tested) when compared to the untreated fibres. Even though some durability improvements were found using certain treatments, the overall durability of sisal fibres in a cement-based matrix remains poor and further research is required to assist improving its behaviour.
4. Conclusions Sisal fibres are proposed as a sustainable alternative for use in fibre reinforced cement-based composites. In this study, the durability of chemically treated sisal fibre in cement-based composites was investigated. In summary, the following conclusions can be drawn: Chemical treatment of sisal fibres by sodium hydroxide and/ or acetic acid/anhydride improved fibre resistance to alkaline attack in the cement-based composites. This improvement in resistance was due to fibre surface improvements as revealed by SEM images for low chemical dosage treatments. From the single fibre pull-out (SFP) tests it was evident that at low concentrations of 2, 6 and 10% NaOH the fibre strength increased and the fibre-matrix bond was improved.
Finally, treated sisal fibres have a potential use in cementitious composites, however special consideration should be made when choosing the pre-treatment approach for the fibres. In this research, it was found that a combination of alkali treatment and acetylation improved the overall flexural behaviour, perhaps due to the marginally improved bond properties, whereas fracture was mostly unaffected.
CRediT authorship contribution statement M.D. de Klerk: Conceptualization, Methodology, Validation, Formal analysis, Writing - original draft, Visualization, Investigation. M. Kayondo: Writing - original draft, Visualization. G.M. Moelich: Formal analysis, Visualization. W.I. de Villiers: Writing - review & editing. R. Combrinck: Writing - review & editing. W.
M.D. de Klerk et al. / Construction and Building Materials 241 (2020) 117835
P. Boshoff: Conceptualization, Validation, Writing - review & editing, Supervision, Project administration, Funding acquisition. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors greatly appreciate the funding from PPC and Wilhelm Frank Bursary Fund which made this research possible. References [1] Y. Li, Y.-W. Mai, L. Ye, Sisal fibre and its composites: a review of recent developments, Compos. Sci. Technol. 60 (2000) 2037–2055. [2] M. Kabir, H. Wang, K. Lau, F. Cardona, Chemical treatments on plant-based natural fibre reinforced polymer composites: an overview, Compos. B Eng. 43 (2012) 2883–2892. [3] G. Ramakrishna, T. Sundararajan, Studies on the durability of natural fibres and the effect of corroded fibres on the strength of mortar, Cem. Concr. Compos. 27 (2005) 575–582. [4] H.S.V. Agopyan, V. John, M. Cincotto, Developments on vegetable fibre–cement based materials in São Paulo, Brazil: an overview, Cem. Concr. Compos. 27 (2005) 527–536. [5] A. Ramzy, D. Beermann, L. Steuernagel, D. Meiners, G. Ziegmann, Developing a new generation of sisal composite fibres for use in industrial applications, Compos Part B: Eng 66 (2014) 287–298. [6] F. de Andrade Silva, B. Mobasher, R. Dias Toledo Filho, Fatigue behavior of sisal reinforced cement composites, Mater. Sci. Eng., A 527 (2010) 5507–5513. [7] A. Belaadi, A. Bezazi, M. Bourchak, F. Scarpa, Tensile static and fatigue behaviour of sisal fibres, Mater. Des. 46 (2013) 76–83. [8] S.R. Ferreira, E. Martinelli, M. Pepe, F.A. Silva, R.D.T. Filho, Inverse identification of the bond behaviour for jute fibers in cementitious matrix, Composites B 95 (2016) 440–452. [9] S.R. Ferreira, M. Pepe, E. Martinelli, F.A. Silva, R.D.T. Filho, Influence of natural fibres characteristics on the interface mechanics with cement based matrices, Composites B 140 (2018) 183–196. [10] F. Pacheco-Torgal, S. Jalali, Cementitious Building Materials Reinforced with Vegetable Fibres: A Review, Construction and Building Materials University of Minho, Guimaraes, 2010. [11] M.A. Aziz, P. Paramasivam, S.L. Lee, Prospects for natural fibre reinforced concretes in construction, Int. J. Cem. Compos. Lightweight Concr. 3 (1981) 123–132. [12] J. Wei, C. Meyer, Improving degradation resistance of sisal fiber in concrete through fiber surface treatment, Appl. Surf. Sci. 289 (2014) 511–523. [13] K. Oksman, L. Wallström, L.A. Berglund, R.D.T. Filho, Morphology and mechanical properties of unidirectional sisal–epoxy composites, J. Appl. Polym. Sci. 84 (2002) 2358–2365. [14] T. Bessell, S. Mutuli, The interfacial bond strength of sisal—cement composites using a tensile test, J. Mater. Sci. Lett. 1 (1982) 244–246. [15] Y. Li, Y. Shen, The use of sisal and henequen fibres as reinforcements in composites, in: Biofiber Reinforcements in Composite Materials, Elsevier, 2015, pp. 165–210. [16] N. Saba, M. Jawaid, O.Y. Alothman, M. Paridah, A. Hassan, Recent advances in epoxy resin, natural fiber-reinforced epoxy composites and their applications, J. Reinf. Plast. Compos. 35 (2016) 447–470.
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[17] K. Senthilkumar, N. Saba, N. Rajini, M. Chandraseka, M. Jawaid, S. Siengchin, O. Y. Alotman, Mechanical properties evaluation of sisal fibre reinforced polymer composites: a review, Constr. Build. Mater. 174 (2018) 713–729. [18] M. Idicula, N. Neelakantan, Z. Oommen, K. Joseph, S. Thomas, A study of the mechanical properties of randomly oriented short banana and sisal hybrid fiber reinforced polyester composites, J. Appl. Polym. Sci. 96 (2005) 1699– 1709. [19] A. Vahan, M.J. Vanderley, Durability evaluation of vegetable fibre reinforced materials, Build. Res. Inf. 4 (1992) 233–235. [20] H.E. Gram, Durability of Natural Fibers in Concrete, Swedish Cement and Concrete Research Institute, 1983. [21] G.V.S. Deniel James, I. Melvyn, A. Galinat, et al., 544.1R-96 State-of-the-Art Report on Fiber Reinforced Concrete, in: ACI Manual of Concrete Practice-Part 6, (2003). [22] R.D. Tolêdo Filho, K. Scrivener, G.L. England, K. Ghavami, Durability of alkalisensitive sisal and coconut fibres in cement mortar composites, Cem. Concr. Compos. 22 (2000) 127–143. [23] S.M. Singh, Alkali resistance of some vegetable fibers and their adhesion with Portland cement, Res. Ind. 15 (1985) 121–126. [24] P. Ganan, S. Garbizu, R. Llano-Ponte, I. Mondragon, Surface modification of sisal fibers: effects on the mechanical and thermal properties of their epoxy composites, Polym. Compos. 26 (2005) 121–127. [25] P. Sreekumar, S.P. Thomas, J. Marc Saiter, K. Joseph, G. Unnikrishnan, S. Thomas, Effect of fiber surface modification on the mechanical and water absorption characteristics of sisal/polyester composites fabricated by resin transfer molding, Compos. A Appl. Sci. Manuf. 40 (2009) 1777–1784. [26] P. Krishnaiah, C.T. Ratnam, S. Manickam, Enhancements in crystallinity, thermal stability, tensile modulus and strength of sisal fibres and their PP composites induced by the synergistic effects of alkali and high intensity ultrasound (HIU) treatments, Ultrason. Sonochem. 34 (2017) 729–742. [27] L. Wu, S. Lu, L. Pan, Q. Luo, J. Yang, L. Hou, et al., Enhanced thermal and mechanical properties of polypropylene composites with hyperbranched polyester grafted sisal microcrystalline, Fibers Polym. 17 (2016) 2153–2161. [28] S. Tragoonwichian, N. Yanumet, H. Ishida, Effect of fiber surface modification on the mechanical properties of sisal fiber-reinforced benzoxazine/epoxy composites based on aliphatic diamine benzoxazine, J. Appl. Polym. Sci. 106 (2007) 2925–2935. [29] A. Orue, A. Jauregi, C. Peña-Rodriguez, J. Labidi, A. Eceiza, A. Arbelaiz, The effect of surface modifications on sisal fiber properties and sisal/poly (lactic acid) interface adhesion, Compos Part B: Eng 73 (2015) 132–138. [30] A. Orue, A. Jauregi, U. Unsuain, J. Labidi, A. Eceiza, A. Arbelaiz, The effect of alkaline and silane treatments on mechanical properties and breakage of sisal fibers and poly (lactic acide)/sisal fiber composites, Composites A 84 (2016) 186–195. [31] L. Mwaikambo, M. Ansell, The Effect of Chemical Treatment on the Properties of Hemp, Sisal, Jute and Kapok for Composite Reinforcement, Wiley University of Bath, Bath, 1999. [32] J.M.L. Reis, Sisal fiber polymer mortar composites: introductory fracture mechanics approach, Constr. Build. Mater. 37 (2012) 177–180. [33] ASTM International, Standard Test Method for Flow of Hydraulic Cement Mortar, ASTM International, Pennsylvania, 2007. [34] M.D. De Klerk, W.P. Boshoff. The durability of natural sisal fibre reinforced cement-based composites. Masters dissertation, Stellenbosch: Stellenbosch University. [35] W.P. Boshoff, V. Mechtcherine, G.P.A.G. van Zijl, Characterising the timedependant behaviour on the single fibre level of SHCC: Part 1: mechanism of fibre pull-out creep, Cem. Concr. Res. 39 (9) (2009) 779–786. [36] ASTM International, Standard Test Method for Hot Water Accelerated Aging of Glass-Fiber Reinforced Cement-Based Composites, ASTM Standards, Pennsylvania, 2009. [37] D.C. Montgomery, G.C. Runger, Applied Statistics and Probability for Engineers, fifth ed., John Wiley & Sons, 2010, pp. 513–523.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Durability of chemically modified sisal fibre in cement-based composites M.D. de Klerk a, M. Kayondo a, G.M. Moelich a, W.I. de Villiers a, R. Combrinck a, W.P. Boshoff b,⇑ a b
Department of Civil Engineering, Stellenbosch University, South Africa Department of Civil Engineering, University of Pretoria, South Africa
h i g h l i g h t s Durability of sisal fibres in cement-based systems. Chemical treatment of sisal fibres. Single fibre pull-out tests on sisal fibres. Ageing tests and performance of fibres.
a r t i c l e
i n f o
Article history: Received 30 April 2019 Received in revised form 5 December 2019 Accepted 9 December 2019
Keywords: Sisal fibre Fibre reinforced concrete FRC Durability Treatment of fibres Mechanical behaviour Single fibre pull-out Mechanical properties Alkali treatment Acetylation
a b s t r a c t The degradation of sisal fibre in a cement-based matrix due to the highly alkaline environment reduces the strength of the composite. In this research, to avert this degradation, alkaline treatment and acetylation were respectively performed with sodium hydroxide (NaOH) and Acetic Acid or Acetic Anhydride to improve the resistance of the fibre to alkaline attack. In addition, single fibre pull-out (SFP) tests were performed to evaluate the influence of chemical treatment on fibre strength, fibre-matrix interaction and also determine the critical fibre length. Specimens were tested in indirect tension (flexure) at 28 days to determine the strength of the composite. Additional ageing tests by extended curing in water at 24 °C, lime saturated hot water at 70 °C, and alternate cycles of wetting and drying were done. Aged samples were further tested at 90 days to evaluate the durability of the fibre. It was found that a combination of alkali treatment and acetylation was the most effective treatment condition, followed by that of alkali treatment at low concentrations of sodium hydroxide. At higher concentrations of sodium hydroxide, a significant reduction in strength was observed. Chemical treatment improves the durability of sisal fibres in concrete, albeit slight degradation still occurs. Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction The global drive towards achieving sustainable concrete infrastructure has led to a concerted effort in the use of various natural fibres in cement-based composites [1]. This is evidenced by the growing literature in this direction [1–6]. Fibre reinforced cement-based composites (FRCC) are widely utilised in several applications in the concrete industry, especially due to their high strength, light weight, and ductile failure mechanisms [5]. Most FRCC are made using synthetic or steel fibres due to their exceptional properties within the cement matrix. The possibility of replacing synthetic and steel fibres in concrete with natural fibres such as sisal, hemp, jute or flax is one of the many areas of research into renewable building materials [7–9]. Natural fibres such as ⇑ Corresponding Author. E-mail address: [email protected] (W.P. Boshoff). https://doi.org/10.1016/j.conbuildmat.2019.117835 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.
sisal are largely available in tropical regions and fit into the sustainable materials category [10]. Some of these natural fibres have tensile strengths in excess of those of polypropylene (PP) fibres and are comparable to polyvinyl acetate (PVA) fibres, and exhibit performance similar to composites of synthetic or steel fibres [11]. Sisal fibre made from the Agave Sisalana plant, one of the most widely used natural fibres, has largely been studied for inclusion in cement-based composites [12–15]. Its abundance and excellent mechanical properties rank it highly among the available natural fibres for use in the construction industry [3,16]. Sisal fibre is low cost, has a high tensile strength, abrasion resistance and toughness, no health-related risk, as well as good thermal and acoustic properties [1,17,18]. This is in addition to being biodegradable and renewable. The mechanical properties of sisal fibre are generally dependent on factors such as the origin of the fibre, fibre orientation, fibre dispersion, type of matrix used, and the bonding between fibre and the matrix [17].
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The use of sisal fibres in cement-based composites enhances the overall mechanical properties of the composites [16]. This material property enhancement largely depends on the critical fibre length, orientation, and fibre cellular make-up. However, despite the several outstanding advantages of sisal fibre, a number of studies have shown that the durability of sisal fibre in cement-based composites is of concern, since the fibres tend to degrade in an alkaline environment [3,10,19–21]. Cement-based composites have high alkalinity resulting from the calcium hydroxide, Ca(OH)2, formed during the hydration of cement, and this presents a corrosive environment for the sisal fibre [22,23]. This degradation (due to alkaline hydrolysis which leads to loss of reinforcing capacity) is what compromises the performance of most natural fibres in cement-based composites. Additionally, at high concentrations of Ca(OH)2, delignification is known to take place and since lignin is what binds the fibres together, absence of lignin leads to defibrillation [2,24–26]. Other flaws of sisal fibre are the low combustion temperature, and poor fibre/matrix adhesion (attributed to the hydrophilic characteristics of natural fibre) [17,27,28]. Despite some of the challenges regarding natural fibre, sisal fibre utilisation in cement-based composites has attracted research with the sole aim of enhancing the material properties of the natural fibre to make it viable for use in cement-based composites [1– 6,12–15]. Techniques such as chemical pre-treatment (such as with sodium hydroxide, silane, acetic acid, and others) have been used to enhance the fibre properties such as bond strength through removal of impurities (fats, waxes, and mineral matter) from the fibre surface [12,29,30]. In this research, two different chemical treatment methods, alkali treatment and acetylation, were performed on sisal fibre. Alkali treatment was performed with sodium hydroxide (NaOH) at concentrations of 2, 6, 10, 20 and 30%, whereas Acetylation was performed with acetic acid (CH3COOH) and acetic anhydride ((CH3CO)2O), both at concentrations of 5 and 10%. A combination of alkali treatment and acetylation was performed by treating fibres pre-treated with 6% NaOH with 5 and 10% acetic anhydride respectively. The improvement of fibre mechanical properties is dependent on both the chemical concentration and exposure periods of the chemical treatment [14]. Longer exposure periods or larger than critical chemical concentration dosages lead to deterioration of the mechanical properties of the fibre. In this research, the viability of sisal fibre in cement-based composites, as a more sustainable building material, was further investigated by reinforcing a cement-based matrix with 1% sisal fibre (by volume) cut to a length of 20 mm. The challenges associated with the durability of natural sisal fibre in an alkaline environment were the main focus of the study thereafter. The experiments conducted and results achieved are discussed in the following sections. 2. Experimental procedure 2.1. Materials The mortar matrix consisted of Ordinary Portland cement (OPC) CEM I and a natural quarry sand, locally known as Malmesbury sand. In addition to fibres, a modified acrylic polymer was used as super-plasticiser and added to maintain workability of the mortar matrix. The sisal fibre was supplied by Rebtex RSA. The specific origin of the fibre is unknown. Sisal fibres were received in bundles, already washed and combed. Using SEM images of six fibres, the average diameter was found to be 260 mm.
then re-combed using a nail comb to remove any entanglement. Untreated fibres were stored in a conditioning chamber at a temperature of 23 ± 2 °C and relative humidity of 65 ± 2%. Two different fibre treatments, namely alkali treatment and acetylation were performed. In this study, 100% concentration is equivalent to one molar. 2.2.1. Alkali treatment Alkali treatment was performed using 2, 6, 10, 20 and 30% NaOH solutions. These concentrations were used since the highest crystallinity index reported by Mwaikambo and Ansell [31] was for fibres treated in a 6% NaOH solution. Higher crystallinity is said to result in stronger and stiffer fibres [31]. Fibres were soaked in the different NaOH concentrations for 48 h in a conditioning chamber, after which they were rinsed in 1% acetic acid to neutralise excessive NaOH and washed in water to remove excess acid from the fibre surfaces. They were then spread out in the conditioning chamber and left to dry over a period of 2–3 days until constant weight measurements were attained. 2.2.2. Acetylation Acetylation was performed using Acetic Acid and Acetic Anhydride, at concentrations of 5 and 10% for both. In addition, a combination of alkali treatment and acetylation was used. Fibres were soaked in Acetic Acid and Acetic Anhydride respectively for 1 h. Fibres, pre-treated with 6% NaOH, were also soaked in 5 and 10% Acetic Anhydride for 1 h. The fibres were removed from the solution and washed in water to remove excess acid from the fibre surface. The fibres were then spread out in the conditioning chamber and left to dry for a period of 2–3 days until constant weight was reached. This treatment procedure is similar to that by Reis [32], except that a hot hair oven was used to dry the treated sisal fibres. 2.3. Specimen production 2.3.1. Mix design and procedure A water/cement ratio of 0.6, with a cement/sand ratio of 0.28, was used for all the specimens. The sand grading is shown in Fig. 1. The workability and consistency were evaluated by the slump flow test performed in accordance to the flow test described in ASTM C1437-07 [33]. A 10-litre pan mixer was used to mix mortar for the single fibre pull-out (SFP) specimens. The mixing pan was initially saturated with water and towel dried before adding the materials. Dry ingredients were first mixed for 30 s, after which water was added and additional mixing of 2 min done. After the matrix was mixed in the 10-litre pan mixer, a 5 L mix, containing 1% fibres by volume, was re-mixed in a 25-litre pan mixer to determine the amount of super-plasticiser required to obtain the same workability as for a mix without fibres. It was found that 1% super-plasticiser (by mass of cementitious material) was sufficient, to avoid material segregation. The compressive, uni-axial tensile and flexural strengths of the matrix were determined to be 27.5 MPa, 2.6 MPa and 5.6 MPa respectively. The fibre volume ratio and length were optimised experimentally by De Klerk and Boshoff [34]. The critical fibre length was determined by analysing the pull-out lengths. For the various treatment conditions, the pull-out length was 10 mm on average. Therefore, a 20 mm fibre was selected to ensure sufficient anchorage. Fibre volume ratios of 0.5, 1, 1.5 and 2% were evaluated by comparing the flexural post-peak strength results. At a fibre volume of 1%, the flexural post-peak strength was the highest for a fibre length of 20 mm.
2.2. Fibre preparation Bundled fibres were separated into smaller bundles before being cut to required lengths using a paper guillotine. Fibres were
2.3.2. Single fibre pull-out (SFP) specimens Single fibre pull-out specimens were cast in a mould similar to the one used by Boshoff et al., [35]. The length, width and depth of
M.D. de Klerk et al. / Construction and Building Materials 241 (2020) 117835
3
Fig. 1. The sieve grading of the sand used.
the mould were 200, 50 and 10 mm respectively. Fibre strands were secured to a frame, spaced at a distance of 10 mm between strands. The mould was then filled halfway, after which the frame with the fibre strands were secured on top of the mortar and the rest of the mould was filled. The specimen was vibrated for 60 s to compact and spread the mortar evenly. Fig. 2 (a–d) shows the detail of the specimen preparation. After one day of curing at room temperature, the specimen was demoulded and cured in water at 24 ± 2 °C until 28 days from the date of casting. At 28 days, the specimen was dry-cut into smaller samples (50 10 10 mm), with a single cast-in fibre protruding on both sides (see Fig. 2 c and d), using a diamond blade cutter. As the fibre was embedded along the 50 mm length of the sample, the sample was simply cut shorter to obtain the desired embedment length.
interlock between the fibres can cause fibre balls which are not separated during the mixing process. Adding the fibres took approximately 10–15 min, after which super-plasticiser followed, and the mixing continued for another four minutes to allow the super-plasticiser to work effectively. The total mixing time was approximately 15–20 min. For flexure, specimens of 300x70x15 mm were obtained by cutting in half bigger specimens cast in 600x70x15 mm beam moulds. After mixing, the fresh mortar was placed in the moulds and vibrated on a vibrating table for two minutes. The surface of the specimen was smoothed using a trowel. After two days at room temperature, the specimens were demoulded and cured in water at 24 ± 2 °C until 28 days from the date of casting. The 600 mm beams were then cut in half to 300 mm beams that were used for flexure.
2.3.3. Flexural test specimens A 50-litre pan mixer was used for preparation of specimens for flexure testing. The addition of fibres was done slowly and by hand, while continuously separating them to ensure that no fibre balling took place. If the fibres are added too quickly, the friction and
2.4. Ageing methods In order to evaluate the durability of SFRCC, specimens for flexure were aged and tested at 90 days. The strength was compared to the reference SFRCC specimens tested at 28 days, both treated and
Fig. 2. Single fibre pull-out specimen preparation: a) Fibre strands secured to a frame at 10 mm spacing; b) Mould filled with mortar; c) Specimen cut into smaller samples; d) Single specimen ready for fibre pull-out test.
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untreated. After 28 days of water curing at 24 ± 2 °C and testing of the reference specimens, other specimens were subjected to three different ageing conditions. One set of specimens was placed in a hot water curing tank saturated with lime and kept at a constant temperature of 70 ± 2 °C. This was done according to ASTM C1560-03 [36]. The addition of lime simulates an alkali-based environment that accelerates the degradation of the sisal fibre in the matrix. Another set of specimens were subjected to alternate cycles of wetting and drying. The specimens were dried in a conditioning chamber for 4 days at 23 ± 2 °C and 65 ± 2% relative humidity (RH), and then saturated in water at 24 ± 2 °C for 3 days to make one cycle of 7 days. This cycle was repeated until 90 days. This ageing condition was used to simulate natural weathering. The third set of specimens was left in the curing tank at 24 ± 2 °C up to 90 days, to determine whether degradation takes place under normal curing conditions. Table 1 shows a summary of the curing and ageing conditions adopted in this investigation.
were continued for shorter embedment lengths. The average pull-out force of six specimens (of the fibre from the matrix) per millimetre of embedded fibre was calculated, as well as the fracture force based on the prevalent failure mechanism.
2.5. Test methods
ry ¼
2.5.1. Scanning electron microscope Scanning Electron Microscope (SEM) images of the fibre were taken using a LEO 1450 VP SEM. The images were used to study the effect of chemical treatment on the fibre appearance as well as the fibre structure and appearance after fibre fracture and pull-out.
2.5.3. Flexural tests Flexural tests were performed on 300 mm beams in a threepoint bending setup to evaluate the flexural strength of the matrix and to study the influence of the fibres on the post-peak (after cracking) behaviour of the specimens. The tests were also performed using the Zwick Z250. A span of 250 mm was used. The beams were loaded at a displacement rate of 5 mm/min. The tests were stopped after a displacement of 10 mm. For most of the series, six specimens were tested. The load and displacement was recorded and represented in graphs of flexural stress of the beam versus the crosshead displacement of the material testing machine. The ultimate flexural stress, ry, is calculated using the following equation:
3PL 2bd
2
ð1Þ
where, P is the applied load in Newton (N), L is the support separation, in this case 250 mm, b is the width of the specimen, in this case 70 mm, d is the thickness of the specimen in mm, in this case 15 mm. 2.6. ANOVA analysis
2.5.2. Single fibre pull-out Single fibre pull-out tests were performed to gain an understanding of the fibre-matrix interaction, the interfacial bond strength and the effect of chemical treatment on the bond strength as well as to determine the critical lengths for the fibres subjected to different treatments. Fibre pull-out tests were performed using a Zwick Z250 material testing machine. The bottom part of the specimen (mortar) was clamped, while the fibre was fixed with superglue to an aluminium surface attached to a Hottinger Baldwin Messtechnik (HBM) 10 N load cell as shown in Fig. 3. The clamp consisted of two plates which were adjustable by bolts. The two plates and the load cell were clamped in the grips of the machine. A tensile force was applied to the specimen at a constant displacement rate of 0.1 mm/s. A linear variable differential transformer (LVDT) was fixed across the clamps of the machine to record the displacement and thus the pull-out of the fibre. The load and displacement were logged using a HBM Spider8 data acquisition system. An initial maximum embedment depth of 50 mm, allowed by the size of the specimen, was used. The fibres were tested starting at embedment depths of 50, 40, 30, 20, 15 and 10 mm respectively. If fibre pull-out was the prevalent failure mechanism at a certain embedment depth, the tests were not continued for shorter embedment depths. Likewise, if fracture occurred, pull-out tests
Table 1 Curing and ageing conditions for test specimen series. Set/Time
Reference Set 1st Set 2nd Set 3rd Set
Curing/Ageing conditions 28 days
Up to 90 days
water curing at 24 ± 2 °C water curing at 24 ± 2 °C water curing at 24 ± 2 °C water curing at 24 ± 2 °C
– Hot lime water curing at 70 ± 2 °C 4 days drying at 23 ± 2 °C and RH 65 ± 2%; water at 24 ± 2 °C for 3 days; (7-day cycle) water curing at 24 ± 2 °C
To account for the large variability of natural fibres, a one-way Analysis of Variation (ANOVA) was conducted. The analysis determines if treating the fibre made a statistically significant difference, at a significance level of 0.05, in both the single fibre pullout and flexural test results. The results were used to compare the untreated to treated fibres as well as to compare the treated fibres to each other. Although the main conclusions were made on statistically significant results, trends were also considered when the results were evaluated. More on an ANOVA can be found elsewhere [37]. 3. Results 3.1. Single fibre pull-out In order to understand some of the results of the fibre pull-out tests, SEM images of the fibres under the different surface treatment approaches have been presented first. Figs. 4–9 show the surface observations for the different fibres before and after the treatment variations. Figs. 4 and 7 show an untreated fibre surface with some impurities, which are observed to be significantly reduced after the treatment process in Fig. 5. The effect of higher dosage of treatment leading to damage as reported by [1,24] are revealed in Figs. 6 and 8. Graphs of the average pull-out force (in N/mm) and fracture force (in N) for the different treatment conditions are presented in Figs. 10–13. The average pull-out force per unit length (in N/mm) was calculated (from six specimens) as the pull-out force (in N) divided by the original embedment depth. This computation assumes a uniform bond shear stress over the length of the embedded part of the fibre. The diameters of the individual fibres are not constant and are difficult to measure, therefore the pullout and fracture force are not expressed as an interfacial shear stress and tensile stress respectively. The error bars on the figures indicate the minimum and maximum force for the specific series. The ANOVA result of the pull-out force test is seen in Tables 2
M.D. de Klerk et al. / Construction and Building Materials 241 (2020) 117835
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Fig. 3. Test set-up for single fibre pull-out tests. The protruding fibre is enhanced in red. Embedded lengths of 50, 40, 30, 20, 15 and 10 mm were tested. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. SEM image of untreated fibre showing surface impurities present (in red circles). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. SEM image of 20% NaOH treated fibre showing damage due to higher dosage of treatment.
Fig. 5. SEM image of 10% NaOH treated fibre, with significantly less impurities than the untreated fibre.
Fig. 7. SEM image of 5% Acetic Acid treated fibre revealing some surface impurities.
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Fig. 8. SEM image of 5% Acetic Anhydride treated fibre showing signs of delignification.
Fig. 9. SEM image of 6% NaOH-5% Acetic Anhydride treated fibre.
and 3. The ANOVA results of the fracture force test is seen in Table 4 and 5. Fig. 10 and Table 2 show that at a NaOH concentration of 2%, treated fibres have a statistically significant higher pull-out strength than the untreated fibre, while at higher concentrations of 20 and 30% NaOH, fibres had a lower pull-out strength. This differential performance is attributed to the surface differences arising from the different treatment approaches leading to improved
bond strength [12]. The low pull-out resistance observed in this research for fibres treated with high concentrations of NaOH is due to the damage to fibres at such alkaline concentrations [24–26]. The untreated and 20% NaOH fibres pulled out at an embedment depth of 15 mm, while the other alkali treated fibres pulled out at 10 mm and had a combination of pull-out and fracture at 15 mm. The increased resistance of the low concentrations NaOH fibres at a smaller embedment depth indicates an improved bond between the fibre and the matrix, due to the removal of surface impurities through alkali treatment. This is further confirmed through observation of the SEM images as in Figs. 4–6 after fibre treatment. Higher dosages of treatment with NaOH however showed declining resistance compared to the untreated, and this is due to delignification as confirmed by the damage visible in Fig. 6. For the untreated fibre, impurities are visible on the surface and encircled in red, whereas for the 10% and 20% NaOH, significantly less impurities are visible. The average pull-out force for the acetylated fibres is presented in Fig. 11 with the ANOVA results presented in Table 3. The resistance of the 5 and 10% Acetic Acid fibres to pull-out is much lower than the untreated and other acetylated fibres. The fibres treated with Acetic Acid pulled out at embedment depths of 15 mm and 20 mm and also fractured at 20 mm, while the fibres treated with Acetic Anhydride pulled out at 10 mm and fractured at 15 mm. A number of fibres pre-treated with 6% NaOH and then with 10% Acetic Anhydride pulled out at 15 mm, but at a higher force than the Acetic Acid specimens, indicating that the 6% NaOH-10% Acetic Anhydride had the highest resistance to pull-out. The higher average pull-out force and the smaller embedment depth at which it pulled out, indicates a stronger bond between the fibres and the matrix for the fibres treated with Acetic Anhydride than for the fibres treated with Acetic Acid. The fibres treated with 6% NaOH10% Acetic Anhydride pulled out at 15 mm while all the other fibres treated with Acetic Anhydride pulled out at 10 mm. In comparison, the fibres treated with only 2% NaOH had the highest resistance to pull-out of all the fibres, followed by fibres treated with 6% NaOH. Pre-treatment with NaOH was more effective in removing surface impurities. This is confirmed by the slightly higher average pull-out force of the 6% NaOH-5% Acetic Anhydride compared to the 5% Acetic Anhydride and also of the 6% NaOH-10% Acetic Anhydride compared to the 10% Acetic Anhydride. Acetic Anhydride improved the bond strength of the untreated fibre, but reduced the bond strength of the 6% NaOH treated fibre. Acetylation was not as effective as alkali treatment in removing surface impurities. This can be seen from the SEM images presented in Figs. 7 and 8, which show surface impurities on both the fibres treated with Acetic Acid and Acetic Anhydride. The fibre pre-treated with 6% NaOH, presented in Fig. 9, however, had a significantly cleaner surface.
Fig. 10. Average pull-out force for alkali treated fibres.
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Fig. 11. Average pull-out force for acetylated fibres.
Fig. 12. Average fracture force for alkali treated fibres.
Fig. 13. Average fracture force for acetylated fibres.
Table 2 Pull-out force ANOVA p-value results for alkali treated fibres. Sample
Mean
2% NaOH
6% NaOH
10%NaOH
20% NaOH
30% NaOH
Untreated 2% NaOH 6% NaOH 10%NaOH 20% NaOH 30% NaOH
0.420 0.697 0.596 0.565 0.287 0.363
0 –
0.052 0.986 –
0.262 0.916 1 –
0.024 0 0.015 0.044 –
0.259 0.014 0.245 0.445 0.997 –
sym.
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Table 3 Pull-out force ANOVA p-value results for acetylated fibres. Sample
Mean
5% A.Acid
10% A.Acid
5% A.Anh
10% A.Anh
6% NaOH, 5% A.Anh
6% NaOH, 10% A.Anh
Untreated 5% A.Acid 10% A.Acid 5% A.Anh 10% A.Anh 6% NaOH, 5% A.Anh 6% NaOH, 10% A.Anh
0.42 0.171 0.166 0.454 0.478 0.479 0.547
0 –
0 1 –
0.563 0.009 0.037 –
0.396 0.003 0.016 1 –
0.369 0.055 0.101 1 1 –
0.057 0.001 0.005 0.982 0.998 1 –
sym.
Table 4 Fracture force ANOVA p-value results for alkali treated fibres. Sample
Mean
2% NaOH
6% NaOH
10%NaOH
20% NaOH
30% NaOH
Untreated 2% NaOH 6% NaOH 10%NaOH 20% NaOH 30% NaOH
6.063 8.301 7.13 5.387 4.589 3.667
0 –
0.131 0.936 –
0.273 0.02 0.483 –
0.005 0.0148 0.264 0.999 –
0 0 0.001 0.466 0.997 –
sym.
Table 5 Fracture force ANOVA p-value results for acetylated fibres. Sample
Mean
5% A.Acid
10% A.Acid
5% A.Anh
10% A.Anh
6% NaOH, 5% A.Anh
6% NaOH, 10% A.Anh
Untreated 5% A.Acid 10% A.Acid 5% A.Anh 10% A.Anh 6% NaOH, 5% A.Anh 6% NaOH, 10% A.Anh
6.063 3.659 4.468 4.354 4.656 6.358 6.839
0 –
0 0.999 –
0.563 1 1 –
0.396 0.996 1 1 –
0.369 0.208 0.658 0.576 0.820 –
0.057 0.090 0.384 0.316 0.563 1 –
sym.
The fracture force for the alkali treated fibres increased for the 2% NaOH fibres compared to the untreated fibres as seen in Fig. 12 and Table 4. The fracture force decreases linearly as the NaOH concentration increased, with 20 and 30% NaOH resulting in a statistical significate decrease in fracture force. Only the 2% and 6% NaOH fibres had higher resistance to fracture than the untreated fibres. Although the strength of the fibres was higher, it fractured at a shorter embedment depth than the untreated fibre, indicating an improved bond through the removal of fats, waxes and mineral matter from the fibre surface. At higher concentrations of NaOH the fibre cells are damaged and the strength is reduced. This further confirmed the phenomenon of delignification which is known to take place at high alkalinity. The fibre structure and morphology is modified by the NaOH. At higher concentrations some of the hemi-cellulose and lignin is removed from the fibre, which results in a loss of strength and rigidity of the fibre. Fibres treated with Acetic Acid and Acetic Anhydride had a significantly lower fraction force than untreated fibres, with Acetic Anhydride fibres having a slightly higher strength than Acetic Acid fibres, as seen in Fig. 13 and Table 5. The fibres pre-treated with 5% NaOH before treatment with 10% Acetic Anhydride had significantly higher strength than the untreated fibres. In comparison, the fibres treated with only 2% NaOH had the highest strength followed by 6% NaOH, 6% NaOH-10% Acetic Anhydride and 6% NaOH-5% Acetic Anhydride in order of decreasing strength. Only treatments of 2% NaOH and 6% NaOH-10% Acetic Anhydride showed a statistically significant increase in strength. Fibres treated with only 6% NaOH had on average higher strengths than fibres treated with a combination of 6% NaOH-10% Acetic Anhydride. The results show that the fibre-matrix bond can be improved and the fibre strength can increase by chemical treatment. The type of treatment is important, as some treatments are more effective than others. The concentration is also important, since higher
concentrations may be detrimental to the fibre strength. Low concentrations of NaOH were more effective than high concentrations of NaOH. The combination of Acetic Anhydride and NaOH also proved to be effective with Acetic Acid being unfavourable. 3.2. Flexural test results The results of the specimens tested for three-point bending are presented in this section. All tests were stopped after a displacement of 10 mm was measured. The results of the unaged specimens tested at 28 days are presented first. Both the peak- and post-peak strength is presented for the unaged, but for the aged specimens tested at 90 days, only the post-peak strength is presented. The ageing was performed in order to evaluate the degradation of the fibres which can be studied best by evaluating the post-peak behaviour. The average peak stress was calculated from the individual peak stress for each specimen in a series. The average post-peak stress for an individual specimen was calculated as an average over a displacement of 2 mm starting at 0.5 mm away from the limit of proportionality (LOP), which is at the peak of the graph for the tested specimens as typically shown in Fig. 14. In flexural test results, the error bars indicate the maximum and minimum values recorded in that series. 3.2.1. Unaged specimens at 28 days Fig. 15 shows the observed trend for the specimens reinforced with alkali treated fibres. The trend does not reveal the expected decrease in strength with increasing NaOH concentration. The trend of a higher peak strength at lower concentrations can be observed. The mean strength of the 2, 6 and 10% NaOH specimens were higher when compared to the untreated specimen but only 6% NaOH was statistically significant as seen in Tables 6 and 7. The 20% NaOH treatment leads to a significant reduction in peak
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Fig. 14. Typical graph for a series of specimens’ test in three-point bending.
Fig. 15. Average peak and post-peak flexural strength for specimens with untreated and alkali treated fibres.
Table 6 Peak flexural strength ANOVA p-value results for alkali treated fibres. Sample
Mean
2% NaOH
6% NaOH
10%NaOH
20% NaOH
30% NaOH
Untreated 2% NaOH 6% NaOH 10%NaOH 20% NaOH 30% NaOH
5.432 5.917 5.931 5.678 4.803 5.044
0.085 –
0.021 1 –
0.229 0.972 0.958 –
0.020 0 0 0.001 –
0.097 0.001 0.001 0.052 0.970 –
sym.
Table 7 Post-peak flexural strength ANOVA p-value results for alkali treated fibres. Sample
Mean
2% NaOH
6% NaOH
10%NaOH
20% NaOH
30% NaOH
Untreated 2% NaOH 6% NaOH 10%NaOH 20% NaOH 30% NaOH
1.682 1.544 1.989 2.050 1.461 1.377
0.639 –
0.257 0.699 –
0.188 0.523 1 –
0.398 1 0.460 0.303 –
0.232 1 0.252 0.148 1 –
sym.
flexural strength. For the post-peak stress, the same trend can be observed if only the mean values are considered. However, the statistical analysis indicated that there is no significant difference in alkali treated post-peak stress. In Fig. 16, the peak and post-peak strengths can be observed for the acetylation treatment. The ANOVA results can be seen in Tables 8 and 9. A treatment of 5% and 10% of Acetic Acid lead to a signif-
icant reduction in peak flexural strength. On average, the Acetic Anhydride specimens had lower peak strength, but higher postpeak strength compared to the untreated specimens, with the 10% Acetic Anhydride also having a higher strength than the 5% Acetic Anhydride. Treatment with 5 or 10% Acetic Anhydride and 6% NaOH resulted in a statistically significant increase in both the peak and post-peak strengths.
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Fig. 16. Average peak and post-peak flexural strength for specimens with untreated and acetylated fibres.
Table 8 Peak flexural strength ANOVA p-value results for acetylated treated fibres. Sample
Mean
5% A.Acid
10% A.Acid
5% A.Anh
10% A.Anh
6% NaOH, 5% A.Anh
6% NaOH, 10% A.Anh
Untreated 5% A.Acid 10% A.Acid 5% A.Anh 10% A.Anh 6% NaOH, 5% A.Anh 6% NaOH, 10% A.Anh
5.432 4.514 5.046 5.232 5.180 5.927 6.146
0.001 –
0.092 0.185 –
0.379 0.016 0.996 –
0.259 0.034 1 1 –
0.052 0 0.001 0.022 0.010 –
0.007 0 0 0.001 0 0.985 –
sym.
Table 9 Post-peak flexural strength ANOVA p-value results for acetylated treated fibres. Sample
Mean
5% A.Acid
10% A.Acid
5% A.Anh
10% A.Anh
6% NaOH, 5% A.Anh
6% NaOH, 10% A.Anh
Untreated 5% A.Acid 10% A.Acid 5% A.Anh 10% A.Anh 6% NaOH, 5% A.Anh 6% NaOH, 10% A.Anh
1.682 1.606 1.623 1.771 1.760 2.075 2.468
0.767 –
0.835 1 –
0.773 1 1 –
0.826 1 1 1 –
0.175 0.632 0.682 0.963 0.953 –
0.061 0.018 0.023 0.117 0.105 0.830 –
sym.
Fig. 17. Average post-peak flexural strength for specimens with untreated and alkali treated fibres.
3.2.2. Aged specimens at 90 days The results of the specimens subjected to ageing and tested at 90 days are presented in Figs. 17 and 18. The 28d data in Figs. 17 and 18 respectively corresponds to the post-peak data shown in
Figs. 15 and 16. The ANOVA results can be seen in Tables 10-15. Only the post-peak strength is presented. Lime saturated hot water was the most severe ageing condition. Specimens, aged in hot lime water, with fibres treated with 10 and 30% NaOH had a statistically
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Fig. 18. Average post-peak flexural strength for specimens with untreated and acetylated fibres.
Table 10 Post-peak flexural strength ANOVA p-value results for alkali treated fibres aged in water. Sample
Mean
2% NaOH
6% NaOH
10%NaOH
20% NaOH
30% NaOH
Untreated 2% NaOH 6% NaOH 10%NaOH 20% NaOH 30% NaOH
1.414 1.815 1.699 1.721 1.261 1.435
0.025 –
0.264 1 –
0.171 1 1 –
0.362 0.439 0.757 0.698 –
0.907 0.845 0.984 0.972 1 –
sym.
Table 11 Post-peak flexural strength ANOVA p-value results for alkali treated fibres aged in wet-dry cycles. Sample
Mean
2% NaOH
6% NaOH
10%NaOH
20% NaOH
30% NaOH
Untreated 2% NaOH 6% NaOH 10%NaOH 20% NaOH 30% NaOH
1.174 1.407 1.188 1.379 1.072 1.181
0.332 –
0.957 0.999 –
0.329 1 0.999 –
0.612 0.980 1 0.977 –
0.971 0.999 1 0.999 1 –
sym.
Table 12 Post-peak flexural strength ANOVA p-value results for alkali treated fibres aged in hot lime water. Sample
Mean
2% NaOH
6% NaOH
10%NaOH
20% NaOH
30% NaOH
Untreated 2% NaOH 6% NaOH 10%NaOH 20% NaOH 30% NaOH
0.719 0.809 0.867 1.018 0.792 0.987
0.296 –
0.169 1 –
0.051 0.841 0.979 –
0.376 1 1 0.768 –
0.005 0.938 0.996 1 0.893 –
sym.
Table 13 Post-peak flexural strength ANOVA p-value results for acetylated treated fibres aged in water. Sample
Mean
5% A.Acid
10% A.Acid
5% A.Anh
10% A.Anh
6% NaOH, 5% A.Anh
6% NaOH, 10% A.Anh
Untreated 5% A.Acid 10% A.Acid 5% A.Anh 10% A.Anh 6% NaOH, 5% A.Anh 6% NaOH, 10% A.Anh
1.414 1.464 1.339 1.400 1.517 2.366 1.962
0.788 –
0.741 1 –
0.949 1 1 –
0.639 1.000 0.999 1 –
0.011 0.009 0.002 0.004 0.019 –
0.052 0.594 0.271 0.415 0.738 0.833 –
sym.
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Table 14 Post-peak flexural strength ANOVA p-value results for acetylated treated fibres aged in wet-dry cycles. Sample
Mean
5% A.Acid
10% A.Acid
5% A.Anh
10% A.Anh
6% NaOH, 5% A.Anh
6% NaOH, 10% A.Anh
Untreated 5% A.Acid 10% A.Acid 5% A.Anh 10% A.Anh 6% NaOH, 5% A.Anh 6% NaOH, 10% A.Anh
1.174 1.563 1.767 1.579 1.781 1.835 1.466
0.08 –
0.02 0.999 –
0.227 1 0.999 –
0.092 0.998 1 0.999 –
0.016 0.987 1 0.992 1.000 –
0.388 1 0.973 1 0.963 0.902 –
sym.
Table 15 Post-peak flexural strength ANOVA p-value results for acetylated treated fibres aged in hot lime water. Sample
Mean
5% A.Acid
10% A.Acid
5% A.Anh
10% A.Anh
6% NaOH, 5% A.Anh
6% NaOH, 10% A.Anh
Untreated 5% A.Acid 10% A.Acid 5% A.Anh 10% A.Anh 6% NaOH, 5% A.Anh 6% NaOH, 10% A.Anh
0.719 0.718 0.741 0.814 1.191 1.033 1.103
0.993 –
0.751 1 –
0.235 0.999 1 –
0.01 0.015 0.026 0.116 –
0.004 0.318 0.425 0.805 0.971 –
0.01 0.101 0.152 0.445 1 1 –
sym.
significant higher post-peak strength than specimens with untreated fibres as seen in Table 12. At concentrations of 2, 6 and 10% NaOH, the average post-peak strength was higher than that of the untreated fibre specimens under all ageing conditions. Similar results were found in a study by Kabir et al. [2]. Mwaikambo & Ansell [31] also found that there are optimum concentrations of NaOH at which the best results are found. In this study, low concentrations of 2, 6 and 10% NaOH showed the best results in terms of post-peak strength with higher concentrations of 20 and 30% NaOH being detrimental to the 28-day strength. For the acetylated fibre specimens, the majority had a significant effect on the post-peak strength as shown in Fig. 18 and Tables 13–15. After ageing in lime saturated hot water, treatment with 10% Acetic Anhydride, 6%NaOH-5% Acetic Anhydride and 6% NaOH-10% Acetic Anhydride showed a higher 90 day post-peak strength. The combination of alkali treatment and acetylation proved to be the best treatment condition under all the tested conditions. At 28 days, the highest strength was recorded for specimens where a combination of the treatments was used. Mwaikambo & Ansell [31] stated that the dimensional stability of sisal fibre is improved through acetylation. Thus, under wet/dry cycle ageing, acetylated fibres were more stable than the alkali treated fibres, leading to the increased durability. At 90 days, the post-peak strengths were significantly higher than the untreated fibre specimens for all ageing conditions.
Higher concentrations of 20 and 30% NaOH caused damage to the fibre structure. Acetylation of sisal fibre with both Acetic Acid and Acetic Anhydride at concentrations of 5% and 10% reduced the fracture strength of the fibre considerably with Acetic Acid also reducing the fibre-matrix bond. However, pre-treatment with NaOH resulted in a statistically significant increase in fraction strength and pull-out force. Chemical treatment improved the flexural post-peak strength of the specimens in the majority of the specimens under all ageing conditions. At low concentrations of NaOH, the post-peak strength of the specimens was improved, but at higher concentrations the strength decreased. The use of Acetic Acid and Acetic Anhydride to treat fibres proved to be less effective in improving the durability of SFRCC. However, when Acetic Anhydride was combined with NaOH the flexural post-stress under all the ageing conditions significantly increased when compared to the untreated fibre specimen. Acetic Anhydride treatment with a NaOH pre-treatment showed an improved fibre-matrix bond strength, fibres fracture strength and post-peak strength (under all the ageing conditions tested) when compared to the untreated fibres. Even though some durability improvements were found using certain treatments, the overall durability of sisal fibres in a cement-based matrix remains poor and further research is required to assist improving its behaviour.
4. Conclusions Sisal fibres are proposed as a sustainable alternative for use in fibre reinforced cement-based composites. In this study, the durability of chemically treated sisal fibre in cement-based composites was investigated. In summary, the following conclusions can be drawn: Chemical treatment of sisal fibres by sodium hydroxide and/ or acetic acid/anhydride improved fibre resistance to alkaline attack in the cement-based composites. This improvement in resistance was due to fibre surface improvements as revealed by SEM images for low chemical dosage treatments. From the single fibre pull-out (SFP) tests it was evident that at low concentrations of 2, 6 and 10% NaOH the fibre strength increased and the fibre-matrix bond was improved.
Finally, treated sisal fibres have a potential use in cementitious composites, however special consideration should be made when choosing the pre-treatment approach for the fibres. In this research, it was found that a combination of alkali treatment and acetylation improved the overall flexural behaviour, perhaps due to the marginally improved bond properties, whereas fracture was mostly unaffected.
CRediT authorship contribution statement M.D. de Klerk: Conceptualization, Methodology, Validation, Formal analysis, Writing - original draft, Visualization, Investigation. M. Kayondo: Writing - original draft, Visualization. G.M. Moelich: Formal analysis, Visualization. W.I. de Villiers: Writing - review & editing. R. Combrinck: Writing - review & editing. W.
M.D. de Klerk et al. / Construction and Building Materials 241 (2020) 117835
P. Boshoff: Conceptualization, Validation, Writing - review & editing, Supervision, Project administration, Funding acquisition. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors greatly appreciate the funding from PPC and Wilhelm Frank Bursary Fund which made this research possible. References [1] Y. Li, Y.-W. Mai, L. Ye, Sisal fibre and its composites: a review of recent developments, Compos. Sci. Technol. 60 (2000) 2037–2055. [2] M. Kabir, H. Wang, K. Lau, F. Cardona, Chemical treatments on plant-based natural fibre reinforced polymer composites: an overview, Compos. B Eng. 43 (2012) 2883–2892. [3] G. Ramakrishna, T. Sundararajan, Studies on the durability of natural fibres and the effect of corroded fibres on the strength of mortar, Cem. Concr. Compos. 27 (2005) 575–582. [4] H.S.V. Agopyan, V. John, M. Cincotto, Developments on vegetable fibre–cement based materials in São Paulo, Brazil: an overview, Cem. Concr. Compos. 27 (2005) 527–536. [5] A. Ramzy, D. Beermann, L. Steuernagel, D. Meiners, G. Ziegmann, Developing a new generation of sisal composite fibres for use in industrial applications, Compos Part B: Eng 66 (2014) 287–298. [6] F. de Andrade Silva, B. Mobasher, R. Dias Toledo Filho, Fatigue behavior of sisal reinforced cement composites, Mater. Sci. Eng., A 527 (2010) 5507–5513. [7] A. Belaadi, A. Bezazi, M. Bourchak, F. Scarpa, Tensile static and fatigue behaviour of sisal fibres, Mater. Des. 46 (2013) 76–83. [8] S.R. Ferreira, E. Martinelli, M. Pepe, F.A. Silva, R.D.T. Filho, Inverse identification of the bond behaviour for jute fibers in cementitious matrix, Composites B 95 (2016) 440–452. [9] S.R. Ferreira, M. Pepe, E. Martinelli, F.A. Silva, R.D.T. Filho, Influence of natural fibres characteristics on the interface mechanics with cement based matrices, Composites B 140 (2018) 183–196. [10] F. Pacheco-Torgal, S. Jalali, Cementitious Building Materials Reinforced with Vegetable Fibres: A Review, Construction and Building Materials University of Minho, Guimaraes, 2010. [11] M.A. Aziz, P. Paramasivam, S.L. Lee, Prospects for natural fibre reinforced concretes in construction, Int. J. Cem. Compos. Lightweight Concr. 3 (1981) 123–132. [12] J. Wei, C. Meyer, Improving degradation resistance of sisal fiber in concrete through fiber surface treatment, Appl. Surf. Sci. 289 (2014) 511–523. [13] K. Oksman, L. Wallström, L.A. Berglund, R.D.T. Filho, Morphology and mechanical properties of unidirectional sisal–epoxy composites, J. Appl. Polym. Sci. 84 (2002) 2358–2365. [14] T. Bessell, S. Mutuli, The interfacial bond strength of sisal—cement composites using a tensile test, J. Mater. Sci. Lett. 1 (1982) 244–246. [15] Y. Li, Y. Shen, The use of sisal and henequen fibres as reinforcements in composites, in: Biofiber Reinforcements in Composite Materials, Elsevier, 2015, pp. 165–210. [16] N. Saba, M. Jawaid, O.Y. Alothman, M. Paridah, A. Hassan, Recent advances in epoxy resin, natural fiber-reinforced epoxy composites and their applications, J. Reinf. Plast. Compos. 35 (2016) 447–470.
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