Enhancing mechanical properties of
mortar with short and thin banana fibers: A sustainable alternative to
synthetic fibers
a
School of
Engineering, Faulty of Science and Technology, Pokhara University, Pokhara,
Nepal b
Paschimanchal Campus,
Pokhara, Nepal
* Corresponding author.
E-mail addresses: lamichhaneniroj36@gmail.com (N.
Lamichhane), 074msste001aadarsha@pcampus.edu.np
(A. Lamichhane), tekrg@pu.edu.np
(T.R. Gyawali).
Abstract
The
use of fiber in mortar/concrete is now common for enhancing the flexural and
ductility properties of structures. However, the utilization of synthetic
fibers contributes to the emission of harmful greenhouse gases. Replacing these
synthetic fibers with natural fibers derived from waste plants is imperative
for sustainable development. The objective of this study was to evaluate the
performance of short and thin banana fibers in enhancing the mechanical properties
of fiber- reinforced mortar, specifically in terms of compressive, flexural,
and splitting tensile strengths. The base mortar, with a water-cement ratio of
0.30 and a unit water content of 298 kg/m3, was employed. The banana
fibers were manually extracted from banana stalks, dried in an oven, and then
cut into 10 mm fibers. The fibers were not treated with alkali. The fiber
content was varied at 0 %, 0.125 %, 0.25 %, 0.5 %, and 0.75 % by weight of
cement. Initially, the fibers were mixed into the viscous mortar along with the
first portion of water and a superplasticizer. Subsequently, workability was
improved by incorporating the second portion of water. The optimal content of
banana fiber was determined to be 0.25 %, which increased the 28-day compressive,
flexural, and splitting tensile strengths by 18.7 %, 29.9 %, and 41.1 %,
respectively, compared to the base mortar. These findings suggest that the
short and thin banana fiber has the potential to serve as a sustainable
alternative to synthetic fibers. However, it is essential to conduct a thorough
assessment of durability properties before implementing it in actual
structures.
Key
words
Sustainable
development, Banana fiber, Compressive strength, Flexural strength, Splitting-tensile
strength
1. Introduction
Concrete is a widely used construction material composed of
cement, water, and aggregate, mixed in specified proportions to achieve the
desired strength, serviceability, and durability of reinforced concrete (RC)
structures [1]. Its widespread availability,
cost-effective production, flexibility for shaping into various forms and
sizes, high compressive strength, rigidity, fire resistance, and durability are
key characteristics of concrete [2].
However, its main shortcomings include low tensile strength, flexural strength,
and brittle failure [3]. To address these shortcomings in RC
structures, steel reinforcement bars have been utilized since the 20th century
[4–7]. Moreover, the flexural and ductility behavior of RC
structures and cement composites has been further enhanced with the use of
short and discrete steel fibers, a practice that began as early as 1874 [8–11]. Nevertheless, a major drawback of using steel fiber in
structures is its susceptibility to corrosion [12].
Table 1
CO2 emissions associated with the production of 1 ton of fibers.
|
Fiber types |
CO2 emission
(ton) |
|
Steel |
1.4 [25] |
|
Carbon |
20 [26] |
|
Glass |
1.7–2.5 [27] |
|
Polypropylene
|
1.34 [28] |
|
Polyethylene |
1.48 [28] |
|
Polyvinyl
alcohol |
2.0 [29] |
Various types of artificial synthetic fibers have been
developed to replace steel fibers and address corrosion issues in structures [13]. The use of such synthetic fibers has not only resolved
corrosion problems but has also significantly reduced the weight of composite
structures, a major concern with steel fiber structures [14]. The inclusion of synthetic fibers in concrete, whether in
small amounts to reduce plastic shrinkage cracking or in larger quantities to
enhance flexural strength, toughness, ductility, and control crack width in the
hardened state, yields several benefits [15].
Polypropylene (PP) fibers, glass fibers, and polyvinyl alcohol (PVA) fibers are
frequently incorporated into concrete to enhance its mechanical and durability
properties [16,17]. The
use of these fibers is particularly advantageous in precast factories for
producing tunnel segments, permanent formwork, wall and flooring panels,
railway wind panels, bridge decks, slope stabilization, and more [18].
Their primary roles in concrete composites are increasing
flexural strength, fracture toughness, ductility, and controlling crack width
while reducing drying and autogenous shrinkage [19,20]. Guler and Akbulut [21]
reported that both single and hybrid combinations of macro and micro basalt
fibers increased the residual compressive and flexural strengths after 24, 48,
and 72 cycles of freeze-thaw tests on mortar, despite a decrease in
workability. Additionally, they found that hybrid basalt fibers outperformed
single fibers in enhancing residual compressive and flexural strengths. In
another experimental investigation utilizing polypropylene fibers, they also
concluded that hybrid combinations of micro and macro fibers were superior to
single macro fibers in enhancing the residual compressive and flexural
strengths of concrete after 150 cycles of freeze-thaw tests [22].
In 2015, the United Nations adopted 17 sustainable goals,
including "Climate Action" and "Life on Land" [23]. These goals have direct or indirect connections to the
construction and production industries. The construction industry alone is
responsible for 36 % of total energy consumption and 39 % of CO2 emissions
[24]. Despite the various advantages of steel fibers and
artificial synthetic fibers in enhancing the mechanical and durability
properties of concrete structures, they are not considered environmentally
friendly materials. In 2018, the International Energy Agency estimated that CO2
emissions accounted for 25 % of total global emissions [25]. Table 1 provides a summary of the CO2
emissions associated with producing 1 ton of different fibers.
Over the past three decades, researchers have explored the
use of natural fibers in cementitious composites [30,31]. Natural fibers are generally
considered eco-friendly construction materials with the potential to replace
conventional steel and other artificial fibers, thereby contributing to
sustainable development and reducing CO2 emissions [32]. Natural fibers should be widely accepted as sustainable
construction materials because they are not only readily available and
cost-effective but also recyclable. They possess high tensile and flexural
strength with low elongation at break [33].
Natural fibers can be sourced from minerals, animals, and plants. Mineral
fibers are derived from sources such as asbestos, graphite, and glass, while
animal fibers come from sources like hair, silk, and wool. Various types of plant
cellulose fibers are extracted from materials such as grass, seeds, leaves,
stalks, bast, wood, and more [34].
Previous research has employed natural fibers like sisal
fiber [35], kenaf fiber [36],
bamboo fiber [37], banana fiber [38], jute fiber [39],
coconut fiber [40], hemp fiber [41], bagasse fiber [42],
abaca fiber [43], and others. In recent years, efforts
have been made to incorporate banana fiber (BF) into mortar/concrete. Bananas
are fruit plants belonging to the Musaceae family [44]. After harvesting banana fruit, the entire banana stalk is
typically left on the ground to decompose as agricultural waste [45,46] The banana stem contains fibers
composed of approximately 50 % cellulose, 17 % lignin, along with hemicellulose
and pectin, resulting in higher tensile and flexural strengths and low
elongation at break [47–49]. Cellulose is the primary
constituent of BF, which predominantly influences its mechanical properties [50]. BF is biodegradable, cost-effective, easily extracted,
and eco-friendly as it emits oxygen (O2) rather than carbon dioxide
(CO2) [51,52]. It
is also lightweight, possesses low density, and exhibits resistance to water
and fire [53]. Additionally, the inclusion of BF
mitigates concrete cracking and spalling while enhancing flexural behavior [54].
BF can be manually or mechanically extracted from banana
stalks [55]. They are used in the form of
reinforcement bars [56] or as discrete fibers [46,48,52,55,57–61] in mortar/concrete. It has
been reported that BF has a drawback related to weaker interfacial bond
strength between the fiber and the matrix [62]. To
address this issue, most researchers have treated BF with alkali to enhance its
surface toughness and reduce its hydrophilic nature [46,52,56,57,61,62].
Nensok et al. [52] observed a decrease in BF diameter by
18.2 % and an increase in density by 7.4 % after treatment with a 6 % NaOH
solution. They also reported an increase in single fiber flexural strength from
166 MPa to 487 MPa and a modulus of elasticity increase from 14.3 GPa to 28.7
GPa. Ernest and Peter [62] also
reported improvements in tensile strength, flexural modulus, and elongation.
Nensok et al. [52]
compared the mechanical properties of mortar using 0.4 % (by weight of mortar)
of 30 mm BF treated with different NaOH solution concentrations. They found
that using BF treated with a 6 % NaOH solution resulted in the highest
compressive, flexural, and splitting tensile strengths of the mortar. Elbehiry
et al. [54] conducted experimental investigations
using BF as reinforcement bars in M25, M35, and M45 grade concrete beams,
showing a 25 % enhancement in concrete flexural strength across all grades. Zhu
et al. [46,56]
reported an increase in flexural strength from approximately 12 MPa–25 MPa when
using 14 % (by mass) of BF strands. Mouli et al. [58] achieved a 35 % enhancement in the 28-day compressive
strength of M30 grade concrete by adding 3 %
Table 2
Fundamental and primary chemical constituents of 43-grade OPC.
|
Chemical
components |
|
|
Components |
Content (%,
by weight) |
|
Basic
compounds: CaO |
62.69 |
|
SiO2 |
21.01 |
|
Al2O3 |
5.31 |
|
Fe2O3 |
3.78 |
|
SO3 |
2.53 |
|
K2O
|
0.57 |
|
MgO |
1.81 |
|
Na2O
|
0.31 |
|
Others |
1.99 |
|
Major
compounds: C3S |
45.1 |
|
C2S
|
26.3 |
|
C3A
|
9.30 |
|
C4AF
|
8.58 |
|
Others |
10.80 |
Table 3
Basic properties of cement.
|
Items |
Values |
|
Colour |
grey |
|
Fineness |
5.53 % |
|
Specific
gravity Normal
consistency |
3.14 28.0 % |
|
Le-Chatelier |
5 mm |
|
Initial
setting time |
145 min |
|
Final setting
time |
245 min |
|
Compressive
strength at 28-day |
52.15
MPa |
BF with a length of 40 mm. similar enhancements of 18.6 %
and 18.2 % were observed with 0.5 % BF content in the research of Keshavraman [60] and Ali et al. [61],
respectively. Akinyemi and Dai [63]
reported a 20 % increase in splitting tensile strength of mortar by using 1.5 %
of 13 mm BF in addition to 10 % wood bottom ash and 0.3 % styrene-butadiene
rubber.
Recently, Rajkohila et al. [38]
conducted a detailed experimental investigation on the influence of banana
fiber (BF) and coir fiber (CF) on the mechanical and microstructural properties
of high-strength concrete. They observed notable improvements in the mechanical
properties of concrete upon the incorporation of BF and CF. Their study
revealed that the addition of 1 % BF notably enhanced the 28-day mechanical properties
of high-strength concrete compared to CF, contributing to the early detection
of potential failures due to its elastic behavior.
When using BF in mortar/concrete, most researchers have
varied their length from 30 to 60 mm. Mugume et al. [57] demonstrated that 40 mm BF yielded better performance than
50 mm and 60 mm in terms of mechanical properties of mortar and concrete. They
also noted that the optimal content increased with longer BF lengths.
Furthermore, previous research has shown that the use of thinner and shorter
polyvinyl alcohol (PVA) fibers enhances the flexural properties of mortar [64]. In the work of Akinyemi and Dai [63], shorter banana fiber (13 mm) was employed, albeit with a
single content of banana fiber (1.5 % by weight of the mortar). Thus, this
paper investigates the impact of different content levels of short BF on the
mechanical properties of mortar, with a fixed length of 10 mm for the BF.
2. Materials
and method
2.1. Materials
and mixture proportions
Ordinary Portland Cement (OPC) of
43 grade, conforming to NS 49:2053 [65], was
utilized in this series of experiments. The fundamental constituents of the
cement were tested in the laboratory according to IS: 4032-1985 [66], and the primary constituents were determined
using Bogue’s equations. The fundamental and primary constituents are outlined
in Table 2.
The specific gravity,
consistency, soundness, setting time, and compressive strength tests were
conducted following the guidelines provided in IS 2720 (Part III) [67], IS 4031 (Part IV) [68],
IS 4031 (Part III) [69], IS 4031 (Part V) [70], and IS 4031 (Part VI) [71], respectively. The test results are presented
in Table 3.
The fine aggregate was sourced
from a nearby aggregate crushing industry in Kotre, Tanahun, Nepal. The sieve
analysis of the fine aggregate was conducted in accordance with IS2386 (Part I)
[72]. Specific gravity, density, and water
absorption tests were carried out following the guidelines provided in IS2386
(Part III) [73]. Table
4 displays the essential properties of the fine aggregate utilized in
the experiment.
Table 4
Physical properties of fine aggregate.
|
Properties |
Values obtained |
|
Maximum aggregate size (mm) |
4.75 |
|
Specific Gravity Bulk density (kg/m3) |
2.6 1647 |
|
Fineness Modulus |
2.8 |
|
Grade Zone |
I |
|
Water Absorption (%) |
0.20 |
|
Silk content (%) |
2.36 |
Fig. 1. Image of
BF after extraction and dried in oven.
Table 5
Mixture proportions of mortar.
|
Mixture condition |
|
Unit content (kg/m3) |
|
|
|
|
W/C |
BF (%) |
Water |
Cement |
Fine aggregate |
BF |
|
0.30 |
0.000 |
298 |
993 |
1006 |
0 |
|
|
0.125 |
|
|
1005 |
0.3725 |
|
|
0.250 |
|
|
1004 |
0.7450 |
|
|
0.500 |
|
|
1003 |
1.4900 |
|
|
0.750 |
|
|
1002 |
2.235 |
The superplasticizer used was
categorized as type F according to the ASTMC 490-10 specification, and it had a
pH level of 6.2, a specific gravity of 1.12, and exhibited a pale yellow
colour. The water used was potable, with a measured pH value of 7.0.
Banana pseudo-stems were gathered
from a local banana farm, washed to remove mud and debris, and subsequently cut
into small pieces to allow for the separation of the leaf sheaths. The fiber
was manually extracted and dried in an oven at 60 ◦C for 4 h. Fig. 1 depicts images of the extracted BF after
the drying process. Once the BF was completely dry, it was chopped into
approximately 10 mm pieces for use in the experiment. The fiber had a
grey/creamy color, was odorless, and had an average measured diameter and
density of 0.208 mm and 1325 kg/m3, respectively. It is worth noting
that previous research has demonstrated that alkali treatment of BF enhances
the mechanical properties of mortar/concrete. However, for this series of
experiments, the aim was to determine the optimal BF content for the given
mortar mixture proportions. As a result, alkali treatment of the banana fiber
was intentionally omitted.
The mixture proportions of the
mortar closely resembled those used in the development of high ductile mortar [64]. They were slightly adjusted to accommodate
the specific characteristics of the ingredients and the need for adequate
workability during the compaction process while casting the test specimens. BF
content varied from 0 % to 0.125 %, 0.25 %, 0.5 %, and 0.75 % by weight of
cement. To account for the volume occupied by BF in one cubic meter (m3)
of the total mixture, the volume of fine aggregate was adjusted accordingly.
The specifics of the mixture proportions are outlined in Table 5. It is worth noting that a uniform 1.0 %
by weight of cement of superplasticizer was utilized in each mix.
Fig. 2. Visual
observation and hand touching checks of BM mortar.
Fig. 3. Finishing condition of the beam specimens.
2.2. Mixing method
The laboratory was maintained at a temperature of 20 ± 2 ◦C and a relative humidity of 65 %. A J-type mortar mixer with a 5-L capacity was used for the experiments. Each batch had a volume of 4 L. The mixing procedure closely followed the method employed for mixing high ductile mortar using PVA fiber [74].
The superplasticizer was premixed with the first part of water (75 %) in a bucket using a spoon. First, half of the fine aggregate was added to the mixer, followed by the entire amount of cement, and then the remaining fine aggregate, creating a sandwich-like arrangement. Dry mixing of the solid ingredients was carried out for half a minute. Subsequently, the first part of the water (containing superplasticizer) was poured and mixed for 2 min to achieve a viscous mixture. The banana fiber (BF) was added gradually, in portions, to prevent agglomeration, with a mixing time of one and a half minutes. Finally, additional mixing was conducted for 2 more minutes with the addition of the second part of the water to achieve a workable BF mortar. It is important to note that the time taken for adding the BF (one and a half minutes) was subtracted when preparing the control mortar without BF.
A few minutes after mixing,
ambient and mortar temperatures were recorded to ensure there was no
significant difference. Visual and tactile checks were performed to confirm the
uniform distribution and coating of BF, as well as to assess the workability of
the BF mortar for casting (refer to Fig. 2).
Before casting the specimens, each mold was thoroughly cleaned, and the inner surface was coated with grease. From each parameter batch, 6 cubes of dimensions 70.6 mm × 70.6 mm × 70.6 mm were cast for the 7-day and 28-day compressive strength tests. Likewise, 6 beams with dimensions of 400 mm (length) × 50 mm (breadth) × 100 mm (depth) and 6 cylinders with dimensions of 100 mm (diameter) × 200 mm (depth) were cast for flexural and splitting tensile strength tests, respectively. The cubes and cylinders were cast in three layers, with 20 tamping strokes at each layer. The beam specimens were also cast in three layers, but with 40 tamping strokes at each layer. Careful attention was given to the finishing of the upper surface to ensure an even surface and prevent voids.
Fig. 4. Wet
curing condition of the test specimens.
Fig. 3 illustrates the
finishing condition of the upper surface of the test specimens.
Following the completion of specimen casting, initial curing
was carried out for 24 h by covering the upper surface with clean, damp cloths
in a controlled environment. After this initial period, all specimens were
demoulded and placed inside a water curing tank, where they remained until the
testing day. The water temperature was maintained at 20 ± 2 ◦C
during the entire curing process. The wet curing of the specimens is depicted
in Fig. 4.
Strength tests were conducted at both 7 days and 28 days. Before the tests, the specimens were removed from the curing tank, and their surfaces were wiped clean with cloths. Subsequently, they were placed on the floor to allow their surfaces to dry. The dimensions and weights of all specimens were recorded to calculate their respective densities. The strength tests were carried out using a universal testing machine with a capacity of 1000 kN, and Indian standard procedures were rigorously followed for specimen setup and loading. Compressive, flexural, and splitting strength tests were performed in accordance with the guidelines outlined in IS: 516 [75]. Following the recording of the maximum load for all test specimens, the respective strengths were calculated using Equations (1)–(3) for compressive, flexural, and splitting tensile strengths, respectively.
fc =aP2 (1)
fb =23bdPl2 (2)
fst =π2LDP (3)
In the equations above, fc,
fb, and fst represent
the compressive, flexural and splitting tensile strengths of the cube, beam,
and cylinder, respectively. a is one
dimension of the cube. l, b and d denote the effective span, width and depth of the beam,
respectively. D and L stand for the diameter and depth of
the cylinder specimen, respectively.
3. Results and discussion
3.1. Density
Based on visual observation and
tactile examination, it was evident that the BF was uniformly distributed and
well-coated throughout the mixed mortar. No agglomeration of BF was detected
within the mortar mix. Notably, despite varying levels of BF content, the
mortar exhibited the necessary workability essential for casting cubes,
cylinders, and beams. The density results measured from all test specimens at 7
and 28 days are depicted in Fig. 5.
The densities at both 7 and 28 days were nearly identical, regardless of the varying percentages of BF. This similarity can be attributed to the relatively low BF content, which had a minimal impact on the density. Interestingly, the densities of the beams were recorded as higher, while the densities of the cylinders were lower than those of the cubes. This discrepancy was a result of the compaction method employed during specimen preparation.
In the case of beams, compaction was achieved by using a
hand trowel at each layer in addition to tamping. However, for cubes and
cylinders, the mortar was only compacted from the upper surface. The lower
density in cylinders can be attributed to the greater thickness of the layers
compared to cubes and beams. It is important to note that no vibration was used
during the compaction process.
3.2. Compressive strength
7-day and 28-day compressive strength test data verses BF
(%) are presented in Fig. 6.
When introducing 0.125 % of BF to the control mortar, both
7-day and 28-day compressive strengths exhibited a significant increase. This
increase was gradual as the BF content ranged from 0.125 % to 0.25 %. At 0.25 %
BF content, the optimum compressive strength was achieved, beyond which the
compressive strength began to decline with higher BF content. Notably, at 0.75
% BF content, the compressive strength was lower than that of the control
mortar.
Nensok et al. [52] used
30 mm length BF and determined that the highest compressive, flexural, and
splitting tensile strengths were attained at the optimum content of 1.2 % by
weight of cement, both for untreated and those treated with 6 % NaOH. Mugume et
al. [57] found the optimum compressive
strengths for M20 and M25 grade concrete with 0.1 % of 40 mm BF, while the
flexural and splitting tensile strengths were maximized at 0.25 % of 60 mm BF.
Mouli et al. [58] achieved the maximum
compressive strength for M30 grade concrete with 3 % of 40 mm BF and flexural
strength with 4 % BF. Ali et al. [61]
obtained the maximum compressive, flexural, and splitting tensile strengths for
M30 grade concrete at 0.5 % (by weight of concrete) of treated BF with 6 %
NaOH.
Based on the analysis of the trend of increasing compressive
strength of the mortar with BF content up to the optimum and then decreasing,
an empirical model was developed as shown in Equation (4).
fc =fcm + ksin 4(BF)0.7 (4)
In the above equation, fc represents the compressive strength of mortar with varying BF content, and fcm signifies the compressive strength of the control mortar. BF denote the percentage of BF by weight of cement, and k stands for the material constant, which set to a value of 10 to align with the experimental data.
Fig. 7. Relation
of flexural strength of mortar with ranging content of BF.
Fig. 8. Relation
of splitting tensile strength with ranging content of BF.
3.3. Flexural strength
The relationship between flexural strength and BF content is
illustrated in Fig. 7. It is evident from
the graph that the pattern of increase and decrease in both 7-day and 28-day
flexural strengths with varying BF content mirrors that of compressive
strengths. The empirical model presented in Equation (4)
also applies to flexural strength, as demonstrated in Equation (5).
fb =fbm + lsin 4(BF)0.7
(5)
Only fb and fbm denote the flexural strengths of BF mortar and the control mortar, respectively. The value of l was set at 2.4 to match the experimental data.
3.4. Splitting tensile
strength
The trend in the relationship between the splitting tensile
strength of the mortar and varying BF content was also observed to be similar
to that of the compressive and flexural strengths (refer to Fig. 8).
The empirical model presented in Equation (6) closely resembles those for compressive and
flexural strength, with the change of the respective symbols. In this case, the
value of m was set at 1.7 to align
with the experimental data.
3.5. Summary of data
Table 6 presents a
summary of all the tested data, including the averages of three test results
and the standard deviations for the
Table 6
Summary of test data.
|
BF (%) |
Age |
Densities (kg/m3) cubes |
cylinder |
beams |
Strengths (MPa) |
|
|
|
fc |
fb |
fst |
|||||
|
0.000 |
7-day |
2323 ± 7 |
2142 ± 41 |
2409 ± 41 |
36.32 ± 1.01 |
5.57 ± 0.07 |
3.48 ± 0.09 |
|
|
28-day |
2297 ± 9 |
2148 ± 5 |
2447 ± 19 |
46.73 ± 0.86 |
8.45 ± 0.19 |
4.48 ± 0.12 |
|
0.125 |
7-day |
2316 ± 23 |
2190 ± 11 |
2450 ± 26 |
44.18 ± 1.21 |
7.31 ± 0.17 |
4.87 ± 0.09 |
|
|
28-day |
2284 ± 13 |
2103 ± 33 |
2469 ± 15 |
53.91 ± 1.22 |
10.41 ± 0.24 |
5.98 ± 0.12 |
|
0.250 |
7-day |
2387 ± 10 |
2124 ± 7 |
2426 ± 56 |
45.72 ± 0.87 |
7.65 ± 0.18 |
5.23 ± 0.06 |
|
|
28-day |
2401 ± 10 |
2141 ± 1 |
2442 ± 17 |
55.45 ± 1.41 |
10.98 ± 0.28 |
6.32 ± 0.17 |
|
0.500 |
7-day |
2365 ± 9 |
2121 ± 9 |
2396 ± 41 |
41.82 ± 1.03 |
7.01 ± 0.19 |
4.38 ± 0.10 |
|
|
28-day |
2366 ± 27 |
2152 ± 2 |
2400 ± 20 |
51.65 ± 1.19 |
9.73 ± 0.21 |
5.47 ± 0.17 |
|
0.750 |
7-day |
2366 ± 21 |
2136 ± 1 |
2409 ± 41 |
35.74 ± 0.67 |
5.47 ± 0.14 |
3.18 ± 0.07 |
|
|
28-day |
2357 ± 7 |
2151 ± 6 |
2448 ± 19 |
45.62 ± 0.70 |
8.07 ± 0.23 |
4.30 ± 0.11 |
fc:
Compressive strength; fb:
Flexural strength; fst:
Splitting tensile strength.
Average density data revealed that there was no significant
difference between the age of specimens and the varying percentages of BF. The
overall average density of the cylinders was 8.7 % lower, while that of the
beams was 3.5 % higher than that of the cube specimens. This discrepancy can be
attributed to differences in compaction methods and the very low percentage of
BF used. Ernest and Peter [62] reported a
decrease in density with an increase in BF content, which can be attributed to
their use of higher BF content, ranging from 5 % to 20 % by volume in cement
composites. Zhu et al. [56] also obtained
similar results, with a decrease in density of polyester composites as the
weight percentage of BF strands increased.
In terms of compressive strength, the 28-day values
increased by 15.4 %, 18.7 %, and 10.5 % compared to the control mortar for BF
percentages of 0.125 %, 0.25 %, and 0.5 %, respectively. However, there was a
2.4 % decrease in compressive strength for 0.75 % BF. The 28-day flexural
strength saw increases of 23.2 %, 29.9 %, and 15.1 % for 0.125 %, 0.25 %, and
0.5 % BF, with a 4.5 % decrease for 0.75 % BF. For 28-day splitting tensile
strength, there was an increase of 33.5 %, 41.1 %, and 22.1 % for 0.125 %, 0.25
%, and 0.5 % BF, respectively, but a 4.0 % decrease for 0.75 % BF. These results
validate that the optimum BF content was 0.25 %, resulting in an 18.7 %, 29.9
%, and 41.1 % increase in compressive, flexural, and splitting tensile
strength, respectively.
Nensok et al. [52]
reported a 13.7 %, 32.7 %, and 45.3 % increase in compressive, flexural, and
splitting tensile strength, respectively, with the use of 1.2 % of 30 mm
untreated BF. They also reported increases of 59.8 %, 117.4 %, and 157 % with
the use of 6 % NaOH treated BF. It is worth noting that their results were for
very low-strength cellular lightweight concrete, with 28-day compressive,
flexural, and splitting tensile strengths of the control concrete measuring
only 5.20 MPa, 0.98 MPa, and 0.64 MPa, respectively.
Fig. 10. Relation of flexural strength with compressive strength.
Mugume et al. [57]
employed 40 mm, 50 mm, and 60 mm of 5 % NaOH treated BF in M20 grade concrete
and reported a 14 % increase in compressive strength at 0.1 % of 40 mm BF,
which decreased with higher BF content. Flexural strength and splitting tensile
strengths increased by 10.4 % and 4 % at 0.25 % of 60 mm and 40 mm BF,
respectively. Mouli et al. [58] achieved a
35 % and 47.4 % increase in compressive and flexural strengths of M30 grade
concrete with the use of 3 % and 4 % of 40 mm BF, respectively. Ali et al. [61] reported an 18.2 %, 16.7 %, and 9.2 % increase
in compressive, flexural, and splitting tensile strengths of M30 grade concrete
with 0.5 % of 50 mm BF treated with 6 % NaOH. Akinyemi and Dai [63] reported a 3.64 % and 20 % increase in flexural
and splitting strengths of C: S = 1:3 mortar with 1.5 % of 13 mm BF. Elbehiry
et al. [54] obtained a 25 % enhancement in
flexural strength when using BF as reinforcement bars in concrete. In contrast,
Afraj and Ali [59] found a 40 % decrease in
flexural strength of concrete with 5 % of 50 mm BF.
Comparing all the aforementioned previous results, two major
points can be drawn from this study. First, shorter fibers are more effective
for enhancing mortar strength and can lead to a lower optimum content to
achieve maximum strengths. Another significant point is that the use of the
High Ductile Mortar (HDM) mixing method likely had a positive effect on the
strength results, as previously verified in a separate study [64]. While BF was not treated with alkali in this
study, it can be inferred from other researchers’ results that strengths can be
further enhanced with the alkali treatment of BF.
3.6. Failure modes
During testing, it was observed that the control mortar
specimens failed with a single crack, resulting in complete brittleness.
However, the inclusion of BF minimized the brittle failure of the test
specimens (refer to Fig. 9). Multiple cracks
were observed in the mortar specimens containing BF. The bridging effect of BF
was clearly noticeable in the early stages of micro-cracking. In the cube and
cylinder specimens, initial multiple cracks appeared in the middle region of
the depth. In the beam specimen, multiple cracks emerged at the bottom of the
mid-span, and the single crack widened and extended to the top as the load
increased until the specimens failed. In all cases, the beam specimens
exhibited flexural failure.
Rehman and Sudheer [76]
observed that nearly 70 % of the BF were pulled out, while the remaining 30 %
were broken at the failure surface. They concluded that the bridging effect of
BF prevented shrinkage cracks during loading. Afraj and Ali [59] revealed that BF enhances the bridging effect
and the bond between the concrete matrix and fibers, leading to increased
energy absorption and toughness index. Zhu et al. [46]
also reported that shorter fibers disperse more uniformly without clumping,
effectively resisting initial micro-cracks. This effect was even more
significant in this study due to the use of short BF and the HDM mixing procedure.
According to Jagadeesh et al. [77], BF
possesses an elliptical cross-section, which enhances tensile strength.
3.7. Relation of strengths
Different types of concrete strength can be estimated from
its compressive strength. Fig. 10 illustrates
the relationship between the flexural strength and compressive strength of
mortar with varying contents of BF. Equation (7) provides
the empirical model for the relationship between flexural strength and
compressive strength:
fb =0.01afca (7)
In the equation fb and fc represent flexural strength and the compressive strength, respectively. The material constant is denoted by a. The values of a was chosen to 1.62 to best fit the experimental data. The figure also displays the empirical model of Gyawali [78], which significantly differs from the data of this study and follows a different trend. This disparity is because the model of Gyawali was developed for lightweight mortar using EPS beads.
The relationship between the splitting tensile strength and
the compressive strength is depicted in Fig. 11.
Fig. 11. Relation
of splitting strength with compressive strength.
The empirical model follows the same form as Equation (7), with a value of 1.5 chosen for a in the case of splitting strength. The
figure also includes the empirical models of two other researchers. The trend
of their models differs from that of this study. The model of Gyawali [78] falls below the experimental data, while Babu
et al.’s model [79] is higher and only
covers the middle range of the data. Their model is based on experimental data
from EPS concrete.
This study has confirmed that a very low content (0.25 %) of
short BF fibers (10 mm) is suitable for mortar applications. The HDM mixing
method has played a significant role in enhancing the strengths of mortar, even
when using untreated BF fibers. However, it is important to note that many
researchers have emphasized the necessity of alkali treatment for BF to further
enhance strength and improve durability. Akinawande et al. [80] reported that alkali treatment reduces the
hydrophilic nature of BF and, consequently, its water absorption capacity.
Additionally, Ernest and Peter [62] have
suggested various chemical treatment options for natural fibers. Therefore,
future research efforts should focus on evaluating the effectiveness of alkali
treatment and assessing durability properties before considering the practical
application of thin and short BF mortar composites in real structures.
4. Conclusion
BF was manually extracted from banana stalk waste and then dried in an oven at 60 ◦C for 4 h. It was subsequently chopped into 10 mm fibers. The mortar was mixed using the high ductile mortar mixing method, with varying fiber contents of 0 %, 0.125 %, 0.25 %, 0.5 %, and 0.75 % by weight of cement. The following key findings were obtained from this experimental investigation.
·
The high ductile mortar mixing method
facilitated uniform distribution and firm coating of the thin and short BF
fibers.
·
Introducing different contents of BF had no
significant impact on the density of the mortar, primarily due to the small
volume of BF relative to the total mortar volume.
·
The optimum content of BF was found to be 0.25
%, resulting in an 18.7 % increase in compressive strength, a 29.9 % increase
in flexural strength, and a 41.1 % increase in splitting tensile strength
compared to the base mortar.
·
The lower content optimum (0.25 %) was
attributed to the use of very short BF fibers (10 mm).
·
The brittle failure mode of specimens was
notably reduced due to the formation of multiple micro-cracks, a result of BF’s
bridging performance.
· Empirical models were proposed to fit the respective experimental data.
Future work should focus on assessing the performance of
alkali-treated BF and investigating the durability properties of BF mortar.
Credit authorship
contribution statement
Niroj Lamichhane: Writing – original draft, Validation,
Methodology, Investigation, Formal analysis, Data curation, Conceptualization.
Aadarsha Lamichhane: Writing – original draft, Methodology,
Investigation, Data curation.
Tek Raj Gyawali: Writing – review & editing,
Supervision, Investigation, Formal analysis, Conceptualization.
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.
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