The usage of green materials in construction activities has become very interesting in recent years due to its advantages in different sectors. Many studies have shown the possible uses of natural fiber, such as the partial replacement of cement and aggregate, as well as fiber reinforcement [1
]. Coconut fiber was known as one of main fifteen plant and animal fibers in the world [7
]. This fiber has a high concentration of lignin among vegetable fibers, up to nearly 50%, which makes it stronger [8
]. Coconut fiber is extracted from the tissues surrounding the envelope of the coconut palm, which is grown on 10 million ha of land throughout the tropics. According to the Food and Agriculture Organization of the United Nations FAO [9
], five countries ranked at the top positions in the world for producing coconut fiber are India, Sri Lanka, Thailand, Vietnam, and Philippines. These countries produce more than 90 percent of the global coconut fiber production. Coconut fiber is commonly utilized in ropes, mattresses, brushes, geotextiles and automobile seats. In contrast, the applications of this fiber in civil engineering are very few and do not commensurate with its potential. In the future, more research and investigation are needed to show coconut fiber as a possible reinforcement in the construction field.
The properties of natural fibers for their applications in civil engineering remain a topic of interest for a wide range of studies. Natural fibers are considered as the prospective material to be used as reinforcement materials in composite products. A detailed literature review on the properties of coconut fiber and its comparison with other natural fibers is reported in present study. Mechanical properties of vegetable fibers depend on factors such as their physical, chemical, morphological and geometrical characteristics. Munawar et al. [10
] have investigated relationships between the density, diameter and mechanical properties of seven non-wood plant fiber bundles (including abaca leaf fiber, pineapple leaf fiber, sansevieria fiber, sisal fiber, coconut fiber, kenaf fiber and bast fiber). The results show that the density and mechanical properties of the fiber decreased with increasing fiber bundle diameter. In addition, only coconut fiber exhibits an almost circular shape on the cross-section, while the cross-sectional shape of other fibers varies widely. Sengupta et al. [11
] analyzed the main physical properties of coconut fiber and indicated that there is a dissymmetrical nature with a big coefficient of variation in the distribution of length, diameter, density, breaking toughness and extension, specific work of rupture and flexural rigidity of untreated fibers. They recommended the usage of long fibers (25.4 mm or 1 inch) to manufacture the traditional ropes and coarse floor covering. Single or with other fibers, medium length fibers (12.7 mm or ½ inch) will better suit to produce finer textile materials with improved characteristics. Contrarily, short fibers (3.8 mm) may be engaged in making not only flexible or semi-rigid composites but also geo-fiber for stabilization of soil. According to Tran et al. [12
], the density of coconut fiber decreases from 1.3 to 0.9 g/cm3
while the fiber porosity increases from 22% to 31% with the increase in the fiber length from 0.05 to 4.0 mm. This phenomenon can be explained by considering the structural characteristic of coconut fiber. Enclosed porosity, which is included in the measured volume of solid material of coconut fiber, is increased with the increase in fiber length. This result shows that the length of fiber has direct effects on its density. The results from a number of studies [10
] confirm that coconut fiber is not very strong and stiff. However, its high strain to failure value may lead to an increase in the toughness of composites while using it as reinforcement for the composites.
Weak bonding between fibers and the matrix and their relatively significant moisture absorption are considered as the main drawbacks of natural fibers in composite products [18
]. Thus, to modify the surface properties of fibers for improving their adhesion with the matrix, treatment of fibers is highly considered. Some techniques, including physical and chemical methods to treat fibers before using them in composites, are presented in several papers [15
]. The researches related to the effects of treated fibers on the properties of fiber-reinforced composites are also available in different papers [13
]. The influence of treatment methods on the physical and mechanical properties of coconut fibers were also reported in [20
], which stated that softening treatment of raw fibers using a solution of Na2
, and NaOH decreases the flexural rigidity value of raw fiber by three fourths without any degradation of desired characteristics.
In terms of thermal conductivity of plant fibers, some studies [2
] introduced the potential use of plant fibers as thermal insulation materials in construction which can reduce environmental impacts in comparison with currently used synthetic thermal insulation materials. Manohar [26
] reported that experimental thermal conductivities of coconut fibers at mean temperature (15.6 °C) in accordance with ASTM C518 lie between 0.04869 and 0.05624 W/m.K for densities varying from 90 and 40 kg/m3
. Researchers then even suggested creating composites with two or more types of fibrous plants for thermal insulation materials. Khedari et al. [30
] mixed durian peels and coconut fibers (ratio of 90:10 coconut fibers and durian peel by dry weight) to manufacture particleboards with low thermal conductivity (in the range of 0.054–0.1854 W/m.K) depending on the board density. This result shows viable options to apply plant fibers in building insulation (wall and ceiling).
Regarding the durability of fiber, Sivakumar Babu et al. [31
] reported that coconut fibers lasted only for 2–3 years without any treatment. Hejazi et al. [32
] indicated that coconut fiber retained 80% of its tensile strength after six months of embedment in clay. Arifuzzaman Khan et al. [21
] conducted a series of experiments indicating that all the treatments using sodium chlorite, sodium hydroxide, and acrylamide monomer could cause changes in the physical and chemical properties. These treatments improve the thermal stability of coconut fiber by causing lower weight loss and shifting of degradation peak to a higher temperature. When exposed to 180 °C, within 5 h, the drop of approximately 36.4%, 24.6%, and 23.2% in the tensile strength was observed for the fibers treated with sodium chloride, sodium hydroxide and acrylamide monomer chlorite, respectively. Thermogravimetric analysis (TGA) was performed to determine the weight loss of some fiber components, along with deterioration products [15
]. From the TGA results, it can be seen that the residual mass of heat untreated coconut fibers is lower as compared to the treated coconut fibers in the temperature range of up to 900 °C, which included the thermo-oxidative effect and evaporation of low molecular weight components in lignocellulose fibers. Hemicellulose is the most sensitive reactive constituent and its thermal degradation is easier than cellulose and lignin, according to Stevulova et al. [33
These coconut fibers could play an important role in reinforcing the building materials or improving/modifying their certain properties. Mechanical, thermal and acoustic properties are highly dependent on the characteristics of the fiber itself. As reported by various authors in the literature, the determination of the characteristics of coconut fibers poses a problem due to the great variability of these. This variability depends on several factors, such as the origin of the coconuts and the storage and processing methods used to obtain these fibers. Each study, like that of the incorporation of coconut fibers in mortars, requires an appropriate determination of the properties of the fibers used, i.e., the potential deposit of fibers at its disposal. The present paper aims at providing further knowledge on the determination of coconut fibers properties in manufacturing reinforced mortars by incorporating different amounts of this type of fibers. The purpose of this present study is the assessment of geometrical, physical and mechanical to thermal properties and durability properties of local coconut fibers (Vietnam). Once determined, these properties are compared with those observed in the literature. This comparative study makes it possible to situate the level of performance of these fibers with respect to other natural fibers, in particular coconut fibers, and to consider using them as reinforcement in mortars. If the fibers tested are suitable for the manufacture of reinforced mortars, it seems necessary to control their preparation. This approach could, therefore, constitute an alternative solution to waste management and contribute to the development of reinforced mortars improving comfort performance in buildings.
The knowledge of the properties of coconut fiber is necessarily required to use it as reinforcement in composite materials. Two treatment approaches were considered: physical and chemical methods. The experimental results obtained put forward the following conclusion:
The treatment process removes a part of the fiber surface, resulting in a rougher fiber surface and increased absolute density as well as the decreased diameter of fiber by roughly 10% and 30% for alkaline and boiled treatment, respectively.
Water absorption capacities of alkali-treated and raw fiber were found to be almost the same, but there was a remarkable reduction in water absorption capacity of boiled fiber.
The experiments to determine the thermal conductivity of the bundles of fiber have shown that both treatment methods do not affect the thermal conductivity values. For all types of fiber, the thermal conductivity was decreased from 0.052 to 0.024 W/m.K with the increase in density of the fiber bundles from 30 to 120 kg/m3.
Both treatment processes decreased the tensile strength of the fiber. This happens due to a reduction in lignin, pectin, fatty acid and cellulose due to the treatment process. By contrast, tensile strains at failure of fibers were increased significantly by 18% and 51% after chemical and physical treatment, respectively. This implies that the ductility of fibers has increased after treatment.
The same trend of thermal behavior of all the types of fibers was found in TGA and DTA tests. However, the higher thermal stability of treated fibers was observed in comparison to raw fiber by virtue of the partial removal of impurities. The residue of treated fibers left at 900 °C is higher than that of raw fibers.
Similarly, higher chemical durability with the application of both treatments was explained by exposing the fibers to the saturated solution of sodium hydroxide NaOH (10%) and calcium hydroxide (Ca(OH)2). The results indicate that the mass loss of raw fiber is 37% and 20% in sodium and calcium hydroxide solution, respectively. Mass loss of treated fibers is roughly half as compared to raw fibers. The higher percentage of hemicellulose, cellulose, and lignin in treated fibers is the reason for this chemical durability.
A careful selection of fibers is a crucial task and required before using coconut fibers as well as other natural fibers in reinforced composites. Different parameters need to be identified, such as the origin and local and seasonal quality variations of the fibers in order to control the retting process, defects, and homogenous batches of fiber.
Considering the circular economy, sustainable development and environmental aspects, a simple method for coconut fibers preparation should be proposed, as illustrated in Figure 14
. In the fiber preparation process, the collection must be installed near the coconut fields or the coconut pulp industry where raw coconut envelopes are collected. Husk retting and fibers extraction are processed using water without chemical additives at the same place. The water used is filtered and impurities gathered for biomass. Water is again reused for the retting process. Fibers are sun-dried and cut to the desired lengths with a knife mill machine equipped with various sieve sizes. In this cutting operation, coarse grains as chips are mixed with fibers that will be separated by means of an air jigging system. Finally, fibers cut to length are bagged like coconut chips.