The construction industry is a major consumer of energy and raw materials and contributes immensely to environmental pollution, especially to greenhouse gas (GHG) emissions [1
]. Since the 1970s, annual GHG emissions have steadily increased and reached 53.5 GtCO2
e in 2017 [2
]. Within the construction industry, concrete is the dominant building material with a global production of 20 × 1012
kg per annum which exceeds the amount of all other construction materials combined. With an increase in the demand for new infrastructure demonstrated by developing countries, the use of Portland cement (PC), the main component in concrete, has been rising rapidly [3
]. Accordingly, the global use of PC has increased from 2.22 to 4.10 Gt/year within the past decade. The production of PC accounts for ~5% of the global anthropogenic CO2
]. Moreover, concrete is commonly reinforced with steel, whose production involves high energy emissions and consumption of fossil fuels that additionally contributes to CO2
emissions. Furthermore, the fast pace of development in many developing countries has led to an increased demand for reinforced concrete for housing and infrastructure projects. Unfortunately, the majority of developing countries lack the resources to produce their own cement and steel for the production of reinforced concrete elements which forces them to import the majority of their needs from highly industrialized countries, and as a result of the import surge, trade deficits, economic slow-downs, and loss of jobs are prominent in those countries. Besides the economic challenges from the cement and steel import, environmental issues also need to be addressed. The construction industry is facing an urgent need for the use of sustainable materials incorporating locally available renewable resources as well as industrial by-products with lower environmental impacts.
One potential material is fly ash, a by-product of the combustion of coal, oil and biomass. Fly ash contains Silicon dioxide (SiO2) and Aluminum oxide (Al2O3) as major components that can contribute to the hydration of cement. Furthermore, low-cost and renewable materials such as bamboo and wood can be found in abundant supply in many developing countries, where the bamboo and wood industry produce a large number of waste products. Replacement of cement with by-products such as fly ash or bamboo and wood waste can enable the reduction of the carbon footprint associated with the cement industry and improve the mechanical and thermal properties of the developed formulations. Other performance aspects such as the ductility of these mixes could be further enhanced via the use of other renewable materials such as natural fibers to avoid the brittle failure that is characteristic of plain concrete.
Previous studies [5
] revealed improvements in the mechanical properties and durability of concrete mixtures, in which PC was partially replaced with wood ash or fly ash. Substituting aggregates with wood process waste such as wood chips, flax or hemp was also shown to enhance the mechanical or thermal properties of concrete mixtures [11
]. Further research on fiber-reinforced concrete reported that the addition of synthetic fibers—such as polypropylene (PP), polyethylene (PE), polyvinyl alcohol (PVA)—or steel fibers could increase the fire resistance, ductility, tensile strength, impact resistance and toughness of concrete mixtures [19
]. However, synthetic fibers, which are mainly derived from petroleum-based sources, and steel fibers require energy-intensive and expensive production processes. In contrast, natural fibers, such as those obtained from wood and bamboo industry by-products, can provide a low-cost and sustainable alternative for the construction industry. Challenges of resource scarcity and the negative environmental impacts of synthetic fiber production have led many researchers to search for alternative, green, sources of fibers for the production of fiber-reinforced concrete. Natural fibers represent a sustainable source of raw materials from renewable resources and can help to alleviate the need for synthetic fibers. While there is growing interest in the use of wood fibers to enhance the mechanical behavior and fracture toughness of concrete [23
], there has so far been relatively little investigation of the use of bamboo fibers for this purpose. Only a few studies [26
] investigated the performance of bamboo fiber-reinforced concrete and mortar mixtures through a series of mechanical tests. The bamboo fibers in those studies were obtained from bamboo forests and were subsequently processed as fibers for concrete mixtures. Furthermore, the studies showed that only concrete’s tensile property had obvious improvement when bamboo fibers were added, while the enhancement to the compression property and flexural property was not obvious. The studies on the application of bamboo fiber-reinforced concrete and mortar mixtures are rather limited. Both bamboo fibers and fly ash present a great opportunity as sustainable and affordable replacements for cement and steel for developing countries. Bamboo belongs to the botanical family of grasses and shows high resistance to tensile stresses. The tensile strength of natural bamboo is superior to that of wood. This attribute marks bamboo as an attractive option to incorporate into fiber-reinforced concrete, especially in developing countries where demand for reinforced concrete is growing rapidly [31
]. Bamboo is a gigantic grass, which belongs to the angiosperms (seed-bearing vascular plants) group and monocotyledon (flowering plants) subgroup. Bamboo attains maturity in 3 to 5 years, in favorable contrast to wood, which takes at least 20 years, depending on the species [34
]. The growth behavior of bamboo culm and the extreme wind loads it has to sustain during its life cycle require a precise mechanical adaptation to the environment. Therefore, material optimization has to be achieved effectively from the bamboo fibers and their cell structures. This results in an optimized microstructure with superior material performance when compared to various wood species. Furthermore, bamboo can directly address global warming as it rapidly grows and sequesters carbon in biomass and soil faster than almost any wood species. The main components of bamboo culms are cellulose, hemicellulose and lignin. The minor components are resins, tannins, waxes and mineral salts. However, the percentage of each component differs from species to species and depends on the conditions of bamboo growth and the age of the bamboo, as well as the location of the section on the culm [31
]. In general, cellulose in bamboo culms accounts for more than 50% of the bamboo chemical components. After cellulose, lignin is the next largest component, and normally accounts for more than 20% of the bamboo’s mass. Bamboo displays a round-shaped cell cross-section, in contrast to the nearly rectangular and relatively large cells of wood species. Furthermore, bamboo culms have a particular multi-layered cell wall structure with alternating thick and thin layers of fibers, unlike the typical three-layered cell wall of wood species which have a structure with a dominating middle layer [34
In recent years various methods have been developed to employ bamboo through new processing technologies for the fabrication of high-performance bamboo-composite materials in such a way that the inherent mechanical capacities of the fibers are retained, while the durability issues, specifically water absorption, swelling, shrinking and chemical resistance, of the composite could be enhanced for application as structural elements in buildings [31
]. The bamboo-composite materials display high mechanical properties and have been used as either reinforcement in concrete, replacing steel or as structural elements in the form of a beam or column. However, the process through which natural bamboo culms transform into bamboo-composite materials employs only certain sections of the culms and therefore the remaining parts usually become part of the waste of the production process which can be safely utilized for applications as sustainable and affordable fibers in fiber-reinforced mortar.
Therefore, the objective of this study is to develop a sustainable and affordable mortar mixture incorporating by-products and renewable materials (i.e., fly ash and bamboo fibers) that show improved mechanical properties and fracture behavior and could be employed for the construction of low-cost and low-rise housing solutions in developing countries. The developed mixture was characterized via compression, splitting and bending tests, whereby the fracture properties including toughness and absorption energy were also assessed. The findings generated through this work set the foundation for further research on bamboo fibers and bamboo-reinforced concrete and mortar mixtures for structural applications.
This study focused on the development of a sustainable and affordable mortar mixture consisting of a high amount of fly ash (656 kg/m3) and varying contents of bamboo fibers (4/6/8 V%) from waste by-products of engineered bamboo-composite fabrication. The bamboo fibers, obtained from bamboo composite production waste, were categorized into two groups of 300 m and 500 m and were incorporated at three different volumes of 4/6/8 V% within each mix. The mechanical performance of the developed formulations was assessed via the measurement of the mechanical and fracture properties. From these results, the following conclusions can be drawn:
The addition of 4/6/8 V% of bamboo fibers has a reasonably modest effect on the compressive strength of the mixtures. Compared to the control mixture without fibers, the reduction of the compressive strength of the mixtures with 300 m fibers is between 7.8% and 19.9%, with larger fiber volume fractions corresponding to a greater reduction of strength.
The mixtures with 500 m fibers were reduced in compressive strength between 9.1% and 27.0% by the same range of 4/6/8 V% of fiber content. The reduction of the compressive strength for both the 300 m
and 500 m fibers could be related to the lower mechanical properties of the bamboo fibers as well as the influence of their aspect ratio.
The compressive strengths of the mixtures with 8 V% of fibers were shown to be 60.2 MPa and 54.6 MPa for the 300 m and 500 m mixtures, respectively, and is within a reasonable range for use in structural members.
The splitting tensile strength of the bamboo fiber-reinforced mixtures displays a reduction of between 6.9% and 31.9% compared to the control mixture depending on the fiber volume and aspect ratio. The mixtures containing the 500 m fibers (lower aspect ratio) show lower strength in comparison with the 300 m mixtures.
All bamboo fiber-reinforced mortar mixtures display a strain-softening behavior. The mixtures with 8 V% of fibers show better crack-bridging effects than those with lower volume fractions, resulting in higher residual strength.
The toughness of the mixtures was evaluated at a mid-span deflection of L/150. The 300 m fiber mixtures show values of 2.6, 2.1 and 1.5 Joule for 8, 6 and 4 V% of fibers respectively, whereas the 500 m mixtures show, in general, lower toughness of 2.40, 1.90 and 1.63 Joule.
Mixtures containing 300 m fibers demonstrate an overall enhanced mechanical performance and post-crack behavior compared to the mixtures with 500 m fibers as a result of the higher bond strength due to their higher aspect ratio.
The findings emerging from this study demonstrate the suitability of using natural bamboo fibers, obtained from process waste, to improve the ductility of high-volume fly ash mortar. The resulting formulations can enable the development of a sustainable and low-cost mixture for structural members. These results can be utilized for the construction of low-cost and low-rise housing units in developing countries, especially in Southeast Asia, Latin and Central America, where there is access to bamboo and low-cost cementitious materials with low demand for ductility. Further studies on the durability of bamboo fibers and the replacement of steel reinforcement with engineered bamboo composites and natural bamboo fiber members are being performed to broaden the application range of these materials on a larger scale.