1. Introduction
With an increasing concern on the energy conservation and environment protection, wood as a natural and sustainable construction material has returned to the spotlight after a long time flagging [
1]. Compared with other conventional construction and building materials, wood has several shortcomings, e.g., relatively low tensile stiffness and strength compared to steel and low compression stiffness and strength compared to concrete. Wood is also susceptible to biological degradations, such as from fungi, bacteria and insects [
2], which weaken its mechanical properties. To overcome the inferior mechanical properties of wood elements, fiber reinforced polymer (FRP) composite [
3,
4,
5] can be one of the solutions. FRP has been widely utilized in the past two decades for rehabilitation and reinforcing of existing structures. FRP materials such as glass or carbon FRP have high strength-to-weight ratio, corrosion-resistance and provide design flexibility [
6,
7,
8].
The commonly utilized FRP composites as reinforcement for wood beams are carbon FRP (CFRP), E-glass FRP (GFRP) and aramid FRP (AFRP) [
3,
4,
5,
9,
10,
11,
12]. However, the production processes of these fibers are energy-intensive and the initial costs are still high. Recently, mineral-based natural FRP, such as basalt FRP (BFRP), has been introduced. BFRP has low material cost, high fire resistance, good thermal, electrical and sound insulating properties [
13,
14,
15]. Furthermore, basalt fiber also has high tensile properties (e.g., tensile strength of 1850–4800 MPa) [
14]. However, similar to glass fiber, the production of basalt fiber also requires a large amount of energy because of the high melting point of basalt rocks (1300 °C–1700 °C) [
13].
As an alternative to glass, carbon and basalt fiber materials, the ecological and economical plant-based FRPs (e.g., flax or jute FRP) have been introduced in civil engineering. Various investigations on plant-based fibers (e.g., flax) have shown that as a single fiber, they have comparable specific mechanical properties (e.g., specific tensile strength and stiffness) compared to those of man-made E-glass fiber [
6]. However, this is somewhat misleading since the length of natural fibers are limited, while carbon or glass fiber can be manufactured to have an endless length. The natural fibers are used in the forms of yarns, which will generally have lower mechanical properties compared to the ones of individual fibers.
Nevertheless, several investigations using the natural fibers in FRP as a reinforcement in civil engineering application have been carried out. Huang et al. [
16] investigated flax FRP (FFRP) strengthened reinforced concrete (RC) beams. Their results revealed that the FFRP increased the ultimate load and maximum strain as well as the ductility of RC beams significantly. It also showed a better interfacial compatibility with the RC beams compared to GFRP and CFRP strengthened RC beams. Yan et al. [
17] investigated the flexural properties of plain concrete beams externally strengthened with FFRP. It has been shown that the bending load capacity of plain concrete beams increased by 100%, 230% and 327% and their fracture energy were increase by 3500%, 4200% and 8160% with two-, four- and six-layer FFRP reinforcement [
17]. In addition, FFRP has been used as external confining materials of natural aggregate concrete [
18], recycled aggregate concrete [
19] and fiber reinforced concrete [
20,
21].
In literature, a large number of studies have investigated FRP as an external reinforcement of wood structures, but only very few have considered plant-based FRPs. For example, Speranzini et al. [
22] investigated solid wood beams externally strengthened with carbon, glass, basalt, hemp and flax FRP under a four-point bending test. No significant difference was observed on the loading capacity of the different FRP composites (i.e., the increase of the bending strength were 42.3%, 24.6%, 23.2%, 24.0% and 35.4% for carbon, glass, basalt, hemp and flax FRP, respectively) although there was a large difference in the tensile strength of these FRPs (i.e., 479, 142, 245, 36 and 25 MPa for carbon, glass, basalt, hemp and flax FRPs, respectively). According to the author, flax and hemp fibers may have better adhesion to wood compared to other FRPs. Borri et al. [
23] investigated flax and basalt FRP strengthened low-grade (bending strength of 18.4 MPa) and high-grade (bending strength of 41.3 MPa) wood beams. The tensile strengths of FFRP and BFRP in the study was 240 MPa and 1880 MPa, respectively. The results showed an increase of bending strength of 38.6% and 65.8%, and maximum mid-span deflection of 58.2% and 40.2% respectively by two-layer FFRP and BFRP strengthened low-grade wood beams. Moreover, the strength increases were 29.2% and 25.9%, the increases of maximum mid-deflection were 9.1% and 14.5% respectively for two-layer FFRP and BFRP strengthened high-grade wood beams. This study concluded that both BFRP and FFRP provided the beams with higher strength and better ductile behavior. Similar results can be found in another research by Borri et al. [
24] for flax and basalt FRP. André et al. [
25] applied FFRP and GFRP with similar fabric density (i.e.,
for flax and
for glass) perpendicular to grain on wood beams. It is reported that the maximum bending load of the entire specimen strengthened with GFRP (45.1 kN) was 23% higher than that one strengthened with FFRP (36.0 kN).
Realizing the advantages and disadvantages of using different types of fibers in FRP, hybrid FRP (HFRP) was proposed in the literature. Hybrid FRP, which consists of two or more combinations of strengthened fibers or fabrics, was designed to inherit the advantages and minimize the disadvantages of the combined fibers. Kim et al. [
26] investigated HFRP made of carbon and glass fabrics to retrofit RC beams. The results showed that the HFRP contributed to higher ultimate bending strength and ductility of the RC beams compared to the single type of CFRP or GFRP. The maximum load in bending of RC strengthened with GFRP–CFRP (G GFRP attached at the tension surface of the RC beam) specimens was 6.6% and 3.9% higher than the one strengthened with two-layer CFRP (CC) and two-layer GFRP (GG), respectively. Moreover, the maximum mid-span deflection was also 27.4% and 18.5% higher than that of CC and GG specimens.
Compared with man-made fiber/fabric materials in conventional FRP composites (e.g., E-glass and carbon), plant-based fiber/fabric has a lower price and positive ecological impact [
27], but it has lower mechanical properties as it has been mentioned before. In order to balance the performance and the cost for proper material design, several studies have investigated the hybridization of a plant-based fabric with a man-made one in FRP composite [
28,
29]. Gupta et al. [
29] have summarized the mechanical properties of this hybrid material reinforcing thermoset polymers. It was concluded that the tensile, flexural and impact strengths of hybrid FRP were higher than those of the single type natural fabric FRP. However, the application of the hybrid FRP with natural fabric for reinforcing wood beams have been scarcely investigated before. Throughout the literature, only very few studies have investigated HFRP strengthened wood beams. Yang et al. [
30] strengthened wood beams with hybrid carbon and glass FRP. Compared to the wood beams strengthened by GFRP or CFRP alone, the HFRP provided a larger energy dissipation for wood beams.
In this study, the flexural behavior of flax FRP strengthened wood beams were investigated. The results were compared with man-made E-glass and mineral-based basalt FRPs. Additionally, hybrid flax/glass/basalt FRPs were also investigated and compared with single type of FRPs (i.e., FFRP, BFRP and GFRP). Various different FRP materials (i.e., FFRP, GFRP and BFRP), FRP thickness (i.e., one-, two- and three-layer) and the arrangement of FRP in the HFRP were considered as experimental variables. As complementary initial investigations, tensile and bending test of flat coupon single type fiber FRPs were also carried out. Furthermore, since the interfacial bonding of fiber/epoxy and FRP/wood are also critical points for the flexural behavior of beams, the microstructures of these interfaces from the fractured specimens were examined under light and scanning electron microscopes.