1. Introduction
Concrete is considered a brittle material due to its low energy absorption capacity and tensile strength. Structural elements made of concrete, such as beams, are primarily subjected to impacts and bending. Therefore, these components must have increased resistance to deformation and impact loads. Concrete’s ability to absorb and distort energy requires additional elements [
1,
2,
3]. To control cracking, discontinuous fibres are employed inside the concrete, mostly as secondary reinforcement. This is not a replacement technique for the main steel reinforcements [
4,
5]. Several types of fibres have been utilised to strengthen and rehabilitate RC structural elements for decades. These fibres are strong against corrosion and result in several structural advantages, including lower cover dimensions and, as a result, structural element thickness [
6]. According to Tysmans et al. [
7], because textile reinforcement has high tensile strength, it might potentially replace steel as the primary reinforcement.
Textile-reinforced concrete (TRC) or textile-reinforced mortar (TRM), a composite material made of high-performance concrete or mortar and textiles fibre, has good corrosion resistance, durability, load-bearing capacity, and ductility. The cover in TRC and TRM is thin owing to textiles’ excellent resistance against corrosion [
6,
8,
9]. The textile reinforcement is oriented toward tension, enhancing the fibres’ utilisation factor [
10]. Therefore, TRC is lightweight and can be used in complicated geometrical shapes and arrangements, thin-walled constructions, and prefabricated sandwich panels. It is critical to investigate TRC’s mechanical properties thoroughly, particularly its flexural behaviour [
11]. Furthermore, incorporating textile fibres in concrete to create TRC has been explored and measured as a novel composite material for applications in buildings [
12]. Alkali-resistant textile fibres, such as carbon and glass fibres, are often made up of multi-filament roving. TRC beams provide many benefits over regular fibre-reinforced concrete (FRC) beams, including being employed in the presence of stresses [
13]. TRC may be utilised entirely in concrete members because it can be situated in essential places, such as following tensile stresses and insufficient amounts.
In contrast, the discontinuous fibre in FRC mixtures is arbitrarily scattered and oriented in the concrete mix, making it less effective. Furthermore, the short fibres are inadequate to regulate fracture development, reinforcement, and strengthening of concrete due to the random arrangement of fibres in traditional FRC mixes. Furthermore, the inclusion of short fibre has a slight effect on beam strength in the compression zone. Textile reinforcement has the same tensile strength as steel reinforcement under tension [
14,
15].
Fibres are distributed randomly over concrete members, resulting in fibre waste; for example, the fibres allotted to compression zones are not entirely utilised. Consequently, TRC is projected to outperform FRC because the significant difference is the behaviour under strain [
16]. Textile reinforcement can be placed where it is desired. Several studies were conducted on the mechanical performance, modelling, and design approach on TRC. Similar to SRC, TRC is regarded as a novel material that might be employed in the architecture and structural sectors because of its high strength and ability to handle large tensile loads [
17,
18]. Researchers discovered that using textile fibres to strengthen concrete components is more efficient and significantly improves structural parts’ deformation performance and energy absorption [
19]. Häußler-Combe and Hartig [
20] found that when concrete is reinforced with textile fibre, the stress-strain trend of TRC beams is comparable to those of SRC beams. However, the TRC has slight, if any, flexibility; consequently, the mode of failure is more brittle, contrasting with the final behaviour seen in most SRC beams. Furthermore, unlike steel bars, the cross-section of the roving textile is non-uniform along the length of the textile reinforcements, according to Hegger et al. [
21] and Hartig et al. [
22], whereas the cross-section of steel bars is consistently homogeneous.
Furthermore, Hannant [
23] observed disparities in post-cracking flexural and uni-axial tensile strengths of FRC beams, emphasising the importance of a thorough understanding of flexure. At post-cracking, the flexural strength is about twice that of the tensile strength. TRC’s behaviour, on the other hand, has not been adequately explored, and more data are needed before it can be employed securely. Given the argument, as mentioned above, the purpose of this research was to investigate the effects of textile fibres in various shapes and geometries on the flexural performance of small-scale concrete beams, as well as to figure out how textile fibres contribute to the reduction in crack formation when compared to plain concrete without fibres. Although this study looks into various textile fibres, the tests and analysis are focused on a single type of fibre, multi-filament carbon fibre. Although the research has emphasised the flexural performance of concrete beams under bending loads, it is considered that technical concerns must be addressed and resolved before these fibres can be used as a major reinforcement in concrete beams. This study compared the flexural performance of plain concrete beams to beams reinforced with textile fibres and steel bars using a four-point bending test.
4. Discussion
The voids ratio was first estimated to study the roving densities of the textile. The voids ratio of roving is the ratio of the voids in the cross-sectional area to the area of the roving’s cross-section. Due to the increase in voids, a rise in the ratio leads to a surge in diffusion. As a consequence, the effect of increasing or decreasing the roving’s width may be measured. The voids ratio may vary for the same number of filaments owing to differences in the roving’s cross-sectional area (warp, weft, or tow). Therefore, Equation (1) was used to compute the void ratios:
where
indicates the void ratio;
is the area of filaments multiplied by the number of filaments in the tow or roving;
is the cross-sectional area of the roving textiles, including the voids.
The results revealed that the voids ratio increased with the rise in the roving’s cross-sectional area for the same number of filaments. This indicates that the filaments have been condensed, resulting in the roving being smaller and having less surface area than the matrix. There are precisely the same number of fibres in both directions of the bi-axial textile to reinforce the concrete beams. Weft-to-warp voids ratios are lower because the filaments are sewed in warp-to-weft directions.
Table 3 demonstrates that reinforcing in the weft direction has no significant effect compared to the warp direction. The voids ratios between warp and weft are very similar, which could explain the capacity comparability. In UT
4, however, increasing the voids ratio leads to a rise in load capacity. When compared to BT
4, the load strength improved by 12%, but the ductility remained unchanged. Accordingly, raising the voids ratio is beneficial since it increases load capacity while retaining ductility.
Furthermore, the effect of changing the orientation of the textile reinforcements with the loading direction on TRC behaviour was examined. The reinforcement was placed in ± 45° different directions. The average findings of 45° TRC are shown in
Table 4. The results show that a noteworthy reduction in the reinforced beam capacity occurs when the loading direction is changed. According to Hegger et al. [
24], the ultimate load reduces as the angle of reinforcement surges. According to the data, the ultimate load decrease from placing the textile reinforcements at 45° is 38%. According to their findings, a 45° orientation reduces the load-bearing capacity of carbon fibre reinforced concrete by 60%. The rise in load capacity seen in this study is due to a surge in active filaments [
25]. In addition, the deflection of the ductility decreased by 89%. Because of the discontinuity in the textile in this example, the deflection and bending moment strength reveal that the textile reinforcements at 45° operate similarly to chopped fibre. This could account for the lower ultimate load and deflection. Consequently, the deviancy of textile reinforcements from the loading directions results consequently in a considerable decrease in the reinforced beam’s capacity.
Table 5 also shows the effect of a rise in the number of woven textile layers and filaments on the performance of TRC beams. This is in line with Yin, Lü, and Xu [
26], who discovered that increasing the number of textile layers does not affect the stiffness of textile reinforced beams before cracking but does improve the stiffness after cracking. However, a deeper examination reveals that the percentage rise for many rovings at the same layer is relatively low compared to a lesser one. For instance, when a small number of rovings are present at the same stratum, BT
4 outperforms BT
3 by 63%. At BT
4, however, there is simply one more layer (a 33% increase in area). The ratio of growth in the strength to rise in the area is said to be high. Because BT
3’s ultimate strength is small, there is a visible difference between BT
3 and BT
4. Compared to BT
4, with a rise in the number of layers of textile fibres by three in BT
7 beams, the ultimate strength increased by about 21%.
The improvement does not reflect the area’s percentage gain. Consequently, having more woven fabrics increases the ultimate loads. It can be explained by a rise in the inner filaments of the woven fabrics, which are not effectively used as reinforcement, as evidenced by the rise in the BT
7 deflection due to internal filament slippage. Owing to piling up the rovings, the contact area of BT
7 diminishes as the number of woven textiles increases [
27]. Notably, the outside filaments have a larger contact area, albeit a smaller one than the internal filaments, which have a smaller contact area. Consequently, some internal warp rovings that resist load are effectively deactivated when applied load is applied; yet, they give ductility due to filament sliding. Furthermore, due to the congestion caused by the stacking of the textiles, the placement of concrete in the formworks is more complicated.
In addition, the study looks at how the weft rovings might affect TRC behaviour. Therefore, the same bi-axial textile reinforcement features were used in the uni-axial direction.
Table 6 presents the ultimate loads and deflections of bi-axial and uni-axial TRC beam samples. It is clear from the data that the influence of weft roving on the load-bearing capacity is negligible. The average ultimate load is improved in both situations, UT
4 and UT
7, respectively, compared to BT
4 and BT
7. The rovings in the loading path effectively resisted the stresses caused by the bending under flexural load [
28]. The bond enhancement and penetration improvement owing to the reduction in reinforcing congestion can explain these findings. Furthermore, tow reinforcement has a larger perimeter than bi-axial reinforcement warp rovings. The tow width of the bi-axial textiles was 16 mm, whereas the warp width was about 5 mm. Due to the slipping inner filaments, the beams of bi-axial reinforcements with high quantities of woven textiles (BT
7) were revealed to be more ductile in nature than those with lower numbers of woven textile beams.
Several researchers have looked at the mechanical properties of TRC [
22,
29,
30,
31]. However, fibre dosages were used to express the consequence of the increase in fibre content on the mechanical performance of concrete. In this regard, Abdulmajeed et al. [
32] discovered that a rise in fibre content does not always imply an improvement in the composite’s flexural strength. As a result, it appears that using volume fraction as a design criterion could lead to inefficient outcomes. When using fibre dosage methods, all the fibres in the concrete member are considered, no matter how they are oriented. Because some of the fibres in TRC, such as those that act perpendicular to the beam’s span and those in the central part of the batch of fibres, are not used in stress resistance, this technique could be profoundly incorrect.
Table 6 displays the findings for various fibre dosages on the ultimate loads of beams with the same dimensions. As can be observed, the increasing volume has a negligible effect on improving ultimate load. BT
4 has a volume fraction of 0.62%, and UT
4 has a volume fraction of 0.31%; nevertheless, the flexural loads remain unchanged despite doubled volume fraction.
On the other hand, a further rise in the fibre content reduced the flexural loads, owing to the lower flowability of concrete and improper placement in formworks. It can be seen that with the rise in fibre content in the same dimension beams of BT
7 and UT
7, the ultimate flexural loads decreased by about 1.08% and 0.54%, respectively. Consequently, it appears that the ultimate load in FRC beams is not linearly related to the fibre dosages. As previously stated, utilising a volume fraction-based technique in TRC may result in improper design.
Table 6 shows that the rise in textiles was primarily responsible for improving the ultimate load. A rise in fibre dosage appears to indicate a surge in failure load at first glance. The ultimate load rises from 27.5 kN to 33.5 kN in UT
4 beams with V
f = 0.31% and UT
7 beams with V
f = 0.54%. However, a deeper examination of these outcomes reveals that the alignment of the fibres is more important than the total fibre content, and the cross-sectional area technique is better for determining this and the ultimate load [
33,
34].
For example, in TRC beams of BT
4 and UT
7, the ultimate loads were increased by about 75% and 36%, respectively, with a reduction in fibre dosages and a surge in the cross-sectional area of fibres. Moreover, the ultimate flexural loads were found to be different in BT4 and UT4 beams with the same fibre area of 30.8 mm
2 and fibres’ dosages of 0.62% and 0.31%. Consequently, in the design methodology of beams, the method of cross-sectional area-based is preferable and must be addressed [
35,
36]. Besides, the fibre dosage parameter is more appropriate for usage with short and randomly dispersed fibres in concrete, where the exact cross-sectional area of fibres cannot be estimated. As a result, the volume fraction should only be used to determine how much fibre is in a beam [
37].