Plant Fibres as Reinforcing Material in Self-Compacting Concrete: A Systematic Literature Review

Abstract

Natural plant fibres have gained growing research interest as a construction material due to efforts to reduce the negative environmental impacts caused by construction activities. Many researchers have investigated the suitability of utilising plant fibres as reinforcement in self-compacting concrete (SCC) as a substitute for synthetic fibres, recognising that the production of synthetic fibres generates significant amounts of CO₂. In this study a bibliometric analysis was conducted to investigate the current research achievements and map the scientific studies where plant fibres were used in SCC. A detailed discussion on the effects of various plant fibres and their underlying mechanisms on the properties of SCC is also provided. The findings indicated that using plant fibres typically reduces the flowability, filling ability, and passing ability of SCC due to the high water absorption of plant fibres, fibre and aggregate interlocking, and the fibre agglomeration effect. Incorporating plant fibres increases the viscosity and enhances the segregation resistance of SCC due to the strong cohesion between plant fibres and the cement matrix. The inclusion of plant fibres usually improves the mechanical properties of SCC because of the synergetic effects of plant fibres on crack-bridging and strain redistribution across the cross-section of SCC. Adding plant fibres to SCC also reduces drying shrinkage and cracking due to the fibre bridging effect, while generally lowering the resistance to sulphate attack, acid attack, and freeze–thaw cycles and increasing the water absorption rate of SCC due to the increased porosity of the mix. A comprehensive overview of research gaps and future perspectives for further investigations is also provided in this study.

1. Introduction

Self-compacting concrete (SCC) is a special type of concrete that flows and consolidates under its own weight and can fill formwork having complicated shapes and congested reinforcement without developing issues including bleeding and segregation [1,2]. A large amount of cement paste is used in SCC to achieve a high level of fluidity and adequate stability during transportation and placement [3], which can make the concrete more brittle and increase shrinkage and cracking [4]. The use of fibres in construction materials dates back to 9000 BC, when straw was used as reinforcement for clay to produce sun-dried bricks [5]. Fibres have been used in concrete to increase the robustness of concrete when subjected to tensile stress [6], and various types, including steel, polyethene, polypropylene, polyolefin, basalt, and recycled plastics, have been used over the last 70 years as reinforcement in concrete and cement composites [7,8,9,10]. The present review is limited to the utilisation of plant fibres in SCC. Studies on other types of fibres in SCC are not considered here, although they have been widely investigated and have contributed significantly to understanding fibre-reinforced SCC [11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29]. Numerous studies have investigated the use of plant fibres in conventional vibrated concrete and other cementitious systems [30,31,32,33,34,35,36,37], which are also excluded from the present study.

Fibres in concrete minimise and delay the development of wide and continuous cracks [6]. The fibres interact with concrete through adhesion, friction, and mechanical anchorage, acting as crack arresters and improving the mechanical properties of concrete, particularly ductility, toughness, and impact resistance after cracking [38,39]. The production of steel and synthetic fibres has significant shortcomings which need to be considered when assessing their advantages [40], including the depletion of natural resources, being energy-intensive, and resulting in the emission of large amounts of CO₂. Steel fibre production generates wastes, including slag and dust, which can harm the environment if not properly managed [41], while the use of fossil fuels and natural gases to manufacture synthetic fibres depletes non-renewable resources [42,43]. Synthetic fibres are non-biodegradable, and the manufacturing processes result in off-cuts and microplastic wastes [44,45], which threaten wildlife and ecosystems when inappropriately dumped [46] and lead to long-term environmental pollution [47]. The rock extraction process for manufacturing basalt fibres causes habitat disruption and landscape alteration [48].

Various recent publications have focused on utilising more sustainable materials rather than fossil resource-based, non-renewable, and non-biodegradable materials. Fibres derived from plants are lightweight and feature a higher tensile strength than synthetic polypropylene fibres, making them suitable for replacing these artificial fibres [49]. The production of plant fibres has lower CO₂ emissions and a lower energy consumption than other types of fibres, with the production of one tonne of plant fibres, namely flax, hemp, kenaf, and jute, emitting approximately 350, 410, 420, and 550 kg of CO_2, respectively [50]. The CO₂ emissions of these plant fibres are relatively low when compared to carbon and glass fibres, in which the production of these synthetic fibres emits approximately 29,450 kg and 2500 kg of CO₂ to manufacture one tonne of fibres, respectively [50,51]. These plants also absorb CO₂ through biogenic carbon uptake ranging from 1270 kg to 1390 kg per tonne of fibre. The energy used to manufacture plant fibres is also lower than required to manufacture synthetic fibres, and the cost of plant fibres is cheaper than other types of fibres [52,53]. The main challenges of utilising plant fibres in SCC are their high water absorption, limited compatibility with the cement matrix, and inadequate long-term durability [54]. The two primary ways to counter these challenges are (1) treating the fibres and (2) partially replacing cement with SCMs. Various chemical and physical treatments of fibres are used to remove the undesirable components and impurities, decrease the water absorption, and enhance the surface roughness of plant fibres [55,56,57,58]. The incorporation of SCMs helps form a denser matrix through the pozzolanic reaction, which reduces the alkalinity of the concrete and improves the fibre–cement matrix interfacial bonding [59,60].

The effectiveness of using plant fibres compared to synthetic fibres has been thoroughly studied for typical concrete, with the main conclusion being that the inclusion of plant fibres improves the mechanical properties of concrete when compared to plain concrete and synthetic polymer-based fibre-reinforced concrete [49,61,62,63,64]. Research on the use of plant fibres in SCC has been comparatively less investigated, and, since SCC is a specific type of concrete, this research area represents a significant gap. Information on the performance of plant fibre-reinforced SCC (PFRSCC) is scattered, and no discussion can be found on the effects of plant fibres and their mechanisms on the rheological and mechanical properties, durability, and microstructure of SCC, and other factors contributing to the performance of PFRSCC. The findings from prior studies suggest that multiple factors, in particular fibre dosage, fibre length, fibre treatment, and the use of SCMs, have a significant influence on the rheological behaviour, mechanical performance, durability, and microstructural characteristics of PFRSCC. This systematic literature review was conducted using the PRISMA method (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) [65] to investigate how variations in fibre dosage, length, and treatment, as well as the inclusion of SCMs, affect the properties of SCC and the mechanisms governing these effects. This review also highlights research gaps in the literature and suggests identifiable future perspectives.

The aim of this research was to answer the following questions:

(a)

What plant fibres are used in SCC, and what are their properties?

(b)

What is the current research maturity regarding the use of plant fibres in SCC?

(c)

What are the effects of plant fibres on the rheological and mechanical properties, durability, and microstructure of SCC?

(d)

What are the effects of fibre lengths, fibre treatments, and SCMs on the properties of PFRSCC?

(e)

What are the limitations, research gaps, and future opportunities for research and innovation in using plant fibres in SCC?

2. Significance of the Review

A comprehensive database of the current uses of plant fibres in SCC is presented in this systematic review. The effects of these plant fibres on the properties of PFRSCC and their underlying mechanisms are investigated in this study, along with detailed discussions explaining the observed behaviour and trends in various properties of SCC when incorporating different plant fibres, offering a novel understanding of both the performance and mechanisms governing the fibre–matrix interactions of SCC. The research findings from various publications demonstrate that the dosage, length, and treatment of plant fibres, as well as the use of SCMs, strongly influence the properties of PFRSCC. These variables are extensively discussed in this study to provide new insights into the optimal uses of plant fibres in SCC. This review is aimed at enhancing the understanding of the suitability of utilising various plant fibres as reinforcing materials in SCC, and to identify research gaps for future studies and innovations in using sustainable materials in SCC applications, providing fundamental insights for engineers, policymakers, and researchers. The comprehension of the use of plant fibres in SCC, as provided in this review, will help promote the advancement and broader adoption of plant fibre-reinforced SCC in both structural and non-structural construction applications, contributing to global efforts to foster environmentally friendly and sustainable construction materials.

3. Methodology

A systematic review process was adopted for the literature selection using the PRISMA framework (see Supplementary Materials), as depicted in Figure 1 [65]. This approach was used to outline primary research on a research topic, in which the relevant evidence and data were systematically identified, screened, selected, and synthesised [66,67]. Scopus and Web of Science databases were used to retrieve data using advanced search queries, as presented in Figure 1. A total of 102 articles were retrieved from the Scopus database, and 64 articles were retrieved from the Web of Science database https://www.webofscience.com/wos/woscc/basic-search (accessed on 7 June 2025). The 66 duplicates from the two databases were removed, and 100 papers were selected for further screening. Three inclusion criteria for the screening were the following: (1) the article must be focused on self-compacting or high-fluidity paste, mortar, concrete, or cement composite; (2) the article must be focused on using plant fibres; and (3) the article must be in English. The inclusion criteria were formulated to ensure that the selected studies were scientifically relevant and methodologically comparable. By limiting the focus to self-compacting or high-fluidity cementitious materials, the review maintained a consistent emphasis on mixtures with similar rheological characteristics. The criterion for high-fluidity cementitious materials is limited to studies that conducted the slump flow test rather than the conventional slump test. Selecting the literature only to those incorporating plant fibres ensured an alignment with the scope of the study. The inclusion of only English-language articles allowed for an accurate interpretation of the review. Following the screening of abstracts of the articles, 75 documents that conformed to all three inclusion criteria were selected for full-text assessment. After thoroughly analysing the full text according to the three inclusion criteria, 66 articles were included in the bibliometric analysis and systematically reviewed, consisting of 45 journal articles, 17 conference papers, 2 review articles, and 2 book chapters.

4. Bibliometric Analysis

4.1. Annual Publication Trend

The number of publications on using plant fibres in self-compacting or high-fluidity paste, mortar, concrete, or cement composite each year from 2010 to 2025 is illustrated in Figure 2, with 66 published articles to date. There has been a growing interest in research on plant fibres in SCC, as illustrated by the ascending trend line in Figure 2. Only 5 articles were published in the first five years, from 2010 to 2014, while the number of publications jumped to 34 articles for the last five years, from 2021 to 2025. The increase in published articles on PFRSCC from 2015 may be related to the adoption of the Paris Agreement, an international treaty on climate change [68]. The increase in publications in recent years may also be related to national policy or industry changes toward green building materials. For instance, in November 2018, the European Commission unveiled its long-term strategy for achieving climate neutrality, outlining how Europe can attain a net-zero greenhouse gas (GHG) emission economy, which involves reducing emissions in the construction sector [69]. Increased interest in PFRSCC is expected in the coming years as the importance of sustainable construction materials, low-carbon concrete, and the use of natural materials in concrete grows.

4.2. Publication Sources

The publication sources were analysed using the VOSviewer 1.6.20 software. The sources with the most published papers and their respective citations are reported in

Table 1. Number of publications by sources.

No.	Sources	Documents	Citations
1	Construction and Building Materials	6	476
2	Materials Today: Proceedings	4	89
3	Journal of Building Engineering	3	69
4	International Journal of Civil Engineering and Technology	3	18
5	AIP Conference Proceedings	3	1
6	IOP Conference Series: Earth and Environmental Science	3	3
7	Case Studies in Construction Materials	2	41
8	Journal of Materials Research and Technology	2	27
9	Computers and Concrete	2	20
10	Asian Journal of Civil Engineering	2	17

. The top six sources in terms of documents published are Construction and Building Materials, Materials Today: Proceedings, Journal of Building Engineering, International Journal of Civil Engineering and Technology, AIP Conference Proceedings, and IOP Conference Series: Earth and Environmental Science. The top three sources with the highest citations are Construction and Building Materials (476 citations), Materials Today: Proceedings (89 citations), and Journal of Building Engineering (69 citations).

4.3. Most Popular Keywords

The keywords used in the published articles were generated using the VOSviewer 1.6.20 software, and the 20 most popular keywords are reported in

Table 2. Occurrences of keywords.

No.	Keywords	Occurrences	Total Link Strength
1	Self compacting concrete	34	655
2	Compressive strength	26	529
3	Fibers	20	391
4	Tensile strength	19	386
5	Self-compacting concrete	19	262
6	Reinforced concrete	12	282
7	Mechanical properties	12	212
8	Natural fibers	10	225
9	Fibre-reinforced	10	219
10	Bending strength	10	217
11	Reinforcement	9	191
12	Durability	9	145
13	Reinforced plastics	8	183
14	Concrete aggregates	7	157
15	Mortar	7	139
16	Jute fibers	6	147
17	Cements	6	145
18	Hybrid fiber	6	144
19	Property	6	137
20	Fly ash	6	120

. The keywords “Self compacting concrete”, “Compressive strength”, and “Fibers” are the top three keywords with the most occurrences. In terms of total link strength, “Self compacting concrete”, “Compressive strength”, and “Fibers” were at the top, with 655, 529, and 391, respectively. Total link strength refers to the number of times a particular keyword appears in conjunction with other keywords in publications [70]. The co-occurrence visualisation of keyword networks, associated with their correlation frequency and occurring at least six times, is shown in Figure 3. The keywords were grouped into four colour-coded clusters to illustrate the main areas of focus within the research on using plant fibres in SCC. The blue cluster highlights the studies concerned with the performance, optimisation, and durability of the mix. The red cluster reflects research on the mechanical behaviour of SCC, including compressive strength, tensile strength, and bending strength, and shows a growing interest in using plant fibres as reinforcement in SCC, particularly jute fibres. The green cluster represents a broader exploration of fibre-reinforced systems, linking themes of natural fibres, hybrid reinforcement, and reinforced concrete. The yellow cluster highlights the investigations on the workability of SCC. The connecting line between two keywords indicates that those two terms have co-occurred within at least one publication, and a thicker connecting line denotes a stronger relationship, indicating that the keywords co-occur with greater frequency [71]. The figure reflects that the abovementioned keywords (“Self compacting concrete”, “Compressive strength”, and “Fibers”) exhibit larger nodes than the others, implying their importance in studying plant fibres in SCC.

4.4. Top Contributing Authors and Countries

The top contributing authors in the PRFSCC research are illustrated in

Table 3. Number of publications and citations of leading authors in PRFSCC research.

No.	Author	Documents	Citations
1	Vivek, S.S.	5	35
2	Barluenga, G.	3	44
3	Kriker, A.	3	44
4	Tioua, T.	3	44
5	Kavitha, S.	3	25
6	Selvaraj, S.K.	3	21
7	Mohamad, N.	3	11
8	Murthi, P.	2	63
9	Poongodi, K.	2	63
10	Venkatesan, G.	2	20

, while the network visualisation of the co-authorship is presented in Figure 4. The co-authorship network visualisation reveals four clusters, each represented by a different colour to represent distinct collaborative communities or research groups. Vivek, S.S. and Selvaraj, S.K. appear at the core of the network, reflecting their roles as key collaborators and central figures within the field. The thickness of the connecting lines indicates the co-authorship ties, in which the thicker the line, the higher the co-authorship.

The countries with the most publications on using plant fibres in self-compacting or high-fluidity paste, mortar, concrete, or cement composite are reported in

Table 4. Number of publications, citations, and total link strength by countries.

No.	Country	Documents	Citations	Total Link Strength
1	India	31	335	12
2	Malaysia	8	183	7
3	Algeria	8	62	4
4	Canada	5	20	7
5	China	4	24	9
6	Indonesia	4	9	3
7	United States	3	50	7
8	Spain	3	44	3
9	Nigeria	3	40	1
10	Brazil	3	39	1
11	South Korea	3	27	3
12	Kuwait	2	64	5
13	Australia	2	21	4
14	Chile	2	20	3
15	Pakistan	2	11	2
16	Iran	1	275	0
17	Bangladesh	1	50	3
18	Mexico	1	42	0
19	Turkey	1	27	0
20	Egypt	1	21	3

. The country with the most contributions to published documents is India, with 31 articles, followed by Malaysia and Algeria, each with 8 articles. Regarding citations, India tops the ranking with 335 citations, followed by Iran and Malaysia, which rank second and third with 275 and 183 citations, respectively. The number of published articles and citations corresponds to India being one of the largest manufacturers of commercial coir and jute fibres [43]. The total link strength indicates the total number of times authors from a country co-authored with researchers from other countries, implying the strength of a country in international research networks and collaborative activity in PFRSCC research [72]. India has the strongest total link strength of 12, followed by China, with a total link strength of 9. The density visualisation of the total link strength of the top contributing countries is depicted in Figure 5, in which the brighter the colour, the higher the total link strength.

5. Characteristics of Plant Fibres Used in SCC

The frequency of the most used types of plant fibres in the reported studies is depicted in Figure 6. The most popular plant fibres used in SCC are coir, sisal, jute, date palm, and bamboo fibres. The amounts of these natural plant fibres were abundant, as evident from the global production volumes in 2023 of jute, coir, sisal, and abaca fibres, which amounted to 2,908,800, 1,112,500, 260,600, and 58,800 tonnes, respectively [73]. The production outputs of these plant fibres were still low compared to various synthetic fibres. The global production volumes of synthetic polyester, polyamide, and polypropylene fibres in 2023 were approximately 71,100,000, 6,700,000, and 3,100,000 tonnes, respectively [74]. The photographs and scanning electron microscope (SEM) images of various plant fibres are depicted in Figure 7. From the investigation of SEM images, plant fibres typically have a porous microstructure with relatively rough surfaces.

5.1. Physical and Mechanical Properties of Plant Fibres

Detailed information on the physical and mechanical properties of plant fibres is reported in

Table 5. Physical and mechanical properties of various plant fibres.

Refs.	Fibres	Classification by Source	Length (mm)	Diameter (µm)	Density (g/cm3)	Tensile Strength (MPa)	Modulus of Elasticity (GPa)	Water Absorption (%)	Elongation (%)
[80,81]	Hemp	Bast	20	110	1.58	600–1100	-	158	-
[82]	Jute		-	-	1.30–1.46	393–800	10–30	-	-
[83]	Jute		12	-	1.30	390	11	-	-
[84]	Roselle		35	200–300	1.35	150–400	2.76	-	-
[85]	Bamboo	Culm	10–30	300–400	1.52	520	24	10	-
[86]	Bamboo		6–18	224–278	1.30	335–500	-	-	2.5–5.5
[87]	Coir	Fruit	20	-	0.97	215	3.50	115	-
[88]	Coir		40	500	1.14	-	20	-	-
[89]	Luffa		-	200–750	0.90–1.30	-	10–20	-	8–13
[90]	Abaca	Leaf	20–25	150–260	1.50	857	41	-	-
[91]	Banana		30–40	150–300	1.29–1.32	275–350	12.00–13.50	-	-
[75]	Date palm		12	400–500	1.58	94.73	2.05	63.58	15.22
[80,92]	Diss		20	900–2480	1.32	376	-	90	-
[93]	Lechuguilla		5	221	1.19	68	-	97.80	-

. The lengths of fibres used in SCC ranged from 5 mm to 50 mm, with 20 mm being the most commonly adopted length. The effects of fibre length on various properties of PFRSCC are extensively discussed in Section 7. Hemp and abaca fibres have a higher tensile strength than other plant fibres. Plant fibres exhibited a lower density than synthetic fibres, giving them relatively superior specific mechanical and physical properties [77,78]. Elongation at break is another essential property of plant fibres, representing the capacity of plant fibres to withstand deformation without forming cracks [79]. Coir and date palm fibres exhibited the highest percentages of elongation at break, which positively contributed to the ductility and toughness of SCC. A common obstacle to using plant fibres in SCC is the high water absorption rate of plant fibres due to their hydrophilic characteristic. Hemp fibres exhibited the highest water absorption rate at 158%.

5.2. Chemical Composition of Plant Fibres

The chemical composition of plant fibres has a significant influence on their properties and applicability, largely depending on various factors, including soil conditions, climate, age, and species [78]. Despite the importance of the chemical composition of plant fibres, only a few studies have investigated these critical parameters. The chemical composition of plant fibres used in the reported studies is shown in

Table 6. Chemical composition of various plant fibres.

Refs.	Fibres	Classification by Source	Cellulose (%)	Hemi-Cellulose (%)	Lignin (%)	Pectin (%)	Pentosan (%)	Moisture Content (%)
[80,81]	Hemp	Bast	56.10	10.90	6	-	-	-
[6]	Jute		68.90	11.89	14.56	0.38	-	-
[82]	Jute		60	22	12	-	-	-
[84]	Roselle		65	17.60	4.50	1.10	3.90	7
[75]	Date palm	Leaf	32.00–35.80	24.40–28.10	26.70–28.70	-	-	-
[80,92]	Diss		43	8	35	-	-	-
[103,104]	Sisal		65	12	9.9

. The plant fibres have four primary components, in particular cellulose, hemicellulose, lignin, and pectin. Cellulose is the main constituent contributing to fibre strength and is resistant to strong alkalis, while cellulose is easily hydrolysed when exposed to acid or water-soluble sugars [78,94,95]. Hemicelluloses form a cementing matrix between cellulose microfibrils, making up the primary structural part of fibre cells. Hemicellulose is a hydrophilic constituent easily hydrolysed by dilute bases and acids [94] and accounts for the biological and thermal degradation of plant fibre [78]. Lignin is a hydrocarbon polymer that aids in rigidity and assists in the water transport of plant fibre [94]. Lignin is hydrophobic, resists microorganism attack, is soluble in acid and hot alkali, is easily oxidised, is readily condensable with phenol, and mainly controls the ultraviolet (UV) and fire degradation of plant fibre [78]. Pectin contains different types of neutral sugars and functions to provide plant flexibility [94], while waxes make up most of the outer part of the fibre, consisting of various types of alcohol, and the benefit of waxes is to protect the fibre surface [78,94,96,97,98,99,100,101,102].

5.3. Treatments of Plant Fibres

Plant fibres exhibit several properties that complicate their potential to be widely used in SCC, including containing impurities, poor fibre–cement matrix interaction, high water absorption, and durability issues in alkaline environments [54,102]. The hydrophilic characteristic of plant fibres results in a weak connection between the fibre and the cement matrix, which is attributed to the interaction between hydroxyl groups in the fibre components and water molecules [105,106,107,108]. The durability issues of plant fibres in the cement matrix are linked to the weakening effects of alkali-induced fibre degradation and fibre mineralisation caused by the migration of calcium hydroxide (CH) into the fibre structure [109]. Attempts have been made to mitigate these limitations by employing various treatments to improve the surface morphology of fibres, thereby preventing undesirable components from adhering to the fibre surface [110,111]. The treatment of plant fibres, including alkaline treatment, heat treatment, and coating, has been utilised to improve the physical and mechanical properties of plant fibres, and these treatments partially remove wax, oil, and other impurities, improving fibre adhesion and surface roughness as well as reducing the water absorption of plant fibres [112,113,114,115]. The various treatments used and their effects on the properties of plant fibres are reported in

Table 7. Treatments of plant fibres.

Ref.	Fibre	Treatments	Procedure of Treatment	Effects on Fibre Properties
[116]	Bamboo	Alkaline treatment	The fibres were soaked in a 2% NaOH solution for 24 h.	Alkaline treatment reduced the water absorption of fibres. Treated fibres demonstrated improved surface roughness and were more compatible with the binding matrix due to their enhanced adhesion and surface.
[117]	Banana	Alkaline treatment	The fibres were immersed in a 5% NaOH solution for 24 h. The fibres were then washed and soaked in distilled water to remove excess NaOH, and subsequently sun-dried for 24 h.	-
[118]	Coir	Alkaline treatment	Coir fibres were treated with a 5% NaOH solution by soaking with occasional stirring. The fibres were then washed with clean water, rinsed, and dried.	-
[119]	Kenaf	Alkaline treatment	The fibres were submerged in a 5% NaOH solution.	Alkaline treatment enhanced the mechanical and compatibility properties of fibres with the cement matrix.
[80]	Hemp and diss	Alkaline treatment	The fibres were treated with a 5% NaOH solution at 20 °C for 2 h. The fibres were then dried in front of a heater at 40 °C to remove the moisture. Furthermore, the fibres were immersed in a styrene–butadiene rubber solution for 20 min, then dried at 25 °C.	Alkaline treatment removed lignin and waxy substances from the fibre surface. The treatment also reduced the water absorption of fibres and improved fibre–matrix adhesion.
[120]	Jute	Alkaline treatment	Jute fibres were soaked in a 1% NaOH solution for 20 min and air-dried at ambient temperature.	-
[121]	Red pine needle	Alkaline treatment	The fibres were immersed in a 5% NaOH solution to remove organic substances.	Fibre colours changed to light green for the middle part and black for the edge parts.
[7]	Sisal	Alkaline treatment	The fibres were submerged in a 4% NaOH solution.	-
[122]	Sisal	Alkaline treatment and coated with a polymer	The fibres were soaked in a 5% NaOH solution for 1 h and then dried. The fibres were consequently soaked in 6% Benzoyl Peroxide in acetone for 30 min and dried again. The fibres were next soaked for 10 min in a polymer solution prepared by diluting carboxylate SBR emulsion with distilled water. The coated fibres were then dried.	-
[123]	Coir	Boiling treatment	The fibres were placed in boiling water for 2 h, then washed and sun-dried.	Boiling treatment induced morphological changes in the fibre surface.
[124]	Date palm	Boiling treatment	The treatment consisted of boiling date palm fibres, draining the water, and thoroughly washing the fibres to remove organic substances.	-
[123]	Coir	Coating with silica fume and metakaolin	Soaked fibres were immersed in an adhesive solution (gum) for 1 min to generate bonding. Silica fume and metakaolin were mixed in equal ratios around the fibres and allowed to dry.	Coating treatment induced morphological changes in the fibre surface.
[125]	Caryota-urens	Silane treatment	The silane solution was prepared by diluting a vinyl tri-ethoxy silane chemical with an ethanol and water mixture (80:20). The fibres were soaked in the silane solution for 15 min and then dried at room temperature. The dried fibres were washed in ethanol three to four times and oven-dried at 105 °C for 12 h to remove excess chemicals.	The treatment reduced hemicellulose and lignin content in fibres.
[123]	Coir	Soaking in water	The fibres were soaked in water for 30 min, washed, and then the process was repeated three times, followed by sun-drying.	Soaking in water induced morphological changes in the fibre surface.
[120]	Jute	Ultrasonic vibration coating	The treatment was conducted using an intelligent ultrasonic processor. A nano-sized silica sand was adopted as a wrapping agent to form a fibre coating with a 0.9% mass ratio of the jute fibre.	-

, while the effects of these treatments and their mechanism on the properties of PFRSCC are comprehensively discussed in Section 8. The treatment of plant fibres leads to adverse environmental effects and higher costs for SCC due to the consumption of electricity or chemicals, depending on the treatment method. A life-cycle assessment (LCA) is required to understand the total environmental impact, sustainability, and economic viability of the fibre treatments over the life span of PFRSCC.

5.3.1. Alkaline Treatment

Alkaline treatment is conducted on plant fibres to partially remove hemicellulose, lignin, pectin, wax, oil, and other impurities from the fibre surface [108,126,127,128]. Plant fibres absorb moisture in the amorphous region of cellulose, hemicellulose, and lignin constituents due to the presence of hydroxyl groups [108]. Alkaline treatment reduces the hydrophilic nature of fibres by removing these hydroxyl groups through a reaction with alkaline solutions, resulting in the formation of water molecules [129]. The effectiveness of alkaline treatment depends significantly on the alkaline concentration, treatment duration, and temperature, while exceedingly high alkali concentrations and long treatment durations lead to the over-removal of surface materials and excessive delignification, which may adversely impact the mechanical strength of plant fibres [130,131].

5.3.2. Coating

Fibre coating is a process where a protective layer is applied over the surface of plant fibres or fibre bundles [132,133,134]. The coating serves as a shield to protect the fibres from cement hydration products, reducing the water absorption of fibres, improving volumetric stability, and minimising the porosity at the fibre–matrix interface [135]. Coating plant fibres with polymers enhances water resistance, improves interfacial adhesion, and strengthens the overall mechanical performance of the cementitious matrix, as the polymer layer acts as a bridge between the fibre and the cement matrix, reinforcing the interfacial bond [132,133]. The combination of a polymer coating and the use of pozzolanic materials enhances both fibre–matrix interaction and fibre durability, as the polymer layer acts as a barrier protecting the fibre. Meanwhile, pozzolanic reactions densify the matrix and shield the coating from degradation caused by CH [135].

5.3.3. Heat Treatment

Heat treatment does not affect the overall composition of plant fibre, with only a slight variation reported in cellulose, hemicelluloses, and lignin content in plant fibres [136]. Heat treatment reduces the moisture content in plant fibres, enhancing their compatibility with the matrix and potentially improving the overall performance of the composite [137]. The heat treatment of plant fibres enhances the mechanical properties of the matrix, resulting in significantly higher strength than for a matrix reinforced with untreated fibres [138]. The tensile strength and Young’s modulus of sisal fibres subjected to heat treatment at 150 °C for 8 h were significantly improved by 45% and 70%, respectively, when compared to untreated fibres [58,139]. The improvement in the mechanical properties of sisal fibre was attributed to the increased cellulose crystallinity, which was also associated with the improved durability of their corresponding concrete composites [139]. The treated fibre matrices exhibited lower reductions in both compressive strength and tensile strength when compared to untreated fibre matrices after 30 wetting and drying cycles [58,139].

6. Effects of Plant Fibres on the Properties of SCC

6.1. Effects of Plant Fibres on the Rheological Properties of SCC

Rheology is one of the most critical properties of SCC, owing to the requirement for high flowability and segregation resistance of this special concrete. The rheology of SCC is even more crucial when plant fibres are introduced into the mix due to the reduced workability and complicated behaviour induced by the presence of plant fibres. Direct rheological parameters, namely the yield stress and plastic viscosity of PFRSCC, have not been investigated in the literature. The rheological tests of PFRSCC are discussed, with particular emphasis on the slump flow, V-funnel, L-box, J-ring, and sieve segregation tests. The apparatuses used for these tests are illustrated in Figure 8. The slump flow test is used to evaluate the flowability and flow rate of SCC without obstructions and is conducted using a slump cone, where the slump flow is calculated as the average of the largest diameter of spread and the diameter of perpendicular spread [140]. The V-funnel test is conducted to examine the viscosity and filling ability of SCC, involving the pouring of fresh concrete into a V-shaped funnel. The duration that fresh concrete takes to flow out is called the V-funnel flow time [141]. The L-box test is used to evaluate the passing ability of SCC through the spaces between reinforcing bars and other obstacles without blocking or segregation, where fresh concrete is allowed to flow through gaps formed by vertical, smooth reinforcing bars and concrete heights are measured in the two sections of the L-box apparatus, and their ratio measures the passing ability of SCC [142]. The J-ring test is another test to assess the passing ability of SCC through obstructions and is conducted by placing a J-ring composed of evenly spaced smooth vertical bars over the slump cone before filling with concrete [143], in which the difference between concrete heights outside and inside the ring is called the J-ring height [144]. The sieve segregation test is conducted to examine the resistance of SCC to segregation [145]. The sieve segregation test is performed by allowing the fresh concrete to stand for 15 min to be observed for any separation of bleed water, and the sample is then poured onto a sieve with a 5 mm square opening, in which the segregated portion is then calculated as a percentage of the mass of material passing through the sieve and the initial mass of concrete. The rheological properties of PFRSCC and their variations compared to the unreinforced mix are reported in

Table 8. Rheological properties of PFRSCC and variations compared to the unreinforced mix.

Ref.	Type of Plant Fibres	Treatment Method	Fibre Length (mm)	Fibre Dosage (kg/m3)	Rheological Properties of SCC Mixes					Variations in the Rheological Properties of PFRSCC Compared to the Unreinforced Mix (%)
					Slump Flow Diameter (mm)	V- Funnel Time (s)	L-Box Ratio	J-Ring Height (mm)	Segregated Portion (%)	Slump Flow Diameter	V- Funnel Time	L-Box Ratio	J-Ring Height	Segregated Portion
[147]	Alfa	Untreated	-	0	730	10.73	0.91	1.50	15.40	0%	0%	0%	0%	0%
				2.0	680	11.98	0.84	17.00	5.40	−6.8%	11.6%	−7.7%	1033.3%	−64.9%
[117]	Banana	Untreated	-	0	565	-	-	-	-	0%	-	-	-	-
				0.6	523					−7.4%
				1.6	325					−42.5%
				2.6	307					−45.7%
		Alkaline treatment	-	0	565	-	-	-	-	0%	-	-	-	-
				0.6	375					−33.6%
				1.6	317					−43.9%
				2.6	310					−45.1%
[60]	Banana	Untreated	-	0	785	-	-	-	-	0%	-	-	-	-
				1.0	760					−3.2%
				2.0	746					−5.0%
				3.0	725					−7.6%
				4.0	711					−9.4%
				5.0	697					−11.2%
				6.0	649					−17.3%
[125]	Caryota-urens	Silane treatment	-	0	680	6.00	1.00	1.50	-	0%	0%	0%	0%	-
				6.3	670	7.00	0.97	2.00		−1.5%	16.7%	−3.0%	33.3%
				12.6	662	8.00	0.95	4.00		−2.6%	33.3%	−5.0%	166.7%
				18.9	655	10.00	0.93	6.00		−3.7%	66.7%	−7.0%	300.0%
				25.2	650	12.00	0.90	8.00		−4.4%	100.0%	−10.0%	433.3%
				31.5	640	14.00	0.85	10.00		−5.9%	133.3%	−15.0%	566.7%
[148]	Coir	Untreated	-	0	690	-	0.86	-	-	0%	-	0%	-	-
				0.9	670		0.84			−2.9%		−2.3%
				1.8	630		0.82			−8.7%		−4.7%
				2.7	600		0.81			−13.0%		−5.8%
[88]	Coir	Untreated	40 mm	0	720	6.00	-	-	-	0%	0%	-	-	-
				2.8	690	8.00				−4.2%	33.3%
				5.6	660	8.50				−8.3%	41.7%
				8.4	645	9.00				−10.4%	50.0%
				11.2	630	10.00				−12.5%	66.7%
[118]	Coir	Alkaline treatment	-	0	760	6.90	0.98	-	-	0%	0%	0%	-	-
				1.2	690	9.00	0.91			−9.2%	30.4%	−7.2%
				1.8	650	12.80	0.88			−14.5%	85.5%	−10.3%
				2.4	605	13.80	0.85			−20.4%	100.0%	−13.3%
[87]	Coir	Untreated	20 mm	0	725	6.15	0.92	-	-	0%	0%	0%	-	-
				4.3	720	6.80	0.89			−0.7%	10.6%	−3.3%
				8.7	720	7.00	0.88			−0.7%	13.8%	−4.3%
				13.2	715	8.65	0.88			−1.4%	40.7%	−4.3%
				17.8	680	9.72	0.82			−6.2%	58.0%	−10.9%
				22.6	640	12.5	0.75			−11.7%	103.3%	−18.5%
[123]	Coir	Soaking in water	25 mm	0	625	-	-	-	-	0%	-	-	-	-
				2.5	615					−1.6%
				5.0	595					−4.8%
		Boiling treatment		0	625	-	-	-	-	0%	-	-	-	-
				2.5	610					−2.4%
				5.0	590					−5.6%
		Coating with silica fume and metakaolin		0	625	-	-	-	-	0%	-	-	-	-
				2.5	615					−1.6%
				5.0	585					−6.4%
[75]	Date palm	Untreated	-	0	750	-	1.00	-	15.80	0%	-	0%	-	0%
				0.6	655		0.83		8.40	−12.7%		−17.5%		−46.8%
				0.9	620		0.82		7.60	−17.3%		−18.0%		−51.9%
				1.2	600		0.80		6.80	−20.0%		−20.0%		−57.0%
[124]	Date palm	Boiling treatment	10 mm	0	790	-	-	-	-	0%	-	-	-	-
				1.1	740					−6.3%
				2.1	700					−11.4%
			20 mm	0	790	-	-	-	-	0%	-	-	-	-
				1.1	670					−15.2%
				2.1	655					−17.1%
[149]	Date palm	Untreated	10 mm	0	700	10.20	0.97	-	-	0%	0%	0%	-	-
				0.5	680	11.40	0.94			−2.9%	11.8%	−3.1%
				1.1	680	11.90	0.92			−2.9%	16.7%	−5.2%
				2.1	670	12.30	0.89			−4.3%	20.6%	−8.2%
			20 mm	0	700	10.20	0.97	-	-	0%	0%	0%	-	-
				0.5	690	10.90	0.95			−1.4%	6.9%	−2.1%
				1.1	680	12.30	0.92			−2.9%	20.6%	−5.2%
				2.1	680	13.70	0.87			−2.9%	34.3%	−10.3%
			30 mm	0	700	10.20	0.97	-	-	0%	0%	0%	-	-
				0.5	700	11.20	0.95			0%	9.8%	−2.1%
				1.1	690	13.40	0.89			−1.4%	31.4%	−8.3%
				2.1	670	15.90	0.83			−4.3%	55.9%	−14.4%
[147]	Date palm	Untreated	-	0	730	10.73	0.91	1.50	15.40	0%	0%	0%	0%	0%
				2.0	690	13.89	0.83	16.1	6.13	−5.5%	29.5%	−8.8%	973.3%	−60.2%
[147]	Diss	Untreated	-	0	730	10.73	0.91	1.50	15.40	0%	0%	0%	0%	0%
				2.0	715	16.30	0.75	14.3	7.87	−2.1%	51.9%	−17.6%	853.3%	−48.9%
[80]	Diss	Alkaline treatment	20 mm	0	730	-	-	-	-	0%	-	-	-	-
				2.0	670					−8.2%
		Polymer-coated		0	730	-	-	-	-	0%	-	-	-	-
				2.0	690					−5.5%
[80]	Hemp	Alkaline treatment	20 mm	0	730	-	-	-	-	0%	-	-	-	-
				2.0	600					−17.8%
		Polymer-coated		0	730	-	-	-	-	0%	-	-	-	-
				2.0	620					−15.1%
[6]	Jute	Untreated	20 mm	0	671	5.50	0.95	6.25	9.50	0%	0%	0%	0%	0%
				1.5	648	8.00	0.88	7.50	8.70	−3.4%	45.5%	−7.4%	20.0%	−8.4%
				3.7	632	10.00	0.82	9.00	8.50	−5.8%	81.8%	−13.7%	44.0%	−10.5%
				7.3	620	11.00	0.75	11.75	8.20	−7.6%	100.0%	−21.1%	88.0%	−13.7%
				11.0	575	11.8	0.71	13.00	7.90	−14.3%	114.5%	−25.3%	108.0%	−16.8%
				14.6	550	13.5	0.70	15.25	6.50	−18.0%	145.5%	−26.3%	144.0%	−31.6%
[150]	Jute	Untreated	9 mm	0	620	-	-	-	-	0%	-	-	-	-
				2.6	380					−38.7%
				5.2	200					−67.7%
				10.3	60					−90.3%
[93]	Lechuguilla	Untreated	5 mm	0	610	-	-	56.00	-	0%	-	-	0%	-
				11.9	600			50.00		−1.6%			−10.7%
[121]	Red pine needle	Alkaline treatment	30 mm	0	735	12.10	-	-	-	0%	0%	-	-	-
				5.9	715	13.40				−2.7%	10.7%
				11.8	725	14.40				−1.4%	19.0%
				17.7	685	16.70				−6.8%	38.0%
				23.6	665	20.00				−9.5%	65.3%
			40 mm	0	735	12.10	-	-	-	0%	0%	-	-	-
				7.4	650	14.30				−11.6%	18.2%
				14.8	670	16.60				−8.8%	37.2%
				22.2	640	18.70				−12.9%	54.5%
				29.6	615	20.60				−16.3%	70.2%
			50 mm	0	735	12.10	-	-	-	0%	0%	-	-	-
				9.8	605	15.80				−17.7%	30.6%
				19.6	615	17.20				−16.3%	42.1%
				29.4	575	18.50				−21.8%	52.9%
				39.2	550	22.30				−25.2%	84.3%
[84]	Roselle	Untreated	35 mm	0	682	7.00	1.00	6.00	-	0%	0%	0%	0%	-
				5.8	677	9.00	0.98	6.50		−0.7%	28.6%	−2.0%	8.3%
				11.7	655	10.00	0.95	7.50		−4.0%	42.9%	−5.0%	25.0%
				17.5	650	11.00	0.93	9.00		−4.7%	57.1%	−7.0%	50.0%
				23.3	638	13.00	0.87	11.50		−6.5%	85.7%	−13.0%	91.7%
[151]	Sisal	Untreated	50 mm	0	685	8.00	1.00	8.00	-	0%	0%	0%	0%	-
				0.5	665	8.00	0.98	7.00		−2.9%	0.0%	−2.0%	−12.5%
				1.0	655	8.00	0.97	8.00		−4.4%	0.0%	−3.0%	0.0%
				1.5	653	8.00	0.95	8.00		−4.7%	0.0%	−5.0%	0.0%
				2.0	650	9.00	0.93	9.00		−5.1%	12.5%	−7.0%	12.5%
[7]	Sisal	Alkaline treatment	25 mm	0	750	8.03	1.00	-	-	0%	0%	0%	-	-
				1.2	740	9.51	0.95			−1.3%	18.4%	−5.0%
				2.4	700	10.18	0.82			−6.7%	26.8%	−18.0%
				3.6	645	11.41	0.75			−14.0%	42.1%	−25.0%
[122]	Sisal	Untreated	20 mm	0	780	7.50	0.94	-	-	0%	0%	0%		-
				1.9	755	8.50	0.89			−3.2%	13.3%	−5.3%
				3.8	720	11.00	0.85			−7.7%	46.7%	−9.6%	-
				5.7	690	12.00	0.81			−11.5%	60.0%	−13.8%
				7.6	655	15.00	0.79			−16.0%	100.0%	−16.0%
		Alkaline treatment		0	780	7.50	0.94	-	-	0%	0%	0%	-	-
				1.9	775	8.00	0.90			−0.6%	6.7%	−4.3%
				3.8	735	9.00	0.86			−5.8%	20.0%	−8.5%
				5.7	715	11.00	0.83			−8.3%	46.7%	−11.7%
				7.6	685	13.00	0.80			−12.2%	73.3%	−14.9%
		Polymer-coated		0	780	7.50	0.94	-	-	0%	0%	0%	-	-
				1.9	775	8.00	0.92			−0.6%	6.7%	−2.1%
				3.8	765	9.00	0.89			−1.9%	20.0%	−5.3%
				5.7	740	11.50	0.85			−5.1%	53.3%	−9.6%
				7.6	695	14.00	0.81			−10.9%	86.7%	−13.8%

.

6.1.1. Slump Flow

Slump flow is the most commonly investigated rheological test of SCC and is conducted to determine the filling ability of SCC, which enables a fresh mix to flow under its own weight and fill the formwork [152]. The European Guidelines for SCC [153] categorised three slump flow classes. The first class is SF1, which refers to the SCC with a slump flow ranging from 550 mm to 650 mm and is suitable for unreinforced or slightly reinforced structures. SF2 is the SCC with a flow diameter from 660 mm to 750 mm, which is appropriate for standard applications, including casting for walls or columns. Lastly, SF3 ranges from 760 mm to 850 mm and is used in congested structures with complex shapes. The incorporation of plant fibres into SCC generally led to a decrease in slump flow as a result of the increased flow resistance due to the interlocking and friction between the fibres and aggregates [84], the reduced amount of mixing water due to the high water absorption rate of hydrophilic plant fibres [7], and the fibre agglomeration effect where the fibres attach to each other at a specific location in the mix, which prevents the flow and increases the yield stress of SCC [154]. The interlocking and friction between fibres and aggregates hinder the flowability by increasing the internal resistance and interrupting the flow of SCC [155]. The hydrophilic nature of plant fibres further impairs concrete flow by absorbing free water from the mix, resulting in increased viscosity and a reduced effective water-to-binder ratio, which compromises the ability of SCC to flow and consolidate [7]. Using a high dosage of plant fibres also leads to fibre agglomeration, forming fibre clusters that reduce workability by increasing internal friction and obstructing particle movement [154]. Reported decreases in slump flow vary from a slight reduction up to approximately 90%. The slump flow was reduced by 90.3% when jute fibres were added into the mix at 10.3 kg/m³ [150], but another study reported that adding jute fibres at 11.0 kg/m³ reduced the flow of SCC by only 14.3% [6]. A significant difference in the decrease in slump flow was observed between these two studies using jute fibres, which may have been due to variability in the density of jute fibres, reported as 1030 kg/m³ and 1460 kg/m³ for studies with a decreased slump flow of 90.3% [150] and 14.3% [6], respectively, meaning that a higher dosage of fibres was added in the former study when compared to the latter study. Using banana fibres in two studies at 2.6 kg/m³ [117] and 3.0 kg/m³ [60] resulted in a decrease in slump flow by 45.7% and 7.6%, respectively, and the variation may be due to the latter study using a small-sized coarse aggregate (maximum size of 12.5 mm), which as a result reduced the fibre–aggregate interlocking effect. Adding sisal fibres at 1.2 kg/m³, 2.4 kg/m³, and 3.6 kg/m³ reduced the SCC slump flow by 1.3%, 6.7%, and 14.0%, respectively [7]. Several studies on utilising coir fibres as a reinforcement in SCC have reported a decrease in slump flow when fibres are added to the mix [87,88,118,123,148].

6.1.2. V-Funnel

The V-funnel test is used to assess the flowability and viscosity of SCC [144]. The European Guidelines for SCC define two viscosity classes based on the V-funnel test results [153]. VF1 is the class for SCC with a V-funnel flow time smaller or equal to 8 s, which shows a demonstrable filling ability even with congested reinforcement and is more likely to suffer from bleeding and segregation. VF2 is the SCC with a V-funnel flow time ranging from 9 to 25 s, which assists in limiting the formwork pressure and improving segregation resistance compared to the VF1 class. On the negative side, VF2 may lead to difficulty in surface finishing and may be sensitive to blockages. The V-funnel flow time of SCC increased with the inclusion of plant fibres and further intensified with higher dosages of plant fibres, resulting in a reduction in flowability and an increase in viscosity. The high water absorption capacity of plant fibres elevates the viscosity of the mix by reducing the effective water-to-binder ratio, resulting in less workable concrete [7]. The V-funnel flow time is also prolonged due to the increased friction between the fibres and aggregates, as well as fibre agglomeration that forms fibre clusters, hindering the uniform flow of the mix [84,154,156]. The increases in V-funnel flow time were reported to increase up to approximately 145% when incorporating various plant fibres. Using jute fibres at 14.6 kg/m³ yielded an increase of 145.5% in V-funnel flow time [6], while using caryota-urens fibres at 31.5 kg/m³ led to an increase of 133.3% [125]. Adding red pine needle fibres at 5.9 kg/m³, 11.8 kg/m³, 17.7 kg/m³, and 23.6 kg/m³ increased V-funnel flow time by 10.7%, 19.0%, 38.0%, and 65.3%, respectively. Various studies on embedded coir fibres in SCC also found an increased V-funnel flow time [87,88,118].

6.1.3. L-Box

An L-box test is conducted to determine the passing ability of SCC [154], and this attribute specifies how the fresh SCC flows through reinforcement, confined and constricted spaces, and narrow openings [152]. The European Guidelines for SCC categorised two passing ability classes by conducting L-box tests. PA1 is the class for SCC with an L-box ratio equal to or higher than 0.8 with two rebars in the L-box apparatus and is suitable for structures with a gap of 80 mm to 100 mm, while PA2 is the SCC with an L-box ratio equal to or higher than 0.8 with three rebars in the L-box apparatus and is appropriate for concrete structures with a 60 mm to 80 mm gap [153]. Using plant fibres in SCC decreased the L-box ratio, which dropped even further with increasing dosages of plant fibres, meaning a reduction in passing ability. The effect of plant fibres is not as significant in the L-box ratio when compared to the V-funnel flow time because the size of coarse aggregates is dominantly attributed to the passing ability of SCC in restricted space and reinforcement [7]. Using smaller coarse aggregates is an effective approach to enhance the passing ability of PFRSCC. The high water absorption rate of plant fibres reduces the available free water and restricts the flow, diminishing the passing ability of SCC through confined spaces [7], and the interlocking between fibres and aggregates also increases internal friction and disrupts the flow, hindering the ability of SCC to navigate narrow or reinforced sections [84,156]. Fibre agglomeration, which occurs when using a high dosage of plant fibres, also negatively affects the passing ability of SCC by forming clusters that obstruct flow, thereby compromising the passing ability through congested areas [154]. The decreases in L-box ratios vary from a slight reduction all the way up to a decrease of approximately 26%. Adding jute fibres at 14.6 kg/m³ decreased the L-box ratio by 26.3% [6], and, when roselle fibres were used at 23.3 kg/m³, there was a 13% decrease in the L-box ratio [84]. The inclusion of coir fibres at 22.6 kg/m³ led to an 18.5% reduction in the L-box ratio [87], while the L-box ratio reduced by 2.3%, 4.7%, and 5.8% when incorporating coir fibres at 0.9 kg/m³, 1.8 kg/m³, and 2.7 kg/m³, respectively [148]. Incorporating sisal fibres at 0.5 kg/m³, 1.0 kg/m³, 1.5 kg/m³, and 2.0 kg/m³ led to a slight decrease in the L-box ratio by 2%, 3%, 5%, and 7%, respectively [151].

6.1.4. J-Ring

The J-ring test is conducted to assess the ability of SCC to pass through reinforcement, and the value of J-ring height is suggested to be smaller than 50 mm to obtain an appropriate flow through rebars [157]. The incorporation of plant fibres into SCC generally led to a significant increase in J-ring height, indicating a reduced passing ability of SCC. The increase in the J-ring height of SCC when incorporating plant fibres is due to fibre and aggregate interlocking and reduced mixing water, resulting from the high water absorption of fibres. The presence of fibres restricts the movement of aggregates and reduces the passing ability of SCC [6]. The J-ring height drastically increases with higher dosages of plant fibres, depending on the fibre type. The SCC resulted in a noticeable blockage in the J-ring test when more than 7.3 kg/m³ of jute fibres were added [6]. The passing ability in the J-ring test drastically decreased when using a high dosage of plant fibres. There was a 566.7% increase in J-ring height when using 31.5 kg/m³ of caryota-urens fibres [125], and the J-ring height also increased by 8.3%, 25.0%, 50.0%, and 91.7% when adding roselle fibres at 5.8 kg/m³, 11.7 kg/m³, 17.5 kg/m³, and 23.3 kg/m³, respectively [84]. Using diss, alfa, and date palm fibres at 2 kg/m³ significantly increased the J-ring height [147], while using lechuguilla fibres at 11.9 kg/m³ led to a slight decrease in the J-ring height when compared to other studies, since the fine and coarse aggregates were reduced in the mix design when fibres were incorporated into the SCC [93].

6.1.5. Segregation Resistance by the Sieve Segregation Test

Segregation resistance is the ability of a fresh mix to maintain the uniform distribution of constituent materials during transport, placing, and compaction processes [152]. A sieve segregation test is carried out to investigate the segregation resistance of SCC [6], and the European Guidelines for SCC distinguish between two segregation resistance classes for SCC by conducting this sieve segregation test, namely SR1 and SR2 [153]. SR1 is the SCC class with a segregated portion less than or equal to 20%, while SR2 is the SCC class with a segregated portion less than or equal to 15%. Using plant fibres in SCC reduced the segregated portion of the mix, meaning an improvement in segregation resistance. The improved segregation resistance results from the reduced fluidity and increased viscosity of the fresh mix when incorporating plant fibres. The incorporation of fibres enhances the segregation resistance of SCC by improving mix cohesion, absorbing excess free water to reduce bleeding, and increasing internal friction, which help maintain the uniformity and stability of the concrete [7,84,156]. An optimum dosage of plant fibres is required to balance the segregation resistance and other rheological properties, particularly the flowability, filling ability, and passing ability. The segregation resistance drastically increases with higher dosages of plant fibres, depending on the fibre type. The incorporation of diss, alfa, and date palm fibres at 2.0 kg/m³ in SCC significantly reduced segregated portions by 48.9%, 64.9%, and 60.2%, respectively [147]. The segregated portion of jute fibre-reinforced SCC also decreased by 8.4%, 10.5%, 13.7%, 16.8%, and 31.6% when adding fibres at 1.5 kg/m³, 3.7 kg/m³, 7.3 kg/m³, 11.0 kg/m³, and 14.6 kg/m³, respectively [6]. The inclusion of date palm fibres at 0.6 kg/m³, 0.9 kg/m³, and 1.2 kg/m³ reduced segregated portions by 46.8%, 51.9%, and 57.0% when compared to the unreinforced SCC, respectively [75].

6.2. Effects of Plant Fibres on the Mechanical Properties of SCC

The mechanical properties of SCC incorporating various plant fibres are discussed, particularly compressive strength, split tensile strength, flexural strength, modulus of elasticity, flexural toughness, and impact strength. The mechanical properties and variations in PFRSCC compared to the unreinforced mix are shown in

Table 9. Mechanical properties at the 28-day age of PFRSCC and variations compared to the unreinforced mix.

Ref.	Type of Plant Fibres	Treatment Method	Fibre Length (mm)	Fibre Dosage (kg/m3)	Mechanical Properties of SCC Mixes				Variations in the Mechanical Properties of PFRSCC Compared to the Unreinforced Mix (%)
					Compressive Strength (MPa)	Split Tensile Strength (MPa)	Flexural Strength (MPa)	Modulus of Elasticity (GPa)	Compressive Strength	Split Tensile Strength	Flexural Strength	Modulus of Elasticity
[147]	Alfa	Untreated	-	0	50.29	-	9.74	-	0%	-	0%	-
				2.0	45.31		7.78		−9.9%		−20.1%
[117]	Banana	Untreated	-	0	32.88	3.15	-	-	0%	0	-	-
				0.6	38.84	3.80			18.1%	8.3%
				1.6	36.17	4.13			10.0%	17.7%
				2.6	25.48	4.21			−22.5%	19.9%
		Alkaline treatment	-	0	32.88	3.15	-	-	0%	0	-	-
				0.6	47.47	4.13			44.4%	17.7%
				1.6	32.88	3.80			0.0%	8.3%
				2.6	30.41	3.64			−7.5%	3.7%
[60]	Banana	Untreated	-	0	49.00	-	-	-	0%	-	-	-
				1.0	51.00				4.1%
				2.0	52.00				6.1%
				3.0	55.00				12.2%
				4.0	57.00				16.3%
				5.0	57.50				17.3%
				6.0	55.00				12.2%
[125]	Caryota-urens	Silane treatment	-	0	50.00	4.30	4.83	33.90	0%	0%	0%	0%
				6.3	51.80	4.37	5.10	35.20	3.6%	1.6%	5.6%	3.8%
				12.6	53.20	4.49	5.25	35.90	6.4%	4.4%	8.7%	5.9%
				18.9	55.40	4.60	5.36	36.80	10.8%	7.0%	11.0%	8.6%
				25.2	53.50	4.52	5.31	35.60	7.0%	5.1%	9.9%	5.0%
				31.5	50.00	4.30	5.09	35.00	0.0%	0.0%	5.4%	3.2%
[158]	Coir	Untreated	-	0	37.70	3.24	13.33	42.40	0%	0%	0%	0%
				2.3	38.04	4.61	22.67	43.13	0.9%	42.3%	70.1%	1.7%
				4.6	36.17	3.98	14.89	40.34	−4.1%	22.8%	11.7%	−4.9%
[148]	Coir	Untreated	-	0	21.20	-	5.48	-	0%	-	0%	-
				0.9	19.55		5.49		−7.8%		0.2%
				1.8	18.25		5.15		−13.9%		−6.0%
				2.7	14.75		5.05		−30.4%		−7.8%
[88]	Coir	Untreated	40 mm	0	42.50	6.20	-	-	0%	0%	-	-
				2.8	43.00	6.65			1.2%	7.3%
				5.6	48.50	7.40			14.1%	19.4%
				8.4	52.50	8.12			23.5%	31.0%
				11.2	54.00	8.38			27.1%	35.2%
[118]	Coir	Alkaline treatment	-	0	51.90	-	7.14	-	0%	-	0%	-
				1.2	53.30		7.48		2.7%		4.8%
				1.8	55.00		7.65		6.0%		7.1%
				2.4	54.40		7.83		4.8%		9.7%
[87]	Coir	Untreated	20 mm	0	27.64	-	-	-	0%	-	-	-
				4.3	27.87				0.8%
				8.7	28.14				1.8%
				13.2	28.28				2.3%
				17.8	28.87				4.5%
				22.6	29.20				5.6%
[123]	Coir	Soaking in water	25 mm	0	22.24	3.24	-	-	0%	0%	-	-
				2.5	20.81	3.69			−6.4%	13.9%
				5.0	22.15	3.90			−0.4%	20.4%
		Boiling treatment		0	22.24	3.24	4.00	-	0%	0%	0%	-
				2.5	27.00	3.64	-		21.4%	12.3%	-
				5.0	32.00	4.44	4.50		43.9%	37.0%	12.5%
		Coating with silica fume and metakaolin		0	22.24	3.24	4.00	-	0%	0%	0%	-
				2.5	32.32	3.86	-		45.3%	19.1%	-
				5.0	37.98	4.67	5.50		70.8%	44.1%	37.5%
[75]	Date palm	Untreated	-	0	48.00	-	5.95	-	0%	-	0%	-
				0.6	43.00		8.05		−10.4%		35.3%
				0.9	37.50		8.50		−21.9%		42.9%
				1.2	35.50		9.00		−26.0%		51.3%
[124]	Date palm	Boiling treatment	10 mm	0	38.00	-	-	-	0%	-	-	-
				1.1	37.00				−2.6%
				2.1	35.50				−6.6%
			20 mm	0	38.00	-	-	-	0%	-	-	-
				1.1	35.00				−7.9%
				2.1	33.00				−13.2%
[149]	Date palm	Untreated	10 mm	0	46.00	-	-	-	0%	-	-	-
				0.5	45.50				−1.1%
				1.1	45.10				−2.0%
				2.1	44.90				−2.4%
			20 mm	0	46.00	-	-	-	0%	-	-	-
				0.5	45.30				−1.5%
				1.1	44.80				−2.6%
				2.1	44.50				−3.3%
			30 mm	0	46.00	-	-	-	0%	-	-	-
				0.5	45.30				−1.5%
				1.1	44.90				−2.4%
				2.1	44.00				−4.3%
[147]	Date palm	Untreated	-	0	50.29	-	9.74	-	0%	-	0%	-
				2.0	46.50		8.98		−7.5%		−7.8%
[147]	Diss	Untreated	-	0	50.29	-	9.74	-	0%	-	0%	-
				2.0	39.53		7.89		−21.4%		−19.0%
[80]	Diss	Alkaline treatment	20 mm	0	50.20	-	9.80	-	0%	-	0%	-
				2.0	44.80		9.10		−10.8%		−7.1%
		Polymer-coated		0	50.20	-	9.80	-	0%	-	0%	-
				2.0	52.00		10.65		3.6%		8.7%
[80]	Hemp	Alkaline treatment	20 mm	0	50.20	-	9.80	-	0%	-	0%	-
				2.0	46.00		9.50		−8.4%		−3.1%
		Polymer-coated		0	50.20	-	9.80	-	0%	-	0%	-
				2.0	50.00		10.30		−0.4%		5.1%
[6]	Jute	Untreated	20 mm	0	20.13	3.09	4.19	-	0%	0%	0%	-
				1.5	20.26	3.43	4.58		0.60%	11.0%	9.3%
				3.7	20.53	3.75	4.95		2.0%	21.4%	18.1%
				7.3	19.13	3.45	4.45		−5.0%	11.7%	6.2%
				11.0	18.05	3.16	4.28		−10.3%	2.3%	2.1%
				14.6	15.85	3.05	3.94		−21.3%	−1.3%	−6.0%
[150]	Jute	Untreated	9 mm	0	26.36	3.30	5.87	-	0%	0%	0%	-
				2.6	40.22	4.34	6.78		52.6%	31.5%	15.5%
				5.2	41.51	4.33	7.07		57.5%	31.2%	20.4%
				10.3	39.88	4.42	7.40		51.3%	33.9%	26.1%
[93]	Lechuguilla	Untreated	5 mm	0	49.20	-	-	-	0%	-	-	-
				11.9	47.00				−4.5%
[121]	Red pine needle	Alkaline treatment	30 mm	0	112.00	-	11.29	-	0%	-	0%	-
				5.9	126.00		21.95		12.5%		94.4%
				11.8	130.00		22.46		16.1%		98.9%
				17.7	128.50		22.25		14.7%		97.1%
				23.6	128.00		22.06		14.3%		95.4%
			40 mm	0	112.00	-	11.29	-	0%	-	0%	-
				7.4	122.00		21.14		8.9%		87.2%
				14.8	124.00		21.63		10.7%		91.6%
				22.2	123.90		21.49		10.6%		90.3%
				29.6	122.00		21.30		8.9%		88.7%
			50 mm	0	112.00	-	11.29	-	0%	-	0%	-
				9.8	106.00		19.85		−5.4%		75.8%
				19.6	106.20		20.44		−5.2%		81.0%
				29.4	105.00		20.06		−6.3%		77.7%
				39.2	104.70		19.95		−6.5%		76.7%
[84]	Roselle	Untreated	35 mm	0	31.11	3.20	3.73	27.90	0%	0%	0%	0%
				5.8	32.02	3.65	4.42	28.64	2.9%	14.1%	18.5%	2.7%
				11.7	33.56	3.78	4.60	29.59	7.9%	18.1%	23.3%	6.1%
				17.5	34.78	3.90	4.75	30.35	11.8%	21.9%	27.3%	8.8%
				23.3	29.47	3.53	4.25	26.96	−5.3%	10.3%	13.9%	−3.4%
[151]	Sisal	Untreated	50 mm	0	42.66	3.40	4.05	27.00	0%	0%	0%	0%
				0.5	41.11	3.67	4.04	27.40	−3.6%	7.9%	−0.2%	1.5%
				1.0	41.57	3.74	4.20	27.90	−2.6%	10.0%	3.7%	3.3%
				1.5	44.19	3.96	4.34	28.30	3.6%	16.5%	7.2%	4.8%
				2.0	42.86	3.53	4.28	28.10	0.5%	3.8%	5.7%	4.1%
[7]	Sisal	Alkaline treatment	25 mm	0	54.50	6.40	5.00	-	0%	0%	0%	-
				1.2	60.00	7.80	6.00		10.1%	21.9%	20.0%
				2.4	52.20	9.10	8.00		−3.7%	42.2%	60.0%
				3.6	45.00	10.60	10.40		−17.4%	65.6%	108.0%
[122]	Sisal	Untreated	20 mm	0	44.00	2.90	6.10	-	0%	0%	0%	-
				1.9	42.00	2.50	7.00		−4.5%	−13.8%	14.8%
				3.8	42.20	2.75	7.25		−4.1%	−5.2%	18.9%
				5.7	33.00	3.40	7.30		−25.0%	17.2%	19.7%
				7.6	30.00	3.25	7.30		−31.8%	12.1%	19.7%
		Alkaline treatment		0	44.00	2.90	6.10	-	0%	0%	0%	-
				1.9	42.90	2.45	7.90		−2.5%	−15.5%	29.5%
				3.8	49.00	3.95	8.00		11.4%	36.2%	31.1%
				5.7	38.00	3.40	8.25		−13.6%	17.2%	35.2%
				7.6	31.50	3.40	8.35		−28.4%	17.2%	36.9%
		Polymer-coated		0	44.00	2.90	6.10	-	0%	0%	0%	-
				1.9	46.00	3.20	8.00		4.5%	10.3%	31.1%
				3.8	51.50	3.45	8.75		17.0%	19.0%	43.4%
				5.7	40.50	3.45	9.00		−8.0%	19.0%	47.5%
				7.6	37.50	3.40	9.75		−14.8%	17.2%	59.8%

.

6.2.1. Compressive Strength

Compressive strength is the most commonly investigated mechanical property of PFRSCC. The 28-day compressive strength of unreinforced SCC and PFRSCC with various plant fibres and fibre dosages is depicted in Figure 9. The literature reported variable results, with about half of the studies reporting an improved compressive strength of SCC, while the other half observed a decreased compressive strength of SCC when incorporating plant fibres. Introducing fibres into concrete may cause two conflicting results in its compressive strength. Fibres delay the crack propagation through fibre bridging, which positively affects the compressive strength of SCC [159], and the primary factors that influence this bridging effect of PFRSCC are fibre dosage and length, fibre–cement matrix bond strength, and fibre types [49]. Conversely, adding fibres increases the porosity of SCC due to the porous nature of plant fibres and the formation of interfacial gaps at the fibre–cement matrix interface. The higher porosity of SCC leads to weakened fibre–cement matrix bonds and premature crack propagation, thereby reducing the compressive strength of concrete [84]. Fibre dosages, fibre types, and concrete mix proportions significantly affect the compressive strength of PFRSCC. Using banana fibres at 3.0 kg/m³ increased the compressive strength of SCC by 12.2% in a study by Poongodi and Murthi [60], while incorporating banana fibres at 2.6 kg/m³ decreased the strength by 22.5% in a study by Chairunnisa et al. [117]. The improvement in the compressive strength of SCC in the former study is attributed to the incorporation of rice husk ash and silica fume to partially replace cement, which made the concrete denser with reduced alkalinity, thereby improving the compatibility of the fibre–cement matrix interface and decreasing the deterioration of fibres in SCC [60]. Using jute fibres at 5.2 kg/m³ improved the compressive strength of SCC by 57.5% [150], and using roselle fibres at 17.5 kg/m³ increased the compressive strength by 11.8% [84]. The compressive strength of the mix was improved by 10.8% when caryota-urens fibres were added at 18.9 kg/m³ [125]. Conversely, other studies found that utilising plant fibres in SCC decreased the compressive strength. The compressive strength was reduced with an increased sisal fibre dosage in SCC, decreasing by 31.8% when 7.6 kg/m³ fibre was added [122], and a similar trend was observed with jute fibres, where the compressive strength reduced by 21.3% with 14.6 kg/m³ fibres [6]. A significant drop in compressive strength was also observed when incorporating coir and banana fibres into SCC [117,148], while several studies demonstrated a slight decrease in compressive strength when incorporating date palm, hemp, and lechuguilla fibres [80,93,149,158].

6.2.2. Split Tensile Strength

The tensile strength of concrete is a crucial property because unreinforced concrete is often susceptible to cracking when subjected to tensile stress [84]. The split tensile strength test is an indirect test conducted to determine the tensile strength of concrete samples subjected to compressive load [160], and the 28-day split tensile strength of unreinforced SCC and PFRSCC with various plant fibres and fibre dosages is shown in Figure 10. The studies reported in the literature depict a demonstrable improvement in the split tensile strength when plant fibres are incorporated into SCC. Incorporating fibres into SCC restrained the development of cracks and preserved the stiffness and the ability to withstand load [161], in which the stress developed is transferred to fibres when the tensile load increases, bridging the cracks and substantially enhancing the split tensile strength [84,162]. The physical interlocking and mechanical bonding of fibres within the concrete contribute to a more uniform distribution of tensile stress and delay the initiation and propagation of cracks [163]. The contribution of fibres to the split tensile strength is more prominent than to the compressive strength and is consistent across the published research. The split tensile strength of SCC improved with a higher dosage of sisal fibres and reached a 65.6% strength improvement at a 3.6 kg/m³ fibre dosage [7], while using coir fibres at 2.3 kg/m³ led to a 42.3% increase in the split tensile strength [158]. The optimum dosage of jute fibres was 3.7 kg/m³, with a 21.4% improvement in the split tensile strength and the failure patterns of jute fibre-reinforced SCC in split tensile strength tests. The unreinforced SCC displayed a brittle failure when subjected to tensile loading, while a ductile failure of specimens due to the fibre bridging effect was observed in all jute fibre-reinforced mixes. Using caryota-urens fibres slightly increased the split tensile strength of SCC, with an optimum dosage of 18.9 kg/m³ [125], while a few studies [151,164] reported that utilising sisal fibres and coir fibres did not significantly affect the split tensile strength of SCC.

6.2.3. Flexural Strength

The 28-day flexural strength of unreinforced SCC and PFRSCC with various plant fibres and fibre dosages is shown in Figure 11, and an improvement in flexural strength was generally reported when adding plant fibres to the mix. The improvement of the flexural strength of PFRSCC depends on fibre types, fibre dosages, and concrete mix proportions. The flexural strength enhancement is due to the combined effects of fibres on crack-bridging and strain redistribution across the cross-section of SCC [107]. The flexural strength of concrete is generally constrained by its inherent brittleness and inadequate performance after cracking [165]. Using plant fibres reduces the propagation of macrocracks by creating a bridging effect on cracks and improving the post-cracking performance of SCC [12]. The effect of plant fibres on the flexural strength of SCC varies from a minor variation to as high as a 108% increment. Using sisal fibres at 3.6 kg/m³ led to a 108% improvement in the flexural strength of the mix [7]. The flexural strength of SCC improved by more than 90% when incorporating red pine needle fibres from 5.9 kg/m³ to 23.6 kg/m³, with higher dosages of a superplasticiser used for the mixes containing red pine needle fibres, and the superplasticiser dosages were increased with higher fibre dosages and longer fibre lengths [121]. The fibres used in these two studies were treated with NaOH solutions before being used, thus enhancing their compatibility with the cement matrix [7,121]. The inclusion of coir fibres also significantly improved the flexural strength of SCC, with a 70.07% increment for a fibre dosage of 2.3 kg/m³ [158], and using caryota-urens fibres was also advantageous to SCC, with the highest flexural strength achieved at 18.9 kg/m³ fibres [125]. The improvement in the flexural strength of plant fibre-reinforced SCC was also observed in various studies utilising sisal [122], jute [6,150,166], and coir [118] fibres. In contrast to the findings of other studies, incorporating coir fibres at 2.7 kg/m³ led to a 7.9% reduction in flexural strength, indicating a potential outlier [148]. Using alfa, diss, and date palm fibres, along with extra water and superplasticiser, to improve the workability of SCC resulted in a drastic drop in the flexural strength due to the increased effective water-to-binder ratio of SCC [147].

6.2.4. Modulus of Elasticity

The modulus of elasticity (MOE) is the ratio of stress to corresponding strain in the elastic region of concrete [125]. Various fibres were used in SCC to enhance stress redistribution and decrease strain localisation [84], and several studies [84,125,151,158] reported that using plant fibres improved the MOE of SCC. Incorporating plant fibres enhances the stiffness of concrete by bridging the microcracks that develop when subjected to stress [159]. The fibres resist further crack propagation and promote a more uniform redistribution of internal stresses throughout the matrix, improving the load-bearing response at early loading stages and resulting in an increased stiffness and a higher MOE of the concrete [163]. Using 17.5 kg/m³ of roselle fibres improved the MOE by 8.8% [84], but adding more than the 17.5 kg/m³ threshold led to a lower MOE, while a similar trend was observed when using caryota-urens fibres at 18.9 kg/m³ improved the MOE by 8.6% [125], which started to decrease when a higher fibre dosage was used. The inclusion of sisal fibres at 2.0 kg/m³ enhanced the MOE of SCC by 4.1% [151], but incorporating coir fibres did not significantly affect the MOE of SCC [158]. Incorporating coir fibres at 2.3 kg/m³ slightly improved the MOE by 1.7%, while adding 4.6 kg/m³ of coir fibres into SCC yielded a decrease in the MOE by 4.9% when compared to unreinforced SCC [158]. The effects of plant fibres on the MOE of SCC depend on many factors, including the physical and mechanical properties of plant fibres, fibre dosage, fibre length, fibre treatment, and concrete mix proportion. The increment of MOE was attributed to crack-bridging by fibres in the elastic region of concrete, where fibres dispersed cracks in multiple directions, held the concrete matrix, and lowered the risk of the sudden failure of SCC [125].

6.2.5. Flexural Toughness

Flexural toughness refers to the ability of concrete to absorb energy and is commonly used to evaluate its ductility and crack resistance [167]. Flexural toughness is a crucial indicator for evaluating the post-cracking properties of fibre-reinforced concrete, including the cracked load-bearing capacity, the energy absorption capacity, and the toughening impact of fibres [168,169]. The flexural toughness of SCC was improved when plant fibres were added due to the bridging effect and strain redistribution induced by the fibres. Incorporating fibres into concrete enhances its flexural performance by bridging the cracks that develop during loading, allowing the material to sustain loads even after cracking and increasing the post-crack load capacity and energy absorption [162,170,171]. The fibres transfer stresses across the crack, reducing the crack propagation rate and promoting a more uniform stress distribution throughout the matrix. The overall toughness of concrete is effectively improved through the use of uniformly distributed and randomly oriented short fibres, which help control the initiation and development of microcracks [172]. The inclusion of roselle fibres in SCC improved flexural toughness indices [84], which were calculated according to the ASTM C1018 standard [173]. The flexural toughness indices of SCC increased when roselle fibres were added from 5.8 kg/m³ to 23.3 kg/m³, with maximum flexural toughness indices being observed at a 23.3 kg/m³ fibre dosage [84], and the enhancement in flexural toughness led to improved impact, fracture, and fatigue characteristics of SCC. The flexural toughness of SCC improved by 51% when sisal fibres were used as a reinforcing material at 1.5 kg/m³ [151], while the flexural toughness of SCC tended to decrease when adding fibres higher than the 1.5 kg/m³ threshold.

6.2.6. Impact Strength

The impact strength of concrete refers to its ability to resist sudden impacts and dynamic loads [174], including falling trees, the crashing of vehicles, and objects thrown at structures during storms [175]. The impact strengths of PFRSCC were investigated by conducting a drop hammer test following the ACI Committee 544 report [176], and the unreinforced SCC displayed brittle failure and minimal resistance after the first crack formation due to impact load, while adding roselle fibres to SCC increased the number of blows for reaching the ultimate crack, improving the impact resistance [84]. The increase in impact resistance is due to fibre bridging across the cracks, which absorbs impact energy and effectively restrains crack propagation within the matrix [84]. Plant fibres act as reinforcements within the concrete matrix by absorbing and dissipating impact energy, thereby reducing the amount of energy transmitted directly to the brittle cement matrix and enhancing the resistance of concrete when subjected to impact loads [177,178]. The unreinforced concrete tends to crack upon impact, and these cracks are bridged when fibres are embedded, leading to a more gradual failure characteristic and significantly improving the impact resistance of concrete [179]. Mixtures containing caryota-urens fibres showed a better performance in arresting microcracks, resulting in higher impact resistance than unreinforced specimens and the maximum number of blows for ultimate crack was observed in SCC containing 25.2 kg/m³ fibres, with the improvement in impact energy being about 76%, 85%, and 25% for 18.9 kg/m³ (3% fibre volume), 25.2 kg/m³ (4% fibre volume), and 31.5 kg/m³ (5% fibre volume) of caryota-urens fibres, respectively [125]. As the fibre dosage increased, the porosity of the concrete also increased, resulting in a lesser improvement in impact energy for the 31.5 kg/m³ fibre mix. SCC reinforced by abaca and polypropylene hybrid fibres also sustained more blows than the unreinforced SCC, and the improvement in impact strength was more substantial in the hybrid fibre mixes than in the abaca fibre mixes [90].

6.3. Effects of Plant Fibres on the Durability of SCC

The number of studies investigating the durability of SCC reinforced with plant fibres is limited; therefore, this review only discusses the durability properties that have been previously studied. These durability properties include sulphate attack resistance, acid attack resistance, drying shrinkage, water absorption, and freeze–thaw cycle resistance.

6.3.1. Resistance to Sulphate Attack

A sulphate attack occurs when dissolved sulphate ions penetrate concrete and react with the hardened cement matrix, resulting in expansion, cracking, and the loss of strength of SCC [180]. The sulphate ions infiltrate concrete voids and react with calcium hydroxide, producing ettringite and gypsum, which occupy the concrete voids, causing abrasion, expansion, and cracks that damage the concrete [121]. Incorporating plant fibres into concrete introduces entrapped air and increases porosity, facilitating the ingress of sulphate ions into the matrix [163]. Additionally, plant fibres degrade in highly alkaline environments within concrete, which generates voids and renders the concrete more susceptible to sulphate attacks [102]. The increased porosity of concrete elevates the penetration of sulphate ions, escalating the risk of deleterious chemical reactions, including ettringite formation, which leads to cracking and expansion [181]. The rate of sulphate attack of SCC was higher when red pine needle fibres were added, and the loss of compressive strength of SCC submerged in a sulphate solution was more significant with a higher fibre dosage [121]. When the red pine needle fibre dosage exceeded 11.8 kg/m³ (0.5% of the concrete volume), the loss of strength dramatically increased due to an increase in the porosity of the SCC.

6.3.2. Resistance to Acid Attack

Constituents of the cement and aggregates are selectively dissolved when concrete encounters acidic solutions, which increases the porosity and affects the mechanical properties of SCC [180]. The chemical reaction between cement paste and acid results in a roughened surface on the specimens after exposure [182,183,184]. Plant fibres are susceptible to degradation when exposed to acidic environments, compromising the integrity and compactness of the concrete matrix [185]. Incorporating fibres into SCC also increases the porosity, facilitating acid ingress and accelerating chemical attacks on the concrete [186]. Using high dosages of plant fibres leads to agglomeration, resulting in an uneven distribution within the matrix and creating weak zones that are more prone to acid attack. Incorporating sisal fibres and sisal and nylon-6 hybrid fibres marginally decreased the acid resistance of SCC [7], although the sisal fibre mixes were more resistant than the sisal and nylon-6 hybrid fibre mixes. The higher resistance to acid attack was due to the surface modification of sisal fibres by an alkaline treatment, which had made the material inert and its pore space intact [7,187]. Using sisal fibres or hybrid fibres led to a slight decrease in weight loss when compared to the unreinforced SCC when submerged in an HCl solution for 60 days, potentially due to the dissolution of calcium silicate hydrate (C-S-H) and calcium hydroxide (CH) into the acidic solution rather than to the effect of the fibres themselves.

6.3.3. Drying Shrinkage

Drying shrinkage is defined as the volume reduction in SCC caused by the loss of moisture from the surface of gel pores during hydration [188], and the deformation caused by drying shrinkage leads to the cracking of SCC, affecting the service life of materials [189]. The paste volume in SCC is relatively high, leading to drying shrinkage cracks, especially at an early age [75]. Plant fibres bridge microcracks that develop when the concrete is drying, thereby mitigating crack propagation and promoting a more uniform distribution of internal stresses, which in turn reduces shrinkage cracking [159]. The inclusion of plant fibres enhances the tensile strength of concrete, making the matrix more resistant to internal shrinkage stresses and shrinkage-induced cracking [7,158]. Early-age shrinkage is particularly critical because the concrete exhibits its lowest strain capacity and highest sensitivity to internal stresses in the initial hours after casting [190]. The well-dispersed nature of plant fibres in the fresh mix provides early mechanical restraint, effectively reducing the width of early-age cracks. Using date palm fibres and polypropylene fibres reduced the drying shrinkage of SCC, with date palm fibres being more effective than polypropylene fibres due to their rough surface [75]. The early-age drying shrinkage of date palm fibre mixes was lower than that of the unreinforced SCC in hot–dry conditions [124]. Date palm fibres also significantly reduced the cracked area when compared to the unreinforced mix, where all tested volume fractions and fibre lengths substantially reduced the early-age cracking risk, and the cracked area consistently decreased with higher dosages of date palm fibres [124]. The results indicate that plant fibres effectively decrease drying shrinkage and reduce crack areas in PFRSCC.

6.3.4. Water Absorption

Water absorption is the quantity of water that concrete absorbs at atmospheric pressure [191], and is a key transport property of concrete and critical to its overall performance [192] because many aggressive ions penetrate concrete through water absorption, leading to durability degradation [193,194]. Incorporating plant fibres into concrete increases the porosity, facilitating greater water ingress and retention [186]. Adding to this, the hygroscopic nature of plant fibres causes them to swell upon water absorption and shrink during drying, which lead to the formation of microcracks and additional voids within the matrix, creating new pathways for water penetration and elevating the overall absorption capacity of the matrix [195]. Two main absorption phases of the capillary water absorption of date palm fibre-reinforced SCC were observed in a study by Derdour et al. [75]. The first phase was relatively rapid, lasting up to 8 h and filling larger pores, whereas the subsequent phase progressed more slowly, filling smaller pores. SCC reinforced by jute–coir hybrid fibres absorbed more water (2.3%) than the unreinforced SCC (1.4%) [196]. Using diss, alfa, and date palm fibres in SCC generated additional porosity at the fibre–cement matrix interface, increasing the capillary water absorption [147]. SCC reinforced by polypropylene–abaca hybrid fibres absorbed less water than those with abaca fibres due to the polypropylene fibres possessing a hydrophobic nature, while abaca fibres were hydrophilic, which caused less water absorption [90]. Using sisal fibres and sisal and nylon-6 hybrid fibres increased the water absorption of SCC, where the initial water absorption immediately after immersion was unaffected by the fibre dosage, while the water absorption after 7 days was solely influenced by the fibre dosage and the hybridisation ratio of sisal and nylon-6 fibres [7].

6.3.5. Freeze and Thaw Resistance

Water is one of the few substances that increase in volume when changing from a liquid to a solid state [180], which leads to damage within concrete pores over repeated freeze–thaw cycles. A higher number of cycles increased the degree of internal damage to concrete due to the continuous freezing and thawing of the pore water in SCC, leading to a loss of strength in the specimens [197,198]. The water absorbed by plant fibres expands upon freezing when subjected to freeze–thaw cycles, generating internal pressure that leads to microcracking and the degradation of the concrete matrix [199,200]. The inclusion of plant fibres typically increases the capillary porosity and permeability of the mix, allowing more water to infiltrate the matrix and intensifying freeze–thaw damage as the water freezes and expands [186]. Moreover, plant fibres are prone to deterioration in moist or fluctuating environments, resulting in additional voids that weaken the internal structure and accelerate the freeze–thaw deterioration [201]. Adding to this, the poor bonding between plant fibres and the cement paste leads to gaps forming at the fibre–matrix interface, which serve as pathways for water ingress and zones of stress concentration during freezing [202]. Using red pine needle fibres in SCC reduced the compressive strength when subjected to freeze–thaw cycles [121]. Incorporating fibres at 11.8 kg/m³ led to a slight decrease in compressive strength compared to an unreinforced mix, while using higher fibre dosages significantly reduced the compressive strength of the SCC when subjected to 50 cycles of freeze–thaw and then tested.

6.4. Effects of Plant Fibres on the Microstructure of SCC

The microstructure of jute fibre-reinforced SCC was investigated through SEM images. The unreinforced SCC exhibited pre-existing cracks even before applying any external force, with small cavities and pores observed in the specimen, potentially leading to a reduction in concrete strength, while the fibre-reinforced mix exhibited a uniform integration of jute fibres with cement paste, which led to fibre bridging, effectively minimising cavities and enhancing the strength of SCC [6]. Incorporating an optimum dosage of jute fibres at approximately 3.7 kg/m³ effectively restricts the propagation of existing cracks in SCC. The microstructure of abaca–polypropylene hybrid fibre-reinforced SCC was investigated, and the SEM images of the unreinforced mix and the abaca–polypropylene hybrid fibre mix are depicted in Figure 12. The findings indicated that hybrid fibres bridged the cracks, effectively restraining the brittle failure of concrete [90]. The rough surface of the plant fibre interacted physically with the matrix interfaces that filled the gaps in SCC. The presence of fibres between the pores indicated that the bridging action of the fibres avoided the discontinuity caused by the pores within the matrix.

7. Effects of Fibre Length on the Properties of PFRSCC

Previous discussions indicate that the types of plant fibres, fibre dosages, and concrete mix proportions significantly influence the fresh and hardened properties of SCC. Another crucial factor determining the performance of PFRSCC is the fibre length. While incorporating plant fibres enhances the tensile strength, flexural performance, and crack resistance, their length influences the dispersion and bonding within the matrix.

7.1. Effects of Fibre Length on the Rheological Properties of PFRSCC

Fibre length has a critical impact on the rheological properties of SCC, with the use of longer plant fibres leading to decreased workability when compared to shorter fibres, resulting in less flowable and more viscous SCC at the same dosage. The reduced workability when using longer fibres is due to the effects of fibre agglomeration, caused by fibre entanglement and fibre–aggregate interlocking, which are more severe in the case of longer fibres [203,204]. Results from the literature show that the slump flow decreased by 9.5% when using 30 mm red pine needle fibres, while the slump flow decreased by 25% when the same fibres were 50 mm [121]. Using date palm fibres with a 20 mm length also reduced flowability when compared to the 10 mm fibre mix [124]. The V-funnel flow time was increased by 65.29% when using 30 mm red pine needle fibres, but escalated by 84.3% when the same fibres were 50 mm, indicating an increased viscosity when longer fibres were used [121]. The mix with 10 mm date palm fibres exhibited a higher V-funnel flow time of 20.6% when compared to the unreinforced SCC when incorporating the fibres at 2.1 kg/m³, and the V-funnel flow time significantly increased by approximately 34% and 56% when 20 mm and 30 mm fibres were used, respectively [149]. Only a few studies [122,149] investigated the effects of the lengths of plant fibres on the passing ability of SCC in terms of the L-box ratio and found that using various lengths of date palm fibres at 10 mm, 20 mm, and 30 mm led to a similar L-box ratio of SCC [149].

7.2. Effects of Fibre Length on the Mechanical Properties of PFRSCC

Various studies have been conducted using different lengths of plant fibres in SCC, while the comprehensive discussions on the effects of fibre lengths on the mechanical properties of PFRSCC are limited. The hypothesis is that plant fibres have an optimum length that maximises the bridging effect while minimising the porosity of SCC. An effective bridging effect cannot be formed when fibres are too short due to a weak fibre–cement matrix interface and low bond strength [49]. When fibres are too long, an additional porosity of SCC is formed due to the fibre agglomeration and fibre–aggregate interlocking effects. The incorporation of the optimum length and dosage of fibres is crucial to maximise the bridging effect and reduce the porosity of concrete. The optimum length varies according to fibre types, the properties of plant fibres, and concrete mix proportions, and should be further investigated to ensure the optimal performance of fibres in PFRSCC. Using 10 mm date palm fibres resulted in a higher compressive strength of SCC compared to mixtures with 20 mm fibres [124]. The compressive strength of SCC increased as high as 14.3% when using 30 mm red pine needle fibres, but decreased by 6.5% when the same fibres were 50 mm [121]. The variations of the 28-day compressive strength of the control mix and mixes with various lengths of date palm fibres are depicted in Figure 13 [149]. The mix with 10 mm date palm fibres exhibited a higher compressive strength than the 20 mm and 30 mm fibre mixes [149]. For the flexural strength, the red pine needle fibres with 30 mm length contributed more significantly to the flexural strength of SCC than longer fibres (40 mm and 50 mm) because the increase in fibre length causes the formation of voids in concrete with the fibre agglomeration effect [121].

7.3. Effects of Fibre Length on the Durability of PFRSCC

Only a few studies [75,121,124] were conducted to investigate the effects of fibre length on the durability of PRFSCC. The rate of sulphate attack of SCC was higher when red pine needle fibres were added, and the loss of compressive strength of SCC was more significant with longer fibres due to an increase in the porosity of SCC, where the mix with 30 mm fibres exhibited a smaller loss of strength than the mixes with 40 mm and 50 mm fibres [121]. Fibre length also played a significant role in the resistance of PFRSCC to freeze and thaw cycles, and the results showed that using 30 mm red pine needle fibres at up to 11.8 kg/m³ (0.5% of concrete volume) in SCC led to a compressive strength comparable to the unreinforced mix when subjected to 50 cycles of freeze and thaw, while using longer fibre lengths caused the formation of voids, which compounded the impact of freezing and thawing on SCC [121]. The decrease in the early-age drying shrinkage of date palm fibre-reinforced SCC depended on the lengths and dosages of fibres [124]. Incorporating the 20 mm fibres reduced the shrinkage more effectively than the 10 mm fibres when incorporating fibres at 1.1 kg/m³, while the 10 mm fibres performed better in shrinkage reduction than the 20 mm fibres when incorporating the fibres at 2.1 kg/m³, and the explanations for these results were not provided. The results indicated that the fibre length and dosage had a combined influence on the durability of PRSCC, and their synergetic effects should be investigated to determine the optimum lengths and dosages of plant fibres to be used in SCC.

8. Effects of Fibre Treatments on the Properties of PFRSCC

The types of plant fibres, fibre dosages, fibre lengths, and concrete mix proportions were identified as critical factors influencing the rheology, mechanical properties, and durability of PFRSCC. Another important factor affecting the properties of PFRSCC is the fibre treatment, which is applied to the fibres to remove impurities, increase surface roughness, and reduce the water absorption rate of plant fibres, thereby improving fibre–matrix bonding.

8.1. Effects of Fibre Treatments on the Rheological Properties of PFRSCC

Treatments of plant fibres have beneficial effects on the rheology of SCC. An alkaline treatment of plant fibres with a 5% sodium hydroxide (NaOH) solution for 1 h improved the workability of SCC in terms of the slump flow diameter, V-funnel flow time, and L-box ratio when compared to the untreated fibre mix [122]. The workability of SCC is enhanced due to the removal of highly hydrophilic hemicelluloses, waxes, and impurities during the alkaline treatment process [77]. Alkaline treatment with a NaOH solution reduces the hydroxyl groups of cellulose, hemicellulose, and lignin by forming water molecules, thereby decreasing the moisture absorption capacity of the fibres [108,129]. Coating the fibres by soaking them for 10 min in a polymer solution prepared by diluting the carboxylate SBR emulsion with distilled water also increased the slump flow diameter and decreased the V-funnel flow time compared to the untreated fibre mixes, but these values were still inferior to those of the unreinforced mix [122]. The polymer acts as a protective layer around the fibre surface and helps to decrease the water absorption capacity of the plant fibre, leading to more available free water to lubricate the mix and increase its flowability [132,133]. Heat treatments by boiling the fibres yielded a similar slump flow for SCC when compared to untreated fibres because the heat treatment does not affect the general composition of plant fibre, except for the slight reductions in hemicellulose, cellulose, and lignin contents in the fibre [123,136].

8.2. Effects of Fibre Treatments on the Mechanical Properties of PFRSCC

Fibre treatments play a significant role in improving the mechanical properties of PFRSCC. The alkaline treatment induces two modifications to the fibre surfaces, including (1) a rougher fibre surface, which improves the mechanical interlocking and enhances adhesion between the fibre and matrix, and (2) the removal of some hemicelluloses, lignin, waxes, and impurities from the fibre surface, facilitating a more chemically homogeneous surface and improving fibre compatibility with the matrix [205,206,207]. The enhanced adhesion and compatibility of plant fibres with the cement matrix improve the mechanical properties, especially the tensile and flexural strengths of SCC [56,208]. Treating banana fibres with a 5% NaOH solution for 24 h led to an improved split tensile strength of SCC when the fibre dosage was 0.6 kg/m³ [117]. The treatment of sisal fibres by submerging in a 5% NaOH solution for 1 h was also found to enhance the split tensile strength of PFRSCC when compared to the untreated fibre mix in a study by Nagarajan and Kannan [122]. Coating the coir fibres with silica fume and metakaolin was highly effective, contributing to a 70.8% improvement in compressive strength when 5.0 kg/m³ of coir fibres were added to the mix, compared to the unreinforced mix. Additionally, heat treatment by boiling the fibres improved the compressive strength of SCC by 43.9% [123]. The polymer coating and alkaline treatment of sisal fibres enhanced the compressive and flexural strengths of SCC when compared to the untreated fibre mix [80,122]. The treatment with fibre coating using silica fume and metakaolin significantly improved the flexural strength, with a 37.5% increase in 5.0 kg/m³ fibres compared to the unreinforced mix [123].

8.3. Effects of Fibre Treatments on the Durability of PFRSCC

The applications of plant fibres in concrete are constrained by their relatively low durability in the cement matrix [58,209]. The plant fibres deteriorate due to alkaline hydrolysis when incorporated into the highly alkaline environment of concrete, adversely affecting the durability of concrete [210]. While the effects of fibre treatments on the durability of PRSCC have not yet been studied, the effects of fibre treatments on the durability of conventional plant fibre-reinforced concrete (PFRC) have been widely explored. Heat treatment and alkaline treatment with Na₂CO₃ solution were found to enhance the durability of concrete, as observed with the improvement of compressive strength by 31.1% and 45.4% and split tensile strength by 36.5% and 46.2% when compared to the untreated fibre mix, respectively, after being subjected to 30 cycles of wetting and drying and then tested [58]. The heat treatment of sisal fibres enhances the crystallinity of cellulose, contributing to an improved initial strength and durability of plant fibres in concrete. For the alkaline treatment with a Na₂CO₃ solution, a protective layer was formed on the fibre surface, protecting the fibres from a highly alkaline concrete environment and enhancing their corrosion resistance and durability. The influences of heat treatment with hot water and alkaline treatment by submerging in a 5% NaOH solution for 2 h on the durability of PFRC were investigated by Tsegaye et al. [211]. The flexural strength tests conducted after 25 wetting and drying cycles demonstrated that untreated fibre composites experience significant reductions in ductility and strength, indicating the increased brittleness of PFRC and deterioration of the plant fibre [211]. The mixes with plant fibres modified by heat treatment and an alkaline solution demonstrated a superior durability in terms of flexural strength and toughness when subjected to accelerated ageing conditions compared to the untreated fibre mix. The concrete reinforced with treated fibres also continued to exhibit multiple cracking behaviours when subjected to accelerated ageing conditions, indicating the effectiveness of these treatments in improving the mechanical properties and durability of PFRC. Further studies should be conducted on the effects of various fibre treatments on the durability of PFRSCC, considering the current gap in this area.

8.4. Effects of Fibre Treatments on the Microstructure of PFRSCC

Surface treatments are commonly applied to plant fibres to enhance their interfacial adhesion with the cement matrix, as plant fibres exhibit a weak chemical bond with the cement matrix [207,212]. The bonding between plant fibres and the cement matrix is primarily governed by weak physical interactions between cellulose and CH [133]. The high water absorption capacity of plant fibres also leads to fibre swelling when incorporated into a fresh cement matrix, causing a weak fibre–matrix adhesion. The alkaline treatment of plant fibres by immersing them in a 0.5% NaOH solution for 24 h effectively removes non-cellulosic substances from the fibre surface, thereby reducing water absorption and enhancing surface roughness, which improves the fibre–cement matrix interface bonding [213]. The effects of alkaline treatment and ultrasonic vibration coating treatment on the microstructure of jute fibre-reinforced SCC were investigated by Zhang et al. [120]. Figure 14 illustrates the SEM images of SCC with (a) untreated fibres, (b) alkaline-treated fibres, and (c) ultrasonic vibration coating-treated fibres. For the alkaline treatment, jute fibres were submerged in a 1% NaOH solution for 20 min and air-dried at ambient temperature. The ultrasonic vibration coating treatment was conducted using an intelligent ultrasonic processor, and nano-sized silica sand was adopted as a wrapping agent to form a fibre coating with a 0.9% mass ratio of the jute fibre. The untreated jute fibre specimens showed noticeable gaps at the fibre–cement matrix interface because untreated jute fibres exhibited a smooth surface texture, which diminished the mechanical adhesion between the fibre and the cement matrix. An alkaline treatment of jute fibres led to a rougher surface when compared to the untreated fibres, which ultimately enhanced the mechanical bond between the fibre and the cement matrix [214,215], while the ultrasonic vibration coating treatment enabled cement hydration products to adhere to the fibre surface, resulting in an enhanced interface bonding structure [120].

9. Effects of SCMs on the Properties of PRSCC

The utilisation of plant fibres adversely affects the workability and long-term durability of SCC. Various SCMs, including silica fume (SF), fly ash (FA), metakaolin (MK), rice husk ash (RHA), slag (SG), and pumice powder (PM), have been used in PFRC and PFRSCC to counter the drawbacks associated with incorporating plant fibres. The chemical composition and physical properties of the investigated SCMs, as reported in the literature, are presented in

Table 10. Chemical composition and physical properties of SCMs used in PFRC and PFRSCC.

	Dávila-Pompermayer et al. [93]		Bingöl et al. [216]	Wei et al. [59]				Booya et al. [217]
SCMs	FA	SF	FA	MK	RHA	SF	FA	FA	SG	SF	MK	PM
Chemical composition (%)
SiO2	67.48	94.93	48.93	51.80	90.45	92.30	32.25	61.30	36.90	85.39	63.49	71.60
Al2O3	23.94	0.76	24.63	42.40	0.02	0.67	17.41	19.91	9.08	6.27	29.85	11.93
Fe2O3	4.63	0.05	7.59	4.15	0.02	0.84	6.85	6.90	0.61	0.19	1.19	1.23
CaO	1.76	0.41	9.06	0.07	0.51	4.89	27.31	1.33	37.6	0.03	0.35	0.99
MgO	1.16	0.36	2.28	-	0.24	0.53	5.32	1.74	10.91	0.01	0.49	0.33
Na2O	1.14	-	0.35	-	0.03	-	1.67	1.02	0.25	0.11	0.14	2.23
K2O	0.99	0.81	2.51	0.22	1.67	0.26	0.37	2.26	0.26	0.04	1.81	5.15
TiO2	0.94	-	-	1.07	0.01	-	1.52	0.90	0.36	0.07	0.68	0.09
P2O5	0.30	-	-	-	0.76	-	0.52	0.15	0.01	0.37	0.03	0.03
MnO	0.04	-	-	-	0.07	-	0.10	0.07	0.33	0.01	0.01	0.03
SO3	-	-	2.48	0.11	0.04	0.30	2.82	-	-	-	-	-
LOI	-	-	1.69	-	-	-	-	3.65	0.26	0.88	2.03	5.86
Physical properties
Density (g/cm3)	2.00	2.23	-	-	-	-	-	-	-	-	-	-
Specific surface area (m2/g)	-	-	-	2.93	-	-	1.46	-	-	-	-	-
Blaine fineness (m2/kg)	330	929	-	-	-	-	-	-	-	-	-	-
Strength activity index at 28 days (%)	71.4	-	-	-	-	-	-	-	-	-	-	-

.

9.1. Effects of SCMs on the Rheological Properties of PRSCC

Various studies have been conducted to investigate the effectiveness of using SCMs in conjunction with plant fibres to mitigate the decreased workability of PFRSCC. Using FA and MK as partial cement replacements in sisal fibre-reinforced SCC led to a similar slump flow when compared to the plain SCC without either SCMs or plant fibres [151], while using FA to replace 30% of cement improved the slump flow of lechuguilla fibre-reinforced SCC by approximately 18% [93]. The slump flow of SCC was found to increase when FA was used to replace cement up to 55% by weight in a study by Bingöl and Tohumcu [216]. The improvement in the workability of SCC is primarily attributed to the spherical particle shape of FA, which acts as a lubricant within the concrete mix, reducing the internal friction between aggregate particles and thereby improving the flow and compaction of concrete [218,219,220]. Replacing 15% of cement with RHA reduced the workability of SCC because RHA is highly reactive and absorbs a large amount of water due to its higher specific surface area, resulting in less free water for lubrication and decreasing the flowability of SCC [221,222,223]. Incorporating RHA in SCC also led to a marginal reduction in filling and passing abilities, as well as a substantial increase in the plastic viscosity and segregation resistance of the mix [224]. Adding 3% SF to SCC decreased the slump flow diameter by 4.4%, whereas adding up to 12% SF increased the viscosity of SCC due to the high pozzolanic reactivity of SF, which enhanced the rate of hydration and increased the water demand [225,226]. The SCMs with higher specific surface areas (RHA and SF) absorb more water, reducing the amount of free water available in the mix and decreasing the workability of concrete [227].

9.2. Effects of SCMs on the Mechanical Properties of PRSCC

The utilisation of suitable SCMs is beneficial to the mechanical properties of PRFSCC in three ways, which are (1) forming a denser matrix due to the pozzolanic reaction, (2) reducing the alkalinity of concrete, decreasing the degradation rate of plant fibres in concrete, and (3) improving the interfacial bonding of plant fibres and the cement matrix [59,60]. When pozzolanic materials are incorporated into the cement matrix, silica reacts with CH and forms a secondary C-S-H gel, refining larger pores into finer pores and resulting in denser concrete [228]. The incorporation of SCMs also reduced the mineralisation and alkaline hydrolysis of fibre cell walls due to the lowered alkalinity of the pore solution and the decreased CH levels in the mix, significantly mitigating the degradation of plant fibre and enhancing the durability of cement composites [59]. Partially substituting cement with SCMs also enhanced the interfacial bonding between plant fibres and the cement matrix, as evident in the lower porosity and reduced thickness of the interfacial transition zone (ITZ), and more cement hydration products adhering to the fibre surface. Using SF to replace 5% of cement improved the 28-day compressive strength of lechuguilla fibre-reinforced SCC by 5.1% in a study by Dávila-Pompermayer et al. [93]. The effects of five different SCMs (SF, FA, MK, PM, and SG) on the compressive strength of PFRC were investigated by Booya et al. [217]. The mix with a partial cement replacement by MK significantly improved the compressive strength of PFRC at 28-day, 56-day, and 91-day testing ages because the MK has a higher specific surface area than cement, which enhances the particle distribution within the matrix, and the fine particles of MK also decrease the pore sizes, reducing the permeability of concrete. Concrete mixes incorporating SF and SG demonstrated a modest increase in compressive strength when compared to the control PFRC at all testing ages, while PM and FA blend mixes exhibited a noticeable reduction in compressive strength at 28-day and 56-day ages [217]. The influence of FA, RHA, SF, and MK on the initial post-cracking of PFRC was studied by Wei et al. [59], and the FA, RHA, SF, and MK blend mixes exhibited an initial higher post-cracking strength with the increments of 14.56%, 32.97%, 34.73%, and 35.73%, respectively, when compared to the control PFRC after 28 days of curing.

9.3. Effects of SCMs on the Durability of PRSCC

Investigations into the effects of SCMs on the durability of PFRSCC are limited, whereas the impacts of SCMs on the durability of PFRC have been extensively studied. Cement reacts with water to form a C-S-H gel during the hydration process, which contributes to strength development and results in CH as a byproduct [229]. The silica in SCMs reacts in the pozzolanic reactions by consuming CH, reducing the availability of CH for harmful chemical reactions [230]. Incorporating SCMs also improves the microstructure of concrete by refining pore size, resulting in a lower porosity and reduced permeability, and the denser pore structure achieved through SCM incorporation slows the penetration of deleterious substances, decreasing the rate and extent of chemical attacks [217,231,232,233]. Incorporating various SCMs into sisal fibre-reinforced cement composites remarkably enhanced the durability, particularly when subjected to accelerated ageing conditions [59]. The mixes containing 30% MK and 10% SF as cement replacements showed decreases in flexural strength by 33.48% and 52.57%, respectively, while the control PFRC mix exhibited an 85.58% reduction after being subjected to 50 cycles of wetting and drying [59]. Incorporating SCMs, including SF, FA, MK, and SG, remarkably improved the resistance to chloride ion penetration of PFRC [217]. The SF and MK blend mixes were the most effective in reducing water sorptivity, with reductions of 15% and 16%, respectively, while the SG blend mix showed a similar value of sorptivity when compared to the control PFRC. The FA and PM blend mixes exhibited a significantly higher sorptivity when compared to the control PFRC, indicating a higher porosity of the mix [217]. Using MK as an SCM significantly reduced the water absorption of concrete by 32% at 28 days, attributed to the improved ITZ and secondary C-S-H formation, while the mixes containing SF, FA, and SG showed similar water absorption values compared to the control PFRC. These findings highlight the superior effectiveness of MK in enhancing PFRC durability through pore refinement and reduced permeability. A study on the effects of FA and SF as an SCM on the drying shrinkage of PFRSCC revealed that the 30% FA blend mix and the control mix exhibited an almost identical drying shrinkage, while the 5% SF blend mix demonstrated a significantly lower drying shrinkage when compared to the control and FA blend mixes [93].

9.4. Effects of SCMs on the Microstructure of PRSCC

Studies examining the impact of various SCMs on the microstructure of PFRSCC are currently limited, whereas the effects of SCMs on the microstructure of PFRC have been comprehensively studied. The SEM analysis of PFRC specimens revealed that the mixes with SF, MK, and SG blends exhibited dense matrices and strong fibre–matrix bonds, as evidenced by fibre tip fractures, instead of the fibre debonding or pulling out, and minimal interfacial gaps, indicating an effective stress transfer from the matrix to the fibres and improved durability of the matrix [217]. The specimens with FA and PM showed brittle failure and fibre debonding, suggesting a limited durability enhancement. An SEM analysis of sisal fibre-reinforced cement composites showed the severe degradation of fibres after 30 wetting and drying cycles, mainly due to the decomposition of lignin, hemicellulose, and pectin, leading to the infiltration of cement hydration products into the fibre structure [234]. Incorporating RHA in concrete improved the durability of fibres by reducing alkalinity through the consumption of CH and enhancing alkali binding via C-S-H formation. When subjected to accelerated ageing conditions, incorporating 30% RHA as a cement replacement effectively preserved the fibre integrity, minimised cellulose fibril peeling, and maintained the crack-bridging ability. The effects of various SCMs, including MK, RHA, SF, and FA, on the microstructure of sisal fibre-reinforced mortar were investigated in a study by Wei et al. [59]. The SEM analysis revealed a severe degradation of sisal fibres in the control cement mortar due to alkaline hydrolysis after 30 wetting and drying cycles, resulting in cellulose fibril separation and mineralisation within the cell walls, which led to an increased fibre brittleness and reduced reinforcement capacity [59]. The 30% MK blend mix showed minimal fibre degradation and tighter fibre–matrix bonding, attributed to the reduced alkalinity and CH content. The 20% RHA blend mix showed a moderate fibre deterioration, while the 10% SF blend mix exhibited minor fibril stripping. The 30% FA blend mix exhibited the most fibre deterioration among the SCMs blend mixes, though still less severe than in the control mix.

10. Life-Cycle Assessment (LCA) of Plant Fibre-Reinforced Concrete

LCA is a widely recognised technique used to study the environmental implications and potential impacts of materials throughout their entire life cycle, from production to disposal [235]. The LCA of polypropylene and sisal fibre-reinforced concrete was comprehensively investigated by Acosta-Calderon et al. (2022) [236]. The polypropylene and sisal materials underwent industrial processing to obtain the fibres, resulting in additional environmental impacts than the unreinforced concrete [236]. The LCA results for 1 m³ of the unreinforced mix, polypropylene fibre-reinforced mix, and sisal fibre-reinforced mix are depicted in

Table 11. LCA results for 1 m³ of concrete mix [236].

Mixes	GWP [kg CO2]	ODP [kg CFC11]	AP [kg SO2]	FE [kg P]	WC [m3]
Unreinforced mix	313.7175	6.2375 × 105	0.5566	0.05118	2.4794
Polypropylene fibre mix	315.0827	6.2712 × 105	0.5609	0.05117	2.4801
Sisal fibre mix	313.7363	6.2444 × 105	0.5571	0.05119	2.4793

. The results illustrate slight variations between the unreinforced mix and the fibre-reinforced mixes in all impact categories, including global warming potential (GWP), ozone depletion potential (ODP), acidification potential (AP), freshwater eutrophication (FE), and water consumption (WC) [236]. The sisal fibre-reinforced mix showed a lower impact in all indicators compared to the polypropylene fibre-reinforced mix, suggesting the potential benefits of using sisal fibres to replace polypropylene and improve the sustainability of concrete. A similar result was observed in a study on the environmental impacts of using kenaf and glass fibres as reinforcements in concrete by Zhou et al. (2018) [235]. Using kenaf fibres to replace glass fibres as a reinforcement in cement wall panels significantly reduced the environmental impact in all impact categories, particularly regarding natural resource depletion, human health, and habitat alteration [235].

11. Contributions of PFRSCC to the Construction Industry and Challenges

Using various plant fibres in SCC substantially contributes to the construction industry in promoting sustainability by providing a more sustainable and cost-effective alternative to synthetic fibres. This review enhances the understanding of critical parameters for incorporating plant fibres into SCC, particularly the fibre types, fibre dosages, fibre lengths, fibre treatments, and the utilisation of various SCMs in PFRSCC. Incorporating plant fibres promotes waste valorisation and reduces the dependence on synthetic fibres, which are energy-intensive and have a high carbon footprint. Using plant fibres also offers cost advantages, making the PFRSCC a promising material for sustainable construction.

The major barriers that must be addressed to optimise the performance of plant fibres in SCC are also identified in this study, including the high water absorption rate of plant fibres, the fibre agglomeration effect, and the fibre–matrix compatibility. The plant fibres absorb a significant amount of water due to their hydrophilic nature, which decreases the amount of water available for the mix and reduces the workability of the concrete. The fibres absorb moisture in the amorphous region of cellulose, hemicellulose, and lignin constituents due to the presence of hydroxyl groups [108]. The reduction in workability is critical when plant fibres are used in SCC, as this special type of concrete requires a high flowability to fill the formwork and pass through obstacles at its own weight. A simple solution to this decreased workability is to account for the water absorption of plant fibres in the mix design so that the fibres do not absorb the water required for the SCC workability. An extra dosage of superplasticiser can also be added to the mix to improve the workability of SCC, which will add additional costs and affect the performance of SCC. Alkaline treatments can also reduce the hydrophilic nature of fibres by removing the hydroxyl groups through a reaction with alkaline solutions [129]. The second obstacle is the agglomeration of plant fibres within the SCC. Fibre agglomeration occurs when fibres are attached at a specific location, resulting in an uneven distribution and reduced workability of the mix. The agglomeration of fibres often occurs when the fibre lengths are long, and short plant fibres can be used in concrete to overcome this issue, especially when high workability is vital in the case of SCC. The effects of fibre agglomeration can also be minimised by using a low dosage of fibres in SCC. Finally, the third issue is the weak fibre–cement matrix interface. The hydrophilic nature of plant fibres leads to a weak connection between the fibre and the cement matrix [105,106,107,108]. The durability issues of plant fibres in the cement matrix are also linked to the weakening effects of alkali-induced fibre degradation and fibre mineralisation, caused by the migration of CH into the fibre structure [109]. Various treatments, including alkaline treatment, heat treatment, and polymer coating, can reduce the water absorption rate and improve the surface roughness of plant fibres, enhancing the compatibility of the fibre–matrix interface.

12. Limitations of the Study

A key limitation in this systematic review on using plant fibres as a reinforcement material in SCC is that most studies were conducted in controlled laboratory settings, while giving little attention to how the PFRSCC performs on actual construction sites, resulting in minimal evidence from field trials or real-world construction projects. This limitation restricts our understanding of how the PFRSCC behaves over time when subjected to environmental conditions in a real construction site. Another limitation of this study is the criterion for English-only articles in the selection process, which may exclude some studies on the PFRSCC published in languages other than English.

13. Conclusions

13.1. Main Findings and Implications

This study was conducted to review the applications and effects of plant fibres as a reinforcement in SCC. The key conclusions are as follows:

• The effectiveness of plant fibres in SCC depends on the fibre type, fibre dosage, fibre length, fibre treatment, concrete mix proportion, and the incorporation of SCMs.
• Three main issues that hinder the extensive use of plant fibres in SCC are the hydrophilic nature of plant fibres, the fibre agglomeration effect, and the weak connection between plant fibres and the cement matrix.
• Using various plant fibres in SCC reduces the flowability, filling ability, and passing ability of SCC because of the high water absorption by plant fibres, the interlocking and friction between fibres and aggregates, and the fibre agglomeration effect. Adding plant fibres to SCC increases the viscosity and improves the segregation resistance of SCC due to the strong cohesion between plant fibres and the cement matrix. The reported decreases in slump flow vary from a slight reduction up to approximately 90%. The V-funnel flow time was reported to increase to approximately 145% when incorporating various plant fibres. The effect of plant fibres is not as significant in the L-box ratio when compared to the V-funnel flow time.
• The inclusion of plant fibres usually improves the mechanical properties of SCC because of the combined effects of fibres on crack-bridging and strain redistribution across the cross-section of SCC. The studies reported in the literature demonstrate a significant improvement in the split tensile strength, up to 65.6%, when plant fibres are incorporated into SCC. The effect of plant fibres on the flexural strength of SCC varies from a minor variation to as high as a 108% improvement.
• Embedding various plant fibres in SCC reduces the drying shrinkage and cracks because of the fibre bridging effect, while lowering the resistance to sulphate attack, acid attack, and freeze and thaw cycles, as well as increasing the water absorption rate of SCC due to the increased porosity of the mix.
• Using longer plant fibres significantly reduces the workability of SCC when compared to the shorter fibres due to greater fibre agglomeration and fibre–aggregate interlocking effects. For the mechanical properties, using the optimum fibre length maximises the crack-bridging effect while minimising the porosity of SCC. Short fibres cannot form effective crack-bridging due to weak bonding, whereas using excessively long fibres causes fibre agglomeration and increases porosity, reducing the long-term durability of concrete when compared to using shorter fibres.
• Alkaline treatments and polymer coatings of plant fibres improve the workability of SCC, while heat treatments have minimal effects on the rheology of SCC. Fibre treatments roughen the fibre surface and remove surface impurities, thereby improving mechanical interlocking and chemical compatibility, and significantly enhancing the mechanical properties of PFRSCC. Heat and alkaline treatments significantly improve the durability of PFRC when subjected to accelerated ageing conditions. Microstructure investigations reveal that alkaline treatments and fibre coatings demonstrate an improved surface roughness and adherence of cement hydration products, leading to stronger mechanical bonding.
• Using suitable SCMs alongside plant fibres helps counteract the reduced workability of PFRSCC. Using appropriate SCMs significantly improves the mechanical properties of PFRSCC by densifying the matrix through pozzolanic reactions, reducing the alkalinity of concrete to slow fibre degradation and enhancing fibre–matrix interfacial bonding. Using suitable SCMs improves the durability of concrete by consuming CH through pozzolanic reactions, refining pore structures, and reducing permeability. The SEM images show that blend mixes with SCMs exhibit dense matrices and strong fibre–matrix bonds, enhancing the durability and reducing the permeability of the mix.

13.2. Research Gaps and Future Perspectives

The research gaps and future perspectives in utilising plant fibres in SCC are provided in

Table 12. Research gaps and future perspectives on the topic of utilising plant fibres in SCC.

Research Area	Research Gaps and Future Perspectives
Feasibility of using various plant fibres in SCC	(a) Many potential plant fibres have not been studied, and these fibres should be investigated for their feasibility in SCC. Natural plant fibres are widely available worldwide, especially in developing countries. Exploring the feasibility of the unexplored plant fibres can contribute to diversifying economic and sustainable materials for SCC applications.
Rheology of PFRSCC	(a) The rheological parameters of PFRSCC, particularly yield stress and plastic viscosity, have not been studied. A detailed investigation of these parameters is necessary to understand the workability and flow characteristics of PFRSCC, which are crucial in evaluating the self-compacting characteristics of SCC.
Durability of PFRSCC	(a) The plastic shrinkage and cracking of PFRSCC have not been comprehensively studied. Synthetic and plant fibres are generally used in concrete to control the cracks caused by plastic shrinkage. A comparison of the efficacy of plant and synthetic fibres should be conducted to investigate the feasibility of using plant fibres in reducing the shrinkage and cracking of SCC. (b) The durability and long-term performance of PFRSCC have not been comprehensively studied. Plant fibres are vulnerable to degradation in the alkaline environment within SCC. The durability properties of PFRSCC should be examined to ensure the long-term performance and safety of the materials.
Optimisation of PFRSCC	(a) The effects of fibre length and fibre treatment on several properties of SCC have not been investigated. The findings in this review indicate that the length and treatment of fibres substantially affect the performance of SCC. These aspects should be thoroughly examined to determine the optimal fibre lengths and treatments of various plant fibres for SCC applications.
Sustainability of PFRSCC	(a) The effect of various SCMs on the performance of PFRSCC has not been comprehensively studied. Numerous studies have demonstrated the benefits of utilising suitable SCMs on the rheology, mechanical properties, durability, and microstructure of concrete. Given the abundance of SCMs and their sustainability and cost reduction, the impact of various SCMs on the properties of PFRSCC should be comprehensively investigated.
Life-cycle assessment (LCA) of PFRSCC	(a) The LCA of PFRSCC has not been studied. An LCA should be conducted to understand the total environmental impact, sustainability, and economic viability over the life span of PFRSCC. The LCA is a critical parameter to be considered when using plant fibres to replace synthetic fibres in SCC.

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