Performance Evaluation of Open-Graded Bituminous Concrete Modified with Natural Fibers

Abstract

Open-graded bituminous concrete (OGBC), also known as open-graded friction course or permeable asphalt layer, is a skid-resistant surface applied to pavements with high bitumen content. This mixture contains more coarse aggregates than fine aggregates, which improves subsurface drainage and indirectly reduces hydroplaning potential during wet weather conditions. The objective of the present study was to enhance the properties of the OGBC mix with fibers. Hence, four distinct natural biofibers, namely, sisal fiber, jute fiber, coir fiber, and bamboo fiber, were considered during experimental investigation at different dosages like 0.15%, 0.3% & 0.45% by weight of mix. Binder content levels ranged from 5 to 6% with an increment of 0.25% between the values in the range. Fiber-reinforced OGBC mixes were tested for air voids (%), draindown, resistance to moisture susceptibility, Cantabro loss, and indirect tensile strength of the compacted mixtures. The experimental findings demonstrate that fibers enhance the performance of OGBC mixes. Fiber incorporation reduced binder draindown and the percentage of air voids in OGBC mixes while maintaining the desired characteristics. The optimal fiber content was determined to be 0.30% for sisal, bamboo, and coir fibers and 0.45% for jute fibers. With the addition of sisal fibers at a dosage rate of 0.30%, the tensile strength of the OGBC mixture increased along with resistance to susceptibility to moisture.

1. Introduction

Around 98% of Indian roads are constructed as flexible pavements using dense graded bituminous mixes as wearing course [1]. The term “dense graded” refers to less scope for air voids in the range of 3–5% of the mix, which causes inconvenience for drivers traveling during the rainy season due to splash and spray from wet roads, lack of friction and also reduced vision, all of which have an impact on drivers’ safety. It is estimated that more than 12.8% of road accidents occur due to wet road conditions [2]. Wheel loads exert hydraulic pressure on the flexible pavement as a result of the waterlogging brought on by heavy rainfall. This pressure weakens the link between the aggregate and the bitumen and strips the bitumen from the flexible pavement [3]. Therefore, in regions in which intense rainfall occurs, it is essential to introduce a highly permeable wearing course (such as “open graded” mixes) [4]. Increased surface permeability is the key benefit of an Open-Graded Bituminous Concrete (OGBC) wearing course, especially in wet conditions. Other benefits include pavement–tire noise reduction and greater surface friction [5]. Generally, the thickness of the OGBC layer ranges from 20 to 40 mm, according to Indian standards [6]. Sun rays, air, and water can effortlessly penetrate the OGBC pavement course due to its unique arrangement of material layers. This accelerates the aging process of the binder and also affects the durability of the pavement [7]. OGBC does not drain water to the base course of the pavement, but allows water to drain from the sides of the pavement. OGBC courses are prone to failure due to temperature fluctuations. The most common failure modes as a result of temperature changes are fatigue cracking and rutting deformation [8]. An open-graded bituminous mixture behaves like a dense-graded mixture once all the voids are clogged and continues to provide service until delamination or ravelling necessitates its rehabilitation [9].

To enhance the engineering properties of the bituminous mixes, additives and modifiers such as textiles, fibers, ionic surfactants, zeolites, and polymers are generally used [10]. Recently, sustainable additives including biochar [11], nanoparticles [12], and bio-oil [13] have also been used. Bituminous concrete mixture with fiber additives are known to enhance the mechanical qualities and durability of the pavement surface [4,14]. Fiber additives act not only as an asphalt stabilizer, but also as a reinforcing agent between aggregates. Permanent deformation/defects can be prevented with the use of fiber additives, and relative draindown can also be controlled even at a high binder content in the asphalt mix [15]. Several researchers have explored the benefits of fiber additives in asphalt mixtures. Previously, Cooley et al. (2000) [16] conducted field and laboratory studies to evaluate the suitability of cellulose fibers versus mineral fibers in open-graded friction courses (OGFC). In terms of rut resistance, it was observed that the performance of the cellulose fiber mix was relatively analogous to the mineral fiber mix. Later, research by Hassan et al. (2005) [17] investigated OGFC mixes that incorporate cellulose fibers and styrene butadiene rubber (SBR) polymer. Compared to cellulose fibers, it was observed that the OGFC mixture with SBR polymer had a significantly higher resistance to ravelling. A follow-up study was conducted by Hassan and Al-Jabri (2005) [18] to evaluate the effect of two diverse organic fibers (date-palm and textile fibers), in combination with SBR polymer (binder additive), on the characteristics of OGFC mixes. It was found that the incorporation of date-palm fibers and SBR polymer into the OGFC mix significantly reduced abrasion and perhaps improved the ravelling resistance of the OGFC. Likewise, the reclaimed polyethylene (PE) fibers were considered as an additive in OGFC mixtures by Punith and Veeraragavan (2011) [19]. The PE fibers in the OGFC mix successfully retarded the draindown of the binder and mineral fillers and showed improved resistance to permanent deformation, moisture susceptibility, and fatigue-induced damage. Many recent studies [20,21] have shown that glass and polypropylene fibers enhance the indirect tensile strength of porous asphalt mixtures. To minimize binder draindown and enhance the ductility and crack resistance of porous asphalt mixtures, Zhang et al. (2021) [22] produced and employed unique innovative fibers made from scraps of cured carbon fiber composite. Collectively, a majority of studies outline that the fiber additives bring ductility to the porous asphalt mixture, thus improving its toughness, resistance to rutting and permanent deformation while retaining its functionality in other areas [4,23]. Several researchers employed a variety of fibers, such as cellulose fibers [24], polyester fibers [25], polypropylene fibers [20], mineral fibers [26], glass fibers [27], nylon fibers [28], basalt fibers [29], aramid fibers [21], lignin fibers [30], and fiber-shaped carbon composites [22] in the production of open graded bituminous concrete. However, the addition of fibers to bituminous concrete needs to be carried out with caution, as excessive fiber content promotes agglomeration with bitumen during mixing and leads to inferior performance [29]. Therefore, fiber properties (such as length, diameter, surface texture, tenacity, flexibility, uniformity, and aspect ratio) and dosage rate play a significant role in its selection as an additive for bituminous concrete. Natural fibers vary in terms of their structure, chemical composition, microfibrillar angle, cell size, stiffness, and fiber defects [31]. These specific properties of natural fibers modify the characteristics of OGBC mixtures.

Motivation and Objectives

While there has been a plethora of research on natural (ligno-cellulosic) fiber-reinforced bituminous concrete mixes over the past three decades, there have been few attempts to compare and contrast the behaviour of Open-Graded Bituminous Concrete (OGBC) mixtures incorporating different natural fibers. In this study, an effort has been made to evaluate OGBC mixes incorporating different natural fibers, namely coir, jute, bamboo and sisal fibers at different dosage rates (0.15%, 0.3% and 0.45% by weight of the mix). To the best of the author’s knowledge, this study is unique from the perspective of the four fibers (derived from agricultural waste) taken into account. The fiber modified Open-Graded Bituminous Concrete mixes were tested for air voids, draindown, Cantabro loss, indirect tensile strength and resistance to moisture susceptibility of compacted mixtures.

2. Methodology and Experimental Programme

2.1. Design of OGBC Mixtures

In the present study, Open-Graded Bituminous Concrete (OGBC) mixes were designed as per the recent IRC:129-2019 [6] guidelines. The basic steps in the OGBC mix design procedure include the selection of aggregates, gradation design, determination of optimum binder content and evaluation of mix properties.

2.1.1. Aggregates

For the experimental study, crushed granite aggregates procured from a nearby quarry were used. The basic properties of the virgin aggregates were tested in accordance with the relevant standards mentioned in IRC:129-2019 [6] and the results are as presented. The aggregate impact value was 14%, the Los Angeles abrasion value obtained was 24.4%, the combined flakiness and elongation index was 26.94% and the water absorption and specific gravity were 0.29% and 2.62, respectively, all of which were well within the specified ranges. Mid-point gradation was considered for the OGBC mixtures in the current study. Figure 1 presents the combined coarse and fine aggregate grading adopted in accordance with the requirement of IRC:129-2019 [6]. OGBC mixes were designed for a nominal layer thickness of 25 mm, with a nominal maximum aggregate size (NMAS) of 9.5 mm.

2.1.2. Binder

VG-30 grade bitumen has been used as a binder in this investigation. The results of the various tests on bitumen quality are reported and all were well within the specified ranges as mentioned in IS:73-2013 [32]. The specific gravity at 25 °C was 0.98, the ductility at 25 °C was 44 cm, the penetration at 25 °C was 56.5 (tenths of mm), the softening point was 48 °C and the absolute viscosity at 60 °C, was 2800 poise. Ordinary Portland Cement of 53 grade conforming to IS:12269-2013 [33] was used as filler material. Binder content levels varied between 5 and 6% with an increment of 0.25% between values in the range. However, the filler content was maintained at a consistent rate of 2% of the aggregate weight.

2.1.3. Fibers

For the experimental study, four readily accessible natural fibers—coir, jute, bamboo, and sisal—were taken into consideration. Coir fibers were harvested straight from the mesocarp of coconuts (Cocos nucifera), jute fibers from the bark of jute plants (Corchorus capsularis), bamboo fibers from the long stem of bamboo grass plants (Bambusa vulgaris) and sisal fibers from the leaves of sisal plants (Agave sisalana). All of these fibers are exceptionally tough and tension resistant due to the presence of lignin and pectin in addition to their cellulose content. The physico-mechanical properties of fibers employed in the study are as presented in

Table 1. The physical and mechanical properties of fibers.

Property	Coir	Jute	Bamboo	Sisal
Diameter (microns)	100–400	14–180	10–30	120–140
Density (g/cc)	1.1–1.3	1.4–1.6	0.54–0.80	1.2–1.5
Tenacity (g/tex)	09–12	22–34	20–60	45–58
Breaking Elongation (%)	30	1–3	20–25	3–6
Tensile strength (MPa)	100–500	120–600	600–850	400–700
Young’s Modulus (GPa)	2–8	15–25	35–45	20–40

. The maximum fiber length was limited to 8 mm and each type of fiber additive was varied at 0.15%, 0.30% and 0.45% by weight of mix to determine the optimal fiber content in the mix. Figure 2 shows the fibers used in the experimental study. As per IRC:129-2019 [6], the length of fiber additive (in the form of pellets) should not be more than 8 mm, as this may lead to a balling phenomenon in which all fibers accumulate in one place, forming lumps, and may not blend well with the asphalt. The cellulose content of coir, jute, bamboo, and sisal fibers varies from 35 to 45%, from 58 to 65%, from 30 to 35%, and from 55 to 60%, respectively. Despite containing cellulose, these fibers are naturally hydrophobic and moisture resistant [34].

2.1.4. Experimental Programme

The Marshall method was adopted to design OGBC mixtures. Marshall samples with five different bitumen contents (5%, 5.25%, 5.5%, 5.75% and 6%) were cast by compacting the OGBC mixture with 25 blows on each face of the specimen. Natural fibers were incorporated into the OGBC mixes at dosage rates of 0.15%, 0.3% and 0.45% of total mixture weight. The aggregates were preheated to 180 °C, and the bitumen temperature was maintained between 140 and 165 °C before the preparation of mix. Before the bitumen was added, the aggregates, filler and fibers were thoroughly dry-mixed. A homogeneous mixture of the aggregates, fibers, filler, and binder was achieved at a temperature of 140 to 160 °C. After that, the heated OGBC mixture was poured into previously heated moulds. The test specimen along with the mould was allowed to cool at room temperature for 24 h and then demolded using an extractor. To meet the optimum design criteria, numerous permutations and combinations were created. To ensure stone-on-stone contact, the volumetric properties of the OGBC mixes were evaluated, namely the percentage of air voids, voids in coarse aggregate fraction (VCA_MIX) and the voids in coarse aggregate in dry-rodded condition (VCA_DRC). Likewise, voids in mineral aggregates (VMA) were determined in the compacted mix as per IRC:129-2019 [6] guidelines. The mix that exceeded the minimum VMA requirement of 25% and (VCA_MIX) < (VCA_DRC) criteria was selected as the desired mix design. Following the procedures mentioned in IRC:129-2019 [6], first, the Schellenberg Binder Drainage test was carried out to determine the amount of draindown from the uncompacted OGBC mix specimens. Next, the abrasion loss of five representative unaged and aged OGBC specimens was determined using the Los Angeles abrasion machine. The optimum binder content (%) was determined based on the test results of air voids (%), draindown (%) and Cantabro loss (aged and unaged specimens) (%). Through experiments, we determined the indirect tensile strength (ITS) of OGBC specimens (produced at optimum binder content) in accordance with IRC:129-2019 [6], to characterize the stiffness of mixes. Later, the ITS of the conditioned and unconditioned specimens was used to determine the tensile strength ratio (TSR) and thereby evaluate the moisture susceptibility of the OGBC mixes with and without fibers.

3. Results and Discussion

3.1. Air-Voids

Stone-to-stone contact was examined in the OGBC mixes of the design gradation that contained various percentages of binder material and included different fibers. The

Table 2. Test results of voids in coarse aggregate.

OGBC Mix	Fiber Content	VCADRC (%)	VCAMIX (%)
			Binder Content
			5.0%	5.25%	5.50%	5.75%	6.0%
Without Fibers		55.9	53.50	52.83	52.58	52.01	51.91
Coir Fiber	0.15%	55.9	52.89	52.83	52.57	52.01	51.78
	0.30%	55.9	52.36	52.34	51.78	51.80	51.46
	0.45%	55.9	52.17	51.99	51.76	51.51	51.52
Jute Fiber	0.15%	55.9	52.80	52.51	52.42	51.81	51.81
	0.30%	55.9	52.45	52.33	51.84	51.64	51.68
	0.45%	55.9	52.27	52.08	51.41	51.54	51.09
Bamboo Fiber	0.15%	55.9	53.31	52.76	52.22	52.23	51.39
	0.30%	55.9	52.94	52.56	52.12	51.75	51.78
	0.45%	55.9	52.40	52.19	51.95	51.26	51.34
Sisal Fiber	0.15%	55.9	52.85	52.79	51.95	52.01	51.40
	0.30%	55.9	52.31	52.31	51.77	51.66	51.73
	0.45%	55.9	52.20	51.87	51.55	51.27	51.29

presents the voids in coarse aggregate under dry-rodded condition (VCA_DRC) and the voids in compacted mix (VCA_MIX). It could be observed that (VCA_DRC) was higher than (VCA_MIX) for all OGBC mixes at all five binder percentages. So, even at the highest trial binder percentage (6%) for each fiber type, stone to stone contact could be maintained in the mixture. Additionally, it was determined that all of the OGBC mixes had enough voids in mineral aggregate (VMA) (

Table 3. Test results of voids in mineral aggregate.

OGBC Mix	Fiber Content	VMA %
		Binder Content
		5.0%	5.25%	5.5%	5.75%	6.0%
Without Fibers		33.37	32.42	32.06	31.23	31.10
Coir Fiber	0.15%	32.50	32.41	32.04	31.24	30.91
	0.30%	31.75	31.72	30.91	30.94	30.45
	0.45%	31.47	31.22	30.88	30.53	30.54
Jute Fiber	0.15%	32.37	31.96	31.82	30.95	30.96
	0.30%	31.87	31.70	30.99	30.71	30.77
	0.45%	31.62	31.34	30.37	30.57	29.92
Bamboo Fiber	0.15%	33.11	32.31	31.53	31.56	30.36
	0.30%	32.57	32.02	31.39	30.09	30.90
	0.45%	31.80	31.49	31.15	30.17	30.28
Sisal Fiber	0.15%	32.44	32.36	31.15	31.23	30.37
	0.30%	31.60	31.66	30.90	30.74	30.84
	0.45%	31.51	31.04	30.58	30.18	30.20

) to meet the IRC:129-2019 [6] criterion of greater than 25%. Figure 3, demonstrates how the increase in the binder content resulted in the decrease in air voids in the OGBC mixtures. The air voids in OGBC mixtures without any fibers ranged from 24.31% to 19.95%. The increase in binder content levels resulted in a decrease in the volume of air voids, which was within the acceptable limits. Likewise, the OGBC with fibers exhibited a relatively lower air void percentage compared to that without fibers. The OGBC mixes with 0.45 percent fiber content had the lowest amount of air voids in comparison to the other two (0.15% and 0.30%) binder content levels.

3.2. Draindown

Different fibers possess varying abilities to minimize the binder draindown. The figure shows the draindown effect of uncompacted OGBC mixes with and without fibers at different trial binder percentages. The OGBC mixtures without any fibers exhibited higher draindown of binder in the range 0.22–0.67%, whereas the mixtures with fibers showed lower binder draindown. As the fraction of fiber content increases, draindown steadily declines. However, the fiber–matrix interface and morphology of fibers to hold the binder within the aggregates determine the optimal fiber content level. The draindown properties of the OGBC mixes were significantly altered by coir and sisal fibers at 0.45% dosage level. From Figure 4, it is evident that the draindown in OGBC mixtures without any fibers exceeds the maximum limit of 0.3% at binder content levels of 5.5% and above, but then with the addition of fibers, the OGBC mixtures stabilize and have a tendency to stiffen or bulk up the asphalt, thereby preventing the binder leakage from the aggregates. The inclusion of coir and sisal fibers at a dosage level of 0.45% contributes to minimize draindown even at higher binder content levels (5.75% and 6.0%). The mixes without any fibers at higher binder percentages fail due to excessive draindown.

3.3. Cantabro Loss

To check for resistance to specimen disintegration and ravelling, the Cantabro abrasion test was performed on both aged and unaged specimens. With an increase in binder content, the abrasion loss often reduces. The Figure 5 shows the abrasion losses of the tested OGBC specimens (both unaged and aged) with and without fibers, for the different trial binder percentages and fiber dose levels. Although improvements vary with each type of fiber, adding fibers generally increases ravelling resistance; all OGBC mixes with fibers showed reduced Cantabro loss. At 5.5% binder content, the OGBC mixes with sisal and coir fibers showed better ravelling and abrasion resistance at 0.3% fiber dosage, whilst the mixes with bamboo and jute fibers performed well at 0.15% fiber dosage. Both aged and unaged OGBC specimens with binder contents of 5.0% and 5.25% failed to provide satisfactory abrasion resistance.

3.4. Determination of Optimum Binder Content (OBC)

The optimum binder content was arrived at based on the test results of the properties of the OGBC mixes, specifically the air voids (%), draindown (%) and Cantabro loss (%). The criteria used were (a) the air voids should be in the range between 18 and 22% (b) the draindown should be no more than 0.3%, and (c) for unaged and aged OGBC specimens, the abrasion loss should not be higher than 20% and 30%, respectively. If one were to take all three criteria into account and look at Figure 6, it is difficult to determine the optimum binder content for the OGBC mix (without any fibers). At 5.33% binder content, the maximum draindown limit of 0.3% was attained, whilst the limits of air voids (%) and abrasion loss (from both aged and unaged specimens) (%) were breached. Likewise, with the exception of the draindown criteria, the requirements for abrasion loss and air voids are met at 5.74% binder content level. As a result, fibers must be added to prevent the draindown of binder content. The graphs plotted (Figure 3, Figure 4 and Figure 5), clearly portray that the OGBC mixes with fibers satisfied all of the aforementioned requirements. The optimum binder content (%) determined for a variety of OGBC mixes incorporating different natural fibers, namely coir, jute, bamboo and sisal fibers at different dosage rates (0.15%, 0.3% & 0.45% by weight of the mix) is outlined in

Table 4. Determination of Optimum Binder Content (OBC) for OGBC with fibers based on test results of trial mixes.

Fiber	Fiber	Binder Content	Air Voids	Draindown	Abrasion Loss (%)		OBC (%)
	(%)	(%)	(%)	(%)	Unaged	Aged
Sisal	0.15	5	23.25	0.16	29.60	38.8	5.5
		5.25	22.72	0.24	25.00	28.6
		5.5	20.90	0.30	18.10	24.9
		5.75	20.55	0.40	17.50	22.6
		6	19.09	0.51	15.40	20.3
	0.3	5	22.38	0.11	28.9	36.8	5.6
		5.25	21.93	0.18	26	27.2
		5.5	20.60	0.27	17	23.6
		5.75	19.98	0.33	11.3	21.8
		6	19.65	0.47	7.7	18.3
	0.45	5	22.19	0.08	30.5	39.2	5.7
		5.25	21.21	0.14	26.3	30.6
		5.5	20.24	0.23	20.3	26.4
		5.75	19.33	0.31	12.5	23.8
		6	18.91	0.41	9.5	21.1
Jute	0.15	5	23.17	0.18	25.07	36.15	5.45
		5.25	22.26	0.26	20.85	31.97
		5.5	21.67	0.32	11.97	30.76
		5.75	20.21	0.44	10.32	23.03
		6	19.78	0.55	7.58	22.07
	0.3	5	22.6	0.12	27.91	39.17	5.6
		5.25	21.97	0.2	22.23	36.23
		5.5	20.71	0.28	20.12	32.24
		5.75	19.94	0.33	11.51	29.24
		6	19.57	0.53	9.11	23.47
	0.45	5	22.32	0.11	29.17	41.65	5.65
		5.25	21.56	0.18	26.06	33.9
		5.5	20	0.25	20.8	30.02
		5.75	19.78	0.33	16.14	23.02
		6	18.58	0.44	10.69	20.36
Bamboo	0.15	5	24.01	0.19	25.1	34.45	5.35
		5.25	22.67	0.28	21.95	31.83
		5.5	21.34	0.35	19.58	29.03
		5.75	20.92	0.48	9.64	24.43
		6	19.08	0.56	7.7	22.28
	0.3	5	23.4	0.15	26.61	37.2	5.60
		5.25	22.34	0.19	23.26	32.15
		5.5	21.18	0.22	20.97	29.55
		5.75	20.12	0.3	11.32	25.17
		6	19.72	0.49	8.21	22.96
	0.45	5	22.52	0.1	29.02	36.47	5.65
		5.25	21.73	0.17	26.22	31.51
		5.5	20.9	0.27	22.69	26.65
		5.75	19.31	0.32	20.39	22.3
		6	19	0.44	9.51	20.34
Coir	0.15	5	23.32	0.14	24.86	35.55	5.55
		5.25	22.78	0.23	21	33.39
		5.5	21.91	0.29	19.26	29.77
		5.75	20.56	0.33	15.55	23.71
		6	19.73	0.49	13.54	21.73
	0.3	5	22.46	0.11	23.04	33.06	5.7
		5.25	21.99	0.16	20.32	30.04
		5.5	20.62	0.22	16.4	28.12
		5.75	20.2	0.31	11.61	22.14
		6	19.19	0.44	8.51	20.8
	0.45	5	22.15	0.06	28.28	34.37	5.75
		5.25	21.42	0.12	25.12	31.89
		5.5	20.58	0.2	20.32	29.01
		5.75	19.73	0.28	16.21	25.57
		6	19.3	0.37	9.73	21.46

. All three of the aforementioned criteria were met in the process of finding the OBC. The properties of the OGBC mixes with fibers, namely the air voids (%), draindown (%) and Cantabro loss (%) at OBC is presented in

Table 5. The properties of the OGBC mixes with fibers at Optimum Binder Content (%).

Fiber	Fiber (%)	Optimum Binder Content (%)	Air Voids (%)	Draindown (%)	Abrasion Loss (%)
					Unaged	Aged
Sisal	0.15	5.50	20.90	0.30	18.10	24.90
	0.30	5.60	20.35	0.29	14.72	22.88
	0.45	5.70	19.51	0.29	14.05	24.32
Jute	0.15	5.45	21.79	0.30	13.75	31.00
	0.30	5.60	20.40	0.30	16.68	31.00
	0.45	5.65	19.86	0.30	18.00	25.82
Bamboo	0.15	5.35	22.13	0.30	21.00	30.70
	0.30	5.60	20.76	0.25	17.11	27.80
	0.45	5.65	19.95	0.30	21.31	24.04
Coir	0.15	5.55	21.64	0.30	18.52	28.56
	0.30	5.70	20.28	0.29	12.57	23.34
	0.45	5.75	19.73	0.28	16.21	25.57

Note: Bold values represent the properties of optimum OGBC mixes.

. The ideal fiber content was determined to be 0.30% for sisal, bamboo, and coir fibers and 0.45% for jute fibers. At 0.3% dosage rate, sisal fibers impart the most desired qualities to the OGBC mix and are thus recommended as the most ideal fiber for OGBC mixes. The ideal binder content in the mix would be sufficient to prevent severe fatigue cracking, too.

3.5. Indirect Tensile Strength (ITS) and Tensile Strength Ratio (TSR)

An indirect tensile strength test carried out on OGBC specimens (conditioned and unconditioned) produced adopting optimum binder content with and without fibers, and the findings are presented in Figure 7. As demonstrated in Figure 7, when both conditioned and unconditioned samples were taken into account, the OGBC with coir fibers produced the highest ITS value, followed by jute, sisal, and bamboo fibers. The ITS of OGBC without fibers was 25.88% and 30.66% lower for unconditioned and conditioned samples, respectively, when compared to that of OGBC containing coir fibers. Likewise, for conditioned samples, the ITS of OGBC incorporating jute, sisal, and bamboo fibers was 42.05%, 40.98%, and 19.65% higher than that of OGBC without fibers; whilst, for unconditioned samples, the ITS of OGBC incorporating jute, sisal, and bamboo fibers was 34.41%, 30.84%, and 14.21% higher than that of OGBC without fibers. To this end, based on ITS criteria, the coir fibers provide OGBC with the highest level of strength qualities, followed by jute, sisal, and bamboo fibers. The Tensile Strength Ratio (TSR) of OGBC mixes serves as a gauge of how sensitive it is to water and as an indicator of resistance to moisture susceptibility. According to test results (Figure 8), all of the examined OGBC mixes—both those made with and without fibers—had a TSR of more than 80%, demonstrating that they are all resistant to moisture damage. The TSR of the OGBC made of sisal fibers was 86.24%, followed by those made of coir (85.55%), jute (84.49%), and bamboo fibers (83.86%). Overall, all the fibers lend optimistic characteristics to OGBC mixes when incorporated at optimal proportions.

4. Conclusions

In this study, an investigation was carried out on Open-Graded Bituminous Concrete incorporating fibers such as sisal, jute, bamboo and coir fibers. The dosage of fibers varied from 0.15% to 0.45% with an increment of 0.15%. Based on the test results and observations, the following conclusions are drawn:

• The addition of fibers slightly reduced the air void content when compared with the reference OGBC mix, but it satisfied the minimum air void content stipulated by IRC guidelines.
• The OGBC mixtures without any fibers exhibited higher draindown of binder in the range of 0.22–0.67%, whereas the mixtures with fibers showed reduced binder draindown property.
• The ideal fiber content was determined to be 0.30% for sisal, bamboo and coir fibers and 0.45% for jute fibers. At 0.3% dosage rate, sisal fibers impart the most desired qualities to the OGBC mix and are thus recommended as the most ideal fiber for OGBC mixes.
• Based on ITS criteria, the coir fibers provide OGBC with the highest level of strength qualities; however, the TSR of the OGBC made of sisal fibers showed increased resistance to moisture susceptibility.

In summary, the study found that fibers can boost OGBC’s performance in terms of strength and resistance to moisture susceptibility, and can reduce binder draindown. Since the study was carried out in a lab setting, additional research is necessary to verify the results in the context of reality. The study did not examine the long-term durability of OGBC with fibers, so further research must be conducted to assess the long-term performance of these mixes.