Performance Evaluation of Recycled Fibers in Asphalt Mixtures

Abstract

This study presents the results of using innovative and sustainable recycled fibers in different asphalt mixtures. Laboratory design and performance evaluation were focused on the cracking and rutting resistance of asphalt mixtures reinforced with recycled fibers. Two mixtures were designed for this research: 1. A dense-graded hot-mix asphalt (HMA) mixture containing 15% reclaimed asphalt pavement (RAP) and a PG 64-22 asphalt binder. 2. A cold-recycled mixture (CRM) incorporating silica fume and Portland cement as a mineral filler and CSS-1H asphalt emulsion. The recycled fibers used in this study included PET, LDPE, and carbon and rubber fibers. A balanced mix design (BMD) approach based on cracking and rutting performance parameters was used to design the control mixtures. The IDEAL-CT (ASTM D8225) was conducted to assess the cracking resistance, and the IDEAL-RT (ASTM D8360) was applied for rutting resistance. For the HMA mixture, results showed that the addition of PET, carbon, and rubber fibers enhanced cracking resistance and influenced the rutting resistance; ANOVA analyses revealed statistically significant differences in both CT index and RT index between the control mixture and the fiber-reinforced mixtures. In the case of the cold-recycled mixtures, the addition of LDPE, PET, and rubber improved cracking resistance; however, a decrease in rutting resistance was also observed among the evaluated CRM samples.

1. Introduction

Asphalt mixture is one of the primary materials utilized in pavement and road construction, with more than 440 million tons estimated to be produced in the USA during the 2022 construction season [1]. During service, pavement is exposed to several aggressive environmental and traffic conditions that affect the asphalt mix’s performance against specific damage phenomena such as cracking, rutting, and moisture damage. Most often, those distresses lead to a reduction in structural capacity to withstand loads, producing a reduction in pavement serviceability and increasing the need for premature preservation and rehabilitation techniques; because of this, different investigations have been conducted [2,3,4,5] to evaluate methodologies to improve asphalt mix performance and increase its service life, including polymer, rubber, plastic, and fiber additives.

Moreover, the increase in waste production, especially for non-biodegradable materials like plastics or some specific industrial waste, and limited disposal techniques [6] have made researchers focus on sustainable materials and green technologies for pavement construction, such as the inclusion of waste fibers or even waste plastics obtained from postconsumer recycled (PCR) and post-industrial recycled (PIR) materials has gained attention as an ecological technology.

On the other hand, cold mix asphalt (CMA) is a highly versatile mixture that comprises unheated mineral aggregate or recycled aggregate and asphalt emulsions. This unique mix is incredibly adaptable and can be tailored to meet various requirements, such as different aggregates, weather conditions, and traffic demands. Additionally, using recycled materials in CMA makes it an economically viable option due to its high production rates relative to the initial investment required [7]. Furthermore, producing cold recycled mixtures (CRMs) involves using nearly 100% recycled asphalt pavement, significantly reducing reliance on virgin material and material costs. This sustainable approach helps reduce costs and minimize emissions, as the production process involves lower temperatures [8]. Moreover, the need for hauling trucks to transport materials from a plant can be significantly reduced or even eliminated, resulting in further environmental benefits [9,10].

The addition of fibers to asphalt concrete increases its tensile strength, since the fibers bridge cracks and micro-cracks and carry tensile forces. Therefore, fiber-reinforced asphalt concrete (FRAC) can sustain higher magnitudes and more repetitions of tensile stresses. This may effectively reduce the potential development/propagation of various types of cracks, such as fatigue cracking, thermal cracking, and block cracking. In addition to increasing the tensile strength, the shear strength of the mix may also improve. Since pavement rutting occurs primarily due to shear, adding fibers may reduce rutting potential under traffic loads. Also, fibers in FRAC keep cracks tight. Tight cracks allow for the interlocking of the two sides of the crack, transferring the load from one side of the crack to the other. Fibers’ geometric sizes also play an essential role in improving mechanical properties such as strength and stiffness [11,12,13,14].

The inclusion of fibers directly into the binder (wet method) or blended with aggregates and binder (dry method) during the production of asphalt concretes is intended to produce a random distribution of fibers within the binder–aggregate matrix to enhance mix performance against specific distresses like cracking as a consequence of the fibers’ mechanical properties [9]. Fibers with different dimensions and physical properties have been reported in asphalt applications in different departments of transportation in the United States [15,16].

Fibers produced from recycled polyethylene terephthalate (PET) bottles have shown improvements in cracking resistance, producing fiber-modified mixtures with higher fracture energy and being capable of resisting higher-fatigue cycles before failure; however, authors have also reported variations in air voids and volumetric parameters, with PET fiber-reinforced mixtures requiring different binder contents to achieve the same volumetric parameters [17,18].

Similarly, several researchers have explored the inclusion of carbon fibers due to their superior mechanical properties, higher tensile strength, thermal stability, and chemical resistance [4]. Improvements in tensile strength and moisture susceptibility reduction have been reported [19], while for cracking resistance, some publications have reported that carbon fibers in dosages from 0.01% to 0.3% enhance fracture toughness, allowing the mix to support higher strain energy before the initiation of fractures. Moreover, three-point flexural test results have demonstrated the improved fatigue life of carbon fiber-modified mixes [19,20].

Despite the increasing research into using fibers as an asphalt mix additive, different concepts and knowledge gaps still need to be addressed, especially considering reinforcement mechanisms and the wide variety of materials available, including recycled materials. Additionally, most of the results of the existing investigations are based on asphalt mixtures designed using Marshall and SUPERPAVE methodologies; however, the development of recent approaches like balanced mix design (BMD) as a method have focused on providing a mix design, using performance tests to make asphalt mixes able to withstand several distresses [21], have not been thoroughly investigated in combination with fiber technology [22,23].

The current mix design methodologies of CRMs do not address the effect on the performance of newer additives, recycled materials, and fibers. Thus, there is a need for the application and implementation of performance-based methodologies. The design process for CRMs could be developed using different methodologies, each considering a different curing process. Since the BMD methodology optimizes the asphalt mix performance to mitigate distress types relevant to the installation region’s climate and traffic conditions [21,24], the combination of these design methodologies with more sustainable materials like CRMs and recycled fibers offers the possibility of designing cost-optimized and sustainable mixtures that meet performance requirements and, at the same time, offer the most significant potential for innovation, since this approach focuses its interest on the mix performance only.

Finally, due to the implementation of novel testing procedures to assess cracking and rutting resistance, like the index-based performance tests the IDEAL-CT and IDEAL-RT [25], and the use of innovative reinforcing and sustainable materials, further investigation is required to evaluate the suitability of recycled fibers and their effect on index results for both types of asphalt mixtures.

Objective

The main objective of this investigation was to evaluate the mechanical performance of HMA and CRMs containing different fibers obtained from recycled materials. Cracking and rutting resistance were evaluated using laboratory index-based performance tests such as the IDEAL-CT and IDEAL-RT. Results were analyzed based on the balanced mix design approach to assess the feasibility of these recycled materials in enhancing mix performance.

2. Materials and Methods

2.1. Virgin Aggregates and Reclaimed Asphalt Pavement (RAP)

Virgin aggregates were obtained from a local asphalt producer in South Carolina. Different aggregate fractions were blended and combined with 1% hydrated lime to produce the gradation for an asphalt mix with a nominal maximum aggregate size (NMAS) of 12.5 mm based on the AASHTO M323 SUPERPAVE criteria.

The recycled aggregate was also collected from a local contractor in South Carolina. The asphalt content of the RAP aggregate was determined following AASHTO T308 in an ignition oven with a value of 6.2% by total weight. For the HMA mixtures, RAP was added in a proportion of 15% by total aggregate weight. For the CRMs, RAP represents 100% of the aggregate composition.

The combined mix gradation for both mixtures, including the virgin aggregate and the RAP, is shown in Figure 1.

2.2. Asphalt Binder

A conventional asphalt binder classified as PG64-22 was obtained from a local supplier in South Carolina; its characteristics are illustrated in

Table 1. Characteristics of asphalt binder.

Binder	Test	Result
Original binder	Dynamic shear G*/sin δ (kPa)	1.37
	Rotational viscosity @135 °C (Pa.s)	0.408
	Rotational viscosity @165 °C (Pa.s)	0.115
	Flashpoint (°C)	320
RTFOT Residue	Dynamic shear G*/sin δ (kPa)	3.73
PAV Residue	Dynamic shear G*sin δ (kPa)	4303
Mixing temperature (°C)		152–158
Compaction temperature (°C)		141–146

, as reported by the supplier.

2.3. Recycled Fibers

For the reinforcement of the HMA, five different recycled fibers were evaluated in the mix at a constant content of 0.05% (0.5 g/kg) by total weight; the selection of this dosage was established based on the literature review and the recommended dosages for commercially available fibers, which typically vary between 0.01% and 0.1% [16]. These included three PET fibers of varying lengths—long (2002B) (Figure 2a), medium (400P) (Figure 2b), and short (1002B) (Figure 2c)—processed from postconsumer bottles via extrusion, post-industrial recycled carbon fibers from the aerospace industry (Figure 2d), and rubber fibers derived from rubber mulch (Figure 2e). Similarly, for CRMs, four recycled fibers were used at the same dosage, including two PET fibers, 1002B (Figure 2b) and 601P (Figure 2f); rubber fibers (Figure 2e); and LDPE fibers (Figure 2g).

Table 2. Summary of fiber properties.

Fiber	Dimensions *	Density (g/cm3) **	Tensile Strength (MPa) **	Modulus of Elasticity (GPa) **	Melting Point (°C) **
Carbon	Average length = 25.4 mm Diameter = 7 µm	1.37 [19]	4900 [19]	230 [19]	1000 [19]
Rubber	Average length = 29 mm Diameter = 2.2 mm	0.83 [26]	600 [26]	2.7 [26]	256 [26]
LDPE fibers	Average length = 40 mm Width = 2.0 mm	0.92 [27]	11 [27]	0.52 [27]	110 [27]
PET-2002B (long)	Average length = 63.5 mm Aspect ratio = 200 denier	1.35 [28]	192.4 [28]	3.9 [28]	250 [28]
PET-400P (medium)	Average length = 44.5 mm Aspect ratio = 40 denier
PET-1002B (short)	Average length = 38.1 mm Aspect ratio = 100 denier
PET-601P	Average length = 57.1 mm Aspect ratio = 60 denier

* Evaluated in the laboratory, ** Based on literature review.

summarizes the dimensions of the evaluated fibers and some fiber characteristics obtained based on the reviewed literature to provide further information of the used materials.

2.4. Fillers

For the cold recycled mix, type I Portland cement and silica fume were considered as fillers for our research. Silica fume was included to enhance the mix’s densification and served as a supplementary cementitious material.

2.5. Asphalt Emulsion

The asphalt emulsion used in this study was a cationic slow-setting emulsion (CSS-1H), and the properties are shown in

Table 3. CSS-1H emulsion properties.

Identification		CSS–1H
Properties	Unit	Value
Presence of sieve, grit	%	0.02
Residue, metler	%	63.64
Particle size distribution, mv	μm	9.91
Particle size distribution, >90%	μm	18.76
Storage stability (24 h)	%	0.61
Settlement (5 days)	%	3.50

, as reported by the supplier.

2.6. Selection of Air Void Levels for Performance Testing

Cold in-place recycling (CIR) asphalt mixtures typically have large numbers of air voids due to moisture content during construction. The air void content of cold-recycled asphalt mixtures can be controlled using a specific number of gyrations during compaction. Some investigations have recommended the application of 25–57 gyrations for mixes to achieve 11% air voids [29]. In addition, researchers have compared air voids between CIR and hot-mix asphalt (HMA) mixtures and determined that CIR specimens contained fewer air voids and a higher number of air pockets when samples were compacted to achieve 11 ± 0.5% air voids [30].

With the known difficulties in obtaining air void levels similar to those of HMA in the field (about 7.0%) and the expected differences in air void sizes, the research team conducted a permeability evaluation of CRMs compacted at air void levels ranging from about 7.0% to 15%. This part of the study aimed to select a target air void level based on an existing maximum permeability threshold of 125 × 10⁻⁵ cm/s established for hot-mix asphalt [31]. Permeability testing was conducted using the Florida Method of Test for Measurement of Water Permeability of Compacted Asphalt Paving Mixtures, designation: FM 5-565 [32]. A falling head permeability test apparatus is used to determine the water flow rate through the specimen (see Figure 3a). Water in a graduated cylinder can flow through a saturated asphalt sample, and the time taken to reach a known change in the head is recorded. The permeability coefficient of the asphalt sample is then determined based on Darcy’s law. The coefficient of permeability, k, is determined using the following equation:

$$k_{20}={\displaystyle \frac{aL}{At}}ln\left({\displaystyle \frac{h_1}{h_2}}\right)t_{20}$$

(1)

where:

K₂₀ = coefficient of permeability at 20 °C, cm/s;

a = inside cross-sectional area of the burette, cm²;

L = average thickness of the test specimen, cm;

A = average cross-sectional area of the test specimen, cm²;

t = elapsed time between h₁ and h₂, s;

h₁ = initial head across the test specimen, cm;

h₂ = final head across the test specimen, cm;

t₂₀ = temperature correction for the viscosity of water.

2.7. Mix Design Phase Using a Balanced Mix Design (BMD) Approach

2.7.1. BMD Approach for HMA

For hot-mix asphalt, the control mix corresponds to a 12.5 mm NMAS dense-graded mix developed following BMD performance design Approach D, where the components and the mix proportions are adjusted based on performance results against cracking and rutting; this approach allows us to consider variations in materials and additives to achieve the desired performance without considering requirements for volumetric properties [24]. The initial binder content and aggregate proportions were selected based on a previous volumetric design, and then variations in the binder content were evaluated to determine the optimum binder that met the selected performance threshold; the specific details of these tests are discussed in a later section.

For all sample preparations, aggregates and binders were preheated at the specified mixing temperature for at least three hours before mixing. Additionally, the RAP (reclaimed asphalt pavement) was added and blended with the preheated virgin aggregates approximately 30 min before the mixing process began, as recommended by the BMD specimens’ fabrication guide to prevent excessive oxidation in the material. HMA test specimens were prepared using the Superpave gyratory compactor (SGC) to a constant height of 62 ± 1 mm, and we aimed to achieve a void content of 7 ± 0.5% according to the protocol of each test for all prepared samples; the duration between sample reduction and compaction (Lag time) was kept constant.

For HMA mixtures, the cracking threshold was chosen based on previous studies with asphalt mixtures containing additives that exhibited good field performance against this distress; a minimum CT index of 55 was required [5,22,25]. Similarly, the rutting threshold was established considering previous works on quality acceptance procedures using the IDEAL-RT; for this investigation, a minimum RT index of 75 was required [5,22,25].

2.7.2. BMD Approach for CRMs

The BMD methodology was conducted on cold-recycled mixtures to create improved asphalt mixtures capable of competing with hot-produced mixtures for low-volume roads and parking lots. BMD approach D was selected for this study because it provides an optimization platform based on the performance test with reduced or zero volumetric requirements [24]. This research considered the employment of recycled asphalt pavement consisting of 97% aggregate and 3% fillers composed of cement and silica fume. Once the final gradation was determined, the water content (considered as the amount that would guarantee acceptable workability, coating, and cohesion of the mixture) was determined at 2%, and then four asphalt emulsion contents were established at 3%, 4%, 5%, and 6% by weight of the RAP mixture. After mixing, the compaction process was conducted with an SGC at a remaining moisture content of 2%, considering the RAP moisture, pre-mix water, and the water contained in the asphalt emulsion. The air void target for the specimens was established at 9.5 ± 0.5% at a height of 62 ± 1 mm; further discussion on the selection of the target air voids is presented in the Results section.

The specimens were fabricated considering all the parameters described above. Once all the specimens were compacted, samples were subjected to the curing stage according to the process recommended by the Asphalt Recycling & Reclaiming Association (ARRA) in the CR201 design guidelines [33]. All the specimens were placed in a forced draft oven with ventilation on the sides and top at 140 ± 2 °F (60 ± 1 °C) for 48 h. Then, the specimens were placed for another additional 24 h at ambient temperature.

The cracking threshold was chosen for CRMs based on previous studies with results specifically aimed at low-volume roads; a minimum CT index of 30 was selected [34,35,36,37]. Similarly, the rutting threshold was established considering previous works on quality acceptance procedures using the IDEAL-RT; for this investigation, a minimum RT index of 75 was selected [34,35,36,37].

2.8. Index-Based Performance Evaluation

The selection of the IDEAL-RT and IDEAL-CT was based on previously conducted investigations, where their results showed a correlation with different existing tests like the Texas Overlay for cracking and the APA and Hamburg Wheel Tracking test for rutting; moreover, both tests are quicker and more straightforward tests that have proven sensitivity to different aspects considered in the BMD framework, like variations in binder content or air voids [5,25].

2.8.1. Indirect Tensile Asphalt Cracking Test (IDEAL-CT)—ASTM D8225-19 [38]

The IDEAL-CT analyzes cracking initiation and propagation in asphalt mixtures, primarily performed at 25 °C. This test provides valuable data, such as deformation tolerance at 75% of the peak load and the mix-behavior curve’s post-peak slope and CT index, calculated based on the final failure energy. CT index is the performance indicator of the cracking resistance [38]. The CT index final parameter was calculated using Equation (2), and the fixture and specimen setup are shown in Figure 3b. The higher the CT index value, the better the cracking resistance of the asphalt concrete.

$${\mathrm{C}\mathrm{T}}_{\mathrm{I}\mathrm{n}\mathrm{d}\mathrm{e}\mathrm{x}}={\displaystyle \frac{t}{62.0}}\times {\displaystyle \frac{G_f}{\left|m_{75}\right|}}\times {\displaystyle \frac{l_{75}}{D}}$$

(2)

where: t is the specimen thickness, G^f is the fracture energy, |m₇₅| is the post-peak slope at 75% of the peak load, and l75 is the displacement at 75% of the post-peak load.

2.8.2. Indirect Tensile Asphalt Rutting Test (IDEAL-RT)—ASTM D8360-22 [39]

The IDEAL-RT determines the rutting resistance properties of an asphalt mixture by applying a monotonic compressive load along a vertical diametral plane of a centrally positioned asphalt mixture specimen. This test is performed at high temperatures (50 ± 15 °C) and considers the elastic/viscoelastic properties of the asphalt mixture and its strain-hardening characteristics. RT index is derived from peak load or shear strength and is the performance indicator for rutting resistance [39]. The RT index final parameter was calculated using Equation (3), and the fixture and specimen setup are shown in Figure 3c. For the rutting resistance testing, a higher RT index value is desired for a better rutting resistance of the asphalt concrete mixture.

$${RT}_{Index}={\displaystyle \frac{P_{max}}{t\times D}}\times 2.356\times {10}^{-5}$$

(3)

where P_max is the applied peak load, t is the specimen thickness, and D is the specimen diameter.

2.9. Inclusion of Recycled Additives

Eleven different asphalt mixtures were prepared for this study based on the additives, including two control mixtures (1 HMA and 1 CRM), five fiber-reinforced HMA mixtures, and four fiber-reinforced CRMs.

Fibers were added at a dosage of 0.05% (0.5 g/kg) using the dry method for both types of fiber-reinforced mixtures. Each batch of aggregate/RAP was equally divided into three portions and added to the mixing bowl in stages (Figure 4a). In comparison, the fibers were divided into two portions and added between the aggregate/RAP sub-batches to create a layered system in the mixing bucket (Figure 4a). Some researchers have remarked that this procedure helps to prevent fiber loss due to flocculation or dispersion in the air [23] and helps to disperse the fibers among the aggregate (Figure 4b). Additionally, the mixture was blended for at least 90 s for all fibers to ensure complete particle coating.

3. Results and Discussion

3.1. Permeability Test Results for CRMs

CRM samples were compacted to achieve air voids ranging from 7.0% to 15%. As shown in Figure 5, the CRM samples with less than 10.4% air voids were below the recommended threshold obtained for hot-produced asphalt mixtures. At the same time, 9.5 mm fine-graded hot-mix asphalt (HMA) samples met the maximum threshold at about 8.0% air voids. The selected range of air voids for the HMA was 6.3% to 8.4%, which is typically obtained from air voids in the field. Figure 4 shows the gyrations needed to compact CRM samples to the selected air void contents. A 9.5% air void content was set as the target for the studied CRMs, and a tolerance level of 0.5% air voids was established, similar to HMA samples for performance testing. The expected permeability for 9.5 ± 0.5% CRM samples is expected to go from 35 × 10⁻⁵ to 95 × 10⁻⁵ cm/s, conservatively below the recommended threshold, and the expected number of gyrations from 100 to 200.

3.2. Balanced Mix Design Results

3.2.1. Hot-Mix Asphalt BMD Results

To develop the control mix design using the BMD performance-modified approach, variations in the mixture proportions were necessary to meet the performance thresholds. Based on a previous volumetric design, the initial binder content was set at 4.6% and evaluated with performance tests. Additionally, 5.1%, 5.6%, and 6.1% binder contents were tested. The results of the balanced design are shown in Figure 6, where increasing the binder content led to higher cracking resistance but reduced rutting resistance. This was expected, as the increased asphalt content made the mixture more flexible and capable of resisting cracking; however, it also increased the lubrication between aggregate particles, reducing shear strength and increasing susceptibility to rutting [23].

At an initial binder content of 4.6%, the mix showed excellent rutting resistance (RT index~130) but was highly susceptible to cracking (CT index = 30.9). Increasing the binder to 5.1% improved the CT index to 46.9 (a 51% increase with respect to the initial value). Further increasing the binder to 5.6% raised the CT index to 90 (a 91% increase with respect to 5.1% binder content); higher cracking resistance was obtained at 6.1% binder content; however, the rutting test did not meet the performance threshold (RT index of 75). Based on this behavior, the minimum binder content to meet the CT index requirement of 55 was set at 5.2%, while the maximum allowable binder content to achieve an RT index of 75 was determined to be 5.6%. Therefore, the optimum binder content was established at 5.4 ± 0.2% by total mix weight, 0.8% more asphalt than initial content (volumetric optimum binder content), ensuring an acceptable balance of cracking and rutting resistance.

3.2.2. Cold-Recycled Mixture BMD Results

The BMD design process was developed with four asphalt emulsion contents. The design results were determined after analyzing the performance (IDEAL-CT and IDEAL-RT) obtained from the compacted specimens. The volumetric calculation on the specimens was conducted exclusively to estimate the air void content (%) and whether the samples were appropriate to continue with the performance phase (air void content of 9.5% ± 0.5%). Figure 7 shows the BMD process curve, considering the four asphalt emulsion contents and the average values of IDEAL-CT and IDEAL-RT for each tested specimen group. The balanced optimum emulsion content (BOEC) value was between 4.9% and 5.5% asphalt emulsion, called the “balanced zone.” Therefore, the CRM’s optimum binder content was established at 5.0%.

3.3. IDEAL-CT Results

3.3.1. Hot-Mix Asphalt Cracking Resistance Results

Regarding the cracking resistance of the HMA, Figure 8 exhibits the CT index results for the conventional and fiber-reinforced mixtures evaluated at the optimum binder content.

Since samples were evaluated at a binder content selected within the balanced zone of the BMD, the conventional mix satisfies the performance criteria established in this investigation. Specifically for the fiber-reinforced mixtures, the inclusion of the PET fibers with different lengths exhibited an improvement in the cracking resistance, where medium PET fibers (PET-400P) had the highest average CT index with a value of 116.6, which represents an improvement of 48% with respect to the conventional mix; however, these samples also exhibited the highest variability in results among the evaluated treatments; some authors may attribute this high variability to limited dispersion of fibers within the asphalt–aggregate matrix [15,23]. Similarly, the long PET fibers also had a higher cracking resistance (CT index = 104), with an improvement of approximately 32% with respect to control.

Among the PET fibers used in HMA, the short PET fibers showed the smallest improvement in cracking resistance, with a CT index of 84, representing an 8% increase over the control. This aligns with prior findings indicating that shorter fibers may not effectively reinforce asphalt mixtures [17]. However, fiber characteristics such as cross-sectional and surface areas may also influence the interlocking, and the overall reinforcement mechanism obtained from fibers.

Similarly, the inclusion of carbon fibers enhanced cracking resistance, achieving a CT index of 88.3 (12% higher than conventional), similar to short PET fibers. Notably, these carbon fibers were the shortest and had the smallest cross-sectional area among those used in HMA. Likewise, rubber fibers also increased the CT index values; however, samples were tested at a higher void content (average air voids of 7.8%) due to a bouncing effect of the rubber after the remotion of the compaction loads, affecting the results of the cracking resistance [25].

This finding suggests that the fibers’ physical properties may influence the material’s reinforcement mechanism against cracking since the fibers have different lengths, surface areas, widths/diameters, and cross-sectional areas. Fiber-reinforced samples exhibited variations in CT index, where the analysis of variance (ANOVA) indicated that reported differences in means between fiber types are statistically significant (F = 3.426, p-value = 0.0374 and α = 0.05), indicating that fibers can significantly influence the cracking resistance of mixtures.

Moreover, to further comprehend the fiber influence on the cracking resistance of the mix, Figure 9 shows the IDEAL-CT interaction diagram for the evaluated samples; this diagram analyzes failure energy and post-peak behavior (the l75/m75 ratio) as a measure of the mixes’ brittleness and toughness. Fiber-reinforced samples demonstrated a higher l75/m75 ratio, indicating a less steep post-peak slope compared to the control mix. While most samples maintained failure energy levels similar to those of the control, long and medium PET fibers showed an increased CT index and failure energy, enhancing the resistance to rupture [28]. This improvement can be attributed to an interlocking mechanism in the fibers, which strengthens the mix and improves its post-peak behavior after cracking initiates.

3.3.2. Cold-Recycled Mix Cracking Resistance Results

Regarding the cracking resistance of the cold mixtures, Figure 10 shows the results for the CT index obtained for the fiber-reinforced CRMs.

Similar to HMA, including the fibers led to an improvement in the cracking resistance of the mix. Specifically, the inclusion of both PET fibers increased CT index. The 601P PET fiber, with an average length of 57 mm, led to a higher increase (75% with respect to control), showing a similar tendency to results for the HMA, in which longer fibers led to better cracking resistance.

Moreover, the rubber fibers exhibited the highest CT index among the evaluated samples, contrary to what was observed for HMA; these samples did not exhibit the bouncing effect due to the presence of rubber, which could be likely due to ambient-temperature production and the higher air void content of CRM samples. As observed in a previous study, the samples prepared with recycled LDPE fibers also exhibited higher values for CT index than the control mix, with an improvement of 54% [27].

Results were also analyzed to determine if fiber inclusion led to variations in means between the CRM samples. Based on the ANOVA analysis, it was determined that the differences observed in the mean RT index among treatments are statistically significant (F = 5.67, p-value = 0.00354 and α = 0.05), suggesting that for the cold mixtures, fibers can also influence cracking resistance.

Finally, Figure 11 shows the IDEAL-CT interaction diagram for the CRM samples. Similar to HMA, the addition of fibers led to a higher l75/m75 ratio, suggesting that mixtures exhibited a less brittle behavior after the initiation of cracks; however, most of the samples also exhibited a reduction in fracture energy, except the PET 601P mixtures, suggesting a decrease in overall mixture toughness despite the improved post-crack resistance characteristics.

3.4. IDEAL-RT Results

3.4.1. Hot-Mix Asphalt Rutting Resistance Results

With respect to rutting resistance, Figure 12 shows the obtained RT index results for the fiber-reinforced HMA. The inclusion of these fiber additives may influence not only the shear strength of the mix but also the particle-to-particle interaction within the asphalt concrete matrix when the fibers are not adequately dispersed and form conglomerates, leading to a reduction in overall rutting resistance; however, for the evaluated samples in this investigation, fiber-reinforced mixtures exhibited similar rutting resistance to the control mix.

Samples with carbon fibers showed the lowest RT index of 82, representing an 8% reduction compared to the conventional. Moreover, all the PET fiber samples registered almost the same RT index as the conventional mix. In contrast, only a 3% reduction was observed for the rubber fiber samples compared to the control mix, even though these samples were evaluated at a higher air void content than specified by the test standard.

Moreover, as part of the multiple comparison process, results from the ANOVA indicated that reported differences in means between fiber types for the RT index are statistically significant (F = 3.48, p-value = 0.0326 and α = 0.05), suggesting that fibers may influence the rutting susceptibility of HMA.

3.4.2. Cold-Recycled Mix Rutting Resistance Results

As previously mentioned, including fibers may positively or negatively influence the rutting resistance of the asphalt concrete. Figure 13 illustrates the RT index results for the CRM samples.

It can be observed in Figure 13 that the inclusion of the rubber fibers, the LDPE fibers and the PET 601P led to a reduction in the RT index to values that are even lower than the established performance threshold; those samples also exhibited significant improvement in cracking resistance, suggesting that fiber inclusion led to more flexible behavior on the part of the mix.

Conversely, the short PET fiber samples improved this parameter, suggesting no clear tendency regarding the addition of fibers and the rutting resistance of the CRM samples. Finally, an ANOVA was conducted to evaluate the RT index across all treatments for the CRMs. The results revealed statistically significant differences (F = 24.29, p-value = 3.03 × 10⁻⁷ and α = 0.05) among the fiber types, indicating that the inclusion of fibers can influence the rutting resistance of CRMs.

However, it must be taken into account that, as mentioned before in the permeability results, achieving the same levels of compaction for these mixtures is more difficult; therefore, they were evaluated at a higher air void content, which may also have affected the RT index results more than the presence of fibers.

4. Conclusions

This investigation was focused on evaluating the cracking and rutting resistance of fiber-reinforced asphalt concrete with fibers from recycled materials; the effect of fibers was explored in two types of asphalt concrete: HMA and CRMs. Both control mixtures were designed following the criteria for BMD approach D, and fiber inclusion was assessed as an alternative to improve mix performance using index-based performance tests. Based on this, the findings of this study are closely associated with the conditions and materials evaluated. Moreover, further investigation is recommended to draw generalizable conclusions. The main findings from the specific laboratory results in this investigation can be summarized as follows:

• It was determined that the permeability of the studied CRMs with 10% air voids met the recommended threshold typically used for HMA, while the number of gyrations in the SGC remained within a practical range (fewer than 200) for fabricating CRM specimens.
• The BMD procedure enhanced the performance of both types of asphalt concrete by increasing the binder/emulsion content, leading to mixtures with improved cracking resistance while maintaining adequate rutting resistance. For the HMA, the optimum binder content varied from 4.6% to 5.4 ± 0.2%, while for the CRMs, the balanced zone was established between 4.9% and 5.5%.
• For the HMA, samples with rubber fibers failed to meet the air void content of 7 ± 0.5% due to a bouncing effect after removing compaction loads, resulting in more air voids, which could affect the index-based results. However, this effect was not observed for the CRM samples.
• Regarding cracking resistance, the HMA fiber-reinforced mixtures exhibited a higher CT index value. Medium PET and long PET fibers showed the highest increase in this parameter. On the other hand, the CRM-reinforced samples also experienced an increase in cracking resistance, where the PET fibers showed a CT index up to 75% higher than the control.
• Based on the IDEAL-CT interaction diagram, both HMA and CRM fiber-reinforced samples exhibited a less steep post-peak slope; HMA samples maintain the same levels of failure energy as the conventional mixture, but CRMs exhibited a reduction in this parameter; fiber-reinforced mixtures exhibited a less brittle behavior of the mix and better resistance to cracking after the initiation of the crack.
• Regarding rutting resistance, the results of the IDEAL-RT exhibited opposite tendencies between the two types of evaluated asphalt concrete. For the HMA, the evaluated recycled fibers exhibited similar performance to the conventional mix, while for the CRMs, the inclusion of recycled fibers led to a significant reduction in the rutting resistance of the mix; however, CRM samples were compacted at a higher air void content which could affect the results.
• PET fiber mixtures performed better overall against cracking and rutting compared to both conventional mixtures (HMA and CRM), suggesting they could be a viable alternative for enhancing mix performance with recycled materials.