Introduction

The rapid development of transportation infrastructure continues to sustain a strong demand for new asphalt materials. Concurrently, the growth of the social economy has led to a substantial rise in traffic volume and load, necessitating enhanced performance from asphalt mixtures1,2,3,4. The reuse of waste materials in asphalt mixture is not only beneficial for reducing the cost of pavement construction but also for reducing solid waste and saving raw materials5,6,7.

Glassphalt is a sustainable paving material using recycled glass in asphalt concrete, reducing natural resource demand and landfill waste. It enhances skid resistance, reduces reflective cracking, and improves durability. Adding fiber materials to asphalt concrete boosts stability, crack resistance, water damage resistance, and fatigue performance, making roads more resilient to heavy traffic and climate variations8,9,10,11.

Study on using glass cullet in ACs has expanded in recent years. Hughes12 found that adding 5% and 15% waste glass to ACs improves their properties by decreasing the voids in aggregate and increasing the voids filled with bitumen. The Federal Highway Administration’s study revealed that up to 15% fine glass can be used to maintain acceptable moisture sensitivity, while 20% waste glass requires significant adjustments with hydrated lime and other additives to prevent stripping13. Su and Chen14 found that the optimal bitumen content depends on glass cullet percentage. Hydrated lime improved the adhesion between glass cullet and bitumen. More glass cullet increased rutting but enhanced skid resistance and safety. Besides, glassphalt had lower water penetration than conventional AC. Moreover, Wu et al.15 found that glass cullet in ACs improves durability against moisture-induced damage and fatigue cracking. Optimal performance was achieved with a maximum aggregate size of 4.75 mm. The optimal content of glass cullet was found to be 10%, as increasing it to 20% resulted in a significant reduction of the TSR.

Dalloul16 studied on glassphalt incorporated aggregates up to 19 mm and glass cullet up to 4.75 mm. The optimal bitumen and glass cullet contents were 1.5% and 7.5%, respectively. Recycled glass bottles provided the necessary glass cullet. Using the viscoelastic model, Arabani and Kamboozia15 evaluated ACs containing waste glass. They found that the optimum glass content for desirable properties was 15% by weight of aggregates. Adding hydrated lime as a bitumen additive and waste glass cullet improved the stiffness of the AC, although the amount of glass cullet inversely affected stiffness. Arabani et al.17 studied the performance of asphalt concrete (AC) against fatigue. They found that samples with waste glass had higher locking and interlocking forces, reducing ultimate strain from tensile stress. Hydrated lime increased the adhesion of bitumen, aggregate, and waste glass cullet. Furthermore, Shafabakhsh and Mirabdolazimi18 proposed a model to determine the stiffness level of ACs. They found that adding lime reduced the moisture damage potential in glassphalt. While glass waste increased stiffness, its brittleness lowered the optimal stiffness with a higher content.

Adding fibers to bitumen enhances its performance. Various fibers, such as cellulose, polyester, glass, and mineral materials, are used in ACs. While fibers increase durability and stiffness, they reduce workability, which makes them excellent additives19. Kim et al.20 investigated different lengths and volumes of artificial fibers (polypropylene, polyester, nylon, carbon) to improve AC’s mechanical properties. The tests showed that polypropylene fibers improved Marshall resistance, tensile strength, and moisture sensitivity by 0.5%. Polyester and nylon fibers outperformed polypropylene and carbon fibers, with carbon fibers enhancing only flexural performance. Pang et al.21 also studied bitumen modified with varying polyacrylonitrile carbon fiber contents. Scanning electron microscopy (SEM) images revealed a three-dimensional network strengthening the bitumen bond, increasing viscosity and shear modulus, thus resisting deformation. Liu and Wu22 found that increasing the graphite content in bitumen decreased Marshall stability and rutting. Adding carbon fibers reversed this trend, improving stability and reducing rutting. Morea and Zerbino23 studied macro glass fibers in AC, finding improved fracture behavior at low temperatures. The fibers increased the first peak stress of fracture and provided higher residual stress capacity. Rutting behavior was significantly improved, reducing pavement deformation by about 50% compared to fiber-free mixtures.

Chen et al.24 examined bitumens modified with corn stalk fibers at various percentages (0, 2, 4, 6, 8, 10). Adding corn stalk fibers increased bitumen viscosity, decreased penetration grade, and raised the softening point. A higher corn stalk fiber content also increased complex shear modulus (G*) and lowered phase angle at high temperatures, enhancing deformation resistance. In addition, Oda et al.25 analyzed natural fibers in stone mastic ACs using coconut, sisal, cellulose, and polyester fibers at 0.3% and 0.5%. Mechanical tests showed high resistance in mixtures with natural fibers, preventing bitumen failure. According to Wu et al.26 research, fiber-reinforced AC has better moisture damage resistance and longer service life. Klinsky et al.27 used polypropylene and aramid fibers to evaluate modified ACs. Fiber-reinforced AC showed similar moisture resistance to fiber-free AC but had about 15% higher stiffness modulus. It also exhibited better resistance to rutting and deformation at high temperatures.

Toraldo and Mariani28 compared two polymers, ethylene vinyl acetate (polymer A) and low-density polyethylene (polymer B), in ACs. The best fatigue life was with 9% ethylene vinyl acetate. Except for 3% low-density polyethylene, the polymers increased rutting resistance. Yao et al.29 assessed AC with polyacrylonitrile fibers using four tests: Texas repeated load lateral, Australian Asphalt Pavement Association, Cantabro, and Cantabro polished stone value wheel tracking. Polyacrylonitrile fibers outperformed cellulose fibers in rutting resistance. Moreover, Ziari et al.30 added polyolefin-aramid fibers (0.25%, 0.5%, 0.75%) to ACs. The tests showed improved rutting and cracking resistance with higher fiber content, with optimal fatigue performance at 0.5% fiber content. Wan et al.31 studied the temperature sensitivity and viscoelastic effect of bitumen modified with ceramic fibers (CF). They found that CF-modified bitumen has a lower penetration coefficient, higher softening point, and less flexibility, indicating reduced softness. Arabani and Shabani32 examined the rheological properties of bitumen modified with 1%, 3%, and 5% CF, replacing asbestos fibers. Based on the tests, the CF-modified bitumen had lower temperature sensitivity and higher resistance against cracking but was unsuitable for low temperatures. Arabani et al.33 also investigated CF’s effect on the mechanical properties of ACs. The results of tests conducted (indirect tensile strength (ITS), indirect tensile fatigue, and repeated load axial tests) on a PG 64 − 16 binder containing 1%, 3%, and 5% CF, indicated that CF improves mechanical performance and significantly enhances resistance to rutting and fatigue.

Necessity of research and objectives

Previous studies indicate that fibers enhance the performance of asphalt concrete (AC) and bitumen by improving durability, fatigue cracking, rutting, low-temperature cracking, and moisture damage. The type and quantity of fibers significantly impact the modification results. In glassphalt, common additives include hydrated lime and polymers. Carbon fibers (CFs) are ideal for modifying ACs due to their high melting point, low thermal conductivity, and resistance to aging, stiffness, corrosion, and compressive strength. Accordingly, the aim of this study was to investigate the performance of glassphalt mixtures modified with various dosages of CF at different temperatures against fatigue cracking, rutting, and moisture-induced damage.

Experimental design

This section details the materials and consumables, including aggregates, bitumen, glass, and CF. The research methodology and experiments are also described (Fig. 1). The tests conducted on the bitumen were performed in the quality control laboratory of Jey Oil Refining Company, and the tests on the asphalt mixture were conducted in the Knowledge-based Laboratory at University of Guilan.

Fig. 1
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The research methodology.

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Materials

Aggregates

The aggregates used were granite, commonly referred to as acidic aggregates. The ASTM D3515 was used for the grading of aggregate materials34. The maximum size and nominal maximum size of aggregates were 19 and 12.5 mm, respectively (Fig. 2). Table 1 lists the mineralogical properties of limestone and granite.

Fig. 2
figure 2

Gradation diagram of aggregates.

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Table 1 Mineralogical properties of aggregates.
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Glass cullet

The glass cullet was obtained from waste glass in a glass workshop, with a maximum particle size of 4.75 mm. A portion of the granite was extracted, and glass was used as a replacement for fine aggregates at percentages of 0, 10, and 20.

Bitumen

PG 58 − 22 bitumen from the Iran Jey Oil Company was used. The specifications of the bitumen are listed in Table 2. This bitumen was procured from the Jey Oil Refining company.

Table 2 Technical specifications of PG 58 − 22 bitumen.
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CFs

In this study, 1, 3, and 5% CF with a mixing time of 20 min at 160 ℃ by a mixer at a speed of 1500 rpm were used for homogeneous modification of bitumen. The details for determining the mixing method are based on the study conducted by Naseri et al.40, which resulted in the production of a homogeneous modified bitumen. CFs are composed of aluminum silicate materials and are prepared by melting and firing molten kaolin with a high percentage of aluminum or a combination of alumina and silica. CFs have different properties such as a lightweight, flexibility, low thermal conductivity, high resistance to thermal shocks and corrosion, thermal and chemical stability, non-toxicity, and lower price compared to other fibers31. Figure 3 displays the CF used in this study.

Fig. 3
figure 3

The CF used in this research.

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Tests and methods

Mix design

The optimal bitumen content for constructing the ACs samples has been determined according MS-2 method. First, the bitumen content corresponding to the maximum specific gravity, maximum stability, and 4% air voids is extracted from the Marshall charts. Then, the average of these three percentages is calculated. The parameters of stability, flow, air voids, and voids in aggregate for the averaged bitumen content are determined from the Marshall charts. These parameters are checked against the AASHTO T245 standard41values, and if they conform, they are selected as the initial mix design values. Otherwise, adjustments are made to the mix design.

Modified Lottman test

A total of 108 samples are required for the moisture sensitivity test using the AASHTO T283 method42. The AASHTO T283 moisture sensitivity test evaluates the tensile strength of AC by measuring the maximum load a sample can withstand before failure. This test was conducted on samples with a diameter of 100 mm and a height of 63.5 mm. The samples were compacted to achieve an air void content of 7 ± 0.5%. After compaction, the samples were divided into dry and wet conditions and subjected to freeze-thaw cycles. Finally, the ITS of the samples was measured to determine the impact of moisture on the AC’s degradation42. The resistance of ACs against moisture damage was evaluated based on the AASHTO T283 or modified Lottman test by reducing the ITS by applying 1, 3, and 5 freeze-thaw cycles.

Wilhelmy plate (WP) test

The WP test was performed to measure the surface free energy (SFE) components of the bitumen. The WP method measures the contact angle between the binder and a liquid by balancing the surface forces of a very thin plate submerged in or pulled out of a liquid at a constant and very low speed. The SFE components of the binders are determined in this method based on two important steps: (1) determining the contact angle and (2) determining the SFE component using the Young-Dupre equation. For the test, a thin glass is uniformly coated with the binder, immersed in a specific solution, and slowly and steadily withdrawn from the container. Simultaneously, the contact angle between the binder and the liquid is measured. The Eq. (1) is used to determine the required force to maintain a suspended plate in the air in a state of equilibrium43:

$$\:F={W}_{plate}+{W}_{asphalt}-V\cdot\:{\rho\:}_{air}\cdot\:g$$
(1)

Where, F represents the force required to hold the plate in place, \(\:{W}_{plate}\) is the weight of the metal plate, V is the volume of the bitumen plate, ρair is the density of the air, and g is the local acceleration due to gravity. The Eq. (2) is used when the plate is immersed in a fluid43:

$$\:F={W}_{plate}+{W}_{asphalt}+{P}_{t}{\varGamma\:}_{L}cos\theta\:-{V}_{im}\cdot\:{\rho\:}_{l}\cdot\:g-\left(V-{V}_{im}\right)\cdot\:{\rho\:}_{air}\cdot\:g$$
(2)

Where, Pt represents the environment pressure of the bitumen-coated surface, ΓL is the total SFE of the liquid, θ is the dynamic contact angle between bitumen and the test liquid, Vim is the volume of the immersed portion of the bitumen-coated surface, ρL is the density of the liquid, V is the total volume of the bitumen surface43.

Equation (3) is obtained by combining Eqs. 1 and 243:

$$\:\varDelta\:F={P}_{t}{\varGamma\:}_{L}{cos}\theta\:-{V}_{im}\cdot\:{\rho\:}_{L}\cdot\:g+{V}_{im}\cdot\:{\rho\:}_{air}\cdot\:g$$
(3)

Where, ΔF is equal to the difference between the weight of the plate and the bitumen and the force indicated by the device during the test.

By rewriting Eq. (3), Eq. (4) is obtained43:

$$\:{cos}\theta\:=\frac{\varDelta\:F+{V}_{im}({\rho\:}_{L}-{\rho\:}_{air})g}{{P}_{t}{\varGamma\:}_{L}}$$
(4)

Based on the Young-Dupree equation, the SFE components of bitumen is measured by Eq. (5)43:

$$\Gamma _{L} \left( {1 + \cos \theta } \right) = 2\left( {\sqrt {\Gamma _{S}^{{LW}} \Gamma _{L}^{{LW}} } + \sqrt {\Gamma _{S}^{ + } \Gamma _{L}^{ - } } + \sqrt {\Gamma _{S}^{ - } \Gamma _{L}^{ + } } } \right)$$
(5)

Where, ΓLiLW, ΓLi+ and ΓLi− are the SFE components of the liquid phase, and ΓSLW, ΓS+ and ΓS− are the SFE components of the bitumen.

By measuring the contact angle between the bitumen-coated surface and at least three research liquids with known SFE components, the above equation will be solved using a system of three equations and three unknowns (ΓSLW, ΓS+ and ΓS−). The system of linear equations is defined as Eq. (9)43:

$$\:\left[\begin{array}{ccc}{A}_{11}&\:{A}_{12}&\:{A}_{13}\\\:{A}_{21}&\:{A}_{22}&\:{A}_{23}\\\:{A}_{31}&\:{A}_{32}&\:{A}_{33}\end{array}\right]\left[\begin{array}{c}{X}_{1}\\\:{X}_{2}\\\:{X}_{3}\end{array}\right]=\left[\begin{array}{c}{Y}_{1}\\\:{Y}_{2}\\\:{Y}_{3}\end{array}\right]$$
(6)

Where, \(A_{{1i}} = ~\frac{{2\sqrt {_{{Li}}^{{LW}} } }}{{_{{Li}} }}\), \(A_{{2i}} = ~\frac{{2\sqrt {_{{Li}}^{ + } } }}{{_{{Li}} }}\), \(A_{{1i}} = ~\frac{{2\sqrt {_{{Li}}^{ - } } }}{{_{{Li}} }}\), and \(X_{1} = \sqrt {_{S}^{{LW}} }\), \(X_{2} = \sqrt {_{S}^{ - } }\), \(X_{3} = \sqrt {_{S}^{ + } }\), and \(Y_{i} = 1 + cos\theta _{i}.\)

Indeed, X1, X2, and X3 are the unknowns, which are the components of the bitumen’s SFE.

Indirect tensile stiffness modulus (ITSM) test

Samples containing CF were subjected to indirect tensile testing to determine the stiffness modulus according to the BS EN 12697-26 standard44. The samples were placed at various temperatures and then loaded at the loading location (Fig. 4-a). Loading was performed twice on the samples, and in the second loading, a force was applied along the diameter of the sample. The deformation of the samples was measured by sensors. In this experiment, the deformation of the samples was maintained within the elastic range, and the modulus of stiffness was of second order, lower than the first order. This difference is more noticeable in materials with low stiffness or at high temperatures. Additionally, plastic deformation in the samples increases with a rise in the number of loading cycles.

The Eq. (7) is used to calculate the modulus of resilience44:

$$\:{M}_{R}\:=\:\frac{P}{Ht}(0.27+\nu\:)$$
(7)

Where, \(\:{M}_{R}\) is the modulus of stiffness in psi or MPa, p is the repetitive load in pounds or Newtons, H is the desired or reversible horizontal deformation in inches or millimeters, t is the thickness of the specimen in inches or millimeters, and ν is Poisson’s ratio.

Indirect tensile fatigue (ITF) test

The sample, according to the BS EN 12697-24 standard45, containing CF, was investigated for the fatigue life of the AC resulting from repetitive traffic loads during the flexible pavement lifespan using the indirect tensile test method. The test was conducted by applying loading on the samples with constant stress at different temperatures and stress levels. Loading was performed sinusoidally with a frequency of 1 Hz and specific loading and rest times. The samples were subjected to fatigue life determination using the indirect tensile test apparatus (Fig. 4-b).

Repeated loading axial (RLA) test

The repeated axial load test for samples containing CF was performed to analyze the ACs against permanent deformation or rutting potential according to the BS EN 12697-25 standard46. The axial stress was repeatedly applied to the sample at a specified temperature, and the axial strain was measured at desired intervals. The samples were subjected to the test according to the BS DD 226 standard at 30 and 50 °C and under a stress of 300 kilopascals (Fig. 4-c).

Fig. 4
figure 4

(a) Placement of the sample in ITSM experiments, (b) Representation of sample placement in fatigue testing under indirect tension, (c) Placement of the sample in repeated axial load testing.

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Results and discussion

The results of the Marshall graphs indicate that the optimum bitumen content for specimens without glass is 6.5%. This content is consistent with the prescribed values of the Marshall parameters. For specimens with 10% and 20% glass added, the optimum bitumen content is 6.3% and 6.1%, respectively, which is also compatible with the prescribed values. As the percentage of glass increases in ACs, the optimum bitumen content decreases due to the lower porosity of the glass surface and its lower absorption of bitumen compared to regular aggregates47,48,49. In another part of the study, the moisture sensitivity of the ACs was evaluated using multiple freeze-thaw cycles.

Statistical hypothesis tests, particularly paired sample tests, were used to evaluate the statistical hypotheses with using the SPSS 16.0 software. If the population follows a normal distribution and the test statistic follows a t-distribution, this test is referred to as a paired sample t-test. The hypotheses for this test pertain to Group 1 (before intervention) and Group 2 (after intervention). The hypotheses can be stated as follows:

$$\:\left\{\begin{array}{c}{H}_{0}=mean\left(1\right)=mean\left(2\right)\\\:{H}_{1}=mean\left(1\right)\ne\:mean\left(2\right)\end{array}\right.$$

When the value of the Sig. is less than 0.05, it indicates that there is a statistically significant difference in the data between the two groups being compared. This means that the observed differences are not likely due to random chance or variability within the data, but rather suggest a real and meaningful difference between the groups. Such a result implies that the intervention or variable being studied has a significant effect, leading to the conclusion that the groups are distinct in some relevant aspect measured by the study.

Results of modified Lottman test

Based on the Fig. 5, increasing the number of freeze-thaw cycles decreased the ITS of glass-free mixtures. Using CF significantly improved the ITS under all freeze-thaw cycles. These fibers enhanced the viscosity of the bitumen, improved the elastic properties of the specimens, and raised their stress tolerance. Moreover, utilizing CF increased the resistance of AC against moisture damage due to the enhancing the adhesion and cohesion properties of the mixture.

Fig. 5
figure 5

ITS of control specimens.

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Adding 10% glass in ACs raised the ITS. Glass particles have high fracture toughness and sharp edges, which raises the internal friction angle within the ACs. This increases the interlocking and bonding between the particles and as a result increases the ITS. Furthermore, utilizing CF also increased the ITS in all applied cycles. Using CF improved the elastic properties and enhanced the stress tolerance of bitumen (Fig. 6). The results indicated that with an increase in the glass content up to 20%, the ITS rose. Furthermore, incorporating CF improved the ITS, and using 3% of CF yielded better results than 1%. This improvement in ITS is attributed to interlocking and bonding between particles and the high fracture toughness and sharpness in glass and CF (Fig. 7).

Fig. 6
figure 6

ITS of glassphalt in specimens containing 10% glass.

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Fig. 7
figure 7

ITS of glassphalt in specimens containing 20% glass.

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Figures 8, 9 and 10 depict the TSR of the examined compounds. With an increase in freeze-thaw cycles, the TSR decreased due to the weakening of sample resistance under moisture condition. Furthermore, utilizing glass in the AC reduced the TSR compared to the control sample, and this reduction became more pronounced with a rise in the percentage of glass. The reasons for this decrease include poor adhesion between glass and bitumen and the chemical properties of aggregates. In samples where the desired additive is used, the TSR was indirectly improved with an increase in the usage percentage. The amount of the TSR index also depended on the percentage of CF and the number of freeze-thaw cycles. In samples containing 10 and 20% glass, utilizing CF enhanced the ITS and TSR index.

When laboratory results show that CF have improved the moisture damage index of the asphalt mixture, it can be expected enhanced field performance in several ways. The improved moisture resistance suggests that the asphalt mixture will have increased durability and a longer lifespan, even in wet conditions. This means reduced susceptibility to moisture-induced damage such as stripping and raveling, leading to fewer maintenance requirements and potentially lower repair costs. Additionally, the improved moisture resistance can contribute to better load-bearing capacity and overall structural integrity of the pavement, ensuring consistent performance over time.

Fig. 8
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TSR of glass-free mixtures under various freeze-thaw cycles.

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Fig. 9
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TSR of samples fabricated with 10% glass under different freeze-thaw cycles.

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Fig. 10
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TSR of samples fabricated with 20% glass under different freeze-thaw cycles.

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The paired t-test was used for statistical analysis to examine the effect of CF use on the TSR index of ACs containing different glass content. Two groups (before and after intervention) were formed, and the impact of CF on the moisture sensitivity index of glass-free samples was investigated (Table 3). The first comparison was made between the control group and the group containing 1% CF, and the second comparison was made between samples containing 1% CF and those containing 3% CF. The results indicated that incorporating CF significantly impacted the moisture sensitivity index of glass-free mixtures.

Table 3 Results of t-test to examine the impact of CF on the moisture sensitivity of glass-free samples.
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The paired t-test was also used to analyze the effect of CF on the TSR index of ACs containing 10% glass (Table 4). The results indicated that utilizing CF as an intervention significantly increased the TSR of ACs containing 10% glass. This effect was observed in all comparative groups, and the null hypothesis was rejected. Consequently, it can be said that the CF significantly enhanced the moisture sensitivity of glassphalt.

Table 4 Results of t-test to examine the impact of CF on the moisture sensitivity of samples containing 10% glass.
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In Table 5, Two groups were identified (before and after intervention), and the results demonstrated that CF elevated the TSR of AC specimens containing 20% glass cullet. The paired t-test results confirmed the significant effect of CF on reducing the moisture sensitivity of glassphalt.

Table 5 Results of t-tests to examine the impact of CF on the moisture sensitivity of samples containing 20% glass.
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Results of WP test

Measuring the SFE components of base bitumen and CF-modified bitumen indicated that bitumen possesses acidic properties, and its adhesion to acidic materials is not desirable (Table 6). Incorporating CF reduced the base bitumen’s acidic components and increased the modified bitumens’ polar components. Furthermore, utilizing CF as a bitumen modifier significantly enhanced the non-polar components of the modified bitumens and stabilized non-polar adhesive bonds. This helps to reduce the moisture-related damages in ACs containing acidic aggregates and glass. Previous studies have also shown that CF have basic (alkaline) properties, and the results of this research are consistent with those of previous studies50,51.

Table 6 Results of SFE measurements of base bitumen and bitumens modified with CF.
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The free energy of cohesion is twice the total SFE component of bitumen, and using CF increased it. This elevation reduced the brittleness of cohesion (Table 7). This resulted a greater resistance in the AC against cracking in the bitumen membrane and reduced the failures due to cohesion brittleness. Additionally, increasing the CF raised the free energy of cohesion.

Table 7 Free energy components of bitumen cohesion.
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Moisture damage occurs through two mechanisms: cohesive and adhesive failures. The results of the WP test indicate that the use of CF increases the cohesive free energy, reducing the likelihood of cohesive failure. On the other hand, the test results show that CF increase the basic properties of bitumen, which enhances its adhesion with acidic aggregates and glass cullet. The non-polar component of SFE results also show that using CF increases the ability of bitumen to form non-polar bonds in the glassphalt, which do not break easily in the presence of water. In fact, CF improve the resistance of the asphalt mixture to both cohesive and adhesive failure, which can be expected to enhance the performance of glass asphalt mixtures in wet climates. It is worth noting that moisture damage is one of the most significant issues concerning glassphalt.

Results of ITSM test

Figure 11 depicts the changes in the modulus of stiffness in control samples and glassphalt samples containing 10% glass modified with CF (1%, 3%, and 5% by weight of bitumen) at different temperatures (5, 25, and 40 °C). With an increase in temperature, the modulus of resilience decreased due to a reduction in elastic deformation. The relationship between the aggregate and bitumen was also temperature-dependent; as the temperature increased, the viscosity and stiffness of the bitumen decreased, and the adhesion between aggregates was reduced. The presence of CF in the bitumen increased the modulus of resilience. The increased viscosity and stiffness of the bitumen enhanced the bond between aggregates and bitumen and improved the elastic properties of the samples. Additionally, the presence of CF increased the absorption of bitumen in the AC and improved the adhesion between the aggregate and bitumen. A higher percentage of CF further elevated the modulus of resilience due to increased viscosity and the creation of a strong skeletal framework in the AC.

Fig. 11
figure 11

Stiffness modulus of glassphalt containing different CF at various test temperatures.

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According to Fig. 12, with a rise in the CF content of glassphalt, the stiffness modulus increases. CF had better performance at higher temperatures than lower one. Additionally, the modulus of stiffness decreased with a rise in temperature for all samples, but the effect of CF was more pronounced at 40 °C. This caused less decrease in rigidity modulus of the modified glassphalt samples at higher temperatures compared to the control samples.

An improvement in the stiffness modulus of the asphalt mixture is a very positive parameter, provided it does not weaken the mixture’s performance against fatigue cracking. The increase in the stiffness modulus of the asphalt layer helps distribute the stress applied to the asphalt layer over a wider area onto the underlying layer, thereby reducing the intensity of the stresses. This means that materials with lower strength and cost can be used beneath the asphalt layer. On the other hand, increased stiffness at high temperatures also increased resistance to rutting.

Fig. 12
figure 12

ITSM ratio of modified sample to control.

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A paired t-test was used to evaluate the effectiveness of CF in increasing the stiffness modulus of glassphalt. In the test, two groups (pre- and post-intervention) were determined, and by examining the results, the significant impact of CF on the stiffness modulus of glassphalt was analyzed. Three comparisons were made for each test temperature. The first comparison was between the control group and the group containing 1% CF. The second comparison included samples containing 1% CF and those containing 3% CF. The third comparison was made between samples containing 3% CF and those containing 5% CF. The results indicated that CF significantly affected the modulus of elasticity of AC samples (Table 8).

Table 8 The results of t-tests to examine the impact of CF on the stiffness modulus of the glassphalt.
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Results of ITF test

Figures 13 and 14 shows the fatigue life of glassphalt containing different CF contents at various test temperatures and stress levels. With increasing the test temperature, the fatigue life of AC decreased due to their high sensitivity to temperature changes. Additionally, an increase in stress reduced the fatigue life of the specimens. Modified glassphalt samples with CF exhibited a longer fatigue life than the control samples due to the fiber’s ability to absorb bitumen and create strong adhesion between the bitumen and aggregate. By adding CF to the AC, the fatigue life of the specimens increased, but after using 3% fibers, fatigue life decreased. The highest fatigue resistance was observed in samples containing 10% glass and 3% CF. Adding more than 3% of CF increased the stiffness of the samples and decreased their elastic properties. CFs are effective in withstanding repetitive stresses and improving the resistance of glassphalt. Therefore, samples containing 5% CF exhibited a greater fatigue life than the control samples. Optimal fatigue performance for glassphalt samples containing 10% glass was achieved with 3% CF.

If laboratory tests reveal that CF have enhanced the fatigue life of the asphalt mixture, it suggests a number of beneficial effects on its filed performance. The increased fatigue life suggests that the asphalt mixture will be more resistant to cracking and other forms of wear and tear over time in intermediate temperature. This means the pavement will be able to withstand repeated loading cycles, such as traffic, without significant deterioration. Consequently, the improved fatigue life leads to extended service life, reduced maintenance costs, and better long-term performance of the asphalt mixture under real-world conditions.

Fig. 13
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Fatigue life of AC containing 10% glass with different CF percentages at 15 °C.

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Fig. 14
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Fatigue life of ACs containing 10% glass with different CF percentages at 25 °C.

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The paired t-test was used to analyze the effect of CF on the fatigue life of glassphalt. Two groups (before and after intervention) were determined, and by examining the results, the significant effect of CF on the fatigue life of the glassphalt specimens was determined. The results showed that incorporating CF significantly affected the fatigue life of the ACs containing 10% glass cullet (Tables 9 and 10).

Table 9 Results of t-tests to examine the impact of CF on fatigue life of glassphalt at 15 °C.
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Table 10 Results of t-tests to examine the impact of CF on fatigue life of glassphalt at 25 °C.
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Results of RLA test

The RLA test was conducted with a stress of 300 kPa at 30 and 50 °C. The results showed that with a rise in temperature, the permanent deformations of the specimens increased, and the specimen’s resistance to permanent deformation decreased. CF reduced axial strain and raised the resistance of the AC against permanent deformation. Furthermore, CF increased the modulus of stiffness of the AC and decreased the permanent deformations. The presence of CF in glassphalt increased the adhesion between the aggregate and the bitumen, enhancing the resistance of the specimens. CFs have a continuous and strong chain-like structure, which forms a strong skeleton in the modified specimens and reduces the resistance of the binder and permanent deformation in the AC (Figs. 15 and 16).

The results of the flow number parameter also indicate that, at both temperatures examined, the control samples entered the tertiary creep phase at a significantly lower number of cycles. This shows that the control samples have a lower resistance to deformation under repeated loading compared to the modified samples. However, in the modified samples, the asphalt mixtures’ resistance to flow improved substantially. The modified samples, containing various percentages of CF, demonstrated a higher resistance by entering the tertiary creep phase at a greater number of cycles. This indicates a delay in the onset of significant deformation, highlighting the effectiveness of fiber modification in enhancing the material’s performance. Furthermore, a detailed comparison of Figs. 15 and 16 reveals that the increase in temperature had a considerable impact on the flow number of all samples. Specifically, it was observed that the flow number decreased for all samples as the temperature increased. This reduction in flow number with rising temperature indicates that higher temperatures reduce the mixture’s resistance to deformation, making it more susceptible to flow. The enhanced performance of the modified samples across different temperatures underscores the potential benefits of using CF in asphalt concrete to improve its durability and longevity under varying environmental conditions.

When tests indicate that ceramic fibers have decreased the likelihood of rutting at elevated temperatures, it implies multiple benefits for the asphalt mixture’s performance in real-world conditions. This improvement suggests better resistance to deformation under heavy traffic loads and high temperatures, ultimately leading to a more durable pavement with a longer lifespan and reduced need for maintenance. This issue also has a significant impact on user costs, as one of the factors affecting vehicle expenses is the smoothness of the road surface. The International Roughness Index (IRI) is recognized as a valid metric in this context. Rutting can greatly reduce this index, leading to increased user costs.

Fig. 15
figure 15

Permanent deformation of glassphalt at 30 °C and 300 kPa stress.

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Fig. 16
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Permanent deformation of glassphalt at 50 °C and 300 kPa stress.

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Figure 17 displays the enduring deformation changes at the end of the test for each sample. At 30 °C, the value of enduring deformation in the control samples at the end of 8000 cycles was 5447 microstrains. This value reduced 3695, 2239, and 1368 microstrains, respectively, due to using 1%, 3%, and 5% CF. These reductions corresponded to 32%, 59%, and 75% reductions, respectively. At 50 °C, the control sample and the sample containing 1% CF could not complete 8000 loading cycles and lost their stability at 6669 and 6552 cycles, respectively. Additionally, at this temperature, the enduring deformation values at the end of 8000 cycles for samples containing 3% and 5% CF were 5584 and 4165 microstrains, respectively, indicating a decrease of 60% and 67% compared to the strains recorded at 30 °C. Overall, the samples modified with 5% CF exhibited the least axial strain and therefore, the highest resistance against rutting in both tested temperatures.

Fig. 17
figure 17

The permanent deformation at the end of the loading period temperature.

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The paired t-test was used to investigate the effect of CF on reducing axial strain in glassphalt specimens. Three groups (before and after intervention) were determined, and the results indicated that using CF significantly reduced the axial strain of glassphalt specimens. In all the comparisons groups, the significance (Sig.) value was less than 0.05, and the null hypothesis was rejected, indicating the significant effect of CF on reducing the axial strain of glassphalt specimens with 10% glass cullet (Table 11).

Table 11 Results of t-tests to examine the impact of CF on the rutting susceptibility of glassphalt.
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Conclusions

This study thoroughly examined the impact of different percentages of CF (1%, 3%, and 5% by bitumen weight) on the stiffness modulus, fatigue life, moisture susceptibility and rutting potential of glassphalt. The most important research findings were as follows:

  1. 1.

    The optimal amount of bitumen decreased in AC samples modified with glass cullet, and with an increase in the percentage of glass cullet from 10 to 20%, this reduction was further increased. The decrease in the optimal bitumen content may be attributed to the properties of glass cullet affecting the mixture, potentially altering its structural integrity, durability, or other performance characteristics.

  2. 2.

    The use of CF, considering their acidic properties, has reduced the acidic characteristics of bitumen and increased its alkaline properties. The non-polar SFE of modified bitumen has significantly increased compared to control samples, forming non-polar bonds with aggregates that do not easily break in the presence of a polar substance like water. Additionally, using CF has increased the cohesive free energy, reducing the likelihood of cohesive failure.

  3. 3.

    The ITS of glassphalt increased with the rise in the percentage of glass cullet due to the increased internal friction angle. Similarly, the ITS of samples modified with CF increased under both wet and dry conditions compared to control samples. The addition of CF increased the viscosity of bitumen, improving the interlock between aggregates and bitumen and consequently increasing their ITS. This likely enhanced the adhesive properties of bitumen, leading to better bonding with aggregates. Furthermore, an improvement in the ITS of glassphalt was observed by increasing the percentage of CF up to 3% by weight of bitumen.

  4. 4.

    Using CF increased the TSR of glassphlt. Therefore, CF-modified glassphalt was more resistant to moisture damage. The consistent improvement in TSR over multiple testing cycles indicated the sustained benefits of CF modification in mitigating moisture-related damage.

  5. 5.

    The stiffness modulus of glassphalt samples made with CF-modified bitumen was higher than control samples at 5, 25, and 40 °C, indicating that using 1% CF positively influenced the stiffness properties of AC across various temperature conditions. The higher stiffness modulus implied increased resistance to permanent deformation and improved structural integrity of CF-modified glassphalt. Additionally, CF-modified glassphalt demonstrated less sensitivity to temperature changes, with enhanced stability of the stiffness modulus, allowing them to maintain their structural integrity and performance across a range of temperatures.

  6. 6.

    The results of repeated axial load testing indicated that utilizing CF increased the resistance of glassphalt against permanent deformation. This finding underscored the beneficial effect of CF on improving the rutting resistance of glassphalt, particularly under repeated loading conditions. By enhancing the glassphalt’s ability to resist permanent deformation at elevated temperatures, CF contributed to the long-term performance and durability of the pavement.

  7. 7.

    The use of CF up to 3% by weight of bitumen has improved the tensile strength of the bitumen and increased the stiffness of the asphalt mixture, resulting in lower tensile strain under indirect tensile loading and an increased fatigue life of glassphalt. At higher concentrations, CFs increase the stiffness of the mixture, enhancing its resistance to deformation under load but simultaneously reducing its ability to flex and accommodate stress, thereby increasing its brittleness. To address this issue, future studies could explore the incorporation of additional modifiers, such as polymers or elastomers, which could potentially improve the flexibility of the glassphalt. These modifiers might help in balancing the material’s stiffness and flexibility, thereby enhancing both its fatigue resistance and overall performance.

  8. 8.

    Based on the results obtained in this research, it can be said that the use of CF improves the performance of glassphalt against moisture damage, rutting, and fatigue cracking. This demonstrates the potential for its use in regions with predominantly moderate and warm climatic conditions. However, future studies should examine the performance of this type of asphalt mixture at low temperatures and with bitumens of different PG. Given the lower cost of CF compared to polymer materials, their field performance can also be evaluated in small-scale projects.