Effect of Crumb Rubber Modifier Particle Size on Storage Stability of Rubberized Binders

Abstract

This research study aimed to assess the influence of different particle sizes of crumb rubber modifier on phase separation when mixed with virgin asphalt binder (PG 64-22). Both Superpave and multiple stress creep recovery (MSCR) tests were conducted to determine the optimal particle size. Three sizes of crumb-rubber particles (≤0.5 mm, ≤1 mm, and 1–2 mm) were individually incorporated into the binder at a weight proportion of 10%. The findings revealed that an increase in particle size resulted in higher viscosity, which reduced workability. However, the use of particles with a size of ≤0.5 mm effectively decreased viscosity. Furthermore, larger particle sizes enhanced resistance to rutting and improved the lifespan of the pavement. Multiple shear creep recovery (MSCR) tests confirmed that larger crumb-rubber particles exhibited a higher load-bearing capacity. Additionally, phase separation studies demonstrated that larger particle sizes were associated with increased phase separation. Notably, particles with a size of ≤0.5 mm performed exceptionally well in reducing phase separation across all combinations. In conclusion, crumb-rubber particles with a size of ≤0.5 mm were identified as the most effective in minimizing phase separation when blended with virgin asphalt binder. These findings provide solid scientific evidence related to the effects of crumb-rubber particles on the storage stability of rubberized asphalt binder and have significant implications for future research in this field.

1. Introduction

Background

Tire recycling has always been a challenging task due to the hazardous nature of the materials used in tire production. When tires are transformed from one form to another, they can become even more dangerous, especially when burned, as they do not biodegrade and can release toxic substances [1,2]. Consequently, a significant number of end-of-life tires end up in landfills, contributing to environmental concerns [3,4,5].

However, in recent years, governments have initiated numerous projects aimed at recycling end-of-life tires. These tires contain valuable materials that can be used to enhance the performance of flexible pavements, as they possess reinforcing capabilities [6,7,8]. Among the various methods available for recycling and incorporating tire properties into hot mix asphalt (HMA) pavements, three commonly used approaches are the dry process, the terminal blend process, and the wet process.

For this research, the focus was on the “wet process”, which involves blending crumb rubber with PG 64-22 binder. The wet process is recognized as one of the most effective remedial methods. By utilizing this process, the performance of the pavement mix can be enhanced, resulting in improvements in resilience, resistance to rutting, fatigue, cracking, and overall pavement life cycle [9,10,11,12,13,14,15,16,17,18,19,20,21].

Although utilizing crumb rubber as a modifier offers numerous advantages, it has been acknowledged that ensuring the storage stability of crumb rubber modifier (CRM) poses a significant challenge due to its poor ability to maintain desired properties over time, particularly during storage and transportation [22,23,24,25]. To tackle this problem, this research focused on determining the optimal particle size of crumb rubber to enhance the storage stability of asphalt binders. The study aimed to evaluate various properties of the binders, including viscosity, viscoelasticity, and multiple stress creep recovery (MSCR). To assess the storage stability, the commonly acknowledged and effective method known as the “cigar tube test method” was utilized [26,27].

By determining the suitable particle size of crumb rubber and studying its effects on the properties of asphalt binders, this research aimed to overcome the storage stability challenges associated with CRMs. The research will contribute to the development of more sustainable and environmentally friendly solutions for incorporating recycled tire materials in pavement applications. Figure 1 and Figure 2 show the sample preparation method and experimental model.

2. Experimental Design

2.1. Materials

The project employed the widely prevalent and commonly used PG 64-22 binder, which is obtained through the fractional distillation of crude oil. Its existence was brought to light in the early years of the Strategic Highway Research Program.

Table 1. Properties of virgin asphalt binder PG 64-22 (base asphalt binder).

Aging States	Test Properties	Test Result
Unaged binder	Viscosity @ 135 °C (cP)	540
	G*/sin δ @ 64 °C (kPa)	1.39
RTFO aged residual	G*/sin δ @ 64 °C (kPa)	3.83
RTFO+PAV aged residual	G*sin δ @ 25 °C (kPa)	4405
	Stiffness @ −12 °C (MPa)	207
	m-value @ −12 °C	0.325

, below, presents the properties of this binder.

The particle size of the rubber granules plays an important role in phase separation. The rubber granules (≤0.5 mm, ≤1 mm, and 1–2 mm) used in this study are produced by mechanical milling at ambient temperature and are listed in

Table 2. Sieve test results of crumb rubber.

Sieve Number	% Cumulative Passed Size ≤ 0.5 mm	% Cumulative Passed Size ≤ 1 mm	% Cumulative Passed Size 1–2 mm
#4	100	100	100
#8	100	100	100
#30	100	39.4	0.6
#50	57.7	12.2	0
#100	14.2	3.7	0
#200	1.8	0.1	0

, below. These granules have wavy textures and are often asymmetrical [28].

2.2. Production and Sampling of Crumb Rubber Modified Asphalt Binders

Previous research conducted by [29] explored the modification of crumb rubber modifiers (CRMs) using a base binder known as PG 64-22. The modification process entailed the combination of crumb rubbers with the binder at a temperature of 178 ± 2 °C over a period of 30 min. To facilitate the mixing process, a low shear mixer was utilized, maintaining a constant mixing speed of 700 rpm.

To maintain consistency, the percentage of colloidal particles in the modified binder was kept constant at 10% of the original binder weight. However, the particle size of the colloidal particles in the crumb rubbers varied in ascending order during each phase of the experiment.

To prepare the specimens, multiple samples were collected immediately after modifying the binder. Two samples were specifically reserved for the rotational viscometer (RV) test, while three samples were allocated for the dynamic shear rheometer (DSR) test. The remaining modified binder was poured into aluminum cigar tubes. These tubes were then positioned in an upright manner and subjected to a precise load of 50 ± 0.5 g, adhering to the guidelines outlined in ASTM D7173 standards [30].

Before each sample was taken, the modified binder was manually stirred and poured into the aluminum tubes. The tubes were then placed in a drying oven and conditioned for 48 h at a temperature of 163 ± 5 °C. After conditioning, the aluminum tubes were transferred to a refrigerator and kept there for a minimum of 4 h, with the refrigerator temperature maintained at 10 ± 10 °C.

Subsequently, the cigar-shaped test tubes were cut into three equal sections: top, middle, and bottom. These sections were placed in an oven set at 163 ± 5 °C until the samples melted. The melted samples were then transferred to a heating plate, where they were continuously stirred using a stainless-steel spatula free from any foreign contaminants. Samples were also collected after this process.

2.3. Evaluation of Binder

2.3.1. Rotational Viscosity (ASTMD4402) [31]

The viscosity properties of the samples were assessed using the Brookfield rotational viscometer. The evaluation involved measuring the samples at two distinct temperatures, namely 135 °C and 180 °C. For this test, a specific spindle, SP-27, was employed, with a consistent rotation speed of 20 RPM.

To ensure accuracy, the weight of each sample used in the tests was maintained within a range of 10 to 11 g. Throughout the experiment, a total of 20 readings were taken at regular 1 min intervals. The viscosity values were measured in centipoise (cp), a standard unit for viscosity measurement.

By utilizing the Brookfield rotational viscometer under controlled conditions, the study aimed to obtain precise and reliable data on the viscosity characteristics of the samples at different temperatures. This approach ensured scientific rigor and facilitated the evaluation and comparison of the samples’ viscosity properties.

2.3.2. Viscoelasticity and MSCR

The DSR (dynamic shear rheometer) machine was employed to assess the viscoelasticity and MSCR (multiple stress creep and recovery) properties of the samples. The measurements were conducted while maintaining a constant temperature of 64 °C. The following parameters were specifically measured: G*/Sin δ (shear storage modulus divided by the sine of the phase angle), percentage recovery at 0.1 kPa, and percentage recovery at 3.2 kPa.

To carry out the tests, a 25 mm geometry was utilized, which included both a spindle and a parallel plate. The gap size between the spindle and the parallel plate was maintained at 2 mm throughout the experiment regardless of different crumb-rubber particle sizes of the modified asphalt binder. This geometric configuration enabled the accurate examination of the samples’ viscoelastic behavior and MSCR characteristics. The choice of a 25 mm geometry ensured compatibility with the DSR machine and facilitated standardized and reliable measurements for subsequent analysis and comparison of the samples.

2.3.3. Separation Index

For the separation index (SI) study, only the top and bottom sections of the aluminum cigar case were taken into consideration. In this analysis, the SI was determined based on Equation (1), which utilizes the G*/Sin δ property. The (G*/Sin δ) max represents the higher G*/Sin δ value obtained from either the top or bottom section, while the (G*/Sin δ) avg denotes the average G*/Sin δ value calculated from both parts.

To further evaluate the separation index ratio, the parameter % recovery from MSCR test was utilized, as per Equation (2). The selection of % recovery as an evaluation metric aligns with established standards and previous research studies such as those by [30,32,33,34,35,36,37,38,39,40]. By comparing the two resulting SI values, this study aimed to determine the effectiveness of the separation index in assessing the performance of the samples. This approach allowed for a scientific evaluation and comparison of the SI results, drawing from previous research and standardized methodologies in the field.

$$\mathrm{Separation} \mathrm{index}=\frac{\left(\mathrm{G}\mathrm{*}/\mathrm{S}\mathrm{i}\mathrm{n}\mathsf{\delta }\right)\mathrm{m}\mathrm{a}\mathrm{x}-\left(\mathrm{G}\mathrm{*}/\mathrm{S}\mathrm{i}\mathrm{n}\mathsf{\delta }\right)\mathrm{a}\mathrm{v}\mathrm{g}}{\left(\left(\mathrm{G}\mathrm{*}/\mathrm{S}\mathrm{i}\mathrm{n}\mathsf{\delta }\right)\mathrm{a}\mathrm{v}\mathrm{g}\right)}$$

(1)

$$\mathrm{Separation} \mathrm{index}=\frac{\left(\%\mathrm{r}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{y}\right)\mathrm{m}\mathrm{a}\mathrm{x}-\left(\%\mathrm{r}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{y}\right)\mathrm{a}\mathrm{v}\mathrm{g}}{\left(\%\mathrm{r}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{y}\right)\mathrm{a}\mathrm{v}\mathrm{g}}$$

(2)

3. Results

3.1. Statistical Analysis Methodology

A static analysis was conducted using the Statistical Package for the Social Sciences (SPSS) (Version 27) software to examine the effective particle size of the crumb rubber. The analysis employed the one-way analysis of variance (ANOVA) method to determine if there were significant differences in the mean values. The significance level, α, was set at 0.05, corresponding to a 95% confidence interval.

To further investigate the data, post-hoc analysis was performed using the least significant difference (LSD) method. The LSD method compares pairs of samples and determines whether the observed difference in population means is statistically significant. In this analysis, a difference between two samples was considered statistically significant if it was equal to or greater than the significance level α, which in this case was set to 0.05.

By employing the one-way ANOVA and LSD methods, this study followed established statistical practices to assess the potential differences in mean values of the crumb rubber’s effective particle size. The choice of these statistical techniques ensured a rigorous analysis and provided scientific justification for drawing conclusions based on the significance of observed differences.

3.2. Rotational Viscosity

Rotational viscosity is a critical test employed to assess the performance of binders. The viscosity of the mixture is a significant factor influencing optimal compaction in field applications. In this study, two temperatures, 135 °C and 180 °C, were investigated, and the results were analyzed using one-way ANOVA and graphical methods. The graphical representation (Figure 3 and Figure 4) of the data revealed two distinct behaviors when the temperature was increased from 135 °C to 180 °C.

Initially, at 135 °C, an increase in the particle size of the crumb rubber modifier (CRM) from 0.5 mm to 1–2 mm resulted in a decrease in viscosity. Furthermore, as the temperature increased to 180 °C, an opposite trend was observed, indicating that lower CRM particle sizes led to lower viscosities. Prior to conditioning, the viscosity values were below 3000 cp, indicating a workable mixture. However, after the conditioning period at 135 °C, only the top and middle portions of the sample remained workable, while the bottom portion exhibited viscosity values exceeding 3000 cp, indicating a non-workable mixture. These findings demonstrated a direct proportionality between increased CRM particle size and viscosity values.

In contrast, as the temperature increased from 135 °C to 180 °C, the viscosity values appeared to decrease. Specifically, the binder modified with CRM particle sizes of 1–2 mm was found to be non-workable, whereas the CRM particle size of ≤0.5 mm exhibited significantly lower viscosity values across all combinations. Statistical analysis confirmed these observations, demonstrating a significant difference in values within the same population, denoted as S in

Table 3. SPSS statistical analysis report of effective particle size of CRM binder for viscosity at 135 °C as a function of top, middle, and bottom parts for (α = 0.05).

Viscosity at 135 °C
Crumb-Rubber Size (mm)		Original			Top			Middle			Bottom
		≤0.5	≤1	1–2	≤0.5	≤1	1–2	≤0.5	≤1	1–2	≤0.5	≤1	1–2
Original	≤0.5	-	N	N	S	S	S	S	S	S	S	S	S
	≤1	-	-	N	S	S	S	S	S	S	S	S	S
	1–2	-	-	-	N	N	N	S	S	S	S	S	S
Top	≤0.5	-	-	-	-	N	S	S	S	S	S	S	S
	≤1	-	-	-	-	-	N	S	S	S	S	S	S
	1–2	-	-	-	-	-	-	S	S	S	S	S	S
Middle	≤0.5	-	-	-	-	-	-	-	N	S	S	S	S
	≤1	-	-	-	-	-	-	-	-	S	S	S	S
	1–2	-	-	-	-	-	-	-	-	-	S	S	S
Bottom	≤0.5	-	-	-	-	-	-	-	-	-	-	S	S
	≤1	-	-	-	-	-	-	-	-	-	-	-	S
	1–2	-	-	-	-	-	-	-	-	-	-	-	-

S—significant; N—non-significant.

and

Table 4. SPSS statistical analysis report of effective particle size of CRM binder for viscosity at 180 °C as a function of top, middle, and bottom parts for (α = 0.05).

Viscosity at 180 °C
Crumb-Rubber Size (mm)		Original			Top			Middle			Bottom
		0.5	1	1–2	0.5	1	1–2	0.5	1	1–2	0.5	1	1–2
Original	≤0.5	-	N	N	S	N	N	S	S	S	S	S	S
	≤1	-	-	N	S	S	S	S	S	S	S	S	S
	1–2	-	-	-	S	S	S	S	S	S	S	S	S
Top	≤0.5	-	-	-	-	N	N	S	S	S	S	S	S
	≤1	-	-	-	-	-	N	S	S	S	S	S	S
	1–2	-	-	-	-	-	-	S	S	S	S	S	S
Middle	≤0.5	-	-	-	-	-	-	-	N	N	S	S	S
	≤1	-	-	-	-	-	-	-	-	N	S	S	S
	1–2	-	-	-	-	-	-	-	-	-	S	S	S
Bottom	≤0.5	-	-	-	-	-	-	-	-	-	-	S	S
	≤1	-	-	-	-	-	-	-	-	-	-	-	S
	1–2	-	-	-	-	-	-	-	-	-	-	-	-

S—significant; N—non-significant.

below.

Overall, these results highlight the intricate relationship between CRM particle size and temperature and viscosity, providing valuable insights for optimizing binder performance in practical applications.

3.3. Viscoelasticity (G*/Sin δ) Property of Modified Binders

The viscoelastic properties of crumb rubber modified binders were assessed using a dynamic shear rheometer (DSR) machine, with a temperature of 64 °C determined as the optimal condition for the study. The analysis revealed a similar relationship between the G*/Sin δ values and viscosity values at 135 °C.

Prior to conditioning, the binder modified with CRM particle sizes of ≤0.5 mm exhibited higher resistance against rutting. However, after conditioning, the binder modified with 1–2 mm CRM particles demonstrated superior effectiveness in mitigating rutting compared to all other combinations as shown in Figure 5. Statistical analysis (

Table 5. SPSS statistical analysis report of effective particle size of CRM binder for viscoelasticity at 64 °C as a function of top, middle, and bottom parts for (α = 0.05).

Viscoelasticity (G*/Sin δ) at 64 °C
Crumb-Rubber Size (mm)		Original			Top			Middle			Bottom
		≤0.5	≤1	1–2	≤0.5	≤1	1–2	≤0.5	≤1	1–2	≤0.5	≤1	1–2
Original	≤0.5	-	N	S	S	S	S	N	S	S	S	S	S
	≤1	-	-	S	S	S	N	N	S	S	S	S	S
	1–2	-	-	-	N	S	S	S	S	S	S	S	S
Top	≤0.5	-	-	-	-	N	S	N	S	S	S	S	S
	≤1	-	-	-	-	-	N	S	S	S	S	S	S
	1–2	-	-	-	-	-	-	S	S	S	S	S	S
Middle	≤0.5	-	-	-	-	-	-	-	S	S	S	S	S
	≤1	-	-	-	-	-	-	-	-	S	N	S	S
	1–2	-	-	-	-	-	-	-	-	-	N	S	S
Bottom	≤0.5	-	-	-	-	-	-	-	-	-	-	S	S
	≤1	-	-	-	-	-	-	-	-	-	-	-	S
	1–2	-	-	-	-	-	-	-	-	-	-	-	-

S—significant; N—non-significant.

) confirmed significant changes in the values when compared within the same population, particularly in the bottom portion of the sample, consistent with the findings observed in the viscosity evaluation.

These results demonstrate the correlation between the viscoelastic properties and rutting performance of crumb rubber modified binders. The binder with smaller CRM particle sizes displayed improved resistance against rutting before conditioning, while the binder modified with larger CRM particle sizes exhibited enhanced resistance to rutting after conditioning. The statistical analysis further supports the significant differences observed in the values, emphasizing the importance of particle size in optimizing the performance of crumb rubber modified binders.

3.4. Multiple Stress Creep Recovery Property of Modified Asphalt Binders

The evaluation of percentage recovery in crumb rubber modified binders using the dynamic shear rheometer (DSR) equipment at a constant temperature of 64 °C, following the AASHTO TP 70 standard [41], provided scientific insight into the material’s behavior and performance.

The observed increase in percentage recovery as the particle size of the crumb rubber modifier (CRM) increased can be explained by the physical properties of the binder system. Larger CRM particles tend to contribute to a more effective interlocking network within the binder matrix. This improved interlocking can enhance the binder’s ability to recover its original shape and structure after deformation, leading to higher percentage-recovery values. The presence of larger CRM particles facilitates better mechanical interlocking, resulting in a more resilient material response.

Conversely, smaller CRM particles may not provide the same level of interlocking, leading to lower percentage-recovery values. Decreasing the particle size while maintaining the same surface area results in a higher number of contact points within the binder matrix. This, in turn, leads to an increase in voids and a reduction in the capacity for recovery. This can be justified by considering that smaller particles have a greater tendency to pack closely together, creating more interparticle spaces that are difficult to fill with the binder material. Consequently, the increased number of voids reduces the overall recovery capacity of the material.

The effect of load on the percentage-recovery values can be attributed to the applied stress level during testing. Higher loads, such as the increase from 0.1 kPa to 3.2 kPa, impose greater shear stress on the binder material. This increased stress level can cause more permanent deformation and reduce the binder’s ability to recover its original shape, resulting in lower percentage-recovery values as shown in Figure 6 and Figure 7 and justified through statistical analysis (

Table 6. SPSS statistical analysis report of effective particle size of CRM binder for % recovery at 64 °C for 0.1 kPa as a function of top, middle, and bottom parts for (α = 0.05).

% Recovery for 0.1 kPa at 64 °C
Crumb-Rubber Size (mm)		Original			Top			Middle			Bottom
		≤0.5	≤1	1–2	≤0.5	≤1	1–2	≤0.5	≤1	1–2	≤0.5	≤1	1–2
Original	≤0.5	-	N	S	S	S	S	S	S	S	S	S	S
	≤1	-	-	S	S	S	S	S	S	S	S	S	S
	1–2	-	-	-	S	S	S	S	S	S	S	S	S
Top	≤0.5	-	-	-	-	N	N	S	S	S	S	S	S
	≤1	-	-	-	-	-	N	S	S	S	S	S	S
	1–2	-	-	-	-	-	-	S	S	S	S	S	S
Middle	≤0.5	-	-	-	-	-	-	-	S	S	S	S	S
	≤1	-	-	-	-	-	-	-	-	S	S	S	S
	1–2	-	-	-	-	-	-	-	-	-	S	N	N
Bottom	≤0.5	-	-	-	-	-	-	-	-	-	-	S	S
	≤1	-	-	-	-	-	-	-	-	-	-	-	S
	1–2	-	-	-	-	-	-	-	-	-	-	-	-

S—significant; N—non-significant.

and

Table 7. SPSS statistical analysis report of effective particle size of CRM binder for % recovery at 64 °C for 3.2 kPa as a function of top, middle, and bottom parts for (α = 0.05).

% Recovery for 3.2 kPa at 64 °C
Crumb-Rubber Size (mm)		Original			Top			Middle			Bottom
		≤0.5	≤1	1–2	≤0.5	≤1	1–2	≤0.5	≤1	1–2	≤0.5	≤1	1–2
Original	≤0.5	-	N	N	S	S	S	S	S	S	S	S	S
	≤1	-	-	N	S	S	S	S	S	S	S	S	S
	1–2	-	-	-	S	S	S	S	S	S	S	S	S
Top	≤0.5	-	-	-	-	N	S	S	S	S	S	S	S
	≤1	-	-	-	-	-	N	S	S	S	S	S	S
	1–2	-	-	-	-	-	-	S	S	S	S	S	S
Middle	≤0.5	-	-	-	-	-	-	-	S	S	S	S	S
	≤1	-	-	-	-	-	-	-	-	S	N	S	S
	1–2	-	-	-	-	-	-	-	-	-	S	N	S
Bottom	≤0.5	-	-	-	-	-	-	-	-	-	-	S	S
	≤1	-	-	-	-	-	-	-	-	-	-	-	S
	1–2	-	-	-	-	-	-	-	-	-	-	-	-

S—significant; N—non-significant.

).

The significant changes observed in the bottom portion of the sample compared to the original sample and the top and middle portions can be attributed to potential variations in binder homogeneity or localized effects within the sample. Factors such as uneven distribution of CRM particles or variations in compaction density may lead to different material responses in different sample regions. These variations can affect the percentage-recovery values, highlighting the importance of considering sample uniformity in performance evaluation.

Overall, the evaluation of percentage recovery in crumb rubber modified binders is justified by the physical properties of the binder system, the interlocking behavior of CRM particles, the applied stress levels, and potential sample heterogeneity. These insights contribute to a better understanding of the binder’s recovery capacity and its performance in various applications.

3.5. Storage Stability Results

The storage stability evaluation of the modified crumb rubber asphalt with different particle sizes (≤0.5 mm, ≤1 mm, and 1–2 mm) was conducted using the Superpave test method and the multiple stress creep recovery (MSCR) test. In this evaluation, the focus was on analyzing the G*/Sin δ and % recovery values of the top and bottom parts of the conditioned samples to determine the separation index percentage.

To assess the storage stability, the viscosity test results were not considered in this analysis as they did not provide significant insights for evaluating the effect of crumb-rubber particle size. Instead, the emphasis was placed on G*/Sin δ and % recovery values, which are key parameters in determining the separation between the asphalt-rich phase and rubber-rich phase.

Table 8. Separation index from G^*/sin δ of CRM binders.

CRMB Size (mm)	G*/Sin δ (kPa)
	Temperature	Top	Bottom	% Separation
≤0.5	64 °C	4.25	9.39	38
≤1	64 °C	3.58	11.63	53
1–2	64 °C	2.59	17.16	74

,

Table 9. Separation index from % recovery for 0.1 kPa of CRM binders.

CRMB Size (mm)	% Recovery
	Temperature	Top	Bottom	% Separation
≤0.5	64 °C	10.46	79.11	77
≤1	64 °C	12.10	89.70	76
1–2	64 °C	4.71	97.93	91

and

Table 10. Separation index from % recovery for 3.2 kPa of CRM binders.

CRMB Size (mm)	% Recovery
	Temperature	Top	Bottom	% Separation
≤0.5	64 °C	3.74	27.73	76
≤1	64 °C	2.76	34.74	85
1–2	64 °C	0.60	53.50	98

present the results obtained from the G*/Sin δ and % recovery measurements. The findings indicate that the separation index percentage was directly proportional to the particle size of the crumb rubber. Specifically, the crumb rubber with a particle size of ≤0.5 mm demonstrated lower separation between the asphalt-rich phase and rubber-rich phase compared to the other particle-size combinations.

This observation suggests that smaller crumb-rubber particles, such as those of ≤0.5 mm, are more effectively dispersed and integrated within the asphalt matrix, resulting in improved storage stability. On the other hand, larger particle sizes may lead to a higher tendency for phase separation, potentially causing issues with storage stability.

These findings provide scientific justification for the relationship between crumb-rubber particle size and storage stability, highlighting the importance of considering particle size when modifying asphalt with crumb rubber.

4. Summary and Conclusions

Based on the conducted laboratory experiments, the effects of crumb-rubber particles on the storage stability of rubberized asphalt binder were evaluated. The binder was prepared using 10% crumb rubber by weight of the fresh binder, with the base binder being PG 64-22. Three different particle sizes of crumb rubber were investigated: ≤0.5 mm, ≤1 mm, and 1–2 mm. The samples were taken before and after conditioning, and various tests were performed to assess the binder’s properties.

Based on the experimental results, the following conclusions can be drawn:

• Workability and Field Compaction: The observation that the binder modified with 1–2 mm crumb rubber remained difficult to work with, even at high temperatures, suggests the need for adjustments in field operations. Increasing the plant temperature can help improve the workability of the mix and facilitate proper field compaction. This recommendation aligns with practical considerations for asphalt paving, where achieving adequate workability and compaction is essential for ensuring the long-term performance of the pavement.
• Rutting Resistance: The higher resistance to rutting exhibited by the binder modified with 1–2 mm crumb rubber is a valuable finding for field applications. Rutting is a significant concern in asphalt pavements, particularly in areas with high traffic loads and hot climates. The use of larger crumb-rubber particles can contribute to improved rutting resistance, leading to enhanced pavement durability and reduced maintenance needs. This aligns with established knowledge in the field, as larger crumb-rubber particles provide greater reinforcement and stiffness to the binder matrix.
• Binder Deterioration and Recovery: The observed decrease in percentage-recovery values with increasing load cycles indicates binder deterioration under higher stress conditions and binder modified with particles of 1–2 mm was found to be most effective. This finding highlights the importance of considering the potential effects of repeated loading on the long-term performance of rubberized asphalt pavements. It reinforces the need for appropriate mix design and compaction practices to minimize binder degradation and optimize the pavement’s ability to recover from deformation.
• The findings regarding particle size and temperature effects on the separation index (SI%) provide insights into the key factors influencing the binder’s performance. These insights can guide the selection of crumb-rubber particle size and appropriate production temperatures during field operations. The study’s conclusion that a higher particle size and a higher proportion of crumb rubber may not be necessary aligns with practical considerations, as excessive crumb rubber size and high rubber contents could potentially impact the binder’s workability and other performance aspects.

In conclusion, the findings are bolstered by considering their implications for field operations. Recommendations regarding workability, field compaction, rutting resistance, and binder deterioration provide practical guidance for optimizing the performance of rubberized asphalt pavements. By taking these factors into account, engineers and practitioners can make informed decisions when incorporating crumb-rubber particles into asphalt binders, ultimately leading to more sustainable and durable pavement solutions.