Optimization of CLS Content of Asphalt Mixtures Based on Balanced Mix Design Approach

Ziaee, Seyed Ali; Fatemi, Seyed Saeed; Shaker, Hamid; Ameri, Mahmoud; Saadatjoo, Seyed Amir

doi:https://doi.org/10.1155/2023/7594186

Advances in Civil Engineering

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Abstract Introduction Results and Discussion Conclusion Data Availability Conflicts of Interest Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2023 | Article ID 7594186 | https://doi.org/10.1155/2023/7594186

Optimization of CLS Content of Asphalt Mixtures Based on Balanced Mix Design Approach

Seyed Ali Ziaee,¹Seyed Saeed Fatemi,¹Hamid Shaker,²Mahmoud Ameri,²and Seyed Amir Saadatjoo²

Academic Editor: Nur Izzi Md. Yusoff

Received07 Jan 2023

Revised07 Apr 2023

Accepted17 Apr 2023

Published03 May 2023

Abstract

Over the years, there have been many studies on the performance improvement of asphalt mixtures using different materials and additives, especially those of the recycled variety, because of the environmental benefits they bring to the design. This study investigated the properties of asphalt mixtures made with different amounts of calcium lignosulfonate (CLS), which is a waste product, and used the Balanced Mix Design (BMD) approach to determine the appropriate mix design for CLS-containing mixtures. In this process, the results of the dynamic creep test were used as a measure of rutting resistance, the results of the indirect tensile test were used as a measure of moisture susceptibility, and the results of the Illinois Flexibility Index Test (IFIT) and Indirect Tensile Cracking Test (IDEAL-CT) were used independently as a measure of cracking resistance. On the other hand, since previous studies have suggested that CLS has antiaging properties, the specimens were made and tested in two groups: those with long-term aging and without long-term aging. So the objective was to determine the amount of CLS that should ideally be used as a bitumen additive in aged and unaged asphalt mixtures. The results showed that using 15% or 20% CLS will always yield acceptable outcomes in terms of BMD criteria for both aged and unaged conditions. However, a CLS content of 10% may also be acceptable for both aged and unaged conditions if IDEAL-CT results are used as the measure of cracking resistance.

1. Introduction

Roads are a primary type of transportation infrastructure that plays a fundamental role in making and keeping destinations accessible. An important part of any road that tends to have a significant impact on its quality and comfort is the pavement. Road pavements can be made with a variety of mixtures, among which the top choice is asphalt mixtures because of their good performance, comfort, and ease of construction and maintenance [1]. Asphalt mixtures consist of two main components: bitumen and aggregate. Naturally, any change in the properties of these two components will affect the properties of the produced mixture. Although bitumen constitutes a relatively smaller portion of an asphalt mixture, its modification can have great impacts on many properties and potential failures of the mixture [2].

The properties of bitumen can change with temperature and time. Because of this dependence, it is crucial to keep track of a phenomenon called aging, which refers to changes in certain properties of asphalt mixtures during their construction and service life [3]. Bitumen aging is studied at two levels: short-term and long-term. Short-term aging occurs at high temperatures during the production, transportation, spreading, and compaction of asphalt mixtures. Long-term aging, however, takes place inside the asphalt over time. The aging process generally involves the transformation of bitumen’s polar and soluble compounds to asphaltene, which will result in reduced bitumen mobility and greatly increased stiffness of the asphalt mixture [4–6]. Eventually, these changes in the constituent parts of the asphalt mixture cause the pavement to undergo fatigue fracture under traffic loads [7].

Over the years, researchers have tried to improve the properties of bitumen with a variety of additives, which can be generally classified into four groups: polymer-based materials, nanomaterials, recycled materials, and biological materials [8–11]. Some researchers have used a class of materials called lignin to improve the properties of bitumen and the resulting mixtures. Lignins can be divided into several types in terms of structure. These include organosolv lignin, which is obtained from wood processing by the use of liquid organic solvents to treat wood chips [12]; Klason lignin, which is produced from the acid hydrolysis of wood [13]; and sulfonated lignin, which is a nontoxic polymer derived from black liquor [14]. Calcium lignosulfonate (CLS) is a brownish powdery amorphous polymer composed of phenylpropane units, which is created as a byproduct in the sulfite pulping process for producing paper from softwood. CLS is generally disposed of as waste, but its improper disposal can cause environmental issues [15].

The first studies on the properties of CLS-containing asphalt mixtures showed an improvement in rutting resistance at CLS contents of 5 to 10 percent. Subsequent studies on this subject found that CLS also increases the viscosity of the bitumen and the brittleness of the mixture [16]. In a study by Batista et al. where kraft lignin was used to modify the bitumen of asphalt mixtures, they reported that the modified specimens showed higher rutting resistance and also higher toughness at low temperatures [17]. A study by McCready and Williams confirmed that CLS increases rutting resistance at high temperatures, but reported slightly decreased performance at low temperatures [18]. In the studies conducted by Fatemi et al. on the modification of bitumen with 5, 10, 15, and 20% CLS, all of these concentrations of CLS exhibited antiaging properties, and the 15% concentration offered the greatest rutting resistance [15, 19, 20]. In a series of studies by Zarei et al. on various properties of CLS, including cracking resistance and moisture susceptibility under different numbers of freeze-thaw cycles in the presence of reinforcement materials such as fibers, the results showed that CLS can improve fracture toughness under multiple loading modes at various temperatures. These researchers stated that CLS can be considered a viable option for increasing the cracking resistance of asphalt mixtures. In these studies, it was stated that CLS can also improve stripping resistance by increasing hardness [21–24].

Given the rapid development of new materials and additives for use in asphalt mixtures, the deficiencies of ordinary performance tests and methods for mix design, and the complexities of the superpave method, the field can always benefit from new mix design methods for asphalt mixtures. Also, with the increasing interest in the use of recycled materials and new technologies in the production of asphalt mixtures, conventional mix design methods have found it increasingly challenging to keep up with these developments.

According to recent studies, to make sure of the reliable performance of asphalt mixtures, one should carefully examine the resistance of these mixtures to fatigue-induced rutting and cracking and select the appropriate mix design accordingly. This consideration is the foundation of the mix design approach called balanced mix design (BMD).

Although rutting and cracking are the two main types of failure in asphalt pavements, other types of failure including moisture damage must also be considered in the design of these pavements. BMD is capable of considering these other factors in the mix design. In this study, the dynamic creep test was used to assess rutting resistance, the indirect tensile test was used to assess moisture susceptibility, and two separate approaches, namely, IDEAL-CT and IFIT were used to assess fatigue cracking at moderate temperatures.

Accordingly, in this research, due to the importance of aging in asphalt pavement, for the first time, the effect of aging on the performance characteristics of asphalt mixtures containing CLS has been investigated. On the other hand, according to the approach of this research, which is optimization based on BMD, the aging characteristics of the mixtures containing CLS were investigated for the first time using the fatigue cracking test, and these results were compared with the IFIT test.

2. Objectives and Scope

In this study, the objective was to determine the amount of CLS that should ideally be used as a bitumen additive in asphalt mixtures. This objective was pursued by making asphalt mixture specimens with CLS contents of 0, 5%, 10%, 15%, and 20% by bitumen weight and subjecting them to dynamic creep test, moisture susceptibility test, Illinois Flexibility Index Test (IFIT), and the Indirect Tensile Cracking Test (IDEAL-CT). Since previous studies have suggested that CLS has antiaging properties in bitumen, the asphalt specimens to be tested were made in two groups: with long-term aging and without long-term aging. The optimum CLS content was determined by the use of the Balanced Mix Design (BMD) method. In this process, the results of the two fatigue cracking tests (IFIT and IDEAL-CT) were used independently as measures of cracking resistance; the results of the dynamic creep test were used as a measure of rutting resistance; and the results of the indirect tensile test and AASHTO-T283 tests were used as a measure of moisture susceptibility. This process involved first determining the acceptable range of results for each test according to past studies and then determining the optimum CLS content with the help of experimental graphs and tables.

3. Experimental Procedure

3.1. Materials

This study used bitumen PG64-22 with the specifications given in Table 1. Using the Superpave method, the optimum bitumen content for the control specimen was determined to be 4.8%. The CLS-modified specimens were also made with the same optimum bitumen content in order to measure how much CLS affects the properties of the mixtures.

3.1.1. Bitumen Additive

In this study, bitumen was modified with CLS in amounts of 0, 5, 10, 15, and 20%. Bitumen and CLS were mixed in a high-speed shear mixer. The CLS used in this study was a light yellow powder produced by LignoTech (South Africa) with the specifications given in Table 2, which was passed through a No. 100 sieve (0.15 mm). An image of this powder is displayed in Figure 1(a). A Field Emission Scanning Electron Microscopy (FESEM) image of this CLS is displayed in Figure 1(b), which shows the rough morphology of CLS particles. To make sure of the complete dissolution of CLS, the authors examined the FESEM images of the unmodified bitumen (without CLS) and the bitumen with the highest CLS content (i.e., 20%), which are presented in Figure 2. As these figures indicate, CLS was found to be completely and uniformly dissolved in bitumen.

(a)

(b)

(a)

(b)

3.1.2. Aggregates

The aggregates used in this study were acquired from a quarry near Tehran. These aggregates were of limestone type with specifications given in Table 3.

The aggregates were obtained with the gradation shown in Figure 3 and the upper and lower limits specified in the Iran Highway Asphalt Paving Code. The maximum nominal size of the aggregates was 12.5 mm.

3.2. Methods

The aggregates were placed in an oven for 16 hours to dry and then mixed with bitumen in a suitable mixer. The resulting mixture was stored for 4 hours at a compaction temperature of ± 3°C as per the instructions of AASHTO-R30 for short-term aging [25], which was necessary for performance tests. The mixing and compaction temperatures for CLS-containing specimens were determined according to ASTM-D4402.

The four tests used to examine the behavior of asphalt mixtures and determine the optimum CLS content were the dynamic creep test, the indirect tensile strength test, the Indirect Tensile Cracking Test (IDEAL-CT), and the Illinois Flexibility Index Test (IFIT), which were conducted on the specimens containing different amounts of CLS.

3.2.1. Aging

Since previous studies have suggested that CLS has antiaging properties, the tests were also performed on a group of specimens subjected to long-term aging to assess the validity of this claim. For this aging, the specimens were subjected to a temperature of 85°C for 120 hours.

3.2.2. BMD

The basic principle of BMD is to use the thresholds of acceptability for test results to determine whether or not a mix design is acceptable. In this study, it was decided to use the following thresholds in the method: FN > 740 for the dynamic creep test [26], TSR > 80% for the moisture susceptibility test [27], CT_index > 35 for IDEAL-CT in the aged condition and CT_index > 70 for IDEAL-CT in the unaged condition [28], and FI > 4 for IFIT in the aged condition and FI > 8 for IFIT in the unaged condition [29].

3.2.3. Dynamic Creep Test

Rutting resistance, which is a common performance characteristic of asphalt mixtures, is typically measured by the creep test. In this study, the dynamic creep test as per AS2891 was used to investigate the rutting resistance of the mixtures modified with different amounts of CLS at high temperatures. In this method, the resistance of asphalt mixtures against permanent deformations is measured in terms of flow number (FN).

The specimens used in this test were made as instructed in the abovementioned standard with a diameter of 100 mm, a height of 50 mm, and a void ratio of 5%. These specimens were kept for 5 hours at a temperature of 50°C and then placed in a universal testing machine (UTM25) and subjected to a haversine load of 400 kPa with 0.5-second loading and 1.5-second unloading cycles. The test output (flow number) was determined from the number of cycles needed for the specimen to enter the third region of the cumulative strain curve as shown in Figure 4 [30].

In order to achieve this goal, in this experiment, by drawing a graph of strain rate against the completed cycles, the value of the cycle equivalent to the minimum strain rate (the desired output is provided by the UTM software) is considered as the value of the cycle entering the third area.

3.2.4. Indirect Tensile Test

In this study, the Indirect Tensile Test (ITS) was used to assess the moisture susceptibility, tensile strength, and hardness of asphalt mixtures made with different amounts of CLS. Six specimens were made as per AASHTO-T283 for moisture susceptibility assessment. These six specimens were divided into two groups of three, one group for conditioned testing and the other for unconditioned testing. All six of these specimens were made as instructed in the aforementioned standard with a diameter of 100 mm and the void ratio of 7 ± 0.5% using a gyratory compactor. The specimens of the first group (unconditioned) were kept at a temperature of 25°C for at least 120 minutes. The specimens of the second group (conditioned) were saturated to 55–80% and kept at a temperature of −18°C for 16 hours, then at a temperature of 60°C for 24 hours, and finally at a temperature of 25°C for 120 minutes. Both groups of specimens were subjected to a constant tensile load at a rate of 50 mm/min to create a relatively uniform tensile stress in the plane normal to the loading direction [31]. For this test, failure was defined as the appearance of cracks and fissures in the loading plane. The output of this test was the maximum load applied to the specimen before failure, from which the indirect tensile strength was determined using.

In this equation, D is the diameter of the specimen (mm), t is the thickness of the specimen (mm), is the maximum load applied to the specimen (kN), and ITS is the indirect tensile strength. In the end, the ITS values obtained for the three specimens of each group were averaged to obtain a single ITS for that group.

Having ITS for conditioned and unconditioned states, moisture susceptibility as measured by TSR was obtained by dividing the conditioned ITS by the unconditioned ITS.

3.2.5. Indirect Tensile Cracking Test

One of the two methods used in this study to examine cracking resistance was the indirect tensile crack test (IDEAL-CT). This test was carried out as per ASTM-D8225 to assess resistance to cracking at moderate temperatures. The main output of this test was CT_index, which represents the cracking resistance of the specimens at moderate temperature (25°C). A higher CT_index indicates a higher resistance against cracking.

The specimens used in this test were made with a diameter of 150 mm, a height of 62 mm, and a void ratio of 7 ± 0.5 using a gyratory compactor. For each required mode of testing, three specimens were made and their results were averaged. Before starting the test, the specimens were kept at a temperature of 25°C for 120 minutes. After this time, the specimens were placed in UTM25 and subjected to a load of 10 N at a rate of 50 mm/min. In the end, CT_index was determined from the following equation:

In this equation, CT_index is the cracking tolerance index, G_f is the failure energy (Joules/m²), |m₇₅| is the absolute value of the postpeak slope (N/m), l₇₅ is the displacement at 75% of the peak load after the peak (mm), D is the diameter of the specimen (mm), and t is the thickness of the specimen (mm). The values of the above parameters in CT_index calculations are given in Figure 5.

3.2.6. Illinois Flexibility Index Test (IFIT)

In addition to IDEAL-CT conducted at a temperature of 25°C, Illinois Flexibility Index Test (IFIT) was also used to assess the cracking resistance of the specimens with different amounts of CLS at a moderate temperature. The outcomes of using the results of these two methods in BMD are discussed and compared later in the paper.

The investigation of cracking resistance has been used in connection with the balanced mix design method, and the output criteria of both tests have been compared in the end.

This test was performed according to AASHTO-TP124. For this test, first, cylindrical specimens with a diameter of 150 mm and a height of 160 mm and a void ratio of 7 ± 1% were made with a gyratory compactor. These cylindrical specimens were then cut across the cross section to create SCB specimens with a diameter of 150 mm and a thickness of 50 mm. Finally, a notch with a thickness of 2.25 mm and a length of 15 ± 1 mm was created in the middle of these specimens. For each mode of testing, four specimens were made and tested and their results were averaged. For the test, the specimens were placed at a temperature of 25 ± 0.5°C for 120 minutes and then subjected to a 1 bar load at a rate of 50 mm/min. The output of this test was the Flexibility Index (FI), which is a function of the fracture energy (G_f) and the slope at the inflection point of the postpeak portion of the load-displacement curve (|m|) as formulated in the following equation:

In this equation, G_f is the fracture energy (Joules/m²), |m| is the absolute value of the postpeak slope (kN/mm), and A is a unit and scale conversion parameter given in the AASHTO-TP124. Figure 6 shows an overview of the parameters that affect the determination of FI in the load-displacement diagram.

To clarify the stages of the research and the experiments, the research process is shown in Figure 7. Also in Table 4 the distribution of the number of samples made for each test for laboratory conditions and each CLS percentage is shown. A total of 150 samples have been made for this research.

4. Results and Discussion

4.1. Dynamic Creep

In this study, the dynamic creep test was used to assess the rutting resistance of asphalt mixtures as a measure of performance in BMD. Figure 8 presents the final results of this test for the aged and unaged specimens with different amounts of CLS. The output of this test is the number of cycles required to reach the third phase of the cumulative strain curve, which is called the flow number (FN). There are two interesting points in the results of this test: one for aged specimens and another for unaged specimens.

In the unaged specimens, FN (and therefore rutting resistance) increased as CLS content increased from 0 to 15%. This increase was quite significant in the specimen containing 15% CLS (compared to the control specimen). However, the specimen with 20% CLS earned a 14% lower FN than the one with 15% CLS. In this regard, multiple studies [11, 15, 16, 34] have reported that adding up to 15% CLS to bitumen increases its hardness and consequently the hardness of the mixture. But adding higher amounts of CLS (>15%) has the opposite effect on FN. The results of the dynamic creep tests of this study suggest that there is some potential for CLS to increase rutting resistance, which is consistent with the findings of [34], regarding lower CLS contents. These findings also indicate a high level of consistency between the results of the dynamic creep test and those of the wheel tracking test, which is another widely accepted test for measuring rutting resistance, because the studies of Jiao et al. and Fatemi et al. also reported improved outcomes following the use of such CLS contents [15, 34].

In the aged specimens, aging changed the hardness of the mixtures and increased their FN (compared to the unaged specimens). This increase was more pronounced in some specimens (those with 5 to 15% CLS) but was more limited in the specimens with 20% CLS.

4.2. Indirect Tensile Test

The results of the indirect tensile test are presented in Figure 9 and the results of the moisture susceptibility test as indicated by TSR are presented in Figure10. ITS has a direct relationship with fracture toughness (it increases as the mixture’s fracture toughness increases) and is also related to the hardness of the asphalt mixture and whether it has been increased or decreased by additives. As Figure 9 shows, the results of this test should be examined in two parts, aged and unaged, and these two sets of findings should then be compared. Among the unaged specimens, there was a significant difference between those containing CLS and those without CLS (control) in terms of both conditioned and unconditioned ITS. The reason for this difference can be found in the test results reported for CLS-containing bitumen. In the study of Fatemi et al. on specimens containing different amounts of CLS, there was a clear increase in the hardness of bitumen modified with CLS, leading to the increased hardness of the mixture. Using a similar approach, Zare et al. also reported an increase in ITS with an increase in CLS content [15, 19–24].

The interesting finding of this study regarding the relationship of aging with unconditioned and conditioned ITS is that in the unaged specimens, ITS increased when CLS content was increased from 15% to 20%, but it had the opposite trend in the aged specimens. As partly explained in previous studies [15], this is because in the specimen with 20% CLS, while CLS itself does increase the hardness of bitumen, this effect is not strong enough to overcome aging-induced hardness reduction, which results in increased brittleness of the mixture. This issue is not clearly reflected in ITS results but is more evident in TSR results.

Another important result in this diagram is the rate of changes in ITS (i.e., how much ITS has increased with each increase in CLS content in aged and unaged specimens), which may provide some insights since CLS content was increased at regular 5% steps. The result of this comparison for unconditioned and conditioned specimens is presented in Figure 11. In this diagram, the initial change for the control specimen is assumed to be zero. This figure shows that for both aged and unaged specimens, adding 5% CLS has caused a significant change in both conditioned and unconditioned ITS. In the unaged specimens, this change is almost uniform across the board. But in the aged specimens, this change is only uniform for CLS contents of up to 15% because ITS has dropped significantly with a further increase in CLS content (>15%).

Another important result of this test is TSR representing the moisture susceptibility of the specimens. This parameter was calculated as a percentage by dividing wet ITS by dry ITS. The TSR results of the specimens are presented in Figure 10. The minimum TSR according to the standard is 80%. The results show that the addition of CLS improved the moisture susceptibility of the asphalt mixture. In both aged and unaged specimens, this improvement had an increasing trend in CLS contents of 0–15%. At higher CLS contents, however, (15–20%), this improvement had an almost constant trend in the unaged specimens and a sharply decreasing trend in the aged specimens. As with the ITS results, this can be attributed to CLS-induced hardness upsurge being overshadowed by aging-induced hardness reduction. This is more clearly evident in the TSR diagram than in the one for ITS. Overall, the aged and unaged specimens that were unmodified and the unaged specimen containing 5% CLS earned substandard results in terms of ITS and TSR.

4.3. Indirect Tensile Cracking Test

As stated earlier in the paper, this study used both CT_index and IFIT for fatigue cracking assessment. This section presents the results related to CT_index. IDEAL-CT is highly regarded by researchers and DOTs for the simplicity of building specimens, not requiring any cutting, and ease of tests [35]. The results of this test for the unaged specimens with different CLS contents are presented in Figure 12. As this diagram shows, adding CLS increased the CT_index, which means an improvement in fatigue cracking resistance. This improvement increased as CLS content increased, ultimately peaking at CLS contents of 15% and 20%. With these two CLS contents, the CT_index of the specimens was almost twice that of the control specimen (without CLS), which is very promising. This improvement can be attributed to the increased hardness of the mixture because of the addition of CLS, which has been able to improve CT_index.

The indirect tensile crack tests were also performed on the aged specimens with different amounts of CLS. The results of these tests are presented in Figure 13. These results clearly show a reduction in the CT_index of the aged specimens compared to their unaged counterparts. These results also confirm the previous reports regarding the antiaging properties of CLS, as both aged and unaged specimens with different amounts of CLS had a lower CT_index than the control specimen (without CLS). These results also show that CT_index increased as CLS content increased from 0 to 15%, which indicates an increase in cracking resistance, but dropped when CLS content was increased to 20%.

To show the effect of aging on the performance of asphalt mixtures, the ratio of CT index for aged to unaged samples is shown in Table 5. As shown, in all additive percentages, the index value is always below 1, which confirms that aging reduces the CT index. However, the addition of CLS has reduced the effect of aging on the reduction of the CT ratio, and on the other hand, this effect has decreased when the percentage of CLS exceeds 15%.

4.4. Illinois Flexibility Index Test

The Illinois flexibility index test is the most reliable and widely used method for assessing cracking behavior at moderate temperatures. Like IDEAL-CT, this test was also used to examine the cracking resistance for BMD. The output of this test is FI, which indicates the overall resistance of the asphalt mixture against cracking at moderate temperature. A higher FI generally indicates a higher longer-term resistance to crack propagation under tensile loading. Hence, this index can be used to rate the asphalt mixtures in terms of resistance to fatigue cracks under tensile loads [35]. The results of this test for the unaged specimens with different CLS contents are presented in Figure 14. The threshold of acceptable values for BMD was also determined from this diagram. As this diagram shows, adding 5% CLS did not make any change in FI, but using higher CLS contents increased this parameter. While the 10% CLS content met the requirements of BMD, higher CLS content surpassed the limits of this method. The interesting difference between this diagram and the diagrams of other tests was the increase in FI as CLS content increased from 15% to 20%.

The results of IFIT for the aged specimens are presented in Figure 15. As can be seen, the addition of CLS had an antiaging effect, which is reflected in the lower reduction of FI in the CLS-containing specimens. While the unaged specimen could not meet the minimum requirements of BMD in terms of FI, the aged specimen with 10% CLS managed to meet these requirements. Another interesting result in this diagram is the significant similarity of the results for the specimens with 15% and 20% CLS.

As a supplementary investigation, the authors also compared the results of fatigue cracking tests at 25°C. The results of this comparison for the aged and unaged specimens are presented in Figure 16. As these results clearly show, there is a significant linear relationship between these two parameters.

(a)

(b)

4.5. Optimization of CLS Content Based on BMD

In this part of the study, the principles of BMD were used to determine the optimum CLS content based on rutting resistance (according to the dynamic creep test and FN), moisture susceptibility (according to the relevant standard and TSR), cracking resistance (according to the FI and CT_index) and the considered thresholds for each index. Out of all results obtained for the five investigated CLS levels (0, 5%, 10%, 15%, and 20%), two groups of results, one based on FN, TSR, and FI and the other based on FN, TSR, and CT_index were considered. In each group, the results for two subgroups, one for the aged specimens and the other for the unaged specimens were examined. These results are presented in Table 6.

In Table 4, the highlighted results could not meet the minimum requirements of BMD. In this step, any mix design that had at least one result outside the acceptable range was excluded (these are highlighted in the mixtures column). As the results of the above table show, mix designs with 15 and 20% CLS are an appropriate choice for all conditions and criteria. If the goal is to gain the best possible properties with the minimum amount of additive, it is better to use 15% CLS. A noteworthy point regarding mix designs with 5% and 10% CLS contents (especially the latter) is that although they have not been rated as acceptable mix designs in terms of FI in the aged state, they have met the requirements of BMD in the aged condition. This can only be attributed to the antiaging property of CLS, which has made it possible for the mix design to work in an aged state. In Figure 17, the results of the above process are presented in a three-dimensional diagram.

5. Conclusion

In this study, the performance characteristics of asphalt mixtures made with different amounts of CLS were investigated, and the BMD approach was used to determine appropriate mix designs for CLS contents of 0–20%. Since previous studies have suggested that CLS has antiaging properties, the specimens were made and tested in two groups: one with aging and the other without aging. For BMD, the dynamic creep test was used to assess rutting resistance; the indirect tensile test and TSR were used to assess moisture susceptibility; and CT_index and Illinois flexibility index were used to assess cracking resistance. A summary of findings is presented.(i)The use of up to 15% CLS in the mixtures improved their rutting resistance, but using more CLS in the mixtures caused a notable decline in rutting resistance. A similar trend was also observed in the rutting test results of the aged specimens.(ii)The use of CLS in all investigated amounts increased the indirect tensile strength of the unaged mixtures. But in the aged mixtures with more than 15% CLS, CLS-induced hardness improvement was overshadowed by aging-induced hardness reduction, resulting in a significant decline in the indirect tensile strength.(iii)The results of the moisture susceptibility test showed that the minimum acceptable TSR in the unaged and aged conditions can be achieved by using, respectively, over 5% CLS and over 10% CLS in the mixture.(iv)The results of the indirect tensile cracking test showed that in the unaged mixtures, CT_index increased with the increase in CLS content from 0 to 15% but remained constant when CLS content was raised from 15% to 20%. In the aged mixtures, a similar trend was observed for CLS contents of 0–15%, but CT_index decreased when CLS content was increased from 15% to 20%.(v)The results of the Illinois flexibility index test showed that the FI of the unaged mixtures increased with any increase in CLS content from 0 to 20%, but in the aged mixtures, it remained constant when CLS content was raised above 15%.(vi)In the process of determining appropriate mix designs based on BMD, the mixtures containing 15% and 20% CLS were found to meet all requirements of this design. The mixture with 10% CLS managed to meet the BMD requirements only when CT_index was used as the measure of cracking resistance.

Data Availability

All data used in this study are available upon reasonable request to the corresponding author.

Conflicts of Interest

The authors declare that there are no conflicts of interest.

Acknowledgments

The research of the corresponding author is supported by a grant from Ferdowsi University of Mashhad (grant No. 58117).

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Copyright © 2023 Seyed Ali Ziaee et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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