Characteristics of Asphalt–Concrete Mixtures Produced by Hot Asphalt Recycling Using Thermal Energy from the Combustion of Waste Automobile Tires

Abstract

The use of resource-saving technology in road construction material production is a current problem, the solution of which will allow us to increase the environmental and economic efficiency of the road construction industry. Nowadays, secondary raw materials are widely used in highway construction, obtained both from the waste of old road construction materials and collected from other industries. During asphalt production, up to 90% of raw materials can be replaced by reclaimed asphalt pavement (RAP). This technology requires residual binder modification to reduce the negative impact on the technological and operational asphalt concrete properties. On the other hand, the use of rubber crumbs or granules obtained from the disposal of old car tires in asphalt–concrete mixtures is widespread. However, some types of car tires cannot be used as raw materials to produce an effective modifier. Truck tires and tires from special vehicles are suitable for use as a modifier for asphalt–concrete mixtures. Tires designed for passenger cars do not contain enough polymer. As an experiment on asphalt–concrete mixture production using secondary resources only, a testing facility was developed. The testing facility uses hot gas obtained by burning automobile tires in a special oven as a heat source. Rubber residues from the recycling of automobile tires are used as fuel, which cannot be used to produce rubber powder or granules. RAP obtained by cold milling of the pavements of city and public roads was used as the object of the research. When studying the characteristics of the asphalt–concrete-mixture-based binder, it was found that the sulfur compounds present in the composition of hot gases change the properties of the binder, leading to a serious deterioration in the technological characteristics of asphalt–concrete mixtures. The asphalt–concrete mixture obtained during RAP processing is characterized by a narrow temperature range in which it can be laid and compacted to the required density values. After laying the pavement, quality control revealed a significant variation (the number of air voids ranged from 0.8 to 5.5%) in the average density of samples taken from the compacted layer. In addition, there were significant violations of the longitudinal evenness of the finished coating. Experiments were carried out to extract the binder from asphalt–concrete mixtures before and after regeneration. The physico-mechanical and rheological characteristics were studied and qualitative analysis of the binder was realized by IR spectroscopy. The data obtained allow us to establish the mechanism of how sulfur-containing gases influence the bitumen binder’s properties in asphalt mixtures. Additionally, the features of thermo-oxidative degradation occurring during the hot recycling of asphalt–concrete mixtures were established. A justification is also given for the need to use anti-aging modifiers to restore the properties of the residual binder.

1. Introduction

The road construction material industry consumes a large amount of non-renewable resources such as natural stone materials and petroleum bitumen. Intensive production of asphalt–concrete mixtures to ensure the functioning of the transport infrastructure leads to an increase in the environmental burden. As a result of asphalt pavement maintenance in standard-compliance conditions, a significant amount of waste is generated, including milled asphalt concrete. The large volumes of asphalt–concrete mixtures produced for various purposes make the use various secondary raw materials possible, which corresponds to the concept of rational use of natural resources and ensures preservation of non-renewable resources [1,2].

Nowadays, recycled materials are extensively utilized in road construction and are derived from both the processing of old road construction materials and other industrial sectors. These recycled materials can be broadly categorized as follows:

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Stone materials obtained from mining waste;

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Fine mineral aggregates from industrial waste;

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Modifiers for organic binders based on oil refinery waste;

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Modifiers for asphalt–concrete mixtures based on recycled car tires;

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Hot recycling asphalt.

The two latter methods not only allow for the rational use of non-renewable raw materials but also help reduce the impact on the region’s natural environment caused by road construction activities. However, the use of rubber powder (either in its disperse or granulated form) requires further research to enhance the interaction effectiveness between the bitumen binder and rubber. Additionally, it has been determined that not all types of rubber are suitable as a modifier [3]. Passenger car tires lack a sufficient polymer content and cannot be used to produce an effective modifier.

The recycling of old asphalt concrete in road construction can be divided into the following technological approaches:

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Cold recycling using granulated old asphalt concrete as a base material for subgrade layers;

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Hot recycling with reclaimed asphalt pavement (RAP) as a raw material for producing new asphalt mixtures.

The latter approach offers a broader range of applications for the resulting materials in various layers of road structures. During the production of asphalt mixtures and the operation of road pavements, bitumen binders are exposed to high temperatures in the presence of oxygen, solar radiation and active pollutants. These factors lead to the appearance and intensification of the binder. As a result, bitumen changes with regard to its qualitative composition and structure. Consequently, the physico-chemical characteristics are also subject to change. Nowadays, the aging processes of bitumen binders are widely studied [4,5,6,7]. However, additional factors arise when asphalt concrete is recycled. RAP heating leads to heating of bitumen to a high temperature, which is mainly present as a film under the active influence of the mineral substrate [8]. The resulting changes may not always be justified by the fundamental principles of building materials science.

Traditionally, natural gas or liquid fuel is used to heat the drying drum in asphalt mixing plants. To save fossil fuels, and to recycle non-recyclable rubber residues, a rubber incinerator was developed. Exhaust gases from this furnace were sent to a drying drum to heat asphalt granulate and mineral materials, if available. The cooled exhaust gases were then removed through filtration units. According to literature analysis, it is difficult to find information about the use of such technology for heating secondary asphalt concrete. During the construction of road pavements at experimental sites, it was found that the behavior of the asphalt mixture when smoothing paver plates and rollers is fundamentally different from the standard behavior of asphalt mixtures from similar work. It is necessary to study the processes occurring during the production of hot asphalt mixtures using this technology.

An experimental facility was designed to produce asphalt–concrete mixtures using recycled materials only. The exhaust gas generated after burning tires in a specialized unit was directed into a drying drum, where the RAP was heated. Subsequently, the cooled gas stream was removed through a gas purification system. The following non-recyclable rubber-containing residues were used as fuel for heating the drying drum: parts after removing the metal cord, rubber powder that cannot be granulated, and residues after cleaning the granulator matrices. In addition, remnants of passenger car tires were used as fuel.

The resulting asphalt–concrete mixtures demonstrate non-standard technological properties, complicating the installation of asphalt concrete pavement. It is well known that the properties of asphalt–concrete mixtures at processing temperatures are primarily determined by the physico-chemical and rheological characteristics of the bitumen binder [9,10]. Clearly, during the production of asphalt mixtures, the bitumen underwent changes related not only to technological aging but also to its interaction with rubber combustion products.

For the effective use of the produced asphalt mixtures, it is necessary to establish the mechanism of the changes occurring in the organic binder during RAP heating in the drying drum of the installation. To achieve this, it is necessary to do the following tasks:

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To study the dependence of the dynamic viscosity of bitumen binder samples on temperature and to establish the nature of the dependencies in the field of technological temperatures characteristic of asphalt mixture preparation and asphalt concrete compaction;

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To establish the effect of the rejuvenating additive on the rheological characteristics of the residual bitumen binder;

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To establish the relationship between the change in qualitative composition and rheological characteristics based on the analysis of the IR spectra of the residual binder.

The scientific novelty of this work lies in the theoretical substantiation and experimental confirmation of the mechanism of absorption of sulfur-containing components contained in the furnace gas by residual bitumen RAP. It has been established that during the interaction of bitumen with furnace gases, the rheological characteristics of the binder change: with a decrease in temperature from 140 °C to 120 °C, a sharp increase in viscosity occurs, which is uncharacteristic of bitumen.

2. Materials and Methods

2.1. Materials

Three samples of RAP were used in this study. The RAP sample no. 1 was obtained by the cold milling of a pavement based on a hot asphalt–concrete mixture from a main city street. By the time the work was completed, the pavement had been in use for over 10 years. The RAP sample no. 2 was obtained by the cold milling of a pavement based on a hot asphalt–concrete mixture that has been in operation for over 15 years. The RAP sample no. 3 was obtained by the cold milling of a pavement based on a stone mastic asphalt concrete from a federal highway. By the time the work was completed, the pavement had been in use for 5 years, no more. The need for repairs is associated with the formation of an abrasive track. The granulometric composition of the samples is shown in

Table 1. Granulometry of the RAP samples.

RAP Samples	Particle Content, %
	11.2	8	4	2	1	0.5	0.25	0.125	0.075
1	11.3	33.57	22.53	9.66	11.26	6.88	3.80	0.75	0.23
2	10.36	34.38	22.26	11.58	10.55	5.85	3.88	0.92	0.20
3	14.90	23.51	21.74	14.82	12.69	6.79	4.42	0.89	0.24

.

The binder content determined by the extraction method is given in

Table 2. Residual bitumen content in RAP samples.

RAP Samples	Binder Content, %
1	5.15
2	4.92
3	5.65

.

In turn, each of the RAP samples was collected from three different stages of the technological process. The first samples were taken from a cold hopper of inert materials (samples 1a, 2a and 3a). These samples are the initial RAP, not exposed to hot gases in the drying drum. The second sampling point is asphalt granulate after heating in a drying drum (samples 1b, 2b and 3b). The third one is asphalt granulate after heating in a drying drum and the introduction of the rejuvenating additive “Revobit” (samples 1c, 2c and 3c).

The steady-state temperature at the outlet of the drying drum was 162°C for sample 1, 170°C for sample 2, and 194°C for sample 3. These temperatures were selected to ensure asphalt concrete compaction, taking into account the transportation distance and weather conditions. The unusually high processing temperature of the 3 RAP samples is associated with the presence of a polymer of the styrene–butadiene–styrene (SBS) type in the residual binder and, as a result, the high compaction temperature of the layer.

2.2. Methods

2.2.1. Bitumen Extraction

Bitumen extraction was performed according to the National Standard GOST R 58401.19 [11], which is equivalent to ASTM D2172 [12]. RAP samples were crushed in a hot state and placed in a binder extraction device. Trichloroethane was used as a solvent. The binder was washed out by the solvent. The transparency of the solvent was used as the criterion for the end of extraction. Next, bitumen with solvent was placed in a rotary evaporator, where the solvent was distilled. The bitumen sample remained in the test container after distillation.

2.2.2. Physico-Chemical Property Testing

The bitumen penetration test was performed in accordance with the National Standard GOST 33136 [13], which is equivalent to EN 1426:2015 [14]. The softening point test was performed according to the National Standard GOST 33142 [15], which is equivalent to EN 1427:2015 [16]. The brittle point test was performed via the Fraas method according to the National Standard GOST 33143 [17], conforming to the requirements of EN 12593:2015 [18]. National Standard GOST 33138 [19], which conforms to the requirements of EN 13398:2017 [20], was used for bitumen elasticity and ductility measurement.

2.2.3. Rheological Property Testing

Determination of dynamic viscosity was carried out via a DV2T Brookfield viscometer (Brookfield Engineering Laboratories, Inc., Middleboro, MA, USA) using a coaxial spindle No 21. The specified test temperature was maintained using a thermocell. Viscosity measurements were taken at the following temperature points: 80 °C, 90 °C, 100 °C, 120 °C, 135 °C, 165 °C, 180 °C. The instrumental accuracy of the viscosity determination is ±1% of the full scale of the instrument measurement; the reproducibility of the measurement results is ±0.2%. The tests were carried out on three samples of each composition; the viscosity value was calculated as an average value.

2.2.4. Fourier Transform Infrared Spectroscopy (FTIR)

The change in the qualitative composition of the binder after aging was studied using Fourier transform infrared spectroscopy (FTIR). FTIR spectra were recorded by a Vertex 70 spectrometer (Bruker Optik GmbH, Ettlingen, Germany). Compressed tablets from a mixture of the material with potassium bromide were used as test samples. As a result, absorption spectra in range of 400–4000 cm⁻¹ were recorded.

3. Results

3.1. Physico-Chemical Properties

To analyze the processes occurring with the binder during the hot recycling of asphalt concrete, a study of the physico-chemical characteristics of bitumen samples was carried out, which included the determination of the softening temperature and the brittleness temperature. The results are shown in

Table 3. Physico-chemical characteristics of various residual bitumen samples.

Bitumen Samples	Softening Point, °C	Brittle Point, °C
1b sample	57.0	−8
1c sample	46.2	−14
1a sample	54.5	−10
2b sample	62.1	−8
2c sample	53.1	−12
2a sample	57.4	−10
3b sample	72.6	−10
3c sample	68.0	−12
3a sample	65.2	−16

.

Changes in the brittleness temperature and softening temperature for binder samples before and after passing through the drying drum can indicate the intensity of aging processes during regeneration. Analysis of the results showed that for samples of series 1a, 1b and 1c, the softening temperature increased by 4.5% after passing through the drying drum. It is necessary to pay attention to the extremely high softening temperature of bitumen, which is uncharacteristic for the grades used in these conditions. This can indicate an initially high binder aging degree as a result of the production of the initial mixture and subsequent operation of the pavement. When using the Revobit modifier, there is a decrease in the softening temperature by 8.3 °C and a decrease in the brittleness temperature by 4 °C. This objectively confirms the effect of bitumen rejuvenation. By introducing an additional maltene fraction into bitumen, as well as stabilizing the bitumen structure, it was possible to approximate these indicators to the required values. A more intense decrease in the softening point indicates a slight excess concentration of the anti-aging additive. In addition, a strong decrease in the brittleness temperature in sample 1c indicates an excessive amount of regenerating additive. Samples 2a, 2b, and 2c showed a similar relationship. At the same time, the increase in the softening temperature of bitumen after passing through the drying drum was 7.57%. The physico-chemical properties of the binder of the third group suggest the presence of a polymer modifier in bitumen, given the high temperatures of softening and brittleness even after operation. Bitumen, regenerated in a drying drum, shows a significant decrease in the brittleness temperature and a further increase in the softening temperature. At the same time, the introduction of the regenerating additive “Revobit” made it possible to lower the softening temperature by 4.66 °C and to lower the brittleness temperature by 2 °C. But these parameters did not return to their original values. These changes indicate the occurrence of intensive aging processes in bitumen during the passage of asphalt through the drying drum.

3.2. Rheological Properties

To assess the intensity of the processes occurring in the binder during passage through the drying drum during asphalt concrete hot regeneration, a study of the dynamic viscosity of residual bitumen samples was performed. The viscosity was determined at various temperatures, which included the following points:

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The temperature of the granulate in the drying drum;

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The temperature range in which asphalt–concrete mixtures are compacted;

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Standard temperature values of 135 °C and 165 °C to determine the optimal temperature ranges for mixing and compaction.

Binder samples were tested in groups of three: before and after passing through the drying drum, and after adding the reducing modifier “Revobit”, in accordance with the nomenclature chosen earlier.

The results of determining the dynamic viscosity of the first group of samples (samples 1b, 1c and 1a) are shown in Figure 1.

The analysis of the rheological curve shows atypical behavior of the binder in the range of 120–140 °C. At higher temperatures, the viscosity of bitumen passed through the drying drum is very low, lower than that of the initial binder. In the range of 120–140 °C. There is a significant increase in dynamic viscosity, which is not observed in the original bitumen, by more than 5%. The introduction of the Revobit modifier led to an even more intense decrease in viscosity in the temperature range of 140–180 °C, but did not smooth out the abnormal increase in viscosity in the range of 120–140 °C.

To simplify the analysis, the section of the rheological curve in the range of 120–180 °C is shown in Figure 2 in logarithmic form.

To quantify the degree of aging, the rheological stability index of the binder was used. The rheological stability index is the ratio of the binder dynamic viscosity before the drying drum to the dynamic viscosity after the drying drum, determined at different temperatures:

$$I_d={\displaystyle \frac{{\mathsf{\eta }}_{ini}}{{\mathsf{\eta }}_{rec}}},$$

(1)

• ${\mathsf{\eta }}_{ini}$—dynamic viscosity of the binder at the reference temperature before the drying drum;
• ${\mathsf{\eta }}_{rec}$—dynamic viscosity of the binder at the reference temperature after the drying drum.

The results of calculating the rheological stability index are shown in

Table 4. Rheological stability indices calculated for different binder.

Binder Sample	Reference Temperature, °C	Rheological Stability Index
Sample 1a	145 165	0.72 0.92
Sample 1b	145 165	0.78 0.98

.

A higher value of the rheological stability index for the studied binder indicates a lower change in dynamic viscosity after aging, and, consequently, a slowdown in destructive processes in it. Analysis of changes in the rheological curves and the obtained rheological stability index in the temperature range of the true liquid state shows that bitumen did not undergo an intensive aging process during passage through the drying drum.

A similar trend is observed in the study of the second series of RAP binder samples (Figure 3 and Figure 4).

To simplify the analysis, a section of the rheological curve in the range of 120–180 °C (the range of process temperatures) is shown in Figure 4 in logarithmic form.

The analysis of the obtained dependence of dynamic viscosity on the temperature allows us to draw similar conclusions to the study of the previous series of samples. In the temperature range from 120 °C to 140 °C, there is a sharp increase in dynamic viscosity, which is unusual for the behavior of bitumen binders. When testing the initial bitumen that has not passed through the drying drum, this form of viscosity dependence on temperature is not observed.

The results of calculating the rheological stability index are shown in

Table 5. Rheological stability indices calculated for different binders.

Binder Sample	Reference Temperature, °C	Rheological Stability Index
Sample 2a	145 165	0.76 0.92
Sample 2b	145 165	0.78 1.15

.

Binder samples in series 3 have a much higher viscosity compared to previous samples over the entire temperature range. The rheological curves demonstrate behavior typical of polymer–bitumen binders. The results of dynamic viscosity determination are shown in Figure 5.

The analysis of rheological curves shows a significant increase in dynamic viscosity in the range of 100–110 °C for samples that have passed through the drying drum. It was not possible to take measurements at a temperature of 80 °C due to the operating range of the device being exceeded. These changes indicate the occurrence of intensive processes of thermo-oxidative degradation when the RAP is in the drying drum, since the dynamic viscosity of the samples before and after the drum varies significantly. At the same time, the introduction of the Revobit modifier made it possible to slightly reduce the viscosity, which indirectly makes it possible to link the changes in viscosity with the processes of the binder thermal oxidative degradation. It should be noted that the behavior of the rheological curve of the RAP binder sample 3 does not exhibit the abnormal character observed in samples from other sources.

3.3. FTIR Analysis of Bitumen Sample Composition

To determine the changes in the bitumen’s quantitative composition that occur during heating in the drying drum, the IR spectra of binders extracted from the initial asphalt granulate and after their heating in the drying drum were compared. Figure 6 and Figure 7 show the IR spectra of samples 1a and 1b before and after heating. Most absorption bands (peaks on the spectrum) can be attributed to various C–H oscillations. Specifically, peaks at 2952 and 2923 cm⁻¹ correspond to valence vibrations of methyl (-CH3) and methylene (-CH2) groups, respectively. The bands at 1459 and 1376 cm⁻¹ can be associated with deformation vibrations of those same groups. The peak at 722 cm⁻¹ indicates the presence of long alkyl chains in saturated compounds, such as (CH2)n, where n is greater than 4. Several other bands at 748, ~802–812, 874 (deformation vibrations of C-H bonds) and 1602 cm⁻¹ (valence vibration of C=C bonds) characterize aromatic compounds. All these bands are present in all compositions and are of no interest from the point of view of describing the processes occurring during heating. Therefore, attention will mainly be paid to peaks describing functional groups based on heteroatoms, primarily oxygen and sulfur. The band at 1699 cm⁻¹ is attributed to valence vibrations of the carbonyl group (C=O) [21,22]. A group of bands in the area of ~1000–1300 cm⁻¹ characterizes sulfur-containing functional groups, i.e., sulfoxides, sulfones, and sulfate esters [23]. Both carbonyl- and sulfur-containing functional groups characterize chemical changes in bitumen binders, including those occurring during aging. As can be seen from Figure 1 and Figure 2, bitumen extracted from RAP (sample 1a) has a rather significant band intensity at 1699 cm⁻¹, which is mainly used to assess the degree of the binder aging [24,25]. Consequently, bitumen was strongly aged before warming up in the drying drum. The degree of bitumen aging is also assessed by the absorption band of the sulfoxide group (S=O), but in this case this is hindered by the presence of small clay particles that did not separate during extraction with benzene and are present in the studied bitumen samples in a minimal amount (one percent). Characteristic peaks of clay particle oscillations were also found in other regions of the spectrum (Figure 6 and Figure 7).

The comparison of bitumen spectra before and after heating in a drying tank was carried out after normalization of the spectra along the absorption band of 1458 cm⁻¹, which describes the deformation vibrations of methyl (-CH3) and methylene (-CH2) groups, the content of which did not change in the technological process. From Figure 6, during the heating of the asphalt granulate in the drying drum at high temperatures, further oxidation of bitumen compounds with oxygen occurs, as evidenced by an increase in the intensity of the 1699 cm⁻¹ band. However, the greatest changes occur in the range of ~1000–1200 cm⁻¹, associated with fluctuations in the bonds between sulfur and oxygen in various organic compounds. In particular, the intensity of the sulfoxide group band (1031 cm⁻¹) increases significantly, minus the clay peaks, and the absorption intensity increases in the range of ~1070~1200 cm⁻¹ with the appearance of new peaks at 1097 and ~1155 cm⁻¹. An analysis of literature sources allowed us to attribute these maxima to compounds with the sulfonyl group R−S(=O)2−R′, which is a functional group found mainly in sulfones [23,26]. It was shown in [26] that the addition of sulfonic-compound-rich products to bitumen leads to a significant increase in the penetration depth of the needle, i.e., a decrease in viscosity. A similar phenomenon is observed in this study.

Similar changes in the qualitative composition occur in bitumen samples 2a and 2b (Figure 8 and Figure 9). There is also a significant increase in sulfur–oxygen organic compounds, including sulfonyl and sulfoxide functional groups.

It should be noted that in sample 2b (bitumen from RAP after the drying drum), in addition to clay particles, there is a certain amount of calcium carbonate grains, most likely limestone mineral powder. The main absorption band of CaCO₃ is wide and is located at ~1425 cm⁻¹. The significant width of this band leads to the underestimation of the intensity of the 1456 cm⁻¹ peak of hydrocarbons, according to which the spectra were normalized (Figure 9). Therefore, the height of the carbonyl group band at 1699 cm⁻¹ (C=O), which describes the aging of bitumen, did not change in sample 2b (Figure 10). However, subtracting the peaks of calcium carbonate from the spectrum for sample 2b (reference sample) demonstrates a more realistic character (Figure 10), which is similar with regard to C=O groups to sample 1b (Figure 7).

In addition to the absorption bands of bitumen, the polymer bitumen binder (PBB) spectrum for samples 3a and 3b contains characteristic peaks of the styrene–butadiene–styrene polymer: 699 cm⁻¹ characterizes the deformation vibrations of aromatic C-H in styrene; 910 and 967 cm⁻¹ are attributed to the =C–H groups in cis- and trans- conformations of butadiene, respectively [22,27]. Figure 11 demonstrates that the changes taking place in PBB differ from those of unmodified bitumen. An increase in the intensity of sulfur–oxygen functional groups also occurs here, but on a much smaller scale and without the appearance of obvious absorption bands of the sulfonyl group. On the other hand, during heating in a drying drum, the bitumen in the PBB is strongly oxidized, as evidenced by a strong increase in the absorption intensity of the carbon group C=O at 1699 cm⁻¹. This indicates that when exposed to the high temperatures of the exhaust gases in the drying drum, the bitumen binder ages significantly (judging by the intensity of the strip, by about 2 times). This is confirmed by the results of dynamic viscosity determination.

The intensive aging of the polymer bitumen binder in the drying drum affects not only the main component—bitumen—but also the polymer additive. According to Figure 11, the styrene block remains stable during aging, while butadiene shows a decrease in the relative peak intensity (peak height minus the absorption bands of the main component). This indicates polymer degradation due to chain breakage, which is mentioned in the works of other researchers [22,28].

4. Discussion

The results of this study confirmed that the bitumen binder changes its properties due to the absorption of SO₂ from the exhaust gases from the bitumen tire combustion furnace in RAP. This has a corresponding effect on the rheological and physico-mechanical properties of bitumen. This leads to a deterioration in the technological properties of the asphalt–concrete mixture in such a way that the temperature range of effective compaction becomes extremely small. Differences in the absorption degree of gases and the appearance of sulfonated compounds have been established depending on the type of binder used: when using PBBs, the content of oxygen–sulfur compounds decreases significantly (several times). Studying the aging processes of bitumen samples with intensive and non-intensive absorption of sulfur compounds allows for us to conclude on the effect of slowing down the aging process through compounds with a sulfonyl group. According to [29], there is evidence that the oxidative processes of bitumen components are suppressed in the presence of sulfur. Sulfur can form in the exhaust gases produced by the combustion of automobile tires, since SO₂ is reduced by carbon monoxide CO. However, at this stage of the research, it was not possible to confirm this assumption. It is not possible to detect the presence of sulfur in bitumen by IR spectroscopy; other methods are used for this, for example, X-ray phase analysis, since sulfur is present in the crystalline state at low temperatures [30]. In addition, information is provided on the behavior of the asphalt–concrete mixture with asphalt–concrete granulate, which decreases when the temperature drops to 120–130 °C. During compaction, sulfur loses mobility, sticks together, and cracks, which may indirectly indicate its presence in bitumen or asphalt concrete: the sulfur melt crystallizes at temperatures below 120 °C. The obtained IR spectroscopy data correspond to the results of studies of the dynamic viscosity of bitumen samples isolated from asphalt mixtures.

During the operation of the asphalt concrete processing plant, according to the accepted scheme, suitable conditions are created for the absorption of sulfur from combustion products. The bitumen present on mineral particles is in a film state, the area of interaction with gases is large, constant mixing occurs, and the time of passage of the granulate through the drum and a high temperature inside the drum are maintained.

When examining asphalt–concrete granulate, which contains a polymer–bitumen binder, there are significantly fewer traces of the addition of sulfur-containing components. At the same time, attention should be paid to the significantly higher heating temperature of the asphalt–concrete granulate (above 190 °C) used to prepare the asphalt–concrete mixture. The following explanations can be given for this:

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Polymer–bitumen binder has a lower reactivity due to the presence of polymer units; as a result of this, it absorbs sulfur-containing compounds less intensively;

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The high temperature in the drying drum leads to the volatilization of sulfur compounds through the smoke extraction system; as a result of this, the concentration of sulfur-containing gases in the presence of asphalt–concrete granulate decreases.

However, the high production temperature of the asphalt–concrete mixture with granulate polymer–bitumen binder led to intensive aging of the binder and degradation of the polymer in its composition, which also negatively affected the strength of the asphalt–concrete mixture and may have led to a deterioration in the performance properties of the asphalt concrete.

To reduce the negative effects of sulfur-containing gases on the material in the drying drum, it is necessary to adjust the technological process. The options for this require experimental verification.

The first option is to remove exhaust gases from the tire combustion furnace by heating the air through an air-to-air heat exchanger. This eliminates the possibility of the binder interacting with sulfur-containing components.

The second option is to increase the temperature in the drying drum to 190–200 °C, at which point the sulfur-containing compounds should volatilize with a one-time decrease in the passage time of the asphalt–concrete granulate through the drying drum (i.e., a variation in the countercurrent movement of gases). At the same time, the aging of the bitumen binder will need to be corrected by the regenerating additive Revobit.

Studies have shown that when using the Revobit reducing additive, the viscosity decreases even more at asphalt concrete compaction temperatures; however, this modifier does not affect the process of an abnormal increase in bitumen viscosity with a further decrease in temperature.

5. Conclusions

The research results show that the use of gas from the combustion of non-recyclable tire residues allows for efficient heating of RAP in the drying drum. However, during such heating, the residual bitumen intensely absorbs sulfur-containing compounds from the combustion products, which leads to a deterioration in the technological properties of the asphalt–concrete mixture. The results showed that by increasing the temperature of RAP processing, it is possible to significantly reduce the intensity of the absorption of sulfur compounds. At the same time, the intensity of thermal and oxidative degradation of the binder increases, which leads to the need for a reasonable selection of an effective concentration of the regenerating additive. In the future, when performing research, the following is required:

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A study of the dependence of changes in the absorption of sulfur compounds on the temperature of RAP processing;

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Identification of differences in the mechanism of the absorption of sulfur compounds by residual bitumen and polymer–bitumen binders;

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Selection of the formulation and technological factors that ensure the production of asphalt mixtures with optimal technological properties;

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Selection and justification of methods for inhibiting the aging of bitumen binders;

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A study of the physico-mechanical and operational properties of asphalt concrete obtained by processing RAP using the investigated method.