Influence of Type of Modified Binder on Stiffness and Rutting Resistance of Low-Noise Asphalt Mixtures

Low-noise asphalt mixtures are characterized by increased air void content. Their more open structure contributes to faster degradation within the operating temperature range. For this reason, binder modification is used in their production. The correct selection of modifiers allows one to significantly improve the technical properties of the mixtures. The article presents the results of tests of six types of mixtures: stone mastic asphalt (SMA8), porous asphalt (PA8), stone mastic asphalt reducing tire/road noise (SMA8 LA) and stone mastic asphalt reducing tire/road noise, with 10%, 20% and 30% content of rubber granulate (RG). Bitumen 50/70 modified with copolymer styrene butadiene styrene (SBS) and crumb rubber (CR) was used for the production of the mixtures. In order to determine the differences in the technical properties of the mixtures, the following parameters were tested: stiffness modules by indirect tensile testing of cylindrical specimens (IT-CY) in a wide range of positive temperatures, and resistance to permanent deformation using the British and Belgian methods with the use of double wheel tracker (DWT). The test results and their analysis confirmed that there was a significant improvement in the IT-CY stiffness modules of SBS and CR modified mixtures. Replacing more than 20% of coarse aggregate with RG causes a significant decrease in the stiffness of the mixture (by 90% in relation to the reference mixture SMA8 LA). The SMA mixtures obtained lower values of rutting resistance parameters (WTS and PRD) in water (Belgian method) compared to the results obtained in the air tests (British method). On the other hand, mixtures of PA, thanks to the compression of stresses in pores filled with water, obtained better results when the rutting resistance test was performed in the water (Belgian method).


Introduction
Development of the communication infrastructure and the increase in road traffic volume have resulted in an increase in the noise level generated by motor vehicles in the road surroundings [1]. "Quiet" road surfaces, which include PA, thin layers of BBTM type (French: Bétons Bitumineux Très Minces) with a maximum grain size 8 mm and layers of grit mastic SMA LA, reduce the level of tire/surface noise even up to 5-6 dB compared to asphalt concrete (AC) or SMA layers with a maximum aggregate grain size 11 mm [2]. However, the greater air void content in such mixtures (in the range of 10-20%) contributes to faster destruction of such surfaces compared to the standard solutions of surface layers [3]. The increased content of air voids requires the use of highquality modified bitumen, which significantly determines the durability of the pavement in operating conditions. Positive results are obtained with the use of binders modified with the addition of SBS copolymer (e.g., Kraton 1192), CR from used car tires and a combination of these two modifiers [4][5][6]. Modifiers of this type contribute to a greater range of viscoelasticity, increasing the softening temperature, improving the resistance to technological and service aging, and increasing low temperature cracking resistance [7,8].
The formation of ruts is one of the most types of common damage to asphalt surfaces [9,10]. This process depends primarily on the physical and mechanical properties of the used

Materials
The test results for bitumen binders and mineral aggregates used for asphalt mixtures are described in publications [7,8], and selected technical properties of modified binders are presented in Table 1. The used copolymer Kraton 1192 contains 30% of styrene, and its molecular weight is 1.38 × 105 g/mol. CR used for modification, with a grain size of 0/0.8 mm, came from used car tires. The bitumen modification process consisted of heating the bitumen 50/70 to the temperature of 180 • C ± 5 • C, then adding 5% SBS copolymer or 10% CR or combined 2% SBS copolymer with 10% CR. where: (a)-before RTFOT; (b)-after RTFOT.
Bitumen binders were subjected to the following tests: penetration (at temperatures: 5 • C, 15 • C and 25 • C), softening point according to ring and ball method, Fraass Breaking Point (T Fraass ,), dynamic viscosity (at temperatures: 90 • C, 110 • C and 135 • C), strain energy with the determination of the maximum tensile force (at temperatures: 5 • C, 15 • C and 25 • C) and elastic recovery (at temperatures: 15 • C and 25 • C). Laboratory tests were carried out for bitumen before and after the technological aging process. The simulation of the technological aging process in laboratory conditions was performed using the rolling thin film oven test (RTFOT) method according to the standard [36]. RG with grain size 1/4 mm was added to asphalt mixtures using the "dry" method in amounts of 10%, 20% and 30% by volume of aggregate, replacing the appropriate part of the mineral aggregate.
Cylindrical specimens (φ = 101.6 mm, h = 63.5 ± 2.5 mm) for testing the stiffness modulus IT-CY were compacted in accordance with the standard [37]. The samples for rutting tests were compacted in accordance with the standard [38] (300 mm × 400 mm × 40 mm plates). The particle size distribution of individual mixtures is shown in Figure 1. The binder content, air void content and bulk density of mixtures are presented in Table 2.

Indirect Tension to Cylindrical Specimens (IT-CY)
Elastic stiffness modulus determined in indirect tension (IT-CY) test according to the standard [23] is an important parameter that allows one to predict the behavior of mixture at positive operating temperatures at which its stiffness modulus decreases. Therefore, the IT-CY test was conducted in the following temperatures: 5 °C, 15 °C, 25 °C and 35 °C. The highest value of 35 °C was determined by the authors as the temperature at which it was possible to conduct a full set of tests of the mixtures. Too high deformation of samples containing mixtures with the addition of RG above this temperature was recorded. A controlled stress test was performed during the measurement. Dynamic load was applied five times to the sample vertically along the diameter. The time of force increase, measured from the moment of applying the force (zero value) to the maximum value, was 0.124 s. The maximum force generated a horizontal displacement of the sample equal to 5 μm. There was a 3-s delay between each force pulse. The test result was calculated automatically by the control program as the arithmetic mean of the stiffness modules for each of the five force pulse measurements. After the test, the sample was rotated 90° about the horizontal axis and tested again. If the average test result was within ±10% of the modulus value tested in the previous position, the stiffness modulus of the sample was calculated as the average of two measurements. If the measurement results did not fall within the acceptable range, they were not taken into account for further analysis. Duplicate test results carried out on the entire series of samples were within the limits of standard deviations. The final result was the arithmetic mean of 5 tested samples.
For the assumed load area coefficient equal to 0.6, the value of the stiffness modulus was determined from the following equations: where F-the maximum force applied to the sample (N); z-the amplitude of the horizontal displacement of the sample during loading (mm); h-sample height (mm);

Indirect Tension to Cylindrical Specimens (IT-CY)
Elastic stiffness modulus determined in indirect tension (IT-CY) test according to the standard [23] is an important parameter that allows one to predict the behavior of mixture at positive operating temperatures at which its stiffness modulus decreases. Therefore, the IT-CY test was conducted in the following temperatures: 5 • C, 15 • C, 25 • C and 35 • C. The highest value of 35 • C was determined by the authors as the temperature at which it was possible to conduct a full set of tests of the mixtures. Too high deformation of samples containing mixtures with the addition of RG above this temperature was recorded. A controlled stress test was performed during the measurement. Dynamic load was applied five times to the sample vertically along the diameter. The time of force increase, measured from the moment of applying the force (zero value) to the maximum value, was 0.124 s. The maximum force generated a horizontal displacement of the sample equal to 5 µm. There was a 3-s delay between each force pulse. The test result was calculated automatically by the control program as the arithmetic mean of the stiffness modules for each of the five force pulse measurements. After the test, the sample was rotated 90 • about the horizontal axis and tested again. If the average test result was within ±10% of the modulus value tested in the previous position, the stiffness modulus of the sample was calculated as the average of two measurements. If the measurement results did not fall within the acceptable range, they were not taken into account for further analysis. Duplicate test results carried out on the entire series of samples were within the limits of standard deviations. The final result was the arithmetic mean of 5 tested samples.
For the assumed load area coefficient equal to 0.6, the value of the stiffness modulus was determined from the following equations: where F-the maximum force applied to the sample (N); z-the amplitude of the horizontal displacement of the sample during loading (mm); h-sample height (mm); ν-Poisson's ratio; ∆V-maximum vertical displacement of the sample (mm).

Rutting Resistance Test Using the British and Belgian Method
The permanent deformation resistance test according to [39] was carried out in the DWT. This test is used to determine the deformability of asphalt mixtures as a result of repeated passage of the loaded wheel through the sample. The plates (height 40 mm, width 300 mm and length 400 mm) were compacted in an electromechanical plate compactor. Loading arms equipped with test wheels (203 mm × 50 mm) performed a reciprocating movement with a total wheel travel length of 230 mm. The test speed was 20 cycles per minute (40 wheel passes), the conditioning time was 4 h and the test temperature was 60 • C. Rutting resistance test was carried out in air according to the British method, while according to the Belgian method-samples were completely immersed in water during the test. Wheel tracking slope (WTS) was calculated based on the Equation (3): where d i and d i/2 -rut depth after i and i/2 load cycles (mm); Percentage of rut depth (PRD) after N cycles of the loading was calculated from the Equation (4): where h 1 -the initial rut depth (mm); h 2 -the final rut depth (mm).
DWT test device with the mounted sample is shown in Figure 2.

Rutting Resistance Test Using the British and Belgian Method
The permanent deformation resistance test according to [39] was carried ou DWT. This test is used to determine the deformability of asphalt mixtures as a repeated passage of the loaded wheel through the sample. The plates (height width 300 mm and length 400 mm) were compacted in an electromechanical pla pactor. Loading arms equipped with test wheels (203 mm × 50 mm) performed a cating movement with a total wheel travel length of 230 mm. The test speed was 2 per minute (40 wheel passes), the conditioning time was 4 h and the test temperat 60 °C. Rutting resistance test was carried out in air according to the British metho according to the Belgian method-samples were completely immersed in wate the test. Wheel tracking slope (WTS) was calculated based on the equation (3): where and / -rut depth after and /2 load cycles (mm); Percentage of rut depth (PRD) after N cycles of the loading was calculated f Equation (4): where ℎ -the initial rut depth (mm); ℎ -the final rut depth (mm).
DWT test device with the mounted sample is shown in Figure 2.

IT-CY Stiffness Modulus
The results of the stiffness modulus test with the descriptive statistics are p in Table 3.
The obtained results of stiffness modulus are similar to the test results desc publications [40,41]. Mixtures with binder modified with SBS and CR achieved modulus values compared to mixtures with unmodified binder. This proves that less susceptible to traffic loads and show higher resistance to deformation. The

IT-CY Stiffness Modulus
The results of the stiffness modulus test with the descriptive statistics are presented in Table 3. The obtained results of stiffness modulus are similar to the test results described in publications [40,41]. Mixtures with binder modified with SBS and CR achieved higher modulus values compared to mixtures with unmodified binder. This proves that they are less susceptible to traffic loads and show higher resistance to deformation. The addition of RG causes a decrease in the stiffness modulus of the mixtures in direct proportion to the amount of granulate introduced. This proves that replacing the aggregate with RG makes the mixture more flexible. Therefore, it can be predicted that they will more effectively dampen vehicle vibrations and road noise.
Based on the results of the tests presented in Table 3, it was found that the highest values of the modulus at 5 • C were obtained for the mixtures: SMA8 with CRM-10 and SBSM-2+CRM-10 (10,183 MPa and 10,156 MPa, respectively) and SMA8 LA with bitumen 50/70 and SBSM-2+CRM-10 (7894 MPa and 7526 MPa, respectively). The lowest values of the modulus at 35 • C were obtained for SMA8 LA (30% RG) with bitumen 50/70 and CRM-10 (28 MPa and 27 MPa, respectively). It was found that the highest dispersion of test results (CoV) was obtained at the temperatures of 25 • C and 35 • C. The SMA8 LA mixtures with the addition of 10%, 20% and 30% rubber granulate with bitumen 50/70 showed scatter of results CoV = 17.6-20.2, SMA8 LA (20% RG) with SBSM-5 (35 • C) CoV = 20.6, SMA8 LA with the addition of 20% RG, and 30% RG with SBSM-2+CRM-10-CoV = 16.6-20.8. Figure 3 shows the results of IT-CY stiffness modulus as a function of temperature. 2+CRM-10 binder. The lowest temperature sensitivity among the mixtures with the addition of RG has SMA8 LA (10% RG) and SMA8 LA (30% RG) with rubber-asphalt binder CRM-10. All analyzed mixtures with the reference bitumen 50/70 showed higher temperature sensitivity compared to the bitumen modified with SBS and CR. The results of the research presented in [5,41] also prove that mixtures with binders modified with SBS and CR are characterized by lower or comparable sensitivity to changes in stiffness modulus as a function of temperature compared to mixtures with unmodified bitumen.  The analysis of the influence of the addition of 10%, 20% and 30% RG on the change of the IT-CY stiffness modulus of asphalt mixtures in relation to the reference mixture SMA8 LA with bitumen 50/70 is presented in Table 4.  The results presented in Figure 3 show that the type of modifier and the amount of RG affect the stiffness modulus of the tested mixtures. SMA8 and SMA8 LA with modified bitumen SBSM-5 showed the lowest temperature sensitivity. PA8 and SMA8 LA (20% RG) mixtures are characterized by the lowest temperature sensitivity when using the SBSM-2+CRM-10 binder. The lowest temperature sensitivity among the mixtures with the addition of RG has SMA8 LA (10% RG) and SMA8 LA (30% RG) with rubber-asphalt binder CRM-10. All analyzed mixtures with the reference bitumen 50/70 showed higher temperature sensitivity compared to the bitumen modified with SBS and CR. The results of the research presented in [5,41] also prove that mixtures with binders modified with SBS and CR are characterized by lower or comparable sensitivity to changes in stiffness modulus as a function of temperature compared to mixtures with unmodified bitumen.
The analysis of the influence of the addition of 10%, 20% and 30% RG on the change of the IT-CY stiffness modulus of asphalt mixtures in relation to the reference mixture SMA8 LA with bitumen 50/70 is presented in Table 4.  Table 4 shows that the greatest changes in the stiffness modulus values in relation to the reference mixture were observed when using 20% and 30% RG. A significant drop in stiffness (about 90%) was obtained in these mixtures, which means that the influence of the type of bitumen was reduced, and the amount of RG added determined the value of the test result. The greatest changes in IT-CY values were observed for the SMA8 LA (30% RG) mixture with GRM-10 binder (decrease by 95%) and bitumen 50/70 (decrease by 94%).
The second-degree polynomial was used to describe the changes in the stiffness modulus: where Z-analyzed parameter of the mixture (IT-CY stiffness modulus); a 0 − a 9 -regression coefficients; X 1 -type of mixture; X 2 -type of binder; The statistical analysis of the obtained results according to ANOVA started with the factorial significance test (Statistica Software, version 13, TIBCO Software Inc., Palo Alto, CA, USA). The results of this analysis are presented in Table 5.
It can be clearly stated based on the analysis of the parameters that the temperature, mixture type and modifier type are important factors affecting the stiffness modulus, because the p-value is lower than the assumed significance level α = 0.05 (p-value < 0.05) ( Table 5).
Analyzing the quadratic term of the modifier type (Type of Binder (Q)) and the factor describing the interaction of the modifier type and temperature (2L*3L), no significant influence was found on the values of stiffness modulus (p-value greater than α = 0.05).
The values describing the parameters of the regression model are summarized in Table 6.  Based on the analysis, it was observed that the value of the corrected determination coefficient was R 2 adj = 97%, which proves that the model was correctly adopted ( Table 6). The developed model of the IT-CY stiffness modulus can be presented using the dependence (6  Temp-test temperature.
The selected graphical interpretation of the IT-CY stiffness modulus change as a function of temperature and mixture type is presented in Figure 4.

Rutting Resistance by British and Belgian Method
The results of rutting resistance tests of the analyzed mixtures using the British and Belgian methods are presented in Figures 5a-f and 6a-f, and in Table 7. The red line mark the maximum allowable rut depth (2.8 mm) after 10,000 cycles according to [42]. The tabl below the graphs presents the calculated values of the WTS and PRD rutting parameters The requirements [42] permit the following limits: WTSair ≤ 0.15 and PRDair ≤ 7.0.

Rutting Resistance by British and Belgian Method
The results of rutting resistance tests of the analyzed mixtures using the British and Belgian methods are presented in Figures 5a-f and 6a-f, and in Table 7. The red line marks the maximum allowable rut depth (2.8 mm) after 10,000 cycles according to [42]. The table below the graphs presents the calculated values of the WTS and PRD rutting parameters. The requirements [42] permit the following limits: WTS air ≤ 0.15 and PRD air ≤ 7.0.

Rutting Resistance by British and Belgian Method
The results of rutting resistance tests of the analyzed mixtures using the British and Belgian methods are presented in Figures 5a-f and 6a-f, and in Table 7. The red line marks the maximum allowable rut depth (2.8 mm) after 10,000 cycles according to [42]. The table below the graphs presents the calculated values of the WTS and PRD rutting parameters. The requirements [42] permit the following limits: WTSair ≤ 0.15 and PRDair ≤ 7.0.    The rutting phenomenon occurs mainly at high temperatures. Therefore, it is important to use binders with appropriate viscoelastic properties at temperatures above 60 • C. On the basis of the rutting results presented in Figures 5a-f and 6a-f and in Table 7, it was found that the test type, mixture type and modifier type have a significant influence on the rutting resistance of asphalt mixture. It was established that the test in water (Belgian method) gives higher values of WTS w and PRD w indexes compared to the results of tests in the air (British method). The highest (unfavorable) results of the WTS w index were obtained for SMA8 LA (10% RG), SMA8 LA (20% RG) and SMA8 LA (30% RG) mixtures with SBSM-2+CRM-10 binder, and the WTS w index increased by 334%, 215% and 89% compared to WTS air values, respectively. The highest (unfavorable) values of the percentage of rut depth PRD w were obtained for SMA8 LA (20% RG) and SMA8 LA (30% RG) mixtures with SBSM-5 bitumen, and PRD w increased by 60% and 65% compared to PRD air values, respectively. Different results of the WTS and PRD tests were obtained for the PA8 mixture. With these mixtures, rutting indexes were lower (favorable) when tested in water. This can be explained by the fact that water that penetrated into the open pores (air void content in PA8 was 24%) acted like "shock absorber". Thus, it improved the resistance of PA8 mixtures to rutting. For PA8 mixtures with CRM-10 binder, the WTS w index decreased by 34% compared to WTS air .
Apart from the test method, the type of binder has a significant influence on the rutting resistance. The best results were obtained for mixtures with SBSM-5 modified bitumen compared to the reference bitumen 50/70. The greatest change in parameters was recorded for the SMA8 LA mixtures, where the WTS w index decreased from 6.97 to 0.06, and the PRD air index decreased from 50.00 to 3.93. In the case of PA8 mixtures, WTS air decreased from 37.79 to 0.61, and PRD w from 50.00 to 17.26.
The rutting resistance tests showed that the amount of addition RG had a direct proportionate effect on the WTS and PRD parameters. The greater the replacement level of the mineral aggregate with the RG, the greater the elasticity of the mixture, and the more significant the deterioration of the analyzed parameters. Taking as an example SMA8 LA mixture with SBSM-5 binder for which WTS air equals 0.05 and PRD air equals 3.93, after adding 30% GR (SMA8 LA (30% RG)), the rutting parameters increased (unfavorable) to the following values: WTS air -2.11 and PRD air -30.31.
A significant improvement in the rut resistance of asphalt-rubber mixtures com-pared to mixtures with conventional binders was also achieved in [43,44]. The authors showed that the addition of CR to bitumen over 10% increases binder stiffness and viscosity. It leads to an increase in stiffness and the rate of increase in the rut depth. The addition of SBS copolymer, as reported by the authors of the publications [34,43,45], in-creases the rut resistance of the mixtures at least twice. When SBS is added to the standard binder, a load-bearing butadiene network is formed that increases its viscosity, stiffness and elasticity.
The second-degree polynomial was used to describe changes in rutting parameters: where Z-the analyzed parameter (WTS air , PRD air , WTS w i PRD w ); a 0 − a 5 -regression coefficients; X 1 -type of mixture; X 2 -type of binder.
The results of the statistical analysis of the influence of mixture type and modifier type on the rutting indexes are presented in Table 8, while Table 9 presents the estimation of the parameters of the model describing the relationship between the type of binder and mixture and the rutting indexes.  The statistical analysis proves that the WTS air value is influenced by the modifier type, the PRD air and PRD w values are strongly affected by the mixture type and the WTS w value is influenced by both the mixture type and the modifier type. This is evidenced by the p-value (Table 8), the value of which is lower than the assumed significance level α = 0.05 (p-value < 0.05). However, it should be noted that there is no interaction between the influence of the mixture type and modifier type (1L * 2L) on the WTS and PRD rutting resistance parameters (p-value greater than α = 0.05). Low values of the determination coefficient result from the high variability of the results in the analyzed groups.
The developed model of the analyzed characteristics can be expressed using the Equations (8)- (11).  Graphical interpretation of the adopted WTS and PRD models for different types of analyzed mixtures and bituminous binders is shown in Figure 7.

Conclusions
Based on the tests of the stiffness modulus and rutting resistance of grit mastic mixtures SMA8, SMA8 LA and porous asphalt PA8, with binders modified with SBS copolymer or crumb rubber, or combined modification with SBS and crumb rubber, the following conclusions were formulated: 1.
The type of modifier used has a significant effect on the stiffness of asphalt mixtures and their temperature sensitivity confirmed by the change in the value of the stiffness modulus as a function of temperature. The highest increase was obtained at the temperature of 35 • C for SMA8 LA mixtures with 10% rubber granulate: by 163% for the binder modified with SBS copolymer, by 92% for the binder modified with crumb rubber and by 104% for the modification with 2% SBS + 10% crumb rubber, compared to mixtures with bitumen 50/70.

2.
Replacing coarse aggregate in the mixture with rubber granulate in an amount exceeding 20% (by volume) causes a significant decrease in the stiffness modulus. The greatest changes were observed in the case of SMA8 LA (30% rubber granulate) with a binder modified with 10% crumb rubber. The decrease in IT-CY stiffness modulus in this case was 95% compared to the reference mixture with bitumen 50/70. 3.
In SMA mixtures, their rutting resistance was found to be lower in Belgian test (in water) compared to the British test (in air). The SMA8 LA (10% rubber granulate) mixture showed an increase in the WTS w rut depth by 334% in relation to the results measured in the air. Percentage of rut depth PRD w of SMA8 LA (30% rubber granulate) mixture increased by 65% compared to PRD air . 4.
The opposite effect was achieved for porous asphalt. Water present in the open pores of PA mixtures during the test is likely to act as a "shock absorber", partially taking the load while improving the rutting resistance of these mixtures. The WTS w index decreased by 34% compared to WTS air for samples with binder containing 10% crumb rubber. Percentage of rut depth decreased by 27% when binder was modified with a combined SBS copolymer and crumb rubber was applied. 5.
The addition of rubber granulate directly affects the WTS and PRD parameters. The higher the replacement ratio of mineral aggregate with rubber granulate, the more the rutting parameters deteriorate. For example, for SMA8 LA mixture with a 5% copolymer modified binder, WTS air is 0.05 and PRD air is 3.93. If the addition of rubber granulate in SMA LA is 30%, the rutting indexes are: WTS air = 2.11 and PRD air = 30.31.

Funding:
The research was carried out as part of the research works No. WI/WB-IIL/10/2020 and No. WZ/WB-IIL/1/2020 at the Bialystok University of Technology and financed from a subsidy provided by the Polish Ministry of Science and Higher Education.
Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.
Data Availability Statement: Data available in a publicly accessible repository.