EFFECTS OF RECLAIMED ASPHALT, WAX ADDITIVE, AND COMPACTION TEMPERATURE ON CHARACTERISTICS AND MECHANICAL PROPERTIES OF POROUS ASPHALT

. This paper describes physical and mechanical properties of porous asphalt mixtures with various RAP amount (0%, 10%, 20%, 30%) containing one WMA additive (organic wax). The samples were prepared using the Marshall compactor at two different temperatures (125 °C, 145 °C) by fabricating six series of porous mixtures. Air void content,


Introduction
Ensuring pavement sustainability is one of the main goals in the road infrastructure planning and management systems (Radziszewski et al., 2016), especially from a life-cycle perspective (Santos et al., 2017). Several technologies have been implemented for this purpose, such as warm mix asphalts (WMAs) and the application of the reclaimed asphalt pavement (RAP) in the production system.
In particular, warm mix asphalts (WMAs) allow limiting the HMA production temperature (e.g., Tatari et al., 2012), while preserving acceptable mechanical properties (e.g., Capitão et al., 2012;Jamshidi et al., 2013;Xiao et al., 2015). WMA technology improves HMA workability based on viscosity changes as a result of the action of organic, chemical, or water-based additives (e.g., Król et al., 2015;Choudhary et al., 2018). For example, long chain additive waxes in a molten form can be added to the aggregate blend or to the HMA mixer (Hurley & Prowell, 2005).
Recycling of the hot mix asphalts (HMAs) through using reclaimed asphalt pavement (RAP) for the production of new pavements is aimed at preserving limited sources of the aggregate and at reducing the amount of virgin asphalt binder. In fact, during the last decades, the content of RAP obtained from road milling or discharged from plant processing in HMA mixtures has been increased to save energy and materials (Lee et al., 2015), still allowing to maintain appropriate mechanical properties of asphalt for road pavement construction (e.g., Kowalski et al., 2016a;Behnood et al., 2015;Su et al., 2014).
The two technologies can be combined (WMA-RAP), and the WMA offers a possible advantage of adding higher percentages of RAP in the mix design process, which should necessarily consider the properties and composition of the added RAPs (see Al-Qadi et al., 2007). The effect of WMA-RAP technology on pavement performance has been recently reviewed by Guo et al. (2020), who considered different types only in the case of chemical additives, compared to the reference HMA. In another study on similar WMA-RAP PAs, Frigio et al. (2016) found lower stiffness values and extensive long-term aging effects compared to the reference HMA, and higher stiffness and lower fatigue slope parameters in the case of mixtures supplemented with organic wax as compared to other additives. Considering WMA-RAP PAs with the added 15% RAP, compacted at 120 °C and treated with chemical additives, Frigio et al. (2017) found lower stiffness values and significant water susceptibility compared to the reference HMA (with 0% RAP). Moreover, better performance in terms of water susceptibility for mixtures with the added surfactants and adhesion enhancers with respect to viscous regulators was reported.
It is evident from the highlighted results of the previous studies that there is insufficient research reporting on the characteristics and mechanical properties of WMA-RAP PAs. Moreover, these studies have used a constant amount of RAP (15%) and a compaction temperature slightly variable (between 110 °C and 120 °C), while using several additive types. Hence, it is necessary to analyze the effects of different RAP contents and compaction temperatures in producing WMA-RAP PAs.
For the above-mentioned reasons, this study aims at answering the following research questions: − Are there any significant changes in the WMA-RAP PAs characteristics and mechanical properties if different RAP contents and compaction temperatures are considered? − Which factors have the most significant effects on each characteristic and mechanical property of WMA-RAP PAs -the use of RAP, different compaction temperatures, or the use of additives? For this purpose, five porous asphalt mixtures were prepared, tested, and compared with a control HMA porous mix (the "ECD" mix, previously used for research purposes by Ranieri et al., 2010). The ECD control mix is a composite that combines the high draining capacity with a reduced thickness that allows for material savings and easier maintenance.
For this study, the wax additive was selected among all existing WMAs because it lowers the viscosity of asphalt bitumen and the working temperatures can be significantly decreased (Capitão et al., 2012). Moreover, the organic wax can provide an increased stiffness to WMA PAs among other possible additives (Frigio et al., 2016).
A detailed test program was conducted in view of the research questions posed, as described in the next section. The test program is focused on the study of macro-characteristics and mechanical properties

1.
Materials and methods

Materials
The characteristics of the mixtures designed for research purposes are presented below.

Characteristics of baseline materials
The baseline porous asphalt mixture used in this study was designed according to the "ECD" gradation previously used by Ranieri et al.  (2010). The aggregate blend was composed of basalt and gabbro. Other mixtures were prepared by adding reclaimed asphalt pavement (RAP).
The used RAP source originally came from a Stone Mastic Asphalt (SMA) pavement, acquired from a Polish road. The binder originally used in this SMA was highly polymer modified.
The selected gradation curves of different mixture types (with different percentages of RAP ranging from 10% to 30%) are presented in Figure 1. They were chosen to be as close as possible to the minimum and the maximum size passing as reported in the ECD granulometric specification, indicated in the figure. In particular, the aggregate gradation was chosen for each mixture type in order to reach the trade-off between the optimum asphalt content minimization and the adherence to the required mix design. It can be noticed that the curve representing 30% RAP is the farthest from the ECD gradation required due to the high presence of fine material in the used RAP. Once that and the RAP aggregate gradation have been determined, RAP aggregates were blended with the virgin aggregates to meet the overall mixture gradation requirements. Clearly, the total amount of material for the production of each specimen changed as the amount of added RAP increased.
The final granulometric composition of the aggregate blend of the different mixtures is reported in Table 1, together with information about the bitumen used as a binder.
All specimens were prepared by using polymer-modified bitumen 45/80-65 (complying with EN 14023 European standard) produced in Poland by Lotos Group. This binder content was set as the reference content for all the prepared samples, considering both 6% of the binder already included in the RAP and the virgin binder added to the mix design. The percentage of the binder was not varied across the sample mixes to reduce the number of study variables (such variability may affect physical and mechanical properties, see Čygas et al., 2011).
Cellulose fibers (stabilizer) were added to the mixtures in the amount of 0.4% by aggregate weight. An adhesion agent was also used (0.3%, related to the bitumen weight). Finally, an organic wax additive (Sasobit®) was added to the binder in the amount of 2.5% by the binder weight.
The aggregates were placed in an oven for four hours. After the mixing process, Marshall samples were produced at two different compaction temperatures (125 °C and 145 °C). Mixture compaction was ensured by application of 50 blows of Marshall hammer per sample side.

Methods
The testing methods used are aimed at inquiring into the variation in physical characteristics and mechanical properties of PAs due to the variation in RAP, wax additive, and compaction temperature.

Testing variation in physical characteristics
Air void content. According to PN-EN 12697-8 specification, the air void content (Vm) was calculated on four specimens of each mixture. It was calculated from the maximum density and the bulk density of the specimen.
Permeability. Permeability was estimated using the following index (Ranieri et al., 2010): where: PI -Permeability Index, the permeability decreases as this index increases; P 2 -percentage of the aggregate passing to the sieve 2 mm, %; P 5 -percentage of the aggregate passing at the sieve 5 mm, %; D max -maximum diameter of the aggregate, mm.
The higher the calculated PI index, the lower is the permeability of the mixture according to the following experimental relationship (Ranieri et al., 2010): k v = 0.008 · exp(−3.8396 · PI), (2) where k v is the vertical permeability, m/s.

Testing variation in asphalt mechanical properties
Particle loss. The Cantabro test is often used for PAs for measuring the durability and the potential for aggregate loss from mixtures (see Tao & Mallick, 2009). The particle loss of porous asphalt mixtures was calculated on three specimens for each mixture according to PN-EN 12697-17 standard. The particle loss was assessed by the loss of sample mass after 300 turns in the Los Angeles machine.
Stiffness. Stiffness was measured according to the specifications of the European test Standard EN 12697-26. The 25 kN Universal Testing Machine was used on two specimens for each mixture. At a temperature of 12 °C, five load pulses were applied to the specimen and the variation of the applied load and horizontal diametric deformation was measured for each load application. The stiffness modulus was obtained for each load pulse according to Equation 3: where: S = measured stiffness modulus, MPa; Cracking potential. The indirect tensile strength (ITS) is a reliable indicator of the asphalt mixture cracking potential (Watson et al., 2004): the higher the ITS, the higher the potential resistance to cracking. For this purpose, a compression (flow/stability) Marshall testing machine was used according to EN 12697-23 on three specimens for each mixture. For each test specimen, the indirect tensile strength was calculated (that is, the maximum tensile stress calculated from the peak load applied at break).
Moisture susceptibility. Moisture susceptibility is the tendency of HMA mixtures to lose the adhesion bond between the asphalt and aggregate particles; it is one of the greatest concerns related to pavement performance independently of its mix design. Testing mixes with and without moisture conditioning can aid in measuring their resistance to moisture susceptibility. Hence, three additional freeze-conditioned specimens for each mixture were prepared besides the unconditioned specimens described above. The conditioning was performed by storing water-saturated samples for three days in a water bath at 40 °C, then they were protected from losing the water by a soft plastic bag and placed in a freezer at −18 °C for one day. Finally, they were placed again in the bathtub at 25 °C for another day. Once tested, all samples were loaded diametrically until failure. The Indirect Tensile Strength Ratio (ITSR), used to assess the resistance to moisture susceptibility (the higher the ITSR, the higher the moisture resistance), was determined by Equation 4: where: ITSR = indirect tensile strength ratio, %; ITSwet = average ITS on the wet group, kPa; ITSdry = average ITS on the dry group, kPa.

Statistical analyses
A one-way analysis of variance (ANOVA) has been conducted for each measure made with regard to physical and mechanical properties. The response variables of each ANOVA test were, namely: − air void content (using 24 observations, with 4 specimens for each mixture, including the reference baseline PA mixture); − particle loss (using 24 observations, with 4 specimens for each mixture); − stiffness (using 8 observations, with 2 specimens for each mixture); − indirect tensile strength (ITS) in dry conditions (using 18 observations, 3 specimens for each mixture); − indirect tensile strength (ITS) in wet conditions (using 18 observations, 3 specimens for each mixture); − ITS ratio (using 18 observations, with 3 specimens for each mixture). Factors considered in the ANOVA test were: − "RAP", that is, presence of RAP (0 = not present, 1 = 10% RAP, 2 = 20% RAP, 3 = 30% RAP); − "WAX", that is, presence of wax additive (0 = not present, 1 = present); − "T", that is, compaction temperature (0 = 145 °C, 1 = 125 °C). In this way, it is possible to assess the contribution of each factor to the differences in the properties of the mixes, accounting for the main effects of the other considered factors.
Due to the unbalanced design, Type-II ANOVA was conducted. Statistically significant differences (at the 5% significance level) between mean values (response variables) of different groups may indicate which factors have the most significant influence on the variation in physical and mechanical properties. Post-hoc tests (Tukey adjusted) were applied in case of statistically significant differences associated with the RAP factor (since it is the only factor with more than two levels). Statistical analyses were run in R environment.
It should be noted that the permeability index (Equation 1) was not used as a response variable for ANOVA tests, since only one measure for each mixture was possible. Moreover, it is clearly related to the porosity, as discussed below. Outlier values were discharged from the dataset before the analysis, excluding one observation in particle loss measures from Cantabro tests and one ITS measure in dry conditions (and then one measure of ITS ratio). The ITS ratio was obtained for each of the 18 observations by applying Equation 4, considering the average ITS measure in dry conditions and the ITS measure in wet conditions for each of the three specimens made with the mixtures investigated. Moreover, due to the limited number of samples, interaction terms were not considered in the ANOVA model specifications. Given the research questions of this study, results from the analyses were used to consider the influence of different factors on the RAP-WMA PA characteristics and mechanical properties. Life cycle assessments (see, e.g., Oner & Sengoz, 2015;Riekstins et al., 2020) are outside the scope of this study, while they may be useful in the future, once the properties of different WMA-RAP PAs will be consolidated based on research outcomes.

Results
The results obtained are presented and discussed below.

Physical characteristics
Results concerning physical characteristics are graphically depicted in Figure 2, while ANOVA results are summarized in Table 3.

Air void content
Mix #4 (R20_T145), which is a RAP PA not supplemented with wax and compacted at 145 °C, has the lowest value of the air void average, while Mix #5 (W_R20_T145) shows the highest value. If an environmental concern is considered and the temperature is lowered (down to 125 °C), the highest voids are achieved with a low RAP content (Mix #1, 10% RAP). Hence, with respect to the baseline PA mixture,  the WMA-RAP PA mixture is related to a general decrease in voids and density, except in the case of using a 145 °C compaction temperature.
In particular, the use of at least 20% RAP (Mixes #2, W_R20_T125 and #3, W_R30_T125) leads to a statistically significant different average air content with respect to the baseline PA mixture.
Concerning the RAP amount in WMA-RAP PAs, the increasing RAP percentage is related to a decrease in the air void content. In fact, there is a decrement of 7% in average air voids between Mix #1 (W_R10_T125) and Mix #3 (W_R30_T125) found to be statistically significant as well.
From the comparison between Mix #4 (R20_T145, without wax additive) and Mix #5 (W_R20_T145, with wax additive), it is possible to notice a great increase (around 9%) in air void. In fact, the presence of wax additive itself is associated with a statistically significant increase in the mean air void content.
Increasing the compaction temperature from Mix #2 (W_R20_T125) with wax additive to Mix #5 (W_R20_T145), the air void content increases by about 6%. In fact, the temperature factor was identified to produce statistically significant differences.

Permeability
In general, as the air void content increases, the permeability also increases (the index P 2 ⋅P 5 ⋅D max decreases).
With respect to the baseline PA mixture, all other mixes show an increased permeability (lower P 2 ⋅P 5 ⋅D max index), except for the WMA-RAP with 30% RAP (mix #3, W_R30_T125). The highest permeability is shown for mix #1 (W_R10_T125).
A linear porosity-permeability trend can be noticed for mixes #1 (W_ R10_T125), #2 (W_R20_T125) and #3 (W_R30_T125). So, increasing the RAP amount (considering the same compaction temperature and the same wax additive quantity), the air void content decreases while P 2 ⋅P 5 ⋅D max increases. Hence, by increasing the RAP amount in WMA-RAP PAs, the permeability should evidently decrease.
Whereas, by looking at differences between Mixes #2 (W_R20_T125), #4 (R20_T145), 5 (W_R20_T145), it is possible to note that for the same amount of RAP (20%), different compaction temperatures or the use of the wax additive does not influence permeability.
It is however important to note that once converted into the vertical permeability values through Equation 2 (see Figure 2), the differences in permeability are very limited (i.e., in the order of 0.1 mm/s). The most significant difference is for WMA-RAP PAs produced with high RAP content (30%) compared to lower contents, which is in the order of 1 mm/s, thus still practically having weak significance for the drainage purposes.

Mechanical properties
Results concerning mechanical properties (particle loss, stiffness, and tensile strength) are graphically depicted in Figure 3, while ANOVA results are summarized in Table 4.  The results show that particle loss is strictly related to the air void content: as the PL index increases, the voids generally increase as well. A significantly increased cohesiveness (decreased particle loss) can be noted for high RAP content (30%) WMA-RAP mixtures (Mix #3) and for RAP mixtures (without wax) compacted at 145 °C (Mix #4). A decreased cohesiveness (increased particle loss) can be observed for low RAP content (10%) WMA-RAP mixtures (Mix #1) instead.
Considering WMA-RAP PA mixes from #1 to #3 compacted at 125 °C, it can be determined that the mixture with higher amount of RAP exhibits better resistance to particle loss: the PL index decreases of 40% in Mix #3 (W_R30_T125) compared to Mix #1 (W_R10_T125). Differences due to the RAP factor were found to be statistically significant (i.e., the higher the RAP content, the lower the PL). However, pairwise comparisons adjusted for multiple comparisons reveal no statistically significant differences between different RAP contents (also with respect to the reference PA Mix #0).
The wax factor was not found to have a statistically significant influence on particle loss. However, adding the wax additive to a RAP PA results in a slight increase in the void content and a less cohesive mixture. This emerges from the comparison between Mix #4 (R20_T145) and #5 (W_R20_T145).
The compaction temperature factor was not found to have a statistically significant influence on particle loss either. In fact, only a slight decrease in PL (−7%) can be noted between Mix #2 (W_R20_T125) and #5 (W_R20_T145), which differ from each other in the compaction temperature only.

Stiffness
It can be immediately noticed that stiffness seems to be strictly correlated to the void content as well: the greater the void content, the lower the stiffness. All mixtures different from the reference standard PA mix show higher stiffness values, except for Mix #5 (W_R20_T145), which is a WMA-RAP with wax compacted at 145 °C. The better result in terms of stiffness is achieved by Mix #4 (R20_T145) and Mix #3 (W_R30_ T125), considering only WMA mixes.
Comparing WMA-RAP PA mixes with different RAP percentages, the higher the RAP percentage (from Mix #1, W_R10_T125 to Mix #3, W_ R30_T125), the higher the stiffness becomes (up to 4% more for Mix #3), even if the differences due to RAP were not highlighted as statistically significant (also with respect to the reference PA mix).
The presence of wax additive lowers the stiffness of the mixtures by about 20% (other conditions being equal, considering the comparison between Mixes #4, R20_T145 and #5, W_R20_T145). Differences due to the presence of wax (the higher the wax content, the lower the stiffness) are indeed highlighted as statistically significant. WMA-RAP PA mixes from #1 to #3 have slightly higher stiffness than the reference PA mix even if wax is added, but this effect is due to the presence of RAP.
The increased compaction temperature (145°C) is related to a decrease in stiffness (−6% if mix #5, W_R20_T145 is directly compared to mix #2, W_R20_T125). However, differences due to the temperature factor are not statistically significant.

Indirect tensile strength
It can be generally noted that the higher the RAP percentage and the compaction temperature, the higher the ITS value. Moreover, there is a weak relationship between air void content and ITS.
In particular, with respect to the reference PA mixture, the presence of RAP significantly affects the ITS (both the dry and the wet measures). In detail, a 30% of RAP (Mix #3, W_R30_T125) is needed to achieve statistically significant differences (in both cases). Moreover, between WMA-RAP Mixes #1 and #3, ITS is increased by 41% (in dry conditions) and by 23% (in wet conditions).
The presence of the wax additive statistically significantly affects the ITS in wet conditions. In fact, in dry conditions, the ITS even decreases by about 5%, while it increases by 12% in wet conditions (comparison between RAP Mixes #4 and #5, at 145 °C compaction temperature, without and with wax additive).
The presence of the compaction temperature significantly affects the ITS only in wet conditions as well. This is clear while comparing Mixes #2 (W_R20_T125) and #5 (W_R20_T145): the ITS increases by 14% in dry conditions, but by 23% in wet conditions.

Moisture susceptibility
From a moisture resistance perspective (related to the ITSR content), the best performance was achieved by Mix #5 (W_R20_T145). Among WMA PAs, the best performance was shown by Mix #1 (with a low RAP content: 10%).
Analysis results are straightforward. In fact, since the RAP content influences both the dry and wet ITS, the pairwise differences in ITSR between mixes with different RAP content are not statistically significant. Hence, while the moisture resistance related to ITSR slightly decreases between Mixes #1 and #3 with different RAP contents, the moisture resistance is comparable, on average, to the performance of reference Mix #0. On the other hand, wax and compaction temperature statistically significantly affected only the ITS in wet conditions. This is reflected in the influence of wax on ITSR (which is not valid for temperature either). The presence of wax clearly leads to an increase in the ITSR (as is evident by looking at the comparison between Mixes #4 and #5) and then to an increase in moisture resistance.

Discussion
As demonstrated by Table 5 below, in general, some RAP PA mixes (in bold or underlined) systematically show better mechanical properties than the reference PA. Moreover, some RAP PA mixes show both an increased porosity (Vm) and permeability (measured through the PI index).
In detail, adding wax and using high compaction temperatures result in significantly increased porosity, other conditions being equal (see Table 5). The result concerning temperatures is surprising if compared with the results for ordinary asphalt mixes (see, e.g., Gao et al., 2014) and wax-treated porous asphalt mixes (see, e.g., Ranieri et al., 2017, Chen & Wang, 2013, which reveal opposite trends. In this case, other conditions being equal, a significant increase in porosity can be observed comparing WMA RAP PAs compacted at high temperature (145 °C) to the one compacted at 125 °C. Hence, porosity differences due to temperature in WMA-RAP-PAs are worth further investigation. On the other hand, high RAP contents (20-30%) result in a significantly decreased porosity (Table 4), which is coherent with the results from Goh & You (2012) and Frigio et al. (2013). In fact, among WMA-RAP PAs, the highest porosity is related to low RAP content (Mix#1, W_R10_T125). Moreover, the highest permeability is reached for the same low RAP WMA RAP PA (Mix#1), followed by all the 20% RAP mixes. It should be noted, however that, as already previously stated, even if statistically significant, the difference in permeability is practically slightly relevant (in the order of 0.1-1 mm/s).
As far as mechanical properties are concerned, clearly, there are advantages and disadvantages in considering different factors alone. In general, from Table 5 it is evident that: − adding wax leads to a significant decrease in stiffness and an increase in moisture resistance; − high RAP contents (30%) are pronouncedly related to an increase in the indirect tensile strength (both in dry and wet conditions); − cohesiveness (and then durability) is not significantly affected by RAP, wax, or compaction temperature.  However, the indications provided in Table 5 can be combined to obtain the following remarks: − with regard to stiffness, a good trade-off between environmental and performance-related aspects is reached by the WMA high RAP content (30%) mix (Mix #3: W_R30_T125), for which the second-best stiffness was recorded; − with regard to moisture resistance, the WMA low RAP content (10%) mix (Mix #1: W_R10_T125) is a good trade-off between the environmental and performance-related aspects, for which the second-best moisture resistance was recorded, which, however, may be lower than the minimum required by some road construction standards. In summary, high RAP content (30%) WMA-RAP PAs (supplemented with wax, mixes from #1 to #3) show an excellent cracking resistance (see also Goh & You, 2012, in case of 15% RAP content), higher stiffness and cohesiveness compared to standard reference PAs, while they have lower porosity, permeability and moisture resistance (see also Guo et al. (2014) or Frigio & Canestrari (2018)). Low RAP content (10%) WMA-RAP PAs (supplemented with wax) show good permeability (related to very high porosity), higher moisture resistance, and relatively higher stiffness compared to standard reference PAs, while they have lower cohesiveness and indirect tensile strength.
The above-stated mechanical properties of WMA-RAP PAs are to a great extent attributable to the asphalt mixtures tested and the type/quantity of additive used (organic wax: Sasobit). In fact, performance may vary and even show opposite tendencies if the asphalt mixture composition and the type/amount of additives vary (see a comprehensive review reported in (Hettiarachchi et al., 2019;Cheraghian et al., 2020) which however mostly refers to dense-graded mixes, or (Sanchez-Alonso et al., 2011;Li et al., 2015). It was shown in the previous research related to WMA-RAP PAs that by varying the additive type, moisture resistance (Frigio & Canestrari, 2018;Frigio et al., 2017) and stiffness and fatigue behaviour (Frigio et al., 2016) can significantly vary.
As far as RAP PAs (Mix #4: R20_T145) are concerned, they show excellent cohesiveness (see also Frigio et al., 2014) and stiffness, higher permeability, and indirect tensile strength (see also Goh & You, 2012), while they have a lower porosity (Goh & You, 2012;Frigio et al., 2013) and moisture resistance. If wax is added (Mix #5: W_R20_T145), RAP PAs demonstrate improved moisture resistance, even at the worsened indirect tensile strength, stiffness and cohesiveness.

Conclusions
The effect of different RAP contents, presence of wax and different compaction temperatures was studied in detail with specific regard of the porous asphalt (PA) specimens. Characteristics and mechanical properties of the produced PAs were studied through a detailed test program. In particular, air void content, permeability, particle loss, stiffness modulus, indirect tensile strength (in dry and wet conditions), and indirect tensile strength ratio, were measured.
Statistical tests were conducted to assess the influence of each factor (RAP, wax, temperature), considering the variation of the other factors. The wax additive is significantly responsible for an increase in porosity (also while using high compaction temperatures), a decrease in stiffness, and an increase in moisture resistance measured through the ITS ratio. High RAP contents (30%) are significantly related to an increase in the indirect tensile strength. Cohesiveness measured through the particle loss seems to be not significantly affected by RAP, wax, or compaction temperature.
As far as advantages and disadvantages of different mixes are concerned: − High RAP content (30%) WMA-RAP PAs show higher indirect tensile strength, stiffness, and cohesiveness compared to the reference PAs, while they have lower porosity, permeability, and moisture resistance; − Low RAP content (10%) WMA-RAP PAs show higher porosity, permeability, moisture resistance, and stiffness compared to the reference PAs, while they have lower cohesiveness and indirect tensile strength; − RAP PAs (20% RAP) show higher cohesiveness, stiffness, permeability, and indirect tensile strength than the reference PAs, while they demonstrate lower porosity and moisture resistance. If wax is added (WMA-RAP), moisture resistance is improved, while indirect tensile strength, stiffness, and cohesiveness get worse. As far as the compaction temperature is concerned, lowering the compaction temperature in the WMA process (down to 125 °C) affects only the porosity, which decreases, and the ITS in dry conditions, which decreases either, while the ITSR is not statistically significantly affected. However, even if the porosity significantly decreases, especially considering the 30% RAP mixture, the corresponding computed permeability is still acceptable (i.e., about 3 mm/s). On the other hand, the cracking resistance could actually be unacceptable, especially considering the 10% RAP mixture. Hence, the decision to use lower compaction temperatures in the case of WMA-RAP PAs, which results in saving energy and thus allows implementing the environmental agenda, could be adopted, even if it is subject to some important remarks. In fact, the appropriate amount of RAP in the mixture should be determined by considering all the requirements provided by specific country standards and, in particular, the strictest requirements. In other words, depending on the strictness of different regulatory requirements in terms of particle loss, stiffness, indirect tensile strength, moisture resistance, porosity, and permeability, different amounts of RAP may be acceptable. In particular, while all porosity, permeability, and stiffness values of WMA-RAP PAs (Mixes from #1 to #3) may be acceptable, one or more measures among particle loss, indirect tensile strength in dry conditions and the indirect tensile strength ratio could not meet specific standards/regulations. In detail, considering the extreme percentages, a 10% RAP content may lead to the development of acceptable moisture resistance, but unacceptable cohesiveness and/or cracking resistance; while a 30% RAP content may lead to a satisfying cracking resistance and cohesiveness, while unacceptable moisture resistance. The 20% RAP mixture clearly shows good performance in terms of cohesiveness, cracking, and moisture resistance, which constitutes a trade-off between the two extreme RAP percentages.

Funding
Part of this work was supported by the General Director for the National Roads and Motorways in Poland and by the National Centre for Research and Development in Poland under the grant agreement RID-I-76. Part of the research leading to these results was supported by the funds from the Kosciuszko Foundation Fellowship obtained by Karol J. Kowalski. Part of this work was supported by the Inter-governmental S&T Cooperation Project of . Also, part of the accommodation costs during the stay of the researchers in Poland was covered by the EU Erasmus Program.

Disclosure Statement
The authors have no competing financial, professional, or personal interests from other parties to declare.