1. Introduction
Newer technologies such as warm mix asphalt (WMA) and reclaimed asphalt pavement (RAP) have gained international approval and have been considered as appropriate solutions that support the sustainability goals of the highway sector. Warm mix technology lowers the production and compaction temperatures which contributes to a reduction in the amount of greenhouse gases emitted and energy consumption [
1,
2]. Reclaimed asphalt technology promotes the recycling of old pavement materials to be used as a replacement of virgin binder and aggregates for the production of new asphalt mixtures. Moreover, RAP technology leads to the preservation of non-renewable resources, reduction of production costs, and other economic and environmental benefits [
3]. However, both technologies present some shortcomings. The lower mixing and compaction temperatures of WMA reduce the binder aging and the bond between the aggregates and the coating binder, thus resulting in less rutting resistance and higher moisture susceptibility [
4,
5,
6]. On the other hand, RAP mixes tend to be stiffer and more brittle than conventional HMA mixtures due to the effect of aged binder. Typically, in recycled pavement mixtures, with the admixing of aged asphalt, the blended asphalt is harder, has higher elasticity, and lower viscosity. The high-temperature performance improves, but the low-temperature performance worsens [
7,
8]. This tends to increase the crack propagation distresses [
9,
10]. In order to overcome these shortcomings, WMA-RAP technology has been investigated [
11,
12]. The aged binder of RAP might efficiently compensate for the soft warm mix binder, so that the fatigue life of RAP mixes gets extended and the rutting performance of WMA mixes is maintained. Ultimately, the most desirable feature of WMA-RAP mixes is the reduction in production and compaction temperatures while incorporating increased proportion of RAP leading to overall saving in energy and economical cost without compromising on the properties of the bituminous mix [
13].
A number of studies have been conducted in recent years to understand the performance of WMA-RAP mixtures. Oliveira et al. [
14] indicated that the rutting resistance of HMA and RAP mixes was improved with the addition of a surfactant-based additive, however, the effect was mostly recognized in RAP mixtures. Gungat et al. [
15] used a multiple stress creep and recovery (MSCR) test to investigate the effect of RH-WMA (combination of polyethylene and wax) on asphalt mixtures incorporating 30, 40, and 50% RAP, and noticed an improvement in the rutting resistance with the increase in RAP content. Zhao et al. [
16] conducted asphalt pavement analyzer, Hamburg wheel tracking, and flow number tests on asphalt samples mixed with a foaming WMA additive, and incorporating RAP contents of 15% and 30%. They found that increased RAP content contributes to a decrease in the rut depth regardless of the source of reclaimed materials. Similar findings were reported by Xie et al. [
17].
Table 1 presents the effect of different WMA additives on the rutting performance of mixtures containing RAP.
In addition to the rutting potential of WMA-RAP mixtures, the fatigue under repetitive loading and low temperature cracking need to be assessed. In that context, Das et al. [
24] conducted dynamic modulus (DM), indirect tensile (IDT), and bending beam rheometer (BBR) tests to measure the stiffness, creep compliance, and tensile strength of WMA-RAP asphalt samples and showed that water foaming WMA and asphaltene B additives had no effect on the fatigue and thermal cracking performance of RAP mixes. Gabchi et al. [
25] found that Zeolite-based additives (ZTWM, CHWM-B, and CHWM-S) enhance the creep compliance of RAP mixtures at −18 °C which implies a better stress relaxation and a better thermal cracking resistance for WMA-RAP. Vaitkus et al. [
26] reported a decline in the cracking performance of WMA-RAP mixtures when Sasobit dosage exceeds 2%. In another study by Singh et al. [
27], the results of the semi-circular bending (SCB) testing indicated that the addition of Ft-wax additive reduced the fracture resistance of RAP mixes. Zhao et al. [
16] found that the cracking performance of asphalt mixtures containing up to 30% RAP content could be improved by the WMA foaming technology; however, when 30% threshold is exceeded, the performance is compromised.
Another concern regarding WMA-RAP mixtures is its moisture susceptibility, especially for foamed techniques that work by injecting water to lower the virgin binder viscosity. Low mixing temperature feature of WMA, non-ductile RAP binder, and aggregate striping concerns of RAP technology are main factors that create the necessity to investigate the moisture susceptibility of WMA-RAP produced mixtures. For this reason, to assess WMA additive impact, Frigio et al. [
28] conducted indirect tensile strength ITS, Cantabro, SCB, and repeated indirect tensile tests on mixes containing 15% of RAP and using different WMA additives and found that the chemical additive provides acceptable resistance to moisture while the organic wax and zeolite performed poorly. The chemical additive performed best because of the inherent antistripping capabilities. Moreover, Guo et al. [
29] indicated that the short-term aged WMA-RAP mixtures provided higher tensile ratio TSR moisture resistance than the corresponding RAP mixture. However, TSR values were drastically reduced after long-term aging.
Further studies should be conducted to characterize the effect of the diversity of available WMA additives on the properties and mechanical performance of asphalt mixtures produced through WMA-RAP technology in order to determine the best additive and with which RAP content. In this regard, the work of this paper addresses this goal and discusses the impact of a chemical WMA additive (Rediset LQ1102CE
®) on the performance of asphalt mixtures incorporating low (15%), medium (25%), and high (45%) RAP contents. The Rediset effect on reclaimed asphalt mixtures needs to be investigated. Rediset additive can be considered as an easy-to-use liquid that not only is a WMA additive, but it also provides an active adhesion that enhances the coating of aggregates and improves the moisture resistance of asphalt mixtures [
30,
31].
Therefore, the objectives of this paper are to: (a) Determine the effect of the chemical WMA additive Rediset on the dynamic modulus and phase angle properties of the asphalt mixture containing different RAP percentages, (b) evaluate the impact of Rediset addition on the permanent deformation and the fatigue behavior of RAP mixtures, and (c) investigate the correlations between the flow number (FN) and dynamic modulus (DM) results used to characterize the asphalt mix rutting resistance potential.
3. Results and Analysis
3.1. Mastercurves Parameters
For all mixes of the study, the summary of the fitting parameters associated with the functions of |E*|, log at, and Φ is presented in
Table 5.
R2 statistics (above 0.99) indicates an excellent curve fitting of the dynamic modulus and phase angle mastercurves to measured data utilizing the sigmoidal model given in Equation (1) and the polynomial shift factor function expressed in Equation (2), as well as the modified phase angle expression of Equation (6).
3.2. Dynamic Modulus Results
Figure 10 and
Figure 11 show the dynamic modulus mastercurves of diverse mixes of the study in log-log and semi-log scale, respectively.
A quick analysis of the plots indicated that the higher the RAP content, the higher is the |E*| of the mixture for both RAP and WMA-RAP technology. Moreover, the values of modulus of all HR mixes are higher than those corresponding to WR mixes. Except of Hz, from the largest to smallest, over the full analyzed frequency range, |E*| of the mixtures can be mostly ranked as follow: HR45, HR25, HR15, WR45, WR25, and finally WR15.
Specifically, the results showed that the addition of Rediset additive reduces |E*| of the corresponding HR mix at both high and low reduced frequencies. Because of the strong correlation between |E*| and the rutting and fatigue distresses [
37], this reduction in |E*| implies that the addition of Rediset might affect the performance of AC mixtures.
To highlight the effect of Rediset on each RAP content separately, the reduction (%) in |E*| against the reduced frequency is presented in
Figure 12. For analysis purposes, the frequency range of |E*| mastercurves is decomposed into three arbitrary zones: (1) lower range from
to
Hz, middle range from
to
Hz, and a higher range from
to
Hz. It is worth noting that the behavior of AC mixtures at the lower, middle, and higher frequency ranges correspond to its behavior at regions with high, normal (operational), and low temperature ranges.
For HMA mixtures incorporating low RAP content (HR15), higher reduction due to Rediset addition occurred in the lower frequency range compared to the middle and higher range. This reduction is significant and increased from 22.98% at Hz to a maximum of 26.97% at Hz then decreased to 24.27% at Hz. In the middle frequency range, the reduction in |E*| decreased from 24.27% at Hz to 9.32% at Hz. In the higher frequency range, the reduction in |E*| is almost constant and not significant ranging between 9.32% at Hz and 7.5% at Hz.
For HMA mixtures incorporating medium RAP content (HR25), the reduction is slight at very low frequency of Hz but it becomes significant and at the highest in this range specifically between and Hz while having a peak of 27.1% at Hz. In the middle range, the reduction in |E*| decreased from 25.74% at Hz to 8.5% at Hz. In the higher frequency range, the reduction in |E*| is almost constant and not significant ranging between 8.5% at Hz and 9.45% at Hz.
In the case of HMA mixtures with high RAP content (HR45), the reduction is significant and occurred also in the lower range of frequencies with a maximum |E*| reduction of 27.85% at Hz. As increases in the lower range, the reduction decreased reaching 16.7% at Hz. In the middle frequency range, the reduction in |E*| decreased from 16.7% at Hz to 9.88% at Hz. In the higher frequency range, the reduction in |E*| is almost constant and not significant ranging between 9.88% at Hz and 10.69% at Hz. As a summary, the reduction in |E*| for HR15 and HR25 caused by Rediset addition is almost equal and higher than that of HR45 in the middle range of frequencies. In the higher range, comparable reduction in |E*| is found for different RAP mixtures assessed. Essentially, it was observed that the effect of Rediset on RAP mixtures was at the highest and mostly significant in the lower frequency range regardless of the RAP content incorporated. Knowing that the behavior of AC mixtures at low frequencies corresponds to its behavior at high temperature in which the rutting distress prevails, it is very critical to assess the impact of the significant reduction in |E*| found on the rutting performance of WMA-RAP mixtures compared to standard RAP mixtures.
In addition, the reduction in |E*| caused by Rediset additive in both the middle and higher range of frequencies indicates, respectively, an improvement in the fatigue and thermal cracking of WMA-RAP mixtures with respect to their corresponding HMA-RAP mixes.
3.3. Phase Angle Results
Figure 13 shows the phase angle mastercurves of diverse mixes of the study in semi-log scale. In the case of phase angle, the effect of the addition of Rediset was not consistent over the range of analyzed frequencies for RAP mixes unlike the case of the dynamic modulus. WR15 mix has the highest Φ at smaller frequencies and at the lower band of middle frequency range indicating highest viscous behavior of this mix among others which correlates well with the lowest dynamic modulus associated to it. At the upper band of the middle range and at higher frequencies, comparable Φ can generally be observed.
In order to show the effect of Rediset on each RAP content separately, the difference (%) in Φ between HR and its corresponding WR mix was plotted against the reduced frequency as illustrated by
Figure 14.
For HMA mixtures incorporating low RAP content (HR15), Rediset addition increased Φ values between of and Hz and decreased Φ values for above Hz. Nevertheless, the only significant effect is observed at very low frequency of Hz. Moreover, a slight increase of about 2.87% in the peak value of Φ is observed for WR15 compared to HR 15, while for both HR15 and WR15 mixes, the peak occurred approximately at the same reduced frequency level indicating that the maximum viscous effect occurs in both mixtures at the same temperature or at the same loading time.
For HMA mixtures incorporating medium RAP content (HR25), Rediset addition increased Φ values for under Hz, slightly decreased Φ values between and Hz, and increased Φ values again above Hz. However, the effect of Rediset is significant only at very low frequency of Hz and very high frequencies above Hz. An insignificant increase of 1.7% in the peak value of WR25 compared to HR25 is observed, while for this WR25 mix, the peak Φ is reached at comparatively lower temperature (higher reduced frequency) than HR25, indicating development of peak viscous effect at relatively lower temperature for the AC mix.
Finally, for HMA mixtures incorporating high RAP content (HR45), Rediset addition increased Φ values for under Hz, decreased Φ values for between and Hz, and is ineffective on Φ values for above Hz. An insignificant decrease of 2.38% in the peak value of WR45 compared to HR45 is observed, while for this WR45 mix, the peak Φ is reached at comparatively lower temperature (higher reduced frequency) than HR45, indicating development of peak viscous effect at relatively lower temperature for the AC mix.
3.4. Flow Number Results
Table 6 shows the results of FN testing for different HR and WR mixes and
Figure 15 presents a graphical plot to compare both FN and FN index data. It is worth noting that the variation in the flow number FN test can be relatively high as the highest proportion of the coefficient of variation in FN-values was 28.97% for the mix HR45. The variation is even higher for the FN index calculation with a maximum COV value of 47.09% for HR45 as well.
From the analysis of the results both parameters indicate the same information, that WR mixes are more susceptible to rutting than HR mixes and by descending order, the rutting resistance of HR45 is the highest, then HR25, HR15, WR45, WR25, and finally WR15. Furthermore, it was found that the addition of Rediset reduced the FN of HR45 by 74.7%, HR25 by 49%, and HR 15 by 56%, and increased the FN index of HR 45, HR25, and HR15 by 136%, 121%, and 206%. Essentially, the results indicated that better rutting performance is associated to increased RAP content for both HMA-RAP and WMA-RAP mixtures. This can be explained by the fact that the RAP contains aged binder having higher PG grade that stiffen the mixture and is indeed contributing to an increase in the rutting resistance.
Ultimately, the results of
Figure 15 indicates that despite the reduction in FN induced by the Rediset effect, both RAP and WMA-RAP mixes except WR15 provided acceptable level of rutting performance with a FN above 190 cycles [
46].
3.5. Comparison between |E*| and FN Test Results of Asphalt Mixtures
|E*| values at higher temperatures are generally used to evaluate the rutting performance since asphalt mixtures are susceptible to this distress at higher temperatures. In line with that, |E*| values at higher temperatures 37.8 °C and 54 °C with different frequency levels of 25, 10, 5, 1, 0.5, and 0.1 Hz were acquired from the dynamic modulus mastercurve of each mixture.
Table 7 and
Table 8 show the results of |E*| at the investigated temperature/frequency condition versus the FN results for the mixtures of the study.
It can be concluded from
Table 7 that at a temperature of 37.8 °C, DM and FN provide mostly similar ranking of the rutting performance of asphalt mixtures. However, as illustrated in
Table 8, at a temperature of 54 °C only at higher frequencies of 25 and 10 Hz, the ranking of rutting performance of DM and FN is comparable. At a temperature of 54 °C and a frequency 5 Hz, DM ranks the rutting resistance of HR15 higher than HR 25 compared to FN. An additional difference is observed between DM and FN at a temperature of 54 °C for smaller frequencies, especially at 0.1 Hz, where DM ranks, respectively, HR15 and WR45 as the highest and lowest resistant mixes to rutting whereas FN ranks HR 45 and WR15 as the highest and lowest resistant mixes to rutting.
Graphical Correlations for the Laboratory Results
The results of the correlations in
Figure 16a reveal that at a temperature of 37.8 °C at which both the dynamic modulus (DM) and flow number (FN) present comparable ranking of the rutting potential of asphalt mixtures, strong correlations (R
2 > 0.84) existed between |E*| and (FN) results, the best correlation being found at a frequency of 0.5 Hz and 0.1 Hz. At a temperature of 54 °C, it is observed in
Figure 16b that there are no strong correlations between the flow number (FN) and |E*| values (R
2 < 0.75) for frequencies of 1 Hz and below, however, strong correlations were found between the FN and |E*| values for higher frequencies of 5, 10, and 25 Hz (R
2 > 0.86). Based on the findings of this section, and since the NCHRP 465 reported a strong correlation between FN results and rutting measured in field [
38], |E*| at 37.8 °C and especially for 0.1 Hz (highest R
2 = 0.857) or at 54 °C and for a frequency of 10 Hz (highest R
2 = 0.889) might be a proper DM laboratory test condition for estimating the rutting-resistance potential of RAP and WMA-RAP mixes in the field.