3.1. Marshall and ITS Tests
The Marshall test results are shown in
Table 1. When samples are subjected to aging processes of STOA and LTOA, the AV slightly increases, and VFA is reduced. In the reviewed literature, no mentions were found in that regard. However, this could have been because during the oven heating processes, part of the volatile, oleous, and lighter components of the asphalt binder are lost [
40]. Even the asphalt binder’s specific gravity increases because asphaltenes begin to prevail [
53] and the molecular weight increases [
42]. Additionally, a small content of asphalt binder is lost in the LTOA process, when it adheres to the oven’s grill that holds the samples. On the other hand, given that mixes retain more time at high temperature inside the oven, the probability that the asphalt binder penetrates and adheres more easily to the superficial pores of the aggregate increases.
In general terms, despite a slight increase in AV during the aging process, the stability of mixes increases and flow decreases, which generates an increase in S/F ratio. This is mainly given because of the increase in stiffness of asphalt binder when it ages. The mix with the greatest monotonic resistance in the Marshall test is HMA-19 (greatest S/F values). In comparison to the HMA-10 mix, HMA-25 presents greater S/F values in control mixes and in STOA. In the LTOA condition, both mixes present statistically similar values according to ANOVA analysis. For the case of the HMA-10 mix, it increases 1.127 and 1.453 times with relation to the control mix when it is subjected to STOA and LTOA procedures, respectively. These increases are of 1.093 and 1.182 times for the HMA-19 mix and of 1.164 and 1.284 times for the HMA-25 mix. According to ANOVA analysis, these S/F increases in mixes as a product of short-term or long-term aging are statistically significant. Compared with the STOA process, the S/F is 1.289, 1.082, and 1.103 times greater in HMA-10, HMA-19, and HMA-25 mixes, respectively, when samples are subjected to the LTOA procedure. The mix that underwent less changes in S/F ratio due to aging was HMA-19. The mix that registered the greatest changes in STOA procedure was HMA-25, while in LTOA, it was HMA-10.
The ITS test results are shown on
Table 2. As in the Marshall test, the HMA-19 mix presents greater ITS values and HMA-10 presents the lowest. Additionally, ITSU and ITSC parameters increase when the mix ages. This is perhaps since in general, increases in stiffness in mixes generate increases in said parameters [
54]. ITSU increases in mixes as a product of STOA aging were statistically significant. For the case of ITSC, said increases were not statistically significant. Increases or changes in ITSU between control samples and samples conditioned in STOA and LTOA are depicted on
Figure 3. The mixes that are most susceptible to changes in their ITSU when they age are HMA-19 and HMA-25.
With regard to moisture damage resistance, the reviewed reference literature is ambiguous when it reports the effect of gradation on mixes. While some studies report that fine gradations tend to be more resistant to moisture damage mainly due to the smaller content of AV these mixes have [
55,
56], others conclude the contrary, arguing that it increases the specific surface of the aggregate, which could form a thinner asphalt film thickness, weakening the adhesion between the asphalt and aggregate [
57]. Additionally, moisture damage resistance is a function of water transport mode within the mix, which is controlled by the air void sizes and their connectivity, diffusivity of water molecules in the mixture and asphalt binder, filler content, aggregate gradation, absorption, and geometry, among others [
58,
59,
60]. In this study, the mix that displayed less moisture damage resistance was HMA-10, and the one that displayed the greatest moisture damage resistance was HMA-19. In addition, the TSR parameter is reduced when the mix ages, indicating a reduction in moisture damage resistance. When asphalt ages, it reduces its capacity to adhere to aggregates and to make the mix cohesive [
8,
61], which is mainly due to the loss of oleous asphalt binder components that contribute to its manageability. Added to this, aged asphalt binders contained a greater quantity of molecules and polar functional groups [
8,
42,
62,
63], which causes asphaltenes to start to prevail (increasing stiffness and viscosity) over other components that contribute with adherence such as resins [
64]. According to [
65], adhesion is directly correlated with the non-polar fractions of asphalt binder while stiffness is likewise with polar fractions.
3.2. Resilient Modulus and Permanent Deformation
The RMs of HMA-10, HMA-19, and HMA-25 mixes (control, STOA, and LTOA) are shown in
Figure 4. It is observable that the mix with lower stiffness (under any temperature, load frequency, and aging condition) is HMA-10, which is mainly due to its smaller particle size, finer gradation, and greater asphalt binder content. When comparing HMA-19 and HMA-25 mixes, there is no clear trend. At 30 °C, the HMA-25 mix presents a slightly superior RM with relation to HMA-19, but said variation is not statistically significant (based on an ANOVA analysis). At 10 and 20 °C, the trend is the same when comparing the control mix with STOA (HMA-25 presents RMs that are slightly higher but not statistically significant). However, under the LTOA condition, the HMA-19 mix tends to be stiffer, although variations continue to not be statistically significant when compared with the HMA-25. The lack of a clearly defined trend between both mixes is rooted in the fact that RM is a parameter that depends on multiple variables. For example, mixes that have a greater particle size tend to have a greater contribution in RM (in this case, HMA-25), while mixes with a lower AV tend to present a greater RM (this is the case of HMA-19).
A parameter used to evaluate the susceptibility of mixes for aging is the relationship between stiffness of the aged and unaged mix [
66,
67]. As a result of such reasons, the RM values of STOA and LTOA aged samples with relationship to the control mix were calculated (RM
STOA/RM
Control, RM
LTOA/RM
Control). The relationship RM
LTOA/RM
STOA was also calculated. These relationships or increases in RM as a product of asphalt binder aging are depicted in
Figure 5 and
Figure 6. Likewise, the averages (with relationship to values reported for each load frequency) of ratios RM
STOA/RM
Control, RM
LTOA/RM
Control, and RM
LTOA/RM
STOA are depicted in
Table 3. It is observable that these relationships are greater when the test temperature increases and load frequency decreases, which is mainly given because of the asphalt binders’ visco-elastic response. With regard to the effect of gradation, there is no existing clear trend. The HMA-10 mix tends to undergo greater RM
STOA/RM
Control and RM
LTOA/RM
Control ratios (greater increases in stiffness or susceptibility for aging) when the test temperature is 10 °C. Under this temperature, the mix that undergoes the smallest increases is HMA-25. At 20 °C, the mix that is more susceptible to changing its RM in STOA condition is still HMA-10; however, in the long term, the most susceptible one is HMA-19. Under this temperature, HMA-25 maintains itself with the least changes in RM. At 30 °C, a greater susceptibility can be observed in the HMA-19 mix when it ages in the short term and in the HMA-25 mix when it ages in the long term. Additionally, the greatest changes in RM between STOA and LTOA condition are presented in the HMA-19 mix when the test temperature is 10 and 20 °C, while for the case of 30 °C, these take place in HMA-25.
The permanent deformation resistance of a mix depends on a combination of factors such as asphalt binder type and stiffness, gradation, type form and texture of the aggregate, AV, and VFA, among others [
68]. The results of the permanent deformation tests that are presented in
Figure 7 are coherent with those obtained in the Marshall test and RM test. It is possible to observe an increase in permanent deformation resistance when mixes age as a product of the increase in stiffness in asphalt binder, increase in S/F ratio, and increase in RM. This increase in resistance can help to aid in resisting the rutting phenomenon in high-temperature climates [
39]. The mix with the least rutting resistance is HMA-10, which is mainly due to its lower RM value. The most resistant mixes are those that present a greater particle size and RM (HMA-19 and HMA-25). With regard to the influence of aging, HMA-25 was the mix that was least susceptible to changing its permanent deformation resistance (displacement of the control mix at 3600 load cycles—Δ
3600 was of 1.03 and 1.09 times with relation to the STOA and LTOA condition, respectively). In the case of the other two mixes, the susceptibility was similar. In control HMA-10, Δ
3600 was 2.19 times greater with relation to the STOA and LTOA condition, respectively, while in control HMA-19, it was 1.17 and 1.90 times greater.
3.3. Fatigue Resistance
The results of fatigue tests are displayed in
Figure 8. The amplitude of stress in kPa necessary for samples to fail at 10
6 load cycles (σ
6) is shown in
Figure 9. For the case of control mixes, the mix with greater fatigue resistance was HMA-19, while the mix that displayed the least resistance was HMA-25. Under controlled stress, in general, the stiffer asphalt mixtures are the ones that have the greatest fatigue resistance [
69,
70,
71,
72]. However, in this case, control HMA-10 with less stiffness undergoes greater fatigue resistance than control HMA-25, which is mainly since this last one displays a superior AV.
On the other hand, fatigue resistance increased when mixes aged, which was mainly given because of what was mentioned above (under controlled stress, asphalt mixes undergo an increase in fatigue life when stiffness increases).
Table 4 shows how the
Nf of mixes increased on average when these aged. HMA-25 presented the greatest increases in fatigue life upon aging (greater
NfSTOA/
NfControl and
NfLTOA/
NfControl ratios). However, HMA-19 displays less AV and greater ITS values than HMA-25, which contributes to positioning it as the mix with the greatest fatigue resistance. The mix that least underwent increases in
Nf was HMA-10. This led to that under LTOA condition, HMA-25 underwent a similar fatigue resistance (even with a greater σ
6) than HMA-10, even though it has greater AV. All these increases were statistically significant based on ANOVA analysis.
3.4. Cantabro Test
The Cantabro test results are presented in
Table 5. Similar results were reported by [
43,
52]. If the results of CL vs. cycles were to be graphed, one would obtain a linear trend, whose approximate slope (named Cantabro Index—CI in this study) may be obtained mathematically using Equation (1):
where
CLCf is the CL obtained in the final cycles (
Cf), and
CLCi is the CL obtained in the initial cycles (
Ci). In this study,
Cf and
Ci are 500 and 100 cycles, respectively.
When the CL and CI parameters increase, it signifies a reduction of abrasion wear resistance. It is observable that both parameters increase when the mix ages. This can take place given that when asphalt binder ages, it reduces its capacity to adhere to the aggregate and to make the mix cohesive [
73]. Additionally, when asphalt binder ages, the proportion of maltenes/asphaltenes is reduced, resulting in a material that is stiffer and brittle [
74,
75], which makes it more susceptible to fissures or cracking under any type of abrasive load [
76,
77].
The control mixes that underwent greater and smaller abrasion wear resistance were HMA-10 and HMA-25, respectively. However, when mixes age, this behavior changes and HMA-19 undergoes greater resistance. Upon aging, HMA-25 continues being the mix with least resistance. This is mainly because the HMA-25 mix has greater AV and lower VFA.
Figure 10 shows how CL increased in mixes when they aged. It is observable that the mix that underwent the least CL changes was HMA-25, while the greatest changes occurred in HMA-10. This was perhaps due to the greater content of asphalt binder that aged in HMA-10, which became brittle and was more easily detached under abrasive load.
Normally, mixes that present greater AV, greater particle sizes, coarse gradations, and lower VFA are more susceptible to aging [
8,
40,
74,
78,
79,
80]. Generally, this conclusion is based on the way it changes in stiffness (mainly in RM tests) of aged mixes with relation to the control mix. However, this study evaluated the change in other properties. Mixes that underwent the least and greatest changes under STOA and LTOA conditions with relation to the control mix are shown in
Table 6. When two mixes appear in a box, it means that they underwent similar changes—in other words, changes that are not statistically significant based on ANOVA analysis.
Generally, the susceptibility of mixes for aging is obtained by measuring increases in its stiffness, in other words, by comparing it or relating the stiffness in an aged state with relation to a control sample (unaged). The increase in RM is especially one that is widely used. Since asphalt is a material that has a viscous behavior, mixes undergo changes in their properties when they are subjected to different temperatures and load frequencies. This study observed that the increases in RM of mixes that occurred due to aging were greater when the test temperature increased and load frequency decreased. In terms of the effect of gradation upon increases in RM, there was no clear trend. It was expected that the greatest changes would be observed in mix HMA-25, which has a greater AV content. However, at 10 °C, the mix HMA-10 tends to display the greatest increases in RM (under STOA and LTOA conditions), while HMA-25 displays the smallest. At 20 °C, the mix that is most susceptible to changing its RM in STOA condition is HMA-10, while in LTOA, it is HMA-19. Under this temperature, HMA-25 undergoes the smallest changes in RM. At 30 °C in STOA condition, the HMA-19 mix undergoes the greatest increases, while in LTOA, it is HMA-25.
Below, there is a summary of the most important aspects of each test:
- ○
In the Marshall test, the mix that underwent greater changes in resistance under monotonic load (greater S/F ratio) was HMA-19. In turn, this mix obtained the smallest increases in S/F ratio in STOA and LTOA conditions (in theory, this property is less susceptibility to change upon aging). The mix that underwent the lowest resistance under monotonic load in this test was HMA-10, and in LTOA condition, it presented the greatest increases. In STOA condition, the greatest increases occurred in HMA-25.
- ○
In the ITS test, the mix that underwent the greatest changes in resistances (ITSU and ITSC) was HMA-19. In turn, this mix obtained the greatest increases in ITS-U under STOA and LTOA conditions (in theory, this property has greater susceptibility to change upon aging). Despite that, this is in theory the mix that is most susceptible to changing its resistance due to the effects of aging in this test; it was the one that underwent the greatest moisture damage resistance. The mix that underwent less resistance under monotonic load in this test and the least increases in ITSU was HMA-10. Although it is the one that is least susceptible to aging, it was the one that underwent the least moisture damage resistance.
- ○
The mixes that were most resistant to the phenomenon of permanent deformation were HMA-19 and HMA-25. The mix that was least susceptible to change resistance to permanent deformation under STOA and LTOA conditions was HMA-25. Under the STOA condition, HMA-10 and HMA-19 mixes underwent the greatest changes, while HMA-10 did likewise in the LTOA condition.
- ○
In general terms, the mix with greater resistance to fatigue under controlled stress was HMA-19, while the one with less resistance was HMA-25. The mixes that underwent greater and smaller increases in fatigue life upon aging were HMA-25 and HMA-10, respectively.
- ○
In the initial state (unaged), the mixes that underwent the greatest and lowest resistance to abrasion wear in the Cantabro Test were HMA-10 and HMA-25, respectively. When mixes age, HMA-19 underwent the greatest resistance, while the one that underwent the least resistance was HMA-25. The greatest and smallest changes in the test (increase in CL) were underwent by HMA-10 and HMA-25 mixes, respectively.