This study aims to evaluate the effect of different rejuvenators and antistripping agents on the healing performance of hot mix asphalt (HMA). Two damage HMA series (e.g., moisture damage and aged damage) were subjected to either induction or microwave heating. A PG64-22 virgin and aged binder were used and modified with several additives. Three long-term aged binders (e.g., PAV5, PAV15, and PAV20) were conducted by pressure aging vessel (PAV) test. The moisture damage series fabricating with a new binder was further categorized into four different freeze-thaw (FT) cycles (e.g., 0FT, 1FT, 3FT, and 5FT). Also, the aged series was fabricated with three different aged binders. A total of eight damage-healing cycles were applied to all asphalt mixtures, examined by the three-point bending test. The moisture resistance of modified asphalt mixture was examined by indirect tensile strength test. Overall, asphalt mixtures modified with either antistripping additives or rejuvenators not only obtained higher moisture resistance but also gained better healing performance under moisture damage. In addition, the study showed a probable correlation between moisture damage and long-term aging in terms of healing performance, such as PAV15 and 3FT cycles and PAV20 and 5FT cycles.
Moisture damage is one of the main factors affecting the durability of asphalt mixtures [
Aging is another factor influencing the performance of asphalt mixture, which occurs during production and construction and continues throughout the services life of asphalt pavements. Long-term aging caused asphalt binder to stiffen and embrittle, which leads to a high potential of cracking [
This research aims to study the effect of different additive agents (e.g., antistripping agent and rejuvenator) on the healing performance of moisture damage asphalt mixture. Induction heating and microwave heating are used to heat asphalt mixture. Long-term aging is considered to find the correlation between moisture damage and aging in terms of healing performance. Two sample series are fabricated to archive the research objectives. First, the unaged asphalt binder is modified with four types of additive agents. These samples are going on to four different freeze-thaw (FT) cycles. Second series was fabricated with three levels of PAV asphalt binder (e.g., PAV5, PAV15, and PAV20). The healing performance of mixture is assumed by the three-point bending (TPB) test, while the moisture resistance is conducted to indirect tensile strength (ITS) test. Steel wool fiber (SWF) is utilized to obtain a prime healing performance. All test samples are applied to eight damage-healing (DH) cycles. During healing process, the infrared camera (FlukeTiS20 model) is used to record the surface temperature of sample. The ANOVA and Tukey HSD post hoc are employed to find the correlation between moisture damage and aging in terms of healing performance.
Laboratory-fabricated HMA mixtures were used to conduct a series of experiments. Table
Aggregate gradation.
Sieve size (mm) | Aggregate | ||
---|---|---|---|
Coarse (33%) | Fine (60.5%) | Filler (6.5%) | |
19 | 100 | 100 | 100 |
12.5 | 83 | 100 | 100 |
9.5 | 35.5 | 100 | 100 |
4.75 | 1.7 | 79.3 | 100 |
2.36 | 41.3 | 100 | |
0.6 | 16.5 | 100 | |
0.3 | 10.5 | 100 | |
0.15 | 6.2 | 56 | |
0.075 | 4 | 47 |
The asphalt mixtures were prepared according to the Superpave Mix Design Method [
Research flowchart. ∗Mixture without additive agent.
Mix proportion of moisture damage series.
New binder (by wt. of total mix) | Additive agent (by wt. of binder) | SWF (by wt. of binder) | |||||
---|---|---|---|---|---|---|---|
C | Type 1 | Type 2 | Type A | Type B | |||
Induction heating | 5.4% | 2% | 6% | ||||
Microwave heating | 5.4% | 2% | 2% |
Mix proportion of aged series.
Aged binder (by wt. of total mix) | Additive agent (by wt. of binder) | SWF (by wt. of binder) | |||||
---|---|---|---|---|---|---|---|
PAV5 | PAV15 | PAV20 | C | Type 2 | Type A | ||
Induction heating | 5.4% | 2% | 6% | ||||
Microwave heating | 5.4% | 2% | 2% |
Asphalt binder-additive mixing process involved preheating the asphalt binder for an hour at 160°C. Once preheated, two percent of additive was then introduced and stirred thoroughly unto the binder. The mixture was allowed to sit in the oven for an additional hour (at 160°C), stirring the mixture at 30 minutes interval. Based on ITS test requirements, a cylindrical test specimen had a dimension of 63.5 mm in height and 100 mm in diameter (Figure
Test setup for (a) ITS test and (b) TPB test.
The asphalt binder underwent short-term and long-term aging. Short-term aging was simulated first, using RTFO (Rolling Thin-Film Oven) in accordance with D2872-12 [
The moisture damage series included 4 groups of freeze-thaw cycle: unconditioned/control (0FT), one cycle (1FT), three cycles (3FT), and five cycles (5FT). According to AASHTO T 283 [
To investigate the effect of additive agents on moisture susceptibility, the indirect tensile strength test was conducted. The indirect tensile strength was recorded by applying a loading rate of 50 mm/min. The indirect tensile strength
To obtain brittle condition before TPB test, samples were placed in a refrigerator at aged binder process −18°C for 2 hours. TPB test contained a loading roller at midpoint on the semicircular arch sample, supported by two fix rollers spaced at 80 mm apart (Figure
Two healing treatments were used unto damaged samples: the induction heating generator and microwave heating machine. The induction heater used in this study has a capacity of 50 kW and a maximum frequency of 35 kHz. The damaged sample was placed under the induction heating coil and heated until 90°C (as shown in Figure
Induction healing test setup.
Microwave healing test setup.
After healing treatment, healed samples were allowed to rest approximately three hours to achieve stable room temperature. Rested and healed samples were again conditioned in a refrigerator for two hours and tested for the TPB test. This equates to one DH cycle. Based on preliminary research recommendations and the author’s team experiment, the healing performance of the test samples was conducted until eight damage-healing cycles. The healing level of the asphalt mixture sample
Figure
Surface temperature after healing.
By using microwave heating, the average surface temperature was increased after one FT cycle and decreased subsequently (see Figure
Under induction heating, the average surface temperature of samples gradually decreased with increasing FT cycles. The highest temperature was recorded in type 2 additive samples with 85°C (at 0 FT cycles), and the lowest was type A with 63°C (after 5 FT cycles). As mentioned before, when the number of FT cycles was increased, there would be higher air void content, thereby breaking the interconnecting bonds of the mixture (especially SWF). This phenomenon which led to heat transfer was interrupted. In addition, the water retained on the samples played a role as a thermal absorbing material, which lowers the temperature. Therefore, the temperature of test samples gradually reduced with an increasing FT cycle when using induction heating method. Temperature attainment after the healing process plays an important role during real-scale heating scenarios. With the results presented, it is conclusive that although it reached the margin of 80–90°C, it is within boundaries of a regular asphalt pavement working temperature.
To better understand and express the indirect tensile strength of samples in this experiment, the tensile strength ratio (TSR) was computed. TSR is defined as the ratio of the tensile strength of both wet-conditioned (i.e., 1, 3, and 5 FT cycles) and dry-conditioned samples (i.e., 0 FT cycles). The tensile strength ratio can be calculated by following equation:
The results from Figure
TSR and IDT strength of moisture-damaged mixtures.
Moreover, (
Figure
Tensile strength improvement.
Table
The initial force of moisture damage asphalt mixtures (kN).
Freeze-thaw cycle | Mixture types | ||||
---|---|---|---|---|---|
Type C | Type A | Type B | Type 1 | Type 2 | |
0 | 2.45 ± 0.10 | 2.56 ± 0.13 | 2.62 ± 0.11 | 2.53 ± 0.11 | 2.55 ± 0.13 |
1 | 2.31 ± 0.12 | 2.52 ± 0.14 | 2.54 ± 0.15 | 2.43 ± 0.13 | 2.47 ± 0.13 |
3 | 2.11 ± 0.09 | 2.45 ± 0.09 | 2.47 ± 0.12 | 2.39 ± 0.12 | 2.38 ± 0.12 |
5 | 1.95 ± 0.11 | 2.21 ± 0.12 | 2.32 ± 0.17 | 2.33 ± 0.16 | 2.21 ± 0.14 |
Healing level of unaged mixtures without moisture damage.
Under the microwave heating treatment, the effect of antistripping additives was less than that of rejuvenators. This can be seen at the fourth DH cycle, with rejuvenator type 2 mixture having a healing level of 76% compared to 55% for antistripping additive type B mixture. Type 2 mixture showed a promising healing performance with a level of 76% and 47% under 4 and 8 DH cycles, respectively. Additionally, the type 1 mixture had the lowest healing level of 43% and 29% corresponding to 4 and 8 DH cycles. It can be explained due to the high viscosity of rejuvenator additive 1, which causes slow and inefficient diffusion into the asphalt. In the early stages of DH cycles, the difference in the healing level of the control sample and additive agent sample was insignificant. Control samples also have healing capabilities effective until four DH cycles. Furthermore, from the fifth DH cycles, the effect of an additive agent, specifically rejuvenators, can have a significant impact on continuing the healing level above 50%. Adding an appropriate rejuvenator (type 2 with lower viscosity) helps asphalt binder reduce its viscosity, which can explain a higher level of healing [
The healing result under induction heating is shown in Figure
The effect of additive agents was proved after one FT cycle (Table
Healing level of unaged mixtures with moisture damage (1 FT cycle).
Healing level of unaged mixtures with moisture damage (3 FT cycles).
Healing level of unaged mixtures with moisture damage (5 FT cycles).
Figure
Similar trend results with microwave heating can be observed, with type 2 additive showing best healing performance and consistency on all mixtures. With moisture damage at 5 FT cycles, type 2 rejuvenator additive samples showed 20% and 32% higher healing level compared to type B antistripping additive samples and control samples, respectively. It is obvious that the healing performance of the mix with rejuvenators was better than that of the mixture with antistripping additives. The antistripping additives can protect the asphalt mixture from water intrusion and increase the adhesion of the aggregate to the asphalt binder. However, in terms of healing performance, rejuvenators with low viscosity can increase the flowability of an asphalt binder; hence, the binder can easily flow to repair microcracks. This observation also explains the high healing level of rejuvenator type 2 additive, which had the lowest viscosity value. The results highlight the important role of the viscosity of an additive agent in providing a self-healing ability.
The containing of additive agents could enhance the initial force of aged asphalt mixtures (Table
The initial force of aged damage asphalt mixtures (kN).
PAV damage | Mixture types | ||
---|---|---|---|
Type C | Type A | Type 2 | |
5 h | 2.36 ± 0.13 | 2.52 ± 0.12 | 2.57 ± 0.14 |
15 h | 2.26 ± 0.15 | 2.41 ± 0.16 | 2.43 ± 0.13 |
20 h | 2.01 ± 0.15 | 2.28 ± 0.17 | 2.25 ± 0.16 |
Healing level of aged mixtures without additive agents.
Figure
Healing level of aged mixtures modified with two additive agents.
The analysis of variance (ANOVA) with Tukey’s HSD post hoc was used to evaluate the statistical significance of the change in healing performance with moisture-damaged and aging time (shown in Table
Tukey’s HSD post hoc summary result.
Types of damaged | PAV 5 hours | PAV 15 hours | PAV 20 hours |
---|---|---|---|
1 FT cycle | S | N | N |
3 FT cycles | S | N | N |
5 FT cycles | S | S | N |
N: nonsignificant; S: significant.
In this study, the indirect tensile strength test and the three-point bending test were conducted to investigate the effect of additive agents on the healing performance of hot mix asphalt under moisture and long-term aging damage. A series of test results showed that adding additive agents can improve the healing performance of asphalt mixture after being subjected to damage. The main research conclusions are presented as follows: Microwave heating method shows a better healing option than that of induction heating. The entire asphalt mixture heats up with electromagnetic radiation, while for magnetic induction, thermal energy disseminates only through conductive materials, that is, steel wool fibers. Mixing asphalt binder with antistripping additive can obtain significant moisture resistance. However, if healing performance is the primary goal of the mixture, a low viscosity rejuvenator additive is best. Three to four damage-healing cycles on asphalt mixture, with or without moisture damage, are able to achieve prime healing performance. The application of rejuvenators leads to the softening of the asphalt binder. This concept may enhance the healing performance of asphalt to some extent. Asphalt binders with rejuvenator may be softened faster under heating treatment, and the use of rejuvenator is expected to accelerate the crack healing process.
Following the statistic results of healing performance, there may have been a correlation between freeze-thaw cycles and long-term aging time in terms of healing performance, such as 3 freeze-thaw cycles with 15 hours of aging time and 5 freeze-thaw cycles with 20 hours of aging time. The mechanism correlation needs to be clarified in further studies.
The experimental data used to support the findings of this study will be made available from the corresponding author upon request.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (no. NRF-2017R1D1A1B03032594).