Effects of short-term aging of base and nano-modified bitumens on surface free energy and their relationship with moisture sensitivity in warm-mix asphalt

Abstract

The warm-mix asphalt (WMA) has become increasingly popular because it reduces the possibility of aging during construction and paving. However, its susceptibility to moisture-related distresses remains a concern, as such damages lead to additional pavement distresses, and the aging that still occurs can further intensify these effects. To this end, this research examined the effect of two types of nano materials, Nano Hydrated Lime and Nano Calcium Carbonate on the moisture sensitivity of base and aged WMA and explored the effects of these additives on thermodynamic parameters. To evaluate these effects, twenty different asphalt mixtures were evaluated using the modified Lottman test under multiple freeze–thaw cycles, along with complementary surface free energy measurements. The results indicate that the additives increased the debonding energy at both base and aged states, indicating a considerable improvement in moisture sensitivity of WMA, which aligns with the indirect tensile strength results. Moreover, bitumen modification with these additives remarkably increased the cohesion free energy by 63% and 53% for PG 64–14 and 58–22 bitumens, respectively. Regarding bitumen-aggregate adhesion, bitumen modification increased the adhesion free energy by 12% on average, indicating the additives’ proper performance for low-adhesion WMA. Finally, the correlation coefficient between the mechanical index of moisture susceptibility and the bitumen-aggregate energy released during the stripping process was 89% and 85% in the fifth freeze–thaw cycle for the base and aged states, respectively. This strong correlation between the results demonstrates the need to use thermodynamic concepts in estimating the effect of bitumen aging on moisture susceptibility of WMA.

Introduction

Advancements in transportation infrastructure have intensified the demand for innovative asphalt technologies. Concurrently, growing socio-economic activity has led to higher traffic volumes and heavier vehicles, highlighting the necessity to improve asphalt mixture performance^1,2. A prominent approach to address this challenge is to incorporate performance-enhancing additives. These materials enhance crack resistance, increase flexibility, and improve thermal and moisture durability, thereby strengthening pavement resilience under increasingly demanding traffic conditions. Widely adopted additives—such as polymers, fibers, and waterproofing agents—are selected based on the specific requirements of each project. Their use contributes to longer pavement service life, reduced maintenance needs, cost efficiency, and enhanced road safety^3,4.

Building on these improvements, the performance of asphalt mixtures largely depends on a key mechanism namely, adhesion. The adhesion between bitumen and aggregates in asphalt mixtures is influenced by various factors, which can be categorized into two groups: external and internal factors. External factors include temperature, moisture, and even traffic conditions. The internal factors include the type of asphalt binder, type of aggregate, percentage of asphalt binder, percentage of air void, and aging of asphalt binder. Asphalt binder aging is a significant internal factor whose effects develop over time^5–7. By altering the chemical and physical properties of bitumen, aging reduces pavement durability, accelerates the onset of early damages, and increases the costs of maintenance and repair. Aging in the asphalt plant and during the implementation process (short-term aging-STA) and aging in the operation period (long-term aging-LTA) can be considered two main types of aging. STA occurs due to oxidation in the vicinity of gases in the atmosphere and evaporation of volatile oils when preparing the materials and mixing bitumen with warm aggregates. LTA happens when the pavement is used during service^8,9.

Bitumen aging is a complex process since its intensity is affected by bitumen itself, the bitumen-aggregate interaction, and their compatibility. Over time, the viscoelastic properties of asphalt undergo a gradual change, becoming more brittle and resulting in a decrease in its ability to adhere to the aggregate^10,11. Thus, depending on the constituting minerals and aggregate surface properties, asphalt mixes with similar bitumens but different aggregates may have different aging resistance. A closer examination of how aggregates influence bitumen aging reveals that the partial selective absorption of bitumen components by aggregate surface disrupts bitumen equilibrium, leading to dispersion and increased bitumen aging rates^12,13.

Literature review

The physicochemical properties of aged bitumen can create tiny cracks on the pavement, which serve as routes for the penetration of water into the bitumen-aggregate bond, thereby accelerating and exacerbating moisture damage in the asphalt mix. Indeed, the contents of resin and asphaltene alter during aging, that deteriorate adhesion, and consequently, increases moisture sensitivity. Increased moisture sensitivity of asphalt mixes may lead to other damages and must be considered in asphalt mixture studies. The WMA allows the creation of asphalt mixtures at a lower temperature than hot asphalt mixture (HMA), thus mitigating primary bitumen aging. Especially, thermal aging that has undeniable effects on the masses of saturates, aromatics, resins and asphaltenes fractions, which are closely linked with adhesion and moisture sensitivity¹⁴. Therefore, research focuses has recently shifted to the application of WMA instead of HMA¹⁵. WMA is finding more applications thanks to its better compaction and efficiency, reduction of energy use and environmental pollution due to its lower construction temperature than HMA to minimize environmental impacts. WMA has great potential for minimizing the destructive impacts of aging, which has motivated research into bitumen modification to promote asphalt mix performance against damages. Fatemi et al.¹⁶ proved that adding calcium lignosulfonate prevents bitumen oxidation reaction, thereby delaying bitumen aging. The Fourier-transform infrared spectroscopy test and the examination of aging indicators revealed that carbonyl groups and sulfoxide groups increase with aging. Also, the aforementioned additive reduces these groups and thus slows aging. This result demonstrates the sensitivity of short-term aging and suggests that some simulation methods may be inapplicable or imprecise¹⁷. Liu et al.¹⁸ investigated the effect of aging on the resistance of WMA modified with two types of polymer materials (low-density polyethylene and high-density polyethylene) against cracking at the contact surface. The results of this research show that there is a relatively close relationship between adhesion energy and separation energy under aging conditions, but the application of aging causes this relationship to weaken.

Izadi et al.¹⁹ investigated the effect of aging on the performance of WMA against intermediate cracking. The WMA was made using two additives, Sasobit and RheoFalt, and the indirect tensile strength test was carried out in strain and stress control mode. The results of this research showed that mechanical tests, regardless of mixture components, loading conditions and aging conditions, cannot predict asphalt against cracking potential. Tabasi et al.²⁰ investigated the effect of moisture and aging condition on the performance of asphalt mixture modified with nano reduced graphene oxide. The results of this research showed that moisture and aging caused a decrease in the resistance of asphalt mixture in the fracture strength test with strain control type. The use of nano reduced graphene oxide has reduced aging and the effect of moisture on bitumen-aggregate adhesion, which improves the performance of the asphalt mixture against cracking, especially at medium temperature. Alvarez et al.²¹ investigated the effect of three types of additives in WMA with the aim of maximizing adhesion on the contact surface of bitumen-aggregate using the surface free energy (SFE) method. The results of this research show that the overall performance of the asphalt mixture cannot lead to the determination of the optimal additive percentage and it shows the necessity of using methods based on the fundamental properties of the materials. Liu et al.²² investigated the effects of the Evotherm additive on warm-mix recycled asphalt mixtures, aiming to improve energy efficiency and sustainability. Their findings showed that while Evotherm had minimal impact on traditional binder properties, it enhanced surface energy and aggregate adhesion. Compared to hot-mix recycled asphalt mixtures, warm-mix recycled asphalt mixtures exhibited better fatigue resistance, improved low-temperature cracking performance, and superior water stability. The study offers practical insights for the safe and efficient use of warm-mix recycled asphalt mixtures technology.

Although due to the grave sensitivity and importance of aging, several laboratory methods i.e., the rolling thin-film oven (RTFO) test have been developed to predict the pattern of aging, these approaches often provide only a general indication of aging behavior and do not fully capture the mechanisms that govern moisture-related deterioration in asphalt mixtures. The negative impact of bitumen aging on the asphalt performance could also be predicted by determining the materials’ SFE; in this way, a compatible bitumen-aggregate system can be designed to promote and optimize their adhesion²³. However, previous works have rarely applied these thermodynamic approaches under conditions that reflect the combined influence of aging and moisture exposure. Therefore, the use of thermodynamic and chemical tests that separately investigate the fundamental properties of materials may help resolve aging-related problems and define more reliable indicators of moisture susceptibility.

Statement of the problem and objectives

It is observed that most studies have concentrated on neat bitumen aging and limited attention has been given to evaluating how aging and nano-modification of base and aged bitumens influence the thermodynamic parameters related to moisture damages, especially under more realistic conditions using different freeze/thaw cycles. Moreover, the relationship between these thermodynamic parameters and mechanical tests of moisture sensitivity in WMA has not been fully addressed. Therefore, the current study explored the STA of base and nano-modified bitumens under conditions more representative of early field exposure. In addition to mechanical tests of the moisture susceptibility of WMA, thermodynamic concepts were adopted to analyze the effect of bitumen STA on molecular components that are uniquely related to WMA moisture susceptibility. Besides the impact of short-term bitumen aging, this research also investigated the incorporation of Nano Calcium Carbonate (NCC) and Nano Hydrated Lime (NHL) additives into bitumen and their effect on the parameters that influence moisture damages according to the SFE theory to provide further insight into their potential to enhance WMA performance. The selection of NCC and NHL was based on their ability to improve moisture resistance and increase surface energy characteristics, which make them useful candidates for reducing moisture-related distresses in WMA.

Experimental plan

The materials (aggregates, bitumens, and additives) were first selected, and the specimens were then made based on the mix design. The specimens were constructed with base and modified materials, and laboratory-aged bitumens were utilized to simulate field conditions. After preparing the specimens, mechanical and thermodynamic tests were conducted. The mix design of the asphalt mixtures of this research is based on the Marshall mix design method (ASTM D6927). The optimal asphalt binder percentage is obtained based on the average asphalt binder percentage in the asphalt mixture with 4% Air Void, the maximum amount of bulk specific gravity and the maximum stability. Other parameters are controlled with MS-2 manual of the asphalt institute. Each mechanical and thermodynamic test was repeated three times to improve the consistency and reliability of the results. Mechanical tests, including ITS and TSR tests were performed according to the AASHTO T283 standard on base and modified specimens. Thermodynamic tests, including the Universal Sorption Device (USD) for the aggregates and the Sessile Drop (SD) for the bitumens were performed separately on the materials to measure their fundamental properties. Figure 1 displays the procedures. Finally, the results were statistically compared and analyzed. Overall, the impact of bitumen STA on WMA and its relationship with the mixes’ moisture sensitivity was examined to show the importance of aging and use of appropriate additives in the process of this distress.

Fig. 1.

Procedure of the tests.

After bitumen modification and preparation of materials based on the given procedure, 24 compositions (Table 1) underwent mechanical and thermodynamic tests.

Table 1.

The compositions of WMA investigated in this study.

Aggregate	Asphalt binder	Conditions	Additive
Limestone	PG 64–16	Unaged and STA aged	–
		Unaged and STA aged	NCC
		Unaged and STA aged	NHL
	PG 58–22	Unaged and STA aged	–
		Unaged and STA aged	NCC
		Unaged and STA aged	NHL
Granite	PG 64–16	Unaged and STA aged	–
		Unaged and STA aged	NCC
		Unaged and STA aged	NHL
	PG 58–22	Unaged and STA aged	–
		Unaged and STA aged	NCC
		Unaged and STA aged	NHL

Materials description

Properties of aggregates

In order to investigate how various combinations of hydrophilic and hydrophobic aggregates affect moisture susceptibility, two types of aggregates with different mineralogical structures have been used. The intermediate gradation of the ASTM D3515 standard for dense-graded materials was used. The maximum and nominal maximum aggregate size of the aggregates are 25 and 19 mm, respectively (Fig. 2).

Fig. 2.

Aggregate gradation curve used in this research.

The performance of stone materials in the asphalt mix depends on numerous factors, including aggregate texture, constituting minerals, and fracture angle. Constituting minerals exert the greatest effect on the mix performance since they directly impact the bitumen-stone materials affinities and the bitumen aging process. The X-ray fluorescence test, according to the ASTM E1621 standard, was carried out to identify the aggregates’ mineralogy structure. The mineralogical properties of the aggregates are briefly listed in Table 2.

Table 2.

The aggregates’ constituting minerals.

Aggregate	Aluminum oxide, Al2O3	Magnesium oxide, MgO	Silicon dioxide, SiO2	Ferric oxide, Fe2O3	Calcium oxide, CaO	Other components
Limestone	0.70	1.20	3.10	0.80	65.50	28.70
Granite	11.30	0.60	70.20	1.80	6.60	9.50

Bitumen

As moisture damage often occurs in intermediate temperatures with heavy traffic, two types of bitumen were used with 60–70 and 85–100 penetration grades and the performance grade of PG 64–16 and PG 58–22, respectively. The rheological behavior of the unmodified bitumens is presented in Table 3.

Table 3.

Main features of PG PG 64–16 and 58–22 and bitumen samples.

Type of materials	Test	Test temperature ()	Results		Standard limits		ASTM standard number
			PG 58–22	PG 64–16	Min	Max
MS7PDT*			58	64
MPDT**			−22	−16
Base bitumen (unaged)
1	Flash point (℃)	0.00	305.00	304.00	230.00	−	D92
2	Viscosity (Pa.s)	135.00	0.320	0.330	−	3.00	D4402
3	G*Sinδ (kPa)	58.00	2.18	1.78	1.00	−	D7175
RTFO aged
4	Dropped weight (%)	163.00	0.09	0.09	−	1.00	D2872
5	G*Sinδ (kPa)	58.00	3.96	3.45	2.20	−	D7175
Pressure aging vessel (PAV) aged
6	G*Sinδ (kPa)	22.00	2650.00	2650	−	5000.00	D7175
7	Creep stiffness (MPa)	− 12.00	109.00	101	−	300.00	D6448
8	Creep rate (m− value)	− 12.00	0.321	0.341	0.300	−	D6648

* Mean seven-day peak pavement design temperature (℃).

** Minimum pavement design temperature (℃).

Additives

Nano calcium carbonate (NCC) has wide-ranging applications across various industries, from paper manufacturing to construction. Synthesized NCC coated with stearic acid with an average particle size of 60 nm, was incorporated. According to the provided specifications, NCC (chemical formula: CaCO³) used in this study had a cubic crystal calcite structure, 2.5 g/cm³ specific gravity, acidity of 8–10, and absorption percentage of less than 0.5%. Table 4 presents the physical properties of the nano-materials.

Table 4.

Physicochemical characteristics of the nano-additives.

Nano-material	Hydrophilicity (%)	Specific weight	PH	Purity (%)
NCC	< 0.5	2.5	8 ~ 10	99
NHL	< 0.2	2.2	12.4	99

The incorporated nano hydrated lime (NHL) had more than 90% CaO, 3% calcium carbonate, a specific density of 2.24 gm/cm³, acidity of 12.4, and was a relatively strong alkali. The main physical properties of NCC and NHL is presented in Table 4.

Bitumen modification with nano-materials

According to the literature, about 0.5–5% of nano-additives are utilized to improve asphalt mix performance. Thus, the current study incorporated 0.5% nano-additives by weight of bitumen for bitumen modification. The high shear mixer available at the Chemical-Polymer Laboratory of Urmia University was used for this purpose at its maximum rotational capacity (12,000 rpm). Nano-materials were slowly incorporated into the base bitumen at a temperature of 140–150 degrees Celsius. Mixing time was 25 min to homogeneously disperse the nano-material particles in the bitumen volume. A heater was also placed around the bitumen cylinder to keep the bitumen temperature constant during mixing. This procedure is consistent with established protocols in the literature²⁴, where nano-modified binders were prepared by heating the binder to ~ 140 °C and mixing with a high-speed shear mixer to ensure uniform dispersion of nanoparticles while preventing localized overheating and minimizing the potential for mixing-induced aging.

Evaluation tests

Modified Lottman moisture susceptibility test

In this test, after compacting the specimens, their diameter and height were measured and their specific gravity and air void percentage were calculated. Specimens with specifications in Table 5 are standard specimens for this test. Six specimens (in two stages) were made for any composition in Table 1. The first stage involves three specimens in wet condition, and the second stage includes three specimens in dry condition. To condition the first three specimens, they were placed in saturated conditions by applying a relative vacuum. The moisture percentage of the samples should be between 70 and 80% to be considered acceptable. The saturated specimens were kept in the freezer at − 18 °C for 16 h. After freezing the saturated specimens, they were transferred to a hot water bath at 60 °C for a full day. Finally, the specimens were maintained at 25 °C for 24 h.

Table 5.

Laboratory specimen standard in AASHTO T283.

Test	Percentage of air voids (%)	Specimen’s size (mm)	Test standard	Number of specimens
Modified Lotman test	7 0.5	100 62.5	AASHTO T283	6

The specimens prepared based on the standard undergo the indirect tension test under loading at the rate of 2 in/min until failure.

Finally, the ratio of wet strength to dry strength was obtained as an index to determine the moisture susceptibility of different WMA. If TSR was lower than 80% in this test, the asphalt mix is regarded as moisture-sensitive²⁵.

STA process in bitumen and asphalt mixture

The RTFO test is performed to induce bitumen STA with two objectives: 1) to use the output bitumen of this test as the input to other tests to further examine bitumen properties; and 2) to determine the bitumen weight drop caused by the evaporation of volatile compounds during the heating process. In this test, a thin layer of bitumen is exposed to hot airflow. This test is based on heat and hot-air transfer (AASHTO R30). It involves a circular container placed around a central axis to which a hot airflow is directed.

To apply STA conditions, the following cycle is performed on non-compacted samples:

• The WMA sample is spread evenly on a PAN so that its thickness is between 2.5–5 cm (1–2 inches).

• The PAN is placed in an oven at 135 ℃ temperature for 5 h.

• Every hour, the asphalt mixture is stirred so that the lower and upper parts of the mixture are exposed to the same temperature and aging conditions.

SFE components’ measurement

Theory of surface free energy

Research evidence suggests that major thermodynamic parameters dependent on HMA moisture damage are obtained by measuring bitumen, aggregate, and water SFE^26,27. According to most theories, each material has three components of SFE, each of which plays an important role in the ability of cohesion and adhesion: A) acidic component of SFE, B) basic component of SFE and nonpolar component of SFE. Based on these three components, the total SFE can be obtained from Eq. 1.

1

where, γ^LW is the non-polar SFE component (ergs/cm²), γ^± is the polar SFE component (ergs/cm²), γ⁺ is the acidic SFE component (ergs/cm²), and γ^- is the basic SFE component (ergs/cm²).

The acidic-basic part comprises the acidic and basic SFE components (Eq. 2).

2

The cohesion and adhesion works can be obtained by having the SFE components of the material attained based on the SD and USD tests with Eqs. 3 and 4.

3

4

where, W_bb is the cohesion work of the bitumen or the mastic (ergs/cm²), and W_ab is the bitumen-aggregate adhesion work (ergs/cm²).

As most damages occur in the presence of a third material, such as water, these equations can be expanded to calculate the adhesion work of a three-phase system, which is the amount of energy released in the stripping process. Equation 5 calculates the work of adhesion in a three-phase system and Eq. 6 calculates the energy released and the tendency of the system in the stripping process.

5

6

where, index a indicates aggregate, index b indicates bitumen and index w indicates water.

Bitumen and asphalt mix SFE measurement

The SD method is used in most previous studies for materials with low SFE components such as bitumen. This method is based on the balance of test liquid drops on the surface of the desired solid substance²⁸.

The SD method requires a precision camera, a syringe system to create test liquid drops, and an image analyzer to determine the contact angle based on the images, an oven with the thermal ability of 150 °C to heat (melt) the bitumen specimens; and a level glass or aluminum plate to be coated with bitumen. To prepare the specimens, the aluminum or glass plate should be immersed in and uniformly coated with bitumen. Then, a probe liquid is dispersed on the bitumen plate. The image of the drop liquid(s) in Table 6 on the bitumen plate is recorded with the software. Finally, these angles with different probe liquids are calculated based on the adhesion work equations to determine bitumen SFE components. Contact angles are calculated by the software or manual analysis of the image.

Table 6.

Properties of liquids used for bitumen SFE determination via the SD approach.

SFE components	Surface free energy components (ergs/cm2)
	Acidic	Basic	Acid–Base	Lifshitz–van der Waals	Total SFE
Water	25.50	25.50	51.00	21.60	72.60
Formamide	2.28	39.60	19.00	39.00	58.00
Glycerol	3.92	57.40	28.80	34.00	62.80

By performing this test for each type of bitumen with the three prob liquids, three equations can be obtained where the index s denotes the SFE of the bitumen (Eq. 7). Three bitumen SFE components are the variables in the equation (denoted with index S). Performing the test three times yields three equations that must be solved to calculate the three variables²⁹.

7

Aggregate SFE measurement

Absorption methods are suitable thanks to their compatibility with highly porous and irregular specimens for aggregates SFE components.

In this method, the container is filled with the absorbent (aggregates), and then the probe vapor–liquid (absorbate) covers the aggregates. After these steps, the absorption responses of the test vapor–liquid over the surface of stone material are measured to yield the aggregate SFE components²⁸.

Aggregates smaller than 4.75 mm and larger than 2.36 mm are used for this test. As the aggregate size does not affect SFE, this value is determined solely for better control. After preparing and dusting the aggregates, 40 g of the material is prepared in a washing cycle, involving washing with distilled water, followed by two other probe liquids. Subsequently, the aggregates are placed in the oven for 4 h to dry. The specimens are kept in a desiccator to cool down. A final step of washing with acetone is then performed, and the specimen is placed inside the device.

Equation 8 can be used to relate adhesion work and vapor of test liquid:

8

where, W_a is the adhesion work between the probe material’s vapor and the aggregate (ergs/cm²), and π_e is the pressure of equilibrium distribution of the probe vapor on the solid surface (ergs/cm²); and γ^T_l is the total SFE of the probe liquids (ergs/cm²).

Equation 9 can be written with the help of Eqs. 63 and 8 and their combination.

9

The value of pressure of equilibrium distribution in USD is obtained from Eq. 10 based on the principles of thermodynamics.

10

where, T is the temperature of USD chamber (Kelvin), R is the gas constant (J/(mol.Kelvin), M is the molecular weight of the probe vapor (gr/mol), A is the specific surface area (SSA) of aggregates (cm²/gr), and n is the weight of the vapor absorbed in the mass unit of the aggregate at the pressure p (gr).

The SSA of aggregate in the Eq. 10 is obtained with Eq. 11.

11

where, N₀ is the Avogadro constant (6.022 × 10²³/mol), n_m denotes the monolayer capacity (cm²/gr), and α is the single molecule projected area (cm²/molecule). The capacity of monolayer is calculated with Eq. 12.

12

Where, I and S parameters correspond to the intercept on the y-axis (in mol) and the slope of the versus diagram (mol), respectively. It should be noted that n is the absorbed vapor mass relative to aggregate mass, P (Pa) is the partial vapor pressure, and P_n (Pa) is the saturated vapor pressure²⁸.

Three test materials to determine the SFE components of aggregate based on the system of three equations with three unknowns according to Eq. 8 are presented in Table 7.

Table 7.

Specifications of the liquids used to measure aggregate SFE via the USD method.

SFE components	Surface free energy parameters (ergs/cm2)
	Acidic	Basic	Acid–Base	Lifshitz–van der Waals	Total SFE
n-Hexane	0	0	0	18.04	18.04
Water	25.5	25.5	51	21.8	72.8
MPK	0	19.60	0	24.70	24.70

Results and discussion

Mechanical tests’ results

The values of ITS test of the compositions under base conditions (without STA) in different conditions are given in Fig. 3, respectively. Evidently, the ITS of the samples considerably decreased under wet conditions. Although dry specimens containing granite had higher resistance, they suffered a marked resistance decline in the wet condition compared to specimens containing limestone, which shows greater moisture susceptibility of samples made with granite aggregates.

Fig. 3.

ITS of samples in different conditions.

In the dry state and for both aggregates, as predicted, specimens produced with PG 64–16 bitumen had greater resistance due to their greater stiffness. After the conditioning cycles, the drop in resistance was 16 and 19% in samples containing PG 58–22 and PG 64–16 bitumens and limestone, while this value was 27 and 30% in samples containing granite aggregates, respectively. Therefore, although PG 64–16 demonstrated greater resistance in combination with both aggregates, the resistance of specimens containing this bitumen under wet condition suffered a greater decline; this reduction was less in specimens containing limestone, demonstrating the greater affinity between PG 64–16 bitumen and limestone aggregates in the presence of moisture.

Another notable point is the effect of NCC and NHL additives. The trend of changes reveals that these two additives increased the ITS of dry specimens and alleviated their resistance reduction. In fact, additives mitigated the moisture susceptibility of the WMA samples. NHL showed a more obvious functional mechanism in increasing the dry samples’ resistance. This is better illustrated especially for granite and PG 58–22 (a 17% rise in resistance compared to the base). However, NCC combined with limestone and PG 58–22 had the smallest resistance reduction, demonstrating its lower moisture sensitivity.

Figure 4 depicts the values of TSR of specimens made with base (unaged) and aged bitumens, respectively. A comparison of the trend of changes in TSR in different states revealed that short-term bitumen aging decreased the specimens’ TSR, expressing an elevation in their moisture sensitivity due to bitumen aging.

Fig. 4.

TSR of base and aged specimens.

Similar to the results of Fig. 3, the TSR values in this diagram show that specimens made with limestone aggregates had more moisture strength than those made with granite aggregates in both states. As noted earlier, bitumen aging increases asphalt mix moisture sensitivity which is influenced by aggregate and bitumen type. Based on Figs. 4, granite aggregates combined with aged bitumens had a lower TSR reduction than limestone aggregates. Meanwhile, the smallest reduction belonged to the combination of granite and PG 64–16 bitumen under base conditions. Bitumen modification with NCC and NHL at all base conditions lead to an increase in the specimens’ TSR. Meanwhile, NHL was superior to NCC in all states, except for combination with PG 58–22 and limestone. These two additives did not alleviate the TSR decline of the specimens containing aged bitumen compared to unaged ones, but even increased it; in other words, they failed to mitigate the destructive effect of bitumen aging on WMA moisture susceptibility.

SFE measurements

Cohesion SFE

The bitumen cohesion free energy obtained at the base and STA states is given in Fig. 5. The bitumen cohesion free energy slightly changed after STA. At all states, this process slightly increased the free energy of cohesion of the specimens containing PG 64–16, but led to non-uniform slight changes in the specimens made with PG 58–22 bitumen. In fact, for this type of bitumen STA slightly increased the free energy of cohesion at the base state but slightly reduced it in the modified state. NCC- and NHL-modified bitumen markedly raised the free energy of cohesion. This considerable increase in free energy of cohesion was 63% and 53% in PG 64–16 and PG 58–22, respectively.

Fig. 5.

The cohesion free energy of base and STA bitumens.

Overall, the additives’ performance in increasing the free energy of cohesion at aged and unaged states was non-negligible and similar. A higher cohesion free energy shows greater mastic resistance to internal film cracking; therefore, these additives were very appropriate for use in WMA. Tiny cracks are usually the main route for the penetration of water into asphalt, and moisture damage frequently occur in WMA.

Adhesion SFE

Figure 6 presents the results of free energy of adhesion of two types of bitumens and aggregates, respectively. The results show that samples made with limestone aggregates had lower adhesion free energy compared to granite, regardless of whether PG 64–16 or PG 58–22 bitumen was used under base or STA conditions. STA diminished the free energy of adhesion in different compositions. This decline in energy was about 3% under baseline conditions for samples made with PG 64–16, but about 1% in similar compositions under modified conditions. These trends show the additives effect in mitigating the free energy of adhesion reduction after STA. These values were 3 and 5% for compositions containing PG 58–22 at both base and modified conditions, respectively. Therefore, NCC and NHL bitumen modification displayed better performance in the adhesion of compositions prepared with PG 64–16 bitumen.

Fig. 6.

The adhesion energy of base and STA bitumens.

Overall, NCC and NHL bitumen modification raised the free energy of adhesion of all the compositions by 12% on average, indicating the proper performance of these additives. The NCC was weaker than NHL in compositions made with PG 64–16. This trend was reversed in the case of PG 58–22 bitumen. In this bitumen, the use of NCC additive has shown better performance.

Adhesion free energy (debonding energy)

The process of the bitumen stripping from the aggregate surface in the presence of water is called a spontaneous process. Therefore, the values of the debonding energy parameter are negative. The greater the absolute value of the debonding energy, it shows that stripping creates a more stable system and the desire to stripping in that asphalt mixture is greater. Therefore, the debonding energy index can be considered as a symbol for the resistance of the asphalt mixture against moisture damage³⁰.

The values of the debonding energy parameter are depicted in Figs. 7 and 8. The compositions made with aged bitumen at the unmodified state exhibited higher debonding energy compared to the other compositions. This indicates the further destruction of these compositions under STA. This means that, the samples’ moisture susceptibility enhanced with STA of bitumen. The major practical finding is that the compositions made with granite had more debonding energy than those made with limestone aggregates, demonstrating the bitumen-limestone composition’s lower moisture sensitivity. A more precise examination revealed that the addition of additives to bitumen greatly decreased debonding energy at the base state, expressing elevated resistance to moisture damage. NHL had greater effects than NCC on base compositions; however, after STA, a different effect was observed for the two additives. Following aging, the values of debonding energy of bitumen-aggregate in the wet conditions decreased by 1.6, 0.8, and 1.3% for unmodified, NCC-modified, and NHL-modified states, respectively. This indicates the greater impact of aging on unmodified bitumen in terms of reduced debonding energy and increased resistance of NCC-containing bitumen to water. Accordingly, PG 58–22 bitumen composition was more compatible than the PG 64–16 containing composition.

Fig. 7.

Debonding energy in bitumen-granite compositions.

Fig. 8.

Debonding energy of compositions of bitumen and limestone aggregates.

Similar to the previous case, the combination of limestone aggregates and PG 58–22 had better affinity (higher moisture resistance) than PG 64–16 at the base, aged, and nano-modified states. After bitumen aging, however, its moisture sensitivity underwent greater changes. In other words, different WMA samples became more moisture-susceptible after aging. The process of bitumen aging had a greater impact on the combination of PG 58–22 with limestone aggregates and decreased the debonding energy to a greater extent. The debonding energy of the base specimens was reduced by 2%, and this reduction was almost twice larger in samples made with modified PG 58–22 than modified PG 64–16.

Relation between the thermodynamic parameters and mechanical tests of moisture susceptibility

The existing tests to determine the moisture susceptibility of WMA, which are mostly mechanical, despite being convenient and simple, have shortcomings. The most important of these shortcomings are: (1) lack of examination of the basic properties of the materials, (2) failure to examine the cause and process of damage, (3) the impossibility of providing a suitable solution in order to modify the mixture, and (4) the difficulty in expressing the mechanism of action of additives^28,31. These problems can be solved by paying more attention to basic experiments such as SFE and using the indices of this theory. SFE parameters can be used to determine mastic and bitumen-aggregate composition sensitivity in dry and in the presence of water conditions.

The results of the analysis of the relationship between thermodynamic parameters (debonding energy in the stripping process) and their relationship with the results of common mechanical tests such as modified Lottman (TSR index) are presented in this research. In order to check the validity between these two categories of results, the coefficient of correlation (R²) for different WMA samples, this analysis has been done for base and aged samples, separately. The results of this investigation are shown in Figs. 9 and 10. As can be seen, the increase in the number of freeze–thaw and STA caused the bitumen-aggregate adhesion and consequently the resistance of the asphalt mixture to moisture to decrease. As the number of freeze–thaw cycles increases, this destructive effect is more severe. The results show that there is a close relationship between thermodynamic and mechanical parameters. This trend indicates that the use of thermodynamic tests can be a suitable alternative to mechanical tests and solve some of the weaknesses of mechanical tests, which were previously stated.

Fig. 9.

Determination of the relationship between debonding energy and TSR (base).

Fig. 10.

Assessment of the relationship between debonding energy and TSR (STA).

The additives greatly increased asphalt mix resistance to cohesive and adhesive failure but slightly weakened the correlation between thermodynamic and mechanical parameters, although this relationship remained valid and strong.

Conclusions

The high construction temperature of asphalt greatly contributes to primary bitumen behavior variation. This viscoelastic behavior change is generally known as STA. WMA is a highly appropriate asphalt mix; thanks to its lower construction temperature, WMA undergoes less behavioral change and STA. In practice, however, WMA has a softer bitumen structure and loses its properties after a slight delay compared to HMA when exposed to environmental conditions and aging. Its softness accelerates aging after this delay. Therefore, the aging factor must be closely examined in WMA. Aging alters thermodynamic and fundamental components of bitumen, impacts the bitumen-aggregate interaction in the long run, and diminishes the asphalt mix resistance; as such, understanding this molecular structure change will promote the understanding of bitumen-aggregate interaction and, by proposing efficient solutions, help improve the bitumen-aggregate compatibility and asphalt quality. This necessitates the further examination of the impact of aging in WMA and its relationship with moisture damages. The current study examined the factors affecting the fundamental properties of bitumen, closely evaluated the STA process, and proposed suitable additives to change bitumen behavior and improve asphalt mix resistance to moisture damage. The most important conclusions are presented below:

1. Granite aggregates are more acidic than limestone aggregates. To create a strong bitumen-aggregate bond, bitumen acidity should be reduced and its basicity should be increased. As such, the basic NHL is more appropriate than NCC for combination with more acidic aggregates. In general, NCC and NHL additives promoted the ITS and, therefore, moisture resistance, which can be attributed to the remarkable rise in the non-polar component which is more prominent than the polar component in asphalt mixes. Non-polar components displayed similar fluctuations in different compositions. Therefore, the rise/decline of the polar component becomes more salient despite its small value.

2. Bitumen aging diminished the resistance of all asphalt mixes. Still, in compositions containing granite, this decline in strength was smaller, indicating the superiority of granite-containing to limestone-containing specimens against bitumen aging. This behavior could be due to the increased adhesion free energy in samples made with granite aggregates. Limestone demonstrated lower free energy of adhesion than granite aggregates when combined with PG 64–16 and PG 58–22 bitumens at the base and STA states. However, the two bitumens displayed a similar rise in cohesion energy. Bitumen modification increased the adhesion free energy by 12%, indicating the proper performance of the additives for WMA which has poor adhesion. The improvement in this factor was more salient for PG 64–16 bitumen.

3. Additives greatly increase the cohesion free energy. A higher cohesion free energy shows greater mastic resistance to internal film cracking; therefore, these additives were very appropriate for use in WMA. Tiny-cracks are the main route for the penetration of water into the asphalt and induction of moisture damages in WMA. The mastic used in WMA is softer than HMA, and increasing the cohesion energy can resolve this problem. Bitumen modification with these additives remarkably increases the cohesion free energy by 63 and 53% for PG 64–14 and 58–22 bitumens, respectively. Therefore, PG 64–16 bitumen becomes stiffer with the additives.

4. Modification of base bitumens led to a smaller decline in the mixes’ moisture resistance. Nevertheless, modification of aged bitumens led to a greater reduction in moisture resistance. Due to aging, the content of polar oils was reduced, which decreased the polar component of the SFE. As aggregates are polar, and polar adhesion between aggregates and asphalt is more effective, so reduction of bitumen polar properties decreases adhesion.

5. The additives increased the debonding energy by about 4% in the presence of water at both base and aged states, indicating an improvement in moisture sensitivity. In this regard, NHL surpassed NCC in the specimen containing PG 64–16 in terms of overall moisture sensitivity, whereas the trend was reversed for PG 58–22.

6. The correlation coefficient between the mechanical index of moisture sensitivity and the bitumen-aggregate debonding energy was smaller for the unaged and aged states after one freeze–thaw cycle than the same state but after three cycles. One cycle induces slight aging; the bitumen is softer; and the aggregate interlocking effect is more prominent. Therefore, the results of previous studies most of which were based on one free-thaw cycle were less precise.

7. The correlation coefficient between the mechanical index of moisture sensitivity and the bitumen-aggregate debonding energy was 89% and 85% in the fifth freeze–thaw cycle for the base and aged states, respectively. This strong correlation between the results demonstrates the necessity of using thermodynamic components in estimating the effect of bitumen aging on asphalt mix moisture sensitivity.

Although this study provides valuable insights, it does not evaluate the complexity of the next step of aging, where bitumen is simultaneously exposed to reactive oxygen species, ultraviolet ultra violet radiation, and prolonged moisture. The fixed nano-additive dosage, the use of specific aggregates and bitumen types, also limit the full representation of real pavement environments.

Future studies should examine a wider range of materials, explore different additive dosages, and include samples subjected to actual field aging. For instance, the percentages of heteroatoms in bitumen, such as sulfur, can vary across different types of bitumen, and the formation of sulfoxide, and therefore the level of STA, can be affected. Moreover, molecular dynamics simulations could be employed to evaluate the influence of nano-additives and aging mechanisms under different conditions at the molecular scale. Such evaluations on bitumen with different heteroatom contents can provide a deeper understanding of bitumen–aggregate interactions and moisture damage processes in different aging conditions. Based on the findings of this study that shows an improvement in adhesion and cohesion which are achieved after modification with nano-additives, the results can be directly used to guide material selection and additive design in real construction projects. Such guideline helps engineers to develop more durable and moisture-resistant asphalt pavements.

Author contributions

Conceptualisation and Methodology: Gholam Hossein Hamedi and Bernhard Hofko; Laboratory test and data analysis: Farhad Sakanlou, Himan Rahmani, Mohsen Sohrabi; Interpretation of Results: Gholam Hossein Hamedi and Bernhard Hofko; Writing Original Manuscript: Farhad Sakanlou, Himan Rahmani, Mohsen Sohrabi; Manuscript review and editing: Gholam Hossein Hamedi and Farhad Sakanlou.

Data availability

The datasets used and/or analyzed during the current study available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.