Study on UV aging characteristics of low-grade asphalt in the desert climate

In figure 3(a), the virgin asphalts were liquefied and subsequently poured into petri dish with a layer of aluminum foil wrapping. An accelerated aging test was performed on the cooled samples using UV radiation, as demonstrated in figure 3(b), with a UV aging device [28]. The device featured a high-pressure mercury lamp as the UV light source, rated at 1 kW, which generated a significant amount of heat and high radiation intensity. To effectively regulate the surface temperature of the samples, the asphalt samples were placed 20 cm away from the lamp.

Zoom In Zoom Out Reset image size

As referenced, the mean annual aggregate solar radiation in the southern region of the Taklamakan Desert is 1700 kWh m⁻², with ultraviolet radiation accounting for approximately 5% of this value [29]. At the project site, the total mean annual UV radiation is 85 kWh m⁻². The central asphalt sample's UV radiation intensity was measured using a UV intensity meter and found to be 400 W m⁻² during indoor testing. Based on the principle of UV radiation energy equivalence [30, 31], the duration of indoor exposure is determined as:

$$\begin{eqnarray}&&{t}_{{indoor}}=\frac{{I}_{{field}}\times {t}_{{field}}}{{I}_{{indoor}}}\end{eqnarray} \tag{ 1 }$$

The duration of indoor exposure, ${t}_{{indoor}},$ is determined by the intensity of ultraviolet radiation, ${I}_{{indoor}},$ and is influenced by the duration of outdoor exposure, ${t}_{{field}},$ and the intensity of outdoor ultraviolet radiation, ${I}_{{field}}.$ By applying the principle of equivalence of UV radiation energy, the duration of exposure required for one year of equivalent UV radiation energy was calculated to be 212.5 h. The asphalt samples underwent a 14-day UV aging test, which is equivalent to 1.58 years of aging in the southern region of the Taklamakan Desert. The images of 30#, 50#, and 70# asphalt after UV aging are shown in figures 3(d)–(f), respectively. A comparative analysis was conducted on the PAV-aged samples, which were prepared in accordance with ASTM D6521.

To investigate the micro-morphology changes of low-grade asphalt resulting from UV aging, atomic force microscopy (AFM) observations were conducted on three different asphalt samples before and after exposure to UV radiation. For observation purposes, asphalt film samples with a diameter of approximately 10 mm were prepared by hot casting. The Bruker Dimension Icon AFM with the RTESPA-150 probe, featuring an elastic coefficient of 5 n m⁻¹, resonance frequency of 150 kHz, and a radius of curvature of 8 nm, was selected for this test. The scanning images were obtained using tapping mode, with a scanning range of 20 μm × 20 μm at a temperature of 25 ± 1 °C and a pre-pressure of 10 nN. Furthermore, adhesion force was obtained by calculating the difference between the lowest point of the curve and the horizontal line. Offline analysis software NanoScope Analysis was utilized to analyze the images and perform calculations.

Gel permeation chromatography (GPC) was employed in this study to evaluate the effects of UV aging on low-grade asphalt by determining the changes in molecular weight and distribution. The GPC analysis was performed using the PL-GPC-50 chromatograph manufactured by Polymer Laboratories, with tetrahydrofuran (THF) serving as the solvent at a mobile phase rate of 1 ml min⁻¹ and a temperature of 40 °C. An injection volume of 100 μl was utilized, and the testing was conducted using three chromatographic columns, each 300 mm in length. The asphalt samples, both pre- and post-UV aging, were dissolved in THF prior to undergoing GPC testing. Through this analytical method, the differential distribution curve of molecular weight was derived, providing insights into the asphalt's structural changes upon exposure to UV radiation.

To investigate the impact of UV aging on the various functional groups of low-grade asphalt, EQUINX55 Fourier transform infrared spectrometer in attenuated total reflection infrared mode was employed to analyze the virgin, UV aged, and PAV aged asphalt samples. Infrared spectroscopy is a versatile technique that can be utilized to determine the molecular structure of compounds, identify unknown substances, and analyze mixture components. The FT-IR test was performed in the wavenumber range of 400 cm⁻¹ to 4000 cm⁻¹. The position and shape of the absorption peaks in the spectrum can provide valuable information about the chemical structure of an unknown compound, while the intensity comparison of the characteristic absorption peaks can aid in determining identifying functional groups present in the samples [32, 33].

Dynamic shear rheological (DSR) tests were conducted to evaluate the high-temperature rheological properties of asphalt, which undergo changes upon aging, particularly when exposed to UV radiation. To investigate the influence of UV aging on the high-temperature rheological properties of low-grade asphalt, dynamic shear rheological tests were carried out on the virgin, UV aged, and PAV aged samples of three kinds of asphalt. The tests were conducted using a dynamic rheometer (TA Instruments) in the mode of temperature sweep, with a test temperature range of 46 °C to 82 °C and an interval of 6 °C. The frequency was set at 10 rad s⁻¹, and the diameter of the parallel plates was 25 mm with a spacing of 1 mm. The tests recorded the complex modulus (G*) and phase angle (δ), from which the rutting factor (G*/sinδ) can be calculated.

Bending Beam Rheometer (BBR) tests were conducted to characterize the low-temperature performance of asphalt samples. The BBR equipment was provided by Cannon Instrument Company and the tests were conducted in accordance with the ASTM D6648 standard. The samples included virgin, UV-aged, and PAV-aged asphalt of three types. The testing temperatures were set at −6 °C, −12 °C, −18 °C, and −24 °C. The low-temperature performance of the asphalt samples was evaluated by measuring the stiffness modulus (S) and creep rate (m) at low temperature, both before and after UV aging. Stiffness modulus reflects the resistance to a constant testing load, while creep rate represents the slope of the stiffness versus time curve. A low stiffness modulus and high creep rate are favorable for resisting low-temperature cracking at a given low temperature.

Figure 4 displays the atomic force microscopy (AFM) images of the surface morphology of 30#, 50#, and 70# asphalt, before and after UV aging, in both two-dimensional (2D) and three-dimensional (3D) microscale. As can be seen from the images, there are no distinct bee-like structures on the surface of the three types of asphalt before or after UV aging, which can be attributed to the origin, composition, and structural makeup of the asphalt.

Zoom In Zoom Out Reset image size

In this study, we aim to investigate the correlation between surface morphology parameters and UV aging in asphalt. To quantify the AFM images, surface roughness was utilized as a characteristic index. The formula (2) was employed to calculate the surface roughness (Ra) of asphalt, where A represents the scanning area of 20 μm × 20 μm, h(x,y) denotes the equation of the topography height parameter, and h0 is the reference height. The surface roughness values of the three asphalt samples under virgin and UV aged conditions were computed and depicted in figure 5.

$$\begin{eqnarray}&&Ra=\displaystyle \frac{\displaystyle \int \displaystyle \int \left[h\left(x,y\right)-{h}_{0}\right]{\rm{dA}}}{\displaystyle \int \displaystyle \int {\rm{dA}}}\end{eqnarray} \tag{ 2 }$$

Zoom In Zoom Out Reset image size

Based on figure 5, it is evident that the surface roughness of the 30#, 50#, and 70# asphalt samples has increased by 15.50%, 5.99%, and 2.70%, respectively, after UV aging compared to their values prior to testing. Moreover, the trend of increasing surface roughness for the 30#, 50#, and 70# asphalt samples appears to be slowing down. This indicates that the surface roughness of the asphalt increases after UV aging. As the asphalt grade increases (i.e., 30#, 50#, 70#), the penetration gradually increases, and correspondingly, the content of light components also increases. Thus, the increase in surface roughness decreases with the rise in asphalt grade after UV aging.

Furthermore, figure 6 presents the force-distance curves obtained from the AFM tests, which reflect the bending state of the free end of the AFM microcantilever probe during contact with and detachment from the sample surface, as the fixed end approaches vertically. By applying Hooke's law, the equation for calculating the adhesion force can be derived, as follows:

$$\begin{eqnarray}&&{F}_{{ad}}=\theta \times {K}_{c}\times {\rm{\Delta }}X\end{eqnarray} \tag{ 3 }$$

Zoom In Zoom Out Reset image size

The displacement of the probe microcantilever from its non-bending state to completely leaving the sample surface is represented by ΔX, while the bending elastic coefficient of the probe microcantilever is represented by K_c . The adhesion force between the asphalt and the probe is represented by F_ad , and θ is the correction factor that accounts for the difference between the actual situation and the theoretical analysis. Therefore, the adhesion force between the probe and asphalt can be calculated by analyzing the force-distance curve. The calculations of the adhesion force can be observed in figure 7.

Zoom In Zoom Out Reset image size

The data presented in figure 7 reveal that the adhesive force of 30#, 50#, and 70# asphalt decreased by 8.35%, 19.16%, and 22.66%, respectively, after undergoing UV aging compared to their original states. It is important to note that the decrease in adhesive force gradually increased for 30#, 50#, and 70# asphalt.

In accordance with the surface physicochemical theory of materials, adhesion work is defined as the energy required to separate a unit adhesion interface between two adhering materials. Among the numerous theories that exist, the well-established John-son-Kendal-Roberts (JKR) contact theory is particularly well-suited for high adhesion systems and imposes minimal restrictions on object rigidity [34, 35]. Consistent with the JKR theory, the conversion relationship between adhesion force and adhesion work can be expressed as follows:

$$\begin{eqnarray}&&{F}_{{ad}}=\frac{3}{2}\pi {WR}\end{eqnarray} \tag{ 4 }$$

F_ad represents the adhesion force, while W represents the adhesion work. Referring to the tip of the probe utilized in this study, the contact radius is 8 nm. Employing equation (4) to calculate the adhesion work between the asphalt and the tip, the results are presented in figure 8.

Zoom In Zoom Out Reset image size

To accurately calculate the change in free energy of the asphalt surface energy, it is imperative to understand the adhesion work between the asphalt and the tip of the probe. In order to do so, the Fowkes model has been selected as the preferred methodology for calculating the surface energy of the asphalt. The calculation methodology is described as follows [36]:

$$\begin{eqnarray}&&{\gamma }_{{ab}}={\gamma }_{a}+{\gamma }_{b}-2\sqrt{{\gamma }_{a}^{b}{\gamma }_{b}^{d}}\end{eqnarray} \tag{ 5 }$$

$$\begin{eqnarray}&&{W}_{{ab}}=2\sqrt{{\gamma }_{a}^{d}{\gamma }_{b}^{d}}\end{eqnarray} \tag{ 6 }$$

The dispersion component of surface energy is represented by γ^d , W_ab refers to the adhesion work between interfaces, and γ_ab is the surface energy between materials. As for asphalt materials, it can be approximated that γ_a is nearly equal to ${\gamma }_{a}^{d},$. The parameter Wab refers to the adhesion work between the asphalt and the tip of the probe as:

$$\begin{eqnarray}&&{W}_{{ab}}=2\sqrt{{\gamma }_{a}{\gamma }_{b}}\end{eqnarray} \tag{ 7 }$$

After utilizing AFM equipment, it was determined that the surface energy γ_b for the tip of the probe is 1649.07 mJ m⁻². Therefore, utilizing equation (7), the corresponding surface free energy γ_a can be calculated. The obtained results are presented in figure 9, which demonstrates the surface free energy values for the three asphalt materials, both before and after undergoing UV aging.

Zoom In Zoom Out Reset image size

As shown in figures 6–8, it was observed that the adhesion force, adhesion work, and surface free energy of the three asphalt types (30#, 50#, and 70#) followed the trend of 30# < 50# < 70# under virgin conditions. After undergoing UV aging, it was found that the adhesion force, adhesion work, and surface free energy of the three asphalt types decreased correspondingly. Moreover, the degree of decrease increased progressively with the grade of the asphalt. In other words, it was observed that the impact of UV aging on the adhesion properties of asphalt was less significant in lower-grade asphalt compared to the conventional asphalt samples.