Abstract
In between thermal-oxidative (heat and oxygen) and photo-oxidative (ultraviolet irradiation and oxygen) aging process of bitumen, photo-oxidative aging mainly depends on the optical properties of the asphalt binder. The higher the reflection (or the lower the absorption of the binders), the better the pavement serviceability. The literature review indicates that there is limited research conducted on the optical properties of the binder with wax additives. In this paper, the optical properties of commonly used binders (PG 64-22, Rubber modified binder, and SBS modified binder) containing wax additives (LEADCAP and Sasobit) were investigated using UV-Vis spectrometer. The result of this study showed that (1) the addition of modifiers (crumb rubber and SBS) with the base binder slightly increases the absorption of the binder; (2) the binder types and aging level have significant contribution on optical properties; (3) in general, the aged binders were observed to have higher reflectivity compared to the unaged binders; and (4) the addition of wax additives is observed to have a significant effect on the optical properties.
1. Introduction
Bitumen has been extensively used as a binder in road construction [1]. Like other organic materials, bitumen is prone to become fragile and stiff due to exposing to ultraviolet (UV) light, oxygen, and heat during storage, mixing, transport and laying down, as well as in service life [2–4]. The absorption of UV light from the sun degrades the bitumen properties, and the effects are different depending upon the binder type. UV radiation and oxidation affect the upper layer of the pavement surface and make the bitumen more brittle and hard, which would lead to reduction in long-term performance, particularly at low temperature, and thus shorten the pavement life. As a result, it is important to improve bitumen’s ability to resist UV radiation and thermal-oxidative aging. The effects of UV irradiation on bitumen aging were ignored in early researches because solar radiation only affects the upper layers of pavement. However, later, the emphasis was given on this topic as it has significant contribution to the bitumen aging [5].
Several studies have been conducted to increase the resistance of UV aging and oxidation of the asphalt binder [6, 7]. Antioxidants, light stabilizers/ultraviolet absorbers, montmorillonite, phosphorus compounds, carbon black, thermochromic materials, and layered silica were implemented to improve the UV and oxidation aging resistance of bitumen [4, 5, 8–11].
Warm mix asphalt (WMA) technologies ensure the reduction of mixing and compaction temperatures of asphalt mixtures [12]. It also improves the working conditions, lowers the paving time, and improves air quality [13, 14]. Rheological, physical, and microstructural properties of the asphalt binder were observed to have significant changes with the addition of wax additives [15–18]. It is important to investigate the interaction between bitumen and wax additives in terms of the optical properties. In this work, UV-Vis spectroscopy is utilized to characterize the optical properties of warm asphalt binders. In order to build a substantial body of knowledge, the main objective of this study is to investigate the change in optical properties of WMA binders mixed with wax additives using UV-Vis spectroscopy.
2. Experimental Design
2.1. Materials
Performance-grade (PG) 64-22 asphalt binder was used in this study. The PG 64-22 binder is commonly used to produce the crumb rubber modified (CRM) and polymer modified asphalt (PMA) binders as a base binder. Table 1 shows the binder properties. The PMA binder containing SBS (approximately 3% by the weight of binder) was used in this study. The crumb rubber was produced by mechanical shredding at ambient temperature. The binder made with ambient CRM is known to result in better performance properties, compared to cryogenic CRM. The rubber gradation is shown in Table 2. The CRM binder was produced in the laboratory at 177°C for 30 min by an open-blade mixer at a blending speed of 700 rpm [19–21].
2.2. Warm Asphalt, CRM, and PMA Binder Production
Two commercial wax additives of LEADCAP and Sasobit were selected to add into the PG 64-22 binder, CRM binder, and PMA binder. The process included the additive addition at a specified concentration (1.5% by the weight of asphalt binder) followed by hand mixing for a minute to produce warm binders (PG 64-22, CRM, and PMA). The concentration of 1.5% was suggested by the manufacturer.
The LEADCAP is an organic additive of a WMA wax-based structure that consists of crystal controller and artificial materials. As polyethylene-based wax is the major component of LEADCAP, the wax material can be melted at over melting temperature due to its crystalline structure. The melting point of LEADCAP is approximately 110°C. Therefore, the LEADCAP in the asphalt binder at 130°C (the temperature at which the asphalt mixture is produced) is liquidized. Since the molecular weight of wax is lower than that of average asphalt molecules, LEADCAP in the asphalt binder can reduce the viscosity of the binder.
Sasobit is a long chain of aliphatic hydrocarbon obtained from coal gasification using Fischer–Tropsch process. Ultimately, it is a result of synthesis of Fischer–Tropsch (FT) wax from methane and also Sasol wax, which melted completely into the asphalt binder at 115°C and reduced the binder viscosity. Sasobit gives poor low-temperature properties because crystalline wax material is very stiff and brittle at temperatures less than the crystallization point, which further expedites the wax-based additive to exhibit a high potential for cracking (Yang et al., 2012). Sasobit forms a lattice structure in the binder after crystallization, which is the basis of the structural stability of the binder containing Sasobit [13, 14]. The PG 64-22, CRM, and PMA binders containing wax additives were then artificially aged using the pressure aging vessel (PAV) for 20 hours at 100°C in order to make aged binders for UV-Vis. Figure 1 shows wax additives used in this study.
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2.3. Superpave Asphalt Binder Tests
The Superpave asphalt binder tests were measured at its original state, after mixing and construction, and after in-service aging. The viscosity test (AASHTO T 316), the dynamic shear rheometer (DSR) test (AASHTO T 315), and the bending beam rheometer (BBR) test (AASHTO T 313) were performed on the sample binder.
Brookfield rotational viscometer at 135°C (the standard test temperature) and at 120°C (the mixing temperature generally used for warm mix asphalt) was used to measure the viscosity of the sample binder. The binder sample, both in the original state (unaged) and short-time aged state, was used to determine the G/sin δ at 64°C using DSR test at a frequency of 10 radians per second, which is equal to approximately 1.59 Hz. The fatigue cracking property for aged binders was measured at 25°C in terms of Gsin δ. The creep stiffness (S) of the binder was measured at a loading time of 60 s using BBR test on asphalt beams (125 × 6.35 × 12.7 mm) at −12°C. Testing was performed on aged sample by applying a constant load of 100 g to the beam of the binder, which was supported at both ends, and the deflection of center point was measured.
Multiple stress creep recovery (MSCR) test was conducted on the RTFO aged binder of warm PG 64-22, PMA, and CRM binders according to AASHTO T 350-14 specification at 64°C. The samples are tested in creep and recovery at a stress level of 3.2 kPa. The nonrecoverable creep compliance (Jnr) and percent recovery (% Rec) parameters are derived by analyzing the MSCR test. The rutting potential of the asphalt binder was evaluated by measuring the nonrecoverable creep compliance (Jnr), which is determined by dividing nonrecoverable shear strain by the shear stress.
2.4. Ultraviolet and Visible Spectroscopy (UV-VIS)
UV-Vis spectrometer measurements were recorded on a UV-2501 spectrophotometer in the wavelength range 200–800 nm, a scanning speed of 300 nm/min, and a response of 120 s. Sample was prepared by pouring melted binder on the surface of a glass slide. All binders were preconditioned by controlled heating at 170°C in an oven. The samples were examined after 24 h of cooling.
The weighted average of reflectance in the entire measurement wavelength range (220 nm–900 nm) was calculated according to ASTM standard E903-96 (ASTM E903-96). The average reflectance R was calculated using the following formula:where (λf – λi) is the spectral wavelength range, R (λ) is the reflectance at wavelength λ, and E (λ) is the spectral irradiance distribution, which is given byHere, is the photon flux, defined as number of photons per square meter per second (#photon m−2·sec−1), and q is a constant with a value of 1.607 × 10 e−19 (electron’s charge).
3. Results and Discussions
3.1. Rheological Properties
Table 3 presents the physical and rheology properties of the warm binders. Viscosity of asphalt binders at high temperatures is considered as an important property because of its ability to be pumped through an asphalt plant and workability. The viscosity test was conducted at two testing temperatures (135°C and 120°C). The addition of SBS and crumb rubber is found to increase the viscosity of PG 64-22, as expected. It is clear that the addition of wax additives into the binder reduced the viscosity, resulting in decrease in the mixing and compaction temperatures. The trend was consistent for both testing temperatures. It was observed that Sasobit has significant effect on reducing the viscosity of binders.
The higher G/sin δ values from the DSR test indicate that the binders are less susceptible to the permanent deformation at high temperatures. Both unaged and RTFO states were showed that the addition of wax additives improved the rutting resistance. As expected, PMA binder has higher rutting resistance compared to the other binders. In general, the binder with Sasobit showed the highest values.
MSCR tests at RTFO state exhibited that the PMA binder has the highest % Rec value compared to the other binders. It was found that the addition of LEADCAP increased the Jnr value, whereas Sasobit reduced the value regardless of binder types.
The product of G and the sine of the phase angle, δ, is used to control the fatigue of asphalt binder, and lower values of Gsin δ are considered desired attribute from the standpoint of resistance of fatigue cracking. The CRM binders exhibited the lowest Gsin δ value among all the binder. The binders with Sasobit have the highest value within each binder type. The addition of LEADCAP into PG 64-22, PMA, and CRM binders reduced the Gsin δ by 11%, 27%, and 21%, respectively.
The stiffness and m-value of the binders with and without wax additives were measured on the aged binders using BBR tests at −12°C. The Superpave standards determine that samples can fail at a given test temperature when the stiffness is greater than 300 MPa or with an m-value lower than 0.300 [22]. The decrease in stiffness reduces the tensile stress in the asphalt binder, which decreases the chance of low-temperature cracking. The addition of crumb rubber reduced the binder stiffness. The binders modified with Sasobit exhibited highest stiffness within each binder type. The addition of LEADCAP is observed to have significant effect on reducing the stiffness of the binders (6%, 7%, and 3% reduction were achieved for PG 64-22, PMA, and CRM binder, respectively).
3.2. Optical Properties
UV-Vis spectroscopy is utilized to examine the absorption and reflection of warm binders. Asphalt binder is vulnerable to aging due to its mixing, paving, and compaction, as well as service life, resulting in the hardening of asphalt. In order to improve the pavement durability, it is important that the binder has antiaging properties. The absorption of UV light from the sun degrades the bitumen properties and the effect is different depending upon the binder type. Figure 2 presents the absorption and reflection spectrum of warm asphalt binders. The base asphalt binder of PG 64-22 has less absorption compared to the binders with wax additives in UV range. The LEADCAP modified binder has the highest absorption rate both in UV and visible range. It indicates that the base binder with LEADCAP is more vulnerable to aging during mixing and compaction. The high peak values of 1.40, 1.45, and 1.60 were observed for the PG 64-22, the binder with Sasobit, and the binder with LEADCAP, respectively. The trend of high absorption rate is consistent throughout the UV and visible spectrum. Although the binder containing Sasobit has higher absorbance compared to the base binder of PG 64-22 at UV range, the binder with Sasobit is found to have lower absorbance ranging from 550 to 800 nm.
Figure 3 illustrates the absorption and reflection of warm binder aged after getting exposed to PAV aging, which simulates the field condition of binder after several years of service period. Like the unaged binder, a similar trend was observed for all the binder types at aged condition. One important phenomenon that was observed for the warm asphalt binders was that after aging, all the binders have higher reflectivity compared to the unaged condition. It means that at aged condition, part of the solar energy that arrives at the pavement is reflected back out into space, thereby lowering the pavement’s temperature. The binder with LEADCAP was found to be more vulnerable to the UV light regardless of the aging level (Figure 4).
The control CRM binder has higher absorption rate compared to CRM binder with LEADCAP. The CRM binder with LEADCAP is found to have significant improvement in terms of reducing the absorption in UV and visible region during mixing and paving of the pavement. Figure 5 presents the absorption and reflection curves of aged warm CRM binders. All the binders after aging are found to follow a similar trend that was observed with unaged binders. The CRM binder containing Sasobit has less reflectivity, whereas the control CRM binder and CRM binder with LEADCAP have similar reflection at the wavelength ranging from 200–800 nm. The control CRM binder and CRM binder with Sasobit are observed to have less reflection after aging, whereas CRM binder containing LEADCAP has slightly higher absorption compared to the corresponding unaged CRM binder.
Figure 6 illustrates the absorption and reflection of warm PMA binder at unaged condition. PMA binder with LEADCAP is observed to have the lowest absorption among all the binders regardless of the UV and visible range. The PMA binder is found to have the lowest reflection compared to the other binder types. Figure 7 presents the absorption and reflection of aged warm PMA binder. PMA binder with LEADCAP seemed to have a similar trend with corresponding unaged PMA binder. However, the binder is observed to have higher absorption compared to the unaged binder. Unlike the unaged binder, the binder containing Sasobit had the lowest reflection among the aged binders. It indicates that Sasobit has significant effect on the PMA binders for its high absorption mechanism in a long run.
CRM binder has slightly higher absorbance compared to the base PG 64-22 binder and PMA binder at unaged condition. Also, PMA binder is observed to have slightly higher absorption compared to PG 64-22. It indicates that modification with CRM and SBS at unaged condition tends to make the control PG 64-22 binder a less reflective binder. A similar trend was observed for PG 64-22, PMA, and CRM binders after aging. The addition of LEADCAP into CRM and PMA binders reduces the absorption of UV light regardless of the aging condition, whereas the opposite trend was observed with the base binder of PG 64-22. A similar trend was observed at the visible region as well. The addition of Sasobit into the PG 64-22 binder increases the absorption in the UV region for both unaged and aged conditions. Generally, CRM and PMA binders containing Sasobit decrease the reflection of the corresponding base binder. The reason might be the fact that LEADCAP dissolved microcrystalline waxes and waxy molecules in the base binder of PG 64-22 easily, which makes the binder absorb more light, whereas CRM and PMA binders may act as a coating, which helps to reflect the light. Sasobit obstructs the movement of asphalt molecule chains and obstructs the crystallization of microcrystalline waxes and waxy molecules in base binder of PG 64-22. It helps to reflect more light, whereas with CRM and PMA, it may dissolve rapidly, resulting in high absorption of the binder. Among all the binder types, the CRM binder containing Sasobit at unaged condition has the highest absorption in both UV and visible regions. Among the aged binders, the PMA binder with Sasobit had the lowest reflection. Table 4 presents the average reflectance (%) of all the WMA binders in the spectral range of 220 to 800 nm.
4. Conclusions
In this paper, UV-Vis spectrometer was used to investigate the effect of WMA additives (LEADCAP and Sasobit) on the optical properties of PG 64-22, CRM, and PMA binders before and after PAV aging. Based on the results of the study, the following conclusions can be drawn:(1)The literature regarding the interaction of wax additives with asphalt binders in terms of optical properties is very limited.(2)The addition of wax additives decreases the viscosity, whereas it increases the rutting resistance. The binders modified with Sasobit exhibited highest stiffness within each binder type.(3)Unaged and aged asphalt binders are observed to have different optical properties. In general, aged asphalt binders reflect more sunlight from the surface of the pavement compared to the unaged binder.(4)Binder types (PG 64-22, CRM, and PMA binders) and the modifiers (crumb rubber and styrene butadiene styrene) are found to have significant contribution towards the change in optical properties. It is observed that modification of PG 64-22 binder with SBS and crumb rubber increases the absorption of the binder.(5)Generally, LEADCAP resulted in increasing the reflection of light of CRM and PMA binders regardless of aging and spectral range.
Data Availability
The data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This study was conducted under research project “Development of High-Performance Concrete Pavement Maintenance Technology to Extend Roadway Life (Project no. 18TLRP-B146699-01)” funded by the Ministry of Land, Infrastructure and Transport (MOLIT) and the Korea Agency for Infrastructure Technology Advancement (KAIA). The authors would like to thank the members of research teams MOLIT and KAIA for their guidance and support throughout the project.