1. Introduction
Cracks are a common disease of asphalt concrete pavements. During road use, cracks are affected by factors such as temperature, precipitation, and loading. Cracks can develop into frost swelling and mineral spalling during stretching, destroying the structural integrity and service life of roads [
1,
2,
3,
4]. Therefore, the timely repair of cracks is very significant. Asphalt pavement crack repair materials can be divided into hot irrigation sealants, cold irrigation sealants, and special sealant materials. The cold-irrigation sealant-type material can be applied at room temperature; however, its failure rate is high, so it is often used for emergency repairs in harsh environments [
5]. Specialized sealant materials offer superior performance but are more costly and complex to work with. Hot irrigation sealants are less expensive, perform much better than cold irrigated materials, and are commonly used crack treatment materials [
6]. Therefore, the use of hot irrigation sealants to seal cracks can effectively prevent surface water and debris from entering the pavement structure and restore the load-bearing capacity of the pavement to a certain extent.
Hot asphalt is a traditional hot irrigation material with some bonding and sealing properties, but its performance is poor, with the elasticity and durability able to be improved [
7]. A survey found that the temperature difference for an area with a sealant life of only about 1 year means the sealant needs to be re-patched, as the performance of the sealant is closely related to the construction of the road and the maintenance funds invested into the situation; this phenomenon has caused a huge waste of manpower and funds [
7,
8,
9]. Modified asphalt sealant materials solve this problem. Many researchers worked on modified asphalt sealants [
10]. With asphalt as the matrix, by adding modifiers to improve the high- and low-temperature stability, anti-aging, elasticity, and other properties of sealant materials. At present, hot irrigation sealants mostly use rubber, polymer, and some inorganic materials to improve the strength of sealants. For example, styrene–butadiene–styrene (SBS) is currently the most widely used asphalt modifier. But rubber and polymer still have shortcomings in performance and engineering applications. Cao et al. [
11] exposed two types of sealants directly to the natural environment for 8 months to study the aging characteristics and the aging mechanism of the sealants. The surface of the sealants hardened, became brittle, and cracked after aging; the polymer particles degraded; and the light component of the asphalt matrix gradually transformed into a granular asphaltene. Kim et al. [
12] modified solvent deasphalting (SDA) residual oil with oil, SBS, and rubber powder, and the results showed that the addition of rubber powder reduced the adhesion of the sealants. The problems of low bonding, a short fatigue life, and a lack of aging resistance in the research of modified asphalt sealants have limited the promotion and engineering application of sealants. To address these problems, it is very essential to find a new modifier to be applied to a crack sealant to optimize its performance and increase its durability.
Carbon nanotubes (CNTs), long hollow cylinders of graphene, have an enormous specific surface area and are stronger, stiffer, and tougher than even most metallic materials [
13]. The fine scale of this fibrous structure also allows it to form a complex mesh structure with only small additions to the asphalt, which improves the strength and toughness of the asphalt [
14]. In addition, CNTs help to improve the fatigue and aging resistance of modified asphalt [
15]. Ibrahim Amin et al. [
16] studied the effect of multi-walled carbon nanotubes (MWCNTs) on the rheology of asphalt by controlling the additive content using MWCNTs as a modifier and found that the temperature sensitivity of asphalt increased when the content of MWCNTs was increased from 1% to 3% against the empirical and rheological properties of asphalt. Wang et al. [
17] used CNTs as modifiers to evaluate the decay of the physical properties of modified asphalt after aging using participation in the needle penetration ratio, the softening point increment, the residual ductility ratio, and the viscosity aging index. The results showed that CNTs reduced the degree of attenuation of the physical properties of the modified asphalt. In addition, the rheological parameters obtained from the rheological tests showed that the addition of CNTs reduced the increase in the complex modulus and the decay in the phase angle of modified asphalt after aging. These results showed that CNTs have some anti-aging effects and can prevent the hardening of asphalt to a certain extent. CNTs act as a reinforcement at the interface between the polymer phase and the asphalt phase, resulting in a dense network structure and influencing the molecular ground motion in the asphalt, thus giving the modified asphalt better mechanical properties [
18].
As an important means of road maintenance, sealants are subject to complex vehicle loads and direct exposure to the natural environment during road use, which requires them to be able to adapt to actual road conditions [
6]. Therefore, it is of great practical engineering importance to explore the research methods used for the performance of sealants. Despite the existence of current specification standards for the study of the performance of sealants, some researchers believe that the specification standards do not adequately reflect the field performance of sealants and can only be used as an empirical standard for conventional performance indicators [
19]. In recent years, the characterization of asphalt materials has gradually shifted from traditional empirical indicators to rheological indicators, and the study of rheological properties based on classical viscoelasticity theory has provided an important basis for the evaluation of the properties of asphalt materials [
20]. Yang et al. [
21] and Ozer et al. [
22] carried out field validation studies on sealants’ performance and found a strong correlation between the rheological index and pavement sealants’ performance. Therefore, the study of rheological properties based on the classical viscoelastic mechanics theory provides an important basis for the performance evaluation of asphalt materials.
In summary, to address the current situation that the aging performance and fatigue performance of asphalt pavement sealants still need to be improved, this paper uses CNTs as the main additive to prepare modified asphalt sealants and compare the performance with two commercially available sealants. The high-temperature stability and aging resistance of SBS/CNT-modified asphalt sealants were investigated by a softening point test, flow value test, frequency scan test, and multiple stress creep recovery test. The main curve of the complex shear modulus was fitted by the CAM model. The creep process of the sealants was fitted by the Burgers model to better understand the viscoelastic intrinsic property of the modified asphalt sealants.
4. Conclusions
In this study, a modified asphalt sealant was prepared by adding SBS and CNTs to asphalt. Firstly, a preliminary evaluation of the high-temperature performance of sealants was carried out. Secondly, the high-temperature rheology and aging resistance of the sealants were further investigated by dynamic shear rheology tests, and the material ratios for the preparation of the sealants were determined by comprehensive evaluation, using 1 wt% CNTs, 5 wt% SBS, and 5 wt% furfural-extracted oil. The creep process of the sealants was then fitted by the CAM model to the main curve and the Burgers model to better understand the dynamic viscoelasticity of the modified sealants. The main conclusions reached are as follows:
- (1)
By comparing the softening point and flow value, C1.0S5F5 and C0.5S5F3 showed excellent high-temperature performance before and after aging compared to the ordinary commercially available sealants. Based on the range analysis, SBS and CNTs play a staple role in strengthening the heat resistance of sealants.
- (2)
The SBS/CNT-modified asphalt sealants were more resistant to rutting than the commercially available sealants, and the addition of CNTs had a more obvious effect on the fatigue resistance of the sealants and can play a toughening role in sealants.
- (3)
The CAM model fitting results show that the model correlates well with the modulus curve and can be used as an effective method for characterizing and predicting the viscoelastic behavior of sealants. The addition of CNTs can increase the glassy composite shear modulus and rheological index of asphalt, thereby increasing the shear strength and reducing the temperature sensitivity of asphalt. C1.0S5F5 had a better relaxation property, which can better avoid secondary cracking after the construction of the sealant.
- (4)
According to the MSCR test, the high-temperature deformation resistance of the C1.0S5F5 sealant before aging was better, and the elastic recovery ability of the C1.0S5F5 and C0.5S5F3 sealants was significantly stronger than that of the MS1 sealant. CNTs act as short-fiber reinforcement between asphalt and SBS.
- (5)
The correlation coefficients fitted using the Burgers model were all greater than 0.99, which is a good predictor of the viscoelastic intrinsic characterization of the sealants. With the Burgers model, C1.0S5F5 showed excellent deformation resistance under heavy traffic conditions.
Based on the above conclusions, SBS/CNTs-modified asphalt sealant can not only meet the high-temperature performance required in the standard, but also has excellent thermal stability and fatigue properties due to the modification of CNTs and SBS. Based on these conclusions, the findings of the study can also provide experimental and theoretical support for the popularization of SBS/CNT-modified asphalt sealant applications.