1. Introduction
As one of the important components of infrastructure construction, highway construction has characteristics of long mileage, large volume, large resource occupation, and high energy consumption. At present, highway pavement surface materials mainly adopt hot-mix and hot-spread asphalt mixtures, and large amounts of these asphalt mixtures need to be produced in the construction process. The production of hot asphalt mixtures consumes much energy and resources, and it causes pollution to the surrounding environment [
1]. Furthermore, due to the rDear Dr. Shirzad, equirement for environmental protection, the number of mixing stations in the construction of hot-mix and hot-pave asphalt mixtures in China is limited at present, which affects long distance transportation of these mixtures. Thus, cold-mix or warm-mix cold-paving asphalt mixtures have attracted increasing attention [
2,
3,
4]. In view of these problems, people have paid increasing attention to the development and application of cold-paving asphalt mixtures with low energy consumption and storability in highway construction and maintenance.
Nowadays, cold-paving asphalt mixtures are mainly used as cold-patching materials in pavement maintenance engineering, and they are called cold-patching asphalt mixtures (CAMs). Scholars have conducted extensive work to investigate the performance of existing CAMs and develop new materials for them [
5,
6,
7]. For example, Chen et al. [
8] analyzed the influence of water-borne epoxy resin, a curing agent, and emulsified asphalt on water-borne epoxy resin-modified emulsified asphalt; then, they determined the reasonable formula of epoxy emulsified asphalt and put forward the improved Marshall experimental design method. Joni [
9] studied the physical properties of two kinds of emulsified asphalt adhesives (cationic and anionic) used as paving mixture adhesives to evaluate the properties of the obtained emulsified asphalt mixture; the results of the Marshall stability test showed that the maximum Marshall stability of the cationic emulsified asphalt mixture is greater than that of the anionic emulsified asphalt mixture, and the optimum emulsification amount is 6.36%. However, due to the demulsification of emulsified asphalt, guaranteeing the long-term storage stability of cold-mix and cold-spread emulsified asphalt mixtures is difficult.
To fully improve the road performance and storage stability of CAMs, research has focused on solvent-based mixtures, with modifiers and diluents added to the base asphalt, to obtain CAMs with excellent performance. For instance, Tan et al. [
10] established the raw material composition formula of CAMs through a series of studies and found that AC-13 gradation CAM has excellent freezing resistance with 70% SBS-modified asphalt, 30% kerosene, and 2% additive in the asphalt solution weight. Rezaei et al. [
11] investigated the permanent deformation and moisture resistance of nine kinds of CAMs by using the Marshall stability test, indirect tensile strength test, and Hamburg rutting test. The results showed that the rutting resistance of CAMs with dense gradation is much higher than that of CAMs with open gradation. Moreover, the dust-to-binder ratio is highly related to the moisture sensitivity of dense-graded CAMs, and the percentage of coarse aggregates plays an important role in open-graded CAMs. Yuan et al. [
12] studied the increase in the performance of CAMs with the extension of soaking time and the increase in cement content; they also put forward a new method to improve the moisture stability of CAMs with cement modification. Zhang et al. [
13] presented a performance evaluation method for solvent-based cold-patching asphalt solutions and developed a new solvent-based cold-patching asphalt liquid (CAL) via the orthogonal design method; the CAL consisted of base asphalt, a diluent, a tackifier, a surfactant, and an anti-stripping agent. Huang et al. [
14] determined the initial amount of CAM through infrared spectroscopy, four-component analysis, lying down method, and column core technology principle then optimized the formula by the orthogonal test design. The results showed that the road performance of self-made CAM is well verified. Xu et al. [
15] evaluated the water-induced damage potential of CAM based on the surface-free energy theory and discovered that CAM has excellent water damage resistance when CAL contains alkali synthetic asphalt, rosin resin, diesel oil, and an anti-stripping agent and when the aggregate is composed of limestone and basalt. In addition, Pei et al. [
16] analyzed the performance changes in CAL and its mixture at each stage and provided some suggestions for the preparation of CAM in cold weather. Gel Permeation Chromatography and Scanning Electron Microscope analysis could explain why the viscosity and strength of CAL increase after curing. The results showed that increasing the filler content can improve the deformation resistance of CAM.
Styrene–ethylene–butadiene–styrene (SEBS) copolymer, as a modifier obtained via styrene–butadiene–styrene block copolymer hydrogenation, is used in many fields because of its excellent physical and chemical characteristics. For instance, Gao et al. [
17] prepared three kinds of nanocomposites and polypropylene/SEBS materials by adding media of different orders. An electrical branch aging test was then conducted with polypropylene, polypropylene/SEBS, and the nanocomposites. The results indicated that SEBS can promote the initiation and growth of the electric branch in polypropylene, and SiO
2 inhibits the growth of polypropylene. Xue et al. [
18] prepared a SEBS/h-SiO
2 superhydrophobic composite coating on an aluminum alloy surface through the Czochralski method. The SEBS/h-SiO
2 composite coating has a binary micro/nano rough structure and low surface energy, and it can form an air cushion when liquid droplets come into contact, thus showing excellent hydrophobicity.
Similarly, SEBS has also been used to improve the road performance of hot-mix asphalt mixtures. Ma et al. [
19] determined the best content of SEBS through a road performance and fatigue resistance test, which showed that the late modulus and fatigue life of SEBS-modified asphalt mixtures are much better than those of base asphalt mixtures and gradually improve with the increase in content, indicating an excellent self-healing performance. Ke et al. [
20] analyzed the physical properties, high-temperature storage stability, rheological properties, aging resistance, and micro-morphology of SEBS/organic montmorillonite-modified asphalt. The results showed that SEBS can considerably improve high- and low-temperature performance, but due to the poor compatibility between SEBS and asphalt, the modified asphalt exhibits serious phase separation during thermal storage.
However, investigations of the improvement in the water stability and storability of CAM after the addition of SEBS remain lacking. Therefore, this study analyzes CAL and its mixture modified with and without SEBS through a series of laboratory tests to obtain the mix design and performance of modified and unmodified CAM and evaluate the modification effect of SEBS on the water damage resistance and storage stability of CAM.
4. Test Results and Discussion
4.1. Determination of Diluent Content
The average values of rotational viscosity with the different modifiers and diluents (calculated according to the weight percentage of the base asphalt) were obtained through the Brookfield rotational viscosity test. The dosage of the SEBS modifier was set to 0%, 1.5%, 3.5%, 5.5%, 7.5%, and 9.5% of the base asphalt weight. A viscosity test on CAL with different diesel diluents was performed for each modifier dosage. The specific results are shown in
Figure 7.
The results in
Figure 7 show that the Brookfield rotational viscosity gradually decreased with the increase in diesel oil content under the same SEBS content, but the decrease gradually slowed down, and the viscosity increased with the increase in SEBS content. To control the usage content of the diesel diluent, this study compared the viscosity of CAL at each modifier content with 40% diesel oil content. The results showed that the viscosity of CAL with 9.5% SEBS content was not between 2 and 3 Pa·s, whereas the other CAL with less than 9.5% SEBS content had a suitable diesel oil content range that can meet the viscosity requirement. The contents of diesel oil under 0%, 1.5%, 3.5%, 5.5%, and 7.5% SEBS contents were about 20%, 23%, 28%, 35%, and 40%, respectively. After determining the corresponding optimum diesel oil content with different SEBS contents in CAL, it was applied to determine the appropriate modifier dosage and the optimum asphalt content.
4.2. Determination of Modifier Content and Optimum Asphalt Content
4.2.1. Preliminary Determination of Asphalt Content
- (1)
Results of the empirical formula method
The asphalt content (i.e., the ratio of modified asphalt and mineral material) in CAM was obtained according to the empirical formula method [
27], as follows:
where
P represents the asphalt content of CAM in %,
a is the weight percentage of particles greater than 2.36 mm in %,
b is the weight percentage of particles between 0.3 and 2.36 mm in %,
c is the weight percentage of particles between 0.075 and 0.3 mm in %, and D is the weight percentage of particles less than 0.075 mm in %.
Given the gradation of CAM in this test, in accordance with Formula (5), the asphalt content of LB-13 gradation CAM was calculated to be 4.6%.
- (2)
Paper trace test results
After the asphalt content in CAM under a given gradation was preliminarily obtained using the empirical formula method, the paper trace test was further performed in a certain range of 4.6% asphalt content to compare with the ink traces on white paper and to verify the rationality of the used asphalt content. The typical results of the paper trace test with the CAM with 7.5% SEBS as an example are shown in
Figure 8.
Figure 8a–c correspond to the presence of ink traces on white paper for the CAM with 5.2%, 4.0%, and 4.6% asphalt content, respectively.
As shown in
Figure 8, compared with the moderate ink traces on white paper for CAM with 4.6% asphalt content, the oil content for CAM with 5.2% asphalt content was too much, indicating that the amount of asphalt in CAM was too much. The oil content for CAM with 4.0% asphalt content was too little, showing that the amount of asphalt in CAM was too little. The paper trace test results for the other SEBS contents were similar to those with 7.5% SEBS content, so the range of asphalt content in CAM with different SEBS contents was determined to be 4.0–5.2%.
4.2.2. Determination of Modifier Content
On the basis of the asphalt content range determined by the above-mentioned CAM with different SEBS contents, a Marshall stability test under each SEBS content was conducted within the determined asphalt content range to obtain the average Marshall stability. Then, the optimal content of the SEBS additive was determined, and the results are shown in
Figure 9, where the bar and line graphs correspond to the results for Marshall stability and average Marshall stability of five asphalt contents at each SEBS modifier content, respectively. The average Marshall stability increased with the increase in SEBS content. Compared with the result without the modifier, the results of 1.5%, 3.5%, 5.5%, and 7.5% SEBS contents increased by 0.12, 0.23, 0.7, and 1.06 kN, respectively. The stability under 7.5% SEBS content was the highest, so the modifier content in CAM was selected as 7.5%.
4.2.3. Determination of the Optimum Asphalt Content
After determining the reasonable content of the modifier in a certain asphalt content range, the modified Marshall method was further used to determine the optimum asphalt content [
28]. The index results of CAM with 7.5% and without SEBS were compared, as shown in
Figure 10.
According to the results in
Figure 10, the asphalt content corresponding to the maximum bulk density, the maximum stability, and the target void ratio (or median) was obtained as a
1, a
2, and a
3, respectively. Then, the optimum asphalt content (OAC) of CAM was calculated as OAC = (a
1 + a
2 + a
3)/3, where the OAC of CAM with 7.5% SEBS and without a modifier was 4.6% and 4.8%, respectively.
4.3. Results and Discussion of Water Stability
On the basis of the mix design of the CAM obtained above, this study further evaluated the water stability of CAM with 7.5% SEBS and without a modifier to verify the water damage resistance modification effect of CAM with SEBS.
4.3.1. Immersion Marshall Test Results
Figure 11 indicates that the stability of the SEBS-modified CAM specimen before immersion was 4.65 kN, and that of the specimen without a modifier was 3.42 kN; the tested result of the former was 1.23 kN larger than that of the latter. After the specimens were immersed in a water bath for about 48 h at 25 °C, the stability of the modified specimen became 5 kN, indicating an increase of 0.35 kN compared with the value for the un-immersed specimen. The stability of the unmodified specimen after immersion was 3.06 kN, which was 0.36 kN lower than that of the un-immersed specimen.
This phenomenon may be due to the fact that although the immersed specimen reduced the CAL viscosity and led to a reduction in specimen strength, the diesel oil content in the modified CAM was twice as much as that in the unmodified CAM (i.e., the former’s content was 40%, and the latter’s was 20%), and the diesel oil loss of the modified specimen was larger than that of the unmodified specimen in the immersion process. For the modified specimen, the increase in CAL viscosity caused by diesel oil loss had a greater effect than the decrease in CAL viscosity caused by water damage after immersion, so the specimen’s stability was greater than that before immersion.
In addition, the residual stability of the modified specimen at 25 °C was 107.5%, and that of the unmodified specimen was 89.5%. Compared with the latter, the former increased by nearly 20.1%, indicating that the water stability of the specimen was improved after adding SEBS to CAM, and the modification effect was remarkable.
4.3.2. Freeze–Thaw Splitting Test Results
Figure 12 shows that the unfrozen splitting strength of the SEBS-modified CAM specimen was 0.16 MPa, and that of the unmodified CAM specimen was 0.09 MPa. The former was 1.78 times the latter. After the freeze–thaw cycles, the splitting strengths of the two specimens decreased. The former’s splitting strength was 0.14 MPa, and that of the latter was 0.068 MPa; the former was still 2.06 times the latter. This result indicates that whether before or after the freeze–thaw action, the strength of the CAM with SEBS was considerably improved compared with that of the CAM without a modifier. Further illustrating the modified effect of water stability, the freeze–thaw splitting strength ratio of the modified specimen was 87.5%, and that of the unmodified specimen was 75.6%. Thus, the splitting strength ratio of the SEBS-modified specimen was higher than that of the unmodified specimen, namely, the former increased by about 15.7% compared with the latter. This result further indicates that the SEBS-modified CAM had a remarkable effect.
4.4. Storage Performance
In addition to the evaluation of the water damage resistance of CAM, this study also compared the construction workability of CAM with and without the 7.5% SEBS content modifier after storage for 0 days, 7 days, 15 days, 1 month, and 2 months. For the state of CAM after storage for a certain period, typical results of the modified CAM are shown in
Figure 13.
As shown in
Figure 13, the SEBS-modified CAM had clear particles and no caking after storage for 7 days, that is, its workability grade was 1. After storage for 15 days, 1 month, and 2 months, CAM exhibited a small amount of caking, but it could be scattered, that is, its workability grade was 2. Meanwhile, the storage and workability grades of the unmodified CAM were similar to those of the SEBS-modified CAM. Therefore, we conclude that CAM does not exhibit agglomeration during short-term storage, but a small amount of agglomeration will occur with the increase in storage time. However, the agglomeration can be stirred and scattered by a shovel, so it has little impact on subsequent usage. In addition, the Marshall stability of the modified and unmodified CAM after storage was tested, and the results are shown in
Figure 14.
After storage for 0 days, 7 days, 15 days, 1 month, and 2 months, the stability of the SEBS-modified CAM was 3.8, 3.82, 3.91, 3.96, and 4.05 kN, respectively, and the respective results of the unmodified CAM were 2.32, 2.34, 2.38, 2.44, and 2.54 kN. Therefore, whether for the modified or unmodified CAM, the stability increased with the increase in storage time, but the increase was small. For example, during two months of storage, the stability of the modified and unmodified CAM increased by only 6.6% and 9.5% compared with the results for 0 days of storage, respectively. This result may be due to the gradual volatilization of the diluent during storage, which increased the CAL viscosity. However, the volatilization amount was small, so CAL had a limited increase in viscosity. According to the comparison of the stability of the two types of CAM, the result of the modified CAM was much higher than that of the unmodified CAM. For instance, when stored for two months, the stability of the former was 1.59 times that of the latter. To sum up, the CAM with the SEBS modifier showed good storage performance, and it could meet construction workability and stability requirements after two months of storage.