High-Performance Semi-Flexible Pavement Coating Material with the Microscopic Interface Optimization

: This paper aims to optimize the asphalt-mortar interface characteristics of semi-ﬂexible pavement coating material (SFPCM), three types of the interface modiﬁers, namely the silane coupling agent, carboxylated styrene-butadiene latex, and cationic emulsiﬁed asphalt, are selected to add into the cement mortar to optimize the asphalt-mortar interface. The microscopic characteristics of the asphalt-mortar interface and the macroscopic pavement performance of the SFPCM after the interface optimization are investigated to reveal the e ﬀ ect of interface modiﬁers and determine the best formula. Results show that: (1) The dry shrinkage of the cement mortar is increased with the increased dosage of the interface modiﬁers, while the strength is decreased accordingly; (2) the optimum dosages of the silane coupling agent, carboxylated styrene butadiene latex, and cationic emulsiﬁed asphalt are 0.5%, 10%, and 5%, respectively; (3) the microscopic asphalt-mortar interface characteristics of the SFPCM with di ﬀ erent interface modiﬁers have signiﬁcant di ﬀ erences; (4) the SFPCM with the cationic emulsiﬁed asphalt presents the best pavement performance, particularly the high-temperature stability. These results can be attributed to improve the pavement properties of SFPCM.


Introduction
Semi-flexible pavement coating material (SFPCM), with the advantage of the flexibility of asphalt concrete pavement and the rigidity of cement concrete pavement materials, is a new kind of composite pavement material, which is made by pouring special cement mortar into the open porous asphalt mixture (void ratio is 20%~28%) [1]. The asphalt concrete pavement forms the material strength through aggregate interlocking [2,3], but the SFPCM forms the material strength through mutual intercalation of aggregate and cement stone, which improves the ability of pavement to resist the load [4,5]. In addition, its high temperature stability and water damage resistance are superior to asphalt concrete pavement, and its shear resistance, fatigue resistance, and slip resistance are also superior to cement concrete pavement. At the same time, it has the characteristics of oil resistance, acid resistance, and easy coloring [6,7].
The semi-flexible pavement originated in France and was originally used for the pavement of the airport runway, showing good road performance [8]. Subsequently, Britain, the United States, the former Soviet Union, and other countries conducted research on SFPM, which proved that the pavement material can extend the service life of the pavement and has good high temperature

Asphalt Binder
In the existing studies, Styrene-butadiene-styrene (SBS) modified asphalt binders and high-viscosity asphalt binders are widely used to prepare the matrix asphalt mixture (open-porous asphalt mixture), because of the advantages in toughness and tenacity compared to other asphalt binders. Moreover, the workable performance of SBS modified asphalt binder is superior to high-viscosity asphalt binder. Hence, SBS modified asphalt binder is adopted to prepare the matrix asphalt mixture in this study.
The classification of the SBS modified asphalt binder is PG76-22. Other technical indexes are listed in Table 2.

Aggregates
The coarse and fine aggregates are both manufactured by basalts. Their technical indexes are listed in Table 3.

Interface Modifier
Three types of interface modifiers are selected in this study, including silane coupling agent, carboxylated styrene-butadiene latex, and cationic emulsified asphalt.
The silane coupling agent is a kind of organic silicon compound that contains two groups with different chemical properties in molecules. It can react with inorganic materials and organic materials at the same time, thereby improving material properties (research institute of highway ministry of transport. 2005). The Kh-550 silane coupling agent (aminopropyl triethoxysilane), produced by Nanjing chengong Organic Silicon Materials Co., Ltd., is adopted in this study. The technical indexes are shown in Table 4. The carboxylated styrene-butadiene latex is a copolymer formed by emulsion polymerization of butadiene and styrene with a small amount of carboxylic acid and other additives. It is a kind of milky white-water dispersion with blue-purple luster [24]. Carboxyl styrene-butadiene latex has a high proportion of styrene, high binding strength and conjunctival strength, and residual benzene monomer is very small, which is an environmentally friendly product. The carboxylated styrene-butadiene latex used in this study is produced by Shanghai Gaoqiao BASF, and its technical indicators are shown in Table 5. Cationic emulsified asphalt is processed by a variety of surfactants and cationic shale inhibitors and a certain range of softening point of asphalt. The cationic emulsified asphalt will react with the cement hydration product after demulsification to form a composite structure product. Its technical indicators are shown in Table 6.

Dosage of the Interface Optimizations
The effects of various interface modifiers on the physical and mechanical properties of cement grout materials were studied based on the research results of cement grout materials in this paper, and the optimal dosage of different interface optimizers was determined by the fluidity, dry shrinkage, and compressive and flexural strength tests of cement. Table 7 shows the test results of different interface optimizer dosages. It can be seen from Table 7 that the properties of the cement grout are lowered when the silane coupling agent is added. Then, with the gradual increase of the amount of the silane coupling agent, the fluidity and the strength of the cement grouting first increases and then decreases, and the dry shrinkage deformation gradually increases. When the amount of the silane coupling agent is in the range of 0.25% to 0.5%, the fluidity is reduced by 5%, the flexural strength is increased by 21%, and the compressive strength is increased by 8%. The dry shrinkage of the grout increases regularly with the increase of the amount of silane coupling agent. When the amount of the silane coupling agent is 0.5%, the performance of the cement grout is optimal.
The flexural strength of the cement grout increases with the addition of carboxylated styrene-butadiene latex, and the shrinkage deformation decreases. With the increase of the content of carboxylated styrene-butadiene latex, the fluidity of cement grouting first increased and then decreased, the flexural strength first increased and then decreased, and the compressive strength and dry shrinkage deformation decreased regularly. When the content of carboxylated styrene-butadiene latex is in the range of 5%-10%, the grout has the lowest fluidity, the flexural strength is increased by 7%, the compressive strength is reduced by 10%, and the shrinkage rate is reduced by 10%. This indicates that the incorporation of carboxylated styrene-butadiene latex can improve the dry shrinkage deformation of cement grout and improve the flexural strength. When the content of carboxylated styrene-butadiene latex is 10%, the performance of the cement grout is optimal.
When the cationic emulsified asphalt is added, the strength of the cement grout is lowered, and the shrinkage deformation is reduced. With the increase of the amount of cationic emulsified asphalt, the fluidity of cement grouting first decreases and then increases, the flexural strength increases first and then decreases, the compressive strength decreases regularly, and the shrinkage rate increases regularly. When the amount of cationic emulsified asphalt is 5%, the fluidity is reduced by 10%, the flexural strength is increased by 13%, the compressive strength is slightly reduced, and the shrinkage ratio is reduced by 5%. This indicates that the incorporation of cationic emulsified asphalt can improve the dry shrinkage deformation of cement grout and improve the flexural strength. When the amount of cationic emulsified asphalt is 5%, the performance of the cement grout is optimal.
The interface optimizer has a great influence on the physical and mechanical properties of the cement grout. Both cationic emulsified asphalt and carboxylated styrene-butadiene latex can reduce the compressive strength and dry shrinkage of the grout, while the addition of silane coupling agent has no obvious effect on the improvement of strength and shrinkage. The optimum dosage of the silane coupling agent was determined by analysis to be 0.5%, the optimum content of the carboxylated styrene-butadiene latex was 10%, and the optimum dosage of the cationic emulsified asphalt was 5%.

Matrix Asphalt Mixtures
The aggregate gradation of the matrix asphalt mixtures is shown in Table 8 and Figure 1. The void ratio of matrix asphalt mixtures is 25%.  The optimum asphalt content is determined by the Schellenberg Leakage test and the Kent-Berg Fly test. The void ratio and mechanical properties of the matrix asphalt mixtures is also verified via Marshall tests. The test results are shown in Table 9, indicating that the matrix asphalt mixture meets the design requirements and has a certain overall strength.

Preparation of Semi-Flexible Materials
The preparation process of the semi-flexible material samples is as follows: Step 1: Prepare the testing slabs (L300 × W300 × H50 mm) of matrix asphalt mixture via rolling the compaction method at 160 °C according to T0703-2011 [21]. These testing slabs must be cooled to lower than 50 °C .
Step 2: Prepare the cement grouting material through mixing the cement mortar (Section 2.1) and interface optimizers for 5 min at room temperature.
Step 3: Pour the prepared cement grouting material into the prepared testing slab of the matrix asphalt mixture with the help of mini plate vibrator until the cement grouting material overflows. Scrape away redundant cement grouting material using the mini rubber scraper. Finally, the test sample of the semi-flexible material can be obtained after curing, as shown in Figure 2. The optimum asphalt content is determined by the Schellenberg Leakage test and the Kent-Berg Fly test. The void ratio and mechanical properties of the matrix asphalt mixtures is also verified via Marshall tests. The test results are shown in Table 9, indicating that the matrix asphalt mixture meets the design requirements and has a certain overall strength.

Preparation of Semi-Flexible Materials
The preparation process of the semi-flexible material samples is as follows: Step 1: Prepare the testing slabs (L300 × W300 × H50 mm) of matrix asphalt mixture via rolling the compaction method at 160 • C according to T0703-2011 [21]. These testing slabs must be cooled to lower than 50 • C.
Step 2: Prepare the cement grouting material through mixing the cement mortar (Section 2.1) and interface optimizers for 5 min at room temperature.
Step 3: Pour the prepared cement grouting material into the prepared testing slab of the matrix asphalt mixture with the help of mini plate vibrator until the cement grouting material overflows. Scrape away redundant cement grouting material using the mini rubber scraper. Finally, the test sample of the semi-flexible material can be obtained after curing, as shown in Figure 2.
Step 2: Prepare the cement grouting material through mixing the cement mortar (Section 2.1) and interface optimizers for 5 min at room temperature.
Step 3: Pour the prepared cement grouting material into the prepared testing slab of the matrix asphalt mixture with the help of mini plate vibrator until the cement grouting material overflows. Scrape away redundant cement grouting material using the mini rubber scraper. Finally, the test sample of the semi-flexible material can be obtained after curing, as shown in Figure 2.

Micro-Characteristics of Semi-Flexible Pavement Material with Interface Optimizer
SEM electron microscopy was used to observe the texture structure of cement-asphalt interface in high performance semi-flexible materials from a microscopic point of view in this paper, and the influence of different interface optimizers on interface microscopicity was analyzed. Figure 3 is an electron microscopic scanning view of cement-asphalt interface modified by a silane coupling agent. The black part is asphalt and the light gray part is hardened cement grout in the figure. It can be seen in the figure that there is a coherent flat crack at the junction of cement and asphalt. When the crack is enlarged, it can be seen that the cement and the asphalt are connected together by a part of the convex products, and the crack between the two is deep. The surface of the cement is uneven and angular, and the surface of the asphalt is uneven at the interface. This indicates that the connection between the silane coupling agent-modified cement and the asphalt is unstable, and there is a long and continuous gap.

Micro-Characteristics of Semi-Flexible Pavement Material with Interface Optimizer
SEM electron microscopy was used to observe the texture structure of cement-asphalt interface in high performance semi-flexible materials from a microscopic point of view in this paper, and the influence of different interface optimizers on interface microscopicity was analyzed. Figure 3 is an electron microscopic scanning view of cement-asphalt interface modified by a silane coupling agent. The black part is asphalt and the light gray part is hardened cement grout in the figure. It can be seen in the figure that there is a coherent flat crack at the junction of cement and asphalt. When the crack is enlarged, it can be seen that the cement and the asphalt are connected together by a part of the convex products, and the crack between the two is deep. The surface of the cement is uneven and angular, and the surface of the asphalt is uneven at the interface. This indicates that the connection between the silane coupling agent-modified cement and the asphalt is unstable, and there is a long and continuous gap.  Figure 4 is an electron microscopic scanning view of cement-asphalt interface modified with carboxylated styrene-butadiene latex. It can be seen from the figure that the connection between cement and asphalt is relatively tight, and cracks appear in some locations. The surface of the cement Asphalt Cement grout   Figure 4 is an electron microscopic scanning view of cement-asphalt interface modified with carboxylated styrene-butadiene latex. It can be seen from the figure that the connection between cement and asphalt is relatively tight, and cracks appear in some locations. The surface of the cement is uneven, and some of the cement particles are covered with asphalt, which does not pierce the asphalt and forms a good connection with the asphalt. This indicates that the interface between the carboxylated styrene-butadiene latex modified cement and the asphalt is better, and the adhesion of the asphalt to the cement particles enhances the connection between the two.  Figure 5 is an electron microscopic scanning diagram of emulsified asphalt modified cement-asphalt interface. It can be seen from the figure that the cement and the asphalt are closely connected, and the asphalt at the interface forms a whole with the cement, and no obvious cracks are generated. The cement surface is relatively flat, and the cement at the edge of the interface is wrapped with asphalt to form a good connection with the asphalt. This indicates that the cationic emulsified asphalt makes the cement and the asphalt at the interface merge with each other, the connection at the interface is integrated, and the stability at the interface is good.  Figure 5 is an electron microscopic scanning diagram of emulsified asphalt modified cement-asphalt interface. It can be seen from the figure that the cement and the asphalt are closely connected, and the asphalt at the interface forms a whole with the cement, and no obvious cracks are generated. The cement surface is relatively flat, and the cement at the edge of the interface is wrapped with asphalt to form a good connection with the asphalt. This indicates that the cationic emulsified asphalt makes the cement and the asphalt at the interface merge with each other, the connection at the interface is integrated, and the stability at the interface is good. Figure 5 is an electron microscopic scanning diagram of emulsified asphalt modified cement-asphalt interface. It can be seen from the figure that the cement and the asphalt are closely connected, and the asphalt at the interface forms a whole with the cement, and no obvious cracks are generated. The cement surface is relatively flat, and the cement at the edge of the interface is wrapped with asphalt to form a good connection with the asphalt. This indicates that the cationic emulsified asphalt makes the cement and the asphalt at the interface merge with each other, the connection at the interface is integrated, and the stability at the interface is good. It can be seen from the above analysis that the interface optimizer makes a large difference in the microstructure of the cement-asphalt interface. The interface of the cationic emulsified asphalt is more tightly connected, no obvious cracks are generated, and the cement surface is relatively flat; the interface of the carboxylated styrene-butadiene latex is tightly connected, and some micro-cracks are present, and the surface of the cement is uneven; the interface at the interface of the silane coupling agent is poorly connected, and there is a large continuous gap, and the surface of the cement is uneven and angular.

Pavement Properties of the Semi-Flexible Material with Micro-Interface Optimization
The macroscopic effect of interface optimizer on the road performance of semi-flexible materials is analyzed through laboratory tests in this paper. Since the semi-flexible material is based on the asphalt mixture and is poured into the cement grout, although it is a rigid-flexible material, it is generally biased toward the flexibility of the base asphalt mixture. Therefore, this study used the road performance evaluation method of asphalt mixture to study the effect of interface optimizer on semi-flexible materials, and the tests measured according to Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering (JTG E20-2011) [21]. Table 10 shows the results of high temperature rutting tests for different semi-flexible materials. It can be seen from Table 10 that the dynamic stability of the high performance semi-flexible pavement material at 60 • C is more than 10,000 times/mm, which is much larger than the conventional modified asphalt mixture, and the interface optimizer type has a greater influence on the high temperature performance of the semi-flexible material. The semi-flexible material incorporating cationic emulsified asphalt has the best high-temperature anti-rutting performance, and its dynamic stability is much larger than the other two interface optimizers, and the rutting depth is the smallest. The results show that the semi-flexible material has relatively high temperature stability. The incorporation of the interface optimizer can significantly improve the high temperature stability of the semi-flexible material [26], wherein the cationic emulsified asphalt has the most obvious lifting effect. Table 11 shows the results of low temperature bending tests for different high performance semi-flexible materials. It can be seen from Table 11 that the stress and strain of the ordinary semi-flexible material at low temperature failure are low, however, the low-temperature crack resistance of the semi-flexible material is improved after the interface optimizer is incorporated. The semi-flexible material incorporating cationic emulsified asphalt has a low temperature failure strain increase of 43% and a stiffness modulus of 19%, and the material has good low temperature crack resistance. The results show that the low temperature crack resistance of ordinary semi-flexible materials is poor. The incorporation of interface optimizer can significantly improve the low temperature crack resistance of semi-flexible materials, and the effect of cationic emulsified asphalt is most obvious. Table 12 shows the results of freeze-thaw splitting tests for different high performance semi-flexible materials. It can be seen from Table 12 that the high-performance semi-flexible material has a relatively large freeze-thaw splitting strength and good water stability. The splitting strength of the semi-flexible material increases with the addition of the interface optimizer. The high-performance semi-flexible material incorporating cationic emulsified asphalt has the highest freeze-thaw splitting strength and a TSR increase of 18%. This indicates that the cation can significantly improve the water stability of the semi-flexible material, followed by the carboxylated styrene-butadiene latex. The results show that the semi-flexible material has good water stability, and its freeze-thaw split ratio is more than 70%. The incorporation of an interface optimizer enhances the water stability of the semi-flexible material, wherein the high performance semi-flexible material incorporating the cationic emulsified asphalt is optimal in stability.

Water Stability
It can be known from the above analysis that the interface optimizer has a great influence on the performance of the semi-flexible material. The properties of high temperature rutting resistance, low temperature crack resistance, and water stability were improved after adding the interface optimizer, which indicated that the interface optimizer could improve the road performance of semi-flexible materials. Among the three kinds of interface optimizers, the emulsified asphalt has the greatest influence on the performance of the semi-flexible material, which indicates that the road performance of the high-performance semi-flexible material doped with the cationic emulsified asphalt is optimal.

Conclusions
The present study determined the optimum dosage of interface optimizers and investigated the effects of interface optimization on the micro-structure and macro-properties of SFPCM. The main conclusions are as follows: (1) The physical and mechanical properties of the cement mortar are changed significantly with the addition of interface optimizers. The interface optimizers improve the dry shrinkage resistance but weaken the strength of cement mortar.
(3) The cement-asphalt interface of semi-flexible materials will be modified obviously by interface optimizers, particularly the cationic emulsified asphalt. The cationic emulsified asphalt greatly improves the interfacial connection between cement and asphalt. The cement-asphalt interface is relatively flat and has no obvious cracks. Moreover, the modification effect of silane coupling agent is the worst.
(4) The proposed semi-flexible materials with interface optimization provide satisfactory high-temperature stability, low-temperature crack resistance, and water stability, particularly high-temperature stability, showing the positive effectiveness of interface optimizers. Particularly, the semi-flexible material modified by the cationic emulsified asphalt presents the best pavement performance.
The effects of interface optimization on fatigue resistance and slip resistance of SFPCM will be conducted in further studies.