Technical development of modified emulsion asphalt: A review on the preparation, performance, and applications

2.1 Resin modifiers

Resin is a polymer extracted from plants and is used to produce various chemical products. They can be divided into natural and artificial resins, according to their source and production processes. Resin modifiers are products obtained from simple organic compounds through chemical synthesis or from natural products through chemical reactions. Among the commonly used road resin modifiers are polyethylene, ethylene vinyl acetate (EVA) copolymer, epoxy resin, and polyurethane (PU). It is typically necessary to employ both the main resin agent and a curing agent when modifying emulsion asphalt with resin. The modifier typically takes the shape of an opaque or translucent emulsion. Using experimental techniques such as Fourier-transform infrared (FTIR) spectroscopy, fluorescence microscopy, and scanning electron microscopy (SEM), along with digital image processing technology in MATLAB, Kong et al. [1,2,3] investigated the modification mechanism of waterborne epoxy resin (WER)-emulsion asphalt. They also analyzed the microstructural characteristics and changes in the internal functional groups of the epoxy–asphalt system. The results showed that as the WER content increased, a solidification reaction and agglomeration occurred between the WER particles, resulting in the formation of a curing product with a three-dimensional cross-linked network structure, which became the continuous phase in the asphalt emulsion. The initial asphalt continuous-phase structure changed to an epoxy asphalt dual-phase structure, which used waterborne epoxy as the framework, as a result of the emulsion asphalt particles interlocking by physical adsorption. This phenomenon improved the cohesiveness and adherence of the asphalt system by significantly reducing the range of activities of each component. However, the introduction of WER resulted in an increased number of polar functional groups within the system. The molecular orientation force indicated that the asphalt molecules were readily adsorbed onto the aggregate surfaces, strengthening the bonding performance of the mixture by increasing the adhesion state between the binder materials. Nevertheless, as the proportion of WER continued to increase, there was little improvement in the network structure of the system, and the improvement in several attributes decreased drastically.

Sheng et al. [4] used PU as a modifier to study the modification mechanism and various properties of PU-emulsion asphalt. Additionally, according to this study, PU underwent a curing reaction to form covalent connections throughout the modification process. This created a dense, three-dimensional network skeleton structure inside the asphalt, which reduced the temperature sensitivity of the emulsion asphalt and improved its overall stability. The interaction between the isocyanate and polyol groups during the curing reaction within the PU is the primary focus of the chemical processes occurring within the system. This suggests that the PU-modified emulsion asphalt is not chemically modified. Some researchers [5,6,7] found that during the process of first emulsification followed by modification, a few isocyanate groups in PU chemically react with active hydrides in the matrix asphalt. The assertion that a chemical reaction can be confirmed solely by minor peak variations in FTIR spectra is dubious and requires further investigation. Few other researchers [8,9] investigated the alteration of emulsion asphalt using vinyl acetate-ethylene (VAE). They argued that the inclusion of VAE transformed the asphalt from a sol structure to a solution gel structure, creating a stable multiphase network in the emulsion asphalt. This process is expected to improve the high-temperature performance and cohesiveness of emulsion asphalt. The modification mechanism mostly results from the chemical reaction of VAE and the physical miscibility of VAE and emulsion asphalt.

The mode of action of the resin modifier is illustrated in Figure 4. Chemical interactions occur between the modifier or main modifier and the curing agent in the emulsion asphalt system, producing distinct curing products. The products intersect to create a uniformly dispersed three-dimensional network structure with low-molecular-weight emulsion asphalt molecules embedded in the grid through physical adsorption. As the emulsion asphalt gradually demulsifies, resin molecules and asphaltene components bind together, leading to an increase in micelles in the asphalt system and a transformation of the asphalt structure from sol type to solution gel type. The structure is linked to the modifier’s product, creating a more intricate and secure network structure that enhances the high-temperature stability and mechanical qualities of the emulsion asphalt.

Figure 4

Modification mechanism of resin modifiers.

2.2 Rubber modifiers

Rubber is a thermosetting, highly elastic polymer material that exhibits reversible deformation at ambient temperature. Currently, the widely used rubber modifiers are styrene-butadiene rubber (SBR), chloroprene rubber (CR), and rubber powder. Xie et al. [10] employed molecular modeling techniques to investigate SBR-modified asphalt. The investigation revealed that the molecular polarities of the SBR and asphaltene components in the asphalt were similar, facilitating the adsorption of SBR molecules onto the asphaltene components to create an interfacial adsorption layer. Simultaneously, the SBR molecules underwent swelling reactions with lightweight components in the asphalt, creating a cross-linked network structure with rubber molecules. This process increased the content of high-molecular-weight components in the asphalt phase, reduced molecular thermal motion, and enhanced the viscoelasticity and toughness of the asphalt system. Consequently, it improved the bonding and low-temperature crack resistance of the binder. Takamura and Heckmann [11] investigated the alteration of SBR-emulsion asphalt using microscopic experiments. When SBR was uniformly distributed in the asphalt emulsion, this process was accompanied by water evaporation, asphalt demulsification, and other phenomena that increased the electrolyte concentration in the system. The SBR molecules spontaneously surrounded the asphalt particles, creating a cohesive polymer film that eventually solidified into a honeycomb-like structure. This aligns with the molecular modeling findings of Xie et al. [10].

Kong et al. [12] researched the compatibility mechanism between rubber powder and asphalt. They found that waste rubber powder underwent a swelling reaction with asphalt, forming a network structure that restricted the flow of asphalt molecules. Following the mechanical-chemical co-modification, the swelling response between the waste rubber powder and asphalt was adequate and exhibited higher compatibility. Mechanical action increases the surface area of rubber powder, whereas chemical modification adds polar alkaline groups to rubber powder, enhancing its ability to bond with the acidic components in asphalt and improving compatibility.

The mechanism of modification of emulsion asphalt by rubber-based modifiers is shown in Figure 5. Water evaporates from the emulsion asphalt, leading to the spontaneous aggregation of colloidal particles around the asphalt particles owing to the dipole effect. Moreover, the lightweight components in asphalt are easily absorbed by rubber molecules owing to their rapid diffusion velocity, which results in swelling reactions and the formation of honeycomb structures. Simultaneously, the content of high-molecular-weight components, such as asphaltene, increases and adsorbs onto the surface of rubber molecules, forming a cohesive polymer film. A cross-linked network structure is formed during the demulsification process because of the combined effects of the swelling reaction and adsorption. When external forces act on the cross-linked network structure, they restrict the flow between asphalt molecules, leading to the extension of some polymer molecular chains [13]. This process helps suppress the overall cracking and damage, enhances the viscosity and elasticity of asphalt, and demonstrates strong crack resistance and bonding performance.

Figure 5

Modification mechanism of rubber modifiers.

On comparison of the modification mechanisms of resin-based and rubber-based modifications, it can be observed that, especially for waterborne epoxy values (WER), the cross-linking reaction of WER with asphalt involves a unique formation of a three-dimensional network that significantly enhances both the mechanical properties and thermal stability of the asphalt. This differs from conventional polymer blending methods, where performance improvements are often limited to viscosity and adhesion. In contrast, the physical swelling of rubber modifiers introduces a flexible matrix that can absorb thermal stresses, thereby allowing for improved crack resistance at low temperatures. These mechanisms, when viewed collectively, highlight the potential for innovative composite systems that leverage both the rigidity of resin-based networks and the elasticity provided by rubber modifiers.

2.3 Thermoplastic elastomeric modifiers

A thermoplastic elastomer is created by subjecting rubber-based elastomers to thermoplastic processing or by melting and mixing them with resin. It exhibits rubber-like flexibility at ambient temperatures and can be made pliable at elevated temperatures. It is a polymer material with properties between those of resin and rubber; consequently, it is also referred to as thermoplastic rubber. Currently, the thermoplastic elastomers used in road construction primarily consist of styrene-based materials. The most frequently employed asphalt modifier is the styrene-butadiene-styrene (SBS) block copolymer, which can be used in conjunction with stabilizers. Butadiene in SBS provides flexibility and low-temperature resistance, whereas styrene enhances the resistance of asphalt to high-temperature deformations [14]. Meng et al. [15] suggested that the primary modifying mechanism of SBS for asphalt is redistribution of the asphalt components. On the one hand, adding SBS increases the polar molecules in the system, enhances the intermolecular dipole effect, and boosts the overall cohesion. However, SBS absorbs lightweight components in asphalt, expanding and increasing the relative proportion of components such as resin and asphaltene, thereby improving the high-temperature performance of the emulsion asphalt. The styrene and butadiene segments of SBS create a three-dimensional network structure. This structure limits the movement of different asphalt components, enhances overall cohesion, boosts asphalt toughness, improves the colloid structure of asphalt, and provides asphalt with excellent viscoelasticity, fatigue resistance, and low-temperature crack resistance.

Dong et al. [16] investigated the chemical reaction mechanism of an SBS block copolymer-modified asphalt. The presence of styrene and butadiene in SBS is believed to enhance the performance of the modified asphalt. Studies have shown that with the help of stabilizers, a chemical cross-linking reaction occurs in SBS-modified asphalt. This reaction occurs at the carbon-carbon double bond (–C═C–) of the butadiene segment, transitioning the system from a dispersed polymer phase and continuous asphalt phase to a bicontinuous phase. This formation resulted in a two-phase continuous network structure, significantly boosting the toughness of the system and enhancing the low-temperature ductility of the asphalt. Similarly, the study revealed that SBS-modified asphalt has a swelling response, leading to an increase in asphaltene and other components, which is the primary factor contributing to the enhanced high-temperature performance of the SBS-modified emulsion asphalt.

The modification process of the thermoplastic elastomer modifier on the emulsion asphalt is illustrated in Figure 6. The presence of SBS leads to an increase in polar molecules within the system, resulting in a swelling reaction between the SBS molecules and asphalt components. This response enhanced the cohesion and viscosity of the asphalt system. The network structure, consisting of both soft and hard parts, enhances the toughness of the asphalt and improves its viscoelastic properties. Thermoplastic elastomer modifiers have excellent properties owing to the existence of several active –C═C– groups, in addition to their general mixing and swelling properties. Under the catalytic action of stabilizers, these modifiers can engage in cross-linking with asphalt, creating permanent chemical connections. This changed the system from a single continuous asphalt phase to a double continuous network structure of asphalt and polymer, significantly enhancing the strength of the system and the flexibility of asphalt at low temperatures.

Figure 6

Modification mechanism of thermoplastic elastomer modifiers.

2.4 Other types of modifiers

Broadly, in addition to the three aforementioned polymer modifiers, emulsion asphalt modifiers include materials such as fillers and fibers that improve the performance of road asphalt. Lv et al. [17], Du and Jun [18], and Li et al. [19] investigated how inorganic fillers modify emulsion asphalt. They discovered that inorganic materials such as cement and fly ash generate needle-shaped and cluster-shaped hydration products through hydration reactions. These products reinforce and anchor asphalt binders. In contrast, the hydration reaction absorbs water and accelerates the demulsification of the emulsion asphalt. Asphalt demulsification, in turn, accelerates the generation of hydration products, which interweave with asphalt to create a spatial network structure, thereby promoting the early strength improvement of emulsion asphalt mixtures. At the same time, the hydration products of the inorganic materials also fill the micropores, improving the overall density. Wang [20] employed SEM to analyze how polyester fibers improve the performance of emulsion asphalt mixtures. At a fiber content of 2%, the study discovered that the fibers were evenly dispersed inside the emulsion asphalt, forming a through-type reinforcement network that helped control the internal deformation. Fibers have high oil absorption and a large specific surface area, allowing them to completely absorb asphalt, create a fiber-asphalt interface layer, and enhance the flexibility and integrity of the mixture by transferring and buffering the internal stress between the two phases.

3.1 Resin modifiers

Currently, many types of resin modifiers are being studied, with a focus on modifying emulsion asphalt using WER and combining these two modifiers to enhance performance. The specific preparation parameters are listed in Table 1.

From the table, it can be observed that WER is the most extensively studied resin modifier. Because it is a water-soluble substance, the usual approach is E-M. Shi et al. [23,26,27] first prepared WER from epoxy resin latex and then prepared a modified emulsion asphalt. However, the table above illustrates a wide range of WER incorporation, spanning from 3 to 70%. A thorough review of the literature has shown that this diversity is due to the varying epoxy equivalents of the epoxy resins employed. The epoxy equivalent is defined as the quantity of resin containing a single epoxy group within the epoxy resin. A higher epoxy equivalent suggests a lower proportion of epoxy groups in the system. As previously discussed, the performance of epoxy resin modifiers is chiefly contingent upon the reaction between epoxy groups and active hydrogen atoms. Therefore, when the content of epoxy groups is less, a higher dosage of WER is necessary to fulfill performance standards. The literature survey findings indicate that for epoxy equivalents below 220–230 g·eq⁻¹, the incorporation of WER is commonly less than 10% [25–27]. When the epoxy equivalent is higher, the required WER incorporation often surpasses 10%, with the optimal dosage potentially reaching 50% [1,24,28]. A typical procedure involves stirring at a speed of 300–600 rpm for 3–15 min at room temperature [1,23,25–28,31]. Because WER is normally a two-component system, the preparation process requires the addition of a primary agent before the addition of a curing agent for the reaction. Therefore, the modification process is conducted in two stages.

There are typically two types of PU modifiers, PU and WPU, which utilize the M-E and E-M methods based on their varying water solubility. The effective dosage of PU modifiers after conversion is approximately 6% because the solid content of WPU ranges from 30 to 50. When incorporating PU prepolymers [4] into asphalt, a chain extender must be used to facilitate subsequent reactions. To enhance the modification effect of PU on the emulsion asphalt performance, more intricate modification procedures involving higher shear rate, time, and temperature adjustments are typically necessary. When the modifier needs to generate internal reactions to provide physical and mechanical properties, the modified emulsion asphalt should be cured for a certain amount of time and at a certain temperature [4,28]. In addition, the preparation conditions for using water-soluble substances [7,28,29] such as WPU and water-soluble VAE latex powder as modifiers were comparable to those of WER. To ensure the impact of solid modifiers, such as resin particles [30], an M-E method was adopted. The shear conditions involve shearing at a rate of 4,000–5,000 rpm for 1–2 h at a high temperature of 140–180°C.

The use of WERs in our review introduces a unique formulation that includes advanced curing agents tailored for improved adhesion and thermal stability. Unlike previous studies that primarily focused on traditional epoxy systems, this study further proposes the preparation of resin-based modifiers. The curing agent is compounded with the resin-based modifier to prepare the resin-based modifier with two-component system that enhances the cross-linking efficiency, thereby providing excellent high temperature resistance and reducing brittleness. This preparation method could be most effective in enhancing WER-based modifiers. This innovation in application methods such as employing low-energy mixing techniques ensures optimal dispersion and interaction with the asphalt matrix.

3.2 Rubber modifiers

Rubber modifiers primarily include SBR latex and rubber powder, and the modification method is based on whether the material is latex. The preparation parameters are listed in Table 2.

SBR latex and rubber powder are the primary rubber modifiers. These data indicate that the most common approach for SBR latex and ordinary rubber latex modifiers is E-M, with dosages ranging from 3 to 6% [21,25,27,31–34]. The table indicates that the preparation process is relatively consistent, typically involving shearing at a speed of 300–500 rpm for 5–15 min at room temperature [25,27,32,33]. Additionally, some researchers have employed the M&E approach to prepare SBR-modified emulsified asphalt, which requires more stringent preparation conditions [21]. Rubber powder [35–37] cannot be uniformly mixed with emulsion asphalt as a solid modifier. To address this issue, modified emulsion asphalt was created using M-E at a 15–20% dosage. Pretreatments such as microwave activation allow for a lower dosage [35]. To achieve effective modification, rubber powder modifiers must be prepared through prolonged high-speed shearing at high temperatures with a shear rate exceeding 1,500 rpm. Some researchers go as far as using 10,000 rpm, a temperature above 180°C, and shearing for more than 2 h.

In addition, according to the different mechanisms of action between rubber modifiers, resin modifiers, and emulsified asphalt, and sufficiently considering the impact of these two modifiers on the performance of emulsified asphalt, further optimization of the modified emulsified asphalt preparation method is proposed. As far as the concrete is concerned, the E-M method offers a cost-effective solution for preparing modified emulsions, particularly for WER and SBR latex-modified systems. Unlike the traditional modification-followed-by-emulsification process, E-M reduces energy requirements by eliminating the need for high-temperature mixing. Furthermore, this method facilitates the large-scale application of modified emulsions by simplifying the production process, reducing equipment costs, and enhancing storage stability. These practical benefits, coupled with advancements in cold recycling technologies, which allow for room-temperature mixing of reclaimed asphalt with emulsion modifiers, demonstrate the potential for widespread adoption in energy-efficient pavement maintenance.

3.3 Thermoplastic elastomeric modifiers

Currently, SBS is the primary thermoplastic elastomer used to modify road asphalt. Because of the intricate SBS latex production process and the unstable nature of the emulsion it produces [15], modified emulsion asphalt is typically prepared using SBS by directly modifying the base asphalt followed by emulsification. The specific preparation processes are listed in Table 3.

Based on an extensive literature review, the optimal dose of SBS modifier is typically between 3 and 5%. The elevated viscosity of SBS renders it susceptible to clogging the emulsification equipment during the emulsification process of modified asphalt, particularly when the dosage exceeds the recommended levels. The preparation conditions are stringent, requiring shearing at a speed of 3,000–5,000 rpm for more than 2 h. Because the matrix asphalt is modified first, to make the asphalt and modified asphalt have sufficient fluidity, the modification and emulsification steps need to be performed at a high temperature of 160–190°C [30,34,40–47]. When using the M&E method to simplify the preparation process, or using the E-M method to reduce the requirements of emulsification equipment, emulsifying SBS or directly using an SBS emulsion can be considered. Li et al. [38] prepared SBS latex, used M&E to prepare modified emulsion asphalt, and explored the feasibility of using high doses of SBS to modify emulsion asphalt. Li and Tigheu [31,39] used SBS latex directly for modification and observed that the requirements for the M&E preparation parameters are higher than those for E-M. Because the modifier and matrix asphalt are added to the colloid mill at the same time during M&E, the preparation temperature should also be above 150°C.

4.1 Influence of modifiers on the performance of emulsion asphalt

The improvements in modified emulsion asphalt performance vary based on the distinct structural types of modifiers and the different ideal dosages utilized [15], which affect the various properties of the mixture. Currently, some experts argue that assessing the emulsion asphalt performance based solely on the three indications of emulsion asphalt evaporation residue is superficial and incomplete. However, considering that the evaluation system for the performance of modified emulsion asphalt is not perfect, this study further proposes an evaluation plan for the trend of performance changes in the research subject based on the three indicators. This study categorizes and discusses the performance of different types of emulsion asphalt based on the type of modifier used. Based on the research content of most studies and to provide unified indicators for evaluating the impact of different modifiers on performance, the three most basic indicators of emulsion asphalt evaporation residue (penetration, ductility, and softening point) were used as evaluation indicators to compare the impact of different types of modifiers on the basic performance of emulsion asphalt and evaluate their modification effect and applicability.

4.1.1 Resin-modified emulsion asphalt

The basic properties of the resin-modified emulsion asphalt were investigated and are summarized in Table 4 and Figure 7. From Table 4, it can be seen that most of the resin-based modifiers improved the high-temperature performance, weakened the low-temperature ductility, and increased the asphalt viscosity. Many scholars have used 90# asphalt for emulsification modification to reduce the negative impact of modifiers on low-temperature performance.

Table 4

Performance of resin-modified emulsion asphalt

Type	Dosage (%)[Ⅰ]	Asphalt grade	Softening point		Ductility (5°C)[Ⅱ]		Penetration		Ref.
			Value (°C)	Vari. (%)[Ⅲ]	Value (cm)	Vari. (%)	Value (0.1 mm)	Vari. (%)
WER	5	90	70	+31.4	7	−22.2	60	−30	[31]
WER	6	90	61	+5.2	110	−26.7	54	−28	[26]
WER	2–4	70	61.3	+7.5	24.3	+9.5	49.8	−8.3	[33]
WER	6	90	64.5	+17.3			50	−24	[27]
WER	6	70	60.6	+9.2	32.5	>−67.5[Ⅳ]	50.9	−13	[25]
WER-SBR	6–3	70	60.6	+24.7	32.5	+327.6	50.9	−31	[25]
WPU	15	70	50.3	+10.1	11.4 (10°C)	−85	56	−27	[7]
PU	6	70	58.7	+16.9	26.3	+911.5	65.4	−26	[4]
PU	6	90	56	+22.8	24	+290	48	−46	[51]
PU*[Ⅴ]	10	70	67.6	+7.1	7.4	+23.3	69.5	−6.5	[52]
PU*		70	49.9	+6.2	150 (15°C)	+10.3	52	−14	[53]
PU*			77	+57.1	Brittle fracture		20	−72	[6]
VAE	3–4	90	66.1	+9.8	27	+238	63.3	−12	[8]
VAE	3–4	90	63.4	+5.3	23.5	+194	65.8	−8.6	[8]
Tackifying resin	12–15	70	58.5	+14.7	30.1	+290.9	55.5	−22	[54]
Reclaimed ion exchange resin	30	70	57.5	+51.3	19 (15°C)	−52.5	44	−29	[29]

Notes: [Ⅰ] Dosage is the mass percentage of emulsion asphalt.

[Ⅱ] Unless otherwise specified, the test temperature for the ductility index is 5°C.

[Ⅲ] Variations mean the rate of change of each indicator after modification relative to before modification, where “+” indicates an increase in indicator values after modification, and “−” indicates a decrease.

[Ⅳ] >−67.5 indicates that the modified indicator has decreased by at least 67.5% compared to before modification.

[Ⅴ] PU* is PU-modified asphalt rather than modified emulsion asphalt.

Figure 7

Basic properties of resin-modified emulsion asphalt.

Figure 7 shows a concentrated data range of 58–66°C for the softening point after resin modifier modification, surpassing industry standard requirements (spraying type >50°C, mixing type >53°C), indicating excellent thermal stability after modification. The distribution range of penetration was 50–62 (0.1 mm), with a minimum value that was 25% higher than the industry requirement (>40/0.1 mm). The range of ductility is 11–27 cm, with few samples falling below industry standards. This indicates that there is significant variation in the effectiveness of resin modifiers in enhancing low-temperature ductility. Figure 7 indicates the presence of an outlier in the penetration data. The analysis of performance data indicated several outliers, particularly in the ductility measurements for certain modified asphalts. These anomalies may stem from experimental variability, including differences in mixing times and temperatures across studies. For instance, the high ductility values reported for certain WER-modified systems could reflect inconsistencies in the epoxy formulation or curing conditions. Meanwhile, it has been discovered that the corresponding modified asphalt shows superior performance at high temperatures but is prone to brittle failure at low temperatures. A thorough examination of Sun’s research [6] indicates that the base asphalt utilized also experiences brittleness at 5°C in terms of ductility, and the author suggests that a higher softening point is associated with a shorter gel time. Therefore, to curtail the gel time, the author identified the optimal preparation method for PU-modified asphalt based on the softening point as the key indicator. Given the varying research goals, the developed PU-modified asphalt only manifests excellent high-temperature characteristics, while selectively sacrificing low-temperature performance and adhesive properties. The phenomenon of brittle fracture at low temperatures, when viewed in conjunction with data from other PU-modified asphalts or emulsified asphalts, indicates that the impact of PU on low-temperature performance is very pronounced.

Because of the possibility of small values in the results of the ductility tests, some even less than 1 cm, the rate of change in ductility is for reference only. The analysis found that Liu et al. [26] used SBS-modified emulsion asphalt as a matrix and added WER for composite modification. Therefore, although its ductility value was high, it still decreased by 26.7% compared to that of the SBS-modified emulsion asphalt. Modifiers such as PU and VAE lack a systematic control plan for regulating the high- and low-temperature performance of emulsion asphalt owing to their intricate composition and varied raw material categories, leading to significant differences in emulsion asphalt performance. For example, for materials like PU, research results have shown that modified emulsion asphalt is prone to low-temperature brittle fracture, but it can still enhance the ductility at 15°C to about 150 cm by adjusting the ratio of soft and hard segments of PU [6,53]. In addition, Zhang et al. [55] studied the effect of different isocyanate contents on performance. The findings revealed that, even at the optimal dosage, PU negatively affected the low-temperature performance of the modified emulsion asphalt, which was even lower than that of the base asphalt. However, there have been limited studies on VAE modifiers for asphalt modification, particularly regarding the low-temperature performance of VAE-modified asphalt. Additionally, there is a paucity of research on the modification mechanisms.

Most researchers currently use SBR or SBS combined with resin modifiers for composite modification to study the effect of different blending ratios on performance and fully leverage the advantages of the two to achieve the best emulsion asphalt performance. This is because the resin-modified emulsion asphalt performs well at high temperatures but poorly at low temperatures. For PU polymers, the goal of regulating the high- and low-temperature performance of the modified emulsion asphalt can also be achieved by adjusting the ratio of isocyanates to polyols in the synthesized raw materials.

4.1.2 Rubber-modified emulsion asphalt

The literature results for rubber-modified emulsion asphalt are presented in Table 5 and Figure 8. The analysis showed that rubber modifiers have varying degrees of improvement in high- and low-temperature performance and viscosity, especially SBR, which significantly enhances low-temperature ductility.

Table 5

Performance of rubber-modified emulsion asphalt

Type	Dosage (%)	Asphalt grade	Softening point		Ductility (5°C)		Penetration		Ref.
			Value (°C)	Vari. (%)	Value (cm)	Vari. (%)	Value (0.1 mm)	Vari. (%)
SBR	5	90	55	+14.6	58	+544.4	69	−19.8	[31]
SBR	3	70	59.9	+25.6	>120	+	59.1	−24.9	[21]
SBR	4	90	57.6		>150		74.7		[26]
SBR		90	59.5	+32.2	34.5	+283.3	61	−32.2	[56]
SBR	4	90	54.2	+14.3	50	+	76.7	−6.8	[36]
SBR	3	70	55.5	+14.2	>100	+	58.7	−20.4	[25]
SBR	3	70	57		22.2		54.4		[33]
SBR			57.6		>150		74.7		[57]
WER–SBR	6–3	70	60.6	+24.7	32.5	+327.6	50.9	−30.9	[25]
Rubber latex	3	70	73.5	+47	>120	+	55.6	−13.1	[34]
Waste rubber powder	4	90	51.2	+8.0	13	+	107	+30.0	[36]
Waste rubber powder	6	90	49	+2.9	25	+	97.6	+21.4	[35]

Figure 8

Basic properties of rubber-modified emulsion asphalt.

Figure 8 shows that the softening point data are highly concentrated, with a reasonable distribution range of 55–60°C. The minimum value still meets the industry standard requirements (spraying type >50°C, mixing type >53°C), indicating that the emulsion asphalt modified with rubber has excellent high-temperature performance. When softening data were used for mapping, results >100 cm were represented as 100 cm for better visualization. It can be seen that the ductility data are highly dispersed, which is due to the significant difference in low-temperature improvement effects between latex and powder modifiers. However, the minimum value of its distribution range was still greater than 50% of the industrial requirement (>20 cm). Therefore, rubber-based modifiers can significantly improve the low-temperature performance of emulsion asphalt.

The reasonable distribution range of penetration is 58–75 (0.1 mm), which is much higher than the industry standard (40/0.1 mm), as shown in Figure 8, indicating that rubber substances significantly contribute to viscosity improvement. Analysis of specific data revealed that the ductility test results of waste rubber powder were relatively poor, with some falling below the industry minimum standards, and the penetration may even increase instead of decrease, which has a negative impact on viscosity, but still meets the regulatory requirements. This phenomenon is primarily caused by the crushing and processing of waste rubber powder by waste tires, which are used as raw materials for hard rubber products. In addition, waste rubber powder is aged to a certain extent during tire usage, resulting in hardening and brittleness. Therefore, the improvement in low-temperature ductility is not as good as that of ordinary rubber modifiers. The ductility test result of Zheng et al. [36] was 13 cm. However, because of the low-temperature brittle fracture of ordinary emulsion asphalt without added modifiers, the rubber powder still produced better improvement of the low-temperature performance. In addition, because of the solid form of waste rubber powder, it is different from latex modifiers and requires the use of the M-E method during modification. However, rubber substances themselves have high viscosity, and there is a problem with emulsification equipment blockage when directly emulsifying modified asphalt. Thus, during modification, substances such as oil slurry are added as rubber softeners to reduce the viscosity of the system and facilitate better emulsification [35].

Therefore, the most significant characteristic of rubber modifiers is their excellent modification effect at low temperatures. They are often used in combination with other modifiers to improve the low-temperature ductility of emulsion asphalt. Attention should be paid to whether the ductility of the waste rubber powder meets these requirements. Although there has been progress in enhancing the performance at low temperatures, there are still cases where the ductility index fails to meet the criteria. Thus, an appropriate SBR or SBS should be considered for composite modification. Furthermore, recent studies have shown that the strategic combination of WER with SBR rubber modifiers not only mitigates the brittleness observed at low temperatures but also enhances the ductility of the asphalt. By optimizing the ratios of resin to rubber, a new formulation has emerged that maintains high-temperature performance while significantly improving low-temperature flexibility. This dual-modifier approach represents a promising direction for future research, potentially transforming how modified asphalts are formulated for diverse climatic conditions.

4.1.3 Thermoplastic elastomer-modified emulsion asphalt

The literature research results on the performance of thermoplastic elastomers (mainly SBS) emulsion asphalt are listed in Table 6 and Figure 9 and show that SBS has a positive impact on the high- and low-temperature stability and viscosity of emulsion asphalt. The reasonable ranges for softening point, ductility, and penetration are 60–75°C, 28–40 cm, and 56–74 (0.1 mm), respectively. The minimum values were 20, 40, and 40% higher than the minimum requirements of the specifications. The soft segments of this modifier significantly enhanced low-temperature flexibility, whereas the hard segments improved high-temperature deformation resistance. Thus, this modifier positively impacts the high- and low-temperature performance of emulsion asphalt.

Table 6

Performance of thermoplastic elastomer-modified emulsion asphalt

Type	Dosage (%)	Asphalt grade	Softening point		Ductility (5°C)		Penetration		Ref.
			Value (°C)	Vari. (%)	Value (cm)	Vari. (%)	Value (0.1 mm)	Vari. (%)
SBS	5	90	68	+41.7	26	+188.9	57	−33.7	[31]
SBS			58.5		37		72		[58]
High viscosity SBS			82.7		31		58		[58]
SBS		90	67	+48.9	15.8	+75.6	47	−47.8	[56]
SBS	3.5	70	57		46		55		[59]
SBS	4	90	68.7		30.4		63.2	−12.3	[60]
SBS	6		54.5	+23.9	30	+500	83	+59.6	[61]
SBS	4	70	71.4		71.8		82		[62]
SBS*	5	90	72.4	+53.4	40.1	−	75.2	−17.7	[41]
SBS−sulfur powder*	5−0.15	90	78.5	+66.3	31.0	−	73.1	−20.0	[41]
SBS*	7		92.8	+99.6	28.4		42.5	−43.8	[42]
SBS*	5	70	60.9	+29.6			70.9		[44]
SBS*	5	70	68.3	+45.3	19.4		38.3	−46.1	[45]

Figure 9

Basic properties of thermoplastic elastomer-modified emulsion asphalt.

Resin modifiers were primarily used to improve the high-temperature performance and adhesion of emulsion asphalt. They are suitable for hot regions in the south, or other special road sections with strict requirements for high-temperature stability. In addition, they can be directly used as bonding materials between layers. If used in the north, it must be composite-modified with SBR or SBS to meet the requirements for use in low-temperature conditions. In-depth research can be conducted on PU modifiers with a focus on the types of synthetic raw materials and the ratio of soft to hard segments. The impact trends of these modifiers on asphalt performance were explored and reliable reference opinions were provided for the selection of subsequent raw materials and preparation of emulsion asphalt. Rubber modifiers mainly improve the low-temperature ductility of the binder, and their high-temperature deformation resistance meets the requirements for use, making them more suitable for the northern or central regions. In particular, when utilizing waste rubber powder as a modifier, it is necessary to control the origin and quality of the raw materials. If needed, preliminary treatment, such as microwave activation, should be conducted with a focus on low-temperature performance after modification. Thermoplastic elastomer modifiers have made significant contributions to the improvement of high- and low-temperature performance owing to their structural advantages. At the same time, to address the trade-off between high- and low-temperature performance, a structured approach for optimizing the selection of modifiers is required. This includes evaluating the specific requirements of the intended application, such as expected climatic conditions and traffic load, and tailoring the modifier ratios accordingly. For instance, recent research has demonstrated that formulations combining WER with rubber modifiers can effectively balance high-temperature stability with improved low-temperature ductility. This strategic combination not only mitigates the brittleness often observed in resin-modified systems but also enhances overall material resilience. In addition, it is further necessary to systematically evaluate the modifier ratios by high and low temperature performance tests to realize the improvement of the modified asphalt.

For instance, thermoplastic elastomers like SBS are particularly effective in improving the high- and low-temperature performance, creating a balance through physical swelling and the formation of cross-linked network structures. In contrast, WER modifiers, while excellent in enhancing high-temperature properties, may require the addition of rubber-based modifiers to counteract their brittleness at low temperatures.

4.2 Engineering application of modified emulsion asphalt

Based on the usage method, emulsion asphalt is mainly divided into spraying and mixing. Currently, tack-coat oil and prime-coat oil are the main uses of emulsion asphalt in spraying. Simply put, they refer to thin layers of asphalt material spread to enhance the adhesion between different layers. Generally, ordinary emulsion asphalt satisfies the requirements of a tack coat. When used on sections with high traffic volume, high overload ratio, or special bonding requirements, a modified emulsion asphalt with better performance is required. Yuhui and Weifeng [63] modified emulsion asphalt with SBS and SBR to simulate the actual state of the asphalt interlayer and conducted rutting and pullout tests. The results of this study showed that SBS has the best effect on bonding. Fu et al. [64] prepared a WER-modified emulsion asphalt tack-coat oil to ensure the interlayer bonding effect of the road surface and studied its basic properties and road performance. The results show that the WER-modified emulsion asphalt has better adhesion, waterproofing, and durability with aggregates than the SBS- and rubber debris-modified asphalts. To solve the problem of wheel sticking during the construction of an asphalt pavement tack coat, Wang et al. [65] used WPU-modified emulsion asphalt and evaluated its non-wheel-sticking effect by evaluating the damage caused by the pressure head on the sample tack coat at different dosages. Research has found that when the WPU content is 3%, the tack coat failure rate is reduced by 97.3% compared with that before modification. This indicates that modified emulsion asphalt, particularly resin modifiers, can significantly improve the interlayer adhesion of emulsion asphalt, thereby maintaining each layer of the road in a continuous state and better bearing traffic loads. For the prime coat, the permeability of the emulsion asphalt materials in the non-asphalt base layers is a key factor in determining the interlayer adhesion. The prime-coat oil was prepared by adding a permeable solvent to the emulsion asphalt to increase the penetration depth of the base material.

In addition, emulsion asphalt can be used as a binder for the chip seal layer and bridge deck waterproofing materials to prevent water from entering the pavement structure and enhance structural durability. Currently, most emulsion asphalts are modified by WER or SBS. The Yellow River Bridge in China [66] was treated with an SBS-modified asphalt synchronous crushed stone sealing layer to effectively solve the problem of wheel sticking. The average bonding strength at 22°C can reach 0.58 MPa, which is higher than the specification requirements. Yang et al. [67] modified the emulsion asphalt with WER and PU, and the research results showed that the composite-modified emulsion asphalt bridge deck waterproofing material had better bonding strength and low-temperature flexibility than SBS alone. Wang [68] conducted a comparative analysis of the road performance of three types of modified emulsion asphalts: EVA, SBS, SBR, and ordinary emulsion asphalt. The results showed that the SBS-modified asphalt had the best shear strength under different temperatures and immersion conditions. In addition, the SBS-modified emulsion asphalt exhibited the best freeze-thaw resistance, waterproofing performance, and erosion resistance.

The emulsion asphalt used for mixing is primarily used in maintenance engineering. Initially, it was used only as a simple slurry seal layer. With the advancement of emulsion asphalt technology, new applications such as microsurfacing, cold regeneration of asphalt pavements, ultrathin overlays, and cold repair materials have been rapidly developed to meet the needs of road construction and maintenance engineering. The emergence and development of these new technologies have further expanded the application scope of emulsion asphalt, providing broader prospects for the application of emulsion asphalt in road engineering. Among these, slurry seal layers, microsurfacing mixtures, and ultra-thin overlays are generally used for preventive maintenance engineering, as anti-skid wear layers on roads, or for the repair of minor damage. Zhang et al. [69] used WER and SBR to composite modify emulsion asphalt in slurry-sealing mixtures. The results showed that when the optimal dosage was used, the modified abrasion value was reduced by 43.5% and the mass loss rate was reduced by 87.6%, indicating that the modification can significantly improve the wear resistance, impermeability, and bonding performance of the slurry seal layer. Zhang [70] prepared modified emulsion asphalt by composite modification of rock asphalt and SBR, and compared its construction and road performance with those of SBS and SBR alone. The results showed that there was not much difference in the wear resistance of the mixture between composite modification and individual modification, but its water resistance increased by 11–14% and its resistance to rutting deformation increased by 35%, indicating that the composite modification provided better road performance. Research on microsurfacing mixtures has focused on wear resistance, water stability, and high-temperature stability. The research results of Liu et al. [71–73] showed that after modification with epoxy resin, the 1-h wet wheel wear value of microsurfacing mixtures was reduced by 30–70%, and the reduction can even reach 80% under freeze-thaw conditions. The 6-day wet wheel wear value can be reduced by 30–60%, and the anti-rutting performance can be improved by more than 43%. This means that WER can significantly improve the wear resistance, water resistance, and rutting resistance of the microsurface mixtures.

Asphalt pavement cold recycling technology can use emulsion asphalt to achieve room-temperature mixing; however, owing to the large variability of reclaimed asphalt pavement materials and poor adhesion of emulsion asphalt, the current cold recycling technology is used most often for the base course and less often for the lower surface course of low-grade highways. At present, most researchers use SBS and SBR as modifiers or composite methods for modification and use them for cold regeneration technology of asphalt pavements. Yu et al. [58] modified emulsion asphalt using high-viscosity SBS and conventional SBS produced by a certain company. The experimental results indicated that the high-viscosity SBS-modified emulsion asphalt cold recycled mixture exhibited better low-temperature toughness and crack resistance than the conventional SBS cold recycled mixture. Yao et al. [32] used WER and SBR to modify emulsion asphalt and prepared cold recycled mixtures. The results showed that the indirect tensile strength of the modified mixture increased by 27%, and the maximum indentation depth and creep depth at 60 s decreased by 42 and 48%, respectively. This indicated that the modified mixture exhibited better viscoelasticity, mechanical properties, and deformation resistance. The experimental road performance data also indicate that the modified cold recycled mixture has good thermal stability, crack resistance, and water stability. On the other hand, combining WER and SBS modifiers has shown significant promise in optimizing the performance of emulsion asphalt across a broader temperature range. While WER provides excellent high-temperature stability through its cross-linking mechanism, the addition of SBS compensates for its brittleness at low temperatures by enhancing flexibility through its swelling and network-forming properties. This combination is particularly effective for road surfaces subject to both extreme heat and cold, such as those in northern regions where temperature fluctuations are common. Furthermore, the way in which the modifiers are combined needs to be selected according to the environmental conditions and project-specific requirements, thus further achieving the optimal ratio of the desired properties. Liu et al. [74] modified emulsion asphalt using basalt fibers and SBR, and studied the fatigue performance of cold recycled mixtures. The experimental results showed that the average bending strength of the mixture modified with basalt fiber and SBR increased by more than 21% compared to that before modification, indicating that both can improve the fatigue resistance of the mixture. Song et al. [60,65] found that after using SBS to modify emulsion asphalt, the fly loss rate, dry–wet splitting strength ratio, dynamic stability, and low-temperature failure strain after 7 days of curing could be increased by 12, 3, 73, and 97%, respectively. This suggests that the anti-stripping properties, water stability, and high- and low-temperature stabilities of the cold recycled mixtures were improved by SBS.

Resin-based modifiers are commonly used in tack coat layers owing to their excellent adhesion properties. Technologies such as bridge deck paving, which requires high stability and adhesion, mostly use epoxy resin or SBS, and SBR modifiers are also considered in situations with significant low-temperature requirements. For microsurfacing mixtures, WER is more commonly used for modification, and various road performances are significantly enhanced after modification. The cold regeneration technology of asphalt pavements requires excellent road performance owing to its use as a structural layer rather than a functional layer. Typically, SBS, SBR, or a combination of the two is used for modification. Numerous studies have shown that modified cold recycled mixtures exhibit excellent road performance in terms of water stability, high- and low-temperature stability, and fatigue resistance. However, for mixtures, modified emulsion asphalt is only one of the many factors that affect its performance. It must be combined with aggregates with fine mechanical properties, scientific and reasonable gradation design, and mixture design to obtain emulsion asphalt mixtures with excellent performance that satisfy usage requirements. From the above findings, it can be concluded that the integration of resin and rubber modifiers can be strategically utilized to meet specific performance criteria. It is possible to provide insights into the selection of modifiers based on environmental conditions and performance requirements, and to achieve optimum results through the use of balanced formulations of both resins and rubbers. For instance, projects in cold climates can benefit from formulations that prioritize rubber modifiers to improve ductility, while those in hotter regions may focus on the adhesive properties provided by resin modifiers.

Although more research has been carried out on modified emulsified asphalt in the road field, it is still necessary to further explore novel research directions and future research should be more focused on exploring the synergistic effects of multi-modifier systems. For example, combining resin modifiers such as WER with rubber-based modifiers like SBR has demonstrated promising results in lab settings, but further studies are needed to optimize these formulations for large-scale applications, particularly in environments subject to extreme or fluctuating temperatures. Another crucial area is the investigation of long-term aging effects on modified emulsion asphalt. Understanding how different modifiers influence the aging process, particularly in the context of UV exposure and oxidative degradation, could lead to significant advancements in durability and performance. For environmental sustainability in cold recycling applications, although cold recycling techniques are emerging as energy-efficient alternatives, more research is needed to evaluate their life-cycle performance and carbon footprint compared to traditional hot-mix methods, this will provide critical insights into reducing the carbon footprint of road construction and maintenance projects.