Performance Evaluation of Highly Modified Asphalt-Based Binders in High Friction Surface Treatment: Comparative Study with Epoxy-Based System

Abstract

High Friction Surface Treatments (HFSTs) are frequently used to increase skid resistance and reduce collisions, particularly in crash-prone zones, including horizontal curves and intersections. Epoxy-based binders traditionally have been the sole option for HFSTs, but their drawbacks, such as high costs and compatibility challenges, have led to the search for substitute binders, including asphalt-based options. This study investigates the comparative performance of highly modified asphalt-based binders, including polymer-modified, mastic, and highly modified emulsions, in HFST applications using two aggregate types, Calcined Bauxite (CB) and Rhyolite with different gradations, with an emphasis on their frictional properties, durability, and resistance to polishing. Laboratory evaluations, including the Pendulum Tester (BPT), Dynamic Friction Testing Equipment (DFT), Surface Texture Measurement Apparatus (CTM), and Binder Bond Strength Test (BBS), were carried out to examine the Coefficient of Friction (COF), Mean Profile Depth (MPD), and aggregate bonding and retention. In terms of durability and friction, this study indicated that highly modified asphalt-based binders performed better than PG binders and conventional emulsions. The highest BPT values, both prior to and following polishing, were consistently observed for CB, with the emulsion containing the highest reactive polymer modifier showing the smallest decrease in BPT value (12.86% for CB and 10.34% for Rhyolite). Epoxy showed a greater COF retention over lengthy polishing cycles; however, highly polymer-modified (PM) binders like PG82-22 (PM) performed better than Epoxy under specific conditions. The macrotexture analysis revealed that Epoxy-based samples retained surface texture for further polishing cycles, while Mastic2 and PG82-22 (PM) also showed strong MPD retention. These findings highlight the importance of optimizing aggregate–binder combinations to ensure durable and effective HFST applications.

1. Introduction

Road accidents, injuries, and fatalities are ongoing challenges for both state DOTs and local transportation authorities, resulting in huge financial and negative social effects. Improving road safety requires more than simply encouraging safe driving behavior; it also requires careful maintenance of pavement surfaces, which is crucial in preventing traffic accidents or minimizing traffic accidents [1]. For almost two decades, High Friction Surface Treatment (HFST) has been used as a beneficial safety countermeasure throughout the United States, particularly in high-risk locations like sharp curves and intersections. These treatments are designed to prevent and reduce crashes, particularly in wet conditions, by greatly enhancing pavement friction [2].

In the United States, HFSTs are typically applied using polymer resin binders that comply with the American Association of State Highway and Transportation Officials (AASHTO) MP 41 standards [3]. A significant component of this treatment is the application of high-quality Calcined Bauxite (CB) aggregate, which was chosen for its exceptional hardness and wear resistance [4]. Traditional HFST systems often rely on Epoxy-based binders, which, despite their effectiveness, come with drawbacks, such as high installation expenses, challenges related to compatibility with current pavements, and concerns about long-term durability, especially under less-than-ideal substrate conditions. Early signs of failure can emerge due to several factors. The condition of the existing pavement is crucial, as a deteriorated surface can reduce HFST effectiveness. Additionally, the Epoxy resin used in these treatments may not always be fully compatible with the pavement beneath. Epoxy-based HFSTs tend to expand and contract more than traditional hot-mix asphalt. This mismatch can lead to movements between the HFST layer and the substrate during temperature fluctuations, potentially causing further damage and deterioration of the road surface [5,6,7]. In response to challenges associated with traditional Epoxy-based HFSTs, state agencies have been actively investigating asphalt-based substitute binders for HFST implementation, including efforts like the National Cooperative Highway Research Program (NCHRP) RFP #NCHRP 10-145. The New Jersey Department of Transportation (NJDOT) developed an alternative approach known as High Friction Chip Seal (HFCS). This method utilizes an asphalt-based binder as a substitute for Epoxy, enhancing compatibility with existing asphalt pavements. Similarly, the Missouri Department of Transportation (MoDOT) conducted studies to evaluate the cost-effectiveness and performance of friction enhancement surfaces employing asphalt-based binders [8,9]. Research on asphalt-based binders as epoxy alternatives shows promise, with optimized aggregate gradations enhancing HFST performance. The interaction between these binders and aggregates is governed by complex adhesive and cohesive mechanisms, along with mechanical interlocking facilitated by the aggregates’ rough texture and angular shape. A few studies investigated the usage of PG-grade binders and conventional emulsions as bonding agents in HFST applications. However, these materials frequently lack the endurance required to withstand harsh-polishing and high-traffic conditions. Standard emulsions, such as CRS-2P, showed considerable degradation as the polishing cycles increased [10].

To address the limitations of Epoxy-based HFSTs and the inadequate performance of conventional PG-grade asphalt binders and emulsions, next-generation binders have been developed. These include highly polymer-modified binders, mastics, and advanced emulsions designed to enhance the compatibility and durability of HFST applications. This study aims to examine the frictional properties, durability over time, and polishing resistance of HFST systems utilizing these innovative binders. Recent studies suggested that highly polymer-modified binders could replace epoxy in HFSTs, offering a comparable COF and MPD when combined with optimized aggregate gradations [9,10].

Mastic is a blend of asphalt, fine mineral fillers, and additives that is compatible with various pavement types and can be used to repair and fill cracks or address substrate issues before HFST application. Highly modified emulsions have high performance polymers and provide more durability and polish resistance, making them suitable for high-traffic areas to address the problems with traditional emulsions used in HFST applications [11].

2. Materials and Methodology

2.1. Aggregate

This research looks at two different aggregate materials: Calcined Bauxite (CB) and Rhyolite (Rhy), each tested in two gradations. The first gradation is called “HFST original size” and is the typical aggregate size used in HFST applications in the US. The second gradation is called “coarse size” and has larger particles and was chosen to see if it would have benefits in asphalt-based HFST systems. The reason for these gradations is because of the differences between Epoxy and asphalt-based binders. Epoxy, being a thermosetting material, forms strong bonds with fine particles, while asphalt, being a viscoelastic material, can be improved by larger size particles to enhance bonding and aggregate retention [12]. CB is the standard aggregate for HFSTs under AASHTO MP 41 and is known for its hardness and resistance to polishing and its ability to retain high friction over an extended period. Rhyolite is a structurally dense rock, known for its durability and ability to withstand weathering, and is an alternative for HFST applications. The properties of the aggregates used are presented in

Table 1. Characteristics of aggregates.

Properties	CB (HFST)	CB (Coarse)	Rhy (HFST)	Rhy (Coarse)
Bulk specific gravity	3.25	3.22	2.57	2.55
Water absorption (%)	1.5	2.2	1.7	1.1
LAA value (%)	Grade D	C	D	C
	16.01	11	15.11	19
MDA value (%)	15 min/30 min	95 min	15 min/30 min	95 min
	2.45/4.2	5.03	2.6/4.74	3.7
Aggregate gradation
3/8″ (9.5 mm)	na	100%	na	100%
1/4″ (6.30 mm)	na	97%	na	97%
No 4 (4.75 mm)	100%	2%	100%	2%
No 6 (3.35 mm)	95–100%	1%	95–100%	1%
No 16 (1.18 mm)	0–5%	na	0–5%	na
No 30 (0.59 mm)	0–0.2%	na	0–0.2%	na

Note: LAA stands for Los Angeles abrasion, MDA stands for Micro-Deval abrasion.

, with their gradations represented in Figure 1 and Figure 2 [13,14].

2.2. Binders

When compared to Epoxy-based HFST applications, traditional asphalt-based binders, notably PG-grade asphalt and emulsions like CRS-2P, have demonstrated minimal efficacy. Previous studies assessed the effectiveness of these conventional binders to address their drawbacks, emphasizing the need for higher performances substitutes [9,10]. This study explores various highly modified asphalt-based binders as potential substitutes for Epoxy resin. The selected binders include two types of laboratory-prepared, highly polymer-modified asphalt binders derived from PG64-22 and PG76-22, both modified with Styrene-Butadiene-Styrene (SBS) polymer—8% for PG64-22 and 6% for PG76-22—along with 0.1% sulfur to promote polymer cross-linking in the modified binder. Several rheological tests were conducted to determine their comprehensive performance grade (PG) [15,16,17,18,19]. Additionally, two mastic binders (M1 and M2) and four types of highly modified emulsions, all were sourced from Pure Asphalt Co. (Chicago, IL, USA), were examined. A stronger alternative was required since a previous study showed that CRS-2P, a typical surface treatment emulsion, was not sufficiently durable at high-polishing conditions [9]. To address this, several emulsions modified with latex and isocyanate-based additives were assessed. Isocyanate-based additive is a modifier that has been shown to increase asphalt emulsion adhesion, which may strengthen the binding between the aggregate and binder [11]. In order to replace Epoxy in HFSTs, an asphalt-based binder needs to be able to tolerate high temperatures and stop aggregate loss in warm weather while being durable in cold weather.

Table 2. Highly modified binders.

Binder
PG64-22 + 8% SBS + 0.1% Sulfur	PG82-22 (PM)
PG76-22 + 6% SBS + 0.1% Sulfur	PG88-16 (PM)
Mastic1	M1
Mastic2	M2
Emulsion + 8% Latex	A8
Emulsion + 16% Latex	A16
Emulsion + 8% Latex + Isocyanate-based additive	B2LA8
Emulsion + 16% Latex + Isocyanate-based additive	B2LA16

Note: PM = polymer-modified.

summarizes the highly modified binders assessed in this study.

2.3. Fabricating HFST Specimens (Coupons and Slabs)

2.3.1. Preparation Process for Aggregate Coupons

The primary steps for preparing the curved HFST specimens, commonly known as coupons, utilized in this study are illustrated in Figure 3. The base of each mold was initially coated with ready-to-use plaster to create a level and smooth base. Aggregates were precisely positioned within the plaster layer to guarantee consistent spreading and appropriate compaction to achieve the best performance from the HFST coupons. To ensure uniform covering and an effective bond, binders were spread over the inlaid aggregates at a controlled application rate. The samples were then allowed to cure, which enhanced the binders and created a durable link with the aggregates. Plaster was rinsed off when the curing process was completed, exposing the surface that was acceptable for testing [20].

2.3.2. Fabricating HFST Slabs

To ensure uniformity and consistency throughout the sample preparation process, a plywood substrate was used. The binder was first heated to the proper temperature, depending on whether mastic or emulsion binders were to be used. The melted binder was then uniformly spread over the prepared sample at a rate of 0.30 to 0.40 gal/yd² (1.36 to 1.81 L/m²) to ensure complete surface coverage. The spread rate required to accomplish aggregates that are embedded at least 50% depends mostly on the aggregate gradation and substrate texture. As seen in Figure 3, the preheated aggregate was broadcasted onto the binder at an application rate of 12–15 lb/yd² (6.51–8.14 kg/m²), continuing until the rejection limit of aggregate occurred or the binder was completely covered [9,21]. A manually operated compactor and a heavy plate were used to embed the aggregate uniformly, ensuring that the samples performed consistently and effectively. Emulsion slabs were placed in a forced mechanical convection oven at 95 °F and 30 ± 3% relative humidity for 24 h to cure and break down the emulsion. Following the drying process, each slab was wiped with a masonry brush to remove any extra aggregate.

2.4. Assessment of Friction Properties Through Performance Tests

To measure the friction characteristics and the impact of different mastics and highly modified emulsions, a variety of tests were conducted. These performance tests were designed to determine the compatibility of binder and aggregate combinations for HFST applications. The British Pendulum Test (BPT), implemented with the British Wheel Accelerated Polishing Machine, provided an early assessment of the interaction between aggregates and binders. It also assessed the skid resistance of the coupons at both the initial and post-10-h polishing stages. The surface friction characteristics of the pavement under simulated roadway scenarios were examined using the Dynamic Friction Testing device (DFT) in combination with the NCAT (Auburn, AL, USA) Polishing Device with Three Wheels (TWPD). In addition, the Circular Track Meter (CTM) was utilized to determine the pavement’s macrotexture and texture depth. The Binder Bond Strength Test (BBS) was also used to assess the adhesion of various binders to the aggregates, as adhesion is crucial in HFST applications. These tests, shown in Figure 4, provided essential information for determining the optimal aggregate–binder combinations for HFST applications. The results contributed to enhancing the performance of HFST samples and enabled a comparative evaluation against Epoxy-based samples [22,23].

2.4.1. Evaluation with the BPT Device (British Pendulum Tester)

The pendulum tester is a well-known method utilized in diverse research applications, following the guidelines of AASHTO T 278 [24]. In this research, the BPT was employed to evaluate and screen the initial effect and frictional properties of HFST samples as well as the aggregate–binder interaction. The test evaluates the sliding resistance produced when a rubber slider moves across the surface of a sample prior to and following polishing. A minimum of five tests were conducted on every specimen to measure initial BPN (British Pendulum Number) value. Once the pre-polishing tests were performed, the samples were then polished for 10 h using the accelerated aggregate polishing device. After 10 h of polishing, BPN values were obtained by re-evaluating the samples with the BPT, which provided information on how the coupons behaved under simulated wear circumstances.

2.4.2. British Wheel-Based Accelerated Aggregate Polishing

In accordance with the AASHTO T 279-18 standard [25], upon completing the initial BPT tests, the aggregate coupons were subjected to polishing with the British Wheel. For each test run, fourteen curved samples were securely attached at the perimeter of the wheel. The wheel rotates consistently at 320 ± 5 rpm. A pneumatic tire wheel was used to provide pressure to the aggregate coupons, resulting in a total load of 391.44 ± 4.45 N. This system enabled a regulated polishing procedure that closely resembled the wear found on actual road surfaces.

2.4.3. Mass Loss Rate

Some aggregate loss may occur on HFST specimens during the rapid polishing test, particularly those containing emulsions. To evaluate the performance of these emulsions, mass loss and BPN values before and after polishing were investigated. The mass loss rate, which is determined using the equation below, is used to quantify the aggregate binder interaction after traffic load simulation.

$$\mathrm{ML}={\displaystyle \frac{\mathrm{m}1-\mathrm{m}2}{\mathrm{m}1}}\times 100$$

(1)

where:

ML = Mass loss rate (%);

m1 = Mass of the coupons before wear (g);

m2 = Mass of the coupons after wear (g)

2.4.4. DFT and the Three-Wheel Polishing Device (TWPD) Evaluation

As per ASTM E 1911-19 [26], the DFT is a rotating round plate equipped with three rubber pads, capable of rotating at up to 100 km/h. When the circular plate has attained the required speed, it comes into contact with the sample surface, and the COF is assessed as it is slowing down. Friction recordings are carried out in wet circumstances, with data averaged across two repeats to guarantee accuracy. The TWPD, developed in accordance with AASHTO PP 104-21 [27], mimics traffic-induced wear on samples. The device has a circular platform with three inflatable rubber tires and a water spraying system to simulate wet roadway conditions. The method minimizes rubber tire degradation and eliminates loose abrasive aggregates, enabling further polishing. The COF is monitored at several cycles in the polishing process, including the initial (0 cycles) and subsequent 30 K, 70 K, and 140 K cycles. This approach gives a full assessment of friction performance at various levels of polishing.

2.4.5. Circular Track Meter (CTM)

The CTM evaluates Mean Profile Depth (MPD) at a specified point according to the ASTM E2157 (2019) [28]. The MPD is the average profile depth of the pavement and features a laser sensor utilizing charge-coupled device (CCD) technology for displacement measurements. This configuration allows the sensor to follow a circular route with a diameter of 284 mm (11.2 inches). By doing so, the CTM delivers a complete assessment of roadway macro-scale texture, providing critical data for analyzing surface properties. The CTM’s capacity to produce reproducible and accurate readings under varying conditions makes it a favored choice for both laboratory and field research contributing to more informed decision-making in pavement design and maintenance.

2.4.6. Binder Bond Strength Test (BBS)

The BBS test in this study was carried out according to AASHTO T361-22 [29], using the Positest AT-A instrument (DeFelsko Co., Ogdensburg, NY, USA) to assess the adhesion strength of different binders. The adhesion tester exerts a pneumatic force via a pressure ring on a pull-off stub connected to a stiff substrate covered with asphalt binder. To imitate real-world settings, the binder is bound to the substrate and cured under various conditions. During the BBS test, the loading rate was carefully regulated at 690 kPa/s (100 psi/s), and the asphalt thickness was kept at 0.2 mm with the stubs, as recommended by AASHTO. Figure 5 outlines the specimen preparation and BBS testing protocol, which included the following steps:

• Substrate and Binder Preparation: Rhyolite substrates were extensively cleaned with an ultrasonic cleaner and dried at 170 °C for at least one hour with the pull-off stubs. To guarantee adequate workability, the binders were heated to their application temperatures—190 °C for mastics and 80 °C for emulsions—for at least an hour.
• Specimen Assembly: A silicone mold of about 40 mm by 40 mm (1.6 in by 1.6 in.) with an 8 mm (0.32-in.) diameter hole and a depth of 2.0 mm (0.08 in.) for mastics and polymer-modified binders was created on the base substrates. Silicone rings with a hole in the center of 21 mm in diameter were used for emulsion. A little drop of binder was poured into the hole of the silicone rings, and pull-off stubs were pushed into the binder, ensuring a consistent asphalt thickness of 0.2 mm.
• Curing and testing: Each specimen was allowed to cool to room temperature for one hour. Some specimens were then cured at ambient temperatures (25 °C, 30% humidity) for 24 h before being evaluated for Binder Bond Strength under dry conditions (BBS_dry). Other specimens were immersed in a 40 °C water bath for 24 h, followed by a 1 h curing phase at room temperature, to replicate wet conditions. The BBS test was then performed on these immersion specimens to determine the Binder Bond Strength in wet conditions (BBS_wet).

3. Results and Discussions

A thorough examination of the experimental test findings was carried out to assess the functional properties of the HFST samples, which include different binders and aggregates of variable sizes. The analysis included a series of tests to see how different aggregate–binder combinations affect the performance of HFST applications. These tests used several ways for assessing performance. The BPT was used to measure skid resistance and for a preliminary evaluation assessing the frictional qualities of the samples before and after accelerated polishing with the British wheel to simulate wear. The DFT provided further insights into the COF in wet conditions, providing an expanded understanding of surface friction in traffic simulation situations. The CTM was used to estimate the MPD, which quantifies the macrotexture and texture depth of the pavement surface and is important for determining skid resistance. The BBS test was used to determine the adhesion strength of various binders to aggregates, which is an important aspect in the longevity and efficacy of HFST applications. The findings emphasized the necessity of choosing the right materials to improve durability and overall HFST performance. Furthermore, the integration of these tests allowed for a detailed examination of how HFST samples operate under simulated traffic. This contributed to the creation of more effective and long-lasting high-friction surface treatments [30,31,32,33,34].

3.1. Influence of Different Aggregate and Binder Combinations on BPN

Figure 5 shows the BPN values obtained prior to and following ten-hour polishing cycles using the accelerated aggregate polishing device. These values were measured for two various aggregate sizes (HFST and coarse) combined with different binders. For each aggregate size, two coupons were fabricated with different highly modified binders (polymer-modified, mastics, and emulsions), as detailed in Table 2, utilizing Epoxy resin as a reference binder. To guarantee reliability and precision, five BPN values were measured prior to and following polishing on each aggregate coupon, and the average BPN values were calculated. This technique enabled a thorough assessment of frictional performance across several binder–aggregate combinations under simulated polishing configurations.

Figure 5. BPN averages for various aggregates and binders, categorized by size.

Figure 5. BPN averages for various aggregates and binders, categorized by size.

CB had the greatest BPN values for both the original HFSTs and coarse-sized aggregates, before and after polishing. Initially, the BPN readings were rather consistent across all samples. However, after 10 h of polishing, samples containing the CRS-2P emulsion exhibited a more significant decline in BPN levels. Specifically, the decrease in BPN for the CB HFST size was 28.57%, and for the Rhy HFST size, it was 22.58%. Among the emulsions, B2LA16 had the lowest drop in BPN after polishing, with reductions of 12.86% and 10.34% for the CB and Rhy HFST sizes, respectively. The mastics, M1 and M2, and the polymer-modified binders performed similarly in terms of BPN retention.

After polishing coarse-sized particles, Epoxy samples showed the highest BPN values. Among the emulsions, B2LA16 had the least decline in BPN while maintaining superior skid resistance compared to other emulsions. PG88-16 (PM) showed the greatest percentage reduction with Rhy, at 39.39%. Previous studies have shown that conventional PG binders and emulsions lost more aggregate [9,23]. The mass loss ratio findings for the highly modified emulsions, given in Figure 6, showed that A16 and B2LA16 performed most effectively, losing the least amount of aggregate over the polishing cycles. This confirms that using the isocyanate-based additive could affect the ability to maintain the surface texture and friction qualities. The paired t-test results showed statistically significant decreases in BPN after polishing for all treatments (p < 0.0001), and the consistency in standard deviations across all the binders suggests a comparable variability in performance.

The choice to use highly modified emulsions in this research was based on the poor performance of CRS-2P after polishing cycles.

Figure 7 depicts samples with CRS-2P that showed significant rutting and bleeding after different polishing stages, highlighting their limitations under wear conditions.

In contrast, as shown in Figure 8, highly modified emulsions, such as A16, demonstrated better durability, with no rutting or bleeding even after intense polishing. This indicates the effectiveness of highly modified emulsions in maintaining surface integrity and skid resistance over time, making them a better option for HFST applications. The results reveal improved adhesion between the binder and aggregate, which is a key feature of modified emulsions. After 10 h of polishing, HFST coupons lost very little aggregate, and the aggregate loss in the coarse size was significantly lower than the HFST size. This demonstrates the effect of coarse aggregate gradation, together with the strength and efficacy of these highly modified emulsions in holding the aggregates and keeping their surface texture, even under harsh-polishing conditions.

3.2. Influence of Aggregate Size, Gradation, and Binder Type on COF

The dynamic surface friction tester was implemented to determine the COF at various phases of polishing: initially (0 cycles), after 30 K cycles, 70 K cycles, and lastly, after 140 K cycles. All tests were done at a speed of 20 km/h. Based on the durability, BPN, and mass loss findings, Epoxy resin, PG82-22 (PM), PG88-16 (PM), M1, M2, A16, and B2LA16 were selected for accelerated friction testing. The DFT was used to obtain a single friction measurement for each polishing cycle and speed combination, and the COF results are the mean of two trials.

As demonstrated in Figure 9, the initial COF values at 20 km/h prior to polishing were roughly equivalent for all binders in both cases, with CB having somewhat higher COF values than Rhy. After 30 K polishing cycles, Epoxy exhibited a lower decline in its COF for the CB HFST size than the remaining binders. However, with the progress in polishing cycles, PG82-22 (PM), followed by M2, demonstrated the least decrease in its COF within the substantially modified binders. Upon completion of 30 K cycles of Rhy HFST, M1 outperformed Epoxy in terms of COF retention. However, after 140 K cycles, Epoxy once again displayed the greatest COF, demonstrating its ability to sustain friction performance under extended polishing. After 140 K cycles, A16 had the highest proportion of COF loss, followed by CB (37.5%) and Rhy HFST (73%). In comparison, Epoxy had the lowest COF loss, with just 16% for CB and 37% for Rhy HFST, demonstrating its better ability to retain friction even under high-traffic and continuous-polishing conditions. This emphasizes the effectiveness of Epoxy in retaining skid resistance over time. Furthermore, CB regularly showed a higher COF in high-traffic conditions, demonstrating its applicability for applications demanding long-term durability and performance. The varying degrees of friction loss across different Epoxy samples can be attributed to the distinct polishing resistances of the aggregates used, which are influenced by their hardness, angularity, and surface texture.

For the larger aggregate size, CB continued to show higher COF values than Rhy, prior to and following polishing. Upon polishing, the COF values for coarse aggregates were different compared to the original HFST-sized aggregates. Specimens fabricated from asphalt-based binders exhibited COF values that were similar to or exceeded those of the slabs made with Epoxy resin. For CB, PG88-16 (PM) had the greatest COF after 140 K polishing cycles. For the Rhy coarse size, both highly polymer-modified binders outperformed Epoxy, with PG82-22 (PM) performing the best. Among the two emulsions, B2LA16 achieved results comparable to Epoxy resin. The results at 30 K and 70 K cycles for both coarse-sized aggregates demonstrated comparable COF values for all binders. This shows that employing larger aggregate sizes can be beneficial in maintaining friction performance over long polishing cycles, particularly for highly modified binders. A previous study has demonstrated that typical emulsions, such as CRS-2P, fail to sustain performance after 30,000 cycles. However, as shown in Figure 9, highly modified emulsions demonstrated better durability even when the number of polishing cycles increased [10].

After 140 K polishing cycles, the COF dropped the least in CB with Epoxy. PG82-22 (PM), on the other hand, performed better for Rhy, with a COF loss of 37.70% vs. 77% for Epoxy.

3.3. Influence of Aggregate Size, Gradation, and Binder Type on MPD (mm)

The CTM served to quantify the surface macrotexture of the slabs, determining the MPD. Figure 10 illustrates the average profile depth assessments taken before and following a series of polishing cycles for various combinations of slabs created with CB and Rhy, each featuring two varying gradations (HFST and coarse). The samples with Epoxy resin demonstrated the lowest change in MPD as the polishing cycles increased to 140 K, indicating more stable texture retention over time. Epoxy, being a thermosetting material, maintains a very stiff bond with the aggregate, effectively keeping it in place throughout the testing process.

Among the highly modified asphalt-based binders for CB HFST, M2 showed the lowest loss in MPD, with only a 22.56% reduction after 140 K polishing cycles. M1 exhibited a similar trend, performing comparably well with a 25.80% reduction in MPD. For the Rhy HFST, M1 demonstrated better performance until 70 K, but when increasing to 140 K cycles, PG82-22 (PM) outperformed the highly modified binders, with a 41.4% reduction in MPD. A16 showed the highest decrease in MPD. Epoxy showed a slight linear decrease in MPD in comparison to other binders in the coarse aggregate sizes, confirming its capacity to maintain a more stable MPD throughout prolonged polishing cycles. After 140 K polishing cycles, PG88-22 (PM) retained the highest MPD among the modified binders, whereas M2 performed at a comparable value. These findings highlight that Epoxy resin has better texture retention as well as the inconsistent performance of asphalt-based binders; PG82-22 (PM) is the most successful substitute among the highly modified binders for preserving surface macrotexture under wear conditions. Among all the binders tested, A16 exhibited the lowest MPD after 140 K polishing cycles for both aggregate types and sizes, indicating a need for improvement in maintaining the macrotexture under simulated traffic conditions.

3.4. Binder Bond Strength (BBS) Test Results

Figure 11 depicts the BBS findings for different binders under both dry and wet conditions. Epoxy resin has the highest BBS values, which is much higher than any other binder. Interestingly, under wet conditions after 24 h in a 40 °C water bath, the BBS of Epoxy resin increased slightly, but the BBS of the other binders declined. With the exception of Epoxy, PG88-16 (PM) and M2 showed the highest BBS values in dry conditions. PG82-22 (PM) and M1 came next. However, after 24 h of conditioning in a water bath at 40 °C, all asphalt-based binders showed a drop in BBS values. CRS-2P had higher BBS values than A8 and A16, suggesting higher adhesive characteristics. However, the stiffness of CRS-2P led to issues, such as bleeding and rutting, which negatively impacted the BPN values, as illustrated in Figure 10. The inclusion of adhesion additives improved the BBS values of B2LA8 and B2LA16 compared to A8 and A16, enhancing their performance during the tests. This improvement highlights the importance of adhesion additives in boosting the bond strength and overall performance of these emulsions, particularly under challenging conditions.

4. Conclusions

HFST is a leading pavement treatment for improving skid resistance and decreasing traffic accidents. Although Epoxy-based HFST systems are commonly adopted, they present with challenges, such as compatibility issues, high costs, and long-term performance challenges, especially on pavements with poor substrate quality. This study evaluated the performance of highly modified asphalt-based binders in HFST applications, comparing them to Epoxy-based systems. A range of durability and performance experiments were conducted, including the BPT, British Wheel Test, DFT, CTM, and BBS test.

Among the tested aggregates, CB performed best across multiple metrics: the BPN, COF, and macrotexture stability. CB exhibited the highest BPN values, both prior to and following polishing, making it suitable for HFST applications. However, the binder selection had a big impact on its performance over time. The CRS-2P emulsion showed a higher decrease in BPN after 10 h of polishing; CB HFST had a 28.57% reduction; and Rhy HFST had a 22.58% reduction. B2LA16 had the lowest BPN loss, 12.86% for CB and 10.34% for Rhy, suggesting it may be a viable option for assuring long-term skid resistance under traffic loads.

For the evaluation of friction performance by COF measurement, CB outperformed Rhy, especially under high-traffic conditions. After 30 K polishing cycles, Epoxy had the highest COF for CB HFST; M1 outperformed Epoxy for Rhy HFST. After 140 K cycles, Epoxy again had the best COF retention, with a 16% loss for CB and 37% for Rhy. A16 showed the highest COF loss at 140 K cycles, with 37.5% for CB and 73% for Rhy.

For the coarse aggregate, CB again had a higher COF than Rhy both before and after polishing. After 140 K cycles, PG88-16 (PM) exhibited the highest COF for CB, and PG82-22 (PM) indicated the best performance for Rhy, with only a 37.70% COF loss compared to the 77% loss of Epoxy. Therefore, highly modified polymer binders can outperform Epoxy in certain conditions, especially with a larger aggregate size.

The macrotexture analysis by CTM supports these trends. The Epoxy samples had the least MPD change after 140 K polishing cycles, so they can retain their surface texture over time. Among the highly modified binders, M2 had the lowest MPD loss (22.56%) for CB HFST, followed by M1 (25.80%). For Rhy HFST, M1 showed good performance initially, but by 140 K cycles, PG82-22 (PM) had the best macrotexture retention with a 41.4% MPD reduction.

Epoxy resin showed the highest BBS values, even increasing slightly under wet conditions. PG88-16 (PM) and M2 followed, with strong dry-condition BBS values, though all asphalt-based binders experienced a decrease after 24 h of water conditioning. CRS-2P showed a higher BBS than A8 and A16, but suffered from stiffness-related issues. The differences in BBS values among the various binders can be partly attributed to their chemical composition and how they interact with the aggregate surface. Key factors include the binder’s polarity, the presence of functional groups that can form bonds, and how well the binder matches the mineral composition of the aggregate.

The results show that the right combination of aggregate size and binder type is important for HFST performance. For long-term performance, Epoxy remains the most effective binder. Nonetheless, the polymer-modified binder (PG82-22 (PM)), the highly modified emulsion (B2LA16), and the mastic binder (M2) demonstrated strong durability and friction retention among the highly modified asphalt-based binders. These binders serve as cost-effective alternatives for HFST applications, especially in pavements with marginal conditions and in sections that require increased friction and are prone to traffic crashes but experience lower traffic volumes. These investigations provide valuable insights for agencies to apply HFSTs on secondary roads, expanding its use beyond high-volume roadways.