Laboratory and Field Performance Evaluation of NMAS 9.5, 8.0, and 5.6 mm SMA Mixtures for Sustainable Pavement

Abstract

This study evaluates the laboratory and field performance of stone mastic asphalt (SMA) mixtures with nominal maximum aggregate sizes (NMAS) of 9.5, 8.0, and 5.6 mm. Aggregates and fine aggregates of these sizes were produced using an impact crusher and a polyurethane screen. Mix designs for SMA overlays on aged concrete pavement were developed. Laboratory tests assessed rutting performance using full-scale accelerated pavement testing (APT) equipment and reflective cracking resistance using an asphalt mixture performance tester (AMPT). Field evaluations included noise reduction using CPX equipment, skid resistance using SN equipment, and bond strength using field cores. Results showed that for 8.0 mm SMA mixtures to achieve the same rutting performance as 9.5 mm SMA, PG76-22 grade binder was required, whereas 5.6 mm SMA required PG82-22. The 8.0 and 5.6 mm SMA mixtures showed 22.2% and 25% reduced crack progression, respectively, compared with the 9.5 mm SMA mixtures. Field evaluations indicated that 8.0 mm and 5.6 mm SMA pavements reduced tire–pavement noise by 1.7 and 0.8 dB, increased skid resistance by 8.5% and 2.0%, and enhanced shear bond strength by 150%, compared with 9.5 mm SMA. Overall, the 8.0 mm SMA mixture on aged concrete pavement demonstrated superior durability and functionality toward sustainable pavement systems.

1. Introduction

Stone mastic asphalt (SMA) was developed in Germany in the 1960s to enhance resistance to rutting by improving aggregate interlocking and increasing asphalt mastic content for greater crack resistance. SMA is known for its ability to resist rutting in high-temperature conditions and heavy-traffic areas, improve skid resistance, reduce noise, and enhance resistance to potholes and cracks [1]. In Superpave mix design, nominal maximum aggregate sizes (NMASs) are defined as “one sieve size larger than the first sieve to retain more than 10% of the aggregate by weight [2]”. Smaller NMAS hot asphalt mixtures are known for their superior fatigue crack resistance, but they are vulnerable to rutting. Research indicates that a 9.5 mm NMAS mixture performs better against fatigue cracking than a 13.5 mm NMAS mixture does [3]. Also, many studies have revealed that NMAS has a significant impact on the performance of SMA mixtures. Laboratory tests on SMA mixtures with a NMAS of 30, 20, 13, and 15 mm showed that increasing NMAS enhances rutting resistance but reduces resistance to cracking and raveling. In addition, it was found that as NMAS decreases, the permeability also decreases [4]. In the Unites States, 19.0 and 12.5 mm NMAS SMA mixtures are commonly used, but studies are also being conducted on the mix designs and durability of fine-SMA mixtures (4.75 and 9.5 mm NMAS) for thin overlays. The results indicate that fine SMA mixtures have rutting resistance and lower permeability [5]. It was found that 16 mm NMAS SMA mixtures performed better than 13 mm NMAS mixtures [6]. Also, 14 mm SMA mixtures showed a higher macro-texture compared with 10 mm NMAS mixtures. Further, 5 and 13 mm NMAS mixtures were more suitable for water-proof pavement surfaces [7,8]. Asphalt overlays on aged Portland cement concrete pavement are prone to reflective cracking. Field performance evaluations of 9.5 mm SMA overlays have revealed excellent reflective cracking performance for over 8 years of service life [9]. Durable and functional SMA mixtures are being considered for overlays on aged jointed cement concrete pavement. The NMASs of 9.5 and 25 mm of SMA mixtures in the USA and China were successfully used. And an NMAS of less than 9.5 mm was limited to field and laboratory performance evaluation [10,11]. Therefore, this study produced aggregates, designed mixes, and conducted laboratory and field tests for SMA mixtures with NMASs of 8.0 mm and 5.6 mm in addition to the typical 9.5 mm.

The study’s objectives were (1) to evaluate the rutting performance and reflective cracking resistance of SMA mixtures with NMAS of 9.5, 8.0, and 5.6 mm through laboratory tests, (2) to assess tire–pavement noise, skid resistance, and bond performance with aged concrete pavement through field tests, and (3) to determine the optimal NMAS for mix designs used in SMA overlays on aged concrete pavement, based on a comprehensive assessment of durability and functionality by NMAS.

2. Experimental Program and Analysis Methods

2.1. Material and Mix Design

This study used SMA mixtures with NMASs of 9.5, 8.0, and 5.6 mm for asphalt overlays on aged cement concrete pavement. Granite aggregates with a maximum aggregate size of 25 mm were produced using the impact crusher and polyurethane screen shown in Figure 1. To produce aggregates of 9.5, 8.0, 5.6 mm and fine aggregate, a vibrating crusher screen with sieve sizes of 15, 11, 9, 7, and 5 mm was installed.

The aggregate gradation for the maximum aggregate size of 9.5 mm used sieve passing percentages of 12.5, 9.5, 4.75, 2.36, 0.59, 0.30, 0.15, and 0.076 mm, as shown in Figure 2a. For the maximum aggregate size of 8.0 mm, the gradation used sieve passing percentages of 9.5, 8.0, 4.75, 2.36, 0.59, 0.30, 0.15, and 0.076 mm, as shown in Figure 2b. Last, the gradation for the maximum aggregate size of 5.6 mm used sieve passing percentages of 9.5, 5.6, 2.36, 0.59, 0.30, 0.15, and 0.076 mm, as shown in Figure 2c.

Modified asphalt with a performance grade (PG) of 76-22 was used according to AASHTO M 320 [12]. In addition, the multiple stress creep recovery test results according to AASHTO T 350 [13] showed that the elastic recovery rate and rutting resistance were 64.2% and 0.29 kPa⁻¹, respectively.

The SMA mix designs were prepared using a Marshall compactor with 75 blows on each side. This method is in accordance with the construction specifications established by South Korea’s Ministry of Land, Infrastructure, and Transport. The mix designs for 9.5, 8.0, and 5.6 mm SMA mixtures were performed separately for the upper and lower layers. The target air voids for the SMA pavement upper layer, which requires rutting resistance, were set at 2–3%. For the lower layer, which requires reflective cracking resistance and enhanced bonding with aged concrete pavement, the target air voids were set at 1–2%. The mix design criteria and results for the asphalt content, voids in mineral aggregate (VMA), and voids filled with asphalt (VFA) for the six mixtures are shown in

Table 1. Mix design results of 9.5, 8.0, and 5.6 mm SMA mixtures.

Item		Criteria	9.5 mm SMA		8.0 mm SMA		5.6 mm SMA
			Upper	Lower	Upper	Lower	Upper	Lower
Air Void (%)		1 to 3	2.8	1.9	2.2	1.6	2.1	1.7
Asphalt Binder Content (%)		Above 6.9	6.9	7.2	7.0	7.2	7.2	7.4
VMA (%)		Above 18	18.2	18.0	18.1		18.5
VFA (%)		Above 75	84.6	85.0	87.9	91.2	88.4	91.0
Cellulous Fiber Content (%)			0.5
Drain Down (%)		Below 0.3	0.18	0.14	0.15	0.17	0.17	0.18
Dynamic Stability (cycles/mm)		Above 2500	7417	6721	7277	6593	6772	5889
Cantabro Loss (%)	Below 12	Below 12	4.3	3.9	4.1	3.8	3.6	3.1
	Below 6	Below 6	10.9	9.1	10.7	8.8	9.6	7.5

. To prevent asphalt draining down in the SMA mixtures, which have a higher asphalt content compared with conventional mixtures, cellulose fiber was added as 0.5% of the mixture. The durability evaluation criteria are presented in Table 1, and the results indicate that all quality standards were met.

The durability evaluation conducted during the mix design of SMA mixtures assessed rutting resistance at high temperatures and raveling resistance at ambient and low temperatures. Rutting resistance was evaluated according to AASHTO T 340 [14], “Determining Rutting Susceptibility of Hot Mix Asphalt Using the Asphalt Pavement Analyzer (APA)”, whereas raveling resistance was evaluated according to AASHTO T 401 [15], “Cantabro Abrasion Loss of Asphalt Mixture Specimen” as shown in

Table 2. Durability test of 9.5, 8.0, and 5.6 mm SMA mixtures.

Drain Down	Dynamic Stability	Cantabro Loss

. The durability evaluation results show that as the maximum aggregate size and design air voids of the SMA mixture decrease, the optimum asphalt content increases, resulting in reduced rutting resistance but enhanced raveling resistance. The cross-sections of the mixtures produced by the gyratory compactor for each maximum aggregate size are shown in

Table 3. Cross-sections of SMA mixtures.

9.5 mm SMA	8.0 mm SMA	5.6 mm SMA

.

2.2. Full-Scale Accelerated Pavement Test

To evaluate the rutting performance of SMA mixtures, a full-scale accelerated pavement test was conducted. Accelerated pavement testing assesses the performance of pavement structures under controlled vehicle and environmental load conditions in a short period. In this study, the rutting performance of SMA mixtures with maximum aggregate sizes of 9.5, 8.0, and 5.6 mm, and using the modified asphalts PG 76-22 and PG 82-22, was evaluated. The test was performed using a dual wheel with a tire pressure of 110 psi (0.76 MPa), and a load of 8.2 tons was applied. The number of load applications was determined based on achieving a rut depth of 13 mm. To maintain the actual road temperature during the vehicle test, a thermocouple installed at a depth of 2.5 cm from the pavement surface was used to keep the pavement temperature at 50 ± 2 °C.

2.2.1. Test Bed Preparation

As shown in Figure 3 and Figure 4, to evaluate the rutting performance of the six types of SMA mixtures, a crushed aggregate sublayer, asphalt base layer, and surface layer were constructed with thicknesses of 37, 10, and 4 cm, respectively. The first lane of the surface layer was constructed using PG 76-22 modified asphalt with SMA pavements with maximum aggregate sizes of 5.6, 8.0, and 9.5 mm, in that order. The second lane was constructed using PG 82-22 modified asphalt with SMA pavements having maximum aggregate sizes of 9.5, 8.0, and 5.6 mm, in that order. Each lane was constructed with a width of 240 cm.

To evaluate the field compaction of the six types of SMA mixtures, the air voids of the mixtures produced in the laboratory during mix design were compared with those of the core samples from the accelerated pavement test, as shown in

Table 4. Compactability results of SMA mixtures.

SMA Mixture		Theoretical Maximum Specific Gravity	Air Void
NMAS	PG		Mix Design	Core
9.5 mm	76-22	2.401	2.8	5.3
	82-22	2.400	2.9	5.3
8.0 mm	76-22	2.385	2.8	4.6
	82-22	2.384	2.8	4.6
5.6 mm	76-22	2.381	2.6	4.4
	82-22	2.380	2.7	4.5

. The air void range for the mix design was 2.5–3.0%, whereas the field core samples showed an air void range of 4.5–5.5%. Compared with the 9.5 mm SMA mixture, the air voids of the 8.0 mm and 5.6 mm mixtures decreased by 13% and 16%, respectively, indicating improved compactability.

2.2.2. Rut Depth Analysis Method

The rutting deformation was measured using a laser profiler at three points: the center of each test section and 50 cm in front and behind it, as shown in Figure 5. To reduce measurement errors caused by the surface texture of the asphalt pavement, the surface was painted before each measurement of the rutting depth. The rutting depth was evaluated by measuring the difference between the lowest and highest points recorded by the laser profiler.

2.3. Reflective Cracking Test

The lower layer of the asphalt overlay pavement must resist the reflective cracking caused by the thermal expansion and contraction behavior of joints within the unreinforced concrete pavement. The reflective cracking resistance of 9.5, 8.0, and 5.6 mm SMA mixtures with 1–2% air voids was evaluated using the Tex-F 248 (2019) method [16].

2.3.1. Specimen Preparation

Reflective cracking test specimens were prepared using the gyratory compactor shown in Figure 6a to create cylindrical specimens with a diameter of 150 mm and a height of 180 mm. After trimming the top and bottom of the gyratory compacted cylindrical specimens by 33 mm each, three test specimens with a diameter of 150 mm and a height of 38 mm were created from the central portion. Finally, each specimen was trimmed to a height of 37 mm on both sides, as shown in Figure 6b. The reflective cracking test specimens were attached to a base plate for the reflective cracking test using epoxy and a 4.2 mm spacer bar, and they were then installed in the asphalt mixture performance tester (AMPT) equipment for testing, as illustrated in Figure 6c.

2.3.2. Testing Protocol and Analysis Method

The reflective cracking resistance of the modified SMA mixtures was evaluated using the AMPT equipment according to Tex-F 248. The test was conducted at 25 °C with a cyclic triangular waveform tensile load, applying a constant displacement of 0.06 cm at a loading rate of 0.1 Hz. The cyclic loading was applied until the maximum initial load decreased by 93%.

The reflective cracking resistance performance was assessed through the critical fracture energy and crack resistance index. The critical fracture energy evaluates the resistance to initial crack occurrence by calculating the fracture energy generated during the initial loading. The crack resistance index evaluates the crack progression rate by calculating the rate of load reduction to 93% of the initial load by using the exponent of a power equation.

The state of Texas proposes that the criteria for the laboratory mix design of asphalt mixtures used in thin asphalt overlays should have a critical fracture energy of 1.5 or higher and a crack progress rate of 0.4 or lower.

3. Field Performance Evaluation and Analysis Methods

3.1. Field Trial Construction

For the remodeling of aged, jointed plain concrete pavement, SMA pavement was overlaid to a thickness of 10 cm, as shown in Figure 7 and

Table 5. Field trial construction of SMA pavements.

9.5 mm SMA		8.0 mm SMA		5.6 mm SMA
Aged jointed plain concrete pavement (JPCP) surfaces

. To evaluate the tire–pavement noise and skid resistance of the SMA pavement, the upper layer of the overlay was constructed with 9.5, 8.0, and 5.6 NMAS SMA pavements, whereas the lower layer was constructed with the commonly used 9.5 mm SMA pavement, as shown in Figure 7a–c. In addition, to assess the bonding between the aged jointed concrete pavement and the SMA overlay pavement, the upper layer was constructed with the commonly used 9.5 mm NMAS SMA pavement, and the lower layer was constructed with 9.5, 8.0, and 5.6 mm SMA pavements, as shown in Figure 7a,d,e. Considering the compactability, the 8.0 and 5.6 mm NMAS SMA pavements were constructed with a thickness of 40 mm, and the 9.5 mm NMAS SMA pavement was constructed with a thickness of either 40 or 60 mm, maintaining a total overlay thickness of 10 cm. Each test section was constructed with a minimum length of 150 m.

3.2. Tire–Pavement Noise Measurement Using Close Proximity Method (CPX)

The tire–pavement noise measurements for the test sections were conducted following the method outlined in ISO 11819-2 [17], “Acoustics-Measurement of the influence of road surfaces on traffic noise—Part 2: The close-proximity method”. The microphones for noise measurement were installed inside a trailer vehicle with a cover, as shown in

Table 6. Tire–pavement noise and skid friction test equipment.

(a) Tire–pavement noise measurement trailer	(b) Skid friction tester

(a). The microphones were positioned to maintain angles of 45° and 135°, relative to the driving direction. Considering road safety during measurements, the test vehicle speeds were set to 80 km/h and 100 km/h. The static load of the test tires was maintained at 3200 N ± 200 N, with a tire inflation pressure of 200 ± 10 kPa. Ambient air temperature was recorded, and noise measurements were conducted three times for each of the three sections shown in Figure 7a–c, averaging the results for analysis.

3.3. Skid Resistance Test

To evaluate the skid resistance of the SMA pavement surface, the test was conducted according to ASTM E 2340 [18], “Measuring the Skid Resistance of Pavements and Other Trafficked Surfaces Using a Continuous Reading, Fixed-Slip Technique”. The skid friction tester used in this study was installed in a trailer format, as shown in Table 6 (b). The trailer was equipped with a water spray system to simulate wet pavement conditions, a braking device for the test wheel, and an instrumentation system to measure traction. During the skid resistance test, the vehicle was driven at a speed of 65 km/h. Approximately 0.5 s after the water spray started, the brake was applied with 100% force to lock the wheel, and traction was measured at intervals of 1 to 3 s. The friction coefficient of the skid number was calculated by dividing the traction by the dynamic vertical load applied to the tire. Measurements were taken three times for each of the three sections shown in Figure 7a–c, and the results were averaged to determine the skid resistance values.

3.4. Shear Bond Strength Test

To measure the shear bond strength between the asphalt overlay lower layer and the aged concrete pavement after the SMA pavement test construction, three 150 mm diameter field cores were extracted from each section shown in Figure 7a,d,e, as illustrated in Figure 8a,b. The test was conducted according to Tex-249-F [19], “Shear Bond Strength Test”. Specimens with a total thickness of 10 ± 0.4 cm, each of which had an upper asphalt layer and a concrete layer of 5 ± 0.2 cm, were prepared and evaluated using the circular jig shown in Figure 8c. The test was performed in an environmental chamber capable of maintaining a temperature of 20 °C, and the direct shear bond strength was evaluated at a rate of 1 mm/min in the actuator displacement mode of the testing machine.

4. Discussion of Results

This section explains the results of evaluating the rutting performance and reflective cracking resistance of SMA mixtures with different maximum aggregate sizes through laboratory experiments. In addition, it presents the results of performance evaluations measured from the test construction of SMA pavements with varying maximum aggregate sizes, including tire–pavement noise, skid resistance, and bonding performance with aged concrete pavement.

4.1. Accelerated Pavement Test

As shown in Figure 9a,b, for SMA mixtures with PG 82-22, the 8.0 mm mixture exhibited the same rutting performance as the 9.5 mm mixture did, whereas the 5.6 mm mixture demonstrated a 15% decrease in rutting resistance. For SMA mixtures with PG 76-22, the rutting resistance of the 8.0 and 5.6 mm mixtures decreased by 2.8% and 10.3%, respectively, compared with the 9.5 mm mixture.

Additionally, as indicated in Figure 9c–e, the rutting performance of the 9.5, 8.0, and 5.6 mm SMA mixtures with modified asphalt PG 82-22 increased by 11.8%, 14.2%, and 7.9%, respectively, compared with those with PG 76-22. Finally, to achieve the same rutting performance as that of the commonly used 9.5 mm SMA pavement with PG 76-22 modified asphalt, the 8.0 mm SMA pavement requires the PG 76-22 grade, and the 5.6 mm SMA pavement requires the PG 82-22 grade.

4.2. Reflective Cracking Test

According to the reflective cracking test results (Figure 10), the initial crack resistance of the 8.0 and 5.6 mm SMA mixtures decreased by 10.5%, compared with the 9.5 mm SMA mixture. Also, the crack propagation rates were found to decrease by 22.2% and 25%, respectively. That is, the 8.0 and 5.6 mm SMA mixtures had similar improvements in reflective cracking resistance, compared with the 9.5 mm SMA mixture.

4.3. Tire–Pavement Noise Measurement Using the Close Proximity Method (CPX)

According to the CPX measurement results of the tire–pavement noise (Figure 11), at a speed of 100 km/h, the noise levels of the 8.0 and 5.6 mm SMA pavements were reduced by 1.7 and 0.8 dB, respectively, compared with those of the 9.5 mm SMA pavement. Similarly, at a speed of 80 km/h, the noise levels of the 8.0 mm and 5.6 mm SMA pavements were reduced by 1.8 and 0.9 dB, respectively, compared with the 9.5 mm SMA pavement. The results of the tire–pavement noise performance evaluation using CPX indicated that reducing the maximum aggregate size did not significantly increase the noise reduction effect. In this study, the 8.0 mm SMA pavement was evaluated to have a greater noise reduction effect than the 6.0 mm SMA pavement because of the noise scattering caused by the texture of the tire and the SMA pavement surface. As new asphalt overlays are aged, the effect of noise reduction is also reduced. Further study should include the tire–pavement abrasion and environmental effect on the noise levels.

4.4. Skid Resistance Test

The results of the skid pavement skid resistance test indicated that the resistance of the 8.0 and 5.6 mm SMA pavements increased by 8.5% and 2.0%, respectively, compared with that of the 9.5 mm SMA pavement. This is consistent with the tire–pavement noise reduction effect, where the 8 mm SMA pavement exhibited higher skid resistance, compared with that of the 5.6 mm SMA pavement. These results indicate that the surface texture of the 8 mm SMA pavement is advantageous for skid resistance.

In the literature, it is well-established that skid resistance is influenced by both the micro and macro texture of pavement surfaces (Figure 12). Microtexture refers to the small irregularities present on the surface of aggregate particles, while macrotexture is characterized by the larger irregularities in the pavement surface, including the voids between coarse aggregate particles [20,21,22]. Microtexture primarily affects skid resistance at low speed levels, whereas macrotexture becomes critical at higher speed levels. Recent findings indicate that macrotexture also significantly influences rolling resistance. Two distinct types of macrotextured surfaces have been described: positively macro-textured surfaces, which are produced by the excessive height of aggregates, and negatively macro-textured surfaces, which arise from voids beneath the interface between tires and pavements [23,24]. In this study, it was observed that an 8 mm SMA pavement provides a positively macro-textured surface compared to the others. However, it is important to note that this study is limited by the amount of data available. Future research should aim to include a larger dataset to validate these findings.

4.5. Shear Bond Strength Test

The test results of the cores taken from the wheel paths and between the wheel paths of the trial construction section indicate that the shear bond strength between the aged concrete and the SMA overlay pavement increased by an average of 150% in the 8.0 mm and 5.6 mm SMA pavements, compared with that of the 9.5 mm SMA pavement (Figure 13). Also, it should be noted that this investigation did not consider the type of tack coat materials. Further research should include additional experiments with types of tack coats, milling methods, and so on.

5. Conclusions

The main findings of this study are as follows.

• The mix design and durability test results for the SMA mixtures indicate that as NMAS and design air voids decrease, the optimal asphalt content increases and thereby reduces the rutting resistance, improving the raveling resistance.
• The results of the indoor accelerated pavement tests show that to achieve the same rutting performance as that of the 9.5 mm SMA pavement with modified asphalt PG76-22, 8.0 mm SMA pavement requires the PG76-22 grade, and 5.6 mm SMA pavement requires the PG82-22 grade.
• Compared with 9.5 mm SMA mixtures, indoor reflective cracking tests indicated that 8.0 and 5.6 mm SMA mixtures had crack progression rate decreases of 22.2% and 25%.
• Compared with 9.5 mm SMA pavements, CPX measurements showed that at a speed of 100 km/h, the noise was reduced by 1.7 and 0.8 dB for 8.0 and 5.6 mm SMA pavements. At a speed of 80 km/h, the noise was reduced by 1.8 and 0.9 dB for 8.0 mm and 5.6 mm SMA pavements, respectively. SN measurements indicated that the skid resistance increased by 8.5% and 2.0% for 8.0 and 5.6 mm SMA pavements. Shear bond strength between the aged concrete and the SMA overlay pavement increased by an average of 150% for 8.0 and 5.6 mm SMA pavement.
• Overall, the 8 mm NMAS SMA mixture had superior durability and functionality for overlaying aged concrete pavements.