Design of Volume Parameters of Large-Particle-Size Asphalt Mixture Based on the Vertical Vibration Compaction Method

Abstract

Volume parameters such as the volume of voids (VV), voids filled with asphalt (VFA), and voids in mineral aggregates (VMA) all have significant impact on asphalt mixtures. In this study, the vertical vibration compaction method (VVCM) was employed to produce a large-particle-size asphalt mixture (LSAM-50). The correlations between the mechanical strengths of VVCM specimens, static compression test (PCT) specimens, and in situ core samples were verified. Additionally, the influence of volumetric parameters on the mechanical properties of VVCM specimens was assessed. Based on the principle of optimal mechanical properties, volume parameter design standards for the LSAM-50 asphalt mixture were proposed. Results indicated that the mechanical properties correlation between VVCM specimens and in situ core samples was substantial, reaching over 90%. With increasing VV and VFA, the compressive strength, splitting strength, and dynamic stability of the LSAM-50 asphalt mixture initially increased and then decreased. The design standards for VV were proposed to be between 3.5% and 4.8%, and for VFA between 49.7% and 52.9%. There was no clear correlation between VMA and the mechanical properties of the mixture; hence, based on the standards, the minimum design value for VMA was set at 7.5%.

1. Introduction

In recent years, large-size asphalt mixtures, characterized by their superior rutting resistance, durability, reduced asphalt usage, and increased single-layer paving thickness, have garnered attention from road engineers and researchers worldwide. Yuan et al. conducted research on the anti-reflective cracking properties of large-particle-size porous asphalt mixtures and established a model for estimating the fatigue life of anti-reflective cracking based on the specimen height and void ratio [1]. In response to the characteristic high porosity of porous asphalt mixtures, Li et al. classified all mineral materials into four categories: large particle aggregates, primary selling materials, Physical Distortion Aggregates (PDA), and interstitial aggregates. Li analyzed the primary framework void structure formed by the main aggregates, determined the particle size range for each type of aggregate, and proposed a method for calculating the void ratio of the primary framework [2]. Jiang et al. conducted numerical tests of the uniaxial penetration of an asphalt mixture to analyze the influencing factors of shear strength and shear cracking mechanism of the asphalt mixture [3]. Mascarenhas, et al., described the rehabilitation of two test sections in Brazil using a composite of dense graded asphalt mixtures with typical (9.5 or 19 mm) and large-stone (32 mm) nominal maximum aggregate size; the results demonstrate that the large-stone asphalt mixture is a promising approach to improve rutting resistance of roads under very heavy traffic [4]. Li et al. analyzed the dynamic properties, crack resistance, and mechanical resistance of asphalt mixtures through indoor experimental methods and a multifunctional pavement material tester [5,6,7,8]. Regarding the forming methods for large-particle-size asphalt mixtures, Jiang, Hu, et al. have found that the vibratory molding method is more suitable for large-particle-size asphalt mixtures and skeleton-dense asphalt mixtures [9]. Takahashi O, et al. observed that when forming specimens of Φ152 mm using the heavy Marshall compaction method, compacting both sides over 75 times can easily lead to extensive damage to the main aggregates [10]. Wang studied the VVCM method for AC-25 mixtures, recommended related parameters, and verified its road performance [11]. Regarding volume parameters, Liu et al. examined the relationship between VV and the high-temperature performance of asphalt mixtures. They found that the maximum penetration load and shear strength decreased with increasing VV when the asphalt type, compaction effort, and gradation remained constant [12]. Lee, Yu, and Ge found that a high VV value can reduce the water stability and fatigue life of asphalt mixtures, while a low VV value can decrease their rutting resistance [13,14,15]. Jiang, Zhang analyzed the influence of volume parameters on asphalt mixtures, particularly examining the effects of VV, VFA, and VMA on the performance of ATB-30 asphalt mixtures. Their findings revealed a strong correlation between VV, VFA, and the mechanical properties of the asphalt mixture, but a weaker relationship with VMA [16,17]. Some researchers believe that VMA is an important recommended value in asphalt mixture design; however, other studies indicate no correlation between the minimum VMA value and road performance, suggesting the evaluation of asphalt mixture durability based on asphalt film thickness instead of VMA [18,19,20,21,22,23].

However, most existing studies focus on asphalt mixtures with a maximum nominal particle size of ≤37.5 mm. Research on asphalt mixtures with a particle size >37.5 mm remains limited. For large-particle-size asphalt mixtures, the change in aggregate size means that traditional forming methods and volume parameter evaluation metrics may no longer be appropriate. In this specific area, there is still limited existing research. To establish volume parameter standards suitable for LSAM-50 asphalt mixtures, this paper evaluates the mechanical properties of specimens formed by VVCM and PCT, comparing them with field core samples. Based on identifying the most reliable forming method, we investigate the influence of VV, VFA, and VMA on the mechanical performance of asphalt mixtures and propose volume parameter design standards for LSAM-50. This research further fills the gap in this field, providing control standards for the design and construction of large-particle-size asphalt mixtures. It contributes to advancing the application technology of such mixtures, offering significant theoretical and practical value.

2. Materials and Test Methods

2.1. Materials

2.1.1. Asphalt

The asphalt used in this study is Zhenhai Transportation 70# road petroleum asphalt, and its technical specifications were measured according to the Chinese specification JTG E20-2011 [24]. The results are shown in

Table 1. Asphalt technical specifications.

Items		Tested Results	Standard Value
Penetration (25 °C, 100 g, 5 s) (0.1 mm)		68	60~80
Ductility (5 cm/min, 15 °C) (cm)		>100	≥100
Softening point(°C)		47.3	≥46
Penetration index (PI)		−0.315	−1.5~+1.0
Flashpoint(°C)		290	≥260
Solubility in trichloroethylene (%)		99.6	≥99.5
Rolling Thin Film Oven Test (163 °C, 5 h)	Quality loss (%)	0.03	≤±0.8
	Ductility (10 °C) (cm)	7	≥6
	Penetration ratio (25 °C) (%)	63	≥61

.

2.1.2. Aggregate

In this study, limestone was used as aggregate and mineral powder, and its technical specifications were measured according to the Chinese specification JTG E42-2005 [25]. The results are shown in

Table 2. Technical indicators of aggregate.

Items	Aggregate Size (mm)
	37.5~53	19~37.5	9.5~19	4.75~9.5	2.36~4.75	0~2.36	Mineral Powder
Apparent relative density(g/cm3)	2.819	2.779	2.755	2.721	2.728	2.728	2.706
Flaky particle content (%)	2.2	7.9	7.3	13.1	/	/	/
Water absorption rate (%)	0.37	0.62	0.98	1.73	0.75	/	/
Crushed value (%)	18.5					/	/
Sturdiness (%)	3.7					7.3	/
Methylene blue value (g/kg)	/	/	/	/	/	3.4	/
Plasticity index (%)	/	/	/	/	/	/	3.7

.

2.1.3. Mixtures

In this study, six distinct aggregate gradation types for molded specimens were chosen. All gradations were based on the findings from the LSAM-50 asphalt mixture with strongly embedded skeleton-dense gradation [26], as detailed in

Table 3. Mineral aggregate gradations of LSAM-50 asphalt mixture.

Gradation	Percentage by Mass Passing through the Sieve Sizes (mm)
	53	37.5	19	9.5	4.75	2.36	1.18	0.6	0.3	0.15	0.075
A	100.0	58.0	46.0	40.0	35.0	29.2	21.7	16.3	12.0	8.2	3.8
B	100.0	70.0	60.0	42.0	34.0	26.0	18.0	14.0	10.0	7.5	4.5
C	100.0	65.0	55.0	37.0	30.0	22.0	14.0	10.0	7.0	5.0	3.0
D	100.0	75.0	65.0	47.0	38.0	30.0	22.0	18.0	13.0	10.0	6.0
E	100.0	72.5	62.5	42.0	32.0	24.0	16.0	12.0	8.5	6.3	3.8
F	100.0	67.5	57.5	42.0	36.0	28.0	20.0	16.0	11.5	8.8	5.3

.

2.2. Specimen-Preparation Methods

The specimens were prepared using the Vertical Vibration Compaction Method (VVCM), employing Vertical Vibration Test Equipment (VVTE) equipped with two symmetrical eccentric blocks to replicate the action of a typical field vibratory roller, as illustrated in Figure 1a. This setup ensures the stability of the VVTE by counterbalancing the horizontal components of centrifugal forces generated by the eccentric blocks, while allowing the vertical components to add up. Consequently, the VVTE experiences only a single vertical force. The operational parameters of the VVTE can be found in

Table 4. The working parameters of VVTE.

Upper-System Weight (kg)	Lower-System Weight (kg)	Work Frequency (Hz)	Static Eccentric Moment (kg/m)	Work Amplitude (mm)
122	180	40	0.215	1.2

.

The test specimens are prepared by drying the required aggregates at (105 ± 5) °C to a constant weight. Coarse aggregates above 19 mm are mixed with asphalt initially, followed by the addition of the remaining aggregates and thorough mixing. Mineral powder is then added to ensure complete mixing within a total duration of 180 s. The mixture is loaded into the mold upon specimen formation, and the VVTE vibration is applied for 90 s.

The LSAM-50 asphalt mixture has a nominal maximum aggregate size of 53 mm, and it appears that the use of large Marshall specimens (Φ152.4 mm × h93.5 mm) may no longer be suitable. Based on experimental findings [27], utilizing larger specimens (Φ200 mm × h160 mm) effectively eliminates the size effect. Therefore, the LSAM-50 asphalt mixture specimens are now standardized to the size of 200 mm × 160 mm.

2.3. Test Method

2.3.1. Uniaxial Compression Test

The compressive strength (R_C), as depicted in Figure 2a, was determined according to the Chinese specification JTG E20-2011 of T0713-2000. The test was conducted using a universal testing machine. The specimen was placed in a constant-temperature chamber at 60 °C for 8 h prior to testing its compressive strength. The loading rate during the test was 2 mm/min. The compressive strength is calculated using Equation (1):

$$R_C=\frac{4P}{\pi {\phi }^2}$$

(1)

where R_C is the compressive strength of the specimen, MPa; P is the maximum pressure at failure, KN; φ is the diameter of the specimen, mm.

2.3.2. Splitting Test

The splitting strength (R_T), as depicted in Figure 2b, was determined according to the Chinese specification JTG E20-2011 of T0716-2011. The test was conducted using a universal testing machine. The specimen was placed in a constant-temperature chamber at 60 °C for 8 h prior to testing its splitting strength. During the experiment, a 25.4 mm arc-shaped press bar was used proportionally, and the corresponding test fixtures were processed. The splitting strength is calculated using Equation (2):

$$R_T=\frac{2P}{\pi ah}\left(\sin 2\alpha -\frac{a}{b}\right)$$

(2)

where R_T is the splitting strength of the specimen, MPa; P is the maximum pressure of the specimen at fracture, N; a is the width of the indenter, mm; b is the diameter of the specimen, mm; h is the height of the specimen, mm; α is the angle of the circle corresponding to the width of the half indenter (°).

2.3.3. Rutting Test

The dynamic stability (DS), as depicted in Figure 2c, was determined according to the Chinese specification JTG E20-2011 of T0719-2011. Before the experiment, the specimens were preheated at 60 °C. The tests were conducted under the conditions of 60 °C and a load of 0.7 MPa.

The nominal maximum particle size of LSAM-50 asphalt mixture was 53 mm, and the existing rutting plate specimens of 50 mm to 100 mm thickness may no longer have been suitable. According to the preliminary test [28], the reasonable thickness, number of rolling passes, and insulation time of the rutting plate with a large thickness of LSAM-50 asphalt mixture were defined, as shown in

Table 5. LSAM-50 asphalt mixture large thickness rutting plate test conditions.

Thickness (cm)	Number of Lapping (Times)	Insulation Temperature (°C)	Insulation Time (h)
18	44	60	7

.

The dynamic stability was calculated according to Equation (3):

$$DS=\frac{\left(t_2-t_1\right)\times N}{d_2-d_1}\times C_1\times C_2$$

(3)

where DS is the dynamic stability of the specimen, times/mm; d₁ is the deformation corresponding to the loading time t₁, mm; d₂ is the deformation corresponding to the loading time t₂, mm; C₁ is the testing machine type coefficient, taken as 1.0; C₂ is the specimen coefficient, taken as 1.0; N is the round-trip crushing speed of the test wheel, times/min.

3. VVCM Evaluation

3.1. Specimen Appearance

The comparative appearance of the VVCM and PCT specimens is illustrated in Figure 3.

As observed in Figure 3, the PCT specimen exhibits large open pore area on its surface. In contrast, the VVCM specimen has relatively fewer open pores. This discrepancy stems from the fundamental difference in the effects of static pressure and vertical vibration excitation force. The VVTE excitation force, while causing aggregate displacement in the vertical direction, disrupts the relative positions of coarse and fine aggregates in all directions. This leads to the coarse aggregate voids being further compacted by the fine aggregates, and under the vertical excitation force, they are further compacted. The static pressure method’s vertical static force can only alter the vertical relative displacement of the aggregates. The efficiency of filling by fine aggregates is largely determined by the tamping work done prior to static pressing. When the coarse aggregate frameworks come into contact, the form of this contact obstructs further compaction by the static pressure. Continual application of static pressure to achieve the desired specimen size often results in significant fracture of the coarse aggregates, affecting the specimen’s mechanical properties.

3.2. Mechanical Properties

To assess the reliability of VVCM, the mechanical strength of both VVCM and PCT specimens was compared to that of field core samples. To mitigate testing discrepancies, both field and indoor LSAM-50 samples shared identical material compositions, as detailed in Table 1 and Table 2, and maintained consistent aggregate gradations, as shown in Table 3.

The mechanical strengths of the LSAM-50 asphalt mixture for VVCM, PCT specimens, and field core samples are presented in

Table 6. Comparison of the mechanical strengths of VVCM specimens and PCT specimens.

Strength Indexes	Forming Method	Parallel Specimen Strength Test Value (MPa)			$\overline{\mathit{R}}$ (MPa)	Cv (%)
		1	2	3
60 °C Rc	Core samples	3.65	3.87	3.46	3.66	5.6
	VVCM	3.32	3.49	3.26	3.36	3.6
	PCT	3.01	2.49	2.72	2.74	9.5
60 °C RT	Core samples	0.225	0.240	0.220	0.228	4.6
	VVCM	0.213	0.199	0.204	0.205	3.4
	PCT	0.178	0.203	0.166	0.182	10.4

. Each test was conducted three times. $\overline{R}$ is the mean intensity value, and C_v is the coefficient of variation.

As indicated in Table 6, the ratios of R_c and R_T between the VVCM specimens and the field core samples are 91.8% and 89.9%, respectively, while those for the PCT specimens are 74.8% and 79.8%, respectively. This suggests that the mechanical strength correlation between the VVCM specimens and the field core samples is stronger than that between the PCT specimens and the field core samples. Furthermore, the R_c and R_T values of the VVCM specimens are 1.23 and 1.13 times those of the PCT specimens, respectively, and the C_v value of the VVCM specimens is notably lower. This indicates that the mechanical strength of the VVCM specimens exceeds that of the PCT specimens and more closely approximates the mechanical strength of the field core samples, with more stable test results. Therefore, VVCM specimens produced in laboratory tests are more reflective of actual field conditions.

The higher mechanical strength observed in VVTM specimens can be attributed to the sinusoidal excitation force they experience during formation. This force ensures substantial relative movement between coarse and fine aggregates. While achieving optimal compaction, it minimizes damage to the internal coarse aggregate, leading to a more robust aggregate framework that provides greater internal friction resistance. In contrast, PCT specimens are exposed solely to a unidirectional vertical pressure during their formation. This limits the lateral relative position change between aggregates, reducing the efficiency of the compaction process. Additionally, this vertical pressure compromises the coarse aggregate structure, resulting in fractures in larger-sized stones and an increase in slip surfaces, consequently diminishing their mechanical strength.

The degree of coarse aggregate damage in VVTM specimens is generally lower, leading to relatively stable test values for specimen strength. In contrast, specimens formed by the static pressure method exhibit varied, and generally higher, degrees of coarse aggregate damage. This results in a pronounced reduction in the interlocking force provided by the coarse aggregate framework, and the extent of this reduction is inconsistent among specimens. This elucidates why the coefficient of variation for the mechanical strength of VVTM specimens is significantly lower than that of PCT specimens.

3.3. Gradation Variation

After forming the VVTM specimens and PCT specimens, a compaction mix extraction test was conducted. The gradation changes before and after extraction can be seen in

Table 7. Comparison of gradation crushing before and after compaction.

Contents	Percentage Passing (%) for Sieve Size (mm)
	53	37.5	31.5	26.5	19	16	13.2	9.5	4.75
Initial gradation	100.0	58.0	54.0	50.0	46.0	44.0	42.0	40.0	35.0
VVCM	100.0	59.2	55.6	51.8	48.1	46.0	43.6	40.6	35.6
PCT	100.0	65.4	58.4	55.7	49.4	47.2	45.0	42.7	37.2

. The differences in the pass rates for each grade after extraction can be seen in

Table 8. Differences in pass rates for each grade after extraction.

Contents	Percentage Passing (%) for Sieve Size (mm)									ΣΔ
	53	37.5	31.5	26.5	19	16	13.2	9.5	4.75
VVCM	0.0	1.2	1.6	1.8	2.1	2	1.6	0.6	0.6	11.5
PCT	0.0	7.4	4.4	5.7	3.4	3.2	3	2.7	2.2	30.0

.

Based on Table 8, it is evident that the degree of aggregate damage inside the PCT specimens is significantly higher than that in the VVTM specimens. The grade with the largest change in aggregate pass rate is 37.5 mm, with a variation of 7.4%, indicating that the damage rate for the 37.5~53 mm grade aggregate reaches 17.6%. This can be attributed to several factors. During the formation of PCT specimens, there is minimal relative displacement of the aggregates. The coarse aggregates embed and intrude each other and are forcibly fractured under vertical pressures exceeding the crushing value of the material. As the primary aggregate of the LSAM-50 mixture, the 37.5~53 mm grade aggregate provides a significant proportion of the specimen’s embedment force. The higher the degree of damage, the greater the decline in this embedment force. Furthermore, the substantial damage to the primary aggregate disrupts the original embedment structure. This not only results in a decreased load-bearing capacity of the overall mixture but also causes the mineral aggregate framework to be propped open by the fractured aggregates. However, without excess asphalt mortar to fill these voids, the porosity of the specimen increases. This subsequently leads to a reduction in the cohesion and internal friction of the mixture, causing a decline in the strength of the specimen.

In summary, specimens of LSAM-50 formed using the VVTM exhibit higher mechanical strengths, and the extent of internal aggregate damage is considerably lower. This ensures greater stability and reliability in indoor mechanical strength tests. Therefore, it is recommended to use the VVTM for molding specimens of LSAM-50 mixtures.

4. Influence of Volume Parameters on the Mechanical Properties of LSAM-50 Asphalt Mixture and Design Criteria

4.1. Influence of VV on the Mechanical Properties of LSAM-50 Asphalt Mixture and Design Criteria

4.1.1. VV Influencing Factors

VV denotes the proportion of void volume to the total volume in compacted asphalt mixtures. This parameter is pivotal in asphalt mixture design, profoundly affecting its mechanical properties. The value of VV is contingent upon aggregate gradation, asphalt–aggregate ratios, and compaction efforts. In the case of a certain compaction effort, the VV is primarily influenced by its aggregate gradation and asphalt–aggregate ratios.

The variation of VV with asphalt–aggregate ratios for LSAM-50 asphalt mixtures with different aggregate gradations is shown in Figure 4.

Figure 4 illustrates that for a consistent compaction effort, both the asphalt–aggregate ratio and aggregate gradation substantially influence the VV of the compacted asphalt mixture. For LSAM-50 mixtures with different aggregate gradations, there is an approximate linear decrease in VV as the asphalt–aggregate ratio increases. This can be attributed to the limited lubrication at lower asphalt contents, resulting in increased friction between mineral particles and insufficient compaction. As asphalt content rises, it fills the mineral voids, leading to a progressive reduction in the void rate.

Under consistent asphalt–aggregate ratios and compaction conditions, the VV for gradation B, which is finer, is notably lower than for the coarser gradation C. This indicates a pronounced filling effect by the fine aggregate within the coarse aggregate framework. As the proportion of fine aggregate increases, the overall VV of asphalt mixtures generally declines. Nonetheless, an excessive amount of fine aggregate can compromise the integrity of the coarse aggregate’s skeleton-embedded structure, resulting in a higher VV; this elucidates the distinction between gradation E and gradation F.

4.1.2. Effect of VV on the Mechanical Properties of Asphalt Mixtures

Based on the experimental data correlating the volume parameters of the LSAM-50 mixture with the mechanical properties of VVCM specimens, the trend graphs were constructed to illustrate the variation in mixture mechanical properties with respect to VV, as depicted in Figure 5.

Figure 5 shows that with an increase in VV, each mechanical property of the LSAM-50 asphalt mixture follows a pattern of initially increasing and subsequently decreasing. Given fixed raw materials, aggregate gradation, and molding methods, a larger asphalt–aggregate ratio in the asphalt mixture corresponds to a smaller VV. At lower asphalt–aggregate ratios, a high VV implies that the asphalt is insufficient to cohesively bind mineral particles, leading to suboptimal mechanical properties in the asphalt mixture. As the asphalt–aggregate ratio increases, the VV diminishes, allowing the asphalt to increasingly envelop the mineral particles. This enhanced binding optimizes the adhesion between the asphalt and mineral components, consequently elevating the mixture’s mechanical properties. However, upon reaching the optimal asphalt–aggregate ratio, any further addition of asphalt reduces VV while increasing the occurrence of free asphalt and excessive lubrication. This scenario undermines the mixture’s cohesion and internal frictional resistance, resulting in a decline in mechanical properties.

4.1.3. VV Design Criteria

To establish a relationship between the mechanical properties of the asphalt mixture and VV, the mathematical models were developed, as illustrated in Figure 6 and

Table 9. Relationship between VV and mechanical properties for LSAM-50 asphalt mixture.

Strength Indexes	Fitting Curve		VV of Smax	VV of 0.95∙Smax
	Fitting Equation	Correlation Coefficient R2	VV	VVmin	VVmax
RC	RC = −0.23394VV2 + 1.94469VV − 0.40744	0.8227	4.2	3.3	5.0
RT	RT = −0.00853VV2 + 0.06197VV + 0.11443	0.8113	3.6	--	4.8
DS	DS = −1435VV2 + 11938VV − 10197	0.8225	4.2	3.5	4.9

, where S_max denotes each curve’s peak value.

From Figure 6 and Table 9, it can be observed that the VV range corresponding to each mechanical property S_max is between 3.6% and 4.2%. Considering the actual construction situation, where factors such as weather, equipment, and personnel can affect the VV values of the asphalt mixture, causing fluctuations that make it difficult to achieve the optimal value, it is essential to ensure that the asphalt mixture maintains sufficient mechanical strength. Therefore, a 95% strength assurance level is introduced to restrict the range of VV values. the VV corresponding to 0.95 S_max is usually adopted as the design criterion. This study, considering the VV requirements for the mechanical properties of the five aggregate gradations, suggests an intersection of the VV value ranges. Hence, the design criteria for the VV of LSAM-50 asphalt mixtures are proposed to be between 3.5% and 4.8%.

The current highway construction industry generally recommends a VV range of 3% to 5%. The rationale behind this is that when VV is too high, the weak areas in the asphalt mixture increase, leading to a larger area affected by stress concentration. This not only can trigger early-stage distresses but also accelerates the progression of these issues, reducing the service life of the asphalt pavement. Conversely, a too-low VV often reflects an excessively high asphalt–aggregate ratio, which, while increasing construction costs, also heightens the risk of pavement bleeding. Compared to the existing standards, the VV design criteria proposed in this paper are more stringent. This approach ensures that the asphalt mixture possesses adequate mechanical strength and service life.

4.2. Influence of VFA on the Mechanical Properties of LSAM-50 Asphalt Mixture and Design Criteria

4.2.1. VFA-Influencing Factors

The VFA represents the effective asphalt volume expressed as a percentage of the void space within the aggregate structure. The variation of VFA as a function of the asphalt–aggregate ratio for various gradations of the LSAM-50 asphalt mixture is depicted in Figure 7.

As seen in Figure 7, it is evident that the asphalt–aggregate ratio and the aggregate gradation significantly influence the VFA of the asphalt mixture. As the asphalt–aggregate ratio increases, the VFA of the LSAM-50 asphalt mixture with different gradations shows an approximate linear growth. This is because, with the increase in the asphalt–aggregate ratio, the asphalt content increases while the porosity of a fixed gradation remains almost unchanged. Under the same asphalt–aggregate ratio and compaction conditions, the VFA of gradation B is significantly higher than that of gradation C.

This is because, compared with Gradation C, Gradation B has a higher content of fine aggregates, which can better fill the voids between the mixtures, resulting in a reduced porosity. With a constant asphalt content, the decrease in porosity leads to an increase in VFA. Similar differences are observed among other gradations as well.

4.2.2. Effect of VFA on the Mechanical Properties of Asphalt Mixtures

Based on the experimental data correlating the volume parameters of the LSAM-50 mixture with the mechanical properties of VVCM specimens, the trend graphs were constructed to illustrate the variation in mixture mechanical properties with respect to VFA, as depicted in Figure 8.

Figure 8 shows that with an increase in VFA, each mechanical property of the LSAM-50 asphalt mixture follows a pattern of initially increasing and subsequently decreasing. When the asphalt–aggregate ratio is low, resulting in a reduced VFA, the effective asphalt content is minimal, leading to inadequate adhesion between the asphalt and mineral particles and compromising the mixture’s mechanical properties. As the amount of asphalt increases, the effective asphalt content in the mixture rises, enhancing adhesion and subsequently improving the overall mixture properties. Upon reaching the optimal asphalt dosage, the mechanical properties of the mixture peak. However, any subsequent increase in asphalt results in a rise in VFA, but with an increased proportion of unbound asphalt. This excessive asphalt can displace mineral particles, weaken the adhesive bond, and ultimately degrade the mixture’s mechanical properties.

4.2.3. VFA Design Criteria

To establish a relationship between the mechanical properties of the asphalt mixture and VFA, the mathematical models were developed, as illustrated in Figure 9 and

Table 10. Relationship between VFA and mechanical properties of LSAM-50 asphalt mixture.

Strength Indexes	Fitting Curve		VV of Smax	VV of 0.95∙Smax
	Fitting Equation	Correlation Coefficient R2	VV	VVmin	VVmax
RC	RC = −0.00165VFA2 + 0.164VFA − 0.4462	0.8449	49.7	40	60
RT	RT = −0.000067VFA2 + 0.00741VFA + 0.02224	0.8433	52.9	43	-
DS	DS = −10.1VFA2 + 1005VFA − 10362	0.8436	49.8	42	58

.

From Figure 9 and Table 10, it can be observed that the VFA range corresponding to each mechanical property S_max is between 49.7% and 52.9%. The Chinese specification of JTG F40-2004 (Standards, 2004) [29] divides the upper and lower limits of VFA based on the nominal maximum particle size of aggregate, and the VFA tends to decrease gradually as the nominal maximum particle size of aggregates increases. For LSAM-50 asphalt mixtures, the median value of VFA technical requirements should theoretically be close to 52.5%. The median value of the VFA design standard determined in this paper is 51.3%, which is close to the theoretical value, so the reliability of this design standard is high.

4.3. Influence of VMA on the Mechanical Properties of LSAM-50 Asphalt Mixture and Design Criteria

4.3.1. VMA Influencing Factors

The VMA is defined as the percentage of the volume, excluding the aggregate and the absorbed asphalt, in relation to the total volume of the specimen. During the design and construction phases, many factors can influence VMA, such as compaction processes, properties of aggregates, aggregate gradation, and the asphalt–aggregate ratio. The curve illustrating the impact of the asphalt–aggregate ratio on VMA can be seen in Figure 10.

As illustrated in Figure 10, the VMA of the asphalt mixture first decreases and then increases with the rising asphalt–aggregate ratio. This trend can be attributed to the following: At lower asphalt–aggregate ratios, the lubricating effect of the asphalt on minerals is limited, amplifying the friction between mineral particles and making the mixture challenging to compact, leading to a larger VMA. As this ratio escalates, the asphalt’s filling and lubricating effects become more pronounced, causing a decline in VMA. Upon reaching the optimal asphalt–aggregate ratio, the VMA reaches its minimum. However, further increases in this ratio boost the proportion of free asphalt, impeding the closeness of mineral particles and subsequently elevating the VMA of the mixtures.

4.3.2. Effect of VMA on the Mechanical Properties of Asphalt Mixtures

Based on the experimental data correlating the volume parameters of the LSAM-50 mixture with the mechanical properties of VVCM specimens, the trend graphs were constructed to illustrate the variation in mixture mechanical properties with respect to VMA, as depicted in Figure 11.

As illustrated in Figure 11, the mechanical properties corresponding to different VMAs are dispersed, making it challenging to discern a pattern. The correlation coefficient R² remains extremely low after data fitting. This suggests that there is no evident correlation between the VMA and the mechanical properties of the LSAM-50 mixture. This is attributed to the fact that the calculation of VMA involves the volume of asphalt absorbed by the aggregate, for which the variations lack a discernible pattern. Additionally, from a design standpoint, VMA tends to be sensitive to various factors, often resulting in an absence of a clear trend.

4.3.3. VMA Design Criteria

From the previous discussion, it is clear that VMA has no evident correlation with the mechanical properties of asphalt mixtures. Currently, in the internationally accepted Superpave design method, the guiding role of the VMA_min standard in mixture design and practical engineering application is not significant. The commonly accepted view is that the VMA is directly related to the nominal maximum size of the aggregate in the mixture. However, a unified standard correlating VMA with the performance of the mixture has not been established. Up to now, in most scenarios, China considers VMA as a reference indicator. A relatively relaxed VMA design standard has been established. After the design is completed, the actual VMA of the mixture is evaluated. For mixtures that do not meet the VMA_min standard, appropriate adjustments should be made to their design process.

In summary, the VMA design standard for LSAM-50 mixture is not recommended as a mandatory design indicator. However, it should still be considered as a reference index to constrain its minimum value. The Chinese specification JTG F40-2004 specifies the technical requirements for VMA corresponding to different nominal maximum aggregate sizes in dense graded asphalt mixtures. Based on the VV design standard discussed in this article, the calculated values for VMA_min are presented in Figure 12.

As seen in Figure 12, the VMA_min decreases with increasing nominal maximum particle size. When the nominal maximum aggregate size exceeds 13.2 mm, VMA_min decreases linearly with the increase in nominal maximum aggregate size. Based on this trend, it can be calculated that when VV is 3.5%, 4.0%, 4.5%, and 5.0%, the VMA_min for the LSAM-50 mixture is 7.5%, 8.0%, 8.5%, and 9.0% respectively. Therefore, taking into account both the mechanical properties and practical engineering applications, the design standard for the VMA of the LSAM-50 mixture is set as VMA_min ≥ 7.5%.

4.4. Criteria for Volume Design of LSAM-50 Asphalt Mixture

Recommended volume design standards for LSAM-50 mixtures using VVTM are presented in

Table 11. Criteria for volume design of LSAM-50 asphalt mixture.

Volume Index	VV (%)	VFA (%)	VMA (%)
Technical standards	3.5~4.8	49.7~52.9	≥7.5

.

5. Analysis of LSAM-50 Test Section

5.1. Project Overview

The Southern Section of the Xi’an Outer Ring Expressway construction project spans approximately 70.16 km. It is built to the standards of a six-lane expressway with two-way traffic, designed for a speed of 120 km/h. The mainline test section is located between K39 + 980 and K41 + 000, covering a total length of 1020 m. The ramp test section is situated at the Yinzhen Interchange from EK0 + 887 to EK1 + 838.5, stretching over 951.2 m.

5.2. Mix Proportion Design of Asphalt Mixture

According to Technical Specifications for Construction of Highway Asphalt Pavements (JTG F40-2004), the designed aggregate gradation is presented in

Table 12. Aggregate gradation.

Aggregate Specifications (mm)	38~54	22~38	11~22	7~11	4~7	0~4	Mineral Powder
Proportions (%)	30	15	10	6	6	29	4

based on the requirements for the aggregate gradation of asphalt mixture. The optimal asphalt–aggregate ratio and the corresponding indicators of the LSAM-50 asphalt mixture at this ratio are presented in

Table 13. Indicators at the optimal asphalt–aggregate ratio.

Project	Optimal Asphalt–Aggregate Ratio (%)	Bulk Specific Gravity	Theoretical Maximum Density	VV (%)	VFA (%)	VMA (%)	Compressive Strength at 60 °C (MPa)
Test value	2.8	2.569	2.683	4.2	51.8	9.0	2.62
Normative value	—	—	—	3.5~4.8	49.7~52.9	≥7.5	≥2.0

. The road performance test results of the LSAM-50 asphalt mixture at this ratio are presented in

Table 14. Test results of road performance for LSAM-50 asphalt mixture.

Project	High-Temperature Performance	Water Stability		Low-Temperature Performance
	Dynamic Stability (Time/mm)	Residual Strength Ratio (%)	Freeze-Thaw Splitting Strength Ratio (%)	Small Beam Bending Strain (µε)
Test value	10,449	86.9	85.7	2608
Normative value	≥4000	≥85	≥80	≥2500

.

5.3. Construction Process

Construction preparation and the asphalt mixture mixing process are omitted. The process of mixture paving and compaction during construction is as follows:

5.3.1. Paving Process

The loose paving coefficient was set at 1.2 with a paving thickness of 18 cm. The laying of LSAM-50 asphalt mixture was carried out using the Volvo P8820DL ABGABG tracked paver, which has a maximum paving width of 13 m and a maximum paving thickness of 300 mm. Prior to commencing work, the screed of the paver should be preheated for 0.5 to 1 h, ensuring the screed temperature is not less than 100 °C. The paver must lay the material slowly, evenly, and continuously without arbitrarily changing speed, to enhance evenness and reduce segregation. The paving process of the LSAM-50 flexible base asphalt pavement is illustrated in Figure 13. Any road surface segregation caused by improper operation during the paving process should be promptly manually corrected.

5.3.2. Compaction Process

The compaction process involves initially using a pneumatic tire roller to compact the surface twice, followed by a single-drum roller to roll until there is no further change in the road elevation, and finally, using the pneumatic tire roller again for two more passes. The pneumatic tire roller used is a 30-ton Xugong XP305S tire roller, and the single drum roller is a 22-ton Xugong XS223J single drum roller. To ensure compaction quality, the rolling must be completed before the road surface temperature falls below 80 °C. The compaction process is illustrated in Figure 14.

5.4. Evaluation of Project Effectiveness

All indicators were measured in accordance with Field Test Methods of Highway Subgrade and Pavement (JTG 3450-2019) [30].

(1)

Road Surface Evenness

The evenness of the LSAM-50 base layer, after compaction, was tested using a continuous evenness measuring instrument. The test was conducted at a traveling speed of 7 km/h. The process of testing the evenness of the LSAM-50 base layer is illustrated in Figure 15. The test results are presented in

Table 15. Results of the evenness test.

Number	Layer Position	Standard Deviation (mm)
		Test Value	Mean Value	Normative Value
1	Road Base Layer	1.77	2.21	≤3
2	Road Base Layer	2.36
3	Road Base Layer	2.20
4	Road Base Layer	2.51

.

(2)

Compaction Degree

After the compaction of the LSAM-50 base layer, core samples were extracted using a core drilling machine with a 200 mm diameter drill bit. The field core samples are shown in Figure 16. The results of the core sample height measurements are shown in

Table 16. Results of the core sample height measurement.

Pile Number	Height of Core Samples (cm)				Height (cm)	Mean Value (cm)	Error (mm)	Normative Value (mm)	Extreme Values (mm)	Error (mm)	Normative Value (mm)
	1	2	3	4
EK1 + 700	14.0	14.2	13.8	14.2	14.1	14.9	−1	≥−8	14.1	−9	≥−10
EK1 + 600	14.9	15.3	15.2	15.1	15.1
EK1 + 500	15.6	15.5	15.5	15.2	15.5
EK1 + 400	15.3	15.0	14.9	15.0	15.1
EK1 + 300	14.5	13.7	14.3	13.7	14.1
EK1 + 200	15.3	15.2	15.5	15.3	15.3

. The results of the compaction degree test are shown in

Table 17. Results of the compaction degree test.

Pile Number	Height of Core Samples h (cm)	Density of Core Sample ρx (g/cm3)	Density of Specimen ρv (g/cm3)	Maximum Theoretical Density ρl (g/cm3)	Compaction Degree (%)
					ρx/ρv	ρx/ρl
EK1 + 700	14.1	2.559	2.569	2.683	99.6	95.4
EK1 + 600	15.1	2.574			100.2	95.9
EK1 + 500	15.5	2.560			99.6	95.4
EK1 + 400	15.1	2.583			100.5	96.3
EK1 + 300	14.1	2.553			99.4	95.2
EK1 + 200	15.3	2.581			100.5	96.2
Mean value	14.9	2.568	-	-	100.0	95.7

.

(3)

Permeability Coefficient

The permeability coefficient of the compacted LSAM-50 base layer was measured using a permeability coefficient meter. The process of testing the permeability coefficient is illustrated in Figure 17. The results of the permeability coefficient test are shown in

Table 18. Results of the permeability coefficient test.

Pile Number	Initial Value (mL)	Final Value (mL)	Permeability Coefficient (mL/min)	Mean Value (mL/min)
eEK1 + 750	100	130	10	12
EK1 + 650	100	109	3
EK1 + 550	100	120	10
EK1 + 450	100	160	20
EK1 + 350	100	154	28

.

(4)

Overall Appearance

The appearance of the LSAM-50 flexible base layer can be seen in Figure 18.

Based on these measurement indicators, it can be observed that the LSAM-50 asphalt mixture, designed according to the volume parameter design standards proposed in this paper, meets the technical requirements of the standard indicators after construction and formation of the road base layer.

6. Conclusions

This paper examined the advantages of the vertical vibration compaction method in comparison to the static compression test. Building on this, interrelationships between the volume of voids, voids filled with asphalt, voids in mineral aggregates, and mechanical properties of VVCM asphalt mixtures were scrutinized. A procedural standard for volume parameters, predicated on the vertical vibration compaction method, for LSAM-50 asphalt mixtures was introduced. The key conclusions of this research are outlined below:

The correlation between the mechanical properties of VVCM specimens and field core samples was up to 90% or more. The compressive strength and splitting strength of VVCM specimens were 1.23 and 1.13 times higher than those of PCT specimens, respectively. The gradation change of VVCM specimens after compaction was less than that of PCT specimens. These results indicate that VVCM is closer to the actual compaction effect at the construction site than PCT. VVCM is better than PCT for asphalt mixture design.

There is a good correlation between the LSAM-50 asphalt mixture’s mechanical properities and VV and VFA. Therefore, the recommended values of VV and VFA design for LSAM-50 asphalt mixture are 3.5% to 4.8% and 49.7% to 52.9%, respectively. Within this range, the mechanical properties of the asphalt mixture achieve their peak values. The correlation between LSAM-50 asphalt mixture and VMA is limited. It is merely used as a recommended design reference. Based on the standard’s minimum value criteria, a VMA greater than 7.5% is recommended for LSAM-50 asphalt mixture.

The practical engineering application has demonstrated that the road base layer formed from the LSAM-50 asphalt mixture, guided by the volume parameter design standards proposed in this paper, meets the requirements of the specified standard indicators.