Study on the influence of matrix asphalt mixture air voids on the performance of semi-flexible pavement materials

In practical applications, semi-flexible pavement materials need to achieve a balance between mechanical properties, high-temperature stability, cracking resistance, and water stability. However, these performance requirements are strongly influenced by the void content of the matrix asphalt mixture, which determines the grouting space and the composite structure of semi-flexible pavement materials. In this study, matrix asphalt mixtures with three different void contents (20.6%, 24.9%, and 29.2%) were prepared. The effects of the matrix void content on the perfusion characteristics and overall performance of semi-flexible pavement materials were systematically evaluated through perfusion tests, Marshall stability and splitting tests, three-point bending tests, rutting tests, and freeze–thaw splitting tests. Subsequently, the impact of the void ratio on each parameter was analyzed. The findings indicate that, within the tested void content range (20.6%–29.2%), increasing the void ratio of the matrix asphalt mixture continuously improved the mechanical properties of the semi-flexible pavement material: the Marshall stability was enhanced by a factor of up to 4.8, and the splitting strength by up to 1.4 times. Results from the trabecular three-point bending test reveal a more pronounced effect of void fraction on the crack resistance of semi-flexible pavement materials with rising temperatures. At 15 °C, lower void fractions lead to superior crack resistance. Conversely, when the void fraction reaches 24.9% at −10 °C, semi-flexible pavement materials exhibit enhanced crack resistance. Rut dynamic stability and freeze–thaw splitting tests demonstrate that higher void ratios in the matrix correlate with superior resistance to high-temperature deformation and water stability. When the void ratio falls below 24.9%, increasing it significantly enhances resistance to high-temperature deformation, with an improvement range of up to 63.6%. Therefore, 24.9% is identified as the recommended balanced air-void content of the matrix asphalt mixture, at which the semi-flexible pavement material achieves the most balanced comprehensive performance in terms of mechanical properties, high-temperature deformation resistance, water stability, and cracking resistance.

Asphalt pavement has emerged as the predominant road surface for high-grade roads in China due to its favorable driving comfort and anti-skid properties. However, being a viscoelastic–plastic material, the asphalt mixture is highly sensitive to temperature fluctuations, making it susceptible to various distresses such as rutting, corrugation, and shoving, especially in urban areas with bus stops, intersections, and other critical sections. Rutting, in particular, tends to occur earlier, more frequently, and with greater deformation depth during hot summers, significantly compromising road durability and driving safety. To address the challenge of insufficient resistance to shear deformation in asphalt pavements at high temperatures, domestic and international scholars have conducted extensive research and proposed a semi-flexible pavement material. This innovative material combines the characteristics of both rigid and flexible pavements by infusing high-fluidity cement slurry into an open-graded porous asphalt mixture with a nominal maximum aggregate size of 13 mm (PAC-13), typically with a void content ranging from 20% to 30%. In terms of performance, it offers not only the smoothness, skid resistance, and ease of repair associated with traditional asphalt pavements but also the high strength and modulus typical of cement concrete pavements. Currently, its primary applications include bus stations, intersections, parking lots, and lengthy longitudinal slopes, demonstrating promising outcomes (Wang, 2020; Zhong et al., 2017; Cheng et al., 2016; Xie et al., 2017).

However, the implementation of semi-flexible pavement also faces several challenges that have not been fully resolved. First, the initial construction cost is relatively high, mainly owing to the expensive grouting materials and the additional grouting process (Hassani et al., 2020). Second, field construction is considerably more demanding than that of conventional asphalt pavement: the grouting temperature and time window, the curing conditions, and the use of specialized grouting equipment and skilled labor must all be strictly controlled to ensure grouting quality (Hassani et al., 2020; Ling et al., 2022a). Third, because semi-flexible pavement is a composite of cement paste and asphalt mixture, its long-term behavior under repeated traffic and climatic loading, along with its life-cycle economy and environmental impact, has not yet been fully investigated (Hassani et al., 2020; Ling et al., 2022a; Antunes et al., 2016). These challenges are closely related to the design of the matrix asphalt mixture, particularly its void content.

The performance of semi-flexible pavement materials is governed primarily by the combined effects of the matrix asphalt mixture and the grouting material. Because the grouting material has much higher stiffness and strength than the matrix asphalt mixture, the mechanical properties of semi-flexible pavement materials can be significantly improved, often exceeding those of conventional asphalt mixtures by more than two times (CAI et al., 2023; Zheng et al., 2023). Therefore, a large body of research has focused on the grouting material itself, mainly optimizing its workability and mechanical properties through various admixtures, solid wastes, and functional additives (Changjiang et al., 2022; Zhang et al., 2022; Liu et al., 2022). However, the void content of the matrix asphalt mixture directly determines the perfusibility of semi-flexible pavement materials, and thus, its influence on the final performance is at least as important as that of the grouting material. Ding et al. (2011) analyzed the effects of the void content and pore structure of matrix asphalt mixtures on the performance of semi-flexible pavement materials and concluded that matrix mixtures with high porosity and single gradation exhibit superior pavement performance. Wang et al. (2020) and Wang and Haiwen (2021) compared the vacuum plastic sealing method and the volumetric method for measuring the void content of matrix asphalt mixtures for semi-flexible pavements and established a conversion relationship between the two methods, thereby improving the accuracy of matrix void content measurement.

These studies indicate that the matrix void content plays a decisive role in the performance of semi-flexible pavement materials. However, existing studies are not fully consistent, and in some respects, even contradictory, regarding how the void content affects a specific performance indicator. Chen et al. (2018) found through semi-circular bending tests that larger matrix voids correspond to better cracking resistance, with the optimum performance observed at 25 °C, implying that increasing void content may have a monotonically beneficial effect. In contrast, Wang and Haiwen (2021) and Wang and Haiwen (2020) showed that the low-temperature cracking resistance of semi-flexible pavement materials is optimized when the void content of the matrix asphalt mixture is approximately 24%, and that further increasing the void content leads to performance deterioration. Meanwhile, the low-temperature performance is also affected by the asphalt binder type and the grouting material type. These inconsistent conclusions indicate that the relationship between the matrix void content and material performance is not simply monotonic but is closely related to the test temperature and performance indicator. A higher void content favors slurry infusion and mechanical enhancement, whereas an excessively high void content may increase the stiffness disparity between the cement slurry and the matrix asphalt mixture, weaken the interfacial transition zone, and, thereby, impair cracking resistance. Yang and Wengjai (2015) further confirmed, through cyclic wheel load tests, the critical role of matrix void content in the durability of semi-flexible pavement materials. Other studies have investigated void-related behavior from individual perspectives, such as fiber-reinforced fracture toughness (Qiu et al., 2020), mineral gradation design based on fractal dimensions (Chun and Feng, 2018), nano-TiO 2 modification for improving low-temperature performance (Fengxia et al., 2020), and reserved grouting depth (Yan et al., 2019). However, most of these studies evaluated only a single performance indicator, a single test temperature, or a single material variable. The trade-off between the improved perfusibility and mechanical properties caused by higher void content and the associated loss of cracking resistance has not yet been systematically quantified within a complete performance framework.

From a design perspective, the practical significance of matrix void content has not been fully reflected in current specifications. Existing standards generally recommend a void content range of 20%–30% for the matrix asphalt mixture of semi-flexible pavement materials, but they do not provide a clear optimum design value. Nor do they consider that although increasing the void content is beneficial for perfusibility, mechanical properties, high-temperature performance, and water stability, low-temperature cracking resistance deteriorates once the void content exceeds a certain level. Therefore, in the design of semi-flexible pavement materials, engineers still lack a clear basis for selecting the matrix void content.

Given that there is still a lack of systematic and comprehensive evaluation of the influence of the matrix asphalt mixture void content on the performance of semi-flexible pavement materials, along with a lack of an optimization design method for matrix void content based on multi-performance synergistic analysis, in this study, we take matrix asphalt mixtures with different void contents as the research object. Laboratory tests are conducted to investigate the effects of the matrix asphalt mixture void content on the perfusion rate, Marshall mechanical properties, cracking resistance, high-temperature performance, and water stability of semi-flexible pavement materials. The primary objective of this study is to clarify, through systematic laboratory tests, the variation patterns of the perfusion rate, Marshall mechanical properties, cracking resistance, high-temperature performance, and water stability of semi-flexible pavement materials under different void contents of the matrix asphalt mixture. The secondary objective is to identify the optimal matrix void content and propose a reasonable design range that coordinates mechanical properties, pavement performance, and cracking resistance, thereby providing a scientific basis for the design and optimization of matrix void content. The findings can provide an experimental basis for the mixture design of semi-flexible pavement materials, the control of matrix void content, and the improvement of relevant specifications. The innovation of this study lies in taking the void content of the matrix asphalt mixture as the core design variable and, under the condition that the grouting material and other test conditions remain unchanged, establishing a multi-index evaluation method covering perfusion performance, mechanical properties, cracking resistance, high-temperature performance, and water stability. In this study, we not only reveal the differentiated effects of the void content on different performance indicators of semi-flexible pavement materials but also propose an optimization design approach for the matrix void content from the perspective of multi-performance synergy, thereby providing a basis for shifting the design of semi-flexible pavement materials from single-index control to comprehensive performance balance.

Grouting material

In this study, we utilized a self-developed grouting material to prepare the semi-flexible pavement material. The grouting material consists of ordinary Portland cement, aluminate cement, fly ash, silica fume, fine quartz sand, latex powder, water-reducing agent, and other constituents in specified proportions. The fine quartz sand exhibits a 100% passing rate through the 1.18-mm sieve and not less than 80% passing rate through the 0.6-mm sieve. The cement paste is formulated by blending water and the grouting material at a 0.3 ratio, followed by high-speed mixing using a high-speed shearing machine at 4,000 rpm for 2 min. Table 1 presents the technical specifications of the cement paste.

Mineral aggregate and asphalt binder

The constituents of the matrix asphalt mixture comprise gabion coarse aggregate, limestone fine aggregate, limestone ore powder, and styrene–butadiene–styrene (SBS) modified asphalt. The coarse aggregate exhibits an apparent relative density of 2.943, a needle-like and flaky particle content of 1.8%, and a crushing value of 9.8%. Meanwhile, the fine aggregate demonstrates an apparent relative density of 2.721 and a sand equivalent of 68%. The mineral powder showcases an apparent relative density of 2.777. Table 2 presents the technical specifications of the SBS-modified asphalt.

Specimen preparation and test methods

Preparation of the matrix asphalt mixture specimen

In this study, the matrix asphalt mixture was formulated using three ore grades labeled G1, G2, and G3, each with varying void ratios as outlined in Table 3. The optimum asphalt–aggregate ratios for the three base asphalt mixtures were determined through laboratory mix ratio design as 3.2%, 3.4%, and 3.5%, respectively. Sample preparation of the matrix asphalt mixture was conducted using the Marshall compaction method, subjecting the mixture to 50 compactions on each side. The preparation procedures adhered to the testing standards outlined in the “Highway Engineering Asphalt and Asphalt Mixture Test Code” (JTG E20-2011, 2026). Aggregate heating was maintained at 175 °C, while the SBS-modified asphalt was heated to 165 °C. The mixing temperature for the base asphalt mixture was set at 165 °C, with compaction temperatures ranging from 155 °C to 160 °C. Six parallel specimens were prepared for each grade.

Preparation of the semi-flexible pavement material specimen

Initially, the Marshall specimens of the asphalt mixture and rut plate were prepared. Following preparation, polyethylene terephthalate (PET) tape was utilized to seal around and beneath the asphalt mixture, as shown in Figure 1a. The prepared cement slurry was immediately used for grouting, and the grouting process was completed within 20 min after mixing. Subsequently, the cement slurry was poured uniformly onto the mixture surface, and a scraper was used to facilitate uniform spreading and penetration of the slurry. The specimens were then vibrated on a vibrating table at 60 Hz for 2 min to promote penetration of the cement slurry into the interconnected voids of the matrix asphalt mixture. After vibration, excess cement slurry was removed from the specimen surface using a scraper until the coarse aggregates were slightly exposed. Upon grouting, the specimens were cured in an environment with a temperature of 20 C ± 2 °C and a relative humidity of at least 90%. The specimens were demolded after 1 day and continuously cured under the same conditions to an age of 7 days before subsequent performance tests. Figure 1 depicts the Marshall specimen of the semi-flexible pavement material and the rut plate specimen post-mold removal. The Marshall specimen measures φ101.6 mm × 63.5 mm, while the rut plate specimen measures 300 mm × 300 mm × 50 mm. Trabecular specimens for three-point bending were extracted from the rutting plate specimens. Each grade yielded 6–8 parallel trabecular specimens, each measuring 250 mm ± 2.0 mm in length, 30 mm ± 2 mm in width, and 35 mm ± 2 mm in height.

Test method for material properties of the semi-flexible pavement

Volumetric parameter test of the matrix asphalt mixture

The volume parameters of the matrix asphalt mixture primarily encompass the air voids ratio and the interconnecting air voids ratio index. Testing is conducted using the volume method and surface dry method by the Chinese standard “Test Procedure for Highway Asphalt and Asphalt Mixture” (JTG E20-2011, 2026). 2. Perfusion rate test

The perfusion rate of the semi-flexible pavement material is defined as the ratio of the actual volume of cement slurry retained within the matrix asphalt mixture after grouting (V f) to the connected void volume of the matrix asphalt mixture before grouting (V c), as expressed by: P r = V f/ V c ×100%, where P r is the perfusion rate (%), V f is the actual volume of cement slurry retained within the matrix asphalt mixture after grouting, and V c is the connected void volume of the matrix asphalt mixture before grouting. The connected voids refer to the voids that are interconnected with each other and accessible from the exterior of the specimen, and, therefore, they represent the effective space available for slurry infiltration. 3. Semi-flexible pavement material performance test

In this study, following the guidelines outlined in the Chinese standard “Highway Engineering Asphalt and Asphalt Mixture Test Protocol” (JTG E20-2011, 2026), the semi-flexible pavement materials underwent a series of tests including the Marshall stability test, Marshall splitting test, trabecular three-point bending test, freeze–thaw splitting test, and rut test. Additionally, the mechanical properties, low-temperature crack resistance, water stability, and high-temperature stability of semi-flexible pavement materials with varying void parameters were evaluated.

Effect of void parameters on the perfusion rate

Base asphalt mixture specimens were prepared for each of the three mineral grades in Table 3 at the optimum asphalt–aggregate ratio, and the void parameters of the base asphalt mixtures were tested after preparation. Then, the cement slurry was filled to test the filling rate of the semi-flexible Marshall specimen. The change rule of semi-flexible pavement material filling rate with the matrix asphalt mixture void ratio is shown in Figure 2.

From Figure 2, it can be seen that the void ratio of the base asphalt mixture has a high linear correlation with the connected void ratio, and the correlation coefficient reaches 0.998. Therefore, there is a consistency in the influence of the void ratio and connected void ratio on the performance of the semi-flexible pavement materials, and in this study, we analyze the index of the void ratio as an influencing factor.

As shown in Figure 2, the perfusion rate of the semi-flexible pavement material gradually increases with the matrix void content obtained by adjusting the aggregate gradation under the same compaction condition. On the other hand, the larger the void ratio, the smaller the standard deviation of the filling rate, indicating that the void ratio has a significant effect on the uniformity of filling; thus, the larger the void ratio, the more uniform the semi-flexible pavement material filling.

This phenomenon can be mainly attributed to the combined effects of the pore structure of the matrix asphalt mixture and the flowability of the cement slurry. Previous studies have shown that connected porosity, pore size, and pore-throat size are important factors governing the infiltration of cement slurry into porous asphalt mixtures (Ling et al., 2022b; Khan et al., 2022). As the void content decreases, the available connected void space and effective channels for slurry penetration are reduced, thereby increasing the resistance to slurry infiltration and decreasing the perfusion rate. In addition, the heterogeneous and discontinuous pore structure of the matrix asphalt mixture may lead to less uniform distribution of the grout at lower void contents.

Effect of void parameters on the mechanical properties

Marshall stability tests and splitting tests were conducted on both the matrix asphalt mixtures and the corresponding semi-flexible pavement materials at three void contents. In this study, PAC-13 denotes the porous asphalt concrete matrix mixture with a nominal maximum aggregate size of 13 mm before grouting, while SFP-13 denotes the corresponding semi-flexible pavement material obtained by infusing cement slurry into the PAC-13 matrix mixture. The Marshall stability tests were performed at 60 °C, and the Marshall splitting tests were conducted at 15 °C. Four parallel specimens were prepared for each void content. The changes in Marshall stability and Marshall splitting strength before and after grouting are compared in Figure 3.

Based on Figures 3a,b, it can be seen that the Marshall stability and split strength of the matrix asphalt mixes were substantially increased after grouting at all three void ratios. When the void ratio of the matrix asphalt mixture is 20.6%, the stability after grouting is increased by 1.2 times, and the split strength is increased by 0.3 times; when the void ratio is 24.9%, the stability after grouting is increased by 2.5 times, and the split strength is increased by 1 time; when the void ratio is 29.2%, the stability after grouting is increased by 4.8 times, and the split strength is increased by 1.4 times. The larger the void ratio, the greater the improvement of the mechanical properties after grouting, especially the stability index increases exponentially. After grouting, the Marshall stability of SFP-13 increased significantly compared with that of PAC-13, with values ranging from 17 kN to 30 kN under different void contents, demonstrating the strengthening effect of cement slurry infiltration on the mechanical properties of semi-flexible pavement materials.

It should be noted that the increased Marshall stability after grouting does not indicate the elimination of asphalt contribution, since semi-flexible pavement material is a composite structure consisting of a flexible asphalt matrix and a rigid cementitious skeleton. In addition, no unified acceptance criteria for Marshall stability and splitting strength have been established for semi-flexible pavement materials. Therefore, these parameters were used as comparative indicators in this study.

As shown in Figure 3, the Marshall stability and splitting strength of PAC-13 decreased with increasing matrix void content, whereas those of SFP-13 exhibited the opposite trend. This indicates that a higher void content provides more space for cement slurry infiltration, thereby improving the mechanical properties after grouting.

The main reason for this phenomenon is that the semi-flexible pavement material is a composite of the base asphalt mixture and cement slurry material, and the strength of the cement slurry is much greater than that of the base asphalt mixture. The larger the void ratio of the matrix asphalt mixture, the higher the filling rate of the slurry, and the higher the strength of the semi-flexible pavement material obtained from the composite; therefore, the mechanical properties of the semi-flexible pavement material are mainly dependent on the strength of the cement slurry and the filling rate and less affected by the mechanical properties of the matrix asphalt mixture. Therefore, in the design of semi-flexible pavement materials, the void ratio of the matrix asphalt mixture should be strictly controlled. Excessively high mechanical properties of the matrix asphalt mixture are unnecessary; only sufficient stability to meet the requirements of field compaction during construction is required.

Effect of void parameters on low-temperature performance

Semi-flexible pavement material is made of rigid cement mortar and flexible asphalt mixture composite, and under the action of temperature and load, the mechanical deformation of the two materials is not coordinated, which causes semi-flexible pavement material interface to develop cracking damage. In this study, the effect of the void ratio and test temperature on the cracking resistance of semi-flexible pavement materials was investigated through the three-point bending test of trabeculae, and the low-temperature trabeculae bending tests at −10 °C and 15 °C were conducted on the semi-flexible pavement materials under three void ratios, respectively. Comparison of flexural tensile strength and maximum flexural tensile strain of semi-flexible pavement materials at different test temperatures is shown in Figure 4, and the variation rule of the maximum flexural tensile strain with the void ratio of semi-flexible pavement materials is shown in Figure 4.

As can be seen from Figures 4a,b, under the same void ratio, the flexural tensile strength of the semi-flexible pavement material at −10 °C is greater than the flexural tensile strength at 15 °C, which is approximately twice the flexural tensile strength at 15 °C; the maximum flexural-tensile strain at −10 °C is much smaller than that of the maximum flexural tensile strain at 15 °C, and the flexural tensile strain of the base asphalt mixture at 15 °C is approximately four times as much as that at −10 °C when the void ratio is 20.6%. When the void ratio is 20.6%, the 15 °C flexural-tensile strain is approximately four times of that at −10 °C, and when the void ratio is increased to 29.2%, the 15 °C flexural-tensile strain is approximately two times of that at −10 °C. It can be seen that the flexural-tensile strain of the semi-flexible pavement material decreases with the first increase of the void ratio, at −10 °C, when the void ratio increases from 20.6% to 24.9%, the flexural-tensile strain reaches the maximum value of 2,020 με and the maximum flexural-tensile strain is enhanced by 8%, and then the flexural-tensile strain decreases rapidly again to only 1,494 με, and the decrease reaches 26%, when the void ratio increases to 29.2%. It can be seen that the crack resistance is optimized when the void ratio reaches 24.9%, but the enhancement is not significant, and the crack resistance of the material decreases rapidly as the void ratio continues to increase, indicating that the void ratio of the base asphalt mixture exceeding 24.9% has a significant effect on the low-temperature crack resistance of the semi-flexible pavement material. At 15 °C, the flexural-tensile strain of semi-flexible pavement materials decreases gradually with the void ratio. There is a significant difference in the variation rule of bending and tensile strains with the void ratio of semi-flexible pavement materials at different temperatures. The two test temperatures under the bending and tensile strain change curve show that the higher the temperature, the more significant the effect of the void ratio on the flexural-tensile strain of semi-flexible pavement materials. The lower the void ratio is, the more significant the effect of the test temperature on the maximum flexural strain of semi-flexible pavement materials.

This is mainly because the asphalt mixture is a kind of viscoelastic–plastic material and its stiffness is greatly affected by the temperature; when the test temperature is lower, the asphalt mixture shows rigidity characteristics, and it is prone to brittle failure, so the lower the test temperature is, the lower the maximum flexural-tensile strain of the semi-flexible pavement material is. At −10 °C, the asphalt mixture exhibits relatively rigid behavior. When the void ratio of the matrix asphalt mixture is low, the perfusion rate of the semi-flexible pavement material is also low, leaving more interconnected voids that can cause stress concentration under loading and accelerate cracking. With the increase of the void rate, the semi-flexible pavement material perfusion rate gradually increased, and semi-flexible pavement material in the communicating interstices gradually reduced; under the action of the load structure, internal force is relatively more uniform, showing better anti-cracking performance; when the void rate is further increased, the perfusion rate continues to increase, the semi-flexible pavement material in the cement slurry content is too high, the asphalt mastic content is too small, and the overall stiffness of the semi-flexible pavement material is increased. The coordinated deformation ability of the asphalt mixture and cement slurry is weakened. This indicates that the improvement in strength and stiffness caused by increased grout content does not necessarily translate into improved cracking resistance because excessive stiffness may reduce the deformation compatibility of the composite structure.

At the test temperature of 15 °C, the asphalt mixture has a certain viscoelastic plasticity and the rigidity of the mixture is relatively low, with good deformation ability; at this time, the lower the void ratio, the higher the content of asphalt mastic, and the overall rigidity of the semi-flexible pavement material is smaller, and the semi-flexible pavement material has better resistance to deformation. With the increasing void ratio, the cement slurry content increases, the asphalt mastic content decreases, the overall rigidity of the semi-flexible pavement material gradually increases, and the semi-flexible pavement material becomes more prone to interface brittle cracking. Therefore, the larger the void ratio is, the higher the perfusion rate of the semi-flexible pavement material and the smaller the maximum flexural tensile breaking strain.

Effect of void parameters on high-temperature performance and water stability performance

High-temperature deformation resistance and water stability performance are the key technical indicators for evaluating the road performance of semi-flexible pavement materials, and they are also important parameters affecting the durability of the pavement. In this study, 60 °C rutting tests and Marshall freeze–thaw split tests were conducted on semi-flexible pavement materials at three void ratios, with three parallel specimens for rutted slabs and four parallel specimens for freeze–thaw splits at each void ratio. The pattern of change of dynamic stability determination and the freeze–thaw splitting tensile strength ratio with the void ratio is shown in Figure 5.

From Figure 5, it can be seen that with the increase of the void ratio, the 60 °C dynamic stability of the semi-flexible pavement material gradually increases; the rate of increase of dynamic stability is faster when the void ratio is lower than 24.9%, and it slows down and stabilizes when it exceeds 24.9%. When the void ratio of the base asphalt mixture increases from 20.6% to 24.9%, the dynamic stability of the semi-flexible pavement material increases by 10,837 times/mm, which is an improvement of 63.6%, and when the void ratio continues to increase to 29.2%, the dynamic stability increases by 2,803 times/mm, which is an improvement of only 10%. (2) With the increase of the void ratio, the freeze–thaw split tensile strength ratio of the semi-flexible pavement material increases linearly, when the void ratio of the matrix asphalt mixture increases from 20.6% to 24.9%, the freeze–thaw split tensile strength ratio increases by 2.1%; when the void ratio continues to increase to 29.2%, the freeze–thaw split tensile strength ratio increases by 2.5%. The increase in the void ratio has an obvious enhancement effect on the water stability of semi-flexible pavement materials.

The main reason for the above is that the larger the void ratio of the base asphalt mixture is, the higher the perfusion rate of the semi-flexible pavement material, the higher the cement slurry content, the greater the overall strength and rigidity of the semi-flexible pavement material, the better the resistance to deformation at high temperatures, and the smaller the amount of deformation under the action of loading. This phenomenon can be attributed to the strong resistance to permanent deformation provided by the cementitious grout skeleton. After grouting, all semi-flexible pavement materials exhibited relatively high dynamic stability, and the difference caused by the matrix void content was, therefore, less pronounced compared with the variation in Marshall stability. With the increase of the void ratio, the semi-flexible pavement material void ratio gradually increases; the increase of perfusion rate means that the volume of communicating interstices in semi-flexible pavement material decreases and less water enters into the interior of the semi-flexible pavement material under the conditions of the freezing and thawing cycle, thus the role of internal damage to the semi-flexible pavement material is smaller, and the freezing and thawing cleavage tensile strength ratio is larger.

In this study, we investigate the influence of the void ratio of the matrix asphalt mixture on the perfusion rate, mechanical properties, cracking resistance, high-temperature performance, and water stability performance law of semi-flexible pavement materials through the semi-flexible pavement material perfusion test, Marshall stability and splitting test, three-point bending beam test, rutting test, and freeze–thaw splitting test. The main conclusions are as follows. Semi-flexible pavement materials have excellent mechanical properties. Increasing the matrix void ratio can greatly improve the mechanical properties of semi-flexible pavement materials; the stability can be improved by 4.8 times, and the splitting strength can be improved by 1.4 times.

The influence of the matrix void content on the cracking resistance of semi-flexible pavement materials varies with the test temperature. At 15 °C, lower void contents lead to better cracking resistance, whereas at −10 °C, the semi-flexible pavement material with a void content of 24.9% exhibits better cracking resistance.

When the void content of the matrix asphalt mixture reached 24.9%, the semi-flexible pavement material exhibited a favorable balance among the mechanical properties, high-temperature performance, water stability, and cracking resistance. Further increasing the void content improved the mechanical properties and water stability but provided limited improvement in high-temperature performance and reduced cracking resistance.