3.1. Effect of Reinforcements on Flexural Tensile Strength
Figure 6 shows the variation curves of flexural tensile stress with loading time for the UN and the RSCB. As can be seen from
Figure 6, the flexural tensile stress–loading time curves of the SCBs can be roughly divided into three stages: the first stage is a slow-growth stage, which shows that the flexural tensile stress increases slowly with the increase in loading time, and the test duration is relatively short. The second stage is the seemingly linear growth stage with the longest test duration. It shows that the flexural tensile stress increases rapidly with the increase in loading time until the peak flexural tensile stress is reached. The third stage is the post-peak drop stage with a short test duration. It shows that the peak flexural tensile stress drops linearly and rapidly with the increase in loading time, and even the flexural tensile stress is almost zero.
It can also be seen from
Figure 6 that the flexural tensile stress of the carbon fiber-based geogrid reinforcement both increase. The time to reach the peak value is gradually prolonged. The flexural tensile strength is also gradually increased, which is greater than that of the UN. In the case of AC-13/AC-20 (AC-20/AC-25) combination, the time to peak for the GCF and the CCF is prolonged by 54.5–70.8% and 65.5–88.0%, respectively, compared to the UN.
According to the lamination theory [
46], the potential reinforcing effect of laying carbon fiber-based geogrids between the layers of the SCBs is applied to increase the modulus of the upper layer [
47], which increases the stiffness ratio of the upper and lower layers of the SCBs. The larger the stiffness ratio of the upper and lower layers of the SCB, the stronger the stress diffusion effect. The slower the rate of stress growth at the bottom of the beam, the longer the stress time required to reach cracking at the bottom of the beam. The greater the load required to reach ultimate stress at the bottom of the beam, the greater the flexural tensile strength. The presence of carbon fiber-based geogrids also reduces the compression–tension stiffness ratio of the SCB. The stress diffusion effect increases as the compressive–tensile stiffness ratio of the SCB decreases. It can be seen that the ultimate tensile strength and stiffness of carbon fiber-based geogrids play an important role in improving the flexural tensile strength of the RSCBs.
As the overall ultimate tensile strength of the carbon fiber-based geogrid increases, the drop rate of the peak flexural tensile stress of the reinforcement decreases, and the residual flexural tensile stress gradually increases. The degree of the post-peak damage brittleness is strong, and the flexural tensile stress drops obviously, showing similar post-peak damage characteristics. That is because the geogrid has a restraining effect on the crack opening through its own high tensile strength, interlayer bonding, and frictional resistance. So that the RSCB has a certain residual flexural stress.
From the above analysis, it can be seen that the flexural tensile strengths of different types of geogrid RSCBs are higher than those of UNs. In order to better compare the flexural tensile strength changes in different types of geogrids after reinforcement under different SCB types to reflect the effect of reinforcement, the strength enhancement factor (
SEF) is introduced:
where
RB* is the flexural tensile strength of the RSCB (in Mpa), and
RB is the flexural tensile strength of the UN (in Mpa).
The flexural tensile strength results of the SCBs are presented in
Figure 7. The
SEFs of different geogrid RSCBs can be calculated by Equation (1), as shown in
Table 5.
The SEFs of different geogrid RSCBs are greater than 0 under different SCB types. The SEF of the CCF RSCB is as high as 25.24% in the case of AC-20/AC-25, which shows the positive effect of the carbon fiber-based geogrid in improving the strength of the SCB. The positive effect of the CCF is higher than that of the GCF.
Under the same SCB conditions, with the increase in the overall ultimate tensile strength of the carbon fiber-based geogrid, the SEF increases. In the case of the AC-20/AC-25 (AC-13/AC-20) combination, the SEF increased by 5.07–5.16% when the geogrid was changed from GCF to CCF, indicating that the performance differences in the longitudinal and transverse ribs of carbon fiber-based geogrids play a certain role in increasing the SEF of the RSCB.
For the same reinforcement conditions, the flexural tensile strengths of the AC-20/AC-25 combination are higher than those of the AC-13/AC-20 combination. Under UN conditions, the flexural tensile strength increases by 5.92% when the SCB changes from AC-13/AC-20 to AC-20/AC-25. Under the condition of GCF(CCF) reinforcement, the flexural tensile strength increases by 11.54–11.88% when the SCB changes from AC-13/AC-20 to AC-20/AC-25. It can be seen that the SCB type has an important contribution in improving the flexural tensile strength of the RSCB.
The reason lies in the fact that the coarse aggregate of the AC-20/AC-25 combination is mainly concentrated in 9.5–19 mm aggregate particles, referred to as the major particle size, which is close to the aperture size of the carbon fiber geogrid. The ratio range of the aperture sizes to the major particle sizes in the AC-20/AC-25 is 1.32–2.63, consistent with previous research findings [
48,
49,
50,
51,
52]. When the aggregate particles are embedded in the openings, the ribs of the geogrid can restrict the free movement of the aggregate particles to better perform the restraining effect and dissipate the temperature stress generated by the low-temperature shrinkage. Meanwhile, the coarse aggregate of the AC-13/AC-20 combination is mainly composed of 4.75–13.2 mm aggregate particles. The ratio range of the aperture sizes to the major particle sizes in the AC-13/AC-20 combination is 1.89–5.26. The aggregate particles are small enough to go through the apertures, even as multiple particles together. The degree of the embedded locking of aggregate particles is improved to be lower, and the restraining effect of the geogrid is lower. Therefore, the ratio range of the aperture sizes to the major particle sizes in the dense gradation is an important factor in improving the flexural tensile strength of carbon fiber-based geogrid RSCBs.
3.2. Effect of Reinforcements on Flexural Tensile Strain
The variation curves of flexural tensile strain with loading time for the UN and the RSCB are shown in
Figure 8a,b.
Figure 8c,d show the variation curves of flexural tensile strain growth ratio with loading time for different SCBs. The flexural tensile strain growth ratio refers to the ratio of flexural tensile strain at a certain moment to the maximum flexural tensile strain at the time of failure. The growth rate refers to the slope of the curve showing the growth ratio of the flexural tensile strain over time. As can be seen from
Figure 8, the flexural tensile strain and the growth ratio of the flexural tensile strain of the SCBs increase approximately linearly with the increase in loading time. The growth rates and the growth ratios of the flexural tensile strain of the RSCBs are lower than those of the Uns, suggesting that reinforcement of the carbon fiber geogrid can retard the growth of the flexural tensile strain of the SCB.
It can also be seen from
Figure 8 that the growth rate and growth ratio of flexural tensile strain of CCF reinforcement are lower than those of GCF reinforcement. That is because the RSCB can be approximated as elastomers at low temperatures, and the flexural tensile strain is directly proportional to the flexural tensile strength. The growth rate of flexural tensile strength of the RSCB decreases from the GCF to the CCF about 100 ms later.
The maximum flexural tensile strain results of the SCBs are presented in
Figure 9. As can be seen from
Figure 9, the effect of geogrid type on the maximum flexural tensile strain of the SCBs is obvious. The maximum flexural tensile strain of the CCF reinforcement is the highest and that of the UN is the lowest at low temperatures, regardless of the SCB type. Compared with the UN, the maximum flexural tensile strain increases by 72.70–93.11% for CCF reinforcement and 50.38–51.36% for GCF reinforcement. This indicates that the laying of a carbon fiber-based geogrid between the layers improves the low-temperature flexibility of the SCB. The increasing effect of the CCF is better than that of the GCF. Due to the performance difference between the longitudinal and transverse ribs of the GCF, uneven stress distribution occurs at the interlayer interface under loading. Compared with CCF, the GCF has a weaker restraining effect on the aggregates and the reinforcing effect is reduced, producing smaller flexural tensile strains.
It can also be seen from
Figure 9 that the SCB type has an effect in increasing the maximum flexural tensile strain of the RSCBs. The increasing effect of AC-20/AC-25 is better than that of AC-13/AC-20 at low temperatures, regardless of the geogrid type. The increasing effect of the maximum flexural tensile strain of the CCF reinforcement improves by 12.65% from AC-13/AC-20 to AC-20/AC-25. The reason may lie in the fact that under low-temperature conditions, the asphalt pavement reinforced surface layer combination can be approximated to an elastomer. As the increment in flexural tensile strength of the RSCB increases, so does the increment in flexural strain.
3.4. Effect of Reinforcements on Fracture Energy
When evaluating the low-temperature cracking resistance of asphalt mixtures in the literature [
53,
54,
55], the fracture energy (
Gf) is considered a comprehensive performance index that takes into account the strength and deformation of the material. Therefore, the fracture energy was used to evaluate the low-temperature cracking resistance of carbon fiber-based geogrid RSCBs. In order to better analyze the reinforcing effect of carbon fiber-based geogrids, the fracture energy (
Gf) was divided into two parts: the bending absorption energy (
QB) and the toughness energy (
Gt) [
56]. The
QB refers to the energy required for cracks to appear in the asphalt pavement SCB under the action of external forces. The
Gt refers to the ability of the asphalt pavement SCB to continue to open and expand under external forces after cracks appear, until a degree of instability is reached, causing fractures.
For the modified load–displacement curves, the load is used to integrate the vertical deformation. The integration range of the
QB is from
d1 to
d2 [
57,
58]. The integration range of the
Gt is from
d2 to
d3. The integration curves are shown in
Figure 11b.
Figure 12 shows the load–deflection curves of different SCBs. The
QB and
Gt values of the SCBs are calculated by Equation (2) and Equation (3), respectively, and the results are shown in
Table 6.
In which QB is the bending absorption energy of the specimen, J/m2; h1 is the height of the specimen, m; b is the width of the specimen, m; d1 is the modified initial deflection, m; d2 is the deflection corresponding to the maximum load after correction, m; Gt is the toughness energy of the specimen, J/m2; h2 is the length of the crack propagation, m; d3 is the deflection corresponding to the termination of the test after correction., m; and PB is the maximum value of the load, N.
The following can be seen from
Table 6:
The geogrid type has a significant effect on the low-temperature cracking resistance of the asphalt pavement SCB. Under low-temperature conditions, the Gf of the CCF is the largest, followed by that of the GCF, and that of the UN is the smallest, regardless of the SCB type. The differences in the Gf of the three reinforcements are significant under the same SCB conditions. Compared with the Gf of the UN, the Gf of the GCF and the CCF increases by 66.75–78.22% and 100.39–129.46%, respectively. It can be seen that carbon fiber geogrid reinforcement improves the low-temperature cracking resistance of the SCB. Compared with it, there is still a certain gap of the glass/carbon fiber composite qualified geogrid reinforcement.
For the same SCB type, the variability of the Gf of the RSCB depends mainly on the variability of the QB of the RSCB. Under the condition of the same SCB, the increment in the QB of CCF reinforcement accounts for about 87.94–90.10% of the increment in the Gf of CCF reinforcement, while the increment in the Gt of CCF reinforcement only accounts for about 9.90–12.06%. At the same time, the increment in the QB of GCF reinforcement accounts for about 86.00–94.17% of the increment in the Gf of GCF reinforcement, while the increment in the Gt of GCF reinforcement only accounts for about 5.82–14.00%. It is evident that increasing the QB is a key element in improving the low-temperature cracking resistance of RSCBs.
For the AC-13/AC-20 (AC/20/AC-25) combination, the
QB of the asphalt pavement SCB under different geogrid reinforcement conditions is not significantly different and the situation of the
Gt is also similar. However, the
QB and
Gt increase with the increase in the overall ultimate tensile strength of the carbon fiber-based geogrids. When the reinforcement material is changed from GCF to CCF, the
QB increases by 18.18–30.27% and the
Gt increases by 19.18–40.54%. It can be seen that the effect of the CCF is superior to that of the GCF from the energy point of view. That is because the planar grid-like structure of carbon fiber-based geogrids is formed by the intersection of longitudinal ribs and transverse ribs. The transverse ribs play an important role in maintaining the planar grid-like structure and improving the flexural rigidity of the longitudinal ribs [
59]. The reduction of transverse rib performance changes the geometry of the GCF mesh, leading to a reduction in stiffness, mesh stiffness, and tensile friction resistance, which changes the reinforcement mechanism of the GCF in the SCB and reduces the reinforcing effect. Therefore, the performance difference between the longitudinal and transverse ribs plays an important role in enhancing the low-temperature cracking resistance of carbon fiber-based geogrid reinforced asphalt pavement SCBs.
The SCB type has a certain effect on the low-temperature cracking resistance of asphalt pavement SCB. Under reinforced conditions, the improvement in low-temperature cracking resistance of AC-20/AC-25 is better than that of AC-13/AC-20, by 16.26%-24.57%. The reason lies in the fact that the improvement in the RSCB is influenced by factors such as aggregate type, aggregate particle size, and the stiffness ratio of the upper and lower layers. Due to the presence of the geogrid, the more the aggregate particle size matches the grid size, the stronger the interlocking effect between the geogrid and the particles. In this situation, the higher the stiffness ratio of the upper and lower layers, the stronger the stress diffusion effect, and the stronger the improvement in low-temperature cracking resistance.
In addition, as can be seen from
Figure 10, the toughness is enhanced, the brittleness decreases accordingly, and the crack propagation is delayed. The flexural tensile displacement significantly increases. However, it still tends to the brittle material fracture characteristics at low temperatures.
3.5. Interface Observations
The bending failure phenomena of the SCB specimens are depicted in
Figure 13 and
Figure 14.
As can be seen from
Figure 13 and
Figure 14, the crack propagation path of the UN specimen rises tortuously, forming a transverse crack of an overall “N-type”, in accordance with the results obtained by Zhang Xiaojing et al. [
60]. On the other hand, the crack propagation path of the RSCB generally shows as the “non-N-type”, forming non-penetrating cracks. Meanwhile, the crack openings of the UNs are relatively large. However, the crack openings of the CCF and GCF RSCBs are relatively small, and the RSCBs do not fracture.
The reason lies in the fact that crack propagation will be blocked and redirected due to the presence of geogrids. The process of crack propagation and the change in direction is accompanied by stress redistribution and energy release, which reduces the degree of singularity at the crack tip and retards the upward expansion of the crack. In low-temperature conditions, the flexural tensile strain of the asphalt mixture is small. When the opening of the crack tip does not reach the deformation limit of the grid but reaches the cracking limit of the upper layer, the crack will continue to expand upward. In the process of expansion, the opening of the cracks is mainly restrained by the “bridging action” of the longitudinal ribs of the geogrid. The specimen eventually forms a non-penetrating crack with a small degree of opening. In addition, when the crack passes through the geogrid, the concentrated force is generated at the intersection of the geogrid and the crack, which reduces the stress intensity factor at the tip of the crack and slows down the expansion of the crack. Consequently, carbon fiber-base geogrids play a very important role in changing the crack propagation mode of asphalt pavement SCBs.
Regardless of the SCB type, the difference in the crack propagation path between CCF and GCF reinforcement is small. The difference between the CCF and GCF fails to significantly change the crack propagation pattern of the RSCB. Therefore, the transverse ribs of carbon fiber-based geogrids play a major role in changing the crack propagation pattern of the RSCB.
In the SCB, there are differences in the crack propagation paths between the upper and lower layers. The variability is large for AC-13/AC-20 and small for AC-20/AC-25, which is probably due to the performance of the aggregates. The aggregates used in AC-13 are basalt and limestone aggregates, in which the coarse aggregates are basalt aggregates. The aggregates used in both AC-20 and AC-25 are limestone aggregates. During the crack propagation process, the cracks easily pass through the limestone. Basalt is a highly wear-resistant material mineral with better crush, abrasion, and wear values than limestone. During crack propagation, it is difficult for the cracks to pass through the basalt; rather, they pass along the edge of the basalt, changing and extending the crack propagation path. Hence, aggregate properties are also an important factor in changing the crack propagation pattern of the RSCB.
The phenomena of “fracture along the stone” and “fracture through the stone” have occurred in carbon fiber-based geogrid-reinforced specimens. It has been shown that the fracture type of the carbon fiber-based geogrid RSCBs under a tension–compression load is a mixed plastic–brittle fracture where plastic fracture and brittle fracture coexist [
61], which has certain significance for the road failure analysis of geogrid-reinforced asphalt pavement.