4.3. Mechanical Properties of the IA
Based on laboratory tests, the mechanical performance of the IA and its mechanical performance in different asphalt mixtures were studied. During the test, the performance of IA was tested with the limestone testing methods and instruments. The technical performance comparison between the IA and traditional limestone aggregates is shown in
Table 9.
From
Table 9, it can be seen that the test values of each index for the IA were close to that of the limestone aggregate, which can indicate that the IA can replace a traditional aggregate to resist external forces and internal load transfer. According to
Section 2.2 “Preparation of specimen”, the ordinary asphalt mixtures and asphalt mixtures embedded with IA were formed: an AC-25 gradation asphalt mixture, AC-25 gradation asphalt mixture embedded with IA, SMA-25 gradation asphalt mixture, and SMA-25 gradation asphalt mixture embedded with IA. Three parallel specimens were prepared for each asphalt mixture. The dynamic modulus tests of the four types of asphalt mixtures were conducted based on the UTM-30 hydraulic servo testing machine, and the results were averaged. The loading frequency was 10 HZ, and the test temperatures were 0 °C, 10 °C, 20 °C, 30 °C, 40 °C, and 50 °C. The results are shown in
Figure 9.
From
Figure 9a, it can be seen that the dynamic modulus of the AC-25 gradation asphalt mixture embedded with IA was slightly lower than that of the AC-25 gradation asphalt mixture at different temperature levels. From
Figure 9b, the dynamic modulus of the SMA-25 gradation asphalt mixture embedded with IA was slightly lower than that of the SMA-25 gradation asphalt mixture at different temperature levels. Based on the SPSS two-factor variance analysis, the difference between the dynamic modulus test results of the asphalt mixtures was analyzed, and the results are shown in
Table 10.
According to the hypothesis of SPSS two-factor variance analysis, the significance of the difference was judged by the F-value. The P-value represents the probability value at the corresponding F-value, and the F-crit is the critical value of F at the corresponding significance level. If the F-value < F-crit and P > 0.05, there is no significant difference between the two groups of data [
22]. It can be seen from
Figure 9 and
Table 10 that the dynamic modulus of the asphalt mixture embedded with the IA was not significantly different from that of the ordinary asphalt mixture of either the SMA-25 gradation or AC-25 gradation. It can be inferred that embedding the IA into an asphalt mixture will not have a significant impact on the original mechanical properties of the mixture. However, the difference of the dynamic modulus between the two SMA-25 gradation asphalt mixtures is less than that between the two AC-25 gradation asphalt mixtures. This is because there is coarser aggregate in an SMA-25 gradation asphalt mixture, which more easily forms an aggregate skeleton under the bonding of asphalt. The particle size of the IA is about 20 mm, and the aggregate with this size has more content in the SMA-25 asphalt mixture. Thus, the traditional coarse aggregate can be replaced more naturally by the IA. Therefore, the difference in the dynamic modulus between the two SMA-25 asphalt mixtures is smaller.
4.4. Compaction Quality Evaluation of Asphalt Mixture Based on the IA
Asphalt mixtures are a particle aggregation, and there are interaction forces between particles. The motion state of an IA will change synergistically with other particles within a certain range. Therefore, the particle situation in an asphalt mixture can be speculated on using the motion data of the IA, and then, the compaction state of the asphalt mixture can be characterized more comprehensively and accurately combined with the compaction degree index. To propose a compaction quality evaluation standard of an asphalt mixture based on the IA motion data and supplement the current single asphalt pavement compaction evaluation index (compaction degree), it is necessary to analyze the relationship between the IA motion data and the compaction index (compaction times and compaction degree) of asphalt mixtures through laboratory tests.
The IA can obtain the attitude angle and acceleration data in the three directions of XYZ. The spatial attitude angle and spatial acceleration were taken as indicators for analysis in this paper. The calculation method is as shown in Equations (1) and (2).
where
C is the spatial acceleration, and the unit is g;
Cx is the acceleration in the X direction;
Cy is the acceleration in the
Y direction;
Cz is the acceleration in the
Z direction;
Φ is the spatial attitude angle, and the unit is degree (°);
Φx is the attitude angle in the
X direction;
Φy is the attitude angle in the Y direction; and
Φz is the attitude angle in the Z direction. The compaction degree is expressed by
K and calculated by Equation (3).
where
K is the compaction degree, with the unit %;
ρs is the actual density of the specimen determined by the laboratory test, with the unit g/cm
3;
ρ0 is the standard density of the asphalt mixture, with the unit g/cm
3.
ρ0 is based on the maximum theoretical density of the sampling test in an asphalt mixing plant, and the values in this paper were 2.67 g/cm
3 for the AC-25 asphalt mixture and 2.56 g/cm
3 for the SMA-25 asphalt mixture.
ρs was obtained by laboratory tests using the bulk density. According to
Section 2.2, “Preparation of specimen”, the asphalt mixture specimen with SMA-25 gradation and AC-25 gradation embedded with the IA was formed by superpave gyratory compactor (SGC), where the compaction angle was 1° the vertical load was 700 kPa and the molding temperature was 170 °C. The instantaneous attitude angle and acceleration data corresponding to each compaction in the process of 1–160 times compaction were obtained through the RDAPS system. In this process, the bulk density of the specimen was tested every 16 times of compaction.
According to the kinematic principle, when the spatial attitude angle and spatial acceleration of IA maintained a stable value in a certain period of time during the compaction process, it can be inferred that the interlocking state between aggregates was better [
23]. The relationship between the IA motion data and compaction times and the compaction degree of two asphalt mixtures is shown in
Figure 10 and
Figure 11.
From
Figure 10a, it can be seen that, with an increase in compaction times, the spatial acceleration of IA showed an unstable decreasing trend at the initial stage (less than 60 times), a stable decreasing trend at the middle stage (61–112 times), and a stable state at the late stage (113–160 times). This is because the early stage of compaction is the stage of the transition of the AC-25 asphalt mixture from a loose to a dense state, and the motion state of the IA in this stage is random [
24]. However, with the gradual compaction of asphalt mixture, the active range of the IA is gradually reduced, and the spatial acceleration is in an overall downward trend in this stage. At the middle stage of compaction, the asphalt mixture is further compacted, and the interlocking state between aggregates is formed at this stage. At this time, the movement of the IA is limited, and the spatial acceleration decreases sharply. When the compaction degree reaches 98% of the specification requirements, the corresponding compaction time is 106, and the spatial acceleration of the IA is not in a stable state at this time. When the compaction was 113 times, the corresponding compaction degree was 99%, and the spatial acceleration of the IA was stable at about 0.05 g/cm
3, due to the common drift phenomenon of the sensors at this time. It can be inferred that the interlocking state between the internal aggregates of the asphalt mixture was in a relatively optimal state.
From
Figure 11a, it can be seen that, with an increase in the compaction times, the spatial acceleration of the IA showed a relative stable decreasing trend at the initial stage (fewer than 60 times). This is because the coarse aggregates in the SMA-25 asphalt mixture more easily form an interlocking effect. As the compaction continues, the spatial acceleration trend of the IA was similar to that in the AC-25 asphalt mixture. When the compaction degree reached 98% of the specification requirements, the corresponding compaction time was 102, and the spatial acceleration of the IA was not in a stable state at this time. When the compaction was at 109 times, the corresponding compaction degree was 99%, and the spatial acceleration of the IA was stable at about 0.05 g/cm
3 at this time, which is consistent with the AC-25 asphalt mixture in
Figure 11a. Therefore, it can be inferred that, when the compaction degree of the asphalt mixture meets the requirements, there is still room for further optimization of the interlocking or contact state between the internal aggregates, which is also consistent with the above viewpoints in this paper [
25].
From
Figure 10b, it can be seen that the attitude angle of the IA decreased due to the gradual narrowing of the movable range during the first 20 compactions; however, the IA was not effectively interlocked in this process and was still free. During the compaction of 23–40 times, the attitude angle of the IA fluctuated clearly, which indicates that the IA was forming an interlocking effect with the other aggregates, and its position in the mixture was gradually fixed. With the continuous compaction, the spatial attitude angle of the IA continued to decline until it reached a stable state of 3° after 113 compactions. When the compaction degree reached 98%, the corresponding compaction times were 106, and the spatial attitude angle of the IA did not reach a stable state. Taking this experiment as an example, when the compaction number was 113 times, the corresponding compaction degree was 99%, and the spatial attitude angle of the IA reached a stable state at this time, which is consistent with the analysis results in
Figure 11a.
From
Figure 11b, it can be seen that the attitude angle of the IA decreased rapidly during the first 76 compactions. It decreased by 91.8% compared with the initial state, and the spatial attitude angle of the IA in the SMA-25 asphalt mixture showed a uniform downward trend in this process. This is because the SMA-25 gradation asphalt mixture formed a coarse aggregate skeleton structure faster than the AC-25 gradation asphalt mixture during the compaction process. As compaction continued, the downward trend of the spatial attitude angle of the IA in the SMA-25 asphalt mixture gradually slowed down until it reached a stable state. When the compaction degree reached 98%, the corresponding compaction time was 102, and the spatial attitude angle of the IA did not reach a stable state. With the continuous compaction, the spatial attitude angle of the IA continued to decline until it reached a stable state of 3° after 109 compactions, and the corresponding compaction degree was 99% at this time. To sum up, whether using SMA-25 or AC-25 gradation, with an increase in compaction times, the final stable values of the spatial acceleration and spatial attitude angle of the IA were consistent. Thus, the effect of the gradation type of asphalt mixture on the IA motion state was not significant.
Based on the above analysis, when the compaction degree met the specification requirements, the motion data of the IA did not reach a stable state, and the interlocking effect between aggregates in the AC-25 asphalt mixture or SMA-25 asphalt mixture could be further optimized. When the motion data of the IA reached a stable state, the corresponding compaction degree met the specification requirements. Based on the developed RDAPS, the evaluation conditions were proposed for the qualified compaction of asphalt mixtures with the compaction degree as the main index and spatial attitude angle and spatial acceleration of the IA as the auxiliary indexes, as shown in Equation (4).
where
K is the compaction degree;
CN is the stable value of the spatial acceleration of the IA after
N times compaction;
ΦN is the stable value of the spatial attitude angle of IA after
N times compaction; and the value of
N is determined by the actual situation. The three conditions in Equation (4) must meet simultaneously. This evaluation method is applicable to AC-25 and SMA-25 asphalt mixtures.