APA rutting tests were conducted for six kinds of asphalt mixtures under air-dry and immersing conditions. The influences of test conditions, including load, temperature, air voids, and moisture, on APA rutting depth were analyzed by using grey correlation method, and the APA rutting depth prediction model was established. Results show that the modified asphalt mixtures have bigger rutting depth ratios of air-dry to immersing conditions, indicating that the modified asphalt mixtures have better antirutting properties and water stability than the matrix asphalt mixtures. The grey correlation degrees of temperature, load, air void, and immersing conditions on APA rutting depth decrease successively, which means that temperature is the most significant influencing factor. The proposed indoor APA rutting prediction model has good prediction accuracy, and the correlation coefficient between the predicted and the measured rutting depths is 96.3%.
Rutting prevention has become one of the most in-demand topics of study with more-extensive research on damage to asphalt pavement. Studies all over the world have established a variety of rutting test methods to analyze and evaluate the antirutting properties of asphalt mixtures. APA (Asphalt Pavement Analyzer) rutting test has gained much international attention in recent years for such advantages as the ability to simulate live load conditions. In 2001, NCAT (National Center for Asphalt Technology) evaluated the applicability of various methods for evaluating the antirutting performance, including APA, HWTD (Hamburg Wheel-tracking Device), FRT (French Pavement Rutting Tester), RLWT, and triaxial repeated load creep tests, and preferentially recommended APA rutting test. Subsequently, numbers of research have conducted the antirutting properties of asphalt mixtures by using APA test.
Xie et al. researched moisture susceptibility through APA and analyzed the results of water-submerged rut testing, from which they presented an index of water submergence stability to assess water resistance of different asphalt mixtures [
Numbers of achievements have been obtained on APA test in the last decades, and most of the previous research focused on the evaluation of asphalt mixture antirutting performance by using APA test. Few attempts have reported the effects of testing conditions on APA rutting depth and are not sufficient in understanding the results of APA rutting tests influenced by test conditions. Therefore, in this paper, the APA experiments were performed under different test conditions for further understanding the influences of load, temperature, and other conditions on rutting depth, and an APA rutting prediction model was established to provide reference for further popularization and application of APA rutting tests.
Four kinds of AC-20 (Asphalt Concrete whose nominal maximum size of aggregate is 20 mm) asphalt mixtures and two kinds of AC-13 (Asphalt Concrete whose nominal maximum size of aggregate is 13 mm) asphalt mixtures (namely, AC-20 coarse-type modified asphalt, AC-20 coarse-type matrix asphalt, AC-20 fine-type modified asphalt, AC-20 fine-type matrix asphalt, AC-13 modified asphalt, and AC-13 matrix asphalt) were designed according to highway project to research the influence of experimental conditions on APA rutting tests. Shell 70 matrix asphalt and SBS modified asphalt were used in the experiments. Their technical properties are given in Table
Technical properties of Shell 70 matrix asphalt.
Test index | Unit | Test result | Technical requirement |
---|---|---|---|
Penetration (100 g, 5 s, 25°C) | 0.1 mm | 76.83 | 60~80 |
Penetration index | — | −1.2 | −1.5~1.0 |
Softening point | °C | 46.2 | ≥45 |
Ductility (5 cm/min, 10°C) | cm | 79.5 | >25 |
Ductility (5 cm/min, 15°C) | cm | 138.0 | >100 |
Wax content (distillation method) | % | 1.9 | ≤2.2 |
Flash point | °C | 316.3 | ≥260 |
Solubility | % | 99.58 | ≥99.5 |
Density (15°C) | g/cm3 | 1.036 | Measured records |
Density (25°C) | g/cm3 | 1.033 | — |
Quality loss after RTFOT | % | 0.170 | ±0.8 |
Residual penetration after RTFOT (100 g, 5 s, 25°C) | 0.1 mm | 52.83 | — |
Penetration ratio after RTFOT (25°C) | % | 68.76 | ≥61 |
Residual ductility after RTFOT (5 cm/min, 10°C) | cm | 10.4 | >6 |
Residual ductility after RTFOT (5 cm/min, 15°C) | cm | 46.8 | >15 |
Technical properties of SBS modified asphalt.
Test index | Unit | Test result | Technical requirement |
---|---|---|---|
Penetration (100 g, 5 s, 25°C) | 0.1 mm | 65.67 | 60~80 |
Penetration index | — | 0.1 | ≥−0.4 |
Softening point | °C | 83.1 | ≥55 |
Ductility (5 cm/min, 5°C) | cm | 33.8 | ≥30 |
Flash point | °C | 272 | ≥230 |
Solubility | % | 99.7 | ≥99 |
Elastic recovery (25°C) | % | 99.67 | ≥65 |
Segregation of storage stability, difference of softening point after 48 h | °C | 1.6 | ≤2.5 |
Density (15°C) | g/cm3 | 1.034 | — |
Relative density (15°C) | — | 1.037 | — |
Density (25°C) | g/cm3 | 1.030 | — |
Relative density (25°C) | — | 1.033 | — |
Quality loss after RTFOT | % | −0.2 | ±1.0 |
Residual penetration after RTFOT (100 g, 5 s, 25°C) | 0.1 mm | 48.7 | — |
Penetration ratio after RTFOT (25°C) | % | 74.16 | ≥60 |
Residual ductility after RTFOT (5 cm/min, 5°C) | cm | 21.46 | ≥20 |
Three kinds of aggregate gradations (namely, AC-20 coarse-type, AC-20 fine type, and AC-13) were selected to mix with two kinds of asphalt binders, respectively, to prepare six kinds of asphalt mixtures. Optimal asphalt contents of asphalt mixtures were determined by the Marshall method; the results are presented in Table
Design gradations and optimal asphalt contents of asphalt mixtures.
Gradation type | Mass percentage (%) through the following mesh (mm) | OAC (%) | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
26.5 | 19 | 16 | 13.2 | 9.5 | 4.75 | 2.36 | 1.18 | 0.6 | 0.3 | 0.15 | 0.075 | Matrix asphalt | Modified asphalt | |
AC-20 C-type | 100 | 92.3 | 82.9 | 69.6 | 58.2 | 39.9 | 27.3 | 18.7 | 13.7 | 10.9 | 6.9 | 4.7 | 4.0 | 4.2 |
AC-20 F-type | 100 | 95.1 | 88.9 | 79.7 | 68.5 | 53.3 | 40.9 | 31.1 | 21.6 | 16.3 | 8.9 | 5.4 | 4.3 | 4.3 |
AC-13 | 100 | 100 | 100 | 97.3 | 75.4 | 45.6 | 27.1 | 20.3 | 14.2 | 10.7 | 8.2 | 5.1 | 5.1 | 4.9 |
Cylinder specimens (
APA rutting tests using SGC cylinder specimens (as shown in Figure
Cylinder specimens in APA testing.
Under immersion conditions, the specimens with the air voids of 4 and 7% were tested, respectively; test temperatures were 50 and 60°C, respectively; load was 445 N; and tire pressure was 690 kPa. All the specimens were placed in APA test machines at the test temperature for 6–24 h before testing to ensure that the specimens would reach the test temperature and maintain temperature equalizing.
Under dry conditions, the APA rutting depth development trends for different types of asphalt mixtures under three different temperatures are shown in Figures
APA rutting depths of asphalt mixtures at 40°C under air-dry conditions.
APA rutting depths of asphalt mixtures at 50°C under air-dry conditions.
APA rutting depths of asphalt mixtures at 60°C under air-dry conditions.
Results show that rutting depth changes with the number of loads and the temperature, according to the APA rutting test results under different temperatures. Thus, the temperature-effects regression model shown in (
For the asphalt mixtures, many factors, such as asphalt properties, asphalt content, air void, aggregate gradation, and others could influence rutting depth
Under dry conditions, the influence from combinations of different loads and tire pressures on the APA rutting depth of asphalt mixtures is shown in Figure
APA rutting depths of asphalt mixtures at different combinations of load and tire pressure.
Matrix asphalt mixture
Modified asphalt mixture
The testing results are consistent with research results of Gu et al., which indicate rutting depths all tend to increase with increases in load and tire pressure [
Development trends of APA rutting depth of asphalt mixtures with the air voids of 4% and 7% under different temperature and humidity conditions are shown in Figures
APA rutting depths of asphalt mixtures with 4% air voids at 60°C under immersing conditions.
APA rutting depths of asphalt mixtures with 7% air voids at 60°C under immersing conditions.
APA rutting depths of asphalt mixtures with 4% air voids at 50°C under immersing conditions.
APA rutting depths of asphalt mixtures with 7% air voids at 50°C under immersing conditions.
The rutting depth ratio is defined as the ratio of dry rutting depth and immersion rutting depth, and the water stability of asphalt mixture with greater rutting ratio is better [
Rutting depth ratio of specimens with 4% air void at 60°C under different rolling times.
Matrix asphalt mixture
Modified asphalt mixture
Rutting depth ratios of specimens with 7% air void at 60°C under different rolling times.
Matrix asphalt mixture
Modified asphalt mixture
Rutting depth ratios of specimens with 4% air void at 50°C under different rolling times.
Matrix asphalt mixture
Modified asphalt mixture
Rutting depth ratios of specimens with 7% air void at 50°C under different rolling times.
Matrix asphalt
Modified asphalt
Analyses show that the rutting depth ratio of the modified asphalt mixture is greater which means that the water stability is better considering the viscosity of modified asphalt is greater than that of matrix asphalt, more polar materials exist in the matrix asphalt, and the matrix asphalt has good wettability. The water stabilities of the AC-13 modified asphalt mixture and the AC-13 matrix asphalt mixture are worse than those of the other limestone mixtures because of the use of acidic stone granite. The AC-13 matrix asphalt mixture is obvious, its rutting depth ratio is the smallest, and the specimens were spilled at the test temperature of 60°C.
Comparisons of the rutting depth ratios at different loading times show that the rutting depth ratios increases, whereas the influence of water decreases gradually with the increase in loading times. The air void of asphalt mixtures decreases and the aggregate skeleton resistance increases with the repeated action of the loading wheel. The increasing amplitude of pore-water pressure is smaller compared to that of the increasing amplitude of the aggregate skeleton resistance, although pore-water pressure also increases. Thus, the influence of water decreases gradually. The results also show that the order of the water stability of the asphalt mixture ordered by rutting depth ratios under 25 or by 4000 loading times is not stable, whereas the order of the water stability of the asphalt mixture ordered by rutting depth ratios under 8000 loading times is more stable. Therefore, 8000 loading times of APA equipment should be ensured when evaluating tests of water stability performance, and test results from loading times under 25 or 4000 are not recommended for evaluating water stability of asphalt mixtures.
Numbers of researchers analyzed influencing factors of rutting deformation characteristics on asphalt pavement. Peilong et al. analyzed the correlation of influencing factors of rut resistance using grey theory, indicating that rut deformation rate has the maximum grey correlation with rate rut depth among five influencing factors of void ratio, graduation index, rut deformation rate, passing rate at 4.75 mm in middle layer, and filler/asphalt ration [
Firstly, pinpoint both the reference sequence and the compared sequence when using the grey correlation method for analysis. Assume that the reference sequence is
The expression of the correlation degree is
Grey correlation degrees of temperature, air void, load, and immersing condition.
The results show that the order of correlation degree is temperature, load, air void, and immersing condition. Thus, temperature is the closest correlation associated with asphalt mixture rutting depth. The main reason is that the asphalt binder in the asphalt mixture is temperature-sensitive, and the deformation depends significantly on temperature. Under high-temperature conditions, the asphalt is prone to flow, and rutting appears. This finding also verifies that asphalt pavement rutting forms mainly under high temperatures. Also, load is closely associated with asphalt mixture rutting depth. Increasing the load might damage the interlock structure between the aggregates in the asphalt mixture, which might affect the temperature of the pavement structure and lead to rutting. The relationship between air void and rutting is that the existence of air voids might cause supplementary compaction under loading. However, the air voids of asphalt mixtures are limited. Hence, the correlation degree of air voids is relatively low. Meanwhile, rutting should be weak. Water damage to the asphalt mixture is mainly attributable to water entering the interface of the asphalt and the aggregate and causing asphalt and aggregate spalling. However, mixtures with dense gradation are mainly used in this research while the water that could enter the interface of the asphalt to aggregate is limited. Thus, water has a relatively low influence on the stability of mixtures.
With reference to the model of APA rutting depth-temperature-loading times in formula (
The APA standard test conditions are as follows:
According to the predicted APA rutting depth in (
Comparison of measured and predicted APA rutting depths.
The orders of rutting depths for 6 kinds of asphalt mixtures under different temperatures and combinations of load and tire pressure are almost the same; that is, AC-20 type matrix asphalt mixture > AC-13 matrix asphalt mixture > AC-20 coarse-type matrix asphalt mixture > AC-20 fine-type modified asphalt mixture > AC-13 modified asphalt mixture > AC-20 coarse-type modified asphalt mixture. Compared with the matrix asphalt mixture, the APA rutting depth of the modified asphalt mixture is smaller. Besides he rutting resistance of the modified asphalt mixture is also better. The effect of temperature on the development of rutting is nonnegligible. At a test temperature of 40°C, the development of rutting of different asphalt mixtures is relatively mild, and the rutting depth is small. Rutting depth at a test temperature of 50°C is 2 to 3 times as big as the rutting at 40°C, whereas rutting depth at 60°C is approximately 1.5 times as large as rutting at 50°C. The regression model of APA rutting depth-temperature-loading times was established and verified on the basis of rutting depth under different test temperatures and loading times. The correlation degrees of temperature, combinations of load and tire pressure, and water on the APA rutting depth were researched through grey correlation method. The results show that temperature is the most significant influencing factor. The indoor APA rutting prediction model considering several factors such as temperature, loading times, loading level, and air void was established. The prediction of this model is precise and convincing. Rutting depth under other conditions could be predicted on the basis of APA rutting depth under these benchmark conditions.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This work was supported by the National Natural Science Foundation of China (no. 51408043), the Department of Science & Technology of Shaanxi Province (no. 2016KJXX-69), the Open Foundation of Key Laboratory of Highway Construction & Maintenance Technology in Loess Region, Ministry of Transport, China (KLTLR-Y11-7), and the Special Fund for Basic Scientific Research of Central College of Chang’an University (nos. 310821153502 and 310821173501). The authors gratefully acknowledge their financial support.