To reduce the temperature of asphalt pavement and improve the antirutting performance of asphalt mixture, a thermal-resistant asphalt mixture (TRAM) was produced, in which a certain proportion of mineral aggregate was replaced by ceramic (CE) or floating beads (FB) featuring low thermal conductivity. Firstly, a parallel plate test was developed to test the thermal conductivity of asphalt mixture added with different thermal-resistant materials. Secondly, the illumination test system was designed to study the visual cooling effect of different TRAM by imitating the natural environment. Finally, the effect of different thermal-resistant materials on asphalt pavement performance was evaluated. The results show that the addition of thermal-resistant materials can reduce the thermal conductivity and the temperature of asphalt mixture. The cooling effect of CE75 and CE100 (coarse aggregate substituted by 75% and 100% CE, respectively) is superior to other aggregates. The temperature reduction rates of CE75 and CE100 reache 6.6°C and 6.8°C, respectively. For FB50 and FB75 (fine aggregate substituted with 50 and 75% FB, respectively), the cooling effect of them reaches 3.9°C and 4.5°C, respectively. In addition, the CE and FB can improve the antirutting performance of asphalt mixture by reducing the temperature inside the pavement. The high-temperature performance of CE75 and FB75 is the best. With the increase of thermal resistance materials, the low-temperature cracking resistance of asphalt mixture decreases gradually. The failure strain of mixture added with 100% thermal resistance materials is close to the lower limit of Chinese specification. The water stability of different TRAM changes with various test methods. Taking into account the results of pavement performance and the cooling effect, the substitution proportion of CE and FB for TRAM is proposed as 50%∼75%, respectively.
Asphalt mixture is a kind of black material, which highly absorbs solar radiation [
Heat-reflective layer and thermal resistance technology have been used to lower the pavement temperature [
Feng and Yi [
Li [
Although there have been considerable researches related to the cooling effect of asphalt mixture added with different materials, at present, many studies just describe the cooling effect of thermal resistance materials on pavement, and the test methods cannot simulate the actual working environment of asphalt pavement. Only some studies relate the influence of cooling effect of different thermal resistance materials on pavement performance. In this paper, the test method, imitating the real temperature environment of asphalt pavement, was designed to evaluate the thermal conductivity and antirutting performance of two kinds of TRAM. Firstly, the thermal conductivity was measured by a parallel plate test. Secondly, the indoor and outdoor illumination tests were developed to evaluate the cooling effect of TRAM by imitating the natural environment. Finally, the effect of different thermal-resistant materials on asphalt mixture pavement performance was also studied.
SK-70# asphalt was used in the TRAM. The coarse basalt aggregate, fine limestone aggregate, and limestone powder were selected. Thermal-resistant materials were shale ceramic (CE) and fly ash floating beads (FE). CE is a mineral material which is produced through the process of burning up and foaming. It is a spherical material with smooth surface and honeycomb porous. In this paper, CE was used to replace the coarse aggregate to produce a thermal-resistant asphalt mixture due to its larger particle size. The shale ceramic was adopted in this paper, and the particle size was between 5 and 15 mm [
The physical properties of CE.
Technical index | Unit | Measured value | Norms requirement |
---|---|---|---|
Crushing value | % | 30.1 | ≤26 |
Los Angeles wear rate | % | 26 | ≤28 |
Polished value | PSV | 50 | ≥42 |
Average particle size coefficient | % | 0.9 | ≤1.6 |
Adhesion | — | 4 grade | ≥4 grade |
Water absorption | % | 5.3 | ≤3 |
Floating beads (FB) [
Major performance indexes of FB used in the test.
Technical index | Unit | Floating beads | Fly ash |
---|---|---|---|
Refractory temperature | °C | 1500 | 1400 |
True density | g/cm3 | 0.721 | 1.95 |
Specific surface area | cm2/g | 2516 | 3120 |
SiO2 content | % | 55–59 | 1.3–65 |
Particle-size distribution |
|
0–600 | 0–300 |
Thermal conductivity | W/(m·K) | 0.058 | 0.110 |
In this paper, AC-13C which was widely used in the upper layer of asphalt pavement was used to analyze the thermal resistance effect of CE and FB. The gradation composition is detailed in Table
Aggregate gradation used for AC-13C mixture.
Mineral aggregate gradation | Passing ratio (by mass) (%) | |||||||||
---|---|---|---|---|---|---|---|---|---|---|
16 | 13.2 | 9.5 | 4.75 | 2.36 | 1.18 | 0.6 | 0.3 | 0.15 | 0.075 | |
AC-13C | 100 | 96.5 | 80.0 | 53.7 | 37.0 | 25.2 | 17.5 | 11.8 | 9.0 | 5.4 |
According to the Marshall test, the optimal asphalt content of CE (CE0, CE25, CE50, CE75, and CE100) can be determined. The results are shown in Table
Results of the mixture with different CE proportions.
Mixture type | Optimal asphalt content (%) | Gross volume density (g/cm3) | Maximum theoretical relative density | VV (%) | VMA (%) | VFA (%) | MS (kN) | FL (0.1 mm) |
---|---|---|---|---|---|---|---|---|
CE0 | 4.7 | 2.384 | 2.493 | 4.4 | 16.1 | 73.00 | 11.58 | 3.24 |
CE25 | 4.8 | 2.214 | 2.320 | 4.6 | 17.0 | 73.12 | 11.44 | 3.37 |
CE50 | 4.8 | 2.059 | 2.156 | 4.5 | 16.8 | 71.10 | 10.77 | 3.17 |
CE75 | 5.2 | 1.956 | 2.052 | 4.7 | 21.7 | 78.44 | 10.43 | 2.96 |
CE100 | 5.5 | 1.817 | 1.902 | 4.5 | 26.9 | 83.30 | 10.31 | 3.52 |
Results of the mixture with different FB proportions.
Mixture type | Optimal asphalt content (%) | Gross volume density (g/cm3) | Maximum theoretical relative density | VV (%) | VMA (%) | VFA (%) | MS (kN) | FL (0.1 mm) |
---|---|---|---|---|---|---|---|---|
FB0 | 4.7 | 2.384 | 2.493 | 4.4 | 16.1 | 73.00 | 11.58 | 3.24 |
FB25 | 4.8 | 2.331 | 2.425 | 3.9 | 16.7 | 76.79 | 10.32 | 2.46 |
FB50 | 4.6 | 2.320 | 2.415 | 3.9 | 12.6 | 68.74 | 9.73 | 2.33 |
FB75 | 4.4 | 2.273 | 2.368 | 4.0 | 10.8 | 62.99 | 9.39 | 2.05 |
FB100 | 4.3 | 2.225 | 2.326 | 4.1 | 8.2 | 51.24 | 9.28 | 1.44 |
The thermal resistance material was applied to reduce pavement temperature due to its low thermal conductivity. To verify the thermal conductivity of thermal resistance materials, the parallel plate test was developed. The schematic diagram is shown in Figure
Schematic diagram of the test system.
The test device is mainly composed of three parts: heating and temperature control system, temperature acquisition system, and insulation system. The test specimens were 300 mm × 300 mm × 50 mm in dimensions. In order to accurately control and measure the heat passing through the specimen, the electric heating plate was sandwiched between two slab specimens; the constant-temperature water tank was controlled by a thermostatic circulation system which was arranged on the outside of the two specimens so that the heat passes through the specimens from the inside to the outside and gradually forms a stable gradient temperature field through which the thermal conductivity state can be reached. The probes of the temperature collector were arranged on both sides of the specimen. Thus, the collector can accurately measure the surface temperature on both sides, and then, the data were imported and stored in a multichannel temperature data acquisition instrument.
Test system of indoor illumination. (a) Schematic diagram of the illumination test and (b) light testing apparatus.
The temperature sensors were installed in the middle of top and bottom surface of the slab specimen, respectively. The temperature of the specimens was measured by using the multichannel automatic temperature recorder at the intervals of 30 min. In the procedure, the specimen was exposed to the lighting system at least 5 hours in the test.
Test system of outdoor illumination.
To evaluate the influence of asphalt mixture with different thermal-resistant materials on pavement performance, the asphalt mixture with different contents of CE and FB was subjected to the illumination rutting test, the little beam bending test, the freeze-thaw splitting test, and the Hamburg wheel tracking device (HWTD) test.
At present, the rutting test is a widely used method for evaluating the high-temperature performance of mixture. In the rutting test, the specimen is conditioned at 60°C for 5 hours in the environment box. By this insulated condition process, the top and bottom of the specimen reaches a thermal equilibrium; the cooling effect of the thermal-resistant material cannot work in the rutting test. Therefore, the test method of illumination rutting was developed to evaluate the high-temperature performance of different thermal resistance asphalt mixtures. In the environment box, the temperature change of the asphalt mixture is close to its regularity under natural illumination during actual use so that the rutting resistance of the thermal resistance asphalt mixture was evaluated more objectively. The illumination rutting test system operates as shown in Figure
Illumination rutting test system.
The addition of thermal-resistant material causes the change of the temperature field in pavement structure under the illumination and ultimately affects the antirutting ability of the pavement.
It can be seen from Figure
Results of the thermal conductivity test.
The slab specimens of CE and FB mixtures were subjected to the indoor illumination test, and the results are shown in Figure
Temperature variation of the indoor illumination test (°C). (a) The top surface for CE type, (b) the bottom surface for CE type, (c) the top surface for FB type, and (d) the bottom surface for FB type.
Results of the indoor cooling effect for TRAM.
Mixture type | Top surface maximum temperature (°C) | Bottom surface maximum temperature (°C) | Temperature difference (°C) | Cooling effect (°C) |
---|---|---|---|---|
CE0 | 64.5 | 64.3 | 0.3 | — |
CE25 | 64.8 | 61.4 | 3.4 | 2.6 |
CE50 | 65.2 | 59.4 | 5.8 | 4.2 |
CE75 | 65.35 | 56.85 | 8.5 | 6.6 |
CE100 | 65.8 | 56.2 | 9.6 | 6.8 |
FB0 | 64.5 | 64.3 | 0.2 | — |
FB25 | 64.8 | 60.9 | 3.9 | 3.1 |
FB50 | 65.7 | 59.2 | 6.5 | 3.9 |
FB75 | 65.9 | 58.4 | 7.5 | 4.5 |
FB100 | 66.0 | 60.6 | 5.4 | 2.2 |
For the mixture added with CE, the order of their cooling effect is as follows: 100, 75, 50, 25, and 0%, while for the mixture added with FB, the order is as follows: 75, 50, 25, 100, and 0%; the cooling effect of specimen CE100 and specimen FB75 reaches 6.8°C and 4.5°C, respectively. By comparison, the cooling effect of CE is better than that of FB.
In order to verify the accuracy of the indoor illumination test, the outdoor illumination test of CE was carried out in this study. The results are shown in Figure
Temperature variation of the outdoor illumination test (°C).
Results of outdoor cooling effect for CE.
Mixture type | Top surface maximum temperature (°C) | Bottom surface maximum temperature (°C) | Temperature difference (°C) | Cooling effect (°C) |
---|---|---|---|---|
CE0 | 51.8 | 45.3 | 6.5 | — |
CE25 | 51.6 | 41.2 | 10.4 | 4.3 |
CE50 | 51.3 | 40.6 | 10.7 | 5.2 |
CE75 | 51.5 | 38.9 | 12.6 | 6.7 |
CE100 | 51.8 | 38.5 | 13.3 | 6.8 |
As can be seen from Figure
Results of the illumination rutting test.
For FB mixture, the change laws of rutting depth and deformation rate are similar to the CE mixture as a whole. With the increasing of FB, compared with FB0, the decrease rates are 8.5% (FB25), 20.1% (FB50), 25.1% (FB75), and 16.5% (FB100). The reason for the change of rutting depth and deformation rate is similar to that of CE mixture.
Under the same proportion of admixture, the antirutting performance of FB mixture is worse than that of CE mixture. Moreover, the cooling effect of CE mixture is also better than that of FB mixture, and the same conclusions are achieved in the thermal conductivity test and cooling effect test. The order of CE mixture performance is conducted as follows: 75, 100, 50, 25, and 0%. It can be seen that the high-temperature performance order of mixture with 75% and 100% CE is not consistent with the cooling effect order. The reason is that hollow ceramic makes the strength of mixture to decrease and the content of asphalt will increase due to more porous materials in the mixture. These two factors comprehensively make the high-temperature performance decrease, so the content of CE in the mixture should be controlled. While for the mixture with FB, the order of antirutting performance is as follows: 75, 50, 100, 25, and 0%, which is not consistent with the cooling effect order of the mixture with 25% and 100% FB. This is because when the substitution proportion of FB reaches 100%, the spherical shape of FB makes the asphalt mixture easily to be compacted and less optimum asphalt content, which improves the high-temperature performance to some extent.
The low-temperature anticracking performance of mixture added with different contents of CE and FB was evaluated by the beam bending test, respectively. The results are shown in Figure
Results of the little beam bending test. (a) Ultimate flexural strength and failure strain and (b) stiffness modulus.
As shown in Figure
As can be seen from Figure
Results of the freeze-thaw splitting test. (a) CE type. (b) FB type.
Results of HWTD.
Mixture type | Loading times (times) | Rut depth (mm) | Sip (times) | Strip slope |
---|---|---|---|---|
HMA | 20000 | 19.4 | 11800 | 450.3 |
CE25 | 20000 | 18.9 | 9800 | 576.4 |
CE50 | 20000 | 17.3 | 12160 | 858.5 |
CE75 | 20000 | 15.6 | 12710 | 999.7 |
CE100 | 20000 | 18.0 | 7360 | 835.9 |
FB25 | 20000 | 15.6 | 12920 | 995.5 |
FB50 | 20000 | 16.4 | 13250 | 910.2 |
FB75 | 20000 | 16.5 | 13900 | 751.7 |
FB100 | 20000 | 19.2 | 8130 | 703.3 |
For the mixture with FB, the TSR of asphalt mixture gradually drops as the proportion of FB increases. When the substitution proportion of FB reaches 100%, water stability of asphalt mixture cannot meet the requirements of specification. In the HWTD test, the order of water stability performance is as follows: 75, 50, 25, 0, and 100%. This means that the conclusions about the water stability evaluation of the mixture with FB are concerned with different test methods. The reason is that the freeze-thaw splitting test is used to evaluate short-term water stability, and the test loading mode of asphalt mixture is different from reality, causing the relatively low result reliability. In comparison, the dynamic loading is used in the HWTD test to simulate real load action which can imitate the effect of dynamic water on adhesion between asphalt and aggregate, so the result is more credible.
In this study, the cooling effect of two kinds of thermal resistance materials was studied through indoor and outdoor illumination tests. The road properties of the mixture, such as high-temperature performance, low-temperature performance, and water stability performance, added with thermal resistance materials were investigated. The main conclusions are summarized as follows: The thermal conductivity of asphalt mixture decreases as the proportion of thermal resistance material increases. The thermal conductivity of CE mixture is smaller than that of FB mixture under the same proportion of admixture. For CE mixture, the cooling effect increases gradually with the increase of CE content. The temperature of the mixture added with 75% and 100% CE drops to 6.6 and 6.8°C, respectively. And, for FB mixture, the cooling effect increases first and then decreases as the proportion of FB increases. The temperature drops to 3.9°C and 4.5°C, respectively, for the mixture added with 50% and 75% FB. The cooling effect of CE is better than that of FB. The high-temperature performance of the mixture added with CE and FB increases first and then decreases. When the proportion of CE and FB is 75%, the mixture obtains the best high-temperature performance. The antirutting performance of FB mixture is worse than that of CE mixture. With the increase of two thermal resistance materials, the low-temperature cracking resistance of cooling mixture decreases. The failure strain of CE100 and FB100 is close to the lower limit of Chinese specifications, so the replacement of two thermal resistance materials in the mixture should be less than 100%. The water stability of different TRAM varies with the test methods. Based on the results of pavement performance and the cooling effect, the substitution proportion of CE and FB for the TRAM is proposed as 50%∼75%.
We confirm that the data submitted in this manuscript are available. All the data provided in the manuscript were obtained from the experiments performed at the Key Laboratory for Special Area Highway Engineering of Ministry of Education of Chang’An University.
The authors declare that they have no conflicts of interest.
This work was supported by the Key Project of Natural Science Research of Anhui Provincial Department of Education (KJ2018A0668) and Demonstration Experiment Training Center (2018FXJS01). The authors gratefully acknowledge their financial support. And special thanks also are due to the Key Laboratory for Special Area Highway Engineering of Ministry of Education.