There have been considerable research studies on the urban heat island effect brought about by asphalt pavements, which is considered risky in permafrost regions. This is because the heat transfer from the asphalt pavement to the frozen soil layers may cause damage to the hydrothermal environment, which in turn reduces the active layer thickness, and as a consequence may cause a geological hazard. Various technologies have emerged to curb this solar heating phenomenon but they are majorly focused on transferring or releasing the absorbed heat in various directions. The solar heating reflective coating layer has emerged as a pre-treatment solution that prevents heat from being absorbed by the top pavement surface layer by reflecting the energy back into the environment.
In a recent paper published in Construction and Building Materials, researchers led by Professor Aimin Sha at Chang’an University in China developed an innovative coating layer technology that aims at cooling the pavement surface. They evaluated the performance of the new material such as skid and aging resistance as well as its cooling characteristics. Professor Sha added that “pavements are not only the passway for vehicles and passengers, but also the living and working environment for human. Hence, the best we can do is to recover the pavements’ function as a natural surface on Earth”.
The research team used normal asphalt concrete as well as epoxy resin and polyvinyl alcohol as the film in the coating layer. Additionally, titanium dioxide was used as pigment and carbon black was added to adjust the color of the coating mixture.
Tests results showed that the temperatures, regardless of top or bottom, were substantially reduced by the applied coatings. After a 1h exposure, the top of both uncoated and coated samples experienced a temperature change from 15°C, to 50°C and 40-45°C respectively. From the 1h point, the temperature raised slowly as compared with the first hour. On the other hand, the bottom temperature raised equably especially during the first hour where it exhibited a linear relationship. Further, the coating layer that contained about 20% titanium dioxide, and 0.5% carbon black had the highest temperature difference between uncoated and coated samples on both top and bottom surfaces after the 7h test period.
From the temperature curves of their laboratory heating setup, during the first hour, the coated samples absorbed the sunlight energy but reflected it back into the environment which explains why the temperature of the uncoated samples increased rapidly as compared with the coated samples. Between 1-2h, the heat overcomes the barrier of the upper layers and is transferred downwards which results in the reduction of the temperature differences between the uncoated and coated samples. The temperature between 5-7h was noted to be stable. The field tests results of the coating layer showed that the temperatures of the coating layer and the control pavement agreed with the laboratory data with a few differences.
To check on the impact that the coating has on the skid resistance, the authors added about 30wt% of sand to the coatings, both on the surface and in the layers. They noted that the sand greatly improved the surface roughness of the pavement which in turn improves the skid resistance.
Further tests on the potential application of the coating layer in permafrost regions showed that carbon black and titanium dioxide are stable under ultraviolet radiation whereas the resins aged. From the accelerated pavement testing, the authors concluded that excellent engineering performance of the solar heating reflective coating layer could be achieved by reducing the difference of modulus and stiffness between resin based layers and asphalt concrete bases. They successfully achieved a resilient system which adapts to the permanent structural deformation. Also, by improving the pavement stiffness, early damage such as cracking and spalling of asphalt mixtures are avoided.
Aimin Sha, Zhuanghuang Liu, Kun Tang, Pinyi Li. Solar heating reflective coating layer (SHRCL) to cool the asphalt pavement surface. Construction and Building Materials 139 (2017) 355-364.Go To Construction and Building Materials