A State-of-the-Art Review on the Functionality of Ultra-Thin Overlays towards a Future Low Carbon Road Maintenance

Highway maintenance mileage reached 5.25 million kilometers in China by 2021. Ultra-thin overlay is one of the most commonly used maintenance technologies, which can significantly enhance the economic and environmental benefits of pavements. To promote the low-carbon development of ultra-thin overlays, this paper mainly studied the mechanism and influencing factors of several ultra-thin overlay functions. Firstly, the skid resistance, noise reduction, rutting resistance and crack resistance of ultra-thin overlays were evaluated. The results indicated that the high-quality aggregates improved the skid and rutting resistance of ultra-thin overlay by 5%–20%. The optimized gradations and modified binders reduced noise of ultra-thin overlay by 0.4–6.0 dB. The high viscosity modified binders improved the rutting resistance of ultra-thin overlay by about 10%–130%. Basalt fiber improved the cracking resistance of ultra-thin overlay by more than 20%. Due to the thinner thickness and better road performance, the performance-based engineering cost of ultra-thin overlay was reduced by about 30%–40% compared with conventional overlays. Secondly, several environmentally friendly functions of ultra-thin overlay were investigated, including snow melting and deicing, exhaust gas purification and pavement cooling. The lower thickness of ultra-thin overlay was conducive to the diffusion of chloride-based materials to the pavement surface. Therefore, the snow melting effect of self-ice-melting was better. In addition, the ultra-thin overlay mixture containing photocatalytic materials could decompose 20%–50% of the exhaust gas. The colored ultra-thin overlay was able to reduce the temperature of the pavement by up to 8.1 °C. The temperature difference between the upper and lower surfaces of the ultra-thin overlay containing thermal resistance materials could reach up to 12.8 °C. In addition, numerous typical global engineering applications of functional ultra-thin overlay were summarized. This review can help better understand the functionality of ultra-thin overlays and promote the realization of future multi-functional and low-carbon road maintenance.