A state-of-the-art review on cold binders for sustainable paving materials

Hot mix asphalt (HMA) has been widely used as a pavement material for decades because of its quick construction process and good engineering performance. However, its construction has to be performed at elevated temperature, causing significant energy consumption and hazardous emissions. Cold mix, which demands no heating in the construction process, is a cleaner and more environment-friendly paving technique. The cold mix binder, which bonds aggregates at ambient temperature, plays a key role in the environment-friendly cold mix pavement. However, in-depth understanding of the working mechanism and applications of cold mix binders is still lacking. To fill this gap, three different kinds of cold binders commonly used in pavement industry are extensively discussed, namely, the conventional bitumen emulsions, and the newly emerging epoxy resin and polyurethane.

Bitumen emulsions are by far the most widely used cold binder in pavement construction for surface dressing, tack coat and cold mix. However, bitumen emulsions are inferior to HMA in terms of early strength and mechanical properties, which limited them from been used in structural layers. To improve the performance of bitumen emulsion, polymer latexes, such as SBR latex and waterborne epoxy resin, are commonly used as modifiers to prepare polymer modified bitumen emulsions. The incorporation of polymer latexes can significantly improve the performance of bitumen emulsion, including high- and low-temperature performance, adhesion with aggregate, and fatigue performance.

Recently, polymer binders like epoxy resin and polyurethane have been introduced into the pavement industry. Epoxy resin and polyurethane are characterized as fast curing, remarkable mechanical strength, and strong adhesion with aggregate and substrates. However, there are still some shortcomings need to be addressed for the resin binders before they can be applied in large quantities, such as limited workability, insufficient resistance to weathering and high initial cost.

This paper set out to provide a state-of-the-art review on the constitutions, properties, applications, and pros and cons of three cold binders, i.e., bitumen emulsion, epoxy resin and polyurethane, paving the way for future research and applications of these cleaner construction materials in pavement engineering.