Skid resistance between vehicle tires and pavements is one of the main factors affecting traffic safety. Reduction in skid-resistance due to the covering of ice or snow on pavement surfaces is the main cause of road accidents during winters. Thus, it is imperative to immediately clear the snow or ice to increase the skid resistance. Typically, road maintenance procedures during winters involve deicing, snow removal and anti-icing applications. Anti-icing technologies are more sustainable and a safer solution compared to the use of salts and chemicals, which are detrimental to both the environments, constructures and materials.
The use of antifreeze asphalt concrete (AFACs) has attracted significant research attention since its innovation decades ago. AFACs are produced by incorporating modified salts that can be grouped into three categories: salt-based fillers, salt-based coatings and salt-based aggregates. Nevertheless, the selection of the most appropriate salt type remains the biggest challenge in the practical application of AFACs. Besides, it is necessary to incorporate modified aggregate-like or filler-like additives to achieve a balanced and efficient control of the salt release rate adequate to ensure antifreeze ability. Notably, AFAC finds it difficult to supply enough salts to provide good friction after a temperature drop in heavy and long-lasting freezing water or snowstorm.
Despite the extensive study on the development of salt-based additives, there is little effort to detail the effects of the softening points of the salt particle modifier despite its implication in the application of AFACs. Most importantly, it is important to link the behavior of salt-based additives during hot mix asphalt (HMA) production to the modifier’s softening point. Additionally, more comprehensive studies on this are still required to establish the relationship between the modifier’s softening point and the engineering performance of the salts and AFACs.
To this note, Professor Zhuangzhuang Liu is leading an international research team composed of Chinese and Norwegian scientists focusing on snow and ice control on pavements. Recently, this team investigated the effects of the softening points of hydrophobic polymers on the behaviors of salt-based fillers used in AFACs. The team commenced by synthesizing resinous modified salts with different softening points. The salt release property, moisture absorption and thermal stability of the salts as well as the antifreeze and moisture susceptibility performance of the AFACs and the aging effects, were then evaluated. Their work is currently published in the journal, Materials and Design.
The authors findings showed the softening points did not affect the conductivity or salt-releasing property of the hydrophobic polymer-modified salt (NaCl) without preheating treatment. However, it did affect the salt-releasing property after being mixed with the asphalt concrete, and the salt conductivity decreased when the softening points were lowered. Other observed effects were binder modification by the polymer, asphalt binder-induced coating layer on salts and the differences in the softened polymer caused by the softening points. Calcium chloride and their hydrates were found not to be appropriate for producing salt-based fillers for AFACs applications because of their high moisture absorption capacity up to 40%, resulting in various anti-skid-related issues.
Since the hydrophobic polymers softened between 100 – 200 °C and decomposed between 250 – 300 °C, the modified salt remained stable during the production of the HMA. As a result, it was possible to ignore the feeding effects of salt-releasing property and anti-freezing performance as they were negligible. The best controlled-releasing property was achieved at 130 °C after first heating and at 150 °C after second heating. This was attributed to four different scenarios of the polymer particles on the salt surface based on the microstructure evolution. Finally, the heating cycle-dependent conductivity trends were divided into three patterns.
In summary, the study firstly reported the impact of softening point of hydrophobic polymer on salt-based fillers commonly used in AFACs. Overall, a better moisture susceptibility performance of the AFACs was realized at the higher softening point of the hydrophober, while preheating the salts damaged the AFACs. The authors confirmed the important role of softening points and aging effects on the performance of additive fillers and AFACs. In a statement to Advances in Engineering, the research team explained that their findings would contribute to the fundamental understanding of the salts filler paticles , as well as the quality control of AFAC asphalt mixtures in plants.
Liu, Z., Hoff, I., & Sha, A. (2021). Effects of the softening point of hydrophobic polymers on the salt-based filler used in antifreeze asphalt concretes. Materials & Design, 204, 109663.