Experimental investigation of optimum properties and conditions for use of anionic emulsions in road maintenance applications

Bitumen emulsions are used for a range of road maintenance applications, including surface dressing, gravel seals, slurry seals, tack coating, cold-mix asphalt and in situ pavement recycling. The emulsions used can be classified as being either cationic (i.e. possessing a positive electrostatic surface charge) or anionic (i.e. carrying a negative electrostatic surface charge). The vast majority of emulsions used for such applications in Europe are cationic, while anionic bitumen emulsions are used widely throughout North America and Africa.
The aims of the experimental study described in this paper were (a) to investigate why one or the other emulsion type is favoured in different global regions, (b) to establish if the aggregates used can be classified as also having a positive or negative electrostatic surface charge and (c) to determine if the surface charge of the aggregate has an effect on the degree of adhesion between the aggregate and bitumen emulsion used. The overall objective of this research project is to study the potential development of new adhesion additives for anionic emulsions used in road applications.
The experimental laboratory work that was performed demonstrated that the electrostatic surface charge of the aggregate plays a significant role in the aggregate–emulsion adhesion mechanism. The laboratory tests results indicated that a higher degree of adhesion was achieved when an aggregate was combined with an emulsion possessing the opposite surface charge. This finding confirms the hypothesis that aggregates possess either an electro-positive or an electro-negative surface charge, depending on their mineralogical composition.