Types and grades
Many different types and grades of polymers can be used to produce PMBs suitable for road surfacing applications. PMBs are generally used on defective or highly stressed pavement surfaces where the use of conventional bitumen binder is not a practical alternative. Polymers can be engineered to develop a range of physical properties. Basically they fall into two categories, elastomers and plastomers.
Elastomers resist permanent deformation when stretched and their strength increases with level of elongation. Natural and synthetic rubbers and SBS block copolymers are common elastomeric polymers used for modifying bitumen used in seals and asphalt to make the binder more resilient and flexible. Natural and scrap rubber do not increase strength with elongation.
Plastomers form a rigid and tough structure that resists deformation. EVA is a common plastomeric polymer used for modifying conventional bitumen to produce a binder with increased stiffness.
The polymer type will determine the general characteristics of the PMB, whilst the particular grade of polymer (intra-classification within a type of polymer and level of modification) will determine the performance of the modified binder. Pavement designers and practitioners need to take account of the physical characteristics, construction constraints and cost benefit/performance balance in order to select the best polymer type and grade for intended application.
FACTORS IN SELECTION OF TYPE AND GRADE
Selection of a particular type and grade of polymer will naturally depend on the performance requirements for the modified binder, as well as the existing surface condition and the anticipated service conditions. The properties of the type and grade of polymer selected for binder modification and its ability to cope with the anticipated service requirements are some of the other factors that need to be considered before PMBs are selected for use in a particular road situation.
The performance of PMBs is highly dependent upon their polymer chemical composition, concentration and molecular arrangement, as well as factors relating to the origin and processing of the bitumen. Wherever possible, the selection of PMB should be based on past performance of binder in a similar situation. In the absence of such information, the results of detailed laboratory characterisation of the modified binder should be taken as a guide to its potential performance rather than the type of polymer alone. The type only indicates the characteristics of the polymer used, whereas in some instances, adequate dispersion of the polymer in the bitumen can be achieved only by the addition of process oil and other chemical additives. These additives can influence the characteristics of the resultant PMB.
When a polymer is added to a bitumen binder, the fluid characteristics of the product are very different from those of conventional bitumen. As the cohesive and adhesive characteristics of PMBs are thought to be opposed, any increase in binder cohesion as a result of polymer modification would lead to a decrease in binder adhesion. In general, PMBs are mom resilient, viscous and cohesive, and consequently less able initially to adhere to the aggregate (the exception being PMB emulsions) than conventional binders and therefore require addition of an adhesion agent in order to promote bonding with the aggregate. When higher concentrations of polymers are used adhesion of PMBs to aggregate becomes increasingly difficult, even when the aggregate is properly precoated. Compatibility and effectiveness of the aggregate precoating materials and adhesion agents for a particular application of PMBs should be tested before selecting a PMB/aggregate/precoating material/adhesion agent combination. Both initial adhesion and stripping tests should be used for the evaluation.