On the majority of roads, conventional bitumen grades possess satisfactory adhesion and mechanical properties for use in asphalt pavements for the vast majority of traffic and climatic conditions that are encountered. However, demands made on roads increase year by year and, in some cases, the limits of performance of conventional bitumens have been reached mainly due to
- increases in rainfall and temperature variations
- increases in axle weight, tyre pressures and freight movement, leading to a higher maintenance requirement and consequent increased costs for road owners
- a tendency to use thinner layers in pavements
- an ongoing demand for financial efficiency and cost savings to reduce the frequency of maintenance required in order to minimise disruption to traffic flow, and to provide an increased service life.
Thus, road failures happen earlier than expected, and maintenance downtime becomes more expensive for road owners. One means of overcoming these challenges is to modify bitumen with polymers or other additives to provide improved pavement performance. Modification technology depends on the specific applications and techniques (surface dressings, thin surface course systems, porous asphalt, surface courses for heavily trafficked roads, bases for heavily trafficked roads, anti-cracking membranes, bridge waterproofing layers, etc.). The degree of improvement required, and hence the cost, will depend on the particular needs of the site where treatment is proposed. In this chapter, the main additives used for bitumen modification for road applications (Brown et al., 1990) are described. The polymers most commonly used are elastomers, plastomers, reclaimed tyre rubbers and, to a lesser extent, viscosity modifiers and reactive polymers. Special asphalts for road and industrial applications, developed through particular processes such as the use of clear pigmentable binders, multigrade bitumens, cold application bituminous adhesives and a special bitumen additive for odour neutralization are also discussed. Finally, basic properties of polymer modified bitumens (PMBs) and the related test methods, the current industry practice for manufacturing and handling PMBs, and some general information on the performance of PMBs for use on road applications are considered.